After the overhaul, handover, and partial replacement of windings in synchronous motors, the insulation and voltage resistance of the stator winding cannot be omitted as a test. Many sites choose 0.1Hz ultra-low frequency, not only because the equipment is small, but more importantly, it has a good detection effect on insulation defects at the end of the motor.

During power frequency testing, the capacitive current flowing out of the wire rod passes through the semiconductor anti corona layer outside the insulation, which forms a certain voltage drop on the anti corona layer, resulting in a decrease in the actual voltage borne by the insulation of the end wire rod. At 0.1Hz, the capacitor current significantly decreases, and the voltage drop on the anti corona layer also decreases accordingly. The voltage borne by the end insulation is more sufficient, and potential defects are more easily exposed.

The synchronous motor test should be conducted in phases: apply pressure to the tested phase, short-circuit and ground the non tested phase. The peak voltage of the experiment can be calculated according to Umax=√ 2 β KUo. In the manual, β is taken as 1.2, K is usually taken as 1.3-1.5, and generally taken as 1.5. Taking a synchronous motor with a rated voltage of 10.5kV as an example, the peak value of the 0.1Hz test voltage calculated is about 26kV. The test time is usually 1 minute, and the overcurrent protection setting also needs to be estimated based on the winding capacitance.

During the pressure resistance process, you cannot just stare at the screen. At the same time, observe whether there is corona, abnormal discharge, odor, smoke or sudden unstable data at the end of the motor. Especially for air-cooled units, external monitoring often reveals insulation abnormalities that are temporarily not reflected by instruments.

After the experiment is completed, stop the machine first, discharge automatically, and then use a discharge rod to fully confirm. The motor winding is a capacitive test sample, and if the discharge is not complete, the risk of disconnecting the wire is very high.