medium voltage - its use control and application by john a kay, cet

8/13/2019 Medium Voltage - Its Use Control and Application by John a Kay, CET

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PRELIMINARY COPY

M E D I U M V O L T A G E

I T S U S E ,

C O N T R O L A N D A P P L I C A T I O N

B Y

-RK-RK-RK-RKQ $1 .D\/ &(7Q $1 .D\/ &(7Q $1 .D\/ &(7Q $1 .D\/ &(7Copyright 1998,1999

rev. 0.9I back up master on LANrev date Nov 09, 1999 22:55


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0.0 Index

1.0 Abstract

2.0 What is Medium Voltage

3.0 Where Medium Voltage is used3.1 Why is medium voltage used3.2 NEMA Classifications

4.0 Types of Medium Voltage Motors4.1 Squirrel Cage Induction4.2 Wound Rotor4.3 Multi-Speed

4.4 Wye-Delta4.5 Part-Winding4.6 Synchronous

4.6.1 Brush type Synchronous Motors4.6.2 Brushless Synchronous Motors

5.0 Starting Medium Voltage Motors

6.0 Starting The Induction Motor at Full Voltage

6.1 Wye-Delta Starting6.2 Multi-Speed Starting6.3 Part Winding Starting6.4 Capacitor Assistance Starting6.5 Wound Rotor Starting

7.0 Reduced Voltage Starting7.1 Reactor Starting7.2 Autotransformer Starting

7.3 Solid State Soft Starting

8.0 Torque Requirements8.1 Reducing the Starting Voltage and Current of an

Induction motor


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9.0 Synchronous Motors and Controls9.1 The Synchronous Motor, Brush type9.2 The Synchronous Motor, Brushless type

10.0 Variable Frequency AC Drive Systems10.1 Current Source PWM10.2 VSI

11.0 Transformer Switching

12.0 Capacitor Bank Switching

13.0 Load Break Switches13.1 Main Switch

13.2 Feeder Switch13.3 Tie Switch13.4 Main-Tie-Main Switch

14.0 Plugging

15.0 Re-Generative Braking

16. 0 Dynamic Braking of Induction Motors

16.1 D.C. Injection Braking16.2 Dynamic Braking of Synchronous Motors

17.0 Medium Voltage Controllers versus Switchgear

18.0 Summary of Medium Voltage Controller Benefits


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Copyright 1998, John A. Kay, CET

All rights reserved. No part of this book may be used or reproduced in any form or by any means,or stored in a database or retrieval system, without prior written permission of the author. Makingcopies of any part of this book for any purpose other than your own personal use is a violation ofthe copyright laws of Canada and the United States.

The material contained within is provided without warranty of any kind either expressed orimplied. The author and his reviewers have taken the time to validate the technical informationcontained within. However, the author does not attest to the accuracy of the entire contents ofthis book and takes no legal responsibility for typographical or technical errors contained within.


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1.0 Abstract

The purpose of this document is to provide a brief technicaloverview of the uses and control of Medium voltage. This document

can be used for the development of sales, marketing, and supportstaff with responsibilities that include the coordination of the quotationand sale of Allen-Bradley Medium voltage Control equipment. Asuggested companion document is The Principle Elements ofRockwell Automation/Allen-Bradley Medium voltage Motor Control,by the same author.


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2.0 What is medium voltage?

Medium voltage, as defined by NEMA*, is a voltage levelbetween 1000 Volts AC and 13,800 Volts AC. The most commonuse of medium voltage, in North America, is at levels of 2400 volts,

4160 volts, 4800 volts and 13,800 volts. Voltages of 7200 and 3300are not widely used in North America. However, in other parts of theworld, these levels of voltage are very commonly utilized incommercial and industrial applications. Table 1.0 illustrates the widevariety of typical industrial medium voltage levels and frequencies ofsome areas of the world. (*NEMA- Nation Electrical ManufacturersAssociation)

Frequencies of 25 Hz, 50 Hz and 60 Hz are typical at mediumvoltage. The majority of power systems in North America are 60 Hz.

However, some older pulp and paper mills that still co-generatepower with older equipment, do still operate parts of their facilities at25 Hz. Also, many countries outside North America, use a 50 Hzpower line frequency. Rockwell Automation currently constructsmedium voltage control equipment for voltage levels up to 7,200 voltsat 50 and 60 Hz applications only. Requirements for 25 Hz controlequipment requires specific definition and clarification with thesupplier.

World Industrial Medium Voltage Electricity Supplies

Country LineFreq.

Line

VoltageCountry Line

Freq.Line

Voltage

Argentina 50 6800 Japan (East) 50 6600

Australia 50 6600 Japan (West) 60 6600

Belgium 50 6000 Philippines 60 4160

Bermuda 60 4160 Romania 50 6000

Czechoslovakia 50 6000 Singapore 50 6600

Egypt 50 6600 South Africa 50 6600

Germany 50 6000 Turkey 50 6300Iraq 50 6600 Venezuela 60 4160

Israel 50 6300 Yugoslavia 50 6600*Many European states have variations of these base standards. Always

check with the local power supplier in the area of the final installation site.

Table 1.0


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3.0 Where is Medium voltage used?

Typical applications for Medium voltage include the control ofmotors larger than 200 horsepower, in industries such as pulp and

paper, steel, rubber, cement, mining, petrochemical refining andpipelines, food processing and utilities. Other commercial applicationsinclude water and waste water plants, public buildings for airconditioning, pumps, fans and compressors.

Medium voltage is also used as the input to transformers that stepdown the medium voltage potentials to lower distribution voltages. Theselower voltages, (240, 360, 480, 575 and 600) are those typically used inmotor control centers (MCC) such as the Allen-Bradley 2100 and 2400product families as well as low voltage drives such as the Allen-Bradley

1300 Products. (Refer to Section 11)


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Medium Voltage Switchgear

Medium Voltage Control Center

Medium Voltage InductionMotors

Low Voltage Motor ControlCenter

Low Voltage InductionMotors

Medium Voltage Drive

Drive IsolationTransformer

Step DownTransformer

Step DownTransformer

Typical Industrial Distribution System


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3.1 Why is medium voltage used?

Generally, a medium voltage motor controller would only be appliedon motors larger than 200 HP. In most large facilities, low voltagemotor controls are used for motors up to and including 200 HP. Onhigher horsepower applications, the full load current, at mediumvoltage, is considerably less than at low voltage, see example 1.0.

Example 1.0 500 HP @ 480 Volts = 625 FLA500 HP @ 4160 Volts = 68 FLA

Therefore, a medium voltage motor and motor controlinstallation could be less expensive than low voltage motors andrequire smaller fuses, current transformers and power wiring.

At 200 HP and above it is usually less expensive to use amedium voltage motor and motor controller than a low voltagesolution, providinga medium voltage distribution system exists inthe facility.


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3.2 NEMA Classifications

The NEMA standards class this type of equipment as class E.This refers to motor starters for synchronous, induction and woundrotor motors rated up to 7200 Volts.

NEMA class E2 controllers incorporate high-interruptingcapacity fast-acting power fuses. These current-limiting fuses protectthe connected equipment, the load cables to the equipment and thecontroller against the high short-circuit current available from powersystems.

NEMA class E1 controllers include instantaneous over currentrelays to signal the contactor or breaker to open on fault current.

These relays are in addition to typical motor protection relays. NEMAclass E1 controllers may be employed on systems having availableshort circuit currents less than the interrupting rating of the contactor.

All Medium voltage motor control equipment, supplied by Allen-Bradley, is typically E2 rated.


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4.0 Types of medium voltage motors

Induction Types

Squirrel Cage Wound Rotor Multi-Speed Wye-Delta Part Winding

Synchronous

Brush type Brushless

4.1 Squirrel cage induction motors

Squirrel cage induction motors, because of their simplicity,ruggedness and reliability, have practically become the acceptedstandard for alternating current, all purpose, constant speed motorapplications. As a result, various kinds of control equipment andstarting methods have been developed for these types of motors.

