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With a hipot tester and an oscilloscope Partial discharge test of the motor stator

With a hipot tester as an application source. Overview of Partial Discharge Test.

For the test circuit, use a stator with the neutral point unconnected, and connect the positive (+) side of the voltmeter to the U-phase and the negative (-) side to the V-phase to apply high voltage. This was done by comparing the change in the partial discharge start voltage when the insulation is good and the U-and V-phase coils are touched.
To confirm the partial discharge phenomenon, partial discharge current pulses were observed using a current probe and an oscilloscope.
 

Measuring Instruments Used and Instrument Settings And DUT info

The instrument used and its setting information and specifications are shown below.

Hipot Tester
Automatic Insulation Withstand Voltage Tester 3153 (manufactured by Hioki Electric)
Test voltage: AC60HZ, voltage rises from 1000V to 100V in increments.

Oscilloscope
Memory Hicoder MR6000 (Hioki Electric)
Sampling speed: 200 MS/s
Storage time: 2.5 seconds
Coupling method: AC coupling

Current Probe
Current-probe CT6711 (manufactured by Hioki Electric)
Range: 0. 5A
Output rate: 10V/A
Frequency band: DC~120 MHz

Differential Probe
7 kV 70 MHz High Voltage Differential Probe
(Made by Keysight)

DUT
Stator for three-phase induction motor

 

Results of partial discharge tests

No error when AC1 of.0 kV is applied to non-defective products.

A AC1 of.0kV was applied to a non-defective stator from a voltage-proof meter, and the wave forms of current (green wire) and voltage (blue wire) were observed. The memory high coder MR6000 can store data for 2.5 seconds (digital recording) even when the sampling speed is set to 200MS/S (=5 ns) at the maximum. When the recorded waveform for 2.5 seconds was checked, the current waveform did not appear to be partial discharge pulses.
Although the peak-to-peak value of the current waveform is a minute signal of about 1mA, the current-probe CT6711 can be output at 10mV / mA, so it was very useful for observing such leakage currents. However, since the DC drifting is severe (in fact, 0A position is shifted in the diagram on the right), it is desirable for the Memory Hicoder to set "AC-Coupling".


Partial-discharge pulsed is detected when AC1 of.5kV is applied to a non-defective product.

When the applied voltage from the withstanding voltage meter was increased gradually, partial discharge pulses appeared in the current waveform of the good stator at about AC1 of.5kV. However, in 2.5 seconds, it appears only in the area shown on the right. In other words, PDIV is 1.5kV.


For pseudo-defective products, AC1of.0kV Partial discharge pulse detected when is applied.

Next, voltage was applied to the pseudo-defective product from AC1 of.0kV. However, partial discharge pulses appeared suddenly as shown in the right-hand figure. That is to say, PDIV was 1.0kV or less, and there was obviously a difference from the non-defective product.

 

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