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Alternative To Liquid Starter


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I understand that by using a soft start on a slip ring motor, the lock rotor torque is very poor and this is not a convenient way to start high inertia load even by adding extra resistance to the rotor circuit. I have heard that some soft starters are able to produce 400-500 % starting torque. How far this is true ?





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Hello Bob


The major advantage of the secondary resistance starter, is that you can alter the slip of maximum torque by altering the rorotr resistance. By doing so in a number of steps, you can achieve a high torque from zero speed to full speed at a relatively low starting torque. With sufficient well placed steps, you can have in the order of 200% torque at less than 250% current right through to full speed.


The only other way that you can achieve this, is by using a shorted rotor and an appropriate variable frequency drive. Note: Most drives will not produce the high torque at less than 5 - 10 Hz!


If you use a soft starter and a single stage of resistance, you essentially make the slip ring motor emulate a standard cage motor and so you have a high start current for a nominal start torque, for example 400% current and perhaps 100% torque.


I do not know of any way that a soft starter can give you 400% start torque. This is higher than the motor will produce DOL or with a secondary resistance starter.


Best regards,

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