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Secondary Resistance Starting


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I'm having problems with a drive that is taking a long time to get up to 100% speed. Would like to know when rotor resistors should change over to next stage to maintain maximum torque?


Specs are:


2 x 630kw 3.3kv drives

2 x 28 stage rotor cubicles

uses either timed start (each stage is preset) or accelerated start (uses tacho pulses to change stages).

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Hello Greg


Welcome to the forum.


When you are using a secondary resistance starter, you are effectively shifting the slip for maximum torque from near full speed to points closer to zero speed, depending on the value of the resistance.

For a given resistance, once the motor speed exceeds the point of maximum torque, the rate of acceleration will slow, and the current will fall.


You could monitor the rate of acceleration and when the rate of acceleration drops below a set point, step to the next resistor stage, however my preference would be to monitor the stator current during start and when this current drops below say 120% of rated current, step to the next resistor stage. (As the current drops, the torque is also falling.) The maximum torque will occur at 200 to 250% rated current. The actual optimium current for stepping will be dependent on the number of stages and the values of these stages, but could be quite easily determined experimentally.


Hope this helps,

Best regards,

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