Speed Control of Induction Motors

Speed Control of Induction Motors:

An induction motor is practically a constant speed motor like a DC shunt motor. But the speed of DC shunt motor can be changed in a smooth manner by using the rheostats. Due to which speed regulation and efficiency of DC shunt motor stay maintained. But when we consider the case of induction motors, it is very difficult to achieve smooth speed control. Somehow, if we achieve smooth speed control, the performance of induction motors in terms of its power factor, efficiency, etc. get adversely affected.

For the induction motors, we know that

N = Ns(1 – s)

From the above expression, we can say that speed control of induction motors generally depends on either slip or synchronous speed of the induction motors.

Similarly, the torque produced in case of induction motors can be written as,

T ∝ (sE22R2) / [R22 + (sX2)2]

So as the parameters like R2, E2 are changed then to keep the torque constant for constant load condition, the motor reacts by the change in its slip. Due which their speed gets changed effectively.

Thus, the speed of the induction motor can be controlled by basically two methods:

(1) .From Stator Side
      • Speed Control by varying supply frequency
      • Speed Control by the supply voltage
      • Speed control by controlling the stator poles
(2) .From Rotor side
      • Kramer System
      • Scherbius System


[1]. From Stator Side

From the stator side of the induction motors, it includes the following methods.

  • Speed Control by varying supply frequency

The synchronous speed of the induction motor is given by

Ns = 120f / P

By controlling the supply frequency, the synchronous speed can become of a wide range. This gives smooth speed control of the induction motors. The air-gap flux of an induction motor is given by,

Фg = (1 / 4.44K1Tph1)*(V / f)

Where       K1 = Stator Winding Constant

Tph1 = Stator Winding turns per phase

V = Supply voltage

f = Supply frequency

From the above expression, if the supply frequency is changed, the value of air gap flux gets affected. This may result in saturation of stator and rotor cores. Such a drastic change will cause a sharp increase in the (magnetization) no-load current of the motor. So, we need to maintain the air gap flux when supply frequency is changing. In order to do that, the supply voltage must also be changed to keep the V/f ratio constant in the above expression which will keep the air gap flux constant also. When this method is implemented practically, the input supply required is of constant voltage constant frequency. Then this supply is fed to the converter to convert to the DC supply. Then this DC supply is given to the inverter which converts it to variable voltage variable frequency supply which is fed to the induction motor to keep the V/f ratio constant and speed is controlled smoothly.

  • Speed Control by the supply voltage

We know that,

T ∝ (sE22R2) / [R22 + (sX2)2]

No E2, the rotor induced emf at standstill depends on the supply voltage V.

Therefore,                      E2 ∝ V

Also, for low slip region which is the operating region of the induction motor,

(sX2)2 << R2

And it can be neglected from the above expression and it can be written as

T ∝ sV2R2 / R22  ∝ sV2/ R2 ∝ sV2             for constant R2.

Now, if the supply voltage is reduced below rated value, the torque decreases as per the above equation but to supply the same load, it is necessary to develop the same torque hence the value of slip increases and the torque produced remains the same. Slip increase means motor reacts by running at a slower speed to decrease in supply voltage. Hence, by varying supply voltage, the speed of the induction motor gets controlled.

  • Speed control by controlling the stator poles:

This method is also named as the concatenation or tandem operation of the induction motors. In this method, two induction motors are mounted on the same shaft. The main requirement for this method is that the one motor should be of the slip ring type which is called the main motor while the other one should of any type named as the auxiliary motor. The stator of the main motor is supplied by AC power supply while the auxiliary motor supply is derived at the slip frequency from the main motor slip rings. This is known as cascading. If the torque produced of both the motors act in the same direction, it is called cumulative cascading but when the torque produced of both the motors act in the opposite direction, it is called differential cascading.

Thus, we can write for cumulative cascading

N = 120f / PA + PB

And for differential cascading,

N = 120f / PA – PB

Where         N = Net speed of the motors

f = supply frequency

PA = number of poles of the main motor

PB = number of poles of the auxiliary motor

[2]. From Rotor side:

In this method, a voltage is injected into the rotor circuit. The frequency of the rotor circuit is the slip frequency and hence the voltage which is being injected must possess the slip frequency. It may be possible that the injected voltage may oppose the rotor induced emf or it may assist the induced emf. If the injected voltage is out of phase of the induced emf then the effective rotor resistance decreases. While on the other, if the opposite of that happens, the effective resistance will increase. Thus, by controlling the magnitude of the injected voltage or emf, rotor resistance and effective speed can be controlled. When it comes to practical implementation, two methods are used which are described in the following.

  • Kramer System

In this system, the main induction motor is to be put in which the speed is to be controlled. The two additional components are also placed which are DC motor and a rotary converter. The slip rings of the main motor are connected to the AC side of the rotary converter. The DC side of the rotary converter feeds the commutator of DC shunt motor which is directly connected to the shaft of the main motor. For the excitation of the field winding of DC motor and exciting winding of the rotary converter, a sperate DC supply is required. And a variable resistance is introduced in the field winding of DC motor which acts as a field regulator.

When the field regulator is controlled, the varying field of DC motor controls the speed of the induction motor. Due to field regulation, back emf of motor changes which causes a change in DC voltage at commutator due to which DC voltage on the DC side of rotary converter change. Thus, the rotary converter has a fixed ratio of AC and DC side voltages. Hence, the voltage on the AC side of it will also change. This AC voltage is given to the slip rings of the main induction motor. Due to this process, the required speed control is obtained.

  • Scherbius System

This method requires an auxiliary 3 or 6-phase AC commutator machine called a Scherbius machine. This machine is excited at slip frequency from the rotor of the main induction motor by regulating the transformer. The taps on regulating transformer can be varied, due to which voltage developed in the rotor of scherbius machine changes which is injected into the rotor of the main induction motor. This controls the speed of the induction motor. The scherbius machine is connected directly to the induction motor supplied from the mainline so that its speed deviates from the fixed value only to the extent of the slip of auxiliary induction motor. For any given set of regulating transformer, the speed of the main motor remains substantially constant irrespective of the load variations.

Related Topics;

  1. Induction Motor Basics
  2. Starting Methods of Induction Motors
  3. Three Phase Induction Motor Basics
  4. Why slip ring induction motor used in crane?
  5. Why slip ring is used in induction motor?