A pole induction motor is working on three-phase 50 Hz AC supply at no load and at full load. When it runs at no-load it has a slip of 4% and when runs at full load then the slip is 3%. By considering these conditions solve the following:
The synchronous speed of induction motor.
Rotor speed at no load
Rotor speed at full load
Determine the rotor frequency at no load
Answers
Answer:
An induction motor, when run as a motor drawing electrical power, runs at a speed little less than the so called synchronous speed where the synchronous speed is given by Ns = (120 * 50)/P . In this case P the number of poles = 6 and hence the synchronous speed will be 1000 RPM. The speed at which the rotor usually runs is less, typically about 4% and the actual rotor speed should be around 960 RPM.
Given that background, can an induction motor run at 1140 RPM? Of course yes. Synchronous speed of an induction motor, however, does not limit the rotor speed to a physically limited maximum running speed. Therefore in the given question, the slip would be
Slip = (Synchronous speed - running speed)
= 1000 - 1140 = -140RPM. The slip is negative!.
When a motor is energized by a certain power source, of a certain frequency, it is developing a torque and because of the torque the motor starts running and a typical induction motor reaches a speed as high as the synchronous speed but less by the slip.
In this condition if the motor is assisted by an external torque typically like wind turbine then because of the assistance of the external torque that works in the direction of the motor generated torque the speed goes beyond the rated synchronous speed.
In normal motoring action the mechanical load is a retarding element. But the wind turbine, while assisting the motor in the same direction of rotation, for example, is an accelerating element. Hence, eventually the speed crosses the synchronous speed. The direction of slip being reversed now , being negative, the direction of power flow is also reversed now. Power now flows from the machine to the source. The machine is no more an induction motor but an induction generator now.
As such, the answer to the given question is that the 6 Pole Induction Motor is now running at a slip of -140 RPM or -14% slip and is functioning as an induction generator.