Induction Motor Speed Control

Induction motor speed control: An overview

Induction motors are among the most widely used electric motors that are helping engineers to convert electrical energy into mechanical energy (in the form of shaft rotation). These AC motors work by the principle of electromagnetic induction; electric current in the rotor is obtained from the rotating magnetic field in the stator winding. This electric current is then used in producing the desired shaft rotation (and torque).

Induction motors offer several advantages, such as simple construction and operation, durability, high efficiency, and starting torque. However, one challenge that has remained constant with induction motors is the difficulty of attaining speed control.

This article will get back to the basics of induction motor speed control. It will present methods and calculations for efficiently controlling motor speed from the stator and the rotor side of the motor.

One challenge that has remained constant in the application of induction motors is the difficulty of attaining effective speed control

Induction motor speed control from the stator side

Method #1 Changing the applied frequencyThe synchronous speed of an induction motor can be estimated using:

    F = incoming line frequency
    P = number of stator poles

Hence, an engineer can control the speed of an induction motor by changing the incoming line frequency. So consider the case where the incoming line frequency is 60 Hz at 120V (typical in the United States). The synchronous speed of an induction motor with four poles (two-pole pair motor) would be 1800 rpm. In reality, the output speed of the motor is usually lesser than this synchronous speed. And this discrepancy is called the slip.

Nevertheless, engineers can achieve lower motor speeds by reducing the incoming line frequency of the motor. This speed control method can be used in applications where there is a dedicated generator supplying the induction motor. In such scenarios, the incoming line frequency can be varied by simply varying the speed of the generator.

Method #2 Constant VF control of induction motorsThe electromotive force (emf) induced by a three-phase induction motor is given by

    Ø = flux
    K = winding constant
    T = number of turns per phase
    f = incoming line frequency

Hence, the flux can be obtained using:

The flux equation shows that the incoming line frequency has an inverse proportionality with the flux. So when the incoming line frequency is increased, the flux is reduced (and vice versa). An increase in flux (resulting from a decrease in the incoming line frequency) causes the air gap flux to saturate, which in turn causes excessive stator current and distorts the stator flux wave.

Therefore, it is essential to also control the motor's EMF (or stator voltage) when controlling the frequency to keep the air gap flux constant. This speed control must be such that the voltage-frequency (or V/f) ratio remains constant. Engineers achieve this voltage-frequency control by using a converter and inverter set (typically found in variable frequency drives).

Not only does this method ensure a relatively constant torque, but it also offers higher run-time efficiency.

Method #3 Changing the number of stator poles
Consider the synchronous speed equation mentioned earlier. It can be seen that induction motor speed control can also be achieved by varying the number of stator poles of the motor. This method of speed control is found in squirrel cage induction motors, which are designed to have several independent stator windings wound for different numbers of poles in the same slots. To control the motor speed, engineers only need to choose which pole winding to connect.

For example, a typical squirrel cage induction motor can have stator windings for two poles, four poles, and six poles. Therefore, for a supply frequency of 60 Hz, the synchronous speeds for when each of the poles are connected are presented below:

    (i) For 2 pole winding connection: Ns = (120*60)/2 = 3600 RPM
    (ii) For 4 pole winding connection: Ns = (120*60)/4 = 1800 RPM
    (iii) For 6 pole winding connection: Ns = (120*60)/6 = 1200 RPM

However, keep in mind that the squirrel cage induction motors usually have a slip of up to 3%.

Method #4 Changing the applied voltage
To understand this induction motor speed control by changing the applied voltage, consider the torque produced by a three-phase induction motor.

The torque equation for induction motors

    E2 = rotor induced emf
    Ns = synchronous speed
    X2 = rotor inductive reactance
    R2 = rotor resistance
    K = constant of proportionality = (3 x 60)/2pNs

Consider the scenario where the slip (s) is so small that the (sX2)2 value can be neglected. The torque equation becomes:

Since the rotor induced EMF is directly proportional to the supply voltage, the torque equation reduces to:

This means engineers can vary the torque by simply varying the rotor-induced emf (or supply voltage). Now, if the torque can be maintained constant, engineers can achieve different slips (difference in synchronous speed and actual induction motor speed) by varying the supply voltage. So if an engineer is looking to achieve a large slip (or much lower motor speed), then the induced emf (or supply voltage) must be reduced accordingly.

Induction motor speed control from the rotor side

Method #1 Cascade operationThis speed control method involves connecting two induction motors to a common shaft. The first motor (called the main motor) is fed from a three-phase supply, while the second motor (also called the auxiliary motor) operates as a result of induced emf from the main motor using slip rings. Engineers can achieve different motor speeds by:
    1. Making only the main induction motor work
    2. Making only the auxiliary induction motor work
    3. Making both the auxiliary and main motor work

Method #2 Rotor rheostat controlThe rotor rheostat control method is quite similar to the armature control method in DC shunt motors: the speed control method involves varying the resistance on the rotor side of the motor through a rheostat. When the rheostat increases the resistance in the rotor side, the speed decreases.

[Learn more about AC induction motor speed controllers with Engineering360]

Induction motor speed control: Reach out to manufacturers
While this article presents helpful information about the different methods for induction motor speed control, there are several other things an engineer must consider to specify induction motors for a particular application. As such, engineers are advised to reach out to induction motor manufacturers to discuss their application requirements.