Stator Shorted Turns - Case History 5
LV, 3-Phase Induction Motor - Inverter Fed (a VSD)

Background and Test Objective
The results in Stator Shorted Turns Case Histories 1 to 4 were obtained from SCIMs fed from a fixed frequency supply.

However, it was important to carry out tests out on an inverter fed SCIM. An 11 kW motor of the same design and rating was used but it was fed from an inverter and had a double layer lap winding. For the tests a complete coil was initially short circuited via an external limiting resistor.

To predict current spectra with and without shorted turns in the stator winding of an inverter fed SCIM.

The tests were carried out at no-load with the inverter set to a base frequency of 50 Hz and at rated volts.

Nameplate details
3-Phase, 415 V, 20.5 A, 11 kW/14.7 h.p., 50 Hz,
1440 r.p.m., delta-connected, SCIM.

Additional Information
36 slot stator,
double layer winding, 8/9 coil pitch,
180turns/phase, 15turns/coil,
Number of rotor slots = 51 (aluminium die cast).

Direct drive coupling to a d.c. dynamometer load machine.

Full-load slip = 0.04
Test Conditions
MCSA FFT Current Spectrum, 0-200Hz span
Dynamic Range = 80dB
Frequency Resolution = 0.5 Hertz/line
Current Spectrum 1
Figure s-20
Time domain current spectrum from
inverter fed SCIM
Figure s-20 shows the input current waveform to the motor supplied from the PWM inverter.
Current Spectrum 2
Figure s-21
Current Spectrum Inverter fed SCIM,
no stator faults, no-load

Test Results
The tests were carried out at no-load with the inverter set to a base frequency of 50Hz and at rated volts.

Figures s-21, s-22 and s-23 show the current spectra with:

a) no fault,
b) a shorted coil (30A short circuit current), and
c) an increased short circuit current up to 45A (full-load phase current = 11.83A).

The difference is clearly evident and the frequency components (125 and 175 Hz) corresponding to n = 3, k = 1 and n = 5, k = 1 respectively from equation (S-1) are not present in Figure s-21 with no shorted turns.

fst = f1((n/p)(1-s) ± k)) ........ (S-1)

As the fault level was increased they clearly became evident and increased in magnitude as shown in Figures s-22 and s-23.


Current Spectrum 3
Figure s-22
Current Spectrum Inverter fed SCIM,
shorted coil, short circuit current 30A, no-load
Current Spectrum 4
Figure s-23
Current Spectrum, Inverter fed SCIM,
shorted coil, short circuit current 45A, no-load

This case history shows that MCSA can also detect shorted turns in a stator winding fed from an inverter. The short circuit current was limited to prevent motor burn out but as was shown in Shorted Turns Case History 2 the components would rapidly increase in magnitude as the severity of the fault increased.
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