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mukeshkumar5609
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Big Capacity Motor Drawing Extra Current Than Small Rating

04/29/2015 5:51 AM

Sir , I have 2 motor . One is 55 kW, 1460 RPM and other is 18.5 KW, 955 RPM. Both work on 440 volt AC, 50 Hz. When we connect these motor in Star than 55 kw motor drawn 10 Amp current in each phase on no load and 18.5 kW motor drawn 13 Amp current in each phase. Kindly guide me why this condition generate.

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SHOCKHISCAN
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#1

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/29/2015 9:13 AM

Because of the difference in magnetic poles (4 pole verses 6 pole )and the differences in motor design.

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SolarEagle
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#2

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/29/2015 10:15 AM

The higher the flux density, the higher the no-load current will be as a percentage of the full load amps...the lower the speed, the higher the percentage of full load amps...the ratio of NLA/FLA is inversely proportional to the hp, approximately....

REF...

http://www.easa.com/sites/files/accreditation_program/NoLoadCurrentBasics_0205.pdf

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Codemaster
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#3

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/29/2015 3:36 PM

You say the motors are connected in star, but is that the correct connection?

The FLC of the 55 kW is about 100 amps, so 10 amp on no-load sounds low. No-load amps is usually 20 - 30% FLC (from memory, somebody might correct me), higher than you might expect because the power is very low on no-load.

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#8
In reply to #3

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/30/2015 4:26 AM

I meant of course "......power factor is very low......"

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SolarEagle
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#4

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/29/2015 8:40 PM

Are these motors on VFD's? ...or maybe just the big one?

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pnaban
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#5

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/29/2015 11:20 PM

Did you compare the prices,types of windings,quality of laminations,cooling,flux density,rotor construction etc

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#6

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/29/2015 11:27 PM

There is nothing unusual in this. Larger horsepower motors always have a lower ratio of no load to full load amps when compared to smaller ones.

The fact that your smaller motor has 50% more poles adds to the difference that you observe, flux path air gap is a major cause of differing no load amps.

1. The greater the number of poles, the more times the flux has to cross the air gap, and the more current this requires to maintain sufficient flux to do so.

2. Rotor clearance - air gap between the rotor and the stator pole faces, the larger the gap, the more current required. Generally a machine with more poles will have a smaller rotor clearance, but this is not always the case, and becomes impractical as motor size decreases, and so a lower HP motor will normally have a greater rotor clearance than a larger motor as a percentage of motor bulk or bore diameter.

3. Flux density is also affected by the quality of steel used in the rotor and cores, the shape of the pole pieces, and by the number and size of the conducting bars in the squirrel cage.

A motor that is designed for delta connection will draw around 1/3rd of the current when connected in star, and no load current is normally around 30-40% of full load current, so 1/3rd of 1/3rd or about 10 amps would be about right for the larger motor, but the smaller motor would appear to be drawing a bit more than would be expected, but is maybe a result of cheaper manufacture, greater air gap tolerances, tighter bearings, comparatively larger cooling fan etc.
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7anoter4
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#7

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/29/2015 11:46 PM

In my opinion, the big motor no-load current ratio with respect to rated [full load] could be less than of the small motor no-load current, but not less than 25-30%.According to O.P. 55 kw motor no-load current it is no-more than 12%.That means the gap between rotor and stator is a half of an usual minimum[??].I think this no-load current has to be 20 A [instead of 10 A].See, for instance:

http://www.easa.com/sites/files/accreditation_program/NoLoadCurrentBasics_0205.pdf

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Geoffrey36
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#9

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/30/2015 5:03 AM

By the way, 440v would be the delta connected voltage. Star connected, the voltage would be 440/√3 = 250v or thereabouts. It doesn't make any difference if you connect the star point to the neutral (but it's probably safer!)
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#10
In reply to #9

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/30/2015 6:23 AM

When you say "Star connected, the voltage would be 440/√3 = 250v" do you mean the volts each winding sees during star-delta starting is 250v? That's correct, though OP hasn't mentioned starting. But if it's wound 440v delta and you want to connect it to run in star, it needs 440*√3 = 760v.

And it depends on the motor. It could be wound 440v delta/440*√3 = 760v star (if connected in star at 760v star-delta starting obviously not a possibility) or 440v star/440/√3 = 254v delta.

Years ago (in UK) most motors were wound 415v delta (415v as it was then, now nominally 400v). Nowadays it's not uncommon to see 400v star. I've seen cases where the sparky hasn't checked the nameplate, assumed delta and got it wrong.

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#11
In reply to #10

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/30/2015 6:51 AM

The initial post spoke of "440v motors - star connected" I assumed they were connected in star on the 440v/250v supply, that's all. They would be star connected during the first stage of an S/D starter, of course.
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#12
In reply to #11

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/30/2015 7:30 AM

Yes you're right, I'd forgotten that, even though I queried in #3 whether the motor was wound for 440v star, or 440v delta but in star during S/D start. OP hasn't confirmed.

It's possible one of his motors is wound 440v delta, the other star, which might account for the current variation.

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Kirk 720kV
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#13

Re: Big Capacity Motor Drawing Extra Current Than Small Rating

04/30/2015 5:51 PM

The smaller motor is producing more torque at starting. Higher torque translates to higher current.
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