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Adding Speed Control and/or Reversing Switch to Older Lathe

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Forum topic by TimH posted 03-26-2009 10:35 PM 7065 views 0 times favorited 16 replies Add to Favorites Watch
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TimH

37 posts in 2999 days


03-26-2009 10:35 PM

I have and older lathe with a 1 hp single phase motor. Is it possible to add a speed control to it and/or a reversing switch? Thanks for the input!


16 replies so far

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Big_Bob

173 posts in 3176 days


#1 posted 03-26-2009 10:46 PM

Yes there is you need a 3 phase AC motor and a variable frequency drive (VFD) with a controller. The other option is a DC motor with a controller. I like the AC motor with a VFD. That being said it is expensive and you need to study up on this so you do not spend a lot of money on the wrong things.
If you shop around on places like eBay and buy the right stuff it will still cost you around $500.

If you are not comfortable doing electrical work and you don’t have a really good lathe I would suggest just looking at buying a new lathe.

-- Bob Clark, Tool Collector and Sawdust Maker

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marcb

768 posts in 3140 days


#2 posted 03-27-2009 03:30 AM

Depending on the VFD design they loose torque pretty quickly on the low end of the speed range.

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TopamaxSurvivor

17676 posts in 3143 days


#3 posted 03-27-2009 04:51 AM

Very few single phase motors are reverseable. If yours happens to be, the reversing swith will probably cost $100 plus as it is a somewhat complicated switch. The others have pretty well covered the variable speed issue. You can buy a 3 phase variable frequency drive, feed it with single phase and run single phase or 3 phase motors.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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Matt

181 posts in 2839 days


#4 posted 03-27-2009 01:44 PM

It’s my understanding that DC motors will not lose torque as RPM increases. I investigated this with my Jet 1220 shortly after I first got it. For $500.00 you would be close to a 3/4 hp VS Jet 1220. Just my two cents. I even think they make a kit (no reverse) to retrofit your 1220 now. I could be way off on that one. :)

-- Matt - My Websites - http://www.bestinwood.com - Hand Tools :: http://www.workshopgarage.com - Small Shops

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marcb

768 posts in 3140 days


#5 posted 03-27-2009 03:28 PM

rick3ddd,

Not to start an argument but it was my understanding that a Scalar VFD Designs loose power pretty quickly regardless of the motor. Not bad drives by any means, but the drop off is pretty severe once you go past a certain point (perhaps the motor dictates the point?).

The vector drives automatically compensate for load and are supposed to be superior for these types of applications.

Perhaps it is not torque that is lost, but the umph is not supposed to be there on scalar drives. I have limited experience with slowing down via VFD, typically just using them for a dialed in constant speed.

Of course like you say you could just gear down and scale the VFD UP from the drop off point.

I guess what I meant to say in my original post is that I wouldn’t just slap a VFD on with no changes and expect great performance at all ranges.

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TopamaxSurvivor

17676 posts in 3143 days


#6 posted 03-27-2009 08:27 PM

I have specialized in motors and controls for 40 years, so I’d like to clear up some miss conceptions.

VFDs are not only variable speed, they are variable horsepower; that is the basic nature of alternating current. DC motors maintain their torque at any speed. DC motors speed is very easily controlled. With the invention of modern electronics, low maintenance, brushless, AC motors are used where DC motors used to be required. There are many applications which are using VFD AC motors which were never dreamed of 25 years ago. DC motors can be a bit dangerous if running without a field. They will try to reach an infinite speed. They will either blow up electrically or physically before they get there! I have seen this happen twice on a 200 hp dc motor. Fortunately, the armature stayed in the case and no one was killed.

As far as single phase motors being reversible, very few have the leads necessary to do the reversing brought out to the connection box. It is true most of them are reversible if you want to take the motor apart to retrieve the proper wires. I suppose you could use a 4-way switch to do the reversing and another switch to turn it off and on. The last switch I bought to start, stop and reverse a single phase motor cost well over $100 in the 80s. Of course it was capable of standing up to an operator accidentally instantly reversing the motor. What is absolutely true and possible is not always good or even standard practice.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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TopamaxSurvivor

17676 posts in 3143 days


#7 posted 03-27-2009 08:48 PM

I should probably add that in the last few years the drive technology has made tremendous advances. Many drives and motors are now avialable for all kinds of special applications. All of our electronic devices cause tremendous electrical noise and inefficiency in power distribution systems if not properly applied.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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marcb

768 posts in 3140 days


#8 posted 03-27-2009 11:49 PM

TopamaxSurvivor,

Thanks for that, so basically as the speed goes down the torque remains constant and the HP goes down?

