# Speed-control of Three-phase Induction Motor

An induction motor is widely used in industries. The speed of an induction motor can be controlled by various methods. In this article, we will discuss different methods of speed-control of Three-phase Induction Motor.

The speed of an induction motor is can be controlled from the stator and rotor side. Different methods of controlling the speed of an induction motor are listed below.

Control from Stator Side: The speed of an induction motor can be controlled from the stator side by changing;

Control from Rotor Side: The speed of an induction motor can be controlled from the rotor side by changing;

• Rotor resistance
• Injection EMF in the rotor circuit
• Scherbius system

## Speed Control from Stator Side

From the stator side, we can control the induction motor by changing various quantities. These methods are briefly explained below.

### Speed Control by Changing Applied Voltage

From the torque equation of the induction motor, the torque is directly proportional to the square of applied voltage.

The slip at maximum torque is independent of the applied voltage. And also, the change in supply voltage does not change the synchronous speed.

The below figure shows the torque-speed characteristics for changing values of the supply voltage.

From the above characteristics, the speed can be varying for a small value by changing a large value in voltage for a given load.

This method is not useful because of the following listed reasons;

If voltages are varied more than the rated voltage, it will result in magnetic saturation and it may create problems. In the worst condition, the core of the motor will damage.

With a reduction in supply voltage, developed torque reduces gradually.

The range of speed control is very limited from rated speed to lower value speed.

From the above reasons, we can conclude that speed control by voltage variation is not popular for large motors. But it can be employed for very small motors.

### Speed Control by Controlling Frequency

The equation of synchronous speed is;

By changing the frequency and number of poles, we can change the synchronous speed, and so on we can change the rotor speed.

But the voltage equation of induction motor is;

Now, for constant voltage, if we change the frequency, the flux is also changing. Hence, to have adequate overload capacity, the motor should maintain constant flux throughout the speed control.

Therefore, only frequency control is not adequate for induction motors.

The flux will remain the same if V/f remains constant. So, it becomes necessary to control the supply voltage and frequency to make the ratio constant. This method is known as the V/f method of speed control.

### V/f Method of Speed Control

This method is a combination of the above two methods.

In this method, we need to control the voltage and frequency provided the ratio V/f remains constant.

The below figure shows the torque-speed curve for the varying frequency with a constant V/f ratio.

From the above characteristics, we can see that the maximum torque is almost constant at a higher frequency.

So, in the voltage equation,

Variable frequency supply can be obtained from a rotary frequency changer. Adjustable frequency generator, and DC link inverter.

### Speed Control by Changing Number of Stator Poles

In the pole changing method of speed control, the stator winding connections are altered in such a way that each type of combination produces a different number of poles.

The equation of synchronous speed is;

From the above equation, the synchronous speed can be changed by changing the number of poles.

This method is only applicable to the squirrel cage induction motor because this motor adopts itself to any number of stator poles provided that the number of poles in the stator is equal to the number of poles in the rotor.

In the case of the slip-ring induction motor, the three-phase winding is wound for a definite number of poles. Once the winding is wound on the rotor, the number of poles cannot be changed.

As shown in the figure above, the number of poles is halved and hence the synchronous speed is doubled.

The pole changing method is used for elevator motors, traction motors, and small motors used in driving machine tools.

## Speed Control Method from Rotor Side

From the rotor side, there are four methods used to control the speed. These methods are listed below.

• By changing Rotor resistance
• By injecting EMF into rotor circuit
• Scherbius system

Now, we discuss these methods in brief.

### By changing Rotor Resistance

The speed of the induction motor can be changed by changing the rotor resistance. In a squirrel cage induction motor, the rotor bars are short-circuited with end rings. Hence, we cannot add extra resistance with rotor winding.

But in the case of the slip-ring induction motor, we can add extra resistance with the rotor winding.

If the rotor resistance increases, the speed of the motor decreases. Therefore, the speed of a motor is controlled by controlling the rotor resistance.

The connection diagram of the rotor resistance method is shown in the figure above.

The main advantage of this method is that the extra resistance cause power loss. And it results in lower efficiency.

Another disadvantage of this method is that the speed regulation with change in load is poor. Therefore, this method is only applicable to applications where speed changes are needed for a short period of time.

Torque-speed characteristics for different values of rotor resistance are shown in the figure below.

• Speed below normal can be achieved.
• Starting torque increases due to additional resistance in the rotor circuit.

• Not useful for squirrel cage motor.
• Large speed changes are not possible.
• Power loss due to additional resistance I2R (efficiency decreases).
• Additional resistance requires oil cooling.
• Speeds cannot be increased above normal.

In the cascade connection method, two induction motors are connected on the same shaft as shown in the figure below.

One motor (Motor-A) is the main motor and the second motor is known as an auxiliary motor (Motor-B). The stator winding of the main motor is connected to supply mains and the stator winding of the auxiliary motor is fed from the rotor of the main motor.

Nsa = Synchronous Speed of motor-A

Where, Pa = Number of poles of motor-A

Hence, slip of motor-A;

The frequency of rotor EMF;

EMF produced in the rotor of motor-A with frequency f’ is fed to the stator of motor-B.

Therefore, its frequency will be the same as f’. This EMF again produces a rotating magnetic field due to which EMF is produced in the rotor of motor-B.

Nsb = Synchronous Speed of motor-B

Where, Pb = Number of poles of motor-B

Hence, slip of motor-B;

The frequency of rotor EMF;

There are three possible combinations for this set;

• Only motor-A connected to supply and in that case Nsa = 120f/Pa
• Only motor-B connected to supply and in that case Nsb = 120f/Pb
• Both motors are connected in a cascade connection.

In cumulative cascade, both motors produce a rotating field in the same direction. Therefore, the torque produced by both motors is in the same direction.

In differential cascade, the direction of a rotating field of both motors is the opposite. In this case, the phase sequence of motor-B is changed.

### By Injecting EMF in Rotor Circuit

In this method, EMF is injected at slip frequency to the rotor circuit by means of an external source. In this way, the speed of motor is controlled by controlling EMF.

If the injected EMF is in the opposite direction of rotor-induced EMF, the speed is decreased. And if the injected EMF is in the same direction as rotor-induced EMF, the speed is increased.

This method consists of a DC motor and rotary converter. The DC motor is connected to the same shaft as the main motor.

The rotary converter is used to convert the low slip frequency AC power into DC power. And this DC power is used to drive the motor. The DC power is fed from the commutator of a rotary converter to the commutator of the DC motor through brushes.

The speed of both motors is controlled by varying field rheostat of the DC motor. Therefore, when field resistance is altered, the DC voltage of the motor and rotary are correspondingly changed. Hence, the AC voltage of a rotary is changed.

Here, the AC and DC voltage of the rotary has a fixed ratio. This method of speed control has a wide range and it is suitable for the applications like steel rolling mills.

The schematic arrangement of this method is shown in the figure below.

### Scherbius System

The scherbius method of speed control of induction motor is suitable for high-capacity machines. The schematic diagram of the scherbius method is shown in the figure below.

In this method of speed control, the main induction motor is connected to a three-phase supply. The voltage developed inside its rotor is fed to the commutating machine through its commutator. This supplied voltage is at slip frequency.

The commutator carries a regular three-phase winding and a three-phase voltage of slip frequency gets applied to it and a rotating magnetic field is produced.

This field is cut by the stator winding of the commutating machine that induces EMF in it.

This EMF is injected into the rotor circuit of the main induction motor through regulating transformer. The magnitude of injected EMF can be varied by changing the taps of the regulating transformer. And hence, the speed of a motor can be controlled.

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