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.
15 years ago