The squirrel cage motor consists of a fixed frame or stator,carrying the stator windings and a rotating member called therotor. The rotor is made up of steel or iron laminations welded tothe motor shaft. The rotor windings consist of many copper oraluminum bars fitted into slots in the rotor member, with each barconnected at each end by a continuous conductive ring. (refer tofig. 4.1a)


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Figure 4.1a

A typical three phase, medium voltage squirrel cage inductionmotor, has three stator windings wound on the stator frame.These three windings are directly connected to medium voltagethrough a motor controller. This type of control applies full voltageto the motor by way of a contactor and as such, this method is

called Full Voltage Starting, figure 4.1b. (See section 5.0)

M

Fig 4.1bFull Voltage Starting


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When three phase alternating current is applied to the statorwindings, a rotating magnetic field is produced within the stator.This field revolves around the rotor at a speed of (120 X line

frequency) number of motor poles. E.g.: With a 60 Hz system

frequency, a motor with 2 poles will rotate at a no load speed of3600 RPM, (120 X 60) 2.

These revolving stator fields induce currents through an air gapto the conductive bars on the rotor. These currents will be highestwhen the rotor is at a standstill and will decrease as the motorcomes up to full speed. These magnetic forces, induced on therotor, will cause the rotor to turn in the direction of the rotating fieldwithin the stator windings. The motor will accelerate until a speedis reached corresponding to the required slip* to overcome

windage and bearing friction. This speed is referred to as thenoload speed. The squirrel cage induction motor can never reach aspeed that is synchronous to the line frequency. If the motor wasto reach synchronous speed, no current would be induced in therotor and the motor would produce no torque. (Refer to sec. 6.0)

The initial inrush currents, locked rotor currents and theresulting torque values produced are the factors that determinewhether the motor can be applied directly across the line or

whether the current has to be reduced to get the requiredperformance to match the load requirements and/or utility linevoltage flicker or voltage dip specifications.

*Slip The difference between the rotor RPM and the rotating magnetic field of an

AC motor


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4.2 Wound rotor motors

This style of induction motor was designed to provide variablespeed operation long before the invention of variable frequency

drives. The stator of the motor is very similar, and in some cases,identical, to that of a standard medium voltage polyphase squirrelcage induction motor. However, the rotor design differssignificantly.

The rotor of such a machine consists of a laminated iron core, theouter periphery of which contains slots into which are inserted theform-wound coils. These coils are grouped to form a 3 phase Y(wye) connected winding having the same number of poles as thestator winding. The open ends of the three rotor phases are

connected to slip-rings mounted on the shaft. Brushes ride on theconductive slip rings to connect the rotor windings to an externalresistor network and shorting contactor arrangement. The rotorslip losses are dissipated in the external resistors versus theinternal rotor windings.

The starting torque can be raised by increasing the resistanceof the rotor circuit. This has the effect of raising the rotor powerfactor so that the rotor currents are more nearly in phase with thestator flux. At a certain critical resistance, where Rr = Xr, themotor can be made to exert maximum starting torque. If theresistance is either above or below this critical value, the startingtorque will be decreased again.

A rotor containing enough resistance to give maximum startingtorque is not satisfactory, after the motor gets up to speed, as itproduces poor running characteristics. An increase of rotorresistance causes the slip to increase for any given value of load.Although the maximum torque which the motor can exert may not

be changed, still the speed at which this torque is exerted islowered, if the rotor resistance is high. If the motor carries its loadat a reduced speed, it is running inefficiently, since the rotorcopper losses are increased. What is desirable is a rotor ofcomparatively high resistance for starting but of low resistance forrunning.


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In contrast with a squirrel cage motor, a wound rotor motorshould not be started on full voltage with zero external resistancein the rotor circuit, that is, with slip-rings shorted. Under theseconditions, the starting current is usually higher than that of asquirrel cage induction motor of the same rating, and the startingtorque is lower. Also, there is a considerable possibility ofdamage to mechanical elements in the rotor, to any solderedconnections of the windings, and to winding insulation, particularlyif the motor is started repeatedly in this manner.

The wound rotor motor is useful where high starting torque withlow starting current is desired. Heavy loads can be started andaccelerated slowly and smoothly, without undue line disturbance.It is also used where speed regulation is desired as on fans and

centrifugal pumps.

Refer to section 6.5 for complete details of how to control of thistype of motor.

Fig 4.2a Typical wound rotor control


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4.3 Types of medium voltage multi-speed motors

There are two different types of medium voltage multi-speed startersoffer by Rockwell Automation/Allen-Bradley. These are the separate

windingand the consequent polestarter. These two types alsohave specific configurations supplying specific output characteristics.Each type may have either a constant horsepower, constant torqueor variable torque. The following sections relate to the design andoperating characteristics of multi-speed motors and their associatedcontrollers.

Operating characteristics of multi-speed motors

Constant Horsepower

(a) Horsepower rating constant.(b) Torque rating varies inversely with speed.e.g., machine tool spindles

Constant Torque(a) Horsepower rating varies as the speed.(b) Torque rating is constant.e.g. conveyer

Variable Torque(a) Horsepower rating varies as the square of thespeed.

(b) Torque rating varies as the speed.e.g. fan, blower or centrifugal

The consequent pole motor has a winding for each of the twospeeds. A typical two speed consequent pole motor controlleruses 3 contactors. (see Figure 4.3a).

A separate winding motor has a separate winding foreach speed. The controller would use only 2 contactors. (see Figure4.3b)


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4.3.1 Consequent pole:

Consequent pole motors get their name from the way thewindings are reconnected to obtain different speeds. For a typicalarrangement for a three phase, four-pole motor, the direction of thecurrent in the various portions of the windings produces north andsouth poles in alternate winding loops. If one-half of the winding isnow reconnected, the direction of the current produces south poles ineach winding loop. Therefore, south poles cannot exist alone, sonorth poles are created between the loops as a consequence. Theresult is now an eight-pole motor that will run at half the speed of thefour-pole arrangement. A two-speed consequent pole motor alwayshas a speed ratio of 2 to 1.

A two-speed consequent pole medium voltage controllerconsists of three, three-pole contactors mechanically and electricallyinterlocked. For one speed, a three-pole contactor connects themotor across the line. For the other speed, another contactorconnects three different leads of the motor across the line. The othercontactor is used to tie together the original three leads. See fig. 4.3a

In Table 4.1, it can be seen that the torque and the horsepowervary with speed for each type of multi-speed motor. It can be notedthat the torque, and therefore the current, of constant or variabletorque motors, is never greater than 100%. That is, the current isnever greater than it is at full speed. This is not true for constanthorsepower motors. Here the torque (and the current) increase atthe lower speeds in order to maintain the constant horsepowercharacteristic. For this reason, the horsepower ratings are lower forconstant horsepower starters than for constant or variable torquestarters for any given size.


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The values shown in Table 4.1 indicate the horsepower andtorque at the full speed of each winding arrangement. They do notindicate the output characteristics as the motor is coming up to theselected speed.

Constant HP Constant Torque Variable Torque

speed Torque HP Torque HP Torque HPfull 1 1 1 1 1 1two-thirds 3/2 1 1 2/3 2/3 4/9half 2 1 1 1/2 1/2 1/4one-third 3 1 1 1/3 1/3 1/9

Table 4.1 Output characteristics for multi-speed motors.


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Consequent pole summary

(a) Single winding, reconnected.(b) Reconnecting creates twice the poles as a

consequence.(c) Speed ratios always 2:1

T1

T4 T2

T5

T3T6

SS

FS

S

SS = Slow SpeedFS = Fast Speed

S = Shorting Contactor

1LNCONPLTQ.VSD

Constant or VariableTorque

Figure 4.3a Two Speed Consequent Pole


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4.3.2 Separate Winding

Separate winding motors have, as the name implies, a separatewinding for each speed. Thus, a two-speed motor has two windings,a three-speed motor has three windings, and so on. These windingsare electrically separate and each is capable of delivering ratedhorsepower at rated speed. The mechanical arrangement of thewindings determines the poles per phase, the more poles the slowerthe rotor speed. Since the windings are independent of each other,so too are the speeds. A two-speed separate winding motor cantherefore have widely different speeds such as 3600/600 RPM. Thespeeds can also be quite close such as 900/720 RPM.

