simple fixed-frequency AC motor speed reduction?

Hi- I'm an electronics novice trying to figure out a way to reduce the
speed on my home drill press for cheap. I'd appreciate any advice or
hints on approaches.
It has an AC motor, labeled as 110V single-phase at 10.2A, with a speed
of 1725 rpm. There is a large capacitor in a lump on the side of the
motor, which sounds like the descriptions I have read of induction AC
motors and not a universal motor.
The motor is connected to a variable-diameter pulley set, giving an
output spindle speed which is fully adjustable from 480 rpm to 4800
rpm. I'd like to get a slower range, ideally down to 200 rpm at the
low end. I really like the variable-diameter pulley setup. I don't
think I have room to fit in a speed-reduction idler pulley, and I'm not
sure how I could even do that with the variable-diameter pulleys.
Based on my reading, to slow down the motor, I can't simply use PWM nor
a typical triac lamp dimmer; I need to reduce the frequency of the AC
power. I also read that it needs to be a sine wave, or close to it;
while a DC motor could compensate for a square wave with a freewheeling
diode, you can't do that in AC. So it sounds like the zero-crossing
aspect of the sine wave is important. I also read that the voltage
needs to drop as the frequency is dropped also.
I've read about variable-frequency drives for AC motors, which are
designed for this. It looks like I could purchase one commercially for
$200, or perhaps construct one using PWM-approximated sine waves and an
H bridge. As a novice, constructing that would be out of my league at
this point. And purchasing one is outside of my hobby budget.
However, since my drill press has the full variable-diameter pulley
system, I don't need full variable speed control at the motor. A
single fixed lower motor speed would work fine for me. Could I make a
simple circuit with a fixed lower frequency and voltage that would
Here is my idea for your critique or advice: approximate a 20 Hz signal
with lower voltage by chopping the 60Hz power with a 33% duty cycle. I
don't know the right terminology, so bear with me.
The way I understand it, the 60 Hz power gives an 8.3 ms sine-shaped
pulse in one direction (a half cycle) followed by an 8.3 ms pulse in
the other direction. An ideal 33% power for the induction motor would
be a 20Hz sine wave with 40V rms; the 20Hz would give a half-cycle of
25ms in each direction.
I could take the input power, at 110v 60Hz, and chop it at every third
half-cycle. This would not produce a real 20Hz sine wave (25ms sine
pulse), but would produce 8.3ms sine-shaped spikes that occur every
25ms. And while the voltage would still be the original 110V rms
during the 8.3ms cycle, it would be followed by a 16.7ms of no voltage,
thereby crudely approximating a lower voltage around 40V rms. Then it
would be followed by the next 8.3ms on plus 16.7 ms off, spiking in the
opposite direction, approximating the other half of the 20Hz cycle.
And so on.
Does this make sense? Would it work?
What would happen to effective motor power and current draw? Would the
motor be one third as strong, or would it suck three times as much
current, or would work and current draw be the same? I'm concerned
about current draw, since this motor is running on a 15A circuit; I
don't want it to try to draw 30A.
Assuming this would work, how could I construct it? It seems like I
might be able to construct a 20Hz oscillator with a 33% duty cycle to
drive some big power transistors, perhaps using a 555. But it also
seems like I would need to make sure it is in phase with the AC power,
so that the on period starts and ends at a zero crossing, right? Any
way I could test it without an oscilloscope?
I'd appreciate any suggestions on components, or other thoughts,
comments, or advice. Thank you.
Reply to
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If you're trying to slow it down, I assume you want to use larger tools, and cut harder materials. If that's true, the best way to do it is to put smaller pulleys on the motor, and/or larger ones on the spindle. That way you retain the horsepower of your motor, and increase your torque. That's exactly what you'll need for larger tools. You'll probably have to buy a different length belt. Check out
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for some pulleys, and belts.
Reply to
Dave Lyon
No, it won't work. Variable frequency drives are usable on three phase motors, not single phase, like you have, at least not over the range you want to go. (Most 60 Hz motors work pretty well on 50 Hz, or even a little less, but at 20 Hz it would have very little power and get awfully hot awfully fast...)
Your choices are to replace the motor with a 3 phase and a VFD, to replace the motor with a DC (universal) motor, to add a jackshaft with all that entails or to add another drill press that will run slower. I would suggest the latter. But even that might be tough because most of them probably turn faster than what you want...
Reply to
Jerry Foster
Yes, changing the pulley on the motor is the simplest solution. Any circuit to reduce the frequency will require higher current. Or, if you take the motor apart, you could find out if you can change the connections of the coils from the present 4 poles into 6 or 8 poles . . . . Good luck.
