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



## Buffalo Bob (Dec 21, 2013)

Long time ago I visited the workshop of an electrical engineer. He had every machine you could think of. Metalworking, wood etc all layed out in his basement. He was selling something and I was checking it out. 

His workbench though was for electronics. He had capacitors? I think, the round tube jobbos with wires at each end. He was building what looked like an accordian of these things all wired together. He called it a Bridge. All his machines were equipped with them. Variable speed.

He had hundreds of those parts segregated in bins. Middle container was whatever the standard part was called. Like it was a "100" that was commonly used in all electronics. However he knew that the actual output was subject to manufacturing variability. He measured somehow each "100" component and the +/- range was maybe -5 to +10 of the stated spec.

And by wiring a group from -5  through the highest +10 maybe, that was all he needed to make all motors VS. 

This was a house in Chicago but I don't recall if the motors were single phase 120v or three phase or whatever. My understanding was his Bridge was the only mod needed. He proceeded to power up his table saw through slow speeds to full on. Scared me. It was whistling. 

What can you tell me about that technology. He made these to sell to other engineers so it was common in the early '70s. I've met some interesting people in my life and he was special. If this rings a bell or is common technology let me know?  

Merry Christmas to you good folks... 
Bob


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## Terrywerm (Dec 21, 2013)

If the wires were coming out both ends, and they were more or less brown in color with colored bands on them, they were resistors, which would make much more sense for a variable speed control. I do not know of a way that capacitors could be used as the primary component in a speed control. One of our more electrically minded individuals would have better information than I can give.


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## Buffalo Bob (Dec 21, 2013)

*Re: Guy I met made his ac drive totally variable using a &amp;quot;Bridge&amp;quot;*



terrywerm said:


> If the wires were coming out both ends, and they were more or less brown in color with colored bands on them, they were resistors, which would make much more sense for a variable speed control. I do not know of a way that capacitors could be used as the primary component in a speed control. One of our more electrically minded individuals would have better information than I can give.



Wermie I think you are correct. Thanks...
I Googled it but no idea what it is. Would be a neat project if I had references or build plans. All the electrical engineers I know have jobs so need to poke around for more info.
https://www.google.com/search?q=cap...ge&rls=org.mozilla:en-US:official&safe=active

BB

- - - Updated - - -

A Diode Bridge http://en.wikipedia.org/wiki/Diode_bridge
can turn "single phase ac into dc current" . Cool.. I keep researching.
Bob

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 http://en.wikipedia.org/wiki/File:Diode_bridge_1000vx4a.jpg

 Detail of a diode bridge, rated at 1000 Volts x 4 Amperes




 
 A hand made diode bridge. The thick silver bar on the diodes indicates the cathode side of the diode.



 A *diode bridge* is an arrangement of four (or more) diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input.

 When used in its most common application, for conversion of an alternating current (AC) input into a direct current (DC) output,[SUP][1][/SUP] it is known as a *bridge rectifier*. A bridge rectifier provides full-wave rectification from a two-wire AC input, resulting in lower cost and weight as compared to a rectifier with a 3-wire input from a transformer with a center-tapped secondary winding.[SUP][2][/SUP]

- - - Updated - - -

On a roll here. YouTube has some DIY instructions.
[video]https://www.youtube.com/results?search_query=how+to+make+a+bridge+rectifie  r%3F&amp;sm=4[/video]


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## Tony Wells (Dec 21, 2013)

Thinking it's not a rectifier bridge. Typical motors like you mentioned probably found on the table saw would not be DC. You cannot run an AC motor on DC, which is roughly what comes out of a rectifier. I say roughly because it is not true, pure DC. 

If what you were seeing is a set of large wattage resistors, you can limit the current (AC or DC) sent through a circuit but that is not the proper way to control motor speed. There must be something more to what you saw.


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## Rbeckett (Dec 21, 2013)

Sounds like he built a PWM generator and is using it to control the motor speed by the duty cycle.  That's how quite a few are done and it works exceptionally well in most instances...

