regarding selection- word 97 format

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Variable-frequency drive

From Wikipedia, the free encyclopedia

Small variable frequency drive

Hitachi J100 VFD
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A variable-frequency drive (VFD) is a system for controlling the rotationalspeed of an alternating current (AC) electric motor by controlling thefrequency of the electrical power supplied to the motor.[1][2][3] A variablefrequency drive is a specific type ofadjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD),

variable-speed drives (VSD), AC drives, microdrives or inverter drives. Sincethe voltage is varied along with frequency, these are sometimes also calledVVVF (variable voltage variable frequency) drives.

Variable-frequency drives are widely used in ventilation systems for largebuildings; variable-frequency motors on fans save energy by allowing thevolume of air moved to match the system demand. They are also used onpumps, elevator, conveyor and machine tool drives.

Contents

1 Benefits of VFD Systemso 1.1 Energy Savings for VFD Systemso 1.2 Extended Equipment Life and Reduced

Maintenance 2 VFD types 3 VFD system description

o 3.1 VFD motoro 3.2 VFD controllero 3.3 VFD operator interface

4 VFD operation 5 Power line harmonics 6 Application considerations

o 6.1 Transmission line effectso 6.2 Motor bearings

7 Available VFD power ratings 8 Dynamic braking 9 Regenerative variable-frequency drives 10 Brushless DC motor drives 11 See also

12 References

13 External links

Benefits of VFD Systems

Energy Savings for VFD Systems
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AC motor-driven applications that do not require full speed can save energyby controlling the motor with a variable speed drive. Energy cost saving withvariable torque can be significant, often paying for the cost of VFD within amatter of months. In variable torque applications such as fans and blowers,the torque required varies roughly with the square of the speed, and the

horsepower required varies roughly with the cube of the speed, resulting in alarge reduction of horsepower for even a small reduction in speed. The motorwill consume only 25% as much power at 63% speed than it will at 100%speed. This is referred to as the Affinity Laws, which define the relationshipsbetween speed, flow, torque, and horsepower.

Extended Equipment Life and Reduced Maintenance

Single-speed starting methods start motors abruptly, subjecting the motor toa high starting torque and to current surges that are up to 10 times the full-load current. Variable speed drives, on the other hand, gradually ramp the

motor up to operating speed to lessen mechanical and electrical stress,reducing maintenance and repair costs, and extending the life of the motorand the driven equipment. Soft starts, or reduced-voltage soft starters(RVSS), are also able to step a motor up gradually, but drives can beprogrammed to ramp up the motor much more gradually and smoothly, andcan operate the motor at less than full speed to decrease wear and tear.Variable speed drives can also run a motor in specialized patterns to furtherminimize mechanical and electrical stress. For example, an S-curve patterncan be applied to a conveyor application for smoother decel/accel control,which reduces the backlash that can occur when a conveyor is acceleratingor decelerating....

VFD types

All VFDs use their output devices (IGBTs, transistors, thyristors) only asswitches, turning them only on or off. Using a linear device such as atransistor in its linear mode is impractical for a VFD drive, since the powerdissipated in the drive devices would be about as much as the powerdelivered to the load.

Drives can be classified as:

Constant voltage Constant current Cyclo converter

In a constant voltage converter, the intermediate DC link voltage remainsapproximately constant during each output cycle. In constant current drives,a large inductor is placed between the input rectifier and the output bridge,
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so the current delivered is nearly constant. A cyclo converter has no inputrectifier or DC link and instead connects each output terminal to theappropriate input phase.

The most common type of packaged VF drive is the constant-voltage type,

using pulse width modulation to control both the frequency and effectivevoltage applied to the motor load.

VFD system description

VFD system

A variable frequency drive system generally consists of an AC motor, acontroller and an operator interface.[5][6]

VFD motor

The motor used in a VFD system is usually a three-phaseinduction motor.Some types ofsingle-phase motors can be used, but three-phase motors areusually preferred. Various types of synchronous motors offer advantages insome situations, but induction motors are suitable for most purposes and aregenerally the most economical choice. Motors that are designed for fixed-speed operation are often used. Certain enhancements to the standardmotor designs offer higher reliability and better VFD performance, such asMG-31 rated motors.[7]

VFD controller

Variable frequency drive controllers are solid stateelectronic powerconversion devices. The usual design first converts AC input power to DCintermediate power using a rectifier or converter bridge. The rectifier isusually a three-phase, full-wave-diode bridge. The DC intermediate power isthen converted to quasi-sinusoidal AC power using an inverter switching
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circuit. The inverter circuit is probably the most important section of the VFD,changing DC energy into three channels of AC energy that can be used by anAC motor. These units provide improved power factor, less harmonicdistortion, and low sensitivity to the incoming phase sequencing than olderphase controlled converter VFD's. Since incoming power is converted to DC,

many units will accept single-phase as well as three-phase input power(acting as a phase converter as well as a speed controller); however the unitmust be derated when using single phase input as only part of the rectifierbridge is carrying the connected load.[8]

As new types ofsemiconductor switches have been introduced, these havepromptly been applied to inverter circuits at all voltage and current ratingsfor which suitable devices are available. Introduced in the 1980s, theinsulated-gate bipolar transistor (IGBT) became the device used in most VFDinverter circuits in the first decade of the 21st century.[9][10][11]

AC motor characteristics require the applied voltage to be proportionallyadjusted whenever the frequency is changed in order to deliver the ratedtorque. For example, if a motor is designed to operate at 460 volts at 60 Hz,the applied voltage must be reduced to 230 volts when the frequency isreduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to aconstant value (460/60 = 7.67 V/Hz in this case). For optimum performance,some further voltage adjustment may be necessary especially at low speeds,but constant volts per hertz is the general rule. This ratio can be changed inorder to change the torque delivered by the motor.[12]

In addition to this simple volts per hertz control more advanced control

methods such as vector control and direct torque control (DTC) exist. Thesemethods adjust the motor voltage in such a way that the magnetic flux andmechanical torque of the motor can be precisely controlled.

