As the load decreases the torque required decreases, so it is possible to reduce the voltage. Torque is proportional to the square of the voltage.
Advantages of reducing voltage
- Power factor is maintained=reduced current
- Iron losses are reduced
- Copper losses are reduced
Induction Motors running at reduced load and under the optimum design load condition are in-efficient. Most LV induction motors are running at below full load therefore running in-efficiently; they may even run for considerable time on very light or no load. Obviously variable speed drives can optimize the load system speed for efficient operation or the driven system but when the load changes it is necessary to optimise the induction motor efficiency and this can be done by reducing the voltage.