# Rpm



## frenchelectrican (Mar 15, 2007)

Julius793 said:


> Being that I am relatively unfamiliar with vfd's I would like to know if I set my VFD with all the nameplate ratings of my motor it will control, will it be able to give me the RPM or will I need in additional sensor to measure rpm?


It depending on what type of VSD you will set up and what kind of load it will be driven .,,

some appacations you will need to add a speed sensor(s) depending on how tight you want to keep the motor speed reguation you want to set it.


in most case with basic set up AFAIK.,, just dail in the HZ on the control panel screen and most case it will pretty much the same speed as DOL ( direct on line or non vsd )

Just hang on I know couple of our members are well verised with VSD so they will reply to ya too.


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## glen1971 (Oct 10, 2012)

One thing to remember, is that the VFD will display the motor RPM, not the load RPM.. Several clients with submersible pumps on oil wells, care more about the shaft to the pump speed than the motor.. Most of those had a parameter or two if I remember right, to adjust for the shiv size..


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## CWDElectric (Jan 28, 2017)

Any quality VFD will provide you with a speed readout on a local display that can also be programmed to an analog output, if supplied, that can be utilized as required, be it a local display or an entire control system. Grab the manual, dig deep, the answers are there somewhere. Good luck.


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## V-Dough (Jul 22, 2014)

The VFD will give you a frequency in Hz, so so you can calculate, the shaft rpm from that. As all asynchronous induction motors have some amount of slip, the actual shaft speed might be slightly lower. If you need the RPM to be dead on accurate, you will need to get an encoder on that motor.


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## Flyingsod (Jul 11, 2013)

If Hertz is nameplate then RPM is nameplate. Unless your vfd can magically change the number of poles in the motor.

Sent from my C6725 using Tapatalk


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## JRaef (Mar 23, 2009)

It will depend a lot on the quality and sophistication of the VFD in question. Some will provide you with internal math capabilities that allow you to display the actual RPM instead of the Hz, cheaper ones will not.

Also, it will depend upon what type of "control method" you use in the VFD. With a basic V/Hz method, the simplest and easiest to use, the VFD will show you what it is TELLING the motor to run at, but it has no idea if the motor is ACTUALLY turning at that speed, because the load can slow it down and the VFD doesn't really know that. This is what glen1971 was referring to (I believe).

If you use what's called "vector control", then the VFD is getting a feedback signal from the motor telling it the actual shaft speed, so what it displays as the speed is going to be very accurate. There are different levels of vector control, some requiring a shaft encoder on the motor for that feedback in order to get the best accuracy, i.e. 0.001% accurate. But for most of us, what's referred to as "sensorless vector control (SVC) is good enough, usually about 0.1% accurate. For this to work, you must first buy a VFD that is capable of it, then you must "tune" the VFD to the motor. Most SVC capable VFDs will allow for an "autotune" procedure to make that easy, but you can't ignore that or they don't work right.

Bottom line: if you buy an SVC capable drive that has a feature to manipulate the speed display, you can get a very accurate display of actual motor RPMs. But that will not be the cheap VFDs.

If you already bought a VFD, post the make and model, I can tell you what it's capable of pretty quickly.


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