The usual method used to achieve variable motor voltage is pulse-widthmodulation (PWM). With PWM voltage control, the inverter switches are usedto construct a quasi-sinusoidal output waveform by a series of narrowvoltage pulses with pseudosinusoidal varying pulse durations.[9][13]

Operation of the motors above rated name plate speed (base speed) ispossible, but is limited to conditions that do not require more power than the

nameplate rating of the motor. This is sometimes called "field weakening"and, for AC motors, means operating at less than rated volts/hertz and aboverated name plate speed. Permanent magnet synchronous motors have quitelimited field weakening speed range due to the constant magnet fluxlinkage. Wound rotor synchronous motors and induction motors have muchwider speed range. For example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole)induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be limited to60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power.[14] At
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higher speeds the induction motor torque has to be limited further due to thelowering of the breakaway torque of the motor. Thus rated power can betypically produced only up to 130...150% of the rated name plate speed.Wound rotor synchronous motors can be run at even higher speeds. In rollingmill drives often 200...300% of the base speed is used. Naturally the

mechanical strength of the rotor and the lifetime of the bearings also limitthe maximum speed of the motor. Consulting the motor manufacturer isrecommended if more than 150% speed is required by the application.

PWM VFD Output Voltage Waveform

An embeddedmicroprocessor governs the overall operation of the VFDcontroller. The main microprocessor programming is in firmware that isinaccessible to the VFD user. However, some degree of configurationprogramming and parameter adjustment is usually provided so that the usercan customize the VFD controller to suit specific motor and driven equipmentrequirements.[9]

VFD operator interface

The operator interface provides a means for an operator to start and stop themotor and adjust the operating speed. Additional operator control functionsmight include reversing, and switching between manual speed adjustmentand automatic control from an external process control signal. The operatorinterface often includes an alphanumeric display and/or indication lights andmeters to provide information about the operation of the drive. An operatorinterface keypad and display unit is often provided on the front of the VFD

controller as shown in the photograph above. The keypad display can oftenbe cable-connected and mounted a short distance from the VFD controller.Most are also provided with input and output (I/O) terminals for connectingpushbuttons, switches and other operator interface devices or controlsignals. A serial communicationsport is also often available to allow the VFDto be configured, adjusted, monitored and controlled using a computer.[9][15][16]
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VFD operation

When an induction motor is first connected to a full voltage supply, it drawsseveral times (up to about 6 times) its rated current. As the load accelerates,the available torque usually drops a little and then rises to a peak while the

current remains very high until the motor approaches full speed.

By contrast, when a VFD starts a motor, it initially applies a low frequencyand voltage to the motor. The starting frequency is typically 2 Hz or less.Thus starting at such a low frequency avoids the high inrush current thatoccurs when a motor is started by simply applying the utility (mains) voltageby turning on a switch. After the start of the VFD, the applied frequency andvoltage are increased at a controlled rate or ramped up to accelerate theload without drawing excessive current. This starting method typically allowsa motor to develop 150% of its rated torque while the VFD is drawing lessthan 50% of its rated current from the mains in the low speed range. A VFDcan be adjusted to produce a steady 150% starting torque from standstillright up to full speed.[17] Note, however, that cooling of the motor is usuallynot good in the low speed range. Thus running at low speeds even with ratedtorque for long periods is not possible due to overheating of the motor. Ifcontinuous operation with high torque is required in low speeds an externalfan is usually needed. The manufacturer of the motor and/or the VFD shouldspecify the cooling requirements for this mode of operation.

In principle, the current on the motor side is in direct proportion to thetorque that is generated and the voltage on the motor is in direct proportionof the actual speed, while on the network side, the voltage is constant, thusthe current on line side is in direct proportion of the power drawn by themotor, that is U.I or C.N where C is torque and N the speed of the motor (weshall consider losses as well, neglected in this explanation).

1. n stands for network (grid) and m for motor2. C stands for torque [Nm], U for voltage [V], I for current [A], and N for

speed [rad/s]

We neglect losses for the moment:

Un.In = Um.Im (same power drawn from network and from motor) Um.Im = Cm.Nm (motor mechanical power = motor electrical power) Given Un is a constant (network voltage) we conclude: In = Cm.Nm/Un

That is "line current (network) is in direct proportion of motor power".

With a VFD, the stopping sequence is just the opposite as the startingsequence. The frequency and voltage applied to the motor are ramped downat a controlled rate. When the frequency approaches zero, the motor is shut
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off. A small amount of braking torque is available to help decelerate the loada little faster than it would stop if the motor were simply switched off andallowed to coast. Additional braking torque can be obtained by adding abraking circuit (resistor controlled by a transistor) to dissipate the brakingenergy. With 4-quadrants rectifiers (active-front-end), the VFD is able to

brake the load by applying a reverse torque and reverting the energy to thenetwork.

