CHAPTER 9 INTRODUCTION TO MOTOR PROTECTION

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.

	Type of Protection	Remarks
	Fuses	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).
	Earth Fault	Against an earth fault in the motor supply circuit or the motor itself.
	Stalling	Against stalling while running still connected.
	Single-phasing	Against loss of one phase, by means of a ‘Negative Phase Sequence’ device or relay.
	Undervoltage	Against loss of supply when running, preventing automatic restart when voltage is restored.
	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’.