Page 160 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

MAINTENANCE OF MOTORS

MAINTENANCE OF MOTORS 8.15

1500 Peak:1500 V

Voltage, V

Rapid shift
between waveform

10 20
s (10 –6 )

(a)

1500 Peak:1500 V

Faulty
Voltage, V

Good

10 20
–6
s (10 )

(b)
FIGURE 8.5 Results of a surge test indicating a fault in the motor.

test is performed at the motor. The larger resistance unbalance (10 percent instead of 5 per-
cent) is allowed when the test is performed from the MCC due to the long cables between
the MCC and the motor.
The voltage and current used during the test for 4-kV and 575-V motors are 5 V dc and 6
A, respectively. This test is used to detect short circuits, ground faults, phase-to-phase faults,
loose connections, open circuits, dirt accumulation at connections, etc. Tables 8.3 and 8.4 list
the tests required and acceptance criteria used for a 575-V and a 4-kV motor, respectively.

Tests for the Detection of Open Circuits in Induction Motor
Rotor Cage Windings
Severe thermal aging and failure of rotor core lamination insulation will occur as a result of
open circuits in induction motor cage windings. Failure analysis of more than 6000 induc-
tion motors confirmed that 10 percent of these failures occurred in the rotors. One-half of
the rotor failures occurred in the cage winding. If a rotor winding cracks, the performance
of the motor will degrade and the stator end windings could also get damaged.

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