Power line harmonics

While PWM allows for nearly sinusoidal currents to be applied to a motorload, the diode rectifier of the VFD takes roughly square-wave current pulsesout of the AC grid, creating harmonic distortion of the power line input,especially in the current waveform. When the VFD load size is small and theavailable utility power is large, the effects of VFD systems slicing smallchunks out of the AC grid generally go unnoticed. Furthermore, in lowvoltage networks the harmonics caused by single phase equipment such ascomputers and TVs are such that they are partially cancelled by three-phasediode bridge harmonics.

However, when either a large number of low-current VFDs, or just a few verylarge-load VFDs are used, they can have a cumulative negative impact onthe AC power waveform available to other utility customers in the same grid.

When the utility voltage becomes misshapen and distorted, the losses inother loads such as normal AC motors are increased. This may in the worstcase lead to overheating and shorter operating life. Also substationtransformers and compensation capacitors are affected, the latter especiallyif resonances are aroused by the harmonics.

In order to limit the voltage distortion, the owner of the VFDs may berequired to install filtering equipment to smooth out the irregular waveform.Alternatively, the utility may choose to install filtering equipment of its ownat substations affected by the large amount of VFD equipment being used. In
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high power installations decrease of the harmonics can be obtained bysupplying the VFDs from transformers that have different phase shift.[18]

Furthermore, it is possible, instead of the diode rectifier, to use a transistorcircuit similar to that which controls the motor. This kind of rectifier is called

an active infeed converter in IEC standards. However, manufacturers call itby several names such as active rectifier, ISU (IGBT Supply Unit), AFE (ActiveFront End) or four quadrant rectifier. With PWM control of the transistors andfilter inductors in the supply lines, the AC current can be made nearlysinusoidal. Even better attenuation of the harmonics can be obtained byusing an LCL (inductor-capacitor-inductor) filter instead of single three-phasefilter inductor.

An additional advantage of the active infeed converter over the diode bridgeis its ability to feed back the energy from the DC side to the AC grid. Thus nobraking resistor is needed and the efficiency of the drive is improved if the

drive is frequently required to brake the motor.

Application considerations

Transmission line effects

The output voltage of a PWM VFD consists of a train of pulses switched atwhat is called the carrier frequency. Because of the rapid rise time of thesepulses, transmission line effects of the cable between the drive and motormust be considered. Since the transmission-line impedance of the cable andmotor are different, pulses tend to reflect back from the motor terminals intothe cable. The resulting voltages can produce up to twice the rated linevoltage for long cable runs, putting high stress on the cable and motorwinding and eventual insulation failure. Increasing the cable or motor

size/type for long runs and using 480V or 600V motors instead of 230V willhelp offset the stresses imposed upon the equipment due to the VFD.(Modern 230v single phase motors are not affected). At 460 V, the maximumrecommended cable distances between VFDs and motors can vary by afactor of 2.5:1. The longer cable distances are allowed at the lower CarrierSwitching Frequencies (CSF) of 2.5 kHz. The lower CSF can produce audiblenoise at the motors. For applications requiring long motor cables VSDmanufacturers usually offer dv/dt filters that decrease the steepness of the
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pulses. For very long cables or old motors with insufficient windinginsulation, more efficient sinusoidal filters are recommended. Expect theolder motor's life to shorten. Purchase VFD rated motors for the application.

Motor bearings

Main article: Shaft voltage

Further, the rapid rise time of the pulses may cause trouble with the motorbearings. The stray capacitance of the windings provides paths for highfrequency currents that pass through the motor shaft and bearings. If thevoltage between the shaft and the shield of the motor exceeds a few voltsthe stored charge is discharged as a small spark. Repeated sparking causeserosion in the bearing surface that can be seen as a fluting pattern. In orderto prevent sparking the motor cable should provide a low impedance returnpath from the motor frame back to the inverter. Thus it is essential to use a

cable designed to be used with VSDs.[19]

In big motors a slip ring and brush can be used to provide a bypass path forthe bearing currents. Alternatively, isolated bearings can be used.

The 2.5 kHz and 5 kHz CSFs cause fewer motor bearing problems than the20 kHz CSFs.[20] Shorter cables are recommended at the higher CSF of20 kHz. (The minimum CSF for synchronize tracking of multiple conveyors is8 kHz.)

The high frequency current ripple in the motor cables may also cause

interference with other cabling in the building. This is another reason to usea motor cable designed for VSDs that has a symmetrical three-phasestructure and good shielding. Furthermore, it is highly recommended toroute the motor cables as far away from signal cables as possible.[21]

Available VFD power ratings

Variable frequency drives are available with voltage and current ratings tomatch the majority of 3-phase motors that are manufactured for operationfrom utility (mains) power. VFD controllers designed to operate at 110 V to

690 V are often classified as low voltage units. Low voltage units are typicallydesigned for use with motors rated to deliver 0.2 kW or 0.25 horsepower (hp)up to several megawatts. For example, the largest ABB ACS800 single drivesare rated for 5.6 MW.[22] Medium voltage VFD controllers are designed tooperate at 2,400/4,162 V (60 Hz), 3 kV (50 Hz) or up to 10 kV. In someapplications a step up transformer is placed between a low voltage drive anda medium voltage load. Medium voltage units are typically designed for usewith motors rated to deliver 375 kW or 500 hp and above. Medium voltage
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drives rated above 7 kV and 5,000/10,000 hp should probably be consideredto be one-of-a-kind (one-off) designs.[23]

Medium voltage drives are generally rated amongst the following voltages :2.3 kV, 3.3 kV, 4 kV, 6 kV, and 11 kV. The in-between voltages are generally

possible as well. The power of M.V. drives is generally in the range of 0.3 to100 MW; this involves a range of several different types of drives usingdifferent technologies.

