Working principle of Slip ring Induction motor

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    An induction motor is a widely used motor in most domestic and industrial applications. It is generally classified into two types, as squirrel cage induction motor and slip ring induction motor. In the before topic, we have seen about the definition of induction motor and the construction and operation of squirrel cage induction motor. In this topic, we are going to see about the construction and operation of slip ring induction motor.

   The induction motor is generally classified into two types because of the operating torque. In squirrel cage induction motor the starting torque is very low, whereas in slip ring induction motor the starting torque is very high. The presence of more inductance in the rotor bars of the squirrel cage induction motor makes it to have very low starting torque. As there is more inductance in the rotor bars, there will be more phase lag between the voltage and the current. So in the squirrel cage rotor, the maximum current will be induced at one bar and the maximum voltage will be induced at another bar. To achieve the maximum torque condition, the maximum current should be in phase with the maximum voltage. This can be done by limiting the inductance or by increasing the resistance. As the squirrel cage rotor is short-circuited by end rings there is no possibility to do these changes. Therefore there is no way to increase the torque in the squirrel cage induction motor. Hence it is not used in high torque applications such as hoists, elevators, compressors, etc. To overcome this problem slip ring induction motor is invented. It provides a high initial torque by introducing resistance in the circuit, which will be well suited for the above applications. Now let’s see the constructional features and the operation of the slip ring induction motor.


Maximum voltage in the rotor bars of Squirrel Cage Induction Motor

 


Maximum Current in the rotor bars of Squirrel Cage Induction Motor


SLIP RING INDUCTION MOTOR

   The Slip ring Induction motor is much more similar to the squirrel cage induction motor. It also has the stator and rotor part. The construction and operation of the stator of the slip ring induction motor is exactly the same as the squirrel cage induction motor. The only difference between them is the rotor construction. The rotor makes the operation different. The rotor of the slip ring induction motor is a wounded type rotor which means windings are used in the rotor; whereas the squirrel cage induction motor is a non-wounded type.

     In addition to that wound rotor, slip rings are provided to the rotor to connect external resistance to the windings, which makes it perfectly suited for many applications. Slip rings are the type of rings which is used to connect a stationary part and a rotating part.


Slip rings

   When the stator is given supply it creates a rotating magnetic field, which begins to cut the rotor conductors. As the rotor bars are replaced with windings in the slip ring induction motor, EMF gets induced on the winding according to Faraday’s Law of Electromagnetic Induction. To make a current flow on the windings, the terminals must be in a closed circuit, so the terminals of the winding are connected in a star formation. Now according to Lorentz force law, as the winding (the current-carrying part) is placed in the magnetic field it begins to rotate. So the motor rotates.  

    When the motor rotates, one of the conductors experiences a maximum EMF (voltage) and another conductor experiences a maximum current, due to the presence of inductance. As mentioned earlier to achieve maximum torque condition, the maximum current induced in the conductor should be in phase to the maximum voltage induced in the conductor. To do this the resistance in the circuit must be increased. When we include resistance, the current through the conductor is got minimized, which means the current value is brought nearer to the voltage value. Therefore, the phase angle between the current and voltage decreases and the current induced will be in phase to the maximum EMF induced. 



   So in this motor, the external resistance is connected to the rotor by means of slip rings and that is why this motor is named as slip ring induction motor. This external resistance is generally added by using a Rotor Resistance Starter. During starting, a high amount of resistance is included in the rotor circuit by the starter, and the rotor is made to start at high initial torque. After the motor picks up speed, the resistance is slowly made to cut and the motor is made to run normally at its rated speed with all its external resistance cut off. This is how a slip ring induction motor works.

 


Slip ring rotor with rotor resistance starter

 

 

 

 

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