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Industrial Equipment

Appendix IV – AC Motors & Variable Speed Drives



Industrial Equipment........................................................................................................... 1

Appendix IV – AC Motors & Variable Speed DrivesRotor Slip in induction motors ....... 1

Rotor Slip in induction motors............................................................................................ 3Motor slip is necessary for torque generation................................................................. 3

Slip depends on motor parameters. ................................................................................. 4Methods to reduce slip.................................................................................................... 5Adjustable speed AC drive is often the best solution. .................................................... 5

Induction Motor Control Theory. ....................................................................................... 7

Stator design.................................................................................................................... 7

Rotor Design. .................................................................................................................. 7Equivalent Circuit. .......................................................................................................... 8

Starting Characteristics. .............................................................................................. 8

Runnng Characteristics. ................................................................................................ 10Design Classification. ................................................................................................... 11

Frame Classification. .................................................................................................... 11

Temperature Classification........................................................................................... 11Power factor correction................................................................................................. 12

Single phase motors. ..................................................................................................... 13

Slip Ring Motors........................................................................................................... 13Variable-frequency drives................................................................................................. 14

Technology development.............................................................................................. 14

Early VFDs ................................................................................................................... 14

Matching Motors and Controls ..................................................................................... 15Nonlinear issues............................................................................................................ 16

VFD Justification: A Mechanical Perspective.............................................................. 17

Recent developments .................................................................................................... 19

Specifying VFDs........................................................................................................... 20Field installation............................................................................................................ 20

Figure 1 AC Motor Cutaway .............................................................................................. 3Figure 2 Induction Motor Speed Curve .............................................................................. 4

Figure 3 AC Motor Torque & Speed .................................................................................. 5

Figure 4 Slip Compensation................................................................................................ 6Figure 5 AC Motor Speed & Torque .................................................................................. 9

Figure 6 VFD & Pumps.................................................................................................... 18

Figure 7 VFD on Centrifugal Fans ................................................................................... 19



Rotor Slip in induction motors

Although slip is a common problem with standard motors, several options exist forreducing its effects

The AC induction motor is often referred to as the workhorse of the industry because it

offers users simple, rugged construction, easy maintenance, and cost-effective pricing.These factors have promoted standardization and development of a manufacturinginfrastructure that has led to a vast installed base of motors; more than 90% of all motorsused in worldwide industry are AC induction motors.

In spite of this popularity, the AC induction motor has two basic limitations. The standardmotor is neither a true constant-speed machine, nor is it inherently capable of providingvariable-speed operation. Both limitations require consideration, as the quality andaccuracy requirements of motor/drive applications continue to increase.

This article will explore the reason for slip and discuss ways to minimize it. In addition, itwill detail the best methods now available for controlling motor speed with powerelectronics, including technology to minimize the negative effects of slip.

Motor slip is necessary for torque generation.

An AC induction motor consists of two basic assemblies: the stator and rotor. The statorstructure is composed of steel laminations shaped to form poles. Copper wire coils arewound around these poles. These primary windings are connected to a voltage sourceto produce a rotating magnetic field. Three-phase motors with windings spaced 120electrical degrees apart are standard forindustrial, commercial, and residentialuse.

The rotor is another assembly made oflaminations over a steel shaft core.Radial slots around the laminations'periphery house rotor bars, which arecast-aluminium or copper conductorsshorted at the ends and positionedparallel to the shaft. Arrangement of therotor bars looks like a squirrel cage,hence the well-known term, “squirrelcage induction motor” (Photo above).The term “induction motor” comes fromthe alternating current (AC) that's“induced” into the rotor via the rotatingmagnetic flux produced in the stator.

Motor torque is developed from theinteraction of currents that flow in therotor bars and the stator's rotating magnetic field. In actual operation, rotor speed alwayslags the magnetic field's speed, allowing the rotor bars to cut magnetic lines of force andproduce useful torque. This speed difference is called slip speed. Slip also increaseswith load and is necessary for producing torque.

Figure 1 AC Motor Cutaway

This cutaway view of a squirrel cage ACinduction motor shows the stator and rotorconstruction, the shaft with bearings, and the

cooling fan.



Slip depends on motor parameters.

According to the formal definition, the slip (S) of an induction motor can be found withthe following equation:

s = [(n2 - n)/ns] x 100%

where ns is synchronous speedand n is actual speed.

For small values of motor slip,the slip is proportional to therotor resistance, stator voltagefrequency, and load torque. It'sinversely proportional to thesecond power of supply voltage.The traditional way to control thespeed of a wound rotor inductionmotor is to increase the slip byadding resistance in the rotor

circuit. The slip of low-horsepower motors is higherthan those of high-horsepowermotors because of higher rotorwinding resistance in smallermotors.

