Difference between Squirrel Cage and Slip Ring Induction Motor Please have a look at the below figures of Squirrel Cage Induction and Wound Rotor / Slip Ring Induction Motor. The figure itself depicts many difference between the two types of motors. A squirrel-cage rotor is the rotating part (rotor) used in the most common form of AC induction motor. It consists of a cylinder of steel with aluminum or copper conductors embedded in its surface. An electric motor with a squirrel-cage rotor is termed a squirrel-cage motor.

A wound rotor induction motor has a stator like the squirrel cage induction motor, but a rotor with insulated windings brought out via slip rings and brushes. However, no power is applied to the slip rings.

Their sole purpose is to allow resistance to be placed in series with the rotor windings while starting. (Figure ) This resistance is shorted out once the motor is started to make the rotor look electrically like the squirrel cage counterpart. Wound rotor induction motor. Why put in series with the rotor? Squirrel cage induction motors draw 500% to over 1000% of full load current (FLC) during starting.

While this is not a severe problem for small motors, it is for large (10’s of kW) motors. Placing resistance in series with the rotor windings not only decreases start current, locked rotor current (LRC), but also increases the starting torque, locked rotor torque (LRT). Figure shows that by increasing the rotor resistance from R 0 to R 1 to R 2, the breakdown torque peak is shifted left to zero speed.Note that this torque peak is much higher than the starting torque available with no rotor resistance (R 0) Slip is proportional to rotor resistance, and pullout torque is proportional to slip. Thus, high torque is produced while starting. Breakdown torque peak is shifted to zero speed by increasing rotor resistance. The resistance decreases the torque available at full running speed.

But that resistance is shorted out by the time the rotor is started. A shorted rotor operates like a squirrel cage rotor. Heat generated during starting is mostly dissipated external to the motor in the starting resistance. The complication and maintenance associated with brushes and slip rings is a disadvantage of the wound rotor as compared to the simple squirrel cage rotor. This motor is suited for starting high inertial loads. A high starting resistance makes the high pull out torque available at zero speed.

For comparison, a squirrel cage rotor only exhibits pull out (peak) torque at 80% of its’ synchronous speed. Speed control Motor speed may be varied by putting variable resistance back into the rotor circuit. This reduces rotor current and speed.

The high starting torque available at zero speed, the down shifted break down torque, is not available at high speed. Programma dlya nastrojki antenni 4g. See R 2 curve at 90% Ns, Figure.

Resistors R 0R 1R 2R 3 increase in value from zero. A higher resistance at R 3 reduces the speed further.

Speed regulation is poor with respect to changing torque loads. This speed control technique is only usefull over a range of 50% to 100% of full speed. Speed control works well with variable speed loads like elevators and printing presses. Rotor resistance controls speed of wound rotor induction motor. Doubly-fed induction generator We previously described a squirrel cage induction motor acting like a generator if driven faster than the synchronous speed. (See ) This is a singly-fed induction generator, having electrical connections only to the stator windings. A wound rotor induction motor may also act as a generator when driven above the synchronous speed.

Since there are connections to both the stator and rotor, such a machine is known as a doubly-fed induction generator (DFIG). Rotor resistance allows over-speed of doubly-fed induction generator. The singly-fed induction generator only had a usable slip range of 1% when driven by troublesome wind torque.

Since the speed of a wound rotor induction motor may be controlled over a range of 50-100% by inserting resistance in the rotor, we may expect the same of the doubly-fed induction generator. Not only can we slow the rotor by 50%, we can also overspeed it by 50%. That is, we can vary the speed of a doubly fed induction generator by ±50% from the synchronous speed. In actual practice, ±30% is more practical.