Dynamic braking

Using the motor as a generator to absorb energy from the system is calleddynamic braking. Dynamic braking stops the system more quickly thancoasting. Since dynamic braking requires that the rotor be moving, itbecomes less effective at low speed and cannot be used to hold a load at astopped position. During normal braking of an electric motor, the electricalenergy produced by the motor is dissipated as heat inside of the rotor, whichincreases the likelihood of damage and eventual failure. Therefore, somesystems transfer this energy to an outside bank of resistors. Cooling fansmay be used to protect the resistors from damage. Modern systems havethermal monitoring, so if the temperature of the bank becomes excessive, itwill be switched off.[24]

Regenerative variable-frequency drives

Regenerative AC drives have the capacity to recover the braking energy of a

load moving faster than the designated motor speed (an overhauling load)and return it to the power system.
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Line regenerative variable frequency drives, showing capacitors (topcylinders) and inductors attached, which filter the regenerated power.

Cycloconverters and current-source inverters inherently allow return ofenergy from the load to the line, while voltage-source inverters require an

additional converter to return energy to the supply.[25]

Regeneration is only useful in variable-frequency drives where the value ofthe recovered energy is large compared to the extra cost of a regenerativesystem,[25] and if the system requires frequent braking and starting. Anexample would be conveyor belt drives for manufacturing, which stop everyfew minutes. While stopped, parts are assembled correctly; once that isdone, the belt moves on. Another example is a crane, where the hoist motorstops and reverses frequently, and braking is required to slow the loadduring lowering. Regenerative variable-frequency drives are widely usedwhere speed control of overhauling loads is required.

[2][3][26]

Brushless DC motor drives

Much of the same logic contained in large, powerful VFDs is also embeddedin small brushless DC motors such as those commonly used in computerfans. In this case, the chopper usually converts a low DC voltage (such as 12volts) to the three-phase current used to drive the electromagnets that turnthe permanent magnet rotor.
http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-Dubey-24http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-Dubey-24http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-Jaeschke-1http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-Siskind-2http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-25http://en.wikipedia.org/wiki/Brushless_DC_motorhttp://en.wikipedia.org/wiki/Chopper_(electronics)http://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Permanent_magnethttp://en.wikipedia.org/wiki/File:KIMO_TRA_U2_mL_Gruppe.jpghttp://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-Dubey-24http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-Dubey-24http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-Jaeschke-1http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-Siskind-2http://en.wikipedia.org/wiki/Variable-frequency_drive#cite_note-25http://en.wikipedia.org/wiki/Brushless_DC_motorhttp://en.wikipedia.org/wiki/Chopper_(electronics)http://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Permanent_magnet


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SOFT STARTER

Soft start is a term describing any electrical circuit which reduces excesscurrent during initial power-up of an electric motor.

Contents

1 Introduction 2 What is a softstarter? 3 Why use a softstarter? 4 Construction 5 Control

o 5.1 Open-Loopo 5.2 Closed-Loop

6Applications/Topologies 7 Need for soft start 8 Soft startingmethods 9 Logic ControllerApplication 10 References

11 See also

Introduction

In industrial applications, electric motors are used extensively and representa significant percentage of total industrial electrical load . There aregenerally three different ways to start a motor: full-voltage, reduced voltage,and inverter. A full voltage, across-the-line, or direct on-line (DOL) start uses

