Speed control of three phase induction motor

Agenda Introduction Requirement of Speed control Types of Methods to control the speed of Induction motor Advantages & disadvantages Industrial applications of AC drives

Introduction 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.

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

Requirement of speed control 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.

Methods of Speed Control of Induction motors Stator voltage Control Stator Frequency Control Stator Current Control V/F Control Static rotor resistance control

Stator voltage Control 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

Continue….. Rotor resistance R2 is constant and if slip s is small then sX2 is so small that it can be neglected. Therefore, T ∝ sE22 where E2 is rotor induced emf and E2 ∝ V And 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.

Variable Terminal Voltage Control

Frequency control 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).

Continue….. By changing the frequency we can control the speed above and below the rated speed. It offers high range of speed control.

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.

Continues….. 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.

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.

STATIC ROTOR RESISTENCE CONTROL 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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Advantages and disadvantages of AC Drives For 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

Continue….. 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.

Industrial applications 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.

IM in lube oil system of steam turbine

IM in coal feeder

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