N2 Electrical Trade Theory|The Easy Way! 65 4.8 Overcurrent protection of motors


When induction motors are started without any type of starter, they draw about 5 to 6 times their full load current and their starting torque is usually less than twice the full load torque. In large motors, this starting current can be very high and starters are required to reduce this starting current to a practical value.


Overcurrent protection for induction motors are oſten set in the region of 20% above the full load current rating. Te use of a starter reduces the starting current but increases the accelerating (starting) time because of the reduced value of the starting torque.


Tis is one of the reasons why overcurrent protection devices must incorporate time delay devices that will allow for the extra time required for acceleration. Overcurrent protection devices must have inverse time delay characteristics, i.e. the greater the overcurrent, the shorter the time delay before tripping.


Te following possible occurrences make it essential for inverse time delay characteristics to be included in overcurrent protection devices:


• If, for some or other reason, the rotor is locked (seized) at starting, there must be very little time delay.


• There must be no time delay in the event of a short-circuit occurring. • In the case of an overload occurring during operation, the time delay must be shortened according to the size of the overload.


▲ 6.16.5.1.3 Te overcurrent protective device shall;


(a) have a tripping value that is as near to the full load rated current of the motor as is practicable,


(b) have sufficient time delay to allow the motor to start and accelerate under normal conditions,


(c) prevent a multiphase motor from continuing to operate under load if single phasing occurs, and


(d) in the case of an automatically controlled motor, have to be manually reset aſter operation before allowing automatic restarting of the motor.


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