As seen in the Table above,smaller motors and lower-speed motors typically have higher relative slip. However,high-slip large motors and low-slip small motors are also available.

You can see that full-load slip variesfrom less than 1% in high-hp motors

to more than 5% in fractional-hpmotors. These variations may causeload-sharing problems when motorsof different sizes are mechanicallyconnected. At low load, the sharing isnormally not a problem, but at fullload, the motor with lower slip takes ahigher share of the load than themotor with higher slip.

As shown in Fig. 1 at right, the rotorspeed decreases in proportion to the

load torque. This means that the rotorslip increases in the same proportion.

Relatively high rotor impedance isrequired for good across-the-line, orfull voltage, starting performance. Inother words, high torque is requiredagainst low current. Low rotor impedance is also necessary for low full-load speed slipand high operating efficiency. The curves in Fig. 2 show how higher rotor impedance in

Motor slip of selected aluminium and cast iron NEMAmotors with synchronous speed ranging from 3,600 RPMto 900 RPM.

Figure 2 Induction Motor Speed Curve

This is a typical speed curve of an induction motor.The slip is the difference in rotor speed relative to thatof the synchronous speed. CD = AD – BD = AB.



motor B reduces the starting current and increases the starting torque, but it causes ahigher slip than in standard motor A.

Methods to reduce slip.

Synchronous, reluctance, or

permanent-magnet (PM) motors cansolve the problem of slip becausethere's no measurable slip in thesethree types of motors. Synchronousmotors are used for very high- andlow-power applications, but to alesser extent in the medium-horsepower range, where manytypical industrial applications fall.Reluctance motors are also used, buttheir output/weight ratio isn't verygood, so they're less competitive than

the squirrel cage induction motor. PMmotors, which are used withelectronic adjustable speed drives(ASDs) offer accurate speed controlwithout slip, high efficiency with lowrotor losses, and the flexibility tochoose a very low base speed, eliminating the need for gear boxes. However, PMmotors are still limited to certain special applications, mainly because of high cost andthe lack of standardization.

Selecting an oversized AC induction motor is another way to reduce slip. Larger motorstypically have a lower slip value to begin with, and slip gets smaller with a partial, ratherthan full, motor load. The disadvantage with oversizing the motor is that with a largermotor comes higher energy consumption, which increases investment and operationcosts.

Adjustable speed AC drive is often the best solution.

The inherent limitations of the AC induction motor can be solved with ASDs. The mostcommon AC drives today are based on pulse-width modulation (PWM). The constant ACline voltage with 60 cycles per second from the supply network is rectified, filtered, andthen converted to a variable voltage and variable frequency. When this output from thefrequency converter is connected to an AC motor, it's possible to adjust the motor speed.

When using an AC drive for adjusting the motor speed, there are many applicationswhere motor slip is no longer a problem. The speed of the motor isn't the primary controlparameter. Rather, it could be the liquid level, air pressure, gas temperature, or someother controlling parameter.

Figure 3 AC Motor Torque & Speed

These curves depict torque/speed and current/speedfor a standard motor (A) and a high-torque motor (B).



High static speed accuracy and/ordynamic speed accuracy are stillrequired in many drive applications,such as printing machines, extruders,paper machines, cranes, andelevators. There are also many

machines and conveyors wherespeed control — between sectionsdriven by separate motors — has tobe synchronized. Instead ofoversizing the motors to eliminate thespeed error caused by slip, it may bebetter to use sectional drive line-upswith separate inverters for eachindividual motor. The inverters areconnected to a DC-voltage bus barsupplied by a common rectifier. Thisis a very energy-efficient solution,

because the driving sections of themachinery can use the braking energy from decelerating sections (regeneration).

Slip compensation can even be added to AC drives to further reduce the effect of motorslip. A load torque signal is added to the speed controller to increase the outputfrequency in proportion to the load. Slip compensation can't be 100% of the slip becauseof rotor temperature variations that may cause overcompensation and unstable control.But the compensation can achieve accuracies up to 80%, meaning slip can be reducedfrom 2.4% to about 0.5%.

Figure 4 Slip Compensation

The addition of slip compensation to the AC drivehelps reduce overall slip.



A bar with a large cross sectional area will exhibit a low resistance, while a bar of a smallcross sectional area will exhibit a high resistance. Likewise a copper bar will have a lowresistance compared to a brass bar of equal proportions.