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TopamaxSurvivor

17676 posts in 3143 days


#9 posted 03-28-2009 12:39 AM

On an AC VFD, the voltage, amps & frequency drop or rise in sync. I can’t remember the exact relationship off the top of my head. As they drop, the speed, hp and torque all drop. On a DC motor, the speed can drop to near “0” and have a nearly constant torque. I think the DC hp drops to, but I’d have to look it up or think about it a bit :-)) I’m too far out of class to remember all the theory stuff.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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Big_Bob

173 posts in 3176 days


#10 posted 03-28-2009 04:23 AM

Tim:
Aren’t you glad you asked?

-- Bob Clark, Tool Collector and Sawdust Maker

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marcb

768 posts in 3140 days


#11 posted 03-28-2009 04:46 AM

I found a chunk of one of the manuals that I was thinking of when I posted my 1st comment on torque drop off.

“This control mode is based on the constant V/F curve(P202=0 – linear
V/F curve). Its performance is limited at low frequencies as function
of the voltage drop in the stator resistance, that causes a significant
magnetic flow reduction in the motor air gap and consequently reducing
the motor torque. This deficiency should be compensated by using
manual and automatic boost torque (IxR compensations), that are set
manually and depend on the user experience.”

This is for a Scalar drive. The vector drives have something in them that allows for automatic torque boost.

Anyways, my electronics learn’n was done too long ago to get too far into this too.

I believe that a DC motor with a good PWM speed controller can maintain near perfect torque at low speeds. I made a couple speed controllers, and the pure voltage controll method had many issues of power drop as you reduced speed. Even a basic PWM design with a dual 555 controller was better than that design.

This is why many of the old small metal lathe restorers (like the Atlas 618) use treadmill motors and a good speed controller to get better speed control than the old jack shafts. coupled with a back gearing you could get super slow feeds for great thread work on a small lathe.

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TopamaxSurvivor

17676 posts in 3143 days


#12 posted 03-28-2009 05:11 AM

march, Good speed control and high bottom end torque where the applications for DC motors before all these hi-tech applications for AC motors were invented. I have worked on the old pre-drive dc motors in steel plants, machine shops and on the Jetways that drive up to airplanes to unload passengers. They all used a plain old rheostat for control when inching tons of steel along. For a brief period, they used a few DC drives, but I haven’t seen one for a long time. But, then I’m not out working every day either :-)) They are getting pretty good at fooling AC motors for use in specific applications.

Anyway, back to the original question; it probably isn’t a very good idea to use a modern drive on an old motor as the insulation is not up to the level stress thousands of cycles of on and off currents per second will put on it. It might be able to be set at as lower rate, but as a general rule rated motors need to be used with these devices. Most important of all, never put the line and load sides of a VFD in the same raceway; ie, conduit or flex.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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Matt

181 posts in 2839 days


#13 posted 04-01-2009 03:03 AM

So, are there any equipment recommendations for a motor controller combo in the 1hp range with good torque, variable speed, and reverse switching? Just curious. I have thought about it for my Jet 1220, as well. Not so much reversing but the variable speed portion. Very interested in this discussion!

Thanks,

-- Matt - My Websites - http://www.bestinwood.com - Hand Tools :: http://www.workshopgarage.com - Small Shops

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TopamaxSurvivor

17676 posts in 3143 days


#14 posted 04-01-2009 03:25 AM

A small variable frequency drive should work just fime. I don’t believe they make a specifically made single phase, but the 3 phase ones are rated for single phase use. Just ignore the third phase. You will have to go to a 3 phase motor to get a reversing function out of it. Why would you want to reverse a lathe?

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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marcb

768 posts in 3140 days


#15 posted 04-01-2009 03:27 AM

The guys I know reverse the lathe to sand.

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