Make a VFD/DC voltage controller - Schematic & writeup

synthesist

synthesist

So many knives so little time
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Sep 14, 2004
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Many years ago, my father-in-law, Frank Mezzatesta ( an electrical engineer mind you ) designed, and made me, a DC motor controller. That device is still going strong today.

I thought I'd share the writeup and schematic here for the electrically experienced, or just plain adventurous, (and perhaps cash strapped) among you.

Here ya go:

DIY DC Variable Voltage Controller

This schematic doesn’t show specific values for any of the components because it is meant to serve a variety of purposes.

This is a typical circuit for converting a constant A.C. voltage to a variable D.C. voltage.

The variac and rectifier ratings are determined by the rating of load to be served.

This particular circuit was designed to operate a 125 V.D.C. SHUNT WOUND MOTOR.

The ratings of the main switch and fuse on the hot side of the line are also determined by the ratings of the load.
The red light on line 1 comes on whenever the main switch is closed, indicating that the system is energized and ready to operate.
Lines 2 and 3 of the schematic show a full wave rectifier and a field discharge diode (D1). The output of the rectifier is used to energize the motor’s shunt wound field with a fixed voltage which will allow the motor to operate at maximum torque at each speed level which is set by varying the armature voltage. Diode (D1) is used to discharge the motor field coil when power is removed. This prevents damage to the full wave rectifier. This part of the circuit is not needed if a series wound motor is used in place of shunt wound motor.

Line 4 shows a variac, zero position limit switch which is normally closed at all times, except when the variac is in the “off” position, at which time it is open. This is a safety switch which prevents the operator from inadvertently applying voltage to the motor armature when he turns on the main switch, and the variac happens to be in the “run” position.

When the variac on line 7 is in the “0” position, the LSW element on line 4 is forced open, while the LSW element on line 5 is forced closed which allows Control Relay (CR) to pick up. The normally open (N.O.) CR contacts on line 6 will close and seal in CR. At the same time, the normally closed (N.C.) CR contacts on line 4 will open, and will remain open until the CR is dropped out of the circuit by opening the main switch. The amber light will be extinguished, indicating that the system is now functional, because the (N.O.) CR contacts on line 7 also closed when CR picked up.

By adjusting the variac on line 7, the operator can vary the voltage output to the full wave rectifier on line 8.

This D.C. output is applied to the armature leads of a shunt wound motor/or the leads of a series motor.

Note: When a series wound motor is used the D.C. power supply on line 2 isn’t needed.


bbuVVlo.jpg



I'm no electrical engineer myself so I can't become technical support for this although I can shoot some pics of the one hanging on the wall in my garage if someone asks for them.

Corey "synthesist" Gimbel
 
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That should work for basic DC motor control. Al components and wiring would have to be heavy enough for the current draw of the motor used.

For those not into electronics:
1) The incoming110-120VAC AC power is converted by the first bridge rectifier into constant voltage DC. Because of diode loss, the output DC should be about 100VDC, which is a common field voltage on DC motors.
2) The Variac ( variable transformer that acts as a variable resistor) takes the incoming AC and makes decreasing voltage AC current that is turned into Dc by the second bridge rectifier. This gives a variable DC voltage for the armature .

While this set-up will work fine for some applications, it isn't quite the same as the output of an electronic controller. I would say that the circuit shown is good for running a disc grinder, drill press, or flex shaft more than a belt grinder. It also would be best on less than 1HP motors, as the component ratings would make the cost high for bigger motors.

To explain DC motors in a very simplified statement:
The field is an electro-magnet that creates the magnetic (Field). Some motors are PM and have Permanent Magnets.
The armature is the motor shaft and its windings. The armature turns when current is run through the windings because of the magnetic field around it.
Raise or lower the armature voltage and the speed raises and lowers.
Reverse the polarity on the field or the armature (but only on one or the other) and the motor reverses direction.
Most larger DC motors run off 90-100 volts or 180-200 volts. This is because the Rectifier Bridge converts the AC to DC with a bit of loss. 110/120VAC=90/100VDC, 220/240=180/200.
 
Thank you Stacy for expanding on what I posted.

