AC motor speed controller

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walter

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Seattle Wa. U.S.A.
I am working on a Lathe motor control for a local custom hot rod shop. They have an older unit that has been ripped apart. It is a box with a speed pot., and three three-way toggle switches for each motor for power and direction. 120VAC. I'll go back and identify the motors and components before I build anything, but I have been doing some research and found some simple motor controllers. I'll guess the original box was Phase Angle control since it was built awhile ago. I found a schematic on-line for a 220VAC motor, and re-drew it, here is a picture of my drawing: ...
 

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walter

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Jun 21, 2006
Messages
412
Location
Seattle Wa. U.S.A.
I need to control three 120VAC motors and direction, so I think this will do it, three motors and three DPTT switches in parallel (I drew one motor) and a beefier Triac.  Do you think this will work?
I read that Snubberless or Alternistor Triacs are good for inductive loads, but didn't see that kind for the  41A Triac. Does anyone have an opinion on that? I'm getting this information from Digikey, they show three kinds of BTA16-600 Triacs, Logic-Sensitive Gate, Snubberless, and Standard. I have no experience with Triacs.
 

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walter

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Seattle Wa. U.S.A.
I'll probably make the coil to keep costs down. The Schematic suggests that's an Iron core coil because of the bar next to the windings. What are my options here? Will a stack of I-lams work, or is there a rod for coil cores? Would air core work? The PWM controller schematic appears to use a Powdered-Iron core, the dashed bar next to the windings. I have no experience with coils.
Here is a PWM assisted AC motor controller schematic that might offer another option.

I have been looking for a custom shop to modernize my 1947 chevy panel truck. There are a few in this area, but nothing close. I found a shop ten miles from my home, but they don't seem to have any adds on-line. I met the owner, Jerry, last week. He has one shop full of old equipment and storage. He said the lathe blew up because the wires to the motors were just hanging and something fell on them. Someone, or more have looked at it. The control box is open and half the components are in a pile. Jerry said the lathe was built by a Boeing engineer who made the frame out of wood, and used that to have it cast in iron. He also designed the controller. I thought I might be able to make it work. I don't have much experience with motor controls. A few years ago I built a couple power tool racers. That was mostly figuring the mechanical linkage between motor and drive wheel, the motors were just plug and go full speed. My last work project involved programming an Arduino to control two motors for an automated test fixture. That was program, plug and play, but I did mess around with PWM for varying speeds. I have found a few off the shelf AC speed controllers from $99 to thousands, but where's the fun in that?
 

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walter

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Seattle Wa. U.S.A.
Here is a picture of the Lathe. The main motor is two wire with a capacitor start, The label shows it is a Dayton 1725rpm 12.6A motor P/N 4K413A, I'm not sure if I got the part number, it was hard to read. I searched for information for that part number, but didn't find any. I did find a similar part number 4K141 listed as a capacitor start power tool motor, this one had different specs, but I think it's the same kind of motor. The other two motors have six wires, he said he thought they were vacuum cleaner motors. I can see two of the wires go to the brushes, the other four, I don't know. I have been reading about motors on-line for a few days, so I can guess the other four wires go to the stators. There is no starting cap, so one set of wires may be for an induction winding, set 90 degrees from the main winding to get the motor turning. Or the motor may be shaded pole, another way to get it turning. At any rate, it appears I have to connect the A/C line hot to one end of one winding, and A/C ground to one end of the other winding. The other end of both windings connect to the brushes. The direction is switched by reversing brush connection. I don't know what polarity to connect the stators, I'll just guess and see if it works. Now I'm ready to order some components. After I get a circuit together, I have a vacuum cleaner motor I can use to test and scope it out.When it works, I'll put it on the lathe and wire up those motors.
 

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walter

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Location
Seattle Wa. U.S.A.
Here is a picture of the waveform across the motor. The diagonal line gets shorter as I reduce the speed. So, it works. I'm not sure what I'm looking at. I'll take some more readings in different parts of the circuit after I make some adjustments. I have a 250k pot, but it's only working with 10k. Of course the lathe motors will likely react different than my test motor, so the final tweaks will be at install. Apparently the o-scope was not happy with the line signal. I ended up using a transformer to get the reading. The speed control works, slow to fast, but I get random bursts at slow.
 

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