Page 388 - Offshore Electrical Engineering Manual

CHAPTER
5
Large Motors

PRECOMMISSIONING
Before commissioning, it is essential that the installation work has been properly
checked before any equipment or cables are energised, or anything is allowed to
rotate. Cable connections are ‘rang through’ and cable insulation is checked. All such
work is termed precommissioning.

MOTOR COMMISSIONING TESTS
Three types of testing are recommended for the offshore commissioning of
motors.

MEGOHMMETER (MEGGER) TEST
This involves the direct current (DC) measurement of the stator winding resis-
tance to earth and between phases at the appropriate test voltage as shown in
Table 7.5.1.
This test is carried out on all motor windings and is of 1 min duration.
The procedure is as follows:

1. Disconnect the neutral point and adequately insulate each phase lead from earth.
On small machines, it will not be possible to disconnect the neutral point.
2. Disconnect and earth all resistance temperature devices (RTDs).
3. Ensure that the motor frame is connected to a known earthing point. Phases not
under test must be earthed.
4. Measure the ambient air temperature in degree Celsius local to the motor before
starting the test. If the temperature of the motor is thought to be significantly
higher than the motor air temperature, then the winding temperature should be
measured, if possible, by using any RTDs fitted to the motor before they are
disconnected for the test.
5. If the temperature of the insulation is cold to the extent that it is below the dew
point, a moisture film will form on its surface and may lower the insulation
resistance, especially if the winding has developed a coating of dirt and/or salt.
Therefore, if this is likely to occur, the motor anticondensation heaters should
be left on for some time before the motor is tested.

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