Each winding requires a three-pole contactor to connect it

across the line. A two-speed starter is constructed similar to astandard reversing starter in the sense that two main contactors arerequired. The exceptions are that two sets of current transformersand overloads are required, one for each speed. The two contactorsare mounted inside one enclosure and are mechanically andelectrically interlocked to a single isolation switch.

Separate winding summary

(a) A separate winding for each speed.(b) Windings are electrically separate.(c) Each is capable of delivering rated HP at rated

speed.(d) Independent speeds.(e) Motor types (constant HP, constant or variable

torque) has no effect on motor connections.


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FS

SS

FS = Fast speedSS = Slow speed

Figure 4.3b Two Speed Separate Winding


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4.3.3 Motor symbols and starter wiring

A) Separate winding

Separate winding starters consist of an assembly of three-polecontactors, one contactor for each speed. When wiring such a starterto its motor, it is not important to know the output characteristics ofthe motor. That is, it is immaterial whether the motor is constanthorsepower, constant torque, or variable torque. The symbols for aseparate winding motor are shown below:

T1 T11 T21

T3 T2 T13 T12 T23 T22

Figure 4.3.3.1

B) Consequent pole winding

For consequent pole applications, it is important to know whether themotor is constant horsepower, constant torque or variable torque.The standard symbols for these three types are shown below infigures 4.3.3.2, 4.3.3.3 and 4.3.3.4


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Constant Horsepower

T3T1

T4

T6T2

T5

Figure 4.3.3.2

Speed line 1 line 2 line 3 open tied

together

LOW T1 T2 T3 T4, T5, T6

HIGH T6 T4 T5 T1, T2, T3

Table 4.3.3.1

Therefore, in the low speed the connections are as in Table 4.3.3.1 inthe parallel star. In the high speed, the connections are in the seriesdelta configuration.


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CONSTANT TORQUE

T1

T4

T3

T5 T2 T6

Figure 4.3.3.3


together

LOW T1 T2 T3 T4, T5, T6


Table 4.3.3.2

The low speed connections are in the series deltaconfiguration where as the high speed connection is the parallel star.


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VARIABLE TORQUE

T1

T4

T3

T2

T6T5

Figure 4.3.3.4


together

LOW T1 T2 T3 T4, T5, T6


Table 4.3.3.3

In the low speed, terminals 1, 2, and 3 are connected in seriesstar. At high speed, terminals 1, 2, and 3 are tied together and 4, 5,and 6 are connected in parallel star.

You will notice that in the figures above that the configurationsare all very similar. In fact, the arrangement is the same for theconstant torque and the variable torque motors. The same motorleads are connected to the lines for each speed. In addition, leads

T1, T2 and T3 are tied together in the high speed. For the constanthorsepower motors, it is leads T4, T5 and T6 that are tied together,and this, occurs for the low speed.


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Because the constant and variable torque starters are the samefor winding arrangement and also for horsepower ratings, they aregrouped together. Physically, the starters look the same, regardlessof the type of motor. A two-speed consequent pole starter consists ofthree contactors, electrically and mechanically interlocked.

There are two types of multi-speed starter that RockwellAutomation/Allen-Bradley builds. The Bulletin.1522is a Non-Reversing Medium Voltage Multi-Speed Squirrel Cage InductionMotor Starter. The other type, Bulletin. 1526, is the ReversingMedium Voltage Multi-Speed Squirrel Cage Induction Motor Starter.


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4.4 Wye-Delta motors

Star or Wye-Delta starting of squirrel cage motors has alwaysbeen popular in Europe. This starting method is now employed inNorth America on large refrigeration compressors and similar

compressor applications. The big advantage is that no resistorsor transformers are present to produce heat. The disadvantage isthat starting torque is low and there are no torque/voltageadjustments.

Any squirrel cage motor can be started by this method,provided that it is wound to run with delta connected windings(with full line voltage applied to each winding). There must be 6wires brought out, both ends of each phase.

During start, the motor is connected in Y(wye or star) so thatthe voltage applied to each winding reduces to one over thesquare root of three (0.577) of rated voltage. The starting torquewill be the square of this value of the full voltage starting torque,or exactly one-third of the normal starting torque. There is no wayto alter this value.

If the motor were started with the windings in their normaldelta configuration, the current taken from each line of the

supply system would be root three times the current in eachwinding of the motor. In the star connection, however, the linecurrent is only one over the root of three, (1/1.73) of what it wouldhave been with the delta connection. The starting current withstar connection is therefore exactly one-third of the line currentwith delta connection.

Star-Delta starting therefore, reduces both starting current andtorque to one-third of the full voltage value. This is even better

than the ratio achieved with transformer starters, because there isno transformer loss. With a typical motor, the starting torquemight be 40 % of the rated full load/full speed torque. This isenough to start a lightly loaded machine, but not enough to moveany sort of loaded machine.


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With Star-Delta starting, the contactors are connected in themotor legs, not in the supply line. This reduces the current to57.7% of the line current. The overload relay settings must beselected with this in mind.

The basic star-delta circuit is closed transition from START (instar) to RUN (in delta). The added resistors are incorporated insuch a way that the circuit is not opened during the transition. Ifthe limited torque is acceptable, star-delta is a very good way tostart a motor. Figure 4.4a outlines the three line diagram for thewye-delta starter. Special attention is required to insure theinterlocking scheme with the position of the transition resistors.


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T6

T1 T2 T3

T5T4

1M

2M

W _D.vsd

Wye-Delta

1A

S

"1M" & "2M" interlocked to isolation switch"2M" interlocked to "S"

Figure 4.4a


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4.5 Part Winding

Part winding is another method traditionally used by the air-

conditioning industry, mostly with motors of medium size. Themethod has little to recommend it except the first cost.

In this scheme, two separate (and usually identical) windings areplaced on the motor frame. In the running position, both windingsare connected to the line (in parallel) and each winding carrieshalf the load. There is a magnetic switch and a set of overloadrelays for each winding.

To start, one of the switches is closed. The inrush current will

naturally be lower than if both windings were energized. Wemight expect the starting current to be cut in half, but in practice, itis more likely to be reduced by only 30 to 35 %.

The starting torque, unfortunately, is always low. Typically, it willbe only 40 % of the torque that would be supplied at starting byboth windings acting together. For this reason, the scheme isapplicable only to unloaded machines.

The starting current, being concentrated in one winding,constitutes a heavy overload. Properly chosen overload relaysare apt to trip out in about 5 seconds. Hence, part-winding startsmust be treated as increment type. The timer which brings inthe second winding must be set for 1 or 2 seconds, not longer.

Part-winding starting must not be tried unless the motor has beenwound for the purpose. Even if the motor is wound with partwindings, there is likely to be considerable noise and vibrationduring the starting period. The transition from START to RUN,

however, is quite smooth and free of transient disturbances.

Because the overload relays are selected for half the total current,CSA requirements limit the branch-circuit fuses to no more thandouble the total full load current. This is the same as saying 4times the rated current of each winding.


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Part-winding starting has the virtue of low cost. There are noresistors or transformers, unless a third starting point is wanted.

In summary, the part winding motor, the stator windings (eachphase) consists of two parallel sections. During starting, half ofthe windings are connected to the line to provide enough torque tobegin acceleration. Then, after a short time, the other parallelwindings are also connected to the line.

The principal virtue of this type of motor is its low cost.However, torque efficiency is low and there may be torque dips inthe torque speed curve. Torque varies significantly depending onthe motors rated speed.