Positioning the bar deeper into the rotor, increases the amount of iron around the bar,and consequently increases the inductance exhibited by the rotor. The impedance of the

bar is made up of both resistance and inductance, and so two bars of equal dimensionswill exhibit a different A.C. impedance depending on their position relative to the surfaceof the rotor. A thin bar which is inserted radialy into the rotor, with one edge near thesurface of the rotor and the other edge towards the shaft, will effectively change inresistance as the frequency of the current changes. This is because the A.C. impedanceof the outer portion of the bar is lower than the inner impedance at high frequencieslifting the effective impedance of the bar relative to the impedance of the bar at lowfrequencies where the impedance of both edges of the bar will be lower and almostequal.

The rotor design determines the starting characteristics.

Equivalent Circuit.

The induction motor can be treated essentially as a transformer for analysis. Theinduction motor has stator leakage reactance, stator copper loss elements as seriescomponents, and iron loss and magnetising inductance as shunt elements. The rotorcircuit likewise has rotor leakage reactance, rotor copper (aluminium) loss and shaftpower as series elements.

The transformer in the centre of the equivalent circuit can be eliminated by adjusting thevalues of the rotor components in accordance with the effective turns ratio of thetransformer.

From the equivalent circuit and a basic knowledge of the operation of the inductionmotor, it can be seen that the magnetising current component and the iron loss of the

motor are voltage dependant, and not load dependant. Additionally, the full voltagestarting current of a particular motor is voltage and speed dependant, but not loaddependant.

The magnetising current varies depending on the design of the motor. For small motors,the magnetising current may be as high as 60%, but for large two pole motors, themagnetising current is more typically 20 - 25%. At the design voltage, the iron is typicallynear saturation, so the iron loss and magnetising current do not vary linearly with voltagewith small increases in voltage resulting in a high increase in magnetising current andiron loss.

Starting Characteristics.

In order to perform useful work, the induction motor must be started from rest and boththe motor and load accelerated up to full speed. Typically, this is done by relying on thehigh slip characteristics of the motor and enabling it to provide the acceleration torque.

Induction motors at rest, appear just like a short circuited transformer, and if connectedto the full supply voltage, draw a very high current known as the "Locked Rotor Current".They also produce torque which is known as the "Locked Rotor Torque". The LockedRotor Torque (LRT) and the Locked Rotor Current (LRC) are a function of the terminalvoltage to the motor, and the motor design. As the motor accelerates, both the torqueand the current will tend to alter with rotor speed if the voltage is maintained constant.



The starting current of a motor, with a fixed voltage, will drop very slowly as the motoraccelerates and will only begin to fall significantly when the motor has reached at least80% full speed. The actual curves for induction motors can vary considerably betweendesigns, but the general trend is for a high current until the motor has almost reached fullspeed. The LRC of a motor can range from 500% Full Load Current (FLC) to as high as1400% FLC. Typically, good motors fall in the range of 550% to 750% FLC.

Figure 5 AC Motor Speed & Torque

The starting torque of an induction motor starting with a fixed voltage, will drop a little to

the minimum torque known as the pull up torque as the motor accelerates, and then riseto a maximum torque known as the breakdown or pull out torque at almost full speedand then drop to zero at synchronous speed. The curve of start torque against rotorspeed is dependant on the terminal voltage and the motor/rotor design.

The LRT of an induction motor can vary from as low as 60% Full Load Torque (FLT) toas high as 350% FLT. The pull-up torque can be as low as 40% FLT and the breakdowntorque can be as high as 350% FLT. Typical LRTs for medium to large motors are in theorder of 120% FLT to 280% FLT.

The power factor of the motor at start is typically 0.1 - 0.25, rising to a maximum as themotor accelerates, and then falling again as the motor approaches full speed.

A motor which exhibits a high starting current, i.e. 850% will generally produce a low

starting torque, whereas a motor which exhibits a low starting current, will usuallyproduce a high starting torque. This is the reverse of what is generally expected.

The induction motor operates due to the torque developed by the interaction of the statorfield and the rotor field. Both of these fields are due to currents which have resistive or inphase components and reactive or out of phase components. The torque developed isdependant on the interaction of the in phase components and consequently is related tothe I2R of the rotor. A low rotor resistance will result in the current being controlled by theinductive component of the circuit, yielding a high out of phase current and a low torque.



