# Driving DC motor with equivalent AC

#### WakelessFoil

Joined Apr 16, 2020
12
I have a small brushed DC motor that I want to be able to vary the speed of with a potentiometer. The motor wants 120V DC but I only have 120V AC available. Can I rectify the AC input, smooth it out, and lower the RMS output using a properly sizes resistor to bring the DC output down from 170V to 120V? Wiring diagram below.

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#### Tonyr1084

Joined Sep 24, 2015
7,210
Lose the capacitor and you won't have the 170V problem. As for lowering the RPM, you might want to try a lower secondary transformer voltage. But until we know how much reduction you want - we can only guess. But I CAN tell you this: Running the motor at half the voltage may or may not reduce the speed. But it WILL cut the torque of the motor by 4 X. As you drop the voltage you double drop the motor output. If it's a slow speed desired you can go with gear reduction. Just a simple 2:1 gear reduction will result in 1:2 greater torque.

#### Tonyr1084

Joined Sep 24, 2015
7,210
You may find this educational:

I had a 3 speed AC fan motor. With no fan blade on it, the motor ran at the same RPM regardless of Low, Med, or High. AC motors like to run at a certain speed when there's no load. But that fan motor - with the fan blade on it, Low, Med, and High were exactly that. When the motor is doing work (moving air in this instance) the amount of power the motor is using is proportional to the amount of work its doing. The low speed coil in the motor drew less amperage. Amperage times voltage equals power (or Watts). So the motor was not able to move as much air as when the motor was on one of the higher speeds. (and higher amperage)

So you also need to consider what the work being done is going to be. How that work is going to affect motor speed. And in a perfect world you get exactly what you want. But in the real world, some of the work being done is heating the motor core, windings and housing as well as the surrounding air. Also there's bearing friction (more heating) going on. Just because you convert watts to watts you never get 1:1. You always lose something to friction. Not to mention the resistance (friction) of the electrons moving through the copper wires.

Joined Jan 15, 2015
7,088
Really need the motor's nameplate data. Things like current are important such as FL (Full Load) Amps.

Ron

#### Tonyr1084

Joined Sep 24, 2015
7,210
What kind of motor? Out of a hair dryer?

#### WakelessFoil

Joined Apr 16, 2020
12
Really need the motor's nameplate data. Things like current are important such as FL (Full Load) Amps.

Ron
0.2 Amps at full load (or at least what is on plate)
It is slightly larger than a hair dryer. See image.
It is for driving the quill down on a tap disintegrator. Not a lot of torque or speed required.

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Joined Jul 18, 2013
26,321
That is a typical Bodine DC motor, I have used one of the KB Drive products to operate those motors, They have full wave SCR bridge or PWM versions.
They operate directly across the 120v line.

#### Tonyr1084

Joined Sep 24, 2015
7,210
It is for driving the quill down on a tap disintegrator.
I have no idea what that is.

A single diode rated to handle 120V who's reverse breakdown voltage is greater than 120V, who's amperage is at least 1 amp should do just fine. Using half the sine wave you'll probably get some reduction in speed. Don't know how much, but seems likely around 50% reduction. Don't take that as gosple. I'm not the motor expert here.

#### LesJones

Joined Jan 8, 2017
3,905
Hi Tony, I think the machine is a simple EDM (Electrical discharge machining.) If so the down feed needs yo be very slow and it is normal to have a closed loop speed control that monitors the voltage across the gap and tries to maintain an constant voltage across the gap. (It also needs to be able to reverse the motor to clear a short across the gap.)

Les.

#### WakelessFoil

Joined Apr 16, 2020
12
Hi Tony, I think the machine is a simple EDM (Electrical discharge machining.) If so the down feed needs yo be very slow and it is normal to have a closed loop speed control that monitors the voltage across the gap and tries to maintain an constant voltage across the gap. (It also needs to be able to reverse the motor to clear a short across the gap.)

Les.
I knew there was more to this system than just a variable speed motor but I did not know exactly what. Thanks!

Joined Jul 18, 2013
26,321
Some of the the tap disintegrators (Electro-Arc) referred to, used a AC 50hz/60hz vibrator, peck-like mechanism to operate the electrode, and this was advanced by a servo like mechanism.
They had a few different models.

#### shortbus

Joined Sep 30, 2009
9,764
I knew there was more to this system than just a variable speed motor but I did not know exactly what. Thanks!
That's what I tried to tell you in your other thread. The speed control is tied in with the actual sparking. No sparking + no speed control, when you select the "auto" position of the switch. The pot that you think is a speed control is controlling the amperage in the spark gap.