Figure 4.5a Part Winding


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4.6.1 Brush Type Synchronous Motor

The rotor contains laminated poles that carry the D.C. field coils thatare terminated at the slip rings. It also has a squirrel cage windingwhich is made up of bars embedded in the pole faces and shorted byend rings. The squirrel cage winding is also known as "damper" or"amortisseur" winding. It is this winding that enables the motor toaccelerate to 95% speed were the D.C. supply can be applied to thefield windings for synchronizing the motor to the line.

S

N

S

N

S

N

S

N

DC fromField

Contactor

3333AC

SYNCMT R.VSD

+

-

The field windings are connected through the slip rings to a dischargeresistor during start up. The ratio of this resistance to the fieldwinding has a significant effect on the starting torque, the torque atpull-in and, to a lesser degree, to the starting KVA. During the startsequence, the discharge resistor is required to dissipate the highvoltages that are induced into the field windings from the stator, andis removed from the circuit when the D.C. field voltage is applied.The synchronous motor can be compared to a transformer, the three

phase stator resembling the primary and the field winding acting likea secondary. In addition, the field winding has more turns than thestator so the medium voltage is actually stepped up to a field windingthat is insulated for low voltage. This transformer action is where theinduced signal is found to determine when the D.C. field can beexcited for synchronization. At zero speed, the frequency inducedinto the field is 60 Hz, at 95% speed the frequency induced is 3 Hz.


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When at approximately 95% speed, the D.C. field, (rotor) is suppliedwith either 125 VDC or 250 VDC and the discharge resistor isremoved from the circuit. The excitation in the field windings createsconstant north and south poles in the rotor, which would lock, into therotating magnetic field of the stator. The slip rings are used toconnect the field windings to the discharge resistor and static exciter.

4.6.2 Brushless Synchronous Motors

By the 1960s, solid state diodes and thyristors had advanced towhere they could carry the current and block the voltages necessaryfor industrial motor control applications. It was at this time thatBrushless Synchronous Motors were developed. The rotor has a

three-phase A.C. armature winding. The stationary field winding is onpoles on the stator and is connected to a variac and rectifier for theD.C. supply. The generated A.C. current is directly connected alongthe shaft to a rotating diode wheel, where it is rectified to D.C. beforegoing to the motor field. The magnitude of the field current is adjustedby changing the current to the stationary exciter field.

Electronic

Control andSensing

Component

s

Armature Rectifier Field Application Circuit Field ArmatureField

Exciter Rotating Heat Sink AssemblySynchronous

Portion

DC to ExciterField

Rotating Portion

3 PhaseAC

DischargeResistor


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Some disadvantages encountered with brush type synchronousmotors are;

More complex motor protection and control system required

The necessity of a DC excitation source and control.

Relatively low starting torque per KVA.

Maintenance requirements of commutation system

Cannot be used in hazardous locations

Features of the brushless motor;

No brushes, no carbon dust, no brush maintenance

No commutator/slip rings to resurface

Completely automatic field application

Automatic resynchronization

No sparking. Can be used in hazardous locations

No additional static exciter cabinet required


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5.0 Starting Methods for Medium Voltage Motors

There are several factors to be considered when selecting thestarting equipment for any electric motor driven load. Theseinclude, but are not limited to:

1. The starting torque requirements of the load

2. The motor starting characteristics (torque) that will closelymatch the load characteristics at full load and speed

3. The source of power and the effects the motor startingcurrent will have on the line or system voltage

4. The effect of the motor starting torque on the driven load

There are three fundamental methods of starting squirrel cageinduction motors; full voltage starting, reduced voltagestarting and variable frequency starting.

Full voltage starting can be used whenever the driven loadcan withstand the shock of instantaneously applying full voltage tothe motor and where line disturbances can be tolerated. Fullvoltage starting uses a main contactor to apply the motor statorwindings directly across the main system voltage.(ref. Fig. 4.1b)This type of starting method provides the lowest cost, a basic andsimple design of controller, resulting in low maintenance and thehighest starting torque.


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Reduced voltage starting may be required if full voltagestarting creates objectionable line disturbances on the distributionsystem or where reduction of mechanical stress to gear boxes orbelt drive systems is required. It must be noted that when thevoltage is reduced from nominal, a decrease in inrush current willoccur at a rate of 12% for every 10% decrease in voltage. Thestarting torque will also decrease at a rate of 20% for every 10%decrease in voltage. This phenomenon also occurs in the oppositemanner when the voltage is increased.

A common rule of thumb is; "If the load cannot beaccelerated to full speed using full voltage and current, itcannot be accelerated to full speed using reduced voltageand current."

Variable frequency startingprovides infinite speed control ofan induction and synchronous motor. The drive converts the linefrequency to a D.C. level. It then recreates an output A.C. waveform at variable frequency levels. Drive types and control will bediscussed in section 10.


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GENERAL EFFECTS OF VOLTAGE AND FREQUENCY VARIATIONS ON INDUCTION M

Efficiencstarting and

maximum

running

torque

sync.

speed

% slip full load

speed

full load

current

starting

current

temp. rise

at full

load

max.

overload

capacity

magnetic

noise, no

load

full load 3/4

loaded

120% 44%

increase

no

change

30%

decrease

1.5%

increase

11%

decrease

25%

increase

5 to 6 C

decrease

44%

increase

noticeable

increasesmall

increase

1/2 to 2

point

decreaseVoltage 110% 21%

increase

no

change

17%

decrease

1%

increase

7%

decrease

10 to

12%

increase

3 to 4 C

decrease

21%

increase

slight

increase

1/2 to 1

point

increase

no

change

Variations function of

voltage(voltage)2 constant

1

(voltage)2(sync.

speed

slip)

voltage (Voltage)2

90% 19%

decrease

no

change

23%

increase

1.5%

decrease

11%

increase

10 to 12%

decrease6 to 7 C

increase

19%

decrease

slight

decrease

2

point

decrease

no

change

105% 10%

decrease

5%

increase

no

change

5%

increase

slight

decrease

5 to 6%

decrease

slight

decrease

slight

decrease

slight

decrease

slight

increase

slight

increaseFrequency Function of

Frequency 1(Freq.)2

frequency (sync.speed

slip)

1

frequency

Variations 95% 11%

increase

5%

decrease

no

change

5%

decrease

slight

increase

5 to 6 %

increase

slight

increase

slight

increase

slight

increase

slight

decrease

slight

decrease

References:

Industrial Electricity, Vol. II; Dawes

Electric Motors in Industry; Shoults, Rife, Johnson

Electric Machinery and Control; Kosow

Electrical Systems and Equipment for Industry: Moore, Elonka

John A. Kay table.doc


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6.0 Starting an Induction Motor with Full Voltage

Figure 6.0a depicts the behavior of the current required by an 1800 RPMinduction motor at various speeds. Two facts stand out: First, thestarting current is quite high compared to the running current; andsecond, the starting current remains fairly constant at this high value asthe speed of the machine increases and then drops sharply during thelast portion of acceleration to full operating speed.

0

100

200

300

400

500

600

0 300 500 900 1200 1400 1700 1800

Motor Current-Speed Curve%

M

ot

o

r

F

u

l

l

L

o

a

d

C

u

r

r

e

n

t

Rotor Speed (RPM)

Figure 6.0a

This means, of course, that the heating effect, to the rotor and thewindings, is quite high during acceleration since it is a function of IT. Italso means that a motor may be considered to be in the locked rotorcondition during nearly all of the accelerating period.


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The example shown in Figure 6.0b depicts the typical characteristicsof a Class B induction motor. As the horsepower gets higher, thepercentage starting torque will be lower and the sag in the torque-speedcurve will become more pronounced.

The torque of an induction motor is a function of the square of itsinduced rotor current, and is therefore approximately the square of itsline current. If the starting voltage is reduced to 50%, the motor currentwill drop to 50% of normal full voltage starting current, but the torque willdrop to 25% which comes from (0.5)2= 0.25 or 25%. Were it not for thisfact, reduced voltage starting methods would not create the problemsthat they do. The inverse is also true. If you increase the voltage thetorque will increase in the same proportion. See Fig. 6.0c.

