The torque produced by an induction motor is roughly proportional to the square of the voltage. When the voltage falls by 10 percent, the available torque drops by about 19 percent (0.9² ≈ 0.81). Since the load stays the same, the motor tries to make up the shortfall by running slightly slower, i.e. with more slip, and drawing more current; that extra current comes back as heat in the winding.

Typical symptoms:

  • Starting time lengthens and the thermal overload relay trips during starting.
  • With a heavy starting load (loaded compressor, crusher, full conveyor), the motor may not reach rated speed.
  • In continuous duty the frame runs hotter than usual and winding life shortens.

Voltage drop often comes not from the grid but from the installation itself: long cables with too small a cross-section, loose terminals, an overloaded transformer or a large motor started direct on line. Measure both at no load and under load, at the motor terminals; if the difference between the two is large, check the cable and connections first. Our electric motor current table helps you compare the measured current with the rated value.