Figures for the locked rotor current and locked rotor torque are almost always quoted inmotor data, and certainly are readily available for induction motors. Some manufactureshave been known to include this information on the motor name plate. One additionalparameter which would be of tremendous use in data sheets for those who areengineering motor starting applications, is the starting efficiency of the motor. By thestarting efficiency of the motor, I refer to the ability of the motor to convert amps into

newton meters. This is a concept not generally recognised within the trade, but onewhich is extremely useful when comparing induction motors. The easiest means ofdeveloping a meaningful figure of merit, is to take the locked rotor torque of the motor(as a percentage of the full load torque) and divide it by the locked rotor current of themotor (as a percentage of the full load current).

i.e

Starting efficiency =Locked Rotor Torque

Locked Rotor Current

If the terminal voltage to the motor is reduced while it is starting, the current drawn by

the motor will be reduced proportionally. The torque developed by the motor isproportional to the current squared, and so a reduction in starting voltage will result in areduction in starting current and a greater reduction in starting torque. If the start voltageapplied to a motor is halved, the start torque will be a quarter, likewise a start voltage ofone third will result in a start torque of one ninth.

Running Characteristics.

Once the motor is up to speed, it operates at low slip, at a speed determined by thenumber of stator poles. The frequency of the current flowing in the rotor is very low.Typically, the full load slip for a standard cage induction motor is less than 5%. Theactual full load slip of a particular motor is dependant on the motor design with typical full

load speeds of four pole induction motor varying between 1420 and 1480 RPM at 50 Hz.The synchronous speed of a four pole machine at 50 Hz is 1500 RPM and at 60 Hz afour pole machine has a synchronous speed of 1800 RPM.

The induction motor draws a magnetising current while it is operating. The magnetisingcurrent is independent of the load on the machine, but is dependant on the design of thestator and the stator voltage. The actual magnetising current of an induction motor canvary from as low as 20% FLC for large two pole machines to as high as 60% for smalleight pole machines. The tendency is for large machines and high speed machines toexhibit a low magnetising current, while low speed machines and small machines exhibita high magnetising current. A typical medium sized four pole machine has a magnetisingcurrent of about 33% FLC.

A low magnetising current indicates a low iron loss, while a high magnetising currentindicates an increase in iron loss and a resultant reduction in operating efficiency.

The resistive component of the current drawn by the motor while operating, changeswith load, being primarily load current with a small current for losses. If the motor isoperated at minimum load, i.e. open shaft, the current drawn by the motor is primarilymagnetising current and is almost purely inductive. Being an inductive current, the powerfactor is very low, typically as low as 0.1. As the shaft load on the motor is increased, theresistive component of the current begins to rise. The average current will noticeablybegin to rise when the load current approaches the magnetising current in magnitude.



As the load current increases, the magnetising current remains the same and so thepower factor of the motor will improve. The full load power factor of an induction motorcan vary from 0.5 for a small low speed motor up to 0.9 for a large high speed machine.The losses of an induction motor comprise: iron loss, copper loss, windage loss andfrictional loss. The iron loss, windage loss and frictional losses are all essentially loadindependent, but the copper loss is proportional to the square of the stator current.

Typically the efficiency of an induction motor is highest at 3/4 load and varies from lessthan 60% for small low speed motors to greater than 92% for large high speed motors.Operating power factor and efficiencies are generally quoted on the motor data sheets.

Design Classification.

There are a number of design/performance classifications which are somewhat uniformlyaccepted by different standards organisations. These design classifications applyparticularly to the rotor design and hence affect the starting characteristics of the motors.The two major classifications of relevance here are design A, and design B .

Design A motors have a shallow bar rotor, and are characterised by a very high startingcurrent and a low starting torque. Typical values are 850% current and 120% torque.

Shallow bar motors usually have a low slip, i.e. 1480 RPM.

Design B motors have a deeper bar rotor and are characterised by medium start currentand medium starting torque. Typical design B values are 650% current and 180%torque. The slip exhibited by design B motors is usually greater than the equivalentdesign A motors. i.e. 1440 RPM.

Design F motors are often known as Fan motors having a high rotor resistance and highslip characteristics. The high rotor resistance enables the fan motor to be used in avariable speed application where the speed is reduced by reducing the voltage. DesignF motors are used primarily in fan control applications with the motor mounted in the airflow. These are often rated as AOM or Air Over Motor machines.

Frame Classification.Induction motors come in two major frame types, these being Totally Enclosed Forcedair Cooled (TEFC), and Drip proof.

The TEFC motor is totally enclosed in either an aluminium or cast iron frame with coolingfins running longitudinally on the frame. A fan is fitted externally with a cover to blow airalong the fins and provide the cooling. These motors are often installed outside in theelements with no additional protection and so are typically designed to IP55 or better.

