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In previous articles I have discussed the construction, operation, starting, speed control, and classes of squirrel cage induction motors. This article is devoted to slip ring induction motors. As discussed earlier, a slip ring induction motor is an asynchronous motor, as the rotor never runs in synchronous speed with the stator poles. Learn more about the construction and operation of a slip ring induction motor.
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The construction of a stator is the same for both the squirrel cage and slip ring induction motor. The main difference in a slip ring induction motor is on the rotor construction and usage. Some changes in the stator may be encountered when a slip ring motor is used in a cascaded system, as the supply for the slave motor is controlled by the supply from the rotor of the other slip ring motor with external resistance mounted on its rotor.
The slip ring induction motors usually have a “Phase-Wound" rotor. This type of rotor is provided with a 3-phase, double-layer, distributed winding consisting of coils used in alternators. The rotor core is made up of steel laminations which have slots to accommodate formed 3-single phase windings. These windings are placed 120 degrees electrically apart.
The rotor is wound for as many poles as the number in the stator and is always 3-phase, even though the stator is wound for 2-phase. These three windings are “starred" internally and the other end of these three windings are brought out and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminal ends touch these three slip rings with the help of carbon brushes which are held against the rings with the help of a spring assembly.
These three carbon brushes are further connected externally to a 3-phase start connected rheostat. The slip ring and external rheostat makes it possible to add external resistance to the rotor circuit, enabling them to have a higher resistance during starting and thus higher starting torque.
When running under normal conditions, the slip rings are automatically short-circuited by means of a metal collar, which is pushed along the shaft, thus making the three rings touch each other. Also, the brushes are automatically lifted from the slip-rings to avoid frictional losses, wear and tear. Under normal running conditions, the wound rotor acts the same as the squirrel cage rotor.
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What happens when external resistance is added?
In the case of a squirrel cage induction motor, the rotor resistance is very low so that the current in the rotor is high, which makes its starting torque poor. But adding external resistance, as in the case of a slip ring induction motor, makes the rotor resistance high when starting, thus the rotor current is low and the starting torque is maximum. Also the slip necessary to generate maximum torque is directly proportional to the rotor resistance. In slip ring motors, the rotor resistance is increased by adding external resistance, so the slip is increased. Since the rotor resistance is high, the slip is more, thus it's possible to achieve “pull-out" torque even at low speeds. As the motor reaches its base speed (full rated speed), after the removal of external resistance and under normal running conditions, it behaves in the same way as a squirrel cage induction motor.
Thus these motors are best suited for very high inertia loads, which requires a pull-out torque at almost zero speed and acceleration to full speed with minimum current drawn in a very short time period.
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Advantages of slip ring induction motors
- The main advantage of a slip ring induction motor is that its speed can be controlled easily.
- "Pull-out torque" can be achieved even from zero RPM.
- It has a high starting torque when compared to squirrel cage induction motor. Approximately 200 - 250% of its full-load torque.
- A squirrel cage induction motor takes 600% to 700% of the full load current, but a slip ring induction motor takes a very low starting current approximately 250% to 350% of the full load current.
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What happens if the motor is started as a normal induction motor?
If the slip motor is started with all the slip rings or the rotor terminals shorted, like a normal induction motor, then it suffers extremely high locked rotor current, ranging up to 1400%, accompanied with very low locked rotor torque as low as 60%. It is not advised to start a slip ring induction motor with its rotor terminals shorted.
In my next article, read about various starting methods & speed control of a slip ring induction motors.
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