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Question about using VFD to reduce motor rpm to spindle speed. Torque/HP


Hot Rolled
Sep 3, 2014
Atlanta GA.
I have a several machine machines I'm in the process of restoring right now. All, of course, were line shaft driven when born and all came to me with various contraptions to power them with an electric motor.

I'd like to remove these contraptions and go with the most direct (simple) method of getting a 1725 rpm motor to work. The machines have various speed requirements but I think my question is generic enough to apply to all. I'll use the Hendey 1B Universal Milling machine as an example. While I don't know the exact spindle speeds for the 1B I saw that Hendeyman posted the speeds for the 2B ranged from 13 rpm to 409 rpm. The largest step on the cone pulley is approximately 11" and the drive pulley on the motor is 4". That reduces the 1725 rpm from the motor to 627 rpm.

My plan is to use the VFD to bring that down to the working range of roughly 100 rpm to 409 rpm by varying the frequency from 10Hz to 40Hz. I know that an electric motor has constant torque and the HP varies with the speed from 0 to its max HP at the rated rpm. What I'm not so sure about is will I have the same ability to remove metal when running the motor at 10Hz (1/6th the rated speed) vs 60Hz?

Part of the reason I'm asking is, like normal, I tried this before I sought advice and it didn't work. I tried to drive my power hacksaw (which wants 150 rpm) with a 1.5HP 3450 rpm motor, a pulley reduction to 690 rpm and then making up the rest with the VFD. It just didn't work. On the cutting stroke the motor would stall. I didn't get any error code from the VFD, the motor just sorta stopped and hummed. I pressed "stop" pretty quick. I tried this multiple times and tried to lift the saw handle some to help it out but it just couldn't do the job. I don't know if it is a bad motor, a motor that doesn't like speed reduction or if my idea is just flawed.

L Vanice

Feb 8, 2006
Fort Wayne, IN
I think it is best to start out with a three phase motor with a lower than normal synchronized speed and possibly a greater than needed HP if you plan to use a VFD. As you know, 1800 and 3600 are the most common speeds for induction motors. But they make 1200 and 900 motors with nominal speeds of 850 or 1150 or so RPM. These motors can safely be run at higher speeds by dialing the VFD to 120 Hz. If you start out with a rating of 1 HP at 850 RPM, you still have .5 HP at 425 RPM (30 Hz.). Motors do not like to run at very low frequency, as you found with the hacksaw.

I used to search eBay for low speed three phase motors of 1 HP because they are ideal for VFD's with 120 V single phase input. Pretty simple to plug into a 120 V outlet instead of running a 240 V line to a machine.