A three-phase motor expects equal voltages between phases. When the voltages differ, the supply applies, besides the forward-rotating field, a small field rotating in the opposite direction. This reverse field does no work; it circulates additional current in the rotor and stator and produces only heat. That is why a small voltage unbalance leads to a much larger unbalance in the phase currents.

The HEM catalogue states this plainly: phase unbalance increases current unbalance and heating; phases must be measured at commissioning.

Where to look in practice:

  • Measure L1-L2, L2-L3 and L3-L1 voltages at the motor terminals.
  • Compare the three phase currents under load with a clamp meter.
  • Check whether large single-phase loads such as welders or heaters are concentrated on the same phase.
  • An oxidised or loose terminal can cause voltage loss on one phase.

A simple way to locate the source: rotate the three supply leads in turn at the motor side. If the high current follows the supply phase as it moves, the problem is in the supply; if it stays on the same motor lead, the winding or motor connection must be examined.