a contactor, which is a heavier duty three-phase relay. Reduced voltagestarting can be accomplished via several different ways: auto-transformer,wye-delta, primary resistor/reactor, or with a solid state soft starter.Inverters are generally referred to as drives. This paper focuses on solidstate soft starters (referred to as soft starter only from here on): what theyare, why they are used, their construction, and applications.
http://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Soft_start#Introductionhttp://en.wikipedia.org/wiki/Soft_start#What_is_a_soft_starter.3Fhttp://en.wikipedia.org/wiki/Soft_start#What_is_a_soft_starter.3Fhttp://en.wikipedia.org/wiki/Soft_start#Why_use_a_soft_starter.3Fhttp://en.wikipedia.org/wiki/Soft_start#Why_use_a_soft_starter.3Fhttp://en.wikipedia.org/wiki/Soft_start#Constructionhttp://en.wikipedia.org/wiki/Soft_start#Controlhttp://en.wikipedia.org/wiki/Soft_start#Controlhttp://en.wikipedia.org/wiki/Soft_start#Open-Loophttp://en.wikipedia.org/wiki/Soft_start#Closed-Loophttp://en.wikipedia.org/wiki/Soft_start#Closed-Loophttp://en.wikipedia.org/wiki/Soft_start#Applications.2FTopologieshttp://en.wikipedia.org/wiki/Soft_start#Applications.2FTopologieshttp://en.wikipedia.org/wiki/Soft_start#Need_for_soft_starthttp://en.wikipedia.org/wiki/Soft_start#Soft_starting_methodshttp://en.wikipedia.org/wiki/Soft_start#Soft_starting_methodshttp://en.wikipedia.org/wiki/Soft_start#Logic_Controller_Applicationhttp://en.wikipedia.org/wiki/Soft_start#Logic_Controller_Applicationhttp://en.wikipedia.org/wiki/Soft_start#Referenceshttp://en.wikipedia.org/wiki/Soft_start#See_alsohttp://en.wikipedia.org/wiki/Inverterhttp://en.wikipedia.org/wiki/Contactorhttp://en.wikipedia.org/wiki/Auto-transformerhttp://en.wikipedia.org/wiki/Wye-deltahttp://en.wikipedia.org/wiki/Solid_statehttp://en.wikipedia.org/wiki/Solid_statehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Soft_start#Introductionhttp://en.wikipedia.org/wiki/Soft_start#What_is_a_soft_starter.3Fhttp://en.wikipedia.org/wiki/Soft_start#What_is_a_soft_starter.3Fhttp://en.wikipedia.org/wiki/Soft_start#Why_use_a_soft_starter.3Fhttp://en.wikipedia.org/wiki/Soft_start#Why_use_a_soft_starter.3Fhttp://en.wikipedia.org/wiki/Soft_start#Constructionhttp://en.wikipedia.org/wiki/Soft_start#Controlhttp://en.wikipedia.org/wiki/Soft_start#Open-Loophttp://en.wikipedia.org/wiki/Soft_start#Closed-Loophttp://en.wikipedia.org/wiki/Soft_start#Closed-Loophttp://en.wikipedia.org/wiki/Soft_start#Applications.2FTopologieshttp://en.wikipedia.org/wiki/Soft_start#Applications.2FTopologieshttp://en.wikipedia.org/wiki/Soft_start#Need_for_soft_starthttp://en.wikipedia.org/wiki/Soft_start#Soft_starting_methodshttp://en.wikipedia.org/wiki/Soft_start#Soft_starting_methodshttp://en.wikipedia.org/wiki/Soft_start#Logic_Controller_Applicationhttp://en.wikipedia.org/wiki/Soft_start#Logic_Controller_Applicationhttp://en.wikipedia.org/wiki/Soft_start#Referenceshttp://en.wikipedia.org/wiki/Soft_start#See_alsohttp://en.wikipedia.org/wiki/Inverterhttp://en.wikipedia.org/wiki/Contactorhttp://en.wikipedia.org/wiki/Auto-transformerhttp://en.wikipedia.org/wiki/Wye-deltahttp://en.wikipedia.org/wiki/Solid_statehttp://en.wikipedia.org/wiki/Solid_state


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What is a soft starter?

A soft starter is a solid state motor starter that is used to start or stop amotor by notching the voltage waveform, thereby, reducing the voltage toeach phase of a motor and gradually increasing the voltage until the motor

gets up to full voltage/speed all at a fixed frequency. The profile of theincrease of voltage depends on the application. The voltage is reduced andcontrolled by 3 pairs of back-to-back silicon-controlled rectifiers (SCRs),which are a type of high speed thyristor. A soft starter takes the place of acontactor and can also take the place of an overload relay in a standardmotor starting application.

Why use a soft starter?

In general, there are two reasons to use a soft starter: the power distribution

network may not be able to handle the inrush current of the motor and/orthe load cannot handle the high starting torque. As a rule of thumb, a motorutilizes around 600-800% of its full load current (FLA) to start. This current isreferred to as inrush current or locked-rotor current. If a large motor is on asmaller power distribution network or on a generator system, this inrushcurrent can cause the system voltage to dip, or to brown out. Brown outscan cause problems with whatever else is connected to the system, such ascomputers, lights, motors, and other loads. Another problem is that thesystem may not even be able to start the motor because it cannot source orsupply enough current. Most industrial businesses run during the day can befined or charged extra (Peak Demand charges) during this peak usage time

for large transients caused by large horsepower (HP)/Kilowatt (kW) motorstart ups. These Peak Demand charges can add up very quickly, especially ifthe motor needs to be started multiple times during any given day. Theinrush current can be controlled one of two ways with a soft starter: eitherwith a current limit (discussed later) or reduced linearly with the reducedvoltage, and follows this approximation:

Applications such as conveyors may not be able to handle a sudden jolt oftorque from an across-the-line start. Utilizing soft starters reduces the wearand tear on belts, conveyors, gears, chains, and gearboxes by reducing thetorque from the motor. The torque decreases as a square of the reducedvoltage, and follows this approximation:
http://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Full_load_currenthttp://en.wikipedia.org/wiki/Inrush_currenthttp://en.wikipedia.org/wiki/Power_outagehttp://en.wikipedia.org/wiki/Horsepowerhttp://en.wikipedia.org/wiki/Kilowatthttp://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Full_load_currenthttp://en.wikipedia.org/wiki/Inrush_currenthttp://en.wikipedia.org/wiki/Power_outagehttp://en.wikipedia.org/wiki/Horsepowerhttp://en.wikipedia.org/wiki/Kilowatt


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Since soft starters are generally controlled and monitored by amicroprocessor, a soft starter can add many features and protections fairlyeasily. It can offer a choice of the starting time, limited speed control, andenergy savings. Power monitoring, such as three-phase current, three-phasevoltage, power, power usage, power factor, and motor thermal capacity

usage, can be implemented with current transformers, a voltage meter, andan internal clock. With the above implementations, protection, for the motoror the soft starter, from the items listed below (Table 1) can also be offeredby stopping the firing of the SCRs, dropping out the bypass contactor (acontactor that carries the motor load after the motor is up-to-speed), and/oralerting a user via some form of communications with the microprocessorand another computer.

