Testing an electric motor below rated voltage 1

[panelmaster]

We have a customer that is testing 10HP 575V fan motors at 460V. I can't see how the motors will even run, but apparently they do. What kinds of problems will they run into?

 

[MiketheEngineer]

I am just a strucutural - but I do know your amperage draw will go way up. You still need the same watts with W = V x A

 

[panelmaster]

Right. I heard speed might be a problem as well.

 

[LionelHutz]

Ya, they might run slower and draw more current. Likely not a big deal unless they are run for a long period of time.

 

[electricpete]

As stated, amps would go up under load, but would actually go down at no load.
Testing at no-load (with blades removed) would not be a problem at all.

Some info to be conidered:
what is the load during the test (or current) in relation to nameplate
as Lionel mentioned, how long
may also be interesting to know if it is DOL start or soft start or vfd.

=====================================
(2B)+(2B)' ?

 

[jraef]

Starting torque varies at the square of the voltage reduction, so at 480V, the applied voltage is at 83.5% of rated, so the starting torque will be reduced to roughly 70% of normal. Running torque will be reduced at the rated voltage drop, so 83.5%. The motor will technically spin at the same theoretical synchronous speed, because the speed has to do with the frequency which I assume has not changed. But because of the loss of torque, the motor is now less capable of keeping the load spinning and you will increase the slip, meaning the RPMs will drop a little, but an increase in slip causes the motor to pull more current until it either accelerates the load back to normal slip speed, or it over loads. If the motor is over designed for the application, i.e. 25% larger than it needed to be, then the loss of torque many not be a problem. Likewise if the motor is not loaded, or the load is reduced commensurately to the loss of torque, it also may run just fine this way.


"Will work for salami"

 

[panelmaster]

This is all great information. Unfortunately I don't have all the details. It comes second hand from one of our technicians. I do know there is no VFD involved, just a motor starter.

We had hoped to sell the customer a test panel that would boost the voltage to 575V from their 460V supply. They decided not ot go with it, for whatever reason. They are a small shop that assembles fans for a large, well known customer. I would have assumed that the end customer being an ISO registered company would require test data. I guess not. Maybe all they require is that it turns!

Thanks for all the help. This is a great resource.

 

[rhatcher]

If they are testing the motors under load at reduced voltage, then a serious overcurrent will result. If the test is a duration test (30 or 60 minutes?), then the motor insulation will be compromised. I would worry about this with respect to motor life and possible warranty issues.

In addition, the slip will be increased and the fan output will be reduced by a cube of the speed change. These circumstances seem to be a poor substitute for an ISO vertified load test.