0

20

40

60

80

100

120

140

160

0 300 500 900 1200 1400 1700 1800

110 % Voltage100 % Voltage

90 % Voltage

50 % Voltage

%o

fR

atedTorque

Rotor Speed (RPM)

Figure 6.0c


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6.1 Wye-Delta Starting

This type of starting method is used for applications where lowstarting torque is required for loads such as fans and compressors.

there are some limitations with this type of starting method. Limitationsinclude a starting characteristic that is not adjustable, as with variousreduced voltage methods and the fact that the motor must have a deltastator winding with extra leads for the wye connections, see Fig. 4.4a.

Star-Delta starting of squirrel cage motors has always been popularin Europe. It is employed in North America on large refrigerationcompressors and similar applications. The biggest advantage versusautotransformer and consequent pole starting is that no resistors ortransformers are present to produce heat. The disadvantage is that

starting torque is low and there are no adjustments.

Any squirrel cage motor can be started by this method, provided thatit is wound to run with delta connected windings (with full line voltageapplied to each winding). There must be 6 wires brought out, both endsof each phase.During the start cycle, the motor is first connected in a Y (wye or star)so that the voltage applied to each winding reduces to one over thesquare root of three (0.577) of rated voltage. The starting torque will bethe square of the value of the full voltage starting torque, or exactly one-third of the normal starting torque. There is no way to alter this value.

If the motor were started with the windings in their normal deltaconfiguration, the current taken from each line of the supply systemwould be root three times the current in each winding of the motor. Inthe star or Y connection, however, the line current is only one over theroot of three of what it would have been with the delta connection. Thestarting current with a star connection is therefore exactly one-third ofthe line current with a delta connection.


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6.2 Multi-Speed Motor Starting

Controllers for multi-speed motors consist of two or more contactorsdepending on the motor type (refer to section 4.3) The configuration ofthe contactors and their associated control circuits are dependent on themotor type and method of the starting sequence.

If the motor is allowed to start at any desired speed and also allowed tochange speed at any time, the control is described as selective start.One run push button is used for each speed and there is only onestop push button. If the control will allow for starting in only one speedbefore switching to the other speed, this control type is compelling. Ifthe motor is started in low speed and cannot be started in high speed,

the control is described as a low speed compelling. But if the controlallows only a start at the higher speed before allowing the motor to go toa lower speed, the control is described as a high speed compelling.

Shown below are the specific control circuit configurations for variousmulti-speed motor controllers

MULTI-SPEED CONTROL CIRCUITS

Fast

Form 'A' Slow speed compelling

StopC R

S

FastS

F

CR

F

Slow

Over- load

S

C R

CR

Slow

Over lapping


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TR1

Stop

C R1

S

TR1

Over- load

Fast

CR1

CR 1

F

S

S

F

CR1

TR1

CR1

Slow

Form 'B' Automatic sequence acceleration

Stop

F

Over load

Fast

F

Slow

C R

Fast

C R

Slow

TR

S

S

F

TR

Form 'C' Deceleration


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Stop

CR A C RC

FastCR C

C RA

F

Slow

Overload

SCR A

Slow

Overlapping

S

TRC

F

TRC

CR A

Form 'AC' Slow speed compelling with automaticsequence deceleration

Fast

Stop

CRB

C RC

TRB

Overload

Fast

CR B

CRB

F

CRC

S

F

C RB

TRB

CR B

TRC

FTRC

TRB

Slow

Form 'BC' Automatic sequence acceleration anddeceleration


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6.3 Part-Winding Starting

This starting method reduces the in rush current to the motor by connectingthe sectionalized, parallel stator windings to the line in two or more steps.To accomplish this starting method, the motor must be designed with part-windings. If the motor can be designed for part-winding starting, theresultant starting torque and reduced in rush currents may meet therequirements for specific load and distribution systems at the least possibleexpense. Refer to section 4.5.

Part-winding is another starting method primarily used in the air-conditioning industry, mostly with motors of medium size. The method haslittle to recommend it except the first cost.

In this scheme, two separate (and usually identical) windings are placed onthe motor frame. In the running position, both windings are connected tothe line (in parallel) and each winding carries half the load. There is acontactor and a set of overload relays for each winding. To start, one of thecontactors is closed. The inrush current will naturally be lower than if bothwindings were energized. You might expect the starting current to be cut inhalf, but in practice it is more likely to be reduced by only 30 to 35 %.

The starting torque, unfortunately, is very low. Typically, it will be only 40% of the torque which would be supplied at starting the motor using bothwindings acting together. For this reason, the scheme is applicable only tolightly-loaded machines.

The starting current, being concentrated in one winding, constitutes aheavy overload. Properly chosen overload relays are apt to trip out inabout 5 seconds. Hence part-winding starts must be treated asincrement type. The timer which brings in the second winding must beset for 1 or 2 seconds, not longer. The load must accelerate quickly.

Part-winding starting must not be tried unless the motor has been woundfor the purpose. Even so, there is likely to be considerable noise andvibration during the starting period. The transition from START to RUN,however, is quite smooth and free of transient disturbances.


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Part-winding starting has the virtue of low cost. There are no resistors ortransformers, unless a third starting point is wanted. The contactors can behalf-size because each one supplies only one half of the motor current.


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6.4 Capacitor Assisted Starting

As was discussed in the previous section, both synchronous andinduction motors draw high current during starting. Starting in-rush

currents for an induction motor can be six or more times the motor fullload current. If the motor is relatively small and/or the distributionsystem is stiff enough to absorb this brief starting burden, no ill effectswill occur to the power system. However, if the starting current is veryhigh and/or if the distribution system is weak , the starting requirementscan cause the voltage on the local distribution system to fall so low thatother equipment may be affected. This voltage drop, in turn, reducesthe motor torque and there may not be enough motor torque toaccelerate the load.

This high current draw at starting will be reflected back into the localutilities power system. It can cause Voltage Flicker that affects thequality of the power being supplied to other customers. This is theprimary reason why most utilities dictate voltage flicker limits during thestarting of large loads.

One method of reducing this demand on the local distribution systemand utility is the application of capacitors to the motor terminals duringstarting. Capacitor assisted starting can provide an attractive solution tothis problem, from the standpoint of initial investment and operatingcosts.

Capacitors can be applied in two methods:

1. As a continuous correction device2. As a source of reactive power during motor starting.


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Figure 6.4a shows a typical Medium Voltage Motor with capacitorsconnected for continuous power factor correction.

L

Figure 6.4a

When power factor correction capacitors are connected across themotor terminals they must be sized no larger that the maximum sizerecommended by the motor manufacturer. The National Electric Code(NEC) also imposes limits on the size of capacitors that can beconnected across the motor and switched with the motor controller. Ifthe capacitors are too large and the motor is overhauled by the load,subjected to plug stopping, rapid reversing or jogging damage to themotor can occur.

Figure 6.4b shows a typical Medium Voltage Motor and controller wherecapacitors are used to assist in the reduction of current demand to thedistribution system during starting.

C1

C2

Figure 6.4b


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In this case, the contactor C2 is closed prior to closing the main motorstarting contactor C1. The capacitor bank is charged to full voltage andC1 is closed. Figure 6.4c indicates graphically the result of applying thecapacitor bank at start. With the capacitors placed across the motorterminals, the voltage will rise as the motor accelerates to full speed andthe required current will fall to full load requirements. If these capacitorswere allowed to remain connected, the voltage will rise in excess of themotors permissible level. Therefore, when capacitors are used to assistin starting only, the controller must have circuitry to sense the systemvoltage and switch part or all of the capacitors out of the motor circuit ifthe system voltage rises to an intolerable level.

Other methods that can be successfully applied to limit startingburden are reduced voltage controllers using autotransformers, reactors

or semi-conductors.

