DC vs AC motors for wood lathes?

[Steve Mathews]

The Woodfast M408H lathe that I just acquired has a DC motor with variable speed control. My other lathe, Oneway 2436, has a 3-phase motor with a VFD. It seems the later is more common with current lathes. What are the advantages/disadvantages with these setups?

[Richard Coers]

Not an expert, but from what I understand you get a straighter hp curve with the VFD vs a DC controller when you slow the motor down. Keeping motor rpm as high as you can with a DC controller is critical as hp drops off dramatically when you get to the minimum motor rpm. So roughing out a big bowl is much easier with a VFD

[Bill Howatt]

I don't think this is a question that has a specific answer because both the AC and DC motors actual operation also depends on their controller design and setup and even the design of the motor. The DC motor actually has more torque at very low RPMs compared to the typical AC motor but the AC motor supplied with a vector VFD can significantly improve low speed torque. Also, AC motors don't have brush wear maintenance problems.
Lathes with VFDs commonly have at least 2 pulley ranges to help with the low speed torque issue.
In "the older days" variable-speed likely mandated a DC motor since VFDs weren't that common and very expensive. VFDs are much cheaper now so now it likely makes economical sense to use them.

[Reed Gray]

I have never had a DC motor on any lathe till now, when I picked up a Rikon 12 inch mini lathe. Supposedly it keeps torque better at low rpm. The old DC motors used to need to be fiddled with to get them to run correctly, and/or the way you wanted them to, but that was 25 years ago. 2 things I don't like about the Rikon. One is the on/off button is a twist to turn on, and a push to turn off. I was used to the old pull for on, and push for off. Broke the knob. The other thing I don't like about it is I want instant speed. With the Rikon, it has a 3 or so second delay before it turns on. All of my other lathes have been AC motors. Love the 3 phase variable speed set ups.

robo hippy

[Steve Mathews]

The first thing I noticed with the Woodfast M408H DC motor was a slight lag when changing speeds. Thought it was going to be an issue but not so much after using it for a short while.

[roger wiegand]

I think the primary difference is that a three phase AC motor with a VFD is a lot cheaper than a similar size DC motor. I've used both, I couldn't say there was any really significant difference in use. My impression is that the 2HP Leeson DC motor on my old Conover lathe had a lot more torque than the 3 HP AC motor on my AB. But that might be a perception artifact of turning much bigger logs on the AB.

[tom lucas]

There seems to be a trend toward more DC motors on lathes. Not sure if driven by cost/simplicity or "product improvement". I suspect the former is the driving reason. Seldom see a "product improvement" just for the sake of the consumer. Advances in servo motors is likely also contributing to use and acceptance.

[Reed Gray]

The Nova DVR is very interesting. Don't know a lot about the type of motor, but it has surprising torque for a motor that is so small and runs on 110. Also interesting is the servo motor on the Harvey lathes. Another small but powerful motor. I haven't had a chance to play on one yet. For me, the ultra slow speeds are essential. I need speeds of 10 rpm or so for sanding my warped bowls. Much above about 15 rpm, and I can't keep the abrasives on the wood. This seems to be a problem for most lathes. The PM 3520A would go down to almost zero before it turned off. Now, they turn off at 50 rpm. Way to fast to sand once turned bowls. This seems to be a problem for the DVR and I don't know about the Harvey lathes. Torque is no issue at the very slow speeds since I am only sanding.

robo hippy