# Over Amp'ing Wiper Motor



## jim6918 (Aug 18, 2006)

Can you over amp a wiper motor? I have surplus 400 watt PC power supply that is rated 15 amp at 12 VDC and 30 amp at 5 VDC. Motor will have some load on it driving a styro cauldron "stirring stick." Thanks to ScaryTerry, I know that it helps to have more amps than needed, but is there such a thing as too much?


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## Vlad (Aug 2, 2005)

It's my understanding that not enough AMPs is bad, but a motor will only draw the Amperage it needs.


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## ScareFX (Aug 18, 2004)

Vlad said:


> It's my understanding that not enough AMPs is bad, but a motor will only draw the Amperage it needs.


That's my understanding as well.


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## DeathTouch (Sep 6, 2005)

I can't remember what the motor is rated for, but it is high. But I know you can use 12vdc on that motor. If for example the motor is rated at 6amps at 12vdc then at power supply that show 15amps at 12vdc is great. 15 amps is the max that the power supply can dish out. The motor will only take as much amps as what it needs. If it works at 6amps, then it will only take 6 amps. The same goes for the 5vdc side with 30 amps. This will work too for the motor. The motor will supply 30 amps max for 5vdc. That is what I used for mine since the other voltages didn't supply the current I needed. Since the power supply is not rated for 12vdc on that side, the current might be a little higher to make it work. Not to worry though. It is just making up for the lost voltage.


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## indamudd (Aug 10, 2006)

Thats what I was told by the electric shop guys at work also. The motor only draws the amps it needs but do not go over the rated DC (12 vdc and you have a 15 vdc hooked up) They said if you do not have enough amps the motor struggles to power its self and you can burn the unit out.


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## gmacted (Jan 18, 2006)

There really is no such thing. 

Let's take the extreme example as that usually helps to explain things. Let's say you have a power supply that can supply 1 Amp at 12 Volts and a wiper motors that requires 2.5 Amps to turn in free air (no load). In this case, the power supply will not be able to supply enough current to turn the motor and the motor will not turn.

Let's take another example. Let's say you have a power supply that can supply 10 Amps at 12 Volts and a wiper motor that requires 2.5 Amps to turn in free air (no load) and 15 amps to turn under a load of 10 lbs. In this case the power supply will be able to turn the motor in free air, but not under the 10 lb. load. If you leave the power supply on when the motor is under the 10 lb load, you will most likely damage the motor. The power supply will not supply more current than it is capable of (rated at). Think of it as a water hose. The hose can't give any more water than it's capable of.


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## jim6918 (Aug 18, 2006)

Thanks for the great response. Electricity was never my forte. So, last question. When you buy a motor, is it usually rated "no load" and/or "full load?" Or, is that something you usually find out too late. The wiper motors I bought have no ratings label on them at all. If the seller hadn't listed the rating in the ad, I wouldn't know what it was.

I imagine crafting motors for haunts is very much try and try again.


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## DeathTouch (Sep 6, 2005)

The best thing to do is find out who made the motor and go on their site and look up the specs on the motor. I think most will say no load.


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## Koumajutsu (Aug 9, 2006)

also, note that a wiper motor is made for automotive applications. though they say 12V DC, a 6 Cell Lead Acid battery (Car battery) under charge from an alternator (i.e. while the car is running) generally has a standing voltage around 14.5V

this is because a Car battery isn't -exactly- 2 volts per cell, and an alternator is just a glorified AC generator with a diode attached (they call it a regulator, but it's just a diode)


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## gmacted (Jan 18, 2006)

jim6918 said:


> Thanks for the great response. Electricity was never my forte. So, last question. When you buy a motor, is it usually rated "no load" and/or "full load?" Or, is that something you usually find out too late. The wiper motors I bought have no ratings label on them at all. If the seller hadn't listed the rating in the ad, I wouldn't know what it was.
> 
> I imagine crafting motors for haunts is very much try and try again.


Typically a motor will have a graph that describes it's rating in Amps versus torque. This graph will usually be linear for the most part. The no load rating will tell you how much current is needed to start the motor. In the case of a wiper motor, you can use your car as a good gauge on how much current it will draw based on the fuse rating for that circuit. I have no idea what it is as I haven't ever looked at it, but I would guess it's either 10 or 15 amps.

I think if your using the motor to turn a styofoam stirring stick you'll have no problems at all. You'll basically be running it under no load. You'll be fine.


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## Spookineer (Aug 20, 2006)

All good and accurate advice. But no one mentioned using a fuse. All circuits in automotive and, for that matter, any direct current applications are protected either by fuse, circuit breaker, or fusable link. This is to protect the circuit from overload. Say for example when your wipers are frozen to the windshield and will not move, The amperage increases as the load increases. When it exceedes the capability of the conductor (wire) the weakest part will burn. This is the purpose of a fuse, to be the weakest link. Much easier and less expensive to replace than the motor or wire harness.


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## roadkill (May 11, 2006)

I have run several wiper motors using the 3V line off a PC power supply for years. Although rated for only a few amps it's more than enough to run my wiper motors and get about 6 RPM. The higher voltages will also get you more speed (but don't go above 12VDC or you'll hurt the motor).

You cannot sent too many amps to a motor. It will only take what it wants and no more. You can send too many (or too little) volts and either cause damage or get no action out of the motor.


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## gmacted (Jan 18, 2006)

That House on Ventura said:


> All good and accurate advice. But no one mentioned using a fuse. All circuits in automotive and, for that matter, any direct current applications are protected either by fuse, circuit breaker, or fusable link. This is to protect the circuit from overload. Say for example when your wipers are frozen to the windshield and will not move, The amperage increases as the load increases. When it exceedes the capability of the conductor (wire) the weakest part will burn. This is the purpose of a fuse, to be the weakest link. Much easier and less expensive to replace than the motor or wire harness.


Good advice, but if you do use a fuse make sure it's a "slow blow" fuse. You usually get some surge when you first turn it on.


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