Methods of Speed Control of 3 Phase Induction Motor

An induction motor is an electric motor which works on Faraday's law of electromagnetic induction. Induction motors are AC fed motors. Specifically, a three-phase induction motor fed by three phase AC supply. Three phase induction motors are the most widely used motor in industries to drive heavy loads (large power applications).

An induction motor is a constant speed motor, means a few percent speed drop from no load to full load like in the case of a DC Shunt Motor. Speed of a DC Shunt Motor can be varied easily with the help of various speed control methods DC Motor with good efficiency, but in case of induction motor changing speed results decrease in efficiency and poor power factor. Then a question arose in our mind, why are we interested in changing the speed of a three-phase induction motor even if it results in decreasing performance of the induction motor.

Why do we need to control the speed of a three-phase induction motor?

Controlling the speed of a three-phase induction motor is essential for various industrial and commercial applications. Induction motors are most commonly used to drive loads in industries. Various industrial applications required different speeds to drive a load. So, these applications require an induction motor that operates on a wide range of speeds. To fulfil the speed requirements of various applications in industry Speed control of an induction motor is required. Controlling the speed of a three-phase induction motor provides flexibility, efficiency, and improved performance in a wide range of industrial and commercial applications.

Methods of speed control of three phase induction motor

Before studying the methods of speed control of a three-phase induction motor we should know the basics of a three-phase induction motor.

As we know, a three phase Induction motor is a three phase AC fed motor which works on Faraday's law of electromagnetic induction. When a 3-phase supply is given to stator winding it produces a flux called rotating magnetic field which rotates at synchronous speed Ns. This rotating magnetic field induces a current in rotor winding. Simultaneously a secondary rotating magnetic field is produced called rotor RMF rotating magnetic field. Due to the interaction of these magnetic field, rotor experience a torque and rotate at speed denoted with N.

N = Ns (1-s) ………………………… (1)

Ns = 120 F / P ……………………….. (2)

Where,

Ns = Synchronous speed at which stator RMF rotates.

F = Frequency of given supply

P = number of poles

N = rotor’s speed

s = slip in induction motor

s = 1 at starting or under standstill condition.

Torque Equation in three phase induction motor

T = K sV² / R_r …………………………………. (3)

Where,

T = Torque in induction motor

K = Proportionality Constant

s = slip in induction motor

V = Voltage applied to induction motor

R_r = Rotor Resistance

It is seen from the equation (1) that there are two basic ways to control the speed of 3-phase induction motor. First is by changing the synchronous speed and the second method is depend on the slip in induction motor.

From equation (2) we can conclude that there are two ways of changing synchronous speed that are by changing the supply frequency and second is by changing the number of stator poles.

From equation (3) it is conclude that slip control method is depends on supply voltage, and rotor resistance.

Apart from above derived method there are three more method for speed control of three phase induction motor. That are slip power recovery method, cascading / Tandem connection method and rotor resistance method.

So, there are seven different method of speed control of 3 phase induction motor

Voltage Control

Frequency Control

Pole Changing Method

Stator Resistance Control

Rotor Resistance Control

Slip Power Recovery

Cascading Tandem Connection

An induction motor has good speed regulation means a few percent speed drop from no load to full load. They are also known as constant speed motors. Then a question arose in our mind that why do we need to control the speed of induction motors even though they have good speed regulation.

Voltage Control / Stator Voltage Control

Voltage control method depend on slip control in which a variable voltage at constant frequency is supplied to the motor stator. A variable supply voltage tends to motor operate at different slip to deliver a constant load torque this relationship follows equation (3). It is obvious that the supply voltage cannot be raised beyond the rated value, it can only be reduced. If the supply voltage is reduced then the motor will operate at higher slip for delivering constant torque (as per eq (3) ) resulting the speed of the motor reduced ( as per equation (1) ).

The voltage control method gives a small range of speed control due to efficiency considerations. Because if slip increases then induced current in the rotor is also increased. This increased current causes the overheating of the motor as a result efficiency decreases.

This method requires a variac or any other device to supply reduced voltage.

Stator Resistance Control

This method is similar to voltage control method in which a rheostat is added in stator circuit. By adding the rheostat supply voltage is reduced simultaneously slip increases to deliver constant load torque as a result motor speed get decreased. (discussed in details in above section).

Frequency Control

This method is a synchronous speed control method In which by altering the supply frequency, the synchronous speed of the induction motor can be changed as per equation (2) simultaneously the speed of an induction motor can be changed as per equation (1).

