# Significantly reducing speed of single-phase AC motor

I have a 1/3hp single-phase AC motor spinning my drill press. With the maximum reductions available through pulleys it still runs 10x too fast for one of my applications (i.e., the chuck turns at 360rpm and I need it around 30rpm).

I'd be happy to cut the output torque proportionately to the speed reduction, but I've been reading about this for hours and I can't even determine:

1. For what types of small AC motors is sustainable speed reduction possible?
2. By what means? Triac? PWM? VFD?
3. To what extent? Is 10x speed reduction possible?

And then there's the problem that I can't even tell for sure what kind of AC motor I have. I can see that it has a 16µF (run?) capacitor bolted to its side. And here's a picture:

I'm further bewildered because typical hacks for this problem involve hundreds of dollars in gearing or special motors. This entire drill press is $100, so I have a hard time believing there's not a sub-$100 solution. E.g., I'd be happy to replace the motor with one that runs on household current at something like 200rpm instead of this one's native ~1800rpm. But I haven't been able to find even that.

Am I missing some fundamental limitation to producing torque off of household AC at very low rotational speeds?

• Use an electric drill, either corded or cordless and clamp it to the motor mount. Some are dirt cheap. – user99600 Feb 7 '16 at 7:17
• @OC71 probably has the cheap-quick-kludge winner here. – Ecnerwal Feb 7 '16 at 15:22
• Slowest run speed I have found in corded drills is 540rpm. Today I am playing with a DC (cordless) drill though to see if I can manage it with voltage reduction. – feetwet Feb 7 '16 at 15:37
• If you have the drill press motor set on the smallest pulley driving the largest pulley of the spindle, you can unplug (and for completeness, isolate the plug) the drill press motor and rig something to hold the portable drill (or other motor) running another small pulley and belt that drives the largest pulley on the original drill press motor (which is just acting as the jackshaft in this configuration.) – Ecnerwal Feb 7 '16 at 20:00

Voice of experience: Use a jackshaft and additional belts/pulleys, &/or change to a different motor (DC with DC speed controller or 3-phase + VFD (variable frequency drive) variable speed.) The DC motor and speed control can often be salvaged from a treadmill that someone gave up on using, for free.

The fundamental limitation of running induction motors slowly on household (60 cycle per second, 3600 cycles per minute) AC is that the motor type you have has 4 poles if it runs 1800 RPM - so you'd need 36 poles to turn 200 RPM on 60-cycle AC. That would be a very rare bird indeed. You're actually fortunate if your drill press motor is not 3600 RPM to start with...

The other limitations (which apply to "universal" motors that can more easily be speed controlled, but are terribly noisy by comparison)(and also to DC, and to a slightly smaller extent 3-phase + VFD) are terrible (worse than a linear reduction) torque, and poor cooling/overheating since the motor's (built-in) fans are not running at a reasonable speed to cool it.

You might find gearmotors (normal motor speed of 3600 or 1800 RPM and an attached reduction gear) that run that slowly, but you won't like the price, especially if you want much power/torque.

The initial cost of the drillpress you are starting from has little impact on the cost of doing non-standard things with it (and may make a more expensive model that has better features such as dual reduction or a wider reduction range already built in look less expensive in the end.) Then again, you may be starting with completely the wrong tool - metalworking lathes are not too hard to find used affordably in moderate sizes, and typically have a "back-gear" setup standard that offers very low speed and high torque. Good for coil-winding (at a guess since you don't say and this is EE.)

• As a side note it is possible to vector control an AC induction motor for variable speed with nearly full torque down to 0 RPM. It's not a \$100 solution either though..... – John D Feb 7 '16 at 2:22
• @JohnD - a three phase induction motor perhaps, but probably not this single phase capacitor start centrifugal switch induction motor. – Chris Stratton Feb 7 '16 at 4:36
• @ChrisStratton Yep, absolutely right, though I did a full bridge PWM V/Hz controller for a single phase AC induction motor that had fairly impressive performance over a wide speed range. The start winding cut out electronically after start up near rated frequency. More suitable for a fan than a drill press though. – John D Feb 7 '16 at 18:32

This is what happens when "good" solutions aren't cheap enough: it's an off-the-shelf 12V 1A geared motor (designed for brass case preparation), clamped to the drill's drive shaft with a rigid coupling. On highest reduction pulleys the chuck is turning about 50rpm. Beauty in kludginess?

• This wins my vote because it embodies the very spirit of engineering: it works. Special brownie points awarded for how the mixing tub is held in place. I wonder what's in the mixing tub, though. – peufeu Sep 14 '17 at 19:25
• @peufeu – thanks ;) IIRC this was a batch of desiccant I needed to dry out. It's sitting on a hot plate. – feetwet Sep 14 '17 at 19:31
• OK! For stirring stuff I usually use the pneumatic hammer drill which has 2 gears, low gear is nice and slow, works well for this. – peufeu Sep 14 '17 at 19:53

The problem is actually in the type of single phase motor you have there, a Capacitor-Start / Induction-Run (CS/IR) version. It has a centrifugal switch inside that removes the starting capacitor and its associated auxiliary starting winding from the circuit at about 75% speed. So as soon as your speed drops below that threshold, that switch re-engages and puts the start winding and starting capacitor back in play. Those are not designed to be used continuously and will burn up (or rather, your motor will trip off line on over-temperature if you are lucky). Bottom line, that type of motor is NOT designed to operate and anything other than full speed, regardless of how you attain a lower speed.

Your viable options, other than the additional mechanical ones, are to change it to a 3 phase motor and use a VFD that converts your single phase source to a 3 phase output; change to a single phase PSC (Permanent Split capacitor) motor and use a special VFD designed to operate that type; or change to a DC motor and DC drive. There is nothing you can do with that particular motor that will work without destroying it.

Give it a try connecting a light bulb in SERIES to the supply cord. Try 200 Watt, 150 Watt, 100 Watt.

• OK, adding a resistor in series: That reduces current, right? So the motor will have less torque, but run at the same speed? – feetwet Feb 13 '16 at 1:03