Electric Motor Speed Control

Thread Starter

mujo

Joined Jan 11, 2021
1
Greetings. First time post from a complete newbie. DIY enthusiast ... would like to buy a mini-lathe for some hobby projects BUT the motor generates 4000 - 7000 rpm. For working wood this is great but I also need it to drop down as low as 250 rpm. I was thinking I'd need to put a potentiometer in the circuit to give me a true variable speed of 0 - 7000 rpm.
The motor runs on mains electricity, and is rated at 96W and 12-24V.
Two questions ...
1. Can it be done?
2. If yes, what are the potentiometer specs I'll need to make it so?
All help very much appreciated.
Cheers.
 

jpanhalt

Joined Jan 18, 2008
10,963
Welcome to AAC.

If it runs on DC, it can be speed controlled.

A pot can be used, but if the motor uses 96W, It will have to be a very hefty pot (I.e., >50W rating) and will get hot. It is better to use a PWM (pulse width modulation) controller. They are cheap on eBay or can be made DIY with a few components. Basically, instead of using a resistor to cut the current, they turn the full voltage on and off rapidly. That reduces speed and produces better power at lower speeds..
 

Dodgy Geezer

Joined Nov 30, 2009
125
There are some mini-lathes which use brushless motors as opposed to brushed ones. Brushless motors usually have better torque at low speeds than brushed, but I do not think you can use a typical brushed PWM unit on them. They CAN be controlled using PWM techniques, but they have more complex control electronics, and the PWM circuitry would neen to be embedded in that.

Your issue for running a mini-lathe slowly will be torque. ANY attempt to slow the rotation of the workpiece by lowering the input power will also lower the torque. This is why many mini-lathes have a high and low gear setting - gearing is the most effective way of running at low speed while still maintaining (or increasing!) the torque. Of course, adding a gear stage is not a trivial task.

You may be better off buying a machine which matches your requirements initially rather than buying one that does not and then modifying it to suit.
 

Dodgy Geezer

Joined Nov 30, 2009
125
Incidentally, using a potentiometer would be much worse than using a PWM unit, since the power is essentially wasted in the pot rather than being delivered to the motor.

What do you want to use the mini-lathe for? My geared one can run down to around 20 rpm, though I would not want to put any load on it at that speed!
 

Dodgy Geezer

Joined Nov 30, 2009
125
Is this the type of lathe you are talking about? https://www.amazon.co.uk/Queiting-M.../B08JGL55G5/ref=psdc_6436900031_t2_B08JCB2QYS

If so, it looks like it uses a brushed motor which will respond well to PWM, as jpanhalt has recommended. These lathes are VERY small, and are unable to turn anything very large - they use a drill chuck which opens to a maximum of 6mm. I believe that the motor has an internal fixed gearbox, but even so the torque is unlikely to be high enough for cutting metal.... in any case, there is no cross-slide.

I have recently been looking at controlling a similar motor, and find that a common PWM unit available on eBay has worked well for me. It is often called a ZK-BMG - here is an example. https://www.ebay.co.uk/itm/DC-9-60V-12A-500W-PWM-Motor-Speed-Governor-Controller-Switch-Digital-Display/133567933932?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

You also might want to consider getting an attachment which turns your power drill onto a small lathe. These used to be very common some years ago, and can probably be found cheaply second-hand. Depending on your drill, they will have a lot more torque and greater range of speeds....
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
22,036
the motor generates 4000 - 7000 rpm. For working wood this is great but I also need it to drop down as low as 250 rpm. I
The only thing you may find, dropping a motor of that nature and RPM down to a very low 250rpm, is you may find you will lose torque to quite a degree, unless some kind of feedback is instituted.
Which I believe has been mentioned.
Max.
 
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