Drip proof motors use internal cooling with the cooling air drawn through the windings.They are normally vented at both ends with an internal fan. This can lead to moreefficient cooling, but requires that the environment is clean and dry to prevent insulation

degradation from dust, dirt and moisture. Drip proof motors are typically IP22 or IP23.

Temperature Classification.

There are two main temperature classifications applied to induction motors. These beingClass B and Class F.The temperature class refers to the maximum allowabletemperature rise of the motor windings at a specified maximum coolant temperature.



Class B motors are rated to operate with a maximum coolant temperature of 40 degreesC and a maximum winding temperature rise of 80 degrees C. This leads to a maximumwinding temperature of 120 degrees C.

Class F motors are typically rated to operate with a maximum coolant temperature of 40degrees C and a maximum temperature rise of 100 degrees C resulting in a potential

maximum winding temperature of 140 degrees C.Operating at rated load, but reduced cooling temperatures gives an improved safetymargin and increased tolerance for operation under an overload condition. If the coolanttemperature is elevated above 40 degrees C then the motor must be derated to avoidpremature failure. Note: Some Class F motors are designed for a maximum coolanttemperature of 60 degrees C, and so there is no derating necessary up to thistemperature.

Operating a motor beyond its maximum, will not cause an immediate failure, rather adecrease in the life expectancy of that motor. A common rule of thumb applied toinsulation degradation, is that for every ten degree C rise in temperature, the expectedlife span is halved. Note: the power dissipated in the windings is the copper loss which isproportional to the square of the current, so an increase of 10% in the current drawn, willgive an increase of 21% in the copper loss, and therefore an increase of 21% in thetemperature rise which is 16.8 degrees C for a Class B motor, and 21 degrees C for aClass F motor. This approximates to the life being reduced to a quarter of that expectedif the coolant is at 40 degrees C. Likewise operating the motor in an environment of 50degrees C at rated load will elevate the insulation temperature by 10 degrees C andhalve the life expectancy of the motor.

Power factor correction

Power factor correction is achieved by the addition of capacitors across the supply toneutralise the inductive component of the current. The power factor correction may beapplied either as automatic bank correction at the main plant switchboard, or as static

correction installed and controlled at each starter in such a fashion that it is only in circuitwhen the motor is on line.

Automatic bank correction consists of a number of banks of power factor correctioncapacitors, each controlled by a contactor which in turn is controlled by a power factorcontroller. The power factor controller monitors the supply coming into the switchboardand adds sufficient capacitance to neutralise the inductive current. These controllers areusually set to adjust the power factor to 0.9 - 0.95 lagging. (inductive)

Static correction is controlled by a contactor when the motor is started and when themotor is stopped. In the case of a Direct On Line starter, the capacitors are oftencontrolled by the main DOL contactor which is also controlling the motor. With staticcorrection, it is important that the motor is under corrected rather than over corrected.

This is because the capacitance and the inductance of the motor form a resonant circuit.While the motor is connected to the supply, there is no problem. Once the motor isdisconnected from the supply, it begins to decelerate. As it decelerates, it generatesvoltage at the frequency at which it is rotating. If the capacitive reactance equals theinductive reactance, i.e. unity power factor, we have resonance. If the motor is criticallycorrected (pf = 1) or over corrected, then as the motor slows, the voltage it is generatingwill pass through the resonant frequency set up between the motor and the capacitors. Ifthis happens, major problems can occur. There will be very high voltages developedacross the motor terminals and capacitors causing insulation damage, high resonant



currents can flow, and transient torque's generated can cause mechanical equipmentfailure.

The correct method for sizing static correction capacitors, is to determine themagnetising current of the motor being corrected, and connect sufficient capacitance togive 80% current neutralisation. Charts and formula based on motor size alone can be

totally erroneous and should be avoided if possible. There are some power authoritieswho specify a fixed amount of KVAR per kilowatt, independent of the size or speed. Thisis a dangerous practice. power factor correction

Single phase motors.

In order for a motor to develop a rotating torque in one direction, it is important that themagnetic field rotates in one direction only. In the case of the three phase motor, there isno problem and the field follows the phase sequence. If voltage is applied to a singlewinding, there are still multiples of two poles which alternate between North and South atthe supply frequency, but there is no set rotation for the vectors. This field can becorrectly considered to be two vectors rotating in opposite directions. To establish adirection of rotation for the vector, a second phase must be added. The second phase is

applied to a second winding and is derived from the first phase by using the phase shiftof a capacitor in a capacitor start motor, or inductance and resistance in an inductionstart motor. (sometimes known as a split phase motor.) Small motors use techniquessuch as a shaded pole to set the direction of rotation of the motor.

