How to achieve the Speed Control of Induction Motor?
The expression for the speed (N) of an induction motor is
Hence, we can obtain control over the speed by varying its synchronous speed (Ns), or by varying its slip (s). The synchronous speed of the induction motor depends on the supply frequency and the number of stator poles. The torque of an induction motor while running is given by
Hence for a given torque and load, we can vary the rotor voltage and rotor resistance to vary the slip to get control over the speed of the induction motor. From the stator side, we can obtain control over the speed of the induction motor by using the following methods namely
- Voltage Control Method
- Frequency Control Method
- Pole Changing Method
- Stator Resistance Method
From the rotor side, the speed of the induction motor can be controlled by
- Rotor Resistance Control Method
- Slip Power Recovery Method
- Cascading (or) Tandem connection
In this article, we will discuss these methods of speed control of the induction motor in a brief.
Voltage Control Method
In this method of speed control of the induction motor, the supply voltage is varied by using an autotransformer. Practically we cannot increase the voltage levels beyond the rated voltage as the stress on insulation will increase and leads to insulation failure.
During running the slip is very small hence (sX2)2 can be neglected
⇒T 𝛼 sE22
And the E.M. F induced in the rotor (E2) is proportional to the stator voltage (V)
⇒T 𝛼 sV22
If the voltage reduces for a given load, then the slip will increase while reducing the speed to maintain the load torque constant. Voltage can be reduced up to a suitable value, if we reduce the voltage below this value then the motor will become unstable.
Frequency Control Method
The flux density of the stator core is inversely proportional to the applied frequency. To reduce the core losses and for the better performance of the motor the maximum flux density (Bm) must be maintained constant.
Bm 𝛼 V/f
So, to maintain the maximum flux density as constant we must vary the voltage along with the frequency. This method cannot be possible for frequencies greater than the rated frequency as voltage also needs to be increased along with it, which is not possible due to insulation constraints. This method requires variable voltage and variable frequency converters, which makes this method an expansive one, but this method offers a wide range of speed control without affecting the efficiency of the motor.
Pole Changing Speed Control
This method of speed control of the induction motor can be applied to the squirrel cage induction motor only. Since the number of poles in the rotor of a slip ring induction motor is fixed whereas the squirrel cage rotor can be adapted to any number of poles. The poles of the induction motor can be changed in two ways
- Multiple winding sets
- Consequent pole changing
In the first method, we use multiple winding sets of stator windings that are designed for different sets of poles. While in the operation any one of them can be connected according to the speed requirements of the user and the other sets will keep in open. We know that
As the number of poles increases the speed will be reduced. This method can vary the speed in steps only and it is expansive as it involves multiple stator windings.
In the method of consequent pole changing, we can obtain another set of poles by reversing the coils. This method can only give two sets of speeds.
Stator Resistance Method
This method of speed control of the induction motor is similar to the voltage control method. It requires three rheostats to be connected in series with each phase of the stator winding to reduce the voltage and achieve the required speed. As there is some power loss due to the rheostats, this method will be preferred for the low-rating machines for a small duration. This method is more advantageous in starting than the speed control.
Rotor Resistance Control Method
This method of speed control can be possible for the slip-ring induction motor only as we cannot access the rotor of the squirrel cage induction motor. In this method, we connect external resistance to the rotor through the slip rings and brushes while it is in running. Hence it leads to the reduction of the torque
For the given stator voltage, the E.M.F induced in the rotor E2 is constant and during running the slip is very small hence (sX2)2 can be neglected.
⇒T 𝛼 s/R2
But to maintain the load torque constant the speed of the rotor will decrease, and the slip will increase. As the operating slip is increasing this method is not efficient and not suitable for a wide range of applications.
Slip Power Recovery Method
In this method of speed control of induction motor, we inject the external voltage into the rotor through slip rings and brushes at a slip frequency (sf) To obtain steady-state torque. This could be done in two ways.
In the first method, we increase the rotor voltage, which leads to an increase in speed for a given load. We know that
T 𝛼 sE22
If the Rotor voltage is increased, then the torque will increase subsequently the rotor speed will increase, and the slip will decrease.
In the second method, we decrease the voltage in the rotor, then torque will decrease and subsequently, the speed will increase, and the slip will decrease. The Scherbius drive is the best example of this kind of operation.
Cascading or Tandem Connection
In this method of speed control of the induction motor, we need two induction motors, one is essentially a slip ring induction motor, and the other is either a slip ring induction motor or a squirrel cage induction motor. Both machines will be mechanically coupled, the first motor is a slip ring induction motor which feeds the second one through its slip rings.
The speed of the slip-ring induction motor is
The speed of the second motor is
In this method, four different speeds are possible they are
- 120 f/P1+P2 in cumulative cascading
- 120 f/P1−P2 in differential cascading
- 120 f/P1 (When motor-1 only operating)
- 120 f/P2 (When motor-2 only operating)
In differential cascading the number of poles in both motors should not be equal.
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