I need to elaborate on the history of this "controller"

I was learning to make knives from Tim Wright when he lived in Chicago. I introduced him to my father in law, Frank Mezzatesta ( BSEE MIT ) who worked at Westinghouse on things like furnaces for the steel mills in Gary Indiana and the " L" ( the electric elevated trains run by the Chicago Transit Authority ). He was a resident expert at Westinghouse on DC electrical applications.

Tim wanted me to have a "professional" grinder with wheels and such. I couldn't talk my wife into the price at the time...................

So Tim and Frank decided to get me one. A Bader II. Tim built/customized/modded and setup the grinder. Frank built the controller out of "junked" parts and came up with a DC motor for it. I have no idea what it would cost to build if you were buying components new today. Originally it had a 1 hp DC motor. It has 1.5 hp Leeson now and runs just fine. It's never so much as blown a fuse. To say it's overbuilt would be a gross understatement.

The reason I posted this is two fold.

One) It's a memorial to Frank in a way. I miss him everyday.

Two) I thought the schematic was simple enough that someone electrically savvy might want to build one of these. At the very least Frank's straight forward writeup might be educational for someone.

I'll post some pics of the controller and the grinder later today. It's cold my shop this morning.

Corey "synthesist" Gimbel
 
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Here are the pics I promised. These will give you an idea of just how MASSIVELY overbuilt this sucker is. As I recall the variac (that's the round thing in the middle of the box which is 10" in diameter) was, in reality, the speed controller off an CTA "EL" train that got salvaged from the discard pile at Westinghouse and rewired.

http://i.imgur.com/dr5ILTP.jpg[/IMG

[IMG]http://i.imgur.com/L2HF8G8.jpg

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I'm glad you enjoyed the history Stacy. It brings back very pleasant memories. The thing is the whole dang grinder is built the same way. Tim mounted the whole thing on a 5/8" steel baseplate, so it wouldn't move around. These days I'd like to rebuild it with a hinge so I could use it horizontally too. That is a project for another day I'm afraid. Someday this grinder will go up on the exchange and continue doing its thing long after I'm gone.......................

Corey "synthesist" Gimbel
 
It's probably not all that overbuilt, autotransformers are big pieces of gear, the 10" coils I've seen were usually rated between 15 and 25 amps.

If someone finds a Variac (or other brand of what is generically called an autotransformer) and decides to build one of these controllers, be aware that the moving part that makes contact with the coil is a consumable carbon brush. The brush needs to be kept in contact with the coil, which may mean periodic adjustment of a spring or the round plate that holds the brush, and occasional replacement of the brush depending on how often the autotransformer gets turned (and how worn the brush already was if it's a used autotransformer). These brushes are still available from suppliers that service old theatrical dimmers and lab equipment. If you hear arcing when you turn the autotransformer stop using it and figure out why. It may be a stuck spring, loose setscrew holding the brush plate to the shaft, worn out brush, or something else. If you allow the arcing to continue eventually you can burn through the coil at one or more points and buying a new coil, if you can find one, is about the same price as a new autotransformer, and more expensive than a new electronic DC controller.
 
Very cool, thank you for posting this Corey.

I don't think I'll ever be building something like it, but the story and those images bring back fond memories of my Grandfather. He was an Electrical Engineer as well... he had all sorts of things like that around his shop that he built or wired up himself. The man could fix anything... my Mom would have a long list of things for him to repair every time he came into town. I remember the TV we'd watch at their house as kids was the same black and white box (much to our complaints and displeasure) they bought even before my mother and uncles were born... he just fixed it up every time it broke (along with everything else in the house I'm sure). Unfortunately, he passed before I was old enough to get to know him and really learn as much from him as I now wish I could. My only possessions of his are an old "Easydriver" ratcheting screwdriver he always carried with him, and his various notes and journals. I flip through the paperwork from time to time... even though they're mostly just random drawings, schematics, budgets, and what-have-you, mostly the sort of stuff most of us probably file away and never plan on looking at again... I feel like it helps me to know him a bit better beyond just the old war stories. It's been a while since I've gone through them... I think I know what I'm doing this afternoon, haha. Sorry for getting sentimental :P

He actually worked among the steel mills in Gary for over 40 years. I have to wonder if they knew each other...
 
That is the coolest grinder I've ever seen! I love it - and all made from cast-off parts at Westinghouse? Even cooler. Thanks for sharing the pics and schematic.
 
This made my wife cry..............................

damnnnnnnnnnnn
 
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