When motor direct-on-line started with the stator winding star connected, it will only take one-third of the starting current that it would take if the windings were delta connected. The starting current of a motor which is designed to run delta connected can be reduced by star-delta starters for small motors may be operated by a manual changeover switch.
For large power motors on ship, the phase windings are automatically switched using contactors controlled by u timing relay.
A choice to time delay relays are available whose action is governed by thermal, pneumatic, mechanical or electronic control devices.
At the instant of starting when the supply has just been switched on and the motor has not yet started to rotate, there is no mechanical output from the motor. The only factors which determine the current taken by the motor are the supply voltage (V) and the impedance of the motor phase windings (Zph).
This shows that the starting current of a delta connected motor can be reduced to one third if the motor is star connected for starting.
The shaft torque is also reduced to one-third which reduces the shaft acceleration and increases the run-up time for the drive but this is not usually a problem.
Induction motor on ship electrical system
When an induction motor is running on load it is converting electrical energy input to mechanical energy output. The input current is now determined by the load on the motor shaft.
An induction motor will run at the same speed when it is star connected as when it is delta connected because the flux speed is the same in both cases being set by the supply frequency.
This means that the power output from the motor is the same when the motor is star connected as when the motor is delta connected, so the power inputs and line currents must be the same when running in either connection.
If the motor is designed to run in delta but is run as star connected, and on full load, then each stator phase winding will be carrying an overcurrent of 1.73 rated phase current.
This is because phase and line currents are equal in a star connection.
This will cause overheating and eventual burnout unless tripped by the overcurrent relay.
Remember that the motor copper losses are produced by the heating effect so the motor will run 3 times hotter if left to run in the star connection when designed for delta running.
This malfunction may occur if the control timing sequence is not completed or the star contactor remains closed while a mechanical interlock prevents the delta contactor from closing.
For correct overcurrent protection, the overcurrent relays must be fitted in the phase connections and not in the line connections.
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