Page 164 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

MAINTENANCE OF MOTORS

MAINTENANCE OF MOTORS 8.19

Manual Rotation Test
The motor is disconnected from its normal three-phase power supply for its off-line test. The
driven equipment should be uncoupled unless it can be manually rotated with the motor. A
single-phase ac supply is connected across two motor terminals. It has voltage rating of 10
to 25 percent of rated line-to-line volts and a kVA rating of 5 to 25 percent of rated kVA.
The rotor is manually turned for one-half revolution while monitoring the variations in the
current. A broken rotor cage winding is indicated by current fluctuations in excess of 10
percent. The current fluctuations can also be monitored on a strip chart recorder connected
to the output of a current transformer.
It is important to note that the test time should not exceed 1 min due to rapid heating in
the stator and rotor windings. The main limitation of this test is that it can only be conducted
off-line. Therefore, breaks in cage windings may not be detected if they close up, giving
low-resistance connections when centrifugal forces are removed.

REPAIR AND REFURBISHMENT OF AC
INDUCTION MOTORS

The repair and refurbishment of an ac induction motor should include the following steps:
1. Perform a visual inspection to assess the general condition of the motor. Check for
cracks, broken welds, and missing parts. Photographs are required in some cases prior
to disassembly of the motor to document the motor construction and accessories.
2. Perform these tests on the stator windings and record the results:
● Insulation resistance (IR) and polarization index (PI)
● Winding resistances (terminal-to-terminal resistances)
3. Rotate the rotor manually, and check for any defects in the bearings and shaft.
4. Run the motor at no load. Measure and record the currents, vibration, bearing temper-
atures, and temperature rises.
5. Measure and record the rotor end play (axial and radial movement of the rotor in the
bearings).
6. Dismantle the motor and remove the rotor.

Stator Work
7. Clean the windings, using low-pressure steam, if they are contaminated with dust, oil,
or grease.
8. Dry the stator in an oven at a temperature of 105°C (220°F) for a period of 6 h.
9. Take IR and PI tests. Note: Following steam cleaning and drying of the windings, the
results of IR and PI tests should improve.
10. Inspect the motor cable insulation for cracks, overheating, and brittleness.
11. Inspect the stator insulation for cracks, brittleness, and puffiness.
12. Inspect the slot wedges and bracing system in the stator for looseness.
13. Inspect the laminations in the stator core for looseness, damage due to rotor rubbing,
localized overheating, and blockage of the vent ducts.

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