Reply to
That's a cap start induction motor. It runs over a narrow speed range of about 1700 to 1800 rpm. You can't speed control it without a lot of expensive electronics.
Reply to
Homer J Simpson
If the motor is a permanent split capacitor motor, which are sometimes fitted to cheap drill presses, they can be controlled to some extent by supply voltage. From various reading permanent split capacitor and shaded pole AC induction motors do respond to voltage control. What this would do for the torque and its effect on the motor in this application I don't know. My experience of this is with a Compair Rotron fan which was shaded pole AFAIK and the controller was quite simple, a phase angle controller I think. It did induce some low level hum at low speed but was supplied for this specific use and was quite effective.
If you can identify the motor as PSC then your options would be a variac or maybe a large dimmer or similar power controller. As the load is inductive a phase angle controller would need to be conservatively rated and input filtering of the supply would be good to remove the noise generated on the mains supply.
Reply to
David Billington
This would be at best a kludge.
You've said that you don't have room to change pulleys. You might revisit that, because this would be the best approach.
Beyond that, the next step would be to fit the DP with a surplus DC treadmill motor. Those are much easier to speed-control, will retain rated torque at rated current regardless of speed, and they're not expensive. They're about $40 locally in Minneapolis. It's much easier to make elex to control a DC motor than it would be to change frequency for an induction motor, and surplus speed controllers are readily available.
Reply to
Don Foreman
The J33 spindle won't take much more torque. gl, John
Reply to
John D. Farr
Wow, lots of misinformation being put out on this. it's a confusing issue, so try to stay with me here.
1) Since the capacitor is on the side of the motor, it is likely NOT a Permanenet Split Capacitor (PSC) version. That means that there is a centrifugal switch on the end of the motor which switches the Start capacitor out of the circuit once the motor gets to speed. If you vary the speed with a VFD, the centrifugal switch closes again and puts the capacitor back into the circuit, the high speed switching of the transistors overheats the capacitor in short order, it shorts, dies, and takes out the transistors all in a spectacular puff of magic smoke. In short, you cannot do this.
2) As mentioned by someone else, if it were a PSC motor by chance, then there are VFDs specifically designed to be used on 1 phase motors, you can buy them from Bardac or Amocon, they are both the same thing (brand labeled from a UK company named Invertek that does not market them directly in the US), Here are the links:
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3) That said, there is another issue with regards to using a VFD to change the motor speed. When you use pulleys, the motor remains at normal speed and power but through speed reduction in the sheaves, the speed is changed one direction, but the torque goes the opposite direction. So when you reduce the speed, you INCREASE the torque at the same ratio. That means if you have a 1/2HP motor and want to reduce the speed to 1/3 of normal, the torque at the work piece will be 3 times what it was at full speed. As someone else mentioned, most likely on a drill press you want the slower speed because you want to drill harder materials, so you WANT the additional torque to keep from stalling the bit. When you change speed with a VFD, you change the speed by altering the voltage and frequency together at a constant rate, refered to as the V/Hz ratio. The motor's output torque is dependent on that V/Hz ratio, so maintaining a constant V/Hz ratio is essential to avoid losing torque and stalling (which would happen if you reduced only the voltage) or over-exciting the motor and burning it up (which would happen if you only reduced the frequency). So if you maintain the torque at a constant level AND reduce the speed, your HP goes down at the same rate as the speed reduction. In your case, if you ran the motor at 20Hz, your HP would be 1/3 of what it is now, meaning if you have a 1/2HP motor, it will become a 1/6HP motor! You will be LESS ABLE to drill harder materials than you were with full speed.
4) Forget what was said about the current increasing as you slow the motor down. That just isn't true with a VFD. The current follows the reduction in HP, not that this means anything though.
Bottom line, first you need to look carefully at your application, i.e. WHY do you want to lower the speed, and are you going to need more or the same torque from the motor at that slow speed. If it is more, belts are your only option. If it is the same or less, you can use a VFD BUT probably not on that motor. If you can use a VFD and you need to change motors anyway, buy a 230V 3 phase motor. You can get any number of off-the-shelf VFD that will take a 120V 1 phase input and deliver a 240V 3 phase output to the motor, they are inexpensive and very good quality.
Reply to
Bob Ferapples
Thanks for the excellent reply.
That makes sense; I can see how it would be a show-stopper.
Yes, as you guessed, I wanted to go slower to drill harder metal.
Your explanations really make sense. I think I'll take another look at the pulley setup.
I appreciate the education. Thanks!
Reply to
If you do a search you should be able to find a replacement pulley that has a built in planetary gear reduction. I think it was Smithy that used to sell this for some of their machines. It was a bit pricey though...