Bob


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## MikeWi (Dec 21, 2013)

Wouldn't PWM  lose a lot of torque at lower speeds?  Don't know a thing about this application, but I know what PWM is, and I'm curious how that would work for this.
Aaaannnnd, knowing me, I guess I should mention that I "know" PWM as Pulse Width Modulation.  Maybe this is something different.


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## Pmedic828 (Dec 21, 2013)

A PWM If that is what you mean is a Pulse Width Modulator.  This is a device that turns on and off for each cycle and limits the duty cycle power.  In other words, normal AC has 60 cycles or Hertz.  When looking at this on a scope, it is a sine wave that crosses the 0 point 60 times a second.  The power curve is full wave.  If you limit the pulse or hertz width to have the wave form, the power is limited so the motor slows down.  I think that I remember that an single phase AC motor will heat up and not run efficiently.  It's been too long out of school for me.


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## Jimw (Dec 22, 2013)

He probably used an H-Bridge. Reversible and able to use Pulse Width Modulation for speed control.

An H-Bridge can be constructed with 4 power transistors driving an motor. He may, be using a micro controller to drive the h-Bridge alternating mode. One could rectify the ac line using a bridge rectifier to produce AC and drive the motor via an h-bridge creating an AC of varying frequency.


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## Jimw (Dec 22, 2013)

Variable frequency speed control of an AC induction motor. AC-DC-AC. Here is a link to a paper on such a system. http://umpir.ump.edu.my/2127/1/ENCON2011_WANISMAIL.pdf


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## SEK_22Hornet (Dec 22, 2013)

Jimw said:


> He probably used an H-Bridge. Reversible and able to use Pulse Width Modulation for speed control.
> 
> An H-Bridge can be constructed with 4 power transistors driving an motor. He may, be using a micro controller to drive the h-Bridge alternating mode. One could rectify the ac line using a bridge rectifier to produce AC and drive the motor via an h-bridge creating an AC of varying frequency.



I agree that using an H bridge and PWM would be a way to do this today, however - the time frame mentioned (1970's) would mean that it would take a lot more to do t that way back then. I worked in electronics for 38 years after graduating from Vocational School in 1975.  There were a few IC's that would do PWM for the then new switching power supplies, and even the old 555 timer could be set up to do that.  The H bridge would have to have been built using multiple transistors, since the power transistors available wouldn't handle much current, especially at high voltage, and a pretty large heat sink would have been needed. Triacs and SCR's were really getting popular for motor control back then. I've even seen the old triac light dimmer circuit used to control motor speed - and considering the time frame, I would guess that he was using some sort of triac circuit - they were cheap and easy to build, and would handle quite a bit of power using only a handful of parts. 

The components mentioned sound like capacitors, and back then, the way to measure the value of these (as was mentioned) would have been to use an LCR Bridge to measure the value.  I'm not sure the capacitors would have been a part of the speed control circuit - maybe just something else he was playing with?  Just my $.02 worth.


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## Tony Wells (Dec 22, 2013)

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.


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## DMS (Dec 22, 2013)

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.


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## Tony Wells (Dec 22, 2013)

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.


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## Jimw (Dec 22, 2013)

Your right about the availability of components in the 70's. one could use a simple variable frequency oscillator circuit to switch on and off power transistors or triacs or SCR's.  Switching power supplies were known on the 30's ( see http://www.righto.com/2012/02/apple-didnt-revolutionize-power.html ), similer thecnology, and produced in the late 60's.


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## Buffalo Bob (Dec 22, 2013)

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


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## bhowden (Dec 22, 2013)

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


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## Tony Wells (Dec 22, 2013)

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.


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## DMS (Dec 22, 2013)

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.



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.


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## Tony Wells (Dec 22, 2013)

Yep, until ESR comes up....then not so good.


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## calstar (Jan 18, 2014)

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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## DMS (Jan 18, 2014)

calstar said:


> 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



It will work, but couple things to keep in mind. First is that those light dimmers are not meant to work with motors, so you will not be able to run them at full power. The other thing is, it will slow things down, but speed will still vary with the amount of load on the motor. I have used the same combination to run a small (1/8 hp) gear motor, and it works just fine. For a cross feed, you may also take a look at BillGruby's power cross feed build for the 9x20 lathe.


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