DC conversion on lathe.

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LX Kid
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DC conversion on lathe.

Post by LX Kid »

I was thinking of modding the lathe AC motor to a DC motor to have variable speed control. I've found a $60 treadmill that has a 1.25 HP motor and was wondering if that would be satisfactory for a modification? I see where new SCR controllers can be bought for under $200. Be nice not to have to mess with the belt changes.
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LX Kid
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Re: DC conversion on lathe.

Post by LX Kid »

Should I forgets it and look for more HP?
chucketn
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Re: DC conversion on lathe.

Post by chucketn »

I'd say 1.25 HP should be fine. Go for it. I have a treadmill motor and controller on my mini mill. Had one on my 7x lathe while it's controller was being repaired.

Chuck
Torch
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Re: DC conversion on lathe.

Post by Torch »

How big is the motor on there now? A bit more power might be nice, too much more power and the castings, etc. may not be sufficient. A chain is only as strong as the weakest link, right?
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LX Kid
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Re: DC conversion on lathe.

Post by LX Kid »

Torch wrote:How big is the motor on there now? A bit more power might be nice, too much more power and the castings, etc. may not be sufficient. A chain is only as strong as the weakest link, right?
12x36 3/4hp on now.
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Re: DC conversion on lathe.

Post by Torch »

I would think anything more than 1.25 or 1.5 hp would be a waste if the stock motor is 3/4 hp.
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LX Kid
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Re: DC conversion on lathe.

Post by LX Kid »

Torch wrote:I would think anything more than 1.25 or 1.5 hp would be a waste if the stock motor is 3/4 hp.
Ok! I'll go buy that sucker this evening and tear it down for the parts. Thanks guys!
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Re: DC conversion on lathe.

Post by Torch »

Hopefully you can just use the one that comes with the treadmill, but if you are willing to spend as much as $200 on a controller, give serious consideration to a PWM controller rather than an SCR controller. Smoother, quieter and more low speed torque. And, IMHO, torque is more important than horsepower.
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LX Kid
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Re: DC conversion on lathe.

Post by LX Kid »

Torch wrote:Hopefully you can just use the one that comes with the treadmill, but if you are willing to spend as much as $200 on a controller, give serious consideration to a PWM controller rather than an SCR controller. Smoother, quieter and more low speed torque. And, IMHO, torque is more important than horsepower.
Ok I'll look into a PWM controller also. Have any suggestions on one? Don't want to get crazy money into it. I have to get the treadmill tomorrow as there are heavy storms there right now.
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Re: DC conversion on lathe.

Post by Torch »

First find out what the motor specs are and determine how much room you have for the controller. Then find a suitable controller. Preferably the one that is currently installed in the treadmill, if possible and money is tight.

I personally have the KBWT-210 controllers, and have found them to be very reliable as well as versatile. Suitable for shunt or permanent magnet motors, a raft of optional features, overload protections, feedback, safety switch inputs, etc. But your treadmill motor is likely to be 110v, not 220, so that would not be a suitable choice for you. KB electronics also makes a couple of 110v versions, one of which will handle up to 12 amps, but at approximately 7" square, they are probably larger than your existing SCR controller. And it requires a separate switch for reverse (just reverse the leads to the motor).

If the motor is a PM design, then you might find a smaller controller elsewhere.
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LX Kid
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Re: DC conversion on lathe.

Post by LX Kid »

I looked at the KB electronics PWM's and I don't see any compatible for a 1.25hp motor at 115v line voltage. They list 115v line voltage PWM's for up to 1hp motors and then jump to 1.5v motors at 230v line voltage. I don't know squat about motors and controllers. Could I use a PWM designed for a 1hp motor on a 1.25hp motor? Of course I will contact them and ask questions but just thought someone might know.
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Re: DC conversion on lathe.

Post by Torch »

What is the amperage and voltage rating on the motor? The HP rating is too approximate and prone to exaggeration by some manufacturers. The electrical HP (not the same as mechanical HP or metric HP) can be derived from the wattage, which depends on the voltage as well as the current. However, I would think the KBWT-112 (12 amp rating) should handle a 1.25 hp motor. It's rated for a 1.5hp (electrical) motor, continuous or 2.5 hp intermittent. It has a maximum AC input current of 18 amps, 3 more than a normal household circuit fuse/breaker so probably more than a treadmill can draw. (The KBWT-110 is rated 1.2hp continuous and 1.5hp intermittent, 15 amp AC draw.)

Note that the actual HP being created by any motor is dependent on the load. In other words, if you only apply a light load, say, 3/4 hp, then that is all the motor will create. The additional HP rating is a reserve. If you try to exceed the HP rating of the motor, it may try to provide it but burn up in the process. That's where some motor manufacturers play with "duty cycle" calculations to make their motors seem more powerful. A motor that can handle 1hp continuously might be able to provide 1.2hp for short periods if given time to cool off between spikes.

The KBWT controllers allow you to set the DC current limit from 0 to 200% with a trim potentiometer, so you can set it to the maximum rating of the motor. If you try to take too deep a cut (ie: apply more HP than the motor is rated for) the controller will simply shut down, protecting the motor and the controller. There is also a time delay adjustment. When I set mine up originally, I set the time delay to 0 and measured the current draw between the motor and controller, taking progressively deeper cuts until the current draw hit the motor's nameplate rating. Then I adjusted the current limit to shut the motor off at that current. Finally, I set the time delay to something like 3 seconds. So the motor can exceed it's maximum HP for no more than 3 seconds before it shuts down. There's also an armature fuse that should be sized to limit the absolute maximum current to the value of your motor's locked rotor current, if that is indicated on the rating plate, up to the maximum capability of the controller.

However, all that said, and considering your comment about your electrical knowledge, the safest alternative is to use the controller that came with the treadmill, if it is still functional.
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