Advice on AC motor speed reduction?

[grunthos]

Hi-  I'm an electronics novice trying to figure out a way to reduce the speed on my drill press.  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.

The motor is connected to a variable-diameter pulley set, giving an output spindle RPM fully variable from 480 rpm to 4800 rpm.  I'd like to get a slower range, ideally down to 200 rpm at the low end.

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 $100 or more, or perhaps construct one using PWM-approximated sine waves and an H bridge.  As a novice, constructing one is out of my league at this point.

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 speed could work fine for me.  My thoughts keep going back to PWM and the triac dimmers: could I make a simple circuit with a fixed lower frequency and voltage that would work?

Here is my idea for your critique or advice.  I don't know the right terminology, so bear with me.  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, but would produce one-third-length sine-shaped spikes that match 20Hz peaks.  And while the voltage would still be the original 110V rms during the 60Hz half wave, it would be followed by a full 60 Hz cycle of no voltage, thereby effectively approximating a lower voltage around 40V rms.  Then it would be followed by the next half-cycle on, which would spike 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.

Assuming this would work, how could I construct it?  It seems like I should be able to construct a simple 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.

[Gazza]

Hey Gruthos,

You are right, you need to vary the frequency to change the speed of your motor. I would use a single phase v/hz drive for this.

The cap on the top of your motor is a starting cap for single phase induction motors

You motor is using approx. 1.5HP, (10.2 x 110)/746

I would just buy one from www.automationdirect.com, http://web2.automationdirect.com/adc/Shopping/Catalog/AC_Drives_-z-_Motors/GS1_(120_-z-_230_VAC_V-z-Hz_Control)/GS1-22P0

That one costs about $150, You just have to make sure you can single phase the output, I dont have enough time to look right now.

[ante]

Hi Guys,

From what I have seen; a single phase motor with a starting cap on the side is actually a 3-phase motor able to run on a single phase by means of this cap!?
In that case it should be possible to run the motor from a three-phase frequency converter if the cap is removed and the jumpers are altered in the junktionbox; this will probably increase the efficiency and torque.

but I might be wrong.