Induction Motor Power Factor with Variable Speed Drive

Question

I know something about pumps, but less about the motors which drive them and am looking for some help in this regard.

The motor in question is a three phase two pole squirrel cage induction motor. I am trying to calculate the power output from the motor based on the supplied current and voltage. To perform this calculation one of the things I need to know is the power factor. The manufacturer has supplied various performance characteristics of the motor including a graph of power factor vs. percentage name plate load (load factor) which I can use. However, I believe the power factor data is specifically for a drive frequency of 60 Hz and, with this in mind, I have two questions please:

1) Given that my motor is connected to a variable speed drive, is the plot of power factor vs. load factor for 60 Hz still valid at other frequencies? Or, to put it another way, is the power factor of a motor running at its name plate load constant with regards to frequency?

2) If the answer to question #1 is 'No' is there a recognised method to correct the power factor curve to other frequencies or is it something which needs to be determined experimentally?

Any words of wisdom would be greatly appreciated.

Answer 1

A variable frequency drive (VFD) will generally try to maintain a constant magnetizing current in the motor with any frequency and load. The results will vary depending on the VFD design and set-up. That will result in an operating power factor that is generally similar to the pf vs load plot provided by the motor manufacturer for rated voltage and frequency sine wave operation.

To calculate output power, motor efficiency must also be considered. That will vary with speed.

You can not accurately determine the output mechanical power from the motor using the method that you propose. If the VFD provides a torque display, that is likely to be more accurate than any determination made by measuring motor current. A power determination using DC bus current of the VFD might also be more accurate. There are clamp-on wattmeters on the market that can accurately measure VFD output power. Of course, that still leaves the problem of estimating motor efficiency. I have some empirical data that might help, but I don't have time to post it right now.

Here is the data that I mentioned above. It is calculated from motor design data, not empirical test data.

I believe this data was calculated in the 1970's by Alberto Abbondanti of Westinghouse, but I can't find where it was originally published. I received it in class notes for a tutorial.

Answer 2

As far as Power Factor of the motor is concerned, I have understood that the motor PF on the load side of the drive is going to be roughly flat at the same value it would be at full load on the fixed frequency chart given by the motor mfr, because the relationship to loading is now tied to speed and as said, the VFD is really only magnetizing the core at a rate commensurate with the required torque. But when Vector technology evolved, that has improved even more because a vector drive can (to varying degrees depending on the technology) separate the flux producing current vectors from the torque producing current vectors and tighten up that relationship to be even better than what the motor may have been capable of without the drive. That is why you often see that a motor being controlled by a vector drive will provide more torque PU of line current than an older V/Hz drive.