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Speed control of Induction Motor:

Speed control of Induction Motor Different methods by which speed control of induction motor is achieved may be grouped under two main headings:- Control from the Stator Side:- By changing the applied voltage By changing the applied frequency By changing the number of stator poles. Control from the Rotor Side:- Rotor rheostat control By operating two motors in concatenation or cascade. By injecting an EMF in the rotor circuit.

Changing the applied voltage:

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 flux density thereby seriously disturbing the magnetic condition of the motor. Hence because of above stated reasons this method of speed control is rarely used.

Changing the applied frequency:

Changing the applied frequency This method is also very rarely used, we have seen that the synchronous speed of an induction motor is given by:- Clearly the speed can be varied by changing the supply frequency ‘f’. This method is used in case where induction motor is the only load on the generator. But again the range over which the motor speed may be varied is limited by the economical speeds of the prime movers.

Changing the number of stator poles:

Changing the number of stator poles We know that the speed of induction motor is given by: From this equation it is clear that the synchronous speed can also be changed by changing the number of stator poles. This change of number of poles is achieved by having two or more entirely independent stator windings in the same slot.

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Each winding gives a different number of poles and hence different synchronous speeds. For example:- a 36 slot stator may have two 3 phase windings, one with 4 poles and the other with 6 poles. With supply frequency of 50Hz, 4 pole windings will give: And 6 poles windings will give:

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Motors with four independent stator windings are also in use and they give four different synchronous speeds. Of course , only one winding is used at a time, the others being entirely disconnected.

Rotor Rheostat control:

Rotor Rheostat control This method is applicable to slip-ring motor and speed is reduced by introducing an external resistance in the rotor circuit. It has been known that near synchronous speed .

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It is obvious that for a given torque, slip can be increased ie . Speed can be decreased by increasing the rotor resistance R2. One serious disadvantage of this method is that with increase in rotor resistance, ohmic loss also increases which decreases the operating efficiency of the motor. In fact the loss is directly proportional to reduction in speed. The second disadvantage is the double dependence of speed not only on R2 but on load as well. This method is used where speed changes are needed for short period of time.

Cascade Connection :

Cascade Connection In this method two motors are used and are ordinarily mounted on the same shaft. The stator winding of main motor ‘A’ is connected to the mains, while that of auxiliary motor ‘B’ is fed from the rotor circuit of motor ‘A’.

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When the cascade set is started, the voltage at frequency ‘f’ is applied to the stator winding of machine ‘A’. An induced emf of same frequency is produced in rotor ‘A’ which is supplied to auxiliary motor ‘B’, both these motor develops a forward torque. As the shaft speed rises, the rotor frequency of motor ‘A’ falls and so does the synchronous speed of motor ‘B’. The set settles down to a stable speed when the shaft speed becomes equal to the speed of the rotating field of motor ‘B’. Synchronous speed is:-

Speed Control by Injection of EMF in Rotor Circuit:

Speed Control by Injection of EMF in Rotor Circuit It is known that power transferred from the stator to rotor is P g. Here (1-s)P g represents the mechanical power developed in the rotor and slip power ‘ sP g ’ appears as rotor ohmic loss if rotor is short circuited. It can be converted by various schemes to useful power which is either returned to the supply or added to the motor shaft. It should be understood that the injected emf must be at slip frequency at all rotor speeds.

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When we inject a voltage which is in phase opposition to the induced rotor emf it amounts to increase in the rotor resistance, while inserting a voltage in phase with the induced rotor emf is equivalent to decreasing its resistance. Hence by changing the phase of the injected emf , the speed can be controlled.

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The effect of an injected emf into the rotor circuit can be better understood with help of an analogy shown below:- Fig(a) Fig(b) In the above fig(a). a current of 2A is flowing in the circuit. If an emf of 5V in opposition to source voltage of 10V is injected, fig(b) the current decreases to 1A.

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In order to raise the current to its original value of 2A, the source emf must be increased to 15V. It can be therefore concluded that injection of an emf in phase opposition to induce emf results in speed less than the normal speed. Similarly fig (c) shows that when 5V aiding the source voltage is injected then the voltage must be reduced to 5V, which is equivalent to a decreases in slip. Thus speed above base speed can be obtained by injecting emf in phase with rotor voltage.

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