Power - Electric Motor

[LSPSCAT]

If two motors of different size are compared in an existing installation - one larger and one smaller should not they both require the same power to perform the work unless there was a great difference in efficiency?

Therefore the current draw would be the same?

All motors are 3 phase / 460 V / 60 Hertz

 

[Skogsgurra]

Power factor also plays a role. A larger motor is operating at a lower percentage of the rated power. Most motors are designed to have best power factor somewhere between 70 and 80 percent rated power. Below that range, power factor decreases and current usually increases.

That doesn't necessarily mean that the larger motor is less efficient - only that cos(phi) is smaller.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

"Power" consumed will be the same, if efficiencies are the same at the operating points. This assumes that load is the same in both cases.

Actual current depends on the power factor as well. Motors tend to have slightly lower power factor at lower % of rated load. But if the ratings are close enough, it should not matter much.

This also apply to efficiencies. You need to look at both the power factor and the efficiencies at the operating points.


Btw, regardless of actual current draw, cables, starters, breakers, fuses etc. have to be sized for the rating of the motor. If cost is a factor, this should be a consideration.

Other thing being equal (or within reason), I would always go with a slightly bigger motor rather than slightly smaller.




Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

If you are doing an actual comparison of two motors driving pumps or fans that "seem" to be similar loads, one factor that may crop up is difference in slip, which will change the power demanded by a pump for a given fixed system (no auto-throttle valve) roughly according to speed^2. And furthermore, if these two pumps happen to be in parallel, I think the load-sharing would be even more unequal.... think worst case the slower pump curve shifts down so when operating in parallel with faster pump the slow pump thinks it's operating near shutoff so carries almost none of the fluid system load (although the motor will still see pump losses).

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[electricpete]

The more general point would be: how do you know these two loads are the "same". That can be a hard determination to make. What are the loads?

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[electricpete]

Also for completeness I'll mention voltage variation is another factor among many to consider... might make your efficiency different from what you're reading off of a data sheet.

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[electricpete]

Correction in bold:
"If you are doing an actual comparison of two motors driving pumps or fans that "seem" to be similar loads, one factor that may crop up is difference in slip, which will change the power demanded by a pump for a given fixed system (no auto-throttle valve) roughly according to speed^2."

should've been

"If you are doing an actual comparison of two motors driving pumps or fans that "seem" to be similar loads, one factor that may crop up is difference in slip, which will change the power demanded by a pump for a given fixed system (no auto-throttle valve) roughly according to speed^3."

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[waross]

You may see a paradox where the larger motor draws slightly more current but consumes slightly less kilo-Watts.
This would be due to power factor and efficiency.
Pete. I suspect that with motors in parallel, the larger motor will have less % load and therefore less slip. This would cause it to hog the load. Load sharing may not be proportional but the largest motor may be expected to carry more load than the smaller motor.
If the slip curves are not very similar, all bets are off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LSPSCAT]

Smaller motor is drawing considerably more amperage - 50%. Loads should be equal but we are looking into it.

I do not have enough specific information on the power factor and efficiencies for the motor curves.

Thanks for the discussion.

 

[electricpete]

Agreed (unless you are suggesting a contradiction to something I said.... I didn't see any contradiction).

For parallel pump operation to determine load sharing one needs to know both motor slip characteristics and pump characteristics. Fluid system load sharing with two pumps in parallel acts in a direction to increase loading differences which would otherwise be created by differences in motor torque vs slip characteristics.

Let's say I have a 100hp motor with 2% nameplate slip and the associated pump draws exactly 100hp BHP at 98% sync speed.

Now connect a 200hp motor with 2% nameplate slip to another identical pump in another identical piping system. Assuming no fluid sytem control valves change position and the fluid system follows a DP~flow^2 relationship, then that 200hp motor will operate at approximately 99% sync speed...and since the pump is about 1% faster than the first, it will draw 103hp. (speed^3 relationship).

Now, cross connect those two piping systems at the suction and the discharge. What happens? We know that before cross connecting the DP of this pump attached to 200hp motor is 2% higher than the one attached to 100hp motor. So we know the 200hp motor is going to send a little more flow into the opposite loop and the 100hp motor is going to pump even a little bit less when we cross connect them. Another way to say it is that the pump connected to 200hp motor pumps a little more and the pump connected to 100hp motor pumps a little less in order to equalize dp. In the end the 200hp motor will draw even more than 103hp and the 100hp motor will draw less than 100hp.

Sorry I got off on a tangent. Blame it on Bill ;-)




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[electricpete]

My last post ("Agree...") was response to Bill

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[Skogsgurra]

LSPSCAT

Is this something that causes problems? There must be some data available, at least motor rated power, rated speed, current drawn, type of load, if motors are connected to same voltage, how you can tell that both motors do the same work? You haven't given us any of these.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

Also knowing what type load are they connected to and how would be useful,


Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

I started to craft a response and began using the term "theoretical" so many times, I came to the same position as Gunnar. Are we having a rhetorical discussion about theoreticals, or is this a real situation with real data associated with it?

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[LSPSCAT]

Both motors are being attached to the same gearbox. Nothing else is changing. Both setups drive a lead screw on a large CNC machine. (Take one motor off and place the second motor on.)

Small motor rated power 15 Hp / Large motor rated power 20 Hp

The smaller motor draws approximately 5 additional amps.

Voltage is the same

Motor Rated Output Speed = 27 RPM

 

[rbulsara]

This still does not cover all basic questions posted so far..

Actual amps, voltage, RPM, and kW (or power factor) of each motor?

25% (or33%) difference in sizes is significant. Perhaps, kW load is more equally shared than their HP ratings. Plus there is always the matter of HP rating stamped on the nameplate vs. actual capability of the motor.

What is the recommendation of the gearbox mfr? To use equal size and probably identical motors, I would guess.

Rafiq Bulsara

http://www.srengineersct.com

 

[LSPSCAT]

I wish I had all this information I just cannot get straight answers right now. The complete data is not published and some of the data I have is apparently wrong from in house testing.

Thanks for responses but in the end there is just too much to guess about.

 

[rbulsara]

Yes, at the end of the day, it is much more economical and far less stressful to follow manufacturer's recommendations and not try to be "cute" by experimenting.

This appears to a self inflicted problem.

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

One time you said smaller motor is drawing 50% more and one time you said 5A more. Would we be correct in deducing that the smaller motor draws 15A and the larger draws 10A?

Motor Rated Output Speed = 27 RPM

This is the output speed of the motor or the gearbox? If it's the motor, then there is obviously something else we should know about this motor, like it has an integral gearbox or it's driven by a vfd?

If you have access to go look at the motor, it would be helpful to go out there and record the nameplate data requested. And let us know what is on the other side of the gearbox (if that's not proprietary).

One more piece of info you might want to check is how balanced are the three phase currents.

Just to add to a long list of possibilities for your consideration (we can't narrow it down without more data);
The motors might be mis-connected. For example if you have dual voltage 460/230 motor it might be connected for 230 in which case it will draw more current than a 460 motor while running. If you have delta-wye choice for starting and you leave it running in delta, the motor winding is seeing a lower voltage and it will draw more current.






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[electricpete]

Actually misconecting the windings can get you in trouble either way (connecting for higher or lower voltage than actual).

Another troubleshooting tool is speed measurement. If you have a vib analyser, you can figure speed from the spectrum.

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