Effect of voltage drop on induction motor current

"We've two motors of 75 kW and 150 kw rating which are driving centrifugal pumps. The normal running current of 75 kW motor is 115A whereas its low current security is set at 90A.
Whenever 150 kW motor cuts in, the low current security of 75 kW motor gets actuated. We've measured the voltage drop during the startup of 150 kW motor which is 15 - 20V.
My question is'
1. Whenever there is a voltage drop on an induction motor, the current should increased or decreased ?"

asked 3/15/2012

2 Answers

The current will increase but it also depends on the starter you are using. I assume you are using a star delta starter, although the are expensive to buy you can use a soft starter or a variable speed drive.
answered 5/15/2012 Interserve 2
As the voltage reduces the motor current will increase (but the speed and torque will decrease). Exactly what happens depends on the magnitude and duration of the dip. Limited decrease in voltages should not be a problems as long as the motor current remains below the nameplate rating. For larger voltage dips the motor may stall (or go into a starting condition), which would increase the current above normal conditions and require the protective devices to operate.

If your 20V drop is related to 400 V (just guessing), that's 5% which should not really cause problems. From your description, it may be that the overload protection is set at less than the name plate full load current. In that case you could increase it to overcome the issue.

If this control voltage is also derived from the system voltage, a drop in this may also cause the contactor to drop out.

If you have measured the voltage drop, can you also measure the current then you will know what is
answered 3/20/2012 Steven McFadyen 246
Steven McFadyen

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