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High starting current


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Dear All,


We have a 200 hp 2900 rpm motor which runs well on No-load and draws 55 amps balanced on no load.


This motor is driving a hammer mill which is free to rotate ( with hand).


Howver when it is started on load it draws high current.,and trips. We are using star delta starter for it.


The insulation resistance of winding is OK. The inductance of one winding is 10 % higher than the other two windings.


What can cause this problem?


Any suggestions?

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Hello chaterpilar


Welcome to the forum.


The start current drawn by an induction motor is a function of the terminal voltage, the motor design and the rotor speed. At any instant in time, the start current is not dependent on the shaft load.


When the start voltage applied to the induction motor is reduced, the start torque is also reduced. If there is insufficient torque under reduced voltage starting conditions for the load to reach full speed, then the step to full voltage will occur at part speed and this will result in full voltage starting current. (typically 6 - 9 times the rated current of the motor.) NB the full voltage start current remains close to locked rotor current until the motor is almost at full speed.


With a star delta starter, the only parameter that you can control to alter the starting conditions, is the time taken for the starter to operate in star then step to delta.

If the motor is still accelerating when the change over to delta occurs, you may find lengthening the time may allow the motor to reach full speed before changeover. If the motor has run out of torque in star, there is nothing that you can do.

A more sophisticated starter such as a soft starter will allow you to accurately match the start conditions to the load.


There is a lot of background information covering this technology on this website. You may find some further answers on http://www.lmphotonics.com/m_control.htm and http://www.lmphotonics.com/m_start.htm


Best regards,

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