100 AmpsLine

Current

ReactiveCurrent

60 Amps

ActiveCurrent

80 Amps

ActiveCurrent

80 Amps

ReactiveCurrent

60 Amps

ActiveCurrent

80 Amps

Capacitor

Induction Motor LoadInduction Motor Load

100

80

Capaid.vsd

Fig 6.4c


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6.5 Wound Rotor Motor Starting

Section 4.2 outlined the configuration of the wound rotor motor.These types of motors are known for their ability to provide limited

speed control and adjustable torque efficiency. The control for thesetypes of motors is more complex than that of a standard induction motor.

The principle of the wound rotor motor is that by varying the rotorresistance at various points in the acceleration period can provide aspecific torque profile. During starting, a set of external resistors areconnected to the rotor through slip rings. As the motor acceleratesportions of the external resistors are shorted by contactors, controlled bytiming relays, until all external resistance is removed. (Figure 6.5a) Insome cases, one portion of resistance is left attached to the rotor if a

specific torque profile is needed for the driven load.

As stated above, wound rotor motors can be operated at specificspeeds. Rotor speed regulation differs from starting duty in two ways.The resistors used, external to the rotor, must be rated for continuousduty and the contactors used to switch in or out specific resistancevalues, must be rated for continuous duty. If a number of speed-regulating points are required, a drum switch is typically used in place ofthe timing relays used in starting duty.

Figure 6.5a


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The external resistors used are classified by the duty cycle required.Resistors designated as class 135, are typically used for starting dutyapplications. Resistors used for speed regulation are typically class 151or higher. Because of the difference in duty cycles, controllersincorporating class 151 and higher class resistors will be significantlylarger and more expensive than controllers incorporating only class 135resistors. The table below outlines various resistor classifications andtheir associated duty cycles.

Starting current in 5 sec. on 10 sec. on 15 sec. on 15 sec. on 15 sec. on 15 sec. onApproximatepercentage

percent of full load torque 75 sec. off 70 sec. off 75 sec. off 45 sec. off 30 sec. off 15 sec. off

of Full-Loadcurrent on Single Three

Generalpurpose Heavy Extra Heavy

first pointstarting phase phase Very light

Generalpurpose Heavy

Intermittentspeed

Intermittentspeed

Intermittentspeed

Continuousspeed

from reset starting starting Start ing duty Starting duty Starting duty regulationduty

regulationduty

regulationduty

regulationduty

25 15% 25% 111 131 141 151 161 171 9150 30% 50% 112 132 142 152 162 172 9270 40% 70% 113 133 143 153 163 173 93

100 55% 100% 114 134 144 154 164 174 94150 85% 150% 115 135* 145 155 165 175 95

200+ --- 200% 116 136 146 156 166 176 96

* Standard starting duty class provided as standard for motor starting


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7.0 Reduced Voltage Starting

Full voltage starting of motors can produce objectionablevoltage flicker. In cases where the supply system does not havethe capacity to meet the starting current requirements, these linedisturbances can be severe. Starting a motor at reduced voltagecan help reduce the amplitude of these disturbances.

Reduced voltage starting can be accomplished in several differentways;

Reactor starting; where a starting reactor is placed inseries with the motor for a period of time during starting.This method also reduces the voltage, current and torque

to the motor according to the reactor tap setting.

Autotransformer starting: automatically switchingbetween taps of an autotransformer reduces the voltage,current and torque to the motor according to the tapsetting used on the auto transformer.

Solid State Starting: a method (soft start, solid state)where a controller ramps the voltage from 0 volts (VoltageRamp) or from a preset level to the full system voltageover an adjustable ramp time. (current limiting). Units canalso be set for specific current level starting as well.

The reduction of starting voltage also reduces the availabletorque, to the driven load, by the square of the voltage. Table 7.0illustrates the effects on the torque available from the motor whenthe voltage is reduced in different starting methods.


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STARTING CHARACTERISTICS

STARTING Voltage to Motor Line Starting Torque **

METHOD Motor Current + Current + Torque + Efficiency

Full Voltage 100% 600% 600% 100% 100%Auto 80%

65%

50%

80%

65%

50%

80%

65%

50%

64% *

42% *

25% *

64%

42%

25%

100%

100%

100%Reactor 80%

65%

50%

80%

65%

50%

80%

65%

50%

80%

65%

50%

64%

42%

25%

80%

65%

50%Part winding 100%

100%

70%

55%

high tap

70%

low tap

55%

50%

50%

72%

90%

Wye-Delta 100% 33% 33% 33% 100%Solid State 0-100% 0-100% 0-100% 0-100% 0-100%

Table 7.0

+ Locked Rotor

* Does not include magnetizing current of autotransformer

** Torque per KVA

This conflict between torque and current requirements ofinduction motors is one typical dilemma facing the user ofreduced voltage starting equipment. It may be only one of severalproblems but is the most common and most important.

% Rated Motor Voltage % Starting torque

0 0

25 6.25

50 25

75 56.25

100 100

Table 7.1


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7.1 Reactor Starting

It is possible to reduce the motor terminal voltage as requiredby using a primary reactor. The use of a reactor during starting,

results in an exceptionally low starting power factor. Reactorsmust be carefully designed and applied since any saturation in thereactor will produce in-rush currents close to those seen during fullvoltage starting.

Reactor starting has one major advantage; the voltage to themotor is a function of the current taken from the line. It cantherefore be assumed that during acceleration the motor voltagewill rise as the line current drops. This relationship results ingreater accelerating energy at higher speeds and less severe

disturbances during the transition to full voltage.

M

1LNReac t .VSD

Figure 7.1


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7.2 Autotransformer Starting

In general, the most widely used method of reduced voltagestarting for squirrel cage induction motors is the autotransformer.

Reduced voltage autotransformer starting is preferred overprimary reactor starting when the starting currents must be held toa minimum and maximum starting torque per line amp is required.(see Table 7.0)

There are two very distinctive characteristics of anautotransformer starter.

1) The motor terminal voltage is nota function of load current andremains constant during the acceleration time. There is some

regulation within the autotransformer, however, this is negligible.As well, from the standpoint of overall power factors, themagnetizing currents are also negligible.

2) Due to the turns ratio advantages, the ratio of primary linecurrent to torque is the same for autotransformer starting as for fullvoltage starting. The primary line current is less than thesecondary motor currents. Since the turns ratio also representsthe voltage ratio, the starting current is reduced by the square ofthe turns ratio

It should be noted however that the motor current and linecurrent are notequal as they are with a primary reactor. A threecoil autotransformer is connected in a wye configuration andconnected to the motor in such a way as to supply reducedvoltage to the motor when the line voltage is applied to theautotransformer. Several sets of taps are usually available to theuser to provide different values of reduced voltage (NEMAstandards are 80%, 65% and 50% of the full line voltage).


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For example, take a motor with a full voltage starting torque at120% and a full voltage starting current of 600%. If the powercompany limitation is 400% current at start, motor current need notbe limited to 400%, but only the line current. Since thetransformers will have a step down ratio, the motor current can belarger for the given line current.

In the example noted, with the line current limited to 400%, onecan apply 80% voltage to this motor, have 80% motor current, andstill have only 0.8 x 80% or 64% line current due to the 1/0.8 turns-ratio for the transformers. The advantage is that the startingtorque is now 80% x 80% of 120%, or 77% instead of the 51%obtained in the reactor scheme. In the preceding example, thismight easily have furnished sufficient accelerating energy to start

the load.

Two types of autotransformer connections or control schemesare in common use today. One is designated closed-circuittransition or "Korndorfer-connection," named after its inventor, andthe other open-circuittransition.

The Allen-Bradley Company has adopted the closed circuittransition autotransformer connection even though it is moreexpensive to manufacture than starters utilizing the open-circuitconnection. Undesirable transient line surges of current areeliminated with this system since the motor is never disconnectedfrom the power supply or transformer.

Figure 7.2a is a typical diagram of the closed-circuit transitionstarter. The taps, of the autotransformer, are permanentlyconnected to the motor. Three contactors are used.


