If I know Hz input of a motor, how do I know RPM?

[Watco]

Hello. We just installed a variable speed drive to our 1150 RPM motor. The drive has a digital readout that reads Hz. We can vary this between 0 and 60. How do I know what my resulting RPMs are for the motor?
Kevin.

 

[PwrCntEE]

You need to know more information: motor type and number of poles, etc.

Assuming you are talking an induction motor,
Syncronous RPM = (120 * Frequency) / (# of Poles)

120 is a constant

example: RPM = 120 * 60 / 4 = 1800 RPM

 

[pablo02]

Check the motor name plate -- the speed listed should correspond to the 60 Hz value (the number of poles dictates this as mentioned above -- the nameplate will adjust for any slip, instead of 1800 rpm, it may state 1750 rpm, as an example)... once you know that, you might be able to reprogram your display to show rpm (if it set up to do so)...

 

[rbulsara]

If you combine the above two answers, you will find that you speed varies proportional to the frequency.

So it appears that 1150 RMP is at rated frequency (50 or 60 Hz.

I assume the motor is rated for 60 hZ, then ,as an example, at 40 hz the speed should be approximately
(40/60)*1150 = 767 RPM. This of course assumes that the slip remains constant over the range of speed involved.

 

[jbartos]

Suggestion: The motor appears to be 60Hz since
3600/1200=3pole
and the slip is (1200-1150)/1200=0.04

 

[ScottyUK]

jbartos,

Do you mean 6 pole motor? As in three pole pairs.

 

[jbartos]

Comment on the previous post: Yes, 3pole=3pole-pairs. I see it often stated as 3pole meaning 3pole-pairs since the 6-pole and digit 6 requires a multiplication by 2; it means an additional explanation.