Induction motor having wound rotor is called slip ring induction motor. It is also known as wound rotor induction motor motor since the rotor is wound with insulated conductors rather than aluminum or copper bars as in the case of squirrel cage induction motor. In this article, we will discuss the working, construction and different applications of slip ring induction motor.
Construction of Slip Ring Induction Motor:
Like in any other AC machine, three phase induction motor has two main parts which are termed as stator and rotor. Construction of stator in any induction motor is almost same but rotor construction is different with respect to the type of motor. At the moment, we will only discuss the construction of wound rotor.
Wound Rotor:
Slip ring induction motors usually have a wound rotor. It consists of laminated cylindrical core and carries 3 phase windings which are usually star connected similar to one on the stator. The below diagram shows the wound rotor along with its connections.
Wound rotor winding ends are connected to the slip rings mounted on the rotor shaft. The reason for connecting the rotor windings to slip rings is to allow the addition of external variable resistance in the rotor circuit. Terminal end of the windings touch these slip rings with the help of carbon brushes which are further connected to 3 phase star connected rheostats (can be seen in the above diagram). At starting, slip rings and external rheostats make it possible to add external resistance in the rotor circuit in order to have large starting torque.
Working of Slip Ring Induction Motor:
Working of slip ring induction motor is almost same as that of simple three phase induction motor. In case of slip ring motor, both stator and rotor have three phase windings. Rotor windings are connected to each other with slip rings. When alternating current is applied to the stator windings, they create a rotating magnetic field. Revolving magnetic field cuts the rotor windings and induces an emf in the rotor coils according to the faraday law of electromagnetic induction. Since armature (rotor) coils are connected with a slip rings in a close electric circuit, current begins to flow. Now, current carrying rotor conductors placed in a magnetic field (produced by the stator) experiences a mechanical force. The mechanical force acting on the rotor conductor produces a rotating effect (torque) on the rotor which causes the motor to rotate.
In order to know the rotor direction and speed of rotor, kindly read this article: Working principle of induction motor↗
Why wound rotor induction motors are introduced?
The main reason for using slip ring induction motor is to obtain high starting torque and low starting current. The main benefit of using this motor is that we can add external resistance in the rotor circuit of slip ring induction motor. By adding resistance in series with the rotor windings, we can achieve a high starting torque and low starting current. First, let understand that why induction motor has low starting torque and high starting current.
Why induction motor has low starting torque and high starting current?
High starting Current :
During the start of induction motor, rotor circuit has a very small resistance. The only resistance to the current flow is the resistance of the rotor windings which is very low. This results in a large flow of current in the rotor circuit at the start of motor.
Low Starting Torque:
At the start of induction motor, rotor is stationary (Slip S= 1)↗. The relative speed of rotating stator field with respect to the stationary rotor is very high (i.e. synchronous speed). Rotor Frequency (Fr) is expressed as:
Rotor frequency = Slip 
Stator frequency
Fr= S 
Fs….. eq.(A)
As slip S = 1(during the start of motor), hence
Rotor Frequency= Stator Frequency
The rotor reactance (Xr) is given as:
Xr= 
(Putting the value of rotor frequency from eq. A)
At standstill, slip s=1.
From the above equation, we can say that rotor reactance is very large compared to its resistance at the start of motor (since rotor frequency is equal to the supply (stator) frequency ). However, it gets reduces as the motor accelerates to attain its specific speed.
Rotor reactance is an inductance effect which delays the rise of rotor current and makes it less effective to react with the stator current in order to develop torque. Hence high reactance makes the rotor current lags the rotor emf by large scale such that the phase difference between the current and voltage is very large. This simply result in low power factor and hence low starting torque of induction motor.
How can we achieve high starting torque in slip ring induction motors?
Mathematical expression of induction motor starting torque↗ is stated as:
Since the torque of induction motor is directly proportional to the resistance of rotor circuit. Hence if one has access to the winding of rotor circuit, the resistance of rotor in slip ring induction motor can be increased.
In case of slip ring induction motor, we can make the rotor resistance high by adding external resistance. Slip rings are used for connecting external resistance. External resistance is connected in series with the rotor windings (see the above diag). Having a large rotor resistance relative to the reactance keeps the current from lagging too much with the rotor emf. In this way, phase difference between the current and emf will be reduced which results in improved power factor. Improve power factor simply leads to high starting torque in slip ring induction motor.
Moreover, adding the proper value of external resistance at the instant of starting in such a way that rotor resistance is equal to rotor reactance (R2=X2), we can obtain the maximum starting torque. As the motor reaches at its rated speed, external resistance can be removed from the rotor circuit. High starting torque of slip ring motor will be very handy in lot of applications.
Applications of Slip Ring Induction Motor:
Slip ring induction motors are used in applications where speed control is require for loads. Hence they are employed for driving loads like pumps, lifts, generators etc.
Slip ring motors are generally used where load requires high starting torque.
Large inertial loads like cranes, elevators, large ventilating fans, compressors demand high starting torque can be driven by slip ring induction motors.
Advantages of Slip Ring Induction Motor:
The main advantage of slip ring induction motor is that it has a high starting torque. Starting torque of slip ring motor is almost 250-300% of its full load torque.
We can easily control the speed of slip ring induction motors.
Starting current of slip ring motor is low.
Power factor of the slip ring induction motor is high (Approximately 0.8-0.9).
Disadvantages of Slip Ring Induction Motor:
Initial cost of slip ring motor is high.
Construction of slip ring motor is complicated due to the presence of slip rings.
Speed regulation of slip ring motor is poor when run with external resistance in the circuit.
Maintenance of motor is often required due to the presence of slip rings and brushes.
In case of slip ring induction motor, rotor copper losses ↗ are high due to which it has less efficiency.