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Associate

Join Date: May 2011
Posts: 43

09/01/2012 1:54 AM

I have replaced a 37 kw, 415V old induction motor which was drawing 40A, 415V, on loading and 30A in decoupled condition.

with a 15 kw, 415V motor which is drawing now 22A on loading and 12A in decoupled condition.

I want to know the actual kw, I saved. Assume PF of 0.85 at MCC bus.

As we know, the power factor of motor varies as per the loading of motor, so we can not calculate kw by simply measuring current.

Is there any thumb rule to calculate kw of motor on no-load condition by measuring its current? or any standard chart is there for motor current vs PF?

I found a chart which indicates 0.35 PF at no load, 0.7 PF on 50% load and .85 PF on 100% load. But my old motor was drawing 50% current on no load!

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Anonymous Poster #1
#1

### Re: Motor running load calculation

09/01/2012 4:37 AM

Well, if nothing else, that was completely humorous.

Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
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#2

### Re: Motor running load calculation

09/01/2012 6:23 AM

Not very much, I wouldn't think. The efficiency of the old motor was lower at the low load, but not vastly. If you're just paying per kWh it would take a long time to get your money back, if the motor didn't need replacing as worn out.

But if part of your tariff is based on kVah, you'll have bigger savings. A meter would give you all the data you need to work it out, can you hire one? It would have been nice to check the old motor too, but you can back-calculate for that as (presumably) the output power to the load is unchanged.

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#3

### Re: Motor running load calculation

09/01/2012 12:19 PM

You don't actually provide enough information to be terribly accurate, but you can make a decent estimate. Forget the unloaded information, that is completely unimportant.

• "I have replaced a 37 kw, 415V old induction motor which was drawing 40A, 415V, on loading and 30A in decoupled condition.

with a 15 kw, 415V motor which is drawing now 22A on loading and 12A in decoupled condition."

Not actually all that unusual really. If you had absolutely no change in your load (which you give zero information on), then we can only assume that your PF and efficiency on the old motor, even when "loaded" was not optimal, which is not that unusual for an older motor. Here is why I can say that.

All we know is that with a 15kW motor, it is pulling 22A with this load. Assuming 4 pole, the FLA should be around 27A. That means this motor is somewhere in the range of 80% loaded now, which means your actual load is around 12kW.

So again, IF THE LOAD HAS ABSOLUTELY NOT CHANGED, then your 40kW motor was pulling 40A when the load was only 12kW, which was only 30% loaded! That would portend a low PF and low efficiency. To whit:

12kW / 415V = 28.9A without PF and eff. If we assume 85% eff, that's 34A without PF, so the PF must have been 36/40 = .85, ergo for that motor to have drawn 40A, the eff and PF was .85, which for an older motor is not that unusual.

Now, what kW did you save? Without knowing the eff of your new motor, there is no way to calculate it. But there is a REASONABLE expectation that a newer energy efficient design motor will be in the range of 95% now. So given all the above assumptions, you can reasonably guess that the 40kW motor 30% loaded was consuming roughly 14.1kW and now your 15kW at 80% loading is consuming roughly 12.6kW. Therefore you have saved roughly 1.5kW when running loaded; not too shabby!

You have also likely improved your power factor, which may have added benefits if you were being penalized.

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#4

### Re: Motor Running Load Calculation

09/02/2012 1:54 AM

Give nameplate data(speed/Amp etc) of both motors. What is the difference between de-coupled and no-load conditions?.

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pnaban
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#5

### Re: Motor Running Load Calculation

09/02/2012 4:17 AM

Just in passing, notice that your incremental load was 10A for both motors. I have to go now but, if no-one else has done the maths before I get back, i'll look at it after morning service (which is actually in the forenoon)!

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#6

### Re: Motor Running Load Calculation

09/02/2012 9:25 AM

JRaef gave an excellent answer to the question you asked, that is how much power you saved. If you replaced the motor to reduce demand on your power lines, then you're done. If you really are concerned about money saved, then the question would have to include energy saved. To answer that, we need to know the times running loaded, unloaded, and off.

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#7

### Re: Motor Running Load Calculation

09/03/2012 2:09 AM

I do agree with the JReaf and very thankful to him. He explains the things very nicely. All his assumptions are matching with my plant condition.

I need more ....

The kw load drawn by 37 kw motor was calculated on the basis of the loading (amps) of 22kw motor.

Is the only way to know the kw of under-loaded motor, like my 37 kw motor, to install energy meter?

Energy meter installed in that section (MCC incomer) shows 90 kwhr reduction per day. Means 90kwhr/24 hrs = about 4 kw reduction in load. I need to run this motor continuously.

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#9

### Re: Motor Running Load Calculation

09/03/2012 11:24 AM

Why don't you give the name plate details of both motors?. This is not a quiz competition.

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pnaban
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#8

### Re: Motor Running Load Calculation

09/03/2012 11:17 AM

746 Watts = 1 horsepower

Old motor = 37KW = 49.6 HP

New motor = 15KW = 20.1 HP

And: Per your new numbers of 22A x 415V x 1.76/1000 = 16KW/746W => 21.5 HP Load

Unless my calculations are wrong the new motor is at least 1.4 HP too small, will not meet the load requirement, is operating in an overloaded condition where the PF will not matter, and your efficiency is lost.

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#10

### Re: Motor Running Load Calculation

09/03/2012 12:55 PM

No. Your formula is leaving out power factor.

you cannot make any calculations of power in an AC motor based only on amps and voltas without factoring in Power Factor somewhere. kW = V x A x PF

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#11

### Re: Motor Running Load Calculation

09/03/2012 1:03 PM

And PF will have been low for the bigger motor running light. Indeed, with the same load at the same speed, the HP and hence the actual kW (not the kVA) will have been the same!

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Posts: 197
#12

### Re: Motor Running Load Calculation

09/03/2012 1:08 PM

Yes, thanks for pointing it out.

I did not factor the PF into the equation as it is actually unknown in either motor's case.

I just wanted to point out that his actual measured running current and voltage values indicate that the application apparently requires more horsepower (KW) than the new motor is capable of producing.

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#13

### Re: Motor Running Load Calculation

09/03/2012 11:34 PM

In the name plate of motors they should give current at no-load,25%,50%,75% load and pf at 25%,50%,75% and 100% of load so that consumer don't need to do guesswork or wrong/approximate calculations.

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#14

### Re: Motor Running Load Calculation

09/04/2012 12:02 AM

Great Idea, but don't hold your breath 'till they do it...

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#15

### Re: Motor Running Load Calculation

09/04/2012 6:25 AM

Yeah, there's only full-load figures on the nameplate, maybe at 2 or 3 voltages and for star and delta connection. He should be able to get the rest of it from motor data sheets though.

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#16

### Re: Motor Running Load Calculation

09/04/2012 8:17 AM

In some countries you may not get motor data sheets if you buy from a dealer. When you order from manufacturer or an appointed dealer only you will get technical data in detail.

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pnaban
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#17

### Re: Motor Running Load Calculation

09/04/2012 9:09 AM

Used to be able to get detailed data sheets from the manufacturer, for the full range for sales purposes, not for a particular one you'd bought. I haven't tried asking recently but I doubt if they're as forthcoming these days. But you might get it from the manufacturer's website.

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