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Power-User

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VFD RPM Scale

12/14/2020 4:57 PM

I noticed that some of our engineers are setting the RPM scale to match the motor nameplate, example they will use a scale of 4-20mA on a range 0 to 1475. My belief was 4-20mA should match the drive setting, which usually is 50Hz, ie 1500RPM.

Am I correct, if yes ,is there an easy way to explain to them why it is incorrect to use nameplate RPM on the scale. I tried to explain slip, but it seems most people forget this when it comes to real scenarios.

Also note PLC setting is set accordingly.

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#1

Re: VFD RPM scale

12/14/2020 7:55 PM

I think they are right. If the top frequency of the VFD is 50 Hz, at 50 Hz, the induction motor turns at 1475 RPM, not 1500 RPM.

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#2
In reply to #1

Re: VFD RPM scale

12/14/2020 8:02 PM

I always believed that nameplate RPM is considering load, if zero load then RPM will be very close to 1500, I will go back to my text books.

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#3
In reply to #2

Re: VFD RPM scale

12/14/2020 9:36 PM

You are correct, nameplate RPM is under load, where the torque required is greater and slip is greater.

https://www.machinedesign.com/motors-drives/article/21828327/basics-of-motor-nameplates#:~:text=The%20speed%20at%20which%20rated,speed%20rather%20than%20synchronous%20speed.

Under these conditions, 50 Hz will result in nameplate RPM. So it boils down to RPM unloaded or RPM at rated load.

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#5
In reply to #2

Re: VFD RPM scale

12/15/2020 4:45 AM

Someone needs to explain the value of running a motor with no mechanical load on it, then.

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#4

Re: VFD RPM scale

12/14/2020 11:31 PM

Some VFDs can be set up to provide slip compensation, so that the output frequency is higher than synchronous speed. So when you tell the motor to turn at 1500rpm, it will adjust the frequency according to the load to turn the shaft at 1500rpm.

This assumes you have no actual motor shaft speed input, a sensorless vector drive.

You may want to check the source of the analog output, to be sure you are forwarding the actual output frequency with no slip compensation, or perhaps actually is slip compensated value. Then your scaling would align correctly with the value the drive is providing. It’s interesting to note that the frequency span could be the full range, which would support your contention, 0 to synchronous speed, or 0 to maximum frequency output, if slip compensation is selected. The actual values forwarded to the analog outputs are poorly defined in some equipment, and may require deep technical reading to determine what it is.

With conventional VFD setups, it is typical to scale motor speed output to the full load slip speed, if there is no slip calculation. That gives you the lowest typical value you might expect. There are few sensorless vector applications where the slip difference is worth noting in the operation of the process. A tachometer is added where the actual shaft speed is really important.

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#6

Re: VFD RPM Scale

12/15/2020 5:49 AM

Surely the VFD would be scaled as 0 to 50Hz or 0 to 100%, the RPM is irrelevant as it will be what it is and anyway I have calibrated many VFDs in a production environment and the operators have no wish to know the RPM of the drive only that they are requesting 100% or some lesser value and they appear to be getting what they requested. The RPM can vary due to the load but all they care about is that at 100% they get so many TPH, LPS, or other value from the plant.

If the RPM of the drive is needed than use a feedback but most operators don't give a fig if the motor is running at 1450 or 1425 or 1475 nor do some even understand slip and other things, only that they are getting an output they expect from the plant. Some have only recently had their tails docked and learnt to walk upright!

After all anything can be Creatively "calibrated", in a PLC, to give the RPM the engineers think should appear. After all I have had 30 years of production PLC interfacing to the real world and understand the perceived values and what is thought to happen.

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#7

Re: VFD RPM Scale

12/15/2020 11:35 AM

<...50Hz, ie 1500RPM...>

Doesn't that rather depend on the number of poles in the motor in question?

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#8

Re: VFD RPM Scale

12/16/2020 5:09 AM

Most users don’t give a monkey’s about motor speed. They care about what the motor is driving.

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#9
In reply to #8

Re: VFD RPM Scale

12/16/2020 5:29 AM

True story, however this data is not for user rather maintenance people.

Example a break in coupling will show close to sync speed.

When motor is replaced, rpm data will always be correct , even if Vfd parameters are not updated.

Trending correct rpm with amps is useful when trouble shooting

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#10
In reply to #9

Re: VFD RPM Scale

12/16/2020 9:05 AM

What has any of that to do with speed scaling?

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#11
In reply to #9

Re: VFD RPM Scale

12/16/2020 1:28 PM

It’s more important to accurately transmit the true value of the parameter being supplied by the drive, which has one, and only one scaling value, not up to the user to set up.

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#12

Re: VFD RPM Scale

12/19/2020 9:35 AM

We use the motor plate rpm. While you are correct that at no load, the rpm will be close to 1500, for sure you're running your motor with load so the actual rpm will be closer to 1475 than 1500.

You also mentioned that you do this for maintenance purposes. If the coupling breaks, the speed will pick up but the drive won't show that. It will continue running at whatever frequency you input to it. It would be better to monitor the current or power.

Besides, if the coupling breaks, there will be other signs that it broke (loss of pressure, loss of flow, etc.). You won't know that the coupling broke but you will know that something's wrong. If you were monitoring the current, however, you'll know that something's wrong on the motor side. (",)

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#13
In reply to #12

Re: VFD RPM Scale

12/22/2020 9:13 AM

What's 25rpm among friends, eh?

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#14
In reply to #13

Re: VFD RPM Scale

12/22/2020 7:10 PM

Yes, thanks for the discount (",)

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