No Load Current of Motors & Pumps

You need to contact the original motor manufacturer. There has been significant improvement in motor efficiency in recent years and some manufacturers have literally left the others in the dust.

You will also need to specify whethere the pumps are coupled to the motors during the test or the motors are free standing since the resistance of the pump will influence the current draw.

Also, if motor current is drawing higher current, then rewinding appears to have used larger diameter wire, or lesser length (fewer turns) either of which will change other motor characterisitics in addition to no-load current.

Fewer turns will increase the no-loas current, but larger diameter wire will decrease the no-load current....

Measure it.

The exercise is a waste of time. Don't bother, you won't know what the data means anyway.

Ideally, the no-load current of all motors is zero. The reason is that if the motor is not doing anything, the best thing to do is switch it off.

Not true.

For AC induction motors you have the magnetizing current, that aproaches a 90 deg phase shift. Depending on the motor, it may be 30% of the total full load current.

So, why not switch it off?

Dear Mr. PWSlack,

You are right, when Motor is switched off, current is zero. But when the motor is started, motor reachs rated speed, and it is not loaded, the current drawn by the motor,will follow the path of Circle Diagram, which you know, and during No-Load, you also know that the Power Factor will be very less.

Pl. explain your point further,

DHAYANANDHAN.S

If motor manufacturers print the value of no load current in the motor name plate there won't be a problem. Why not BEAMA,NEMA,ISO do it?.

"Why not BEAMA,NEMA,ISO do it?."

Because:

1. There is no value to the information
2. If it is shown, ignorant people will be taken advantage of by unscrupulous salespeople who will try to use that information to make them believe there IS value to it, and that their numbers (higher or lower) mean something valuable compared to someone else' numbers (higher or lower).

Bottom line, it would just create needless confusion.

Value of no-load current of a new motor serves as a guideline for testing a motor after repairs,although it need not be exactly same due to damage to stator,bearing etc

Thanks for your suggestion i have tried to get the no load current detail from motor manufacturerer & from internet but not find anything.now i have got somw brand new motors in our company i will checked there no load currrent & then compare it with the installed motor on machine.if suppose the no no load current is 30 percent high after replacing it i will gert the 30 percent energy dave. Is that s

Some manufacturers mention power factor,noise level,bearing sizes too in the name plate. Instead of writing to manufacturers or referring to their website users should get information at sight. Before internet was introduced imagine the situation-you write to them & wait?. I suggest to print size of capacitor required for PF correction as well as that for shunt capacitor type starter in the name plant.

Rewinding a motor should not significantly change its characteristics.

I used to work for Reliance Electric in their large AC motor plant in Canada. (500 to about 5000HP)

Occasionally we did do a rewind - burn out the old coils in an oven, and then rewind.

Depending on the insulation system, wire size, etc, you could actually end up with a superior motor.

Don't bother about the no load current. This varies widely from maker to maker and local conditions.

As already suggested, try to check the kW reading when pumping against the expected kW theoretically needed to transport the required flow under the back pressure (or to what height...). This will give the overall efficiency of pump-motor assembly. Check against the pump chart that the pump supplier should give.

Dear Mr. ashutosh goyal,

The No-Load Current will be in the range of 30% to 40% depending upon the Power Factor, and you draw the circle diagram, for your self and see. If it is above 40%, then re-winding is not proper and some error has taken place, knowingly or unknowingly.

DHAYANANDHAN.S

Poster,

There is a lot of information you left out of your question.

1. Why do you need to know the no load current?
2. If it is for reference, then do as earlier posters have suggested.
3. What is the HP of the motor?
4. What information is on the data plate on the motor?
5. What type of pump is the motor connected to? (Centrifugal, Positive displacement, Etc?)
6. What type of fluid are you pumping?

Why don't you do this,

1. Measure the current with the motor running not connected to the pump.
2. Measure the current with the motor running connected to the pump without fluid. Don't run it too long dry! Just long enough to get your current reading.
3. Measure the current with the motor running and pumping fluid at your rated flow and back pressure.

You're going to get 3 different numbers and that is the data you want. Next time you have a technical question, we need as much information as possible to help us help you!
BA