Higher current drawn by motor at Low Voltage - WHY ?

Back EMF? Check it out!

Al

The counter electromotive force created in the rotor is less, and current goes up to a limit then drops off as voltage is reduced.

If an induction motor works at all It will run close to but below synchronous speed. Lowering voltage may result in somewhat further reduction of speed but the motor will still run close to synchronous speed. The load still must still provide about the same amount of torque. Thus, mechanical output power will typically be reduced only slightly. To keep the power up at reduced voltage, in phase current must be increased.

Bill

------------- You have to look at the equations for the usual induction motor model (which is a transformer equivalent). Simplify them by ignoring stator resistance and play with them. There are more detailed models but these are also much more complex.

Starting current will be directly proportional to the voltage but the full load current will be higher above and below rated voltage (in range +/_15%). This is because the current is a sum of the "Load" current and the exciting current, and one decreases when the other increases. For a given output torque the total current increases as the voltage is reduced and there will be a small speed increase. Above rated load, the exciting current increases and the load current will decrease. The total current will decrease a bit as voltage rises between 0 and 5% (typically) then will increase as the voltage rises further.

Explanations using "back emf (or speed voltage)" as with a DC or a synchronous motor don't work because, unlike these machines, there is no excitation source other than the stator supply with nothing equivalent to the field winding and the apparent "back emf plus impedance drop" will match the applied voltage at any speed-just as in a coil or a transformer.

As for the data- just what data are you looking at? Starting torque and current? Full load current? Peak torque? Torque speed curves? Note that the torque-speed curve will be lower at reduced voltage than at full voltage. --

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

There are a couple good answers to question 1.

For question 2, at 80% voltage there will be much lower speed and available torque, so there is much lower available horsepower, resulting in electrical power also being much lower.

Do they give 80% voltage for other than reduced voltage starting?

Wowwwww

Thanks for all for posting their replies

(Mr. Bill answer is little favour to me - please give me more clarity by speaking about torque, i.e if applied voltage is less O/P has to reduce (in terms of torque) rather why motor drawing more current tends to supply rated torque - "motor is not knowing that voltage is less: :) :) ! )

I agree with all of your answers

Here i want to highlight my question again that

***If u observe "manufacture data sheet" - current drawn by the motor at 80% of voltage is less than the 100% voltage

this lead to my confusion

I again asking about here that what manufacture data sheet exactly indicates

please put some light on the above matter

"I agree that current drawn by the motor is more if the voltage reduces during running - by observation in site"

Please advise !

Regards N.A.Govind

One thing to keep in mind is not all motors are exactly the same. There are a few oddball designs out there like 'D' class that don't follow exactly the same rules. (well, in a nod to Don Kelly, they do follow the same formulae, but with a much higher rotor resistance, the resulting torque/speed curves don't look much like the others).

It might help us if you could tell us more about the specific example you have in mind. Or even find the data sheet on line and share it with us. Then we might be able to focus a bit more on your particular question.

daestrom