Prepared by: Ashvani Kumar Shukla

Dy. Manager(C&I) BGR EnergyME Modular, Electrical(I&C) 2015

Roll No-151503,NITTTR Chandigarh1

Speed control of three phase induction motor

Introduction Requirement of Speed controlTypes of Methods to control the speed of Induction

motorAdvantages & disadvantages Industrial applications of AC drives Conclusion Research

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Agenda

A three phase induction motor is basically a constant speed motor .

It is widely used in industry due to low cost and rugged construction .

The speed control of induction motor is done at the cost of decrease in efficiency and low electrical power factor.

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Introduction

Induction motor driving seal air fan for supercritical boiler at Adani TPP Mundra , Gujrat. 4

Speed control means change the drive speed as desired by the process to maintain different process parameter at different load .

Energy Saving.Speed control is a different concept from

speed regulation where there is natural change in speed due change in load on the shaft.

Speed control is either done manually by the operator or by means of some automatic control device.

Low speed starting requirement.5

Requirement of speed control

Stator voltage Control Stator Frequency Control Stator Current Control V/F Control Static rotor resistance control

Methods of Speed Control of Induction

motors

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Synchronous speed Ns = 120 f P Slip = Ns-N Ns

Torque =

Where E2 is the rotor emf Ns is the synchronous speed R2 is the rotor resistance X2 is the rotor inductive reactance

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Stator voltage Control

Rotor resistance R2 is constant and if slip s is small then sX2 is so small that it can be neglected. Therefore, T ∝ sE2

2 where E2 is rotor induced emf and E2 ∝ VAnd hence T ∝ V2, thus if supplied voltage is decreased, torque decreases and hence the speed decreases.

This method is the easiest and cheapest, still rarely used because-

A large change in supply voltage is required for relatively small change in speed.

Large change in supply voltage will result in large change in flux density, hence disturbing the magnetic conditions of the motor.

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Continue…..

Variable Terminal Voltage Control

ms

mLT

T

V decreasing

Synchronous speed of induction motor Ns = 120 f

P

where, f = frequency of the supply and P = number of stator poles.

Thus, synchronous speed changes with change in supply frequency, and thus running speed also changes.

This method is not widely used. This method is used where, only the induction motor is supplied by a generator (so that frequency can be easily change by changing the speed of prime mover).

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Frequency control

By changing the frequency we can control the speed above and below the rated speed.

It offers high range of speed control.

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Continue…..

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Stator current control

Starting torque of IM Ts is proportional to square of stator current.

It is independent of supply frequency. It is independent of rotor resistance.

A constant current for 3 phase IM can be obtained from 3 phase CSI .

Inductor convert the dc voltage as constant current source.

CSI regulate the output frequency and hence torque of induction motor.

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Continues…..

VVVFD: EXISTING TECHNOLOGY

HOW SPEED IS CONTROLLED USING VFD Rectifier: The rectifier in a VFD is used to convert

incoming ac power into direct current (dc) power. DC bus: DC output of rectifier flows through the dc link to

inverter input. Inverter: The “Insulated Gate Bipolar Transistor” (IGBT) is a

common choice in modern VFDs. The IGBT can switch on and off several thousand times

per second and precisely control the power delivered to the motor.

The IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine wave at the desired frequency to the motor.

V/F = C v is voltage and f is called frequency, c= constant In sinusoidal pulse width modulation there are

multiple pulses per half-cycle and the width of the each pulse is varied with respect to the sine wave magnitude corresponding to that duration. 

In V/f control we have to maintain the constant ratio of voltage and frequency.

In order to achieve the constant ratio ,we have to fire the (IGBT )in such way that the amplitude of output voltage waveform in a cycle will maintain the v/f ratio according the frequency change.

Waveforms are shown on next slide 16

SPWM TECHINIQUE FOR v/f

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A slip ring motor or a phase wound motor is an induction motor which can be started with full line voltage, applied across its stator terminals. The value of starting current is adjusted by adding up external resistance to its rotor circuit.

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STATIC ROTOR RESISTENCE CONTROL

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Continue…..

Advantages of ac drivesFor the same rating, ac drives are lighter in weight as

compared to dc drive.AC drive require low maintenance.AC drives are less expensive.Provides the most efficient means of motor speed

control.Reduces the thermal and mechanical stresses on the

motor.Provides low speed motor starting facility.Saves more energy

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Advantages and disadvantages of AC Drives

Disadvantage of ac drives Power converters for the control of ac motors are more

complex. Power converter for ac drives are more expensive. Power converters for ac drives generate harmonics in the

supply system and load circuit.

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Continue…..

Induction motors with squirrel cage rotors are the workhorse of industry .

When Squirrel cage induction machine is operated directly from the line voltages an Induction motor is operated at constant speed. However in the industry we required to vary the speed of an Induction motor. This can be done by Induction motor drive.

Fans, Compressor, Pumps, blowers, machine tools like lathe, drilling machine, lifts, conveyer belts etc.

Industrial applications

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25IM in lube oil system of steam turbine

26IM in coal feeder

In Case of Squirrel cage induction motor the slip cannot be increase above certain limit, the operating speed range is very less. By applying the V/F control we can get the large operating range by keeping V/F ratio constant.

Research The research of speed control for induction motor based on

dynamic recurrent neural network. Speed Control of induction motor using predictive current

control and SVPWM.

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Conclusion

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THANK YOU