Motors, like any other item of electrical equipment,
lend themselves readily to automatic protection against abnormal operation and
possible damage. This invariably takes
the form of quick disconnection from the supply to prevent - or at least to
limit - any damage. The protection can
vary from a simple set of fuses to a sophisticated protection system consisting
of sensors or relays, or both.
Protection in general, and motor protection in
particular, is dealt with in detail in the manual ‘Electrical Protection’. It will be sufficient here to enumerate the
possible types of protection which may be found with motors on offshore or
onshore installations.
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Type of Protection
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Remarks
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Fuses
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Against
overcurrent from any cause such as mechanical overload, short-circuit or
prolonged stalling. Also to back-up
contactors.
|
|
Overcurrent
|
Instantaneous or
inverse-time, electromagnetic or thermal relay. Against overcurrent from any cause (as for
fuses).
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|
Earth Fault
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Against an
earth fault in the motor supply circuit or the motor itself.
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|
Stalling
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Against
stalling while running still connected.
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Single-phasing
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Against loss
of one phase, by means of a ‘Negative Phase Sequence’ device or relay.
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|
Undervoltage
|
Against loss
of supply when running, preventing automatic restart when voltage is
restored.
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|
Motor
Protection
|
A combination
of thermal overcurrent (inverse-time) and single-phasing protection. Sometimes with a stall relay included.
|
|
Winding
Overtemperature
|
Against
excess temperature in the motor windings, by embedded thermocouple,
thermistor or resistance temperature device elements in the windings. These sensors are described in the manual
‘Electrical Control Devices’.
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