VFD Torque Question

[Hank Knight]

I downsized my lathe from a Conover to an older English Myford ML8 to free up some space in my shop. I changed out the motor on the Myford this past weekend and installed a new 1 hp Westinghouse DC motor with a TECO VFD. So far I love it - very quiet and smooth. At low RPMs, however, the torque drops off pretty dramatically. I can stop the rotation with my hand on the hand wheel. Is this characteristic of the VFD/DC setup? I thought these motors were supposed to maintain high torque throughout the speed range.

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PLEASE DISREGARD THE QUESTION. I'm new to VFDs, so I did some research and found the answer to my question above. Should have done it before I posted. I apologize fir my laziness!

[robo hippy]

So, what was the answer? I know the older DC motors had problems with torque at low speeds, but the newer ones seem to have gotten away from that at least some what. Was it just programming the VFD?

robo hippy

[Alan S]

I think the answer is that the torque remains pretty much constant, but as the speed decreases, so does the horsepower, which is proportional to torque times speed.

Edit: I was referring to 3 phase motors. DC may be different.

[AHill]

I think the answer is that the torque remains pretty much constant, but as the speed decreases, so does the horsepower, which is proportional to torque times speed.

Edit: I was referring to 3 phase motors.  DC may be different.

Torque is proportional to speed squared.  HP is proportional to speed cubed.  A disadvantage of VFD motors at low speed is they generate a lot of heat.  Too much time at low speeds, and you'll reduce the life of your motor.

One advantage of my Nova DVR is the motor retains its torque at lower speeds.

[MichaelMouse]

It's the reason lathes fitted with VFD's usually want 1/3 or so more HP rating than their mechanical brothers.  Distances between frequency peaks increases - distance between nulls does as well.

[Hank Knight]

Alan nailed it. With a common AC VFD, the horsepower output of the motor decreases as the speed decreases. Recently, technology has advanced and produced the AC "sensorless vector drive" that helps boost horsepower as motor speed decreases. The boost does not eliminate the problem, but it reduces the rate of decrease, so the horespower drop is not directly proportional to the decrease in motor speed as it is with a common VFD.

I helped my problem a little bit by putting a smaller pulley on the motor and using the largest diameter sheave availabke on my spindle. This allows me to keep the motor speed a little higher than ran with a larger motor pulley.

[Spence30]

I had a Powermatic 90 lathe that I raised up and put a VFD on it. I fixed this problem by using a 3hp motor instead of 1 hp and then got a teco 3hp VFD never had a problem after that. This is also the reason I took the VFD off of my Powermatic 1150 drill press lost to much torque.