I think that this is the part, but it doesn't say that it has a planetary gear set up:
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The price is about what I remember it to be though. I recall seeing some plans and/or some other info for a planetary setup like this too. If you want to go this route I'm sure it can be located...
Reply to
Leon Fisk
I can see how it would be a show-stopper.
The other easiest solution is to buy a VFD and a Motor for it... You might even be able to get a larger motor, and thus retain enough torque for the application. This eliminates fitting new pulleys, and/or fitting a DC motor and control approach.
Reply to
According to :
Yes -- you do.
[ ... ]
[ ... circuit ideas snipped ... ]
No -- it would not work. At least while you might be able to approximate a 20 Hz sine wave, your motor would not react gracefully to that. This is a capacitor start motor, which requires a specific frequency (or pretty close -- a 60 Hz motor will often work on 50 Hz, and a 50 Hz motor will almost always work on 60 Hz -- but to work at 20 Hz, the value of the capacitor would have to be significantly increased to probably three times the capacitance.
Motor speed changes by variable frequency works well with *three* phase motors. You notice above that I put a series of '^^^' under your "single-phase" (though they may be shifted if you read this in a proportional space font. I used a fixed space font to type this.
Well -- assuming that you replaced your single phase motor with a three-phase one, you would retain about the same torque -- but the horsepower is a function of the RPM as well as the torque. Without looking it up, I *think* that the horsepower varies as the square of the RPM, so you would have about 11% of the horsepower. (The variable speed pulley, in contrast, will increase the torque as it decreases the speed.)
So -- my first real question -- do you really need the full speed of the drill press at its top speed setting? If you could live with *that* being reduced by the same factor, you could replace your 1725 RPM (four-pole) single phase motor with a motor wound for a lower speed -- a bit under 600 RPM (probably close to 575 RPM). This would allow you to get your lower speed without needing a variable frequency drive and a three phase motor.
Also -- you could wire the motor for 240 VAC instead of 120 VAC, thus allowing half the current. It is a lot easier to add a 240 VAC circuit in a home shop than to add three phase or a higher current circuit.
Or -- you could buy a three phase motor for the drill press, and a VFD -- an things would be a lot simpler.
You would certainly need an oscilloscope. And I'm not at all sure that your design (which I snipped above) would actually work. The normal process for a VFD is to rectify and filter the incoming power to get a DC supply equal to the peak voltage of the incoming AC, and then to switch that with high voltage power MOSFETs in a PWM (pulse width modulation) approximation of the desired frequency. There are lots of other things taken care of, including sensing the output current and switching off the VFD if there is excess current drawn, to protect the expensive MOSFETs.
Note that there is one entirely passive way to get 20 Hz from 60 Hz. It used to be common in developing ringing current (20 Hz) for telephone bells from key-tel units (the pushbutton multi-line phones and their hold circuits). The brand was "Sub-Cycle" -- and they were (as far as I know) only transformers, inductors, and capacitors. However, all that I have seen were too low powered to run a drill press motor.
I would suggest that you either look into a motor which runs at 1/3 the RPM of the current one, and swap it in in place of the current motor, or get a three-phase motor and a VFD. The three-phase motor will certainly need 240 VAC, and a VFD to generate the three phase from your single phase. The slower single phase motor may well draw more current so you will want to re-wire it for 240 VAC so you can drop the current by a factor of two. (I run my 1-1/2 HP lathe motor from 240 VAC, because it was tripping the 15A breaker a bit too often on starting.
Good Luck, DoN.
Reply to
DoN. Nichols
He'd do better to get an AC/DC motor if he is going to replace it (which seems unlikely). Other substitutions are too expensive or complex.
That will be simple to speed control - you can buy an off the shelf unit.
Reply to
Homer J Simpson
Another alternative is to physically switch the motor out with say a 575 rpm induction motor of the same size.
Reply to
Actually three phase motors are available very cheaply if you can find a local source or don't mind the shipping cost. Surplus 1 and 2 HP VF drives are also pretty inexpensive, and many will convert single phase to three phase. Then speed control is very easy. If you need more torque, you can get a higher HP motor or one with more poles.
There are also multi-speed motors in sizes like 1/2 to 1 HP used in HVAC fans.
Reply to
Paul E. Schoen
According to Paul E. Schoen :
[ ... ]
Agreed. Of course more poles means less high-end speed.
Not good for a drill press or other machine tool. Those speeds are depending on limited torque (depending on which winding is selected) fighting against the drag of the squirrel-cage blower. Such a motor with no blower mounted will spin at its natural speed (just a bit under 3600 RPM for a two-pole, just a bit under 1800 RPM for a four pole and so on. Hang it on a drill press, and you can only control the speed by pushing the drill harder (more load) to slow it down.
Enjoy, DoN.
Reply to
DoN. Nichols

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