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

Your Answer

In order to provide an answer to this question, you must Login

I am flagging this question because...

10 inform moderator flags remaining

I am deleting this question because...

I am flagging this answer because...

10 inform moderator flags remaining

Flagging a Post

You have already flagged this post. Clicking "Remove Flag" below will remove your flag, thus reducing the count by one as well.

I am deleting this answer because...

Harmonised Cable Codes and Colours

Within Europe the European Committee for Electrotechnical Standardization (CENELEC) has standardised the both the designation and colour of cables.   ...

Meeting room of the future

The IET site has a video of a visit showing of a high tech meeting room developed at Napier University in Edinburgh. It a good demonstration of innovative...

IEEE Winds of Change

IEEE TV has a part series of videos on wind power and it's implication. For a really good overview to the technologies and issues around wind power, these...


Multimeters are undoubtedly the most common item of electrical test equipment in use.  Often it is the first piece of equipment people will turn to when...

Microsoft OneNote

A couple of months ago I came Microsoft's OneNote and downloaded the 60 day free trail. Since then I have been using it regularly and now have a full license...

Arc Flash Calculations

Working in the vicinity of electrical equipment poses an hazard. In addition to electric shock hazard, fault currents passing through air causes Arc Flash...

Windows Live Writer and myElectrical

When making adding a Note to our site we have a great online WYSIWYG editor and things are pretty simple.  However, if you prefer you can write, manage...

Lead us, Warleader

Delum, who had watched all in silence, his face empty of expression, now spoke in turn. ' "Lead us, Warleader, into glory."' Reading is something I do...

Introduction to Current Transformers

Current transformers (CTs) are used to convert high level currents to a smaller more reasonable level for use as inputs to protection relays and metering...

Network Theory – Introduction and Review

In electrical engineering, Network Theory is the study of how to solve circuit problems. By analyzing circuits, the engineer looks to determine the various...

Our website uses cookies so that we can provide a better experience.
To learn more about what cookies are and how to manage them visit AboutCookies.org