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M

1LNAUTOTX.VSD

Figure 7.2a


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25% Voltage 50% Voltage 100% Voltage0% Voltage

Figure 7.3a

It is because of this voltage drop, (energy loss), that it is becomesmore economical to bypass the SCRs after the motor is up tospeed. The Solid Starter controller is NOT used for continuousspeed adjustment. These controllers are used strictly for smoothlystarting and stopping of a motor.

If the solid state controller is used for ramp stopping duty, theSCRs must be fired full on before the bypass contactor is opened.The SCRs are then phased back to reduce the voltage to the motorthus allowing the motor to smoothly come to a halt,

In the case of medium voltage solid state starters, the SCRs areoperating at true medium voltage levels. As such, the device mustbe physically isolated from the low voltage controls. The gate firingsignals are transmitted from the main microprocessor controlmodule, to a separate gate firing printed circuit board by way ofoptical signal carried through fiber optics cables. This method oftriggering the SCRs to conduct provides the utmost in isolation forpersonnel safety.

The main control module allows for specific firing characteristics.The Rockwell Automation/Allen-Bradley SMC provides thefollowing starting and stopping features.


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Soft Start with Kickstart

This is the general method of soft starting. The initial torque valueis set between 5-90 % of locked rotor torque. The motor voltage isincreased steplessly during the acceleration ramp period which isadjustable from 2 to 30 seconds.

A kickstart torque pulse is dip switch selectable. This provides acurrent pulse of 500% of full load current and is adjustable from 0.4to 2 seconds. This feature allows the motor to develop additionaltorque at start.

Current Limit Start

This starting mode is used when it is necessary to limit themaximum starting current. This can be adjusted from 50 to 500 %of full load amperes. The current limit starting time is set by theuser.

Full Voltage Start

This mode is used in applications requiring across-the-line starting.The ramp time is less than 1/4 second. The SCRs are phasedfrom their full off position to full on in this 250 msec. time frame.

Soft Stop Option

This function can be used in applications that require an extendedramp to rest. The voltage ramp down time can be set from 2 to 60seconds. The starting and stopping times are independentlyadjusted. The load will stop when the voltage drops to a pointwhere the load torque is greater than the motor torque.


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Pump Control Option

This function is used to reduce surges in a pumping system duringthe starting and stopping of centrifugal pumps. The Allen-BradleyMV SMC PLUS Controller controls the speed of the motor duringstarting and stopping without feedback devices.

SMB Smart Motor Braking Option

This function provides motor braking for applications which requirethe motor to stop quickly. It is a microcomputer based brakingsystem which applies three phase braking current to a standardsquirrel cage induction motor. The strength of the braking current

is adjustable from 150 % to 400 % of full load current.

M

Bypass Contactor

Main Contactor

1LNSMC.VSD

ControlLogic

Figure 7.3b Typical Solid State Motor Controller c/w bypass


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8.0 Torque Requirements

When the rotor is at a standstill, the rotating flux caused by thestator induces voltages in the rotor bars and since the rotor bars

are short circuited, significant currents are induced to flow. Therotor currents react with the air gap flux to generate forces that tryto turn the rotor and that try to reduce the induced effects in therotor. The rotor begins to turn in the same direction as the rotatingflux fields produced by the stator.

S

S

N

N

`

Induced armature currents

Direction of rotation of armature with

respect to fieldDirection of rotation of

field structure

Actual rotationof armature

Torque.vsd

Figure 8.0a

It is important to reiterate that when the voltage is reduced forstarting a motor, so are the current and torque values. It shouldbe apparent that a motor that will not start a load at fullvoltage, will not start that same load under reduced voltageconditions.


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8.1 Reducing the starting voltage and current of aninductionmotor

As outlined earlier, when the voltage is reduced from nominal(100%), a decrease in inrush current will occur at a rate of 12% forevery 10% reduction in voltage while starting torque will bedecreased at a rate of 20% for the same reduction of voltage.Refer to Table 7.0. and table 8.1 below

Table 8.1

% Rated Motor Voltage % Starting torque0% 0.00%

25% 6.25%

50% 25.00%

65% 42.00%

75% 56.25%

80% 64.00%

100% 100%


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9.0 Synchronous Motors and Controls

Synchronous motors are used on continuous load applications(such as motor generators, compressors, mills etc.) where there

is a need to improve the power factor of a plant distributionsystem and/or where constant speed is required. Synchronousmotors are usually larger than 40 horsepower and, unlike aninduction motor, they have an additional rotor field winding whichrequires DC excitation. Therefore, a motor controller for asynchronous motor not only connects the AC Line voltage to themotor stator but must also apply a DC excitation voltage to therotor field winding. It must also protect the rotor field and statorwindings from overload conditions during the starting cycle, andwhile operating at synchronous and at sub-synchronous speed.

An application where constant speeds are required are in logchippers for the pulp and paper industry. Constant speed of thechipper drum will result in consistent size wood chips. Consistentsized chips will allow the consistent processing of the raw chip intostock product.

There are two types of synchronous motors, brush andbrushless, (see section 4.0). The brush type employs anexternal DC excitation control circuit which is connected to therotor mounted field winding through brushes and slip rings (Figure9.0b). The brushless type employs an internal rotor mounted DCexcitation control circuit. The more common is the brush type onexisting motors. However, newer motor designs incorporate abrushless type of excitation as standard.


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9.1 The Synchronous Motor Controllers, Brush Type

The synchronous motor has a stator winding that is identical tothat found in a squirrel cage induction motor. The rotor, with a fieldwinding wound around field poles, is energized from a DC source,(Static Exciter), to produce alternating north and south poles, thatmate with opposite magnetic polarity poles in the rotating field setup in the stator. During the start sequence, the motor also actssimilar to a standard squirrel cage induction motor.While the motor is accelerating, the rotor winding is connected toan external discharge resistor. The purpose of this resistor is twofold.

1. To provide a load path for the currents induced into the rotor

from the rotating stator windings

2. Determines the starting torque profile of the motor during theacceleration phase. (Refer to figure 9.0b)

3. Determines the pull-in torque profile of the motor.

4. Effects the starting KVA, to some degree.

When the rotor reaches a speed that is approximately 95% ofsynchronous speed, the DC supply is connected to the rotorwinding (amortisseur).


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S

N

S

N

S

N

S

N

DC fromStatic Exciter

3A C

SYNCMT R.VSD

+

-

Figure 9.0a

The synchronous motor, therefore, is supplied with two sources ofenergy, 3 phase AC to the stator and DC to the rotor field.Maximum torque generation only occurs when the rotor fieldwinding is supplied with DC power and the rotor is rotating insynchronism with the rotating field in the stator.


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is in series with the motor field windings. The 0-5OmV signalprovides a reference to the regulator circuit which will indicate to thecircuit how to fire the SCR'S. Current regulation will provide aconstant current by varying the voltage supplied. This constantcurrent will overcome the effects of motor winding heating and smallload variations to provide stable motor control.

Although available on the current style A exciter, voltage regulationis not usually used since it does not provide a very stable powerfactor. The three phase style "B" system provides voltage regulation.

Power factor regulation requires a 0-20 mA signal be fed into theregulation system. This is the ideal form of regulation since it willprovide a stable power factor, and overcome motor winding heating

and load changes.

On synchronous motors the power factor, D.C. field excitationcurrent, and AC stator line current are related. This is reflected in aset of V curves. Under full load with rated excitation the statorcurrent should be at 100%. if the field is over excited the statorcurrent will go up, if under excited the stator current will also go up.


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9.2 Brushless Synchronous Motor Controllers

The rotor, of a brushless synchronous motor, has a three-phase A.C.armature winding identical to that of a brush type motor. The

stationary field winding is on poles on the stator and is connected toa variac and rectifier for the D.C. supply. The generated A.C. currentis directly connected along the shaft to a rotating diode wheel, whereit is rectified to D.C. before going to the motor field. The magnitude ofthe field current is adjusted by changing the current to the stationaryexciter field. This stationary exciter current is typically in the range of1-10 amps only.