Electroarc is still in business have you contacted them? They may answer all of your questions. https://electroarc.com/

#### MisterBill2

Joined Jan 23, 2018
13,826
Variable speed is not that difficult, but for EDM work it needs to be controlled like shorty said. So it is a bit more complicated. The goal is to advance the wire to hold a constant current
For starters, use a transformer so that your control circuit is not connected to the mains. That makes working with the control circuit easier and safer, and avoids several problems.

Joined Jul 18, 2013
26,321
Variable speed is not that difficult, but for EDM work it needs to be controlled like shorty said. So it is a bit more complicated. The goal is to advance the wire to hold a constant current
For starters, use a transformer so that your control circuit is not connected to the mains. That makes working with the control circuit easier and safer, and avoids several problems.
You are thinking of EDM, The OP is referring to a tap buster and they work on a much cruder principle.
You do not advance a wire, but an electrode that 'stabs' the busted tap,drill,bolt etc.

#### MisterBill2

Joined Jan 23, 2018
13,826
That is true, I was. But certainly there is still a requirement to have some feedback to avoid jamming or starving. So there does need to be some feedback, it seems to me.
I did do an experiment with a smaller DC powered winch, which was running if from a 12 volt battery charger, which is totally unfiltered full wave rectified DC. The motor ran slowly but smoothly. Reversing the polarity reversed the direction.

A very simple PWM circuit using a dual 555 timer IC with the first stage generating a ramp sawtooth wave and the second section serving as a comparator can provide your variable speed signal to drive a switching transistor to control the rectified DC to set the motor speed. I suggest using a transformer to reduce the mains voltage to whatever the motor requires, and to make the controller safer to work with. Of course, I am guessing that the motor would not be required to run at full speed.

Joined Jul 18, 2013
26,321
In the pic of the motor shown, it appears that as well as the power conductors, there is two exiting the back of the motor, which may indicate some kind of rotation sensor, also shown what appears to be a control board adjacent to the motor.
I would also second contacting Electro-Arc support for a manual etc.

#### Tonyr1084

Joined Sep 24, 2015
7,210
Hi Tony, I think the machine is a simple EDM (Electrical discharge machining.) If so the down feed needs yo be very slow and it is normal to have a closed loop speed control that monitors the voltage across the gap and tries to maintain an constant voltage across the gap. (It also needs to be able to reverse the motor to clear a short across the gap.)

Les.
That makes sense. I'm familiar with EDM. I trained on one for a short while. Would have loved to stick with the job but the group leader thought that since my background is in inspection that I could do nothing more than inspect. I could have been as good an EDM operator as he was, or at the very least one of the top machinists, but he never gave me a chance. He spoke to me like I was a red headed step child. I was there only three months before I went to HR and told them there's no way I could work for him. I quit.

OK, so regardless of what sort of EDM (or other) this comes from, what are you going to use it for? Is this an EDM machine repair? Or are you intending to use it in a totally different application? I did notice a few worm gear drives, which the output would be very low RPM on the final drive. Are you after an even slower drive? Identifying what we're discussing is important, but even more important is what the goal is; how the machine is going to be used. So I raise the question; what is your ultimate goal?

Joined Jul 18, 2013
26,321
That makes sense. I'm familiar with EDM. I trained on one for a short while. ?
Unlike EDM which is usually capacitor discharge for the erosion, a Tap Disintegrator is usually the same construction as a spot welder, but instead of applying pressure, it comes to the work piece live, very LV, high current AC , so the resultant low pressure and arc creates the metal erosion.

#### MisterBill2

Joined Jan 23, 2018
13,826
Indeed, in post #6 the TS explains that "it drives the quill down in a Tap Disintegrator . THAT is a device I have nneither used nor even seen.
And I see the tag on the motor just states DC and not volts. The wires just below the motor continue to the potentiometer below, they are not part of the motor. There does appear to be a bit of a circuit board associated with the motor, possibly a previous speed control?

The simplest scheme will be a variable voltage regulated power supply that covers the needed speed range With a maximum current draw of 200 miliamps that should not be very difficult. It will need a heat sink for the pass transistor but not a massive one.

#### Tonyr1084

Joined Sep 24, 2015
7,210
Indeed, in post #6 the TS explains that "it drives the quill down in a Tap Disintegrator . THAT is a device I have nneither used nor even seen.
I guess I missed the point in #6. I have seen the machine, and would have been trained on it at some point, but as stated, I quit before I got that far. Otherwise I would have been more familiar with the terminology and would have recognized that.

Some people are just too difficult to work with. Or for. And I'm sure someone would say the same thing about me. That's life.