2026-09-20
Stator Winding Inter-Turn Short: How We Found One and How the Surge Test Catches Them
A customer sent a 48V 10kW AC asynchronous traction motor back to us. Their note said it starts, runs for a second, stops, and the shaft feels notchy when turned by hand. They had already replaced the inverter, which told us the drive was probably innocent.
We checked the two obvious suspects first: the controller and encoder pairing, then the shaft and bearings. Both came back clean. The fault was inside the stator — a few turns in one phase had lost the insulation between them and were acting as one thick turn.
That is an inter-turn short, and it is the failure a multimeter, a megger and a hipot test can all miss. This article walks through how we found it, why the usual tests miss it, and how the surge comparison test works: the stator as a resistor, inductor and capacitor, the three stages of one pulse, how to choose peak voltage and wavefront time, and how to read pass or fail.
We checked the two obvious suspects first: the controller and encoder pairing, then the shaft and bearings. Both came back clean. The fault was inside the stator — a few turns in one phase had lost the insulation between them and were acting as one thick turn.
That is an inter-turn short, and it is the failure a multimeter, a megger and a hipot test can all miss. This article walks through how we found it, why the usual tests miss it, and how the surge comparison test works: the stator as a resistor, inductor and capacitor, the three stages of one pulse, how to choose peak voltage and wavefront time, and how to read pass or fail.
