# slowing the motor



## greghaa (Jan 27, 2013)

I have a lathe I rebuilt from an old combo machine.  Northern tool #143352.  I only have the lathe portion working as I plan to get a stand alone mill.  My question is about the lathe motor though.  It is 3/4 HP and uses a belt system to adjust the speed.  ~600 RPM to about ~1800 RPM.  Some material requires a slower speed though so I was wondering if I could use a variac like the one in this photo.

http://www.weisd.com/store2/PHI48-1205.jpg

Will it cause any undue stress to the motor?  Has anyone else ever done anything like this to achieve slower speeds?  How would it affect the torque if at all?

Thanks for any help you can provide.


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## swatson144 (Jan 27, 2013)

A variac will decrease the voltage so it will decrease the power proportionately.  You don't have a back gear on the lathe?

Without a back gear I'd probably just make a jack shaft to add another reduction. Slow rpm, increased torque all is good.

Steve


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## twstoerzinger (Jan 27, 2013)

I believe this lathe has a standard AC induction motor (no brushes) as it comes from Northern Tool.

The Variac will reduce the single phase voltage (and hold 60 hz). At reduced voltage, the induction motor has to increase its slip rate to induce more current. More slip means lower RPM. The problem is that the current induced in the rotor as well as the current in the windings starts to go up pretty fast. Essentially, you are putting your motor into a "brown out" condition to get lower speeds.

This technique works OK on small blowers and mixers where the required HP falls off quickly as the RPM is reduced. Machining operations, however, tend to be more constant torque with changing speeds. I'm not sure the Variac is a good long term solution for speed control. You may only get 10% or 20% reduction without overheating the motor. If you try it, keep a close watch on the motor temp.

Terry S.


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## Alan Douglas (Jan 27, 2013)

Yes, the best you could do is 10 or 20% reduction in speed, not worth the risk.  If the speed drops much more, the centrifugal starting switch might kick back in, burning up the start winding or capacitor.

An induction motor is a (substantially) constant-speed device, locked to the frequency of the power line.  Its power is the product of voltage times current.  If you reduce the voltage, the current increases, and it's the current that heats the wire windings.  It's the same reason, as already noted, that air conditioners tend to burn out during brownouts.


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## greghaa (Jan 27, 2013)

I was reading another thread where someone is using a treadmill motor and control unit to do a similar thing.  Control the speeds better.
If this motor I have is 3/4 HP, and I did something like swap to a treadmill motor, the HP would be a lot more.  Is that a concern
for the bearings on the spindle or anything?  I am thinking it might be easier to do that.

I wish I could just add a gear but it is a single belt from the motor to the pulley on the end of the spindle.  I got this combo from a 
old warehouse.  It was all rusted and junked.  So I rebuilt it and got it working.  So far it is pretty good.  There is only about .006
backlash on the cross feed where the table and tool post is, as well as about .006 on the main screw.  

Thanks guys


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## twstoerzinger (Jan 27, 2013)

The spindle bearings, gear drives, etc. should be designed to deliver the necessary torque for the size machine that it is.

Treadmill motors are designed to be variable speed. Often they are brush style, series wound, AC/DC motors that will run from an SCR or a low-cost DC drive.
The series wound motors are able to deliver high torque a low speeds. As you say, there are some threads here on adapting treadmill motors to machines.

Another example of a variable speed AC/DC motor is a sewing machine motor. I believe that at least one member is experimenting with installing an industrial duty sewing machine motor on a lathe. Do a search on "sewing machine motor."

Terry S.


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## greghaa (Jan 30, 2013)

So I grabbed a treadmill off craigslist for free.  The motor and controller work great.  Got it hooked up and I am seeing the same issues that others have seen.
Have to trun the 10K pot about 25% before it engages.  Then it is fine.  Can go up or down.

At this point I could put it in a box and mount everything and have a speed controlled lathe.  Have to get a mounting bracket for motor but I think I can adapt
what I have with what I took from treadmill.  

Wondering what others have done.  Is this really the best way or is there a better controller out there?
One I have found on ebay is this one

http://www.ebay.com/itm/Baldor-KB-G...820&pid=100011&prg=1005&rk=1&sd=300545535232&


Not sure if that one will work better than the treadmill controller.  Any input from anyone?

Thanks

-greg


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## twstoerzinger (Jan 30, 2013)

I will leave it to others to comment on the treadmill DC drive experience.
As far as the Graham DC drive on Craig's list: this is a "professional" grade drive that can be configured to do many jobs. $30 seems like a really low price. It probably has a lot of features that you won't need for lathe speed control. The only watch-out, is that I think you still have to buy a few additional components to make it work. Such as: current sense resistor for motor size, and possibly a heat sink depending on the HP of the treadmill motor.
Terry S


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## greghaa (Jan 30, 2013)

I managed to figure a way to not have to zero the pot each time I turn it on.  So I am thinking for now I will just stick with the MC-60 controller.

Working on the belt issue.  The flywheel fits on a 17mm shaft with left hand threads.  Trying to work out if I can just cut the v-groove in the flywheel
for the belt, or if I am going to have to figure out another way.  Maybe get another v-pulley and bore it to the correct size to fit the shaft with retaining screws
or something.  

Thanks!


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