ConstructionGenerally, a soft starter is constructed with three pairs of SCRs reverseparallel connected to allow the current to flow to or from the motor. Softstarters can be made by controlling just one or two phases, but this page willfocus on the most prevalent implementation, three-phase control. Eachphase of a soft starter can be controlled with an SCR pair reverse parallelconnected, an SCR/diode pair reverse parallel connected, or a triac,depending on cost and/or quality. The most prevalent switch in industry isprobably the SCR pair and will also be the focus of this paper. Soft startersare used almost exclusively for starting and stopping and not during the run

time because of the heat loss through the SCRs from the voltage drop acrossthem.

A standard assembly of a soft starter uses one SCR pair per phase and oncethe voltage gets to within approximately 1.1V of full voltage (depending onthe voltage drop across the SCR) a bypass contactor (internal or external tothe soft starter), running parallel to the SCR pairs, pulls in. Once pulled in theSCRs stop firing. Typically, the bypass contactor is much smaller thancompared to what is needed for a full voltage start as the contacts only needto be able to handle the full load current of the motor. Since the mechanicalcontacts cannot handle the inrush current, the SCRs must be sized correctly

to handle the motors locked-rotor current. The transition from SCRs tobypass should also be near full speed to minimize the jump in current.

Control

A soft starter reduces the voltage by notching the applied sinusoidalwaveform. A notch is a non-technical term for the zero voltage area in the
http://en.wikipedia.org/wiki/Microprocessorhttp://en.wikipedia.org/wiki/Triachttp://en.wikipedia.org/wiki/Microprocessorhttp://en.wikipedia.org/wiki/Triac


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middle waveform. As the notch decreases in size, the Vrms increases alongwith Irms. An initial voltage, determined by the user, is ramped up to fullvoltage by varying the firing angle depending on the preset profile of the softstarter. Soft starters can be controlled via open-loop or closed-loop control.All control schemes listed monitor back EMF of the motor as to not become

unstable.

Open-Loop

An example of open loop control is the voltage ramp; the voltage ramps froman initial voltage to full voltage in a linear fashion without regards to theload. Pump start is another form of open-loop control. The pump startersfiring circuit ramps up the voltage with a profile that allows the speed/torqueto ramp in a more efficient manner and helps protect against waterhammering, a common problem in pump applications.

Closed-Loop

Applications such as current limit use feedback from the motor or the linecurrent/voltage to change the firing angle of the SCRs as necessary, henceclosed-loop.

Applications/Topologies

Soft starters can be made for a reversing application by adding two extraSCR pairs that switch two phases. For example, line phase b is connectedto load phase c and vice versa. L2 (b) is connected to T3 (c) and L3(c) is connected to T2 (b) in Fig. 17. A delta configuration motor can alsobe controlled with a soft starter, but it will see more current than a lineconnected motor. As a way to get around the larger current switching, a softstarter can be wired inside the delta. Wiring in this configuration will allowthe soft starter to control a larger motor than even line connected by a 3advantage. For instance, an inside the delta soft starter can switch a 277Aload versus a line connected soft starter needs to be able to switch 480A tocontrol the same rated motor load. A disadvantage of inside the delta is

that it requires six leads coming from the motor which can be an addedexpense with larger HP motors. In contrast, kick starting and low speedramps are some other applications that can be implemented via differentprogramming of SCR firing angles.

Need for soft start


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Many electronic devices will consume a large initial current when first turnedon (i.e. Inrush current) that can cause voltage fluctuations and affect theperformance of other circuits connected to a common power supply. Thesource for this problem is often large capacitors with very low Inputimpedance. To counteract this issue, components can be added in series to

throttle back the current initially as the device comes online.

Soft starters are also used to start some types oflamps. A tungsten filamenthas a positive temperature coefficient of resistance: a cold filament has asmaller resistance by a factor of 8-10 than a hot filament, and allows a largeinrush of current. This inrush coupled with uneven filament wear causes localtemperature overshoot in hotspots during startup, further evaporating thethinner filament sections. While soft start has little effect on GLS lamp life, itcan make a sizeable difference to a halogen lamp's life.

High initial current can cause damage to other components such as

semiconductors if they are not rated for the initial high current of loads suchas filament lamps, motors or capacitors.

Soft starts are sometimes used on larger equipment as well, such as electricmotors in various applications. The current drawn by an electric motor duringa start can be 2 to 10 times the normal operating current, and this canexceed the supply's ratings if not controlled.

Finally soft start is widely used on hand held tools to prevent the tool beingjerked out of position when switched on. Jumping ofelectric drills was awidespread issue in the 1970s when soft start was usually not fitted.

Soft starting methods

There are various ways to implement soft starting.

The most popular for appliances is triac control which ramps up duty cycleover several cycles.

A method used with some motors is to start with windings in series,switching them to parallel when partial speed has been reached. This iscommonly known as Y- start.

Increasing supply impedance has a semi soft start effect on motors, withinitial current still being above run current, but by a much reduced amount.This method is not widely used.

