Answer MET Question 9

Question: Describe how protection against short circuit is provided in a 3 phase induction motor circuit.

Answer: The protection of a motor form any fault causing high current drawn can be explained from the Inverse time graph

Protecting an electric motor basically involves preventing the motor from getting too hot. Remember, every 10 deg. C. above the maximum recommended temperature of the insulation can reduce its working life by half. If the temperature exceeds the maximum set value for the motor insulation its contactor is tripped to stop the motor and allow it to cool down.
Three main types of direct temperature sensors can be used. These are Thermocouple, Resistance temperature device (RTD) and Thermistor.
The thermistor sensor is probably the most common as its thermal characteristic more closely matches that of a motor than the other types. Thermistors are small pellets of semiconductor materials which are embedded into the insulation of all three motor stator windings during manufacture. When a thermistor gets hot its resistance changes dramatically. They are connected so that if the motor
temperature gets too high the starter contactor will be tripped by an electronic protection relay to stop the motor.
Direct thermistor protection is usually only fitted to large motors, e.g. bow thrusters, FD fans, air conditioning compressors, etc.
Most motors are protected by monitoring the temperature indirectly by measuring the current flowing in the supply lines.
This method uses electronic, thermal or electromagnetic time-delayed overcurrent relays (OCRs) in the motor starter. The system is designed so that if the motor takes too much current because it is mechanically overloaded, the OCR will trip out the contactor coil, after a pre-set time delay, before severe overheating can occur. The largest overcurrent possible is the current taken when the motor has stalled.
This, of course, is the starting current of the motor which will be about five times the full load current. The contactor is capable of tripping this stalled current quickly and safely.
lf a short-circuit occurs in the motor, the starter, or the supply cable, then a huge fault current will flow. If the contactor tries to open under short-circuit conditions, serious arcing will occur at its contacts such that it may fail to interrupt the fault current. The prolonged
short-circuit current will cause serious damage to the motor, starter and cable with the attendant risk of an electrical fire. To prevent this, a set of fuses or a circuit breaker is fitted upstream of the contactor which will trip out almost instantaneously thereby protecting the
contactor during a short-circuit fault.
It is important that the tripping characteristics, of the OCR and fuses/circuit breaker are co-ordinated so that the contactor trips on thermal overcurrent while the fuses/circuit breaker interrupt shortcircuit fault currents. This contactor + fuse arrangement is usually called back-up protection.To protect a modern CMR (Continuous Maximum Rating) motor the thermal OCR should be set at the full-load current (FLC) rating of the motor. The motor fuses are not chosen for their rated current but for their inverse current/time
(I/t) characteristic. This means that the current rating of fuses used to protect a motor does not appear to have any direct relationship to the FLC rating of the motor.
Fuses used for back-up protection for motor circuits have a special time/current characteristic. They are generally carrying steady currents well below their rated capacity to allow for short duration DOL starting currents without blowing.
Consequently they do not protect against normal overloads but do protect the motor and supply system against a short-circuit fault. Fuses designed for motor circuit back-up protection have a restricted continuous current rating (called "M" rating) as compared with their fusing characteristic.
Hence a typical fuse designation for motor circuits could be "32M63" which indicates a continuous rating of 32 A but a rating of 63 A for the starting period.