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Commentator

Join Date: Mar 2010
Location: Islamic Republic of Pakistan
Posts: 98
Good Answers: 12

No Load Power consumption of different Induction Motors

07/31/2010 8:38 AM

I have observed that if Induction motors in the range between 1KW to 10 KW operated at no load, It take 30-40 % of the rated current. As we go up to the higher rating this consumption in percentage of current goes on decreasing.

Now take an example of 3400 KW ( 3400 KW, 6300 V, 50 Hz, 362 A, P.F 0.89, 994 RPM, Rotor Voltage 1490 V, Rotor Current 1370 A). Yesterday we operate this motor at Decoupled condition (No Load) It was taking only 60 KW (i-e 1.76 % of its rated power).

I want to know why Low rating motors have worst efficiency at no loads ?

Why they have very high consumption at No load as compared to High rating motors?

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Power-User

Join Date: Mar 2010
Posts: 106
#1

Re: No Load Power consumption of different Induction Motors

07/31/2010 9:00 AM

Dear Niazi,

You are right. Power is required to run the motor even at no-load against

bearing power loss, windage loss, plus the electrical losses like magnatising loss,

no-load copper loss etc. At lower range of motors the percentage (%) of this loss

to the total power may be high .In high power motors the % may be less when

compared to the total power. This is the reason to the best of my knowledge.

Manroop.Chennai

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Guru
Engineering Fields - Electrical Engineering - Analog and Digital Circuit Design Engineering Fields - Electromechanical Engineering - Transformers, Motors & Drives, EM Launchers Engineering Fields - Engineering Physics - Applied Electrical, Optical, and Mechanical

Join Date: Jan 2008
Location: NY
Posts: 1217
Good Answers: 119
#2

Re: No Load Power consumption of different Induction Motors

07/31/2010 1:31 PM

As a general rule, electromagnetic machines (transformers, motors, and generators) all benefit from economies of scale. The larger they are the more efficient they will be.

While efficiency should be expected to be lower in the smaller motors, I bet most of that no-load current in your first example was reactive and the efficiency wasn't quite as bad as you think.

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Guru
United States - Member - New Member Engineering Fields - Power Engineering - New Member

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 3803
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#3
In reply to #2

Re: No Load Power consumption of different Induction Motors

07/31/2010 8:56 PM

That's why looking at current alone is never a good method of determining motor loading. At low load, you have high current, very low Power Factor, so the POWER consumption is actually low. The smaller the motor, the lower the PF is at low loading, because a grater relative percentage of the incoming power is being used just to create the magnetic field.

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Guru

Join Date: Oct 2009
Location: sometimes Wales,UK.. was Libya, now Oman!
Posts: 1352
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#4

Re: No Load Power consumption of different Induction Motors

08/01/2010 4:15 AM

there is also the kinetic energy stored in the rotor of larger motors that assists the field in rotating the rotor in the larger motor.

This can be observed by the run down time once the power is switched off, a larger motor will take longer to stop than a smaller one.

Also if you're really interested, measure the air gap between rotor and stator on both motors and see if the gap is relative to the frame size/rotor diameter

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