Display RPM of a drive, convert from frequency how to do

Hello,

I have some drives which are controlled by pressure transmitters, PID controllers etc..
I'm a bit confused about the frequency of the drive and how to convert it to RPM..

I send a control word with a speed reference 4000Hex = 100%, 0HEX = 0%. It affects the frequency. But how can I convert this to the RPM of the motor.. I have the motor nominal RPM, but the drive calculates the RPM by itself?

Since I don't have a lot of experience... Is RPM even used? or is it all about setting the correct frequency --> If an operator set the SP at 50%, the drive will speed up to 50% of the maximum frequency... This could be 50Hz or 70Hz... But what does it actually stand for?

Hope someone can give me a clear answer,

Thanks

It depends on the drive control mode. Most ordinary drives operate as you describe, controlling only frequency. It is entirely up to the motor to convert frequency to rpm.

The formula, rpm = 120 x frequency/poles easily gives you the synchronous speed for any motor assuming you know the number of poles wound into the stator.

However, that is synchronous speed. The motor only runs at that speed when completely unloaded. As the motor loads, the rotor "slips" more thus reducing its speed until, at full nameplate load, the motor shaft has slowed to the nameplate speed. For example, a two pole motor running on 50hz power will run at 3000rpm at no load. If the nameplate says 2940rpm, then at full load the speed will be down to 2940rpm. The 60rpm speed reduction is called slip and is present in the motor at any frequency depending upon the torque loading on the shaft.

Again, for an example, let's take the same motor and run it at 30hz. Using the above formula, we find that the synchronous speed is 1800rpm. At no load at this frequency, this motor will now turn at 1800rpm but, when fully loaded it will slow by the same 60rpm slip to 1800 - 60 = 1740rpm.

Some drives are capable of running as speed regulators rather than the above frequency regulators. In that case, the motor shaft speed must be found either by direct measurement or by estimation. The speed command on these "vector" drives actually represents speed, not frequency. The drive still uses frequency to control motor speed but the control is speed so the drive simply uses frequency as a means of holding shaft speed to the setpoint. Of course, the drive can only do this accurately if it knows exactly the motor nameplate data.

Hope this makes drive control a bit clearer.

If you don't know the number of poles your motor has then use this rule of thumb for three phase induction motors:

• If the nameplate speed is about 3000 rev*min^-1 at 50hz then its a two pole motor.
• If the nameplate speed is about 1500 rev*min^-1at 50hz then its a four pole motor.
• If the nameplate speed is about 1000 rev*min^-1at 50hz then its a six pole motor.

For simple code, use a constant c = 120/#poles so that speed = (c*freq)-slip

------------------------------------------------------
Just in case any North American readers may need it, I'm tacking on the following for reference.

• If the nameplate speed is about 3600 rev*min^-1 at 60hz then its a two pole motor.
• If the nameplate speed is about 1800 rev*min^-1at 60hz vthen its a four pole motor.
• If the nameplate speed is about 1200 rev*min^-1at 60hz then its a six pole motor.