Thread: Wiper motor producing AC current? Possible hazzard?

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Ok, so this is odd - posting here to see if I have a bad motor or what may be going on.

I'm finishing up my spider victim... just made a video to edit and post (thats coming up soon).

First let me explain the setup. Wiper motor is running at 106 RPM by hooking both the high and low leads to the + and - of a 12v current coming from a computer power supply. Through some metal linkages, the wiper motor arm is in contact with some metal (exact details in the video to follow soon).

So I'm messing with adjusting it, and I touch the metal brace, and I feel this bizzare and uncomfortable... tingle feeling... and I'm thinking... WTF? Electricity? There? So I break out my multimeter. Nothing. Wait. DC current doesnt do that unless its REALLY strong... so I switch the multimeter to AC.

I've somehow got 70 volts AC coming out of the ARM of the wiper motor.

Wtf?

Is that normal? Now I know I have the motor hooked up "wrong" (using High and Low instead of either + ground), and I know that a motor is also a generator (hook a DC motor up to a lightbulb, and spin it... the bulb will glow faintly). But to be converting 12v DC to 70vac and send it out the wiper motor arm?!

Is this normal? I mean, its not really an issue since I'm now going to insulate the motor arm from any metal framework and it'll all be wrapped up, but that stunned me to get a mild shock from the 70vac coming out the moving arm.

Woah there, buddy! You've wired the wiper motor WRONG! Do NOT power both the low speed and the high speed pins at the same time! You will eventually burn the motor out very quickly if nothing else. It was not designed to have both the low speed and the high speed pins powered at the same time. Also note that the ground pin is wired to the case of the motor, so you do not want the motor or anything that can conduct that is attached to the motor to come into contact with anything else that might be generating electricity. Please refer to this from the MonsterGuts website for the proper way to wire a wiper motor:

http://monsterguts.com/extra/prodImages.php?productId=4

By the way... when I was first starting out, I did the same mistake on my first FCG. The motor eventually burned out after only a couple of days.

Except it's pretty common practice to overdrive the motor that way. Search the forums. People everywhere do it without any real consequence. I'll post links to threads here in a bit. Believe me, I didn't do it without a lot of research first.

If you put a lot of torque on that motor with it overdriven, it is going to generate a lot of heat and eventually the motor will burn out. At least it happened to me and now I know better. Your results may vary. Good luck.

Just a guess, but I'd imagine the A.C. voltage you're reading is the result of the magnetic field in the commutators collapsing and the resultant EMF being shunted through the case of the motor, which would normally be common to ground. (It's probably actually pulses of D.C.) Shouldn't be a problem for the motor, but might be a bit dangerous - be careful & keep the TOTs away.
As for decreasing the longevity - the only way to know for sure is to try

Originally Posted by hedg12

Just a guess, but I'd imagine the A.C. voltage you're reading is the result of the magnetic field in the commutators collapsing and the resultant EMF being shunted through the case of the motor, which would normally be common to ground. (It's probably actually pulses of D.C.) Shouldn't be a problem for the motor, but might be a bit dangerous - be careful & keep the TOTs away.
As for decreasing the longevity - the only way to know for sure is to try

A lot of that was over my head... all I know is I read 70 volts AC, and it buzzed the crap out of my hand as if I touched a live 120v wire, which unfortunately, I have before.

I just didnt know if anyone else had experienced it before. I know a lotta folks here overdrive a wiper motor like that.

Well I'll say that I don't think its normal. I've had a wiper motor hooked up as you have it and no ac shock resulted. That would freak the crap outta me!

Have you touched the wiper arm while its in motion? Thats kinda my point - at 106rpm not many people are touching it, or have it linked up through an all-metal system to where they could touch something metal. I have another motor I'm gonna try it out lateron and see if that does it too.

since you do not have an actual ground on the motor and you have the motor hooked up the way you do, the voltage that you are seeing is most likely a pulsating DC put out by the motor as you discussed earlier. The ground wire is actually connected to the stator of the motor...electrically connected to the "generator action in a motor" that you are seeing.

For safety's sake drive a metal stake in the ground and bolt a wire to it. Attach the other end of the wire to the ground lead of the motor. This will at least relieve the hazard that you spoke of and should prevent anyone from getting bitten if they come in contact with your little beast. I do not know how or IF this will affect the operation of the motor...theoretically it may slow down some as a new path is introduced into the circuit, but I would have to see it to be sure...

As for motor longevity...i've never hooked a motor up like that and would think that it would indeed burn it out sooner...but hey, wiper motors are cheap.

I second DannyK, ground that motor and you will lose the shock value (so to speak) I had a similar problem with an inadvertently ungrounded appliance and connecting the ground bled the stray ac to ground and eliminated the problem. There is no real amperage behind the ac voltage you are seeing (and feeling) but you will, more than likely, hurt yourself trying to get away from it.