CAD CAM EDM DRO - Yahoo Group Archive

Re: [CAD_CAM_EDM_DRO] AC Speed Controller for Lathe: Meat or Accident?

Posted by Jon Elson
on 2003-09-17 10:47:12 UTC
Abby Katt wrote:

>Swarfer-breatheren,
>
>I seemed to have (accidentally) bought myself a massive big old lathe
>at an auction. When I picked it up however, I found that the motor
>was some horrid 3-phase electical fire with a rats nest in it, so I
>dropped this and decided to just buy a new one of ebay. However: as
>I'm now missing the belt-drive and motor assembly I'll have to make
>this up.
>
>I thought this would be the best place to ask this: I'm thinking
>about building my own AC motor speed controller (with some
>thyristors) and just leaving the motor belted on one setting or the
>other. I was wondering what advice people would have on this? I'd
>have asked this on another forum, but as you all are so terribly
>nice, and experienced with this type of thing I thought I'd ask here
>(and hey! I'll probably convert the lathe to CNC in the future
>anyway. :) )
>
>So what's the deal? Is this good science, or bad science? Will the
>lower voltage mean the motor will turn really slow, but also have no
>torque what-so-ever?
>
No, this does not work. AC motors speed depends on FREQUENCY, not
voltage. Yes, if you turn the voltage down until the motor is insufficently
excited, there will be NO torque! It will hum, and not start.

Because of the starting switch, single-phase AC motors do not take
to speed control well.

> Should I use some other kind of circuit? Or is
>this is all just a terrible idea that's going to wreak havoc on the
>electrical grid for blocks around as I throw rude-pulses at a 2KW
>motor? Should I use a DC motor instead? (I want to do this on the
>cheap, really)
>
>
Your choice is a 3-phase AC motor and a surplus VFD (there are 20-30
of them on eBay at any time) or a DC motor. DC will be hard to come
by, and you will easily burn up several hundred $ getting a DC speed
controller
to work reliably.

>BTW> This is a BIG old lathe! 5foot long and weighs.. well.. the
>tailstock weighs about a third of my weight. :P
>
>
A VFD runs the 5 Hp motor on my 3500 Lb Sheldon 15" lathe VERY
nicely. Hey, you also get braking with a VFD! Stops the spindle in
a couple of seconds, instead of waiting 20 seconds for it to coast down.
(And, I can't even lift PART of the tailstock on it without extreme
exertion!)

Jon

Discussion Thread

Abby Katt 2003-09-17 03:32:50 UTC AC Speed Controller for Lathe: Meat or Accident? Marv Frankel 2003-09-17 05:56:05 UTC Re: [CAD_CAM_EDM_DRO] AC Speed Controller for Lathe: Meat or Accident? turbulatordude 2003-09-17 06:25:05 UTC Re: AC Speed Controller for Lathe: Meat or Accident? Harvey White 2003-09-17 09:04:21 UTC Re: [CAD_CAM_EDM_DRO] AC Speed Controller for Lathe: Meat or Accident? Jon Elson 2003-09-17 10:47:12 UTC Re: [CAD_CAM_EDM_DRO] AC Speed Controller for Lathe: Meat or Accident?