Slip Ring Motors.

Slip ring motors or wound rotor motors are a variation on the standard cage inductionmotors. The slip ring motor has a set of windings on the rotor which are not shortcircuited, but are terminated to a set of slip rings for connection to external resistors andcontactors. The slip ring motor enables the starting characteristics of the motor to betotally controlled and modified to suit the load. A particular high resistance can result in

the pull out torque occurring at almost zero speed providing a very high locked rotortorque at a low locked rotor current. As the motor accelerates, the value of theresistance can be reduced altering the start torque curve in a manner such that themaximum torque is gradually moved towards synchronous speed. This results in a veryhigh starting torque from zero speed to full speed at a relatively low starting current. Thistype of starting is ideal for very high inertia loads allowing the machine to get to fullspeed in the minimum time with minimum current draw.

The down side of the slip ring motor is that the sliprings and brush assemblies needregular maintenance which is a cost not applicable to the standard cage motor. If therotor windings are shorted and a start is attempted, i.e the motor is converted to astandard induction motor, it will exhibit an extremely high locked rotor current, typicallyas high as 1400% and a very low locked rotor torque, perhaps as low as 60%. In most

applications, this is not an option.

Another use of the slipring motor is as a means of speed control. By modifying the speedtorque curve, by altering the rotor resistors, the speed at which the motor will drive aparticular load can be altered. This has been used in winching type applications, butdoes result in a lot of heat generated in the rotor resistors and consequential drop inoverall efficiency.



Variable-frequency drives

Electric motors have driven mechanical-system loads for many years. For most of thattime, constant-speed turning of the loads and the simplest and safest on-off controls

provided all the oversight a user needed. Today, though, energy-efficiency concerns andprecise control requirements are boosting the popularity of variable-frequency drives(VFDs).

Technology development

The merit of the basic polyphase induction motor was always its simple configurationand control: Energize the motor at its base speed on the line frequency of 60 hertz; setits base speed by selecting the number of pole-pairs in design; and switch poles forneeded two-speed operation.

In the past, some building equipment, such as boiler fans and conveyors, requiredadjustment of driven speed. Cumbersome solutions accommodated these adjustments.

Wound-rotor motors and large, variable rotor resistors would set speeds externally, or aneddy-current drive inserted between motor and load allowed speed-slip to increase.These methods tossed off a lot of energy in heat losses, but that was not the issue.Rather, it was the need to vary speed that was important.

Direct-current (dc) motors have been available with silicon-controlled-rectifier drives forvariable-speed operation since the 1960s, but the higher maintenance needs of dcmotors deterred their use for building mechanical systems. Keeping brushes andcommutators fresh as they accumulate thousands of operating hours is simply notpractical.

The energy crisis of the mid-1970s, however, changed the variable-speed terrain,probably forever. A lot of power was being wasted both in electrical drives and the

mechanical systems they served. Changing induction-motor speed by varying voltagefrequency to the motors kept these otherwise excellent drivers in business andincreased power efficiencies to an acceptable level. What was needed was a frequency-conversion method of similar reliability and reasonable cost. The efficiency of both theunderlying mechanical process and the motor-drive system serving the mechanicalequipment could be improved later.

But there is more to application efficiency than just dividing output watts by input watts.Efficiency also includes evaluating any risks introduced to surrounding systems,managing escalating parts counts, handling potential instabilities and counteractingmaintenance woes.

Early VFDs

As the 1980s opened, variable-frequency drives (VFDs) became available in ratingsappropriate to motor applications for building fans and pumps. Typically, this mechanicalequipment incorporates centrifugal designs, for which the following Affinity Laws relatespeed changes to flow volumes, static pressures and driving horsepower:

Flow volume moved varies directly with the driving speed ratio.

Static pressure developed varies with the square of the driving speed ratio.



Horsepower required varies with the cube of the driving speed ratio.

This means that, for centrifugal equipment, required driving power drops quickly asmotor speed slows. The counter side of this observation is that a rather slight increase inspeed--above rated maximum--leads quickly to a large overload on driving equipment.

Many early building-system VFDs came from other fields, such as conveying and

machine tools, and carried 150 percent overload ratings and speed ranges that were notmatched to centrifugal-load requirements. In time, variable-torque rated drives appearedto meet the needs of this market, since equipment offerings vary directly with the"speed" of the market and a little lag.

