Overload protection of motors onboard ship


  • 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.


4 comments:

Powered by Blogger.