Page 228 - Offshore Electrical Engineering Manual

CHAPTER

Motor Faults and 3

Protection

The various types of motor used offshore are discussed in Chapter 4. In this chapter,
the various types of motor protection devices and their application offshore are dis-
cussed. In general, there are seven main categories of drive system fault which need
to be protected against. These are:

MOTOR WINDING ELECTRICAL FAULTS
Such faults are usually catered for by means of fuses or circuit breakers fitted with
instantaneous overcurrent relays. Sensitive earth fault relays set at about 20% of full-
load current are normally provided on motor starters rated at more than about 40 kW.

MOTOR MECHANICAL FAULTS
Bearing failures:
Without some form of vibration monitoring, it is not very practicable to detect the
incipient failure of a ball or roller bearing and shut down the machine before the bear-
ing disintegrates. However, monitoring devices are being developed which detect the
effect of various forms of abnormal vibration on a machine’s magnetic ‘signature’. This
information is then processed to provide various alarms and trips. The incipient failure
of a sleeve bearing can usually be detected by a resistance temperature device or similar
device which detects the rise in bearing temperature before seizure or disintegration.

ABNORMALITIES IN THE DRIVEN MACHINERY
The control logic of the driven machinery package should prevent the occurrence of
severe changes in torque, negative torques or reverse running before the motor is ener-
gised. An example of such a fault would be if an extra water injection pump was being
brought into service and seawater, pressurised by the running pumps, was allowed to
flow backwards through the pump being started, thus driving the pump and the motor
backwards. The extra time at elevated current required by the motor to decelerate before
reaccelerating in the forward direction may be sufficient to overheat the machine. It
may be necessary to fit a tachometric device to the machine such that reverse run-
ning will result in an ‘inhibit start’ signal. Although valve actuator interlocking logic
should be designed to prevent the water valves being opened in the wrong sequence,
it is worth checking the driven equipment control scheme to ensure that this or any
other likely cause of motor failure is catered for by one or more protective measures.

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