# Thread: Measuring AC motor RPM for line side frequency

1. Junior Member
Join Date
Oct 2009
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6

## Measuring AC motor RPM for line side frequency

Maybe I haven't had enough coffee this morning but I am stuck in a quandary this morning.

I am thinking of ways to measure the RPM of a 50hp AC motor without using a shaft encoder or any "downstream" measurement device. Ideally, I would like to have it on the line side of the power to the motor where it is less likely to break/fail/give faulty readings (stupid encoders...). I am thinking, if I measure the AC line frequency of one of the legs (3phase, 480VAC, reference 60Hz) then I can calculate out the RPM easily. Problem is, the only frequency transmitters I have found so far will only handle max 240VAC. So naturally, my thought is RELAYS! Everyone loves relays! But is there a relay that will open/close at the same frequency of the AC frequency across the coil (I have not tried to do this before, and my experience in motors is limited)?

Thoughts?

2. Originally Posted by CKMEYER
Maybe I haven't had enough coffee this morning but I am stuck in a quandary this morning.

I am thinking of ways to measure the RPM of a 50hp AC motor without using a shaft encoder or any "downstream" measurement device. Ideally, I would like to have it on the line side of the power to the motor where it is less likely to break/fail/give faulty readings (stupid encoders...). I am thinking, if I measure the AC line frequency of one of the legs (3phase, 480VAC, reference 60Hz) then I can calculate out the RPM easily. Problem is, the only frequency transmitters I have found so far will only handle max 240VAC. So naturally, my thought is RELAYS! Everyone loves relays! But is there a relay that will open/close at the same frequency of the AC frequency across the coil (I have not tried to do this before, and my experience in motors is limited)?

Thoughts?

The drive that is controlling the motor should have some monitoring capabilities.

3. Senior Member
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Sep 2005
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Greenville SC
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Originally Posted by mcclary's electrical
The drive that is controlling the motor should have some monitoring capabilities.
+1. And a small CPT will give 120 from 480. If you are planning to exceed 60Hz, a 400Hz rated CPT (there is no load) may be the better choice.

4. Originally Posted by GeorgeB
+1. And a small CPT will give 120 from 480. If you are planning to exceed 60Hz, a 400Hz rated CPT (there is no load) may be the better choice.
Except that a 400Hz CPT may saturate at 60Hz or below. Do transmitters require relatively high input voltage or could you find one that works with a current transformer?

5. What's the purpose of this?

If you are measuring the line side frequency, that isn't really going to change is it? If you are down stream from a VFD, no inexpensive relay or transformer is going to be able to accurately read the PWM output of a VFD. But as mentioned, if you have a VFD, it should be able to give you an output signal that will tell you the motor speed, either as a percentage or a hard number based on motor parameters you program into it.

6. Senior Member
Join Date
Oct 2008
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673
As Jraef mentioned it would be extremely difficult if not impossible to measure the frequency at the ouput of the VFD with some sort of meter or relay. (Except for really expensive ones). The best way as mentioned is to look at the output frequency on the drives display.

Even with knowing the drives output frequency (measured or displayed on drive) this will not necessarily give you the rpm's of the motor. The motor is going to have slip which will vary with loading. Therefore the actual speed or rpm's of the motor will be a function of both the output frequency from the vfd and the slip in the motor depending on the load

An encoder on the other hand is measuring the actual shaft speed so it is measuing the actual motor speed with the slip and thefore is a function of both frequency and motor slip. Depending on what you need this for you may be able to deal with just looking at the drive output frequency or you may need to install an encoder if tigheter control is necessary.

7. Originally Posted by CKMEYER
Maybe I haven't had enough coffee this morning but I am stuck in a quandary this morning.

I am thinking of ways to measure the RPM of a 50hp AC motor without using a shaft encoder or any "downstream" measurement device.
What is there, if anything, between the supply and the motor?

8. gar
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Besoeker:

That is my question as well.

CKMEYER:

A piece of retro-reflective tape on the shaft. An optical RPM gage that works with the tape.

How often do you need RPM information, and why do you need it?

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