DC vs AC Motors

[Bob Jeppson]

I have been reading some threads about DC and AC motors and would appreciate confirmation on my understanding of both.

DC motors offer variable speed control at a lower price though you sacrifice torque on the lower speeds. You find these more on the mini lathes

AC motors (220v) with inverters are more expensive, offer variable speed control and you have good low end torque. You find these on the larger lathes.

Are my assumptions correct?

Thanks,

Bob

 

[MichaelMouse]

Your assumptions are not unique, but they're not necessarily correct. http://www.clrwtr.com/Leeson-Motor.htm

http://lancet.mit.edu/motors/motors3.html#tscurve

AC motors are a bit different both in their controls and curves. http://www.reliance.com/prodserv/motgen/b7097_2.htm

Name of the game is to use the mechanical selection features - pulleys - to help keep the motor in its best speed range.

 

[john lucas]

I don't know the math but I've turned on a lot of lathes with both DC and AC variable speed. It's not even close in terms of the power you have at low speeds. I put a 1 1/2 hp DC motor on my Nova 3000. At low speeds even on the small pulley I could almost stall it with my hands. I put a 1 horse 3phase AC motor on with a variable frequency drive and would leave the belt on the middle setting. I could barely stall it using extremely heavy cuts at the slow speeds.
I spent 3 days demonstrating bowl turning on a jet mini with 1/2hp DC motor. At the slow speed of 500 rpm the cuts had to be so light I didn't think I would ever get the bowl round. I switched to a Jet mini with non variable speed and at the slowest speed of 500 was able to take at least a reasonable cut.
I'm probably wrong in my wording of this but DC motors vary the voltage when you dial them down and you lose lots of power. Variable Frequency Drives vary the frequency but leave the voltage closer to full power. You also have the advantage of having electronic braking which is really beneficial when turning large items.