As with a brush type synchronous motor, the field windings must beshorted with a discharge resistor and the D.C. must be blocked until

the rotor is up to near full speed. When the motor is near full speed,the D.C. is then applied and the discharge resistor is disconnectedfrom the circuit. All of this control, on a brushless synchronous motor,is found on the rotor of the motor. Conversely, on a brush typesynchronous motor all of these controls are located in a separatemotor control unit.


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M S

1LINBSYN.VSD

20 Amp.Full Wave

BridgeRectifieir

DischargeResistor, field contactor, and

static exciter mounted on rotorshaft

Powerstat

D.C. Relay

Control CircuitTransformer

Fig 9.2a


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10.0 Medium Voltage Variable Frequency Motor Drive Systems

Traditionally variable voltage drives have been used primarilywith DC motors. However, with the increased need for variable speedcontrol of AC motors throughout industry, the use of AC variablefrequency drive systems has overtaken the use of DC drives. Thereare primarily two basic types of AC motor drives; current source andvoltage source.

The primary operating difference between the two is the speedresponse rate (rads/sec.). In the case of a current source(fig. 10a),all of the current required by the inverter is pulled through a large linkreactor that places a high impedance between the rectifier andinverter. In a voltage source (fig. 10b), instantaneous current is

supplied by a large capacitor connected between the rectifier andinverter sections. With little impedance, the capacitor will provideinstantaneous currents when called upon by the inverter. Thus, veryquick changes in the inverter outputs can be realized. These veryquick speed changes, in the order of >30 rads/sec, can be veryimportant for system drive applications such as rolling mills, paperprocessing and test stands. Where rapid speed changes are notnecessary, the current source drive provides an inexpensive and lesscomplex alternative. Typical applications for current source drivesinclude fans, pumps and blowers. These types of loads traditionallydo not require quick speed changes, but rather smooth speedchanges and tight control.


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RectifierDC LinkInductor Inverter

GTOsSCRs

SCRsGTOs

Motor FilterCapacitors

TransformerPowerFuses

IsolationSwitch

InputContactor Motor

OPTIONAL

Fig. 10.0a Typical Medium Voltage Current Source Inverter,A.C. Drive

In both the current source and voltage source drive, the mainsystem voltage is rectified to DC. This DC supply is fed to a threephase inverter that reconstructs the AC wave form at frequenciestypically between 5 to 70 Hz. In some cases drives are available withinverters that output maximum AC frequencies as high as 200 Hz formedium voltage drives. Low voltage drives use much higherswitching frequencies (>2K Hz).

To provide the conversion of power from one type to another,(AC-DC-AC), a variety of high current semiconductor switchingdevices are used. The most common devices used for mediumvoltage drives are silicon controlled rectifiers or thyristors (SCR) andgate turn off thyristors (GTO). The thyristor is an established high

power switching device that can be easily turned ON but is verydifficult to turn OFF. The GTO has similar attributes as the thyristorwith the added ability to turn OFF using a gate control signal. In lowvoltage drives (600V), the most common semiconductor used is theIGBT.


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The rectifier portion of the drive converts the three phase fixedAC voltage line supply into variable DC voltage. This is accomplishedby switching the appropriate power semiconductors at intervals of120 degrees on the positive portions of the AC sine wave. Theopposing semiconductors are switched during the negative portion ofthe wave form.

In the current source drive, the inverter converts the DC linkcurrent back into variable frequency AC to the motor. Appropriateswitching of the semiconductors transfer the DC link current to theAC output terminals. The wave shape of the output current dependson the time of operation of each semiconductors. The actual outputconsists of variable duty width square wave pulses. A motor filtercapacitor along with the inductive components of the motor load

cables and stator windings filters the output wave form to be nearsinusoidal.

MDRIVE1.VSD

Figure 10.0b Typical Medium Voltage, Voltage Source Inverter,VSI


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11.0 Transformer Switching

Medium voltage controls are primarily used for motor controland protection, however, they can be safely applied to control largedistribution or isolation transformers. These styles of transformersusually range in sizes between 150 KVA and 10000 KVA at voltagesbetween 2300 to 7200 Volts.

Large transformers must be adequately protected in the followingbasic areas:

1. Overload protection2. Winding faults (secondary and primary)3. Single phasing

Other styles and types of protection may also be specified. Thesecould include, but are not limited to, undervoltage, ground fault,internal pressure fault and differential faults.

In medium voltage controllers, current limiting fuses are used toprotect the transformer primary windings and the cable running fromthe controller to the transformer, from faults.

The power fuses used in this application are typically E ratedhaving the ability to clear high level fault currents very quickly. Theyare also capable of carrying high in-rush (transformer magnetizing)currents in excess of 9 to 14 times rated full load current withoutopening or sustaining damage. These in-rush currents are typicallyonly seen for approximately 0.1 seconds (100 milliseconds) duringenergization of the transformer.

These types of power fuses should not be considered overloaddevices and as such should be coordinated with an appropriate

overload relay to provide proper overload protection of the fuse.


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12.0 Capacitor Bank Switching

In section 6.4, we discussed the use of capacitors to correctpower factor and their use to assist in starting large loads on weakpower systems. Vacuum contactors can be safely applied to switchlarge banks of capacitors. The main concern is the time it takes forthe contacts to close. Because there is inherently some inductancein the circuit, either due to purposely installed air core inductors orinherent circuit inductance, a resonant series circuit is formed. Whenthe contacts of the vacuum bottle begin to close, the inrush currentsare proportional to the RMS voltage across the contacts of thevacuum bottle and the circuit impedance at the point the arc begins.This inrush current can easily be ten times the running current.

Increasing the inductance of the circuit helps reduce the inrushcurrents and resonant frequency. Allen-Bradley includes air corereactors in all power factor correction capacitor bank switchingconfigurations to limit the initial in-rush current.


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13.0 Load Break Switches

In areas where repeated energization and de-energization, of a load,is not required, as with a motor controller, a load break switch may bea more appropriate switching means. A load break switch is amechanically operated switch capable of making or breaking, andoperating continuously at its rated currents. Typical sizes include400, 600 and 1200A. Unlike the non-load break switch within astandard motor controller, the load break switch is not mechanicallyinterlocked to any other device with the exception of the safety doorinterlocks. Load break switches use a stored energy system toprovide a means of a quick make and quick break cycle. Load breakswitches should not be used for motor loads due to safetyconsiderations.

The following sections describe some typical load break switchconfigurations.


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13.1 Main Switches

A load break switch used as a Main Switch is used to switchpower to a group of devices operating at the same potential voltage.Figure 13.1a illustrates a typical example.

O pt ion a l fus ing

Figure 13.1a MAIN


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13.2 Feeder Load break Switches

A load break, used as a feeder, obtains its power from acommon system bus. It is sized to switch only a single load device,typically a step down transformer. Figure 13.2a illustrates a typicalfeeder load break configuration.

O pt ion a l fus ing

Figure 13.2a FEEDER


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13.3 Tie Load break Switches

Load break switches can also be used to tie two distributionbusses together. This type of configuration is typically used inapplications where the user wants the ability to feed essentialequipment in the event of a main power feed failure. Figure 13.3aillustrates a typical configuration. Normally the load break switchshown would be locked in the open position. If either feed A or Bwas lost, all essential loads could be operated from on main feed.

Feed A Feed B

Optionalfusing

K

* * * * * *

* Essential loadK Kirk-Key Interlocked

Figure 13.3a TIE


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14.0 Plugging

Plugging is used to bring a rotating motor to an abrupt stop by brieflyreversing any two of the three stator connections. The pluggingtorque varies between the limits of one half to full motor startingtorque, the exact amount depending on the resistance of the rotorwindings. The higher the resistance the higher the ratio of pluggingtorque to starting torque. During the plugging cycle, the motor currentis slightly higher than the starting current.

Any revers

medium voltage - its use control and application by john a kay, cet

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