Motor RPM Sizing

[TEGGER]

Qusetion, if a OEM motor is size at 208/230v 1050 rpm. Could you replace it with the same voltage but a 1075 rpm motor. How much is the different between cfm with a 1050 verses a 1075 rpm? Thanks

 

[electricpete]

If it is a centrifugal fan, flow rate will go up in proportion to speed, dp will go up in proportion to speed^2, and power will go up in proportion to speed^3

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[SteveKW]

Most of that RPM change, I believe, is just to throw the homeowner off from doing the work himself! On a PSC motor like that the load would dictate the RPM (just restrict/open the blowers output and you can verify that).

If you were to remove the load completely off of both motors there would be very little difference, if any, in RPM between the two.

So you can replace the motor with the 1075. You just may need to consider, as Pete pointed out, the new CFM output of the blower and possibly (I doubt if you will need to) restrict the output a bit!

 

[pugap]

Don't forget to look at the capacitor, HP, and amp rating of the motor you are replacing.

 

[jbartos]

Suggestion: The electricpete posting is supposed to be supported by adding information to the original posting.
There are additional factors or parameters needed for the correct answer, namely:
1. NEMA Design Code Letter, either A, B, C, D, or E often appearing on the motor nameplate
2. The motor loading, e.g. 80%
3. Motor manufacturing tolerance since the 1075rpm/1050rpm=1.0238 suggesting 2.38% increase only, which is a very small increase. I.e. the motor marked with 1075rpm may actually run slower than the motor with 1050rpm depending on the motor manufacturing tolerance. Also, meters often have a 5% accuracy, 2% accuracy, 1% accuracy, etc.

 

[TEGGER]

I forgot to state that it is a 1/3hp 1050rpm, 208-230v that is used on a Liebert condensing unit with a P66 pressure sensor control that controls the speed of the motor.

 

[electricpete]

fan motor or compressor motor?

Controls the speed (vfd) or switches the motor on/off based on pressure?

I think Steve has a lot of experience with small motors and I believe him most likely the 2.5% change in speed will not cause any problems.



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

... but as always, you have the final responsibility to make sure that what you do is safe.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[jbartos]

Suggestion: Considering the given parameters and the pressure sensor controlling the speed of the compressor motor, the speed difference is tolerably small.