Guy I met made his ac drive totally variable using a "Bridge"

If they were caps, the only reasonable purpose I can see is as part of the DC conversion section of the circuit. I agree that in that time frame, things were a bit more limited, but it could have been Triac or SCR control. That's what I was alluding to in my earlier comment about there possibly being more to it than what Bob was able to see.
 
Capacitors in an AC circuit will actually act a lot like a resistor. If you ever disassemble a ceiling fan, the multi-speed units actually have a large capacitor that is used on the "low" setting.
 
That's true, but they also discharge during the downslope of the AC sine wave. As a filter, that's the theory of operation to smooth ripples. If the value is correct, the present a load while charging that drops the voltage that flows through them. Then during discharge, normal voltage passed. They do not put out more than what is put in. Cool if they would (speaking in absolute power terms). So the net effect is that part of the inbound power is sidetracked into charging the cap instead of powering the fan, in your example. Normally, we think of a cap as a device that simply allows AC to pass, while blocking DC, but there is more to it than that, and other properties can be manipulated.
 
Holy Cow..... I have NO idea what you smart guys are discussing but thank you! It's possible that guy way back then talked about how he did that. I probably looked as dumb as I feel now. And I was way smarter and better lookin' back then. :)
Bob
 
We might be overthinking this. My guess is that he changed the motors to universal motors that will run just fine on variable voltage. Resistors would burn up a lot of heat but some combination of half wave rectifier to get half speed and resistors to fill in the blanks might work. It would take a lot of resistors to absorb that much heat. It would also explain why top speed seemed much higher than the normal 1725 rpm of an induction motor.

Brian
 
Yeah, I thought about universal motors, and it's possible. I don't think I want a TS with one on it though. Although, you used to be able to buy a table to clamp a handheld circular saw (Skilsaw style) to it upside down.....

And of course, the grinders on a welder's rig are usually universal and run just fine on the current of the aux port of the welding machine.
 
Tony Wells said:
That's true, but they also discharge during the downslope of the AC sine wave. As a filter, that's the theory of operation to smooth ripples. If the value is correct, the present a load while charging that drops the voltage that flows through them. Then during discharge, normal voltage passed. They do not put out more than what is put in. Cool if they would (speaking in absolute power terms). So the net effect is that part of the inbound power is sidetracked into charging the cap instead of powering the fan, in your example. Normally, we think of a cap as a device that simply allows AC to pass, while blocking DC, but there is more to it than that, and other properties can be manipulated.
Click to expand...

All true. I was mainly referring to the OPs question about an AC drive. Putting capacitors in series with the motor would reduce the voltage (and shift the current out of phase) and reduce the running speed of the motor. If you had a bank of these in series with taps, selecting the right tap would let you select the right speed. In this application, capacitors are superior to resistors in dropping the voltage because less power is dissipated in the capacitor than would be in a resistor doing the same job.
 
Is this what you're talking about, using a bridge rectifier to go from AC to DC and give it vs? Would this method work to to run a crossfeed on a lathe or mill if you have the appropriate size DC motor? Would the dimmer cut down the power going to the DC motor enough to reduce its torque at low voltage? Not sure how all this works but trying to figure it out.

http://www.youtube.com/watch?v=LtXtmVV1kjY

thanks, Brian
 
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