It was clear that the drive-makers were on to something that could produce realefficiency improvements for air-handling and water-pumping systems. Theseapplications for VFDs, however, were rather expensive and often involved complicateddesign considerations. There was much room for development.

Heating-water pumps (left) and variable-frequency drives (right) inPharmacia & Upjohn’s Kalamazoo, Mich., Building 300 are located close to each

other to ease maintenance and lessen the potential for electrical disturbances.

(Photo courtesy The Austin Company.)

Matching Motors and Controls

Motor-to-drive relationships long have been a concern for engineers specifing VFDs.Previous motor applications did not require precise matches of motors to controllers(such as magnetic starters). The National Electrical Manufacturers Association (NEMA)ratings system for motors and starters--together with manufacturers' good designs--placed motors in the customary electrical environment, where standardization was welldeveloped.

There are two general VFD-design categories, which differ in how they pass power tothe motor after it is processed by the drive:

Voltage-source inverter (VSI) drives treat motors as parallel-connected loads,controlling the overall performance envelope by adjusting drive output voltage.

Current-source inverter (CSI) drives, which are much more concerned aboutmotor impedance--inductive reactance in particular--since it drives current



through the motor as part of the performance envelope. CSI drives must bematched carefully to their motors.

This motor-matching process always has been less important with VSI drives. Manyengineers have tied VFDs to motors of uncertain origin, particularly when retofittingsystems. But the best approach is to specify, purchase and install VFDs and associated

motors as a team--from one source, whenever possible. It is also important to keep trackof the operating motor's thermal budget, especially in rooms housing large groups ofdrives.

Nonlinear issues

Harmonics proved to be another concern. Despite the efficiency gained from turningmechanical loads at varying and lower speeds, the process of forming and deliveringvariable-frequency power elicits some concern over losses. The drive power line-upcomprises basically three units: a rectification input section, changing 60 hertzalternating-current (ac) supply to dc; a dc bus supported by capacitors; and an invertersection, which recreates ac power of varying frequency and voltage, as directed by the

drive's controller.

Motors once operated, by design, from a sinusoidal voltage wave form. The wave formsto which today's models are exposed are distinctly not sinusoidal. These waves includea fundamental frequency of alternating voltage, to which the motor's base speedresponds, and a rather rich mixture of harmonics formed from the attempt of the six-stepswitching process in the inverter to form a sine wave.

These harmonics produce two important effects. At higher frequencies, they lead to eddycurrents and skin-effect losses that produce no usable work, but do produce heat, whichmust be dissipated. Second, harmonics cause motor currents that produce additive andsubtractive torques, which are neither useful nor efficient in their production.

While torque effects have seemed minor, motor-heating problems have remained aconcern. The first solution was to derate motor capacity and control ambientenvironment. Low-temperature-rise U-frame motors and service-factor rated motorswere common steps to thermal survival. Integrated motor-cooling fans face effectssimilar to those of the loads driven by the motors: reduced cooling flow at reducedspeeds. However, as drive inverters for variable-torque centrifugal applicationsdeveloped, so did the motors suited to these applications.

Harmonics also can travel up to the feeder power system supplying the drives. Invertersdid not allow input current to flow as a regular sine wave. Instead, current to the rectifiercame in bursts, and harmonics moved around the entire connected power-distributionsystem. These harmonics were confined by isolating the power system for mechanicalloads from the rest of the facility. Identifying the "point of common-coupling" for this

somewhat separate system and enforcing a no-connection barrier around themechanical distribution performed well, especially on larger projects.

Placing inductance in the input circuit to a VFD also can control harmonic proliferation onpower systems and is used routinely. Isolation transformers are an even betterapproach, but cost more and require more space.

Another problem was displacement power factor. Capacitors installed to correct for low,lagging power factor on motor drives had a good operating and application history, bothin preserving system capacity and in satisfying electric-utility requirements. But



capacitors, shunt-connected to correct motor loads, could not work in the rich harmonicenvironments around VFDs. Drives typically delivered good 90 percent-plus power-factorvalues at high load, but dropped off as loads dropped off with reduced frequency andspeed.

Through the 1980s and into the 1990s, the population of personal-computer (PC) work

stations increased exponentially. Switch-mode power supplies common to these PCscaused another level of nonlinear, nonsinusoidal power-system effects to appearthroughout building distribution and into utility distribution installations.

Together with a steadily rising VFD load, concerns over harmonic levels spurred thedevelopment of standards and recommendations for control of these phenomena,notably Standard 519 from the Institute of Electrical and Electronics Engineers. Eversince, the industry has focused on measuring, developing, and meeting a structure oflimiting values for harmonics on power distribution systems. And, although it fosterscomplaints and arguments, Standard 519 has served for many years as a specificationand applications keystone for VFDs.

