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

 

[LSPSCAT]

It is not self-inflicted or experimentation for the sake of experimentation. It is an attempt to identify the problem using limited resources of what is available. Limited data has been provided by the motor manufacturer therefore it is difficult to have all of the information. In addition some of the data has been provided has been found to be definitively incorrect.

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

 

[rbulsara]

LSPSCAT:
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

 

[waross]

A couple of general statements that should apply.
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
--------------------
"Why not the best?"
Jimmy Carter