Current limiting and voltage ramp-up in electronic supplies are commonmethods of soft starting low voltage loads.
http://en.wikipedia.org/wiki/Inrush_currenthttp://en.wikipedia.org/wiki/Power_supplyhttp://en.wikipedia.org/wiki/Capacitor_(component)http://en.wikipedia.org/wiki/Input_impedancehttp://en.wikipedia.org/wiki/Input_impedancehttp://en.wikipedia.org/wiki/Lamp_(electrical_component)http://en.wikipedia.org/wiki/Electrical_filamenthttp://en.wikipedia.org/wiki/Temperature_coefficient#Temperature_coefficient_of_electrical_resistancehttp://en.wikipedia.org/wiki/Resistance_(electricity)http://en.wikipedia.org/wiki/Halogen_lamphttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_drillhttp://en.wikipedia.org/wiki/Triachttp://en.wikipedia.org/wiki/Duty_cyclehttp://en.wikipedia.org/wiki/Series_and_parallel_circuits#Series_circuitshttp://en.wikipedia.org/wiki/Parallel_circuits#Parallel_circuitshttp://en.wikipedia.org/wiki/Electrical_impedancehttp://en.wikipedia.org/wiki/Current_limitinghttp://en.wikipedia.org/wiki/Inrush_currenthttp://en.wikipedia.org/wiki/Power_supplyhttp://en.wikipedia.org/wiki/Capacitor_(component)http://en.wikipedia.org/wiki/Input_impedancehttp://en.wikipedia.org/wiki/Input_impedancehttp://en.wikipedia.org/wiki/Lamp_(electrical_component)http://en.wikipedia.org/wiki/Electrical_filamenthttp://en.wikipedia.org/wiki/Temperature_coefficient#Temperature_coefficient_of_electrical_resistancehttp://en.wikipedia.org/wiki/Resistance_(electricity)http://en.wikipedia.org/wiki/Halogen_lamphttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_drillhttp://en.wikipedia.org/wiki/Triachttp://en.wikipedia.org/wiki/Duty_cyclehttp://en.wikipedia.org/wiki/Series_and_parallel_circuits#Series_circuitshttp://en.wikipedia.org/wiki/Parallel_circuits#Parallel_circuitshttp://en.wikipedia.org/wiki/Electrical_impedancehttp://en.wikipedia.org/wiki/Current_limiting


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Logic Controller Application

This soft start is provided by a dedicated output of a programmable logiccontroller. They can be used to start motors where Variable-frequency drive(VFD) are not used. This allows a cheaper starting option for motors that will

run at a constant speed.[1]
http://en.wikipedia.org/wiki/Variable-frequency_drivehttp://www.latticesemi.com/documents/I0191.pdfhttp://en.wikipedia.org/wiki/Variable-frequency_drivehttp://www.latticesemi.com/documents/I0191.pdf


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Motor soft starter

From Wikipedia, the free encyclopedia

Examples of motor soft starters [1]

A motor soft starter is a device used with AC electric motors to temporarilyreduce the load and torque in the power train of the motor during startup.This reduces the mechanical stress on the motor and shaft, as well as theelectro dynamic stresses on the attached power cables and electricaldistribution network, extending the lifespan of the system.[2]

Motor soft starters can consist of mechanical or electrical devices, or acombination of both. Mechanical soft starters include clutches and severaltypes ofcouplings using a fluid, magnetic forces, or steel shot to transmittorque, similar to other forms oftorque limiter. Electrical soft starters can beany control system that reduces the torque by temporarily reducing thevoltage or current input, or a device that temporarily alters how the motor isconnected in the electric circuit.

Electrical soft starters can use solid state devices to control the current flowand therefore the voltage applied to the motor. They can be connected inseries with the line voltage applied to the motor, or can be connected insidethe delta () loop of a delta-connected motor, controlling the voltage appliedto each winding. Solid state soft starters can control one or more phases ofthe voltage applied to the induction motor with the best results achieved bythree-phase control. Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled rectifiers (thyristors), but in some circumstanceswith three-phase control, the control elements can be a reverse-parallel-connected SCR and diode.
http://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-0http://en.wikipedia.org/wiki/AC_motorhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Powertrainhttp://en.wikipedia.org/wiki/Mechanical_stresshttp://en.wikipedia.org/wiki/Electrodynamicshttp://en.wikipedia.org/wiki/Electricity_distributionhttp://en.wikipedia.org/wiki/Electricity_distributionhttp://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-Siskind-1http://en.wikipedia.org/wiki/Clutchhttp://en.wikipedia.org/wiki/Couplinghttp://en.wikipedia.org/wiki/Fluid_couplinghttp://en.wikipedia.org/wiki/Lead_shothttp://en.wikipedia.org/wiki/Torque_limiterhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_circuithttp://en.wikipedia.org/wiki/Solid_state_(electronics)http://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Series_circuitshttp://en.wikipedia.org/wiki/Y-%CE%94_transformhttp://en.wikipedia.org/wiki/Three-phase_electric_powerhttp://en.wikipedia.org/wiki/Parallel_circuitshttp://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/File:Soft_starters1.jpghttp://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-0http://en.wikipedia.org/wiki/AC_motorhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Powertrainhttp://en.wikipedia.org/wiki/Mechanical_stresshttp://en.wikipedia.org/wiki/Electrodynamicshttp://en.wikipedia.org/wiki/Electricity_distributionhttp://en.wikipedia.org/wiki/Electricity_distributionhttp://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-Siskind-1http://en.wikipedia.org/wiki/Clutchhttp://en.wikipedia.org/wiki/Couplinghttp://en.wikipedia.org/wiki/Fluid_couplinghttp://en.wikipedia.org/wiki/Lead_shothttp://en.wikipedia.org/wiki/Torque_limiterhttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_circuithttp://en.wikipedia.org/wiki/Solid_state_(electronics)http://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Series_circuitshttp://en.wikipedia.org/wiki/Y-%CE%94_transformhttp://en.wikipedia.org/wiki/Three-phase_electric_powerhttp://en.wikipedia.org/wiki/Parallel_circuitshttp://en.wikipedia.org/wiki/Silicon-controlled_rectifierhttp://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Diode


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Another way to limit motor starting current is a series reactor. If an air core isused for the series reactor then a very efficient and reliable soft starter canbe designed which is suitable for all type of 3 phase induction motor[ synchronous / asynchronous ] ranging from 25 KW 415 V to 30 MW 11 KV.Using an air core series reactor soft starter is very common practice for

applications like pump, compressor, fan etc. Usually high starting torqueapplications do not use this method.

