AC Motors Induction Motor Speed - Torque Characteristics

The figure below shows induction motor speed - torque characteristics.

Induction Motor Speed - Torque Characteristics

Under no load, the motor rotates at a speed close to synchronous speed. As the load increases, the motor's speed drops to a level (P) where a balance is achieved between load and motor torque (TP).
If the load is further increased and reaches point M, the motor can generate no greater torque and stops at point R.
In other words, the motor can be operated in a stable range between M and O, while the range between R and M is subject to instability.

Induction motors are available in two types: permanent-split capacitor single-phase induction motors and three-phase induction motors. With the single-phase motor, the starting torque is generally smaller than the operating torque. The three-phase motor features a relatively greater starting torque.

  • Single-Phase Induction Motors
  • Three-Phase Induction Motors

The motor generated torque changes proportionally to roughly twice the power supply voltage. For example, if 110 VAC is applied to a motor whose rated voltage is 100 VAC, the torque generated by the motor increases to approximately 120 %. In this case, the motor temperature will rise and may exceed the permissible range.
If 90 VAC is applied to the same motor, the torque produced by the motor decreases to approximately 80 %. In this case, the motor may not be able to operate the automated equipment as expected.
For the above reasons, the power supply voltage should be kept within ±10 % of the rated voltage. Otherwise, when the power supply voltage fluctuates beyond the aforementioned range, the motor temperature may rise beyond the permissible range or the motor torque may drop and thereby make the equipment operation unstable.

Technical Reference