To start with, let us assume that the squirrel cage induction motor has been started without any load on it (i.e., no electrical devices are connected to the generator, no metal is being cut by a machine tool, or the conveyor belt is completely disengaged). The motor will come to its no-load speed, which may be at a slip as low as 0.1 per cent. Load is now placed on the motor and the following reactions will be observed:
- Speed decreases.
- Slip increases (because the stator field continues to rotate at synchronous speed and therefore cuts the rotor bars more rapidly).
- As a consequence of increased cutting the rotor induced e.m.f. increases.
- The current in the rotor bars increases.
- Torque developed by the rotor increases until it is sufficient to carry the load placed on the motor.
- The motor continues to rotate at that slip which will develop the required torque for that particular load.
The motor may be loaded continuously till pull-out (or break-down) torque is developed, at which point the motor will stop if more load is placed on it. Pull-out occurs at a slip well beyond the