Affinity laws and PMMs

[The Hobbit]

Tinkerers and those of technical knowledge. I come to you with a question and hopes of education.

For work we are testing a cooling tower equipped with permanent magnet motors. The motors are allegedly 10hp/60hz/480v units. They are being ran through a vfd to control speed and we are also running lower hz to simulate lower hp motors to collect data across the product range.

typically when we have done this with induction motors it is a relatively simple equation based on fan affinity laws.

Desired hp/ rated hp = ratio
cube root of ratio * 60 hz= required hz.

When following this equation for the PMM, the output hz results in a significant decrease in power ie 50% instead of the 25% we are looking for.

So does anyone here have experience applying affinity laws to PMMs and able to assist me in “derating” this motor to the desired hp?

thanks

[XEMON]

For best precision, put a power analyser on the incoming power and adjust the vfd to the desired power consumption of the motor.

[flyweightnate]

I know a ton about fan affinity laws so i came here... and i know a lot less about PMMs.

I got stuck in the past with the V/F mapping on our VFD. I wasnt aware that, when running off speed, the VFD was also changing voltage. (Imagine my surprise when the 440V service was only feeding 108V to the motor, because I was at 15hz. Suddenly the results made sense.) It was a Yaskawa, so the documentation was decent, but more importantly after running the motor tune, it provided an analog output that was a very accurate calculation of motor output power.

The new guy got bit when he put a different motor on the same drive and his performance test data was all a mess... he forgot to tune to the new motor.

See what your VFD can do for you, and if it doesn't do your math, follow Xemon's advice. At least then you'll know how much power is going IN, and it's just motor losses to figure out.