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#### Goldmast

##### New Member
Hello,
I have very limited understanding of voltage, amps and electronics. I am
Working on a project and have a 12v windshield wiper motor that I was running off a 12v rechargeable 30 amp/hrs battery. I tested the motor and it looks like it pulls 6amps at max load. I want to buy a wall power supply and ditch the battery. I bought a 12v 5amp power supply (I figured out that it needed 6amps after). Should the motor at least turn over and run at limited efficiency at 5amps? Or is it as simple as buying a 12v 10amp power supply to get it going? On the 5amp it would make a small pulse every second or so. -- I may be leaving out nessessary information. Please ask me questions if you think you can help and I will do my best to answer them. Thank you

#### Pommie

##### Well-Known Member
The start current is much more than the run current. Can you measure the resistance of the motor. Your current supply is probably being overloded, shutting down and then trying again after a short stupdown period and this will cause your motor to pulse.

Mike.
You could try starting it with the battery added and see if it will run with just the power supply

#### Goldmast

##### New Member
I'll try to figure out the resistance. Is this something that would come up on the multi meter if started it and ran it off the battery to check?

#### KeepItSimpleStupid

##### Well-Known Member
Totally agree with Pommie. Measuring the motor resistance, which isn't trivial, is a really good way of estimating the starting current.
The current in an inductor cannot change instantaneously so for a short time, the motor is seen as a low value resistor.

The way to measure this resistance is by measuring the current through the motor and wiring using a shunt, for eaxmple, and then measuring the voltage drop across the motor. You don;t want to use something that can supply the rated current.

Measuring low resistances with a standard multi-meter will have errors.

Assume the motor has 0 resistance and you have 12 V and say you wanted about 0.1 A, so a 120 ohm resistor would be required.
The wattage would have to be > (0.1)(0.1)*120 = 1.2 W. So lets say it has to be greater than 2 W. Use a 1% resistor. Use your 12 V voltage source in series with the resistor. Then measure with your voltmeter, the voltage across the 120 ohm resistor and the voltage across the motor close to the motor. Rm = V/I.

Then > 12/Rm would be the power supply current required for start-up.

Many power supplies go into current limiting, but simetimes you can add capacitance to the power supply.

#### Pommie

##### Well-Known Member
As a "bodge" you could also try a 2Ω 10W resistor in series with your motor but it may not work at all.

Mike.

#### dr pepper

##### Well-Known Member
If the said 5a supply is current limited then it would probably do as you said, run at lower torque.
However if said supply supply does not have current limit, you could easily blow it up or burn it out.
To do it 'right' you want something like a 10a supply min.

#### JonSea

##### Well-Known Member
Many (most?) switching power supplies reduce the voltage as they go over their design output current, which is exactly what this poster is seeing.

#### fezder

##### Well-Known Member
Inrush current in both capacitive (capacitors) and inductive (motors) loads can cause headache. I had trouble with isolation transformer that took several hundred peak current during start, which 10A mains fuse didn't like. It was solved using series resistor, which limited inrush current. After small period, this resistor was short-circuited with relay. Happy fuse now. You could try similar thing, depending on what torque is needed and whether or not motor likes being bit stalled during start.

Another method that comes in to mind is using soft starter, which basicly increases current slowly. Some machines like angle grinders (in region around 2kW) use these.

#### audioguru

##### Well-Known Member
I suspect that your 12V battery is not fully charged or is old and works poorly.

#### unclejed613

##### Well-Known Member
if you put a large capacitor across the power supply, and then use a switch to connect the motor, the cap discharge current might provide enough kick to get the motor running. large capacitors are often used to provide a short burst of high current that's beyond the capabilities of the power supply by itself.

#### Goldmast

##### New Member
No gonna lie; I didn't understand most of that. I will try to re-search and apply what you have said. But, I'm mostly interested in just buying a power supply with enough 'get up and go' to start the motor and keep it running. this is almost exactly the motor i bought: https://www.autopartswarehouse.com/...components/wiper-motor/replacement/repj361105 --- I know there a too many variables to say EXACTLY. But, can it be guesstimated what would run this type of motor? or do i miss the point completely. Ill also re-assert that perhaps "novice" was giving my self too much credit.

#### audioguru

##### Well-Known Member
How can you power a motor that has absolutely no spec's??

#### Goldmast

##### New Member
Yes, sorry. I shared that just so people would know the type of motor. What specs specifically would you need to know? Maybe I can track down the model/manufacturer.

#### rjenkinsgb

##### Well-Known Member
To give you an idea of the power "overhead" needed to allow for starting, on my car the windscreen wiper motor circuit uses a 30 Amp fuse....

Wiper motors are somewhat crude and car electrical systems have massive current capability.
If you can use a different motor it may make things a lot simpler overall.

eg. a smaller, high speed one with a built-in gearbox to give a reasonable output speed. Depending what you are using it for, you may possibly be able to get a similar result with a motor that takes an amp or less.

It's all down to the speed and torque needed at the mechanical load.

#### Goldmast

##### New Member
30amps? Deal lord. I'll do some more testing. Unfortunately some of the smaller motors I have looked at can't handle the 3lbs I need this to move

#### Pommie

##### Well-Known Member
Maybe you should describe what you are trying to do. Not, I'm trying to power this motor but I'm trying to move this weight in this way in this time.

Mike.

#### rjenkinsgb

##### Well-Known Member
I reiterate -
It's all down to the speed and torque needed at the mechanical load.
Without knowing that, it's impossible to know whether any particular motor / gearbox combination will be suitable.

A tiny motor that takes a fraction of an amp can pull or lift a 3lb load, just not very fast...

#### JonSea

##### Well-Known Member
The Post Apocalyptic Inventor on YouTube recognizes the value of windshield wiper motors and has used them in some great applications. The lack of a data sheet and full specs doesn't hold him back.

You will find good information these two videos.

Build Powerful Linear Actuators From Windshild Wiper Motors and Car Jacks

Build A Robot With Windshield Wiper Motors

TPAI also has a lot of other videos about repurposing washer and dryer motors and salvaging mechanical parts from printers and copiers. Hope this helps.

#### rjenkinsgb

##### Well-Known Member
The Post Apocalyptic Inventor on YouTube recognizes the value of windshield wiper motors and has used them in some great applications. The lack of a data sheet and full specs doesn't hold him back.
OK - but, he's using a large bench supply in some sections, or a lead-acid battery in others.
He's not concerned with efficiency.

I have a 25A bench supply that could do the same thing - I'm not going to permanently build it in to project, though!
Demonstrating ideas and making practical projects are two rather different things.

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