Power - Electric Motor
Power - Electric Motor
(OP)
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
Therefore the current draw would be the same?
All motors are 3 phase / 460 V / 60 Hertz





RE: Power - Electric Motor
That doesn't necessarily mean that the larger motor is less efficient - only that cos(phi) is smaller.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Power - Electric Motor
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
RE: Power - Electric Motor
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(2B)+(2B)' ?
RE: Power - Electric Motor
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RE: Power - Electric Motor
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RE: Power - Electric Motor
"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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(2B)+(2B)' ?
RE: Power - Electric Motor
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
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"Why not the best?"
Jimmy Carter
RE: Power - Electric Motor
I do not have enough specific information on the power factor and efficiencies for the motor curves.
Thanks for the discussion.
RE: Power - Electric Motor
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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RE: Power - Electric Motor
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RE: Power - Electric Motor
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Power - Electric Motor
Rafiq Bulsara
http://www.srengineersct.com
RE: Power - Electric Motor
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RE: Power - Electric Motor
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
RE: Power - Electric Motor
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
RE: Power - Electric Motor
Thanks for responses but in the end there is just too much to guess about.
RE: Power - Electric Motor
This appears to a self inflicted problem.
Rafiq Bulsara
http://www.srengineersct.com
RE: Power - Electric Motor
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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RE: Power - Electric Motor
Another troubleshooting tool is speed measurement. If you have a vib analyser, you can figure speed from the spectrum.
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RE: Power - Electric Motor
The basic question originally was:
"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?
(The current draw for all three legs is essentially equal when testing either motor)
All motors are 3 phase / 460 V / 60 Hertz"
I believe this basic request has been answered:
rbulsara (Electrical) 14 Oct 10 14:33
"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.
Thank you
RE: Power - Electric Motor
Start with basics that is with the gearbox manufacturer's recommendations as to what size and type of motors to use on their gearbox.
Your information is inadequate and not very clear. Nor is the circumstances of how you got to the "problem". Was there ever the situation when there was no problem before this happened?
Rafiq Bulsara
http://www.srengineersct.com
RE: Power - Electric Motor
Yes, the power used depends on the load, not on the motor size. In your installation, either motor should use the same power.
Current: There are a number of reasons why the current draw may not be the same.
5 Amps more doesn't tell us squat. For a motor application, there is a huge difference between 2A and 7A, 5A and 10A, and 15A and 20A, even though all share the characteristic of 5 Amps more.
That said, the load on the motor driving a lead screw is usually much less than the rated load. The motor is often sized to provide enough breakaway torque and runs at less than full load.
To that end, the original motor may be a design D and the replacement a design B. This would easily explain the lower amps for the larger motor.
BUT, a 20HP design B compared to a 15HP design D would have a disproportionate increase in starting current.
It may also have less breakaway torque which is what you really need.
It is seldom a good idea to replace a properly applied design D motor with a design B motor.
I understand the frustrations when you are not able to visit the field but must rely on second hand information.
Bill
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"Why not the best?"
Jimmy Carter