One Method of Speed Control of 3-phase Induction Motor


SPEED CONTROL OF INDUCTION MOTORS



•    A 3-phase induction motor is practically a constant speed machine.

•    The speed regulation of an induction motor is usually less than 5% at full load.

•    D.C. shunt motors can be made to run at any speed within wide limits, with good efficiency and speed regulation

•     Different methods by which speed control of induction motor is achieved is grouped as
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1. Control from stator side
       
                        a) By changing applied voltage

        b) By changing applied frequency

        c) By changing the number of stator poles
   
2. Control from the rotor side
       
                        a) Rotor rheostat control

        b) By operating two motors in concatenation

        c) By injecting an emf. in the rotor circuit

1. Control form Stator side
   
        Changing the applied voltage
       
•    This method, though the cheapest and the easiest, is rarely used because a large change in voltage is required for a relatively small change in speed.

•     This large change in voltage will result in a large change in the flux density thereby seriously disturbing the magnetic conditions of the motor.
   
    Changing the applied frequency
        
The synchronous speed is given by
Ns = 120f/P

•    The synchronous and hence the running speed of the induction motor can be changed by changing the frequency f.

•    This method can be used in cases where induction motor happens to be the only load on the generators.

•    This method can be used to some extent on electrically driven ships.    


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•    This method is used in elevator motors. Traction motors and also for small motors driving machine tools.

2. Control from the Rotor side
  
Changing the number of stator poles
      
•    This method is applicable to squirrel cage motors.

•    This change of no. of poles can be achieved by having two or more entirely independent stator windings in the slots.

•     Each winding gives different number of poles and hence different speeds.

a) Rotor rheostat control
  
•    This method is applicable to slip ring induction motors only introducing an external resistance in the rotor circuit reduces the motor speed.

•    For this purpose, the rotor starter may be used.

•    Disadvantage of this method is that increase in the rotor resistance increases the I2R loss, which decreases the efficiency of the motor.

b) Cascade or Concatenation or Tandem operation
  
•    In this method two or more motors are used and are mounted on the same shaft, so that both run at the same speed.

•    The stator winding of the main motor A is connected to the mains in the usual way, while the auxiliary motor B is fed from the rotor circuit of A.

    There are three ways in which the combination may be run

•    Main motor A may be run separately from the supply

•    Auxiliary motor B may be run separately from the mains.

•    The combination may be connected in cumulative cascade, in such a way that the phase rotation of the stator fields of both the motors is in the same direction.

3. Injecting an emf in the rotor circuit
  
•    In this method the speed is controlled by injecting a voltage in the rotor circuit.

•    When we insert a voltage in phase opposition to the induced rotor emf, it leads to increase in rotor resistance and if we insert voltage which is in phase with the rotor emf it leads to decrease in rotor resistance.

•    Hence speed can be controlled. Hence these are the various techniques by which the speed of motors can be controlled.

Speed – Torque and braking characteristics of induction motors






Rotor angular speed (rad /sec) = +r

The slip is defined as

            

The rotor current to the stator is (I)




Where Vph = voltage per applied to the stator ,

•     r1 and r2’ are the stator and rotor resistance per phase respectively referred to the stator.

•     x1 and x2’ are the stator and rotor leakage reactance per phase respectively referred to the stator.

The developed torque in N-m is












The figure below shows a typical torque-speed characteristic of the three phases induction motor operating at constant voltage and frequency. 













Control of three phase induction motors using stator voltage control


•    The developed torque of an induction motor at any slip is approximately proportional to the square of the applied voltage.

•    The speed therefore can also be controlled by stator voltage variation. But for an ordinary induction motor operating with constant load torque, the range of speed control is small.

•    A continuous control of speed of an induction motor can be obtained by step less adjustment of stator voltage, if its rotor resistance is high. The drawback here is that the torque per ampere is small at low speeds.

•    However in a fan or pump drive the load torque varies as the square of the speed. The power requirement for these drives decreases rapidly with decrease in speed, thus surmounting this problem.

•    Solid iron rotors may be employed for satisfactory operation at low speeds

                           

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