Contents

1 Applications 2 Motor and machine 3 Motor start-up

o 3.1 Direct on-linestartingo 3.2 Star-deltastart-upo 3.3 Soft start-up

4 See also

5 References

Applications

Soft starters can be set up to the requirements of the individual application.In pump applications, a soft start can avoid pressure surges. Conveyor beltsystems can be smoothly started, avoiding jerk and stress on drivecomponents. Fans or other systems with belt drives can be started slowly toavoid belt slipping. In all systems, a soft start limits the inrush current and soimproves stability of the power supply and reduces transient voltage dropsthat may affect other loads. [3][4][5]

Motor and machine

Across-the line starting of induction motors is accompanied by inrushcurrents up to 7 times higher than running current, and starting torque up to3 times higher than running torque. The increased torque results in suddenmechanical stress on the machine which leads to a reduced service life.Moreover, the high inrush current stresses the power supply, which may leadto voltage dips. As a result, the operability of sensitive consumers may beimpaired.[2]
http://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Motor_soft_starter#Applicationshttp://en.wikipedia.org/wiki/Motor_soft_starter#Motor_and_machinehttp://en.wikipedia.org/wiki/Motor_soft_starter#Motor_start-uphttp://en.wikipedia.org/wiki/Motor_soft_starter#Direct_on-line_startinghttp://en.wikipedia.org/wiki/Motor_soft_starter#Direct_on-line_startinghttp://en.wikipedia.org/wiki/Motor_soft_starter#Star-delta_start-uphttp://en.wikipedia.org/wiki/Motor_soft_starter#Star-delta_start-uphttp://en.wikipedia.org/wiki/Motor_soft_starter#Soft_start-uphttp://en.wikipedia.org/wiki/Motor_soft_starter#See_alsohttp://en.wikipedia.org/wiki/Motor_soft_starter#Referenceshttp://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-Bartos21st-2http://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-3http://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-4http://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-Siskind-1http://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Motor_soft_starter#Applicationshttp://en.wikipedia.org/wiki/Motor_soft_starter#Motor_and_machinehttp://en.wikipedia.org/wiki/Motor_soft_starter#Motor_start-uphttp://en.wikipedia.org/wiki/Motor_soft_starter#Direct_on-line_startinghttp://en.wikipedia.org/wiki/Motor_soft_starter#Direct_on-line_startinghttp://en.wikipedia.org/wiki/Motor_soft_starter#Star-delta_start-uphttp://en.wikipedia.org/wiki/Motor_soft_starter#Star-delta_start-uphttp://en.wikipedia.org/wiki/Motor_soft_starter#Soft_start-uphttp://en.wikipedia.org/wiki/Motor_soft_starter#See_alsohttp://en.wikipedia.org/wiki/Motor_soft_starter#Referenceshttp://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-Bartos21st-2http://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-3http://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-4http://en.wikipedia.org/wiki/Motor_soft_starter#cite_note-Siskind-1


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Motor start-up

A soft start-up eliminates the undesired side effects. Several types based oncontrol of the supply voltage or mechanical devices such as slip clutcheswere developed. The list provides an overview of the various electric start-up

types. The current and torque characteristic curves show the behavior of therespective starter solution.

Direct on-line starting

Three-phase motor with low to medium power rating 3 conductors to the motor High starting torque High current peak Voltage dip One simple switching device

Star-delta start-up

Three-phase motor with low to high power rating Six conductors to the motor Reduced starting torque, 1/3 of the nominal torque High mains load due to current peak during switchover from Y to D High mechanical stress due to torque surge during switchover from Yto D Two or three switching devices, more maintenance

Soft start-up

Three-phase motor with low to high power rating 3 conductors to the motor Variable starting torque No current peak No torque peaks Negligible voltage dip One simple switching device Optional: Guided soft stop, protective functions, etc.

Zero maintenance Compared to contactor solutions, soft starters, sometimes also referredto as soft starting devices, offer considerable advantages.

Torque surges entail high mechanical stress on the machine, which results inhigher service costs and increased wear. High currents and current peakslead to high fixed costs charged by the power supply companies (peakcurrent calculation) and to increased mains and generator loads.


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A soft starter continuously controls the three-phase motors voltage supplyduring the start-up phase. This way, the motor is adjusted to the machinesload behavior. Mechanical operating equipment is accelerated in a gentlemanner. Service life, operating behavior and work flows are positivelyinfluenced.

Braking chopper

Space Vector Modulation
http://en.wikipedia.org/wiki/Braking_chopperhttp://en.wikipedia.org/wiki/Space_Vector_Modulationhttp://en.wikipedia.org/wiki/Braking_chopperhttp://en.wikipedia.org/wiki/Space_Vector_Modulation

regarding selection- word 97 format

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