VSD Allen Bradley 753

Please include following information:
- Drive size (kW or HP)
- Motor size (kW or HP)
- How is the setpoint set (local HMI panel of the drive, analog input, preprogrammed speeds, fieldbus...)?
Is the feeedback speed you mention the measured shaft speed (using a mechanical or optical device) or the Hz (output frequency) displayed by the drive?
Was there a specific reason to switch to scalar mode? Normally unless there is a good reason to no set vector mode scalar mode is avoided.

Use slip compensation and/or analog output offset or scale, their function should be listed in the parameters section (read the manual).

Hi,

As Yahlasit eluded to, it is motor slip. Running a VSD in V/Hz without any feedback or compensation will have some value of slip. I would think 3% would be about right. Tuning the drive in Sensorless Vector will not really provide benefits in V/Hz mode. Why did you not leave the drive in Sensorless Vector mode? I have found the PF753 easy to setup and tune, with the help of the software tools, and will good speed regulation with our plant.

Regards, Trevor.

At Tooz says this is due to motor slip. A typical 4 pole motor on 50Hz should run at 1500rpm typically runs at about 1485rpm. This slip is all related to the torque from the motor. You say sensor less and then mention feedback. Upshot is without a sensor you do not have a true indication of speed - you do not have feedback.

Most VSDs contain a 'model' within them so that when you program them it knows that at 50Hz the motor should be turning at 1485RPM. Without feedback this is no more than an assumption with knowledge gained from the model.

Just about slip?

Slip percentage depends on the motor but for an asynchronous motor 3% slip (at full load) corresponds to a relatively small nominal power, i.e. a couple of kW (IIRC roughly around 2 - 3 kW). Slip at full load decreases with motor size.

Check the rating plate, the speed is usually mentioned. Also check the current and voltage in the VFD display (measuring with a multimeter can be misleading as you need to measure TRMS values, amount of measuring error depends on the used multimeter).

I asked my question in my message above because in some VFD you can access different speed values within the digital control loops.