In an induction motor induced EMF in Rotor E_r is given by

E_r = 4.44 ɸ. F. T

And E_r is proportional to V

So, V = 4.44 ɸ. F. T …………………………… (4)

ɸ α V/F …………………………… (5)

Where,

F = frequency of supply voltage

V = Applied Voltage

ɸ = Flux

T = Number of Turns.

Equation (5) indicates that if we vary the frequency, flux density is also changed. It has to be taken care that flux density in an induction motor should be constant because if flux density increases then the motor will get saturated and draw a large amount of magnetizing current and if flux density decreases the machine fails to operate.

So, with the variation in frequency supply voltage is also varied to make flux density constant. This method is also called variable voltage variable frequency control of induction motor or V/F control method of induction motor.

If frequency increases then voltage should also be increased to keep v/f constant. If voltage increases beyond the rated value, then insulation failure may occur and the motor will get damaged. So, frequency cannot be increased to high value.

If frequency decreases then voltage should also be decreased in order to keep v/f constant. If voltage is decreased the machine fails to operate at lower voltage. So, frequency cannot decrease to very low values.

So, v/f control is used over the specific range. This method provides the highest speed control range among all the speed control methods.

In modern practices variable voltage and variable frequency supply from a constant voltage and frequency supply is achieved by Power Electronics converter - inverter arrangement shown in the given figure. This converter - inverter arrangement employs SCR or any other controlled switch for getting variable voltage variable frequency supply. This whole arrangement refers to variable frequency drive (VFD).

Frequency Control of 3 phase Induction Motor

Pole Changing Method

Pole changing method of speed control is a synchronous speed control method in which we can alter the synchronous speed by changing the number of the stator poles This relationship is shown in equation (2). This method requires two or more stator windings wound for different numbers of poles and are isolated to each other. These two stator windings generate RMF rotating at different synchronous speeds. By simply connecting one winding to the supply and leaving the other open, desired synchronous speed simultaneously motor speed can be obtained.

This method is applicable only for squirrel cage induction motors because in a squirrel cage induction motor rotor automatically creates the same number of poles as the stator.

This method is not so popular and rarely preferable because it requires another winding which increases the cost as well as size of the motor.

Rotor Resistance Control

As the name implies, in this method rotor resistance is altered for getting the desired speed of the induction motor. It is obvious that rotor resistance cannot be decreased but it can only be increased by adding external resistance. This type of speed control is only possible for slip ring induction motors in which rotor winding is essentially star connected.

In this method additional resistance is added to the rotor in each phase through slip rings. By adding the additional resistance in the rotor, we will force the motor to operate in high slip region to deliver the constant load torque. In high slip region motor will be operated in low speed. In this method speed of the 3-phase induction motor cannot be increased to the rated value. (refer torque slip characteristics to get more details about effect of adding rotor resistance)

In this method additional power losses occur in rotor additional resistance. So, for efficiency consideration this method does not offer a wide range of speed control of three phase induction motors. However, this method will help in increasing the starting of the three-phase induction motor by increasing the starting torque.

Slip Power Recovery

This method will help to overcome the power losses that occur during the speed control of a three-phase induction motor by rotor resistance control method. In this method additional voltage is injected into the rotor through the slip ring at the same frequency of the rotor. By injecting the additional voltage into the rotor, we will force the motor to operate at a low slip region. If slip of the three-phase induction motor then the speed of the motor increases to deliver a constant load torque.

Cascading / Tandem Connection

This method of speed control of a three-phase induction motor requires two motors which are mechanically coupled as well as electrically connected. Mechanically coupled means these two motors are connected with a common shaft and the second motor is electrically connected with the slip ring of the first motor as shown in figure. In this method the first motor is essentially a slip ring induction motor or the second motor can be any type, either slip ring or squirrel cage induction motor. The first motor is called main motor which is used to give supply at slip frequency to the second motor which is called auxiliary motor whose speed control is required.

Let

Ns₁ = Synchronous speed of first motor

F = Supply frequency of first Motor

F₁ = Frequency of induced EMF in first Motor’s rotor

P₁ = Number of pole of first motor

N₁ = Speed of the first motor

s₁ = Slip of the first motor

Ns₂ = Synchronous speed of second motor

F₂ = Supply frequency of second Motor

P₂ = number of pole of second motor

N₂ = Speed of the second motor

S₂ = Slip of the second motor