Why is efficiency greater at no load than full load for an induction motor?

Efficiency at no load

Induction motors have very low efficiency when operating without load. Specifically, the efficiency of an induction motor at no load is close to zero. This is because at no load, the motor does not carry an actual load, so the output mechanical power is very small. However, the motor still needs to consume energy to maintain its internal magnetic field and other necessary operations, and these energy losses are manifested as copper loss, iron loss, and mechanical loss, etc. Due to the existence of these losses, although the input power is relatively small, the output power is almost negligible, resulting in extremely low efficiency.

Full load operating efficiency

Comparatively, when an induction motor is in full-load operation, it will bear the actual load and deliver mechanical power. Although the total losses (including copper loss, iron loss, and mechanical loss) increase during full-load operation, the overall efficiency may actually improve due to the significant increase in the useful output power (i.e., mechanical power) at this time. The efficiency at full load typically falls within the range of 74% to 94%.

Conclusion

In summary, the efficiency of an induction motor is not higher during no-load operation compared to full load. In fact, the efficiency of an induction motor operating at no load is close to zero, while the efficiency at full load significantly increases. This is mainly due to the fact that at full load, despite the increase in losses, the output useful power increases dramatically, thereby improving overall efficiency.