VFD Justification: A Mechanical Perspective

VFDs offer real improvements to the operating efficiencies of many mechanical fan andpumping systems. Mechanical systems are sized for performance at maximum designconditions, but large parts of the total operating time are spent at conditions well awayfrom maximum. Slowing equipment drives often results in surprising opportunities foroperating savings, especially when a large number of hours accumulate.

In early variable-speed applications, high VFD expense had to be justified. Althoughcosts have decreased significantly, specifying variable-speed operation still requirescare to reduce electrical power distribution risks. Further, VFDs may never reach thereliability level of magnetic motor starters. Bypass contactors on VFDs--an attempt todeliver that level of reliability--are not appropriate for all applications (for example,pressure-controlled supply-air). However, VFDs have become an accepted method for

meeting variable-flow needs and efficiency demands.



Figure 6 VFD & Pumps



Figure 7 VFD on Centrifugal Fans

Recent developmentsElectronics also have contributed an important component to the further developmentand proliferation of efficient VFDs. Insulated gate bipolar transistors (IGBT) alloweddistribution systems to switch large current levels at high kilohertz rates. Now, pulse-width-modulated (PWM) inverter drives are becoming popular. Displacement powerfactor at drive inputs can remain at mid-90 percent levels throughout the speed-adjustment and power-load ranges, because the rectification stage is a diode bridge.



Distortion levels are reasonable, units are compact and operating efficiencies are athigh-90 percent levels.

PWM inverters control the kilohertz chopping rate of a wave--varying its envelope involtage and fundamental frequency--to control induction-motor operation. Pulse rate canbe set high enough to eliminate audible vibration. The range of fundamental frequencies

through which the drive accelerates and operates can be arranged to skip criticalfrequencies where mechanical-equipment and structural-support resonances might beexcited to damaging levels.

VFD drive speed or frequency traditionally is positioned by external control systems,typically the building temperature-control system. These settings have been passed tothe drive controller section by an analog 4-to-20 milliampere dc instrument loop and readby an analog receiver at the VFD.

Building temperature-control systems also have adopted the ever-present PC as theirhost controller, so widespread use of digital speed setting on a communications line andprotocol for VFDs appears. This simplifies speed transmission and should delivercalibration-less speed-control loops.

In today's PWM inverters, most drive parameters and set-up requirements are controlledby software. As a result, these units need little field calibration.

The power circuit through a VFD, and the attending electronic hardware, is highly similarto other system applications of solid-state motor controllers (or "soft starters") anduninterruptible power supply systems, as used for large computer systems and datacenters. Improvements in design for one system eventually will help all applications.

Specifying VFDs

As VFDs have become more popular, motor manufacturers have begun offeringproducts developed specifically for VFD applications. These units apply higher insulationratings--nominally 1,600 volts--to windings and provide adequate temperature-rise

headroom. However, some of these products can develop etching of their rotor bearings.The cause: rapid PWM inverter output rise-times can excite the capacitance of bearinglubricants and create a circuit to ground for stray currents.

Safety-disconnect switches between inverters and motors should include an auxiliarycontrol-circuit switch to disable drive output. Closing back into a drive will provokeunnecessary faults. In addition, VFDs should be near motors wherever possible.

Also, specifiers should be cautious when applying VFD-powered motors in NationalElectrical Code (NEC) Classified Hazardous areas. Thermal management of motors atreduced speeds may press the hazard limits, particularly with increased surfacetemperatures. Underwriters Laboratories Standards 674 and 1836 for Division 1 andDivision 2 locations should be consulted to confirm appropriate products for the

application. Minimum motor-speed interlocking and motor-winding temperaturemonitoring should be specified for safer operation.

Field installation

Despite harmonic-distortion concerns, bringing VFDs online in the field is much morestraight-forward today than it has been in the past. Harmonic line analyzers can makethese installations even more trouble free. Documenting line conditions near and around



VFDs can aid future maintenance activities. Base-line data showing line conditionswithout VFDs operating should be included in any records.

The oldest VFD installations are now 15 to 17 years. Most of these installations havebeen on HVAC air-handling fans and pumps in the range of 10 to 450 hp, with thelargest number in 100- to 150-hp sizes. Most included input line reactors. Although these

units from the mid-1980s were very touchy to set up, problems have been minimal.Further, the benefits of adjustable speed have now added up and matured throughseveral generations of development.

appendix5 vsd

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