Fuses are used to protect motors against short circuit faults.
Fuses of sacrificial devices used to protect components from being damaged.
The speed of operation of the fuse is very high at higher short circuit fault currents.
Protection against relatively small overloads is provided by using small current relays.
There are three types of overcurrent relays namely, magnetic, thermal and electronic relay.
The given circuit shows a 3 phase motor with direct on-line starter.
A contactor coil is connected in series with start button, stop button and over current trip contact.
This forms a control circuit which is energized from two phases of three-phase supply.
In case of short circuit on motor windings, the current drawn by the motor will be very high and this will blow the fuse, which in turn isolate the motor.
The fuse should be replaced after investigating and rectifying the fault.
The motor will draw a high current when it is mechanically overloaded.
The current release will open the trip contact, when the current increases above the set limit, after a set time delay.
This will break the control circuit to de-energise the contactor coil, which will open the motor starter contactors and trip the motor.
Single phasing of a motor is the loss of current through one phase in a three-phase supply, due to fuse blow off, faulty contactor or broken wire.
If a 3 phase motor is switched on to a supply with a single phasing condition, it will not start rotating but will draw a high current in the healthy phases.
The overcurrent relays or negative sequence relays will detect the overcurrent or unbalanced current respectively.
These relays will isolate the motor from the supply, preventing the motor coils from excessive damage due to the heat developed.
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