# Problem with wiper motor...



## DarkOne (Oct 16, 2012)

Hello everyone,

I have a salvaged wiper motor out of a 2007ish Jeep Liberty that only works on HIGH speed. When attaching voltage to LOW, it pulses, moving forward then stopping, etc at about 1 second intervals. Any ideas what could be wrong? Maybe a capacitor? Just wondering if anyone ever had the same problem and was able to fix it. Thanks.


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## corey872 (Jan 10, 2010)

What type of power are you running it from? When it 'pulses forward' does it always make one complete revolution of the arm, or just a small jump?


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## Lunatic (Oct 3, 2006)

Corey has a good point....
Is the amperage to low from the power source?
Under load it would be the biggest problem but if you're at least 1.5 amps it should run. I run 5.0 amps to run my motors under load. Lower voltage lets say 3 or 5 volts may require a higher amperage to satisfy the motor.


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## DarkOne (Oct 16, 2012)

Just tested it again, turns out it works at LOW speed. It ran for a few seconds at HIGH, then returned to pulsing. Takes about 13 pulses for a 1/4 turn of the arm. I'm using a walwart rated 5A at 12VDC. The light on the wallwart pulses with the motor motion. 

Connected it to an automotive battery charger set a 12V 6A and it runs fine. I put the multimeter on it with both the wallwart and the charger, it draws about 1.2A on low and 2.2A on high when I can get it going, with a pretty good load on it, I only got it to draw 4.33A. This was running on the walwart. It seems if I hook up the positive terminal and then ground, high will only work after there is a spark from the ground terminal.???? I set the multimeter to record the max draw, so I know it's not drawing more upon startup. 

Do these motors have a Starting Capacitor or something that could be burned out? Or maybe I have a short?

Any other ideas? I probably only need LOW speed but it still bugs me!


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## DarkOne (Oct 16, 2012)

Just realized if it's working fine with the battery charger, the power supply must be the culprit. 

Any bad reasons to use an automotive battery charger as the power supply?


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## corey872 (Jan 10, 2010)

Sounds like you are getting some 'inrush' current and tripping a current overload on the power supply. For a split second, the motor is likely drawing well over the 5A rating. It's probably too fast for the meter to catch, but the power supply is tripping. 

If you measure the resistance of the motor coil, then divide the applied voltage by that resistance, it would approximate the instantaneous inrush current. Once the motor starts spinning, it produces a 'back EMF' which limits the current flow.

Using the battery charger is OK - it's probably able to deal with the inrush current a little better. If you want to try and stick with the WM power supply, I believe some people have mentioned putting a resistor in one of the motor lines to limit the inrush, too. (likely serves the same effect as your spark) Possibly something rated at a few ohms, which can handle several watts of power... which starts to look a lot like a 12v light bulb.


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## bfjou812 (Aug 15, 2008)

Personally I would go with a larger amperage power supply. Here is a link for some reasonably priced ones. I've used several of them and have no complaints.

http://www.mpja.com/power-supplies.asp


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## Lunatic (Oct 3, 2006)

I've had no problems running the motor under load using a cheap 1.5 amp 12v supply. However it would over heat in time and shut off until it cooled down. That is why I use computer power supplies for their amperage and voltage options. I love those damn wiper motors...they're work horses!


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## corey872 (Jan 10, 2010)

Lunatic said:


> I've had no problems running the motor under load using a cheap 1.5 amp 12v supply. However it would over heat in time and shut off until it cooled down....


I'm not trying to be a smart a$$... but that does seem like a problem, then??

I think the main problem we run into is all power supplies may be a little different, and all our non-standard applications may be a bit different too.

Some PS's may have over voltage / over current protection, others may not, some supplies are over or under rated voltage, some may not produce full amperage, or may only be designed to do so for an intermittent period. Some may run a simple bridge rectifier/capacitor to get a rough DC waveform, while others are switch mode / SMPS which produce a fairly smooth DC supply. Even a brand which seems to be a good power supply one year, could have an undocumented re-design and not work in the same capacity the next year.

Surprisingly, for this application, some of the lower end / dumb supplies may actually work better. They may not respond to the 'blip' of inrush current and the voltage/current regulation isn't critical. Whereas a 'smarter' or high end supply would trip/reset at the high inrush current.

The ATX computer supplies mentioned above are probably going to be the best on a 'dollars per watt' basis, plus you get 12V for most motors, 5V for other motors and could run some battery operated props designed for 3 alkaline cells, and 3.3V which is about perfect for most LEDs and props which use 2 alkaline cells. It might even work out to have a central ATX supply which branches out to multiple props.

I've been converting my lighting over to LEDs - so far one ATX supply has proven plenty to run 1700 RGB nodes, which is equivalent to 5100 individual LEDs. So they definitely have some 'potential'.


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## DarkOne (Oct 16, 2012)

Thanks for all the input! I have a couple power resistors laying around, maybe I'll try figuring out how to put one inline and see if that works. 
I also have a couple PC power supplies sitting around, just too lazy to wire them up like bench supplies.
I'll also try a cheaper power supply, the one I've been trying was made for an external harddrive or harddrive vault I believe, so it may have protection built in that's tripping an internal breaker. I think I have a 12V 2A one that may work...

Thanks again, if I figure out anything definitive, I'll post it.


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## Hippofeet (Nov 26, 2012)

Your motor is building up a magnetic field, but the current is not enough to allow it to cross the gaps between the magnets on the inside of the outer case (stator). The pulsed rotation is equal to the face width of 1 of the magnets on the stator (actually the center to center distance of any two magnets, but same-o same-o). The rotor moves under starting load to the next set, the field collapses (well, sort of, it actually builds up a sufficient field to jump a set,then immediately stalls as starting load kicks in) , builds back up, it moves to the next set, and on and on...

I think lol.

Anyway,I don't think it's the power supply voltage/current cut-off causing the pulses. That would trip right off, and never even turn the rotor at all. It happens to me with the absolute cheapest 12VDC PS's I can get, when I run insufficient amps. 

I bet if you wanted to, you could measure the c to c of the E-magnet faces on the stator and rotor, and bust out some gears in train math, and calculate the rotation from that c to c distance, after going through the gears.

If you wanted to. I know I sure don't.


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## scarybill (Oct 7, 2012)

Most wiper motors in cars run on a 20amp circuit. Must be a reason the automakers do that.
Hippo, I think you hit it right on the head


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