Electronics How do I reduce rpm on 12volt DC motor

Hi to everyone new boy here. Got a problem I need solving, can anyone help me ? Please!!

Here goes:

I have a 12volt Dc motor, its a windscreen wiper motor off a car. I am powering with a 12volt 7Ah battery. The RPM of the motor is way too fast for what I need it to do. How do I reduce the RPM of the motor ? I have been told connecting a resistor in series would reduce the voltage, thus reducing the RPM of the motor, if so what would be the correct type & size of resistor. Also I may require variable speed on the motor later on. Is there a simple way to reduce the motor RPM.

Many thanks in anticipation

Jonah204

 

You can use gears (or belts) so the torque goes up as the speed goes down.

 

I need to reduce RPM electrically not mechanically. I have a spinning flat piece on metal attached to the motor shaft, so thereis no room to modify it mecahnically. Besides it would be too heavy. The motor is only small, about the size of my fist. I need to reduce RPM down to about 28 to 33 RPM. At the moment, well its way too fast

Any advice please

Jonah

 

They make smaller gear motors that may be more suited to your needs, check those out. but if it must be that motor, I would use PWM to slow it down. Do you need it to be a either on/off at a constant speed, or do you need it to be variable?

Remember www.google.com is your friend, check out things like PWM, motor control, motor speed control, and other things you can think of.

 

For such low speeds gearing is the best answer. Problem with voltage control is, at 6V you've only got 1/4 the power, at 3V only 1/16, so you soon get to a point where the motor hasn't the torque to run.

You could try a simple PWM controller but I wouldn't expect to get lower than 1/4 the original speed with any smoothness.

 

I agree. Those kind of speeds are not going to be able to be achieved (with good results) electronically. Some form of gearing down is required.

 

If you really are insistent on using that motor, and electronically reducing the speed, try hooking up a potentiometer (pot) between the battery and motor. make sure you get a linear pot, not an audio pot. audio pots have a non-linear resistance curve, i.e. it doesn't change very much for most of the rotation, then changes a LOT right near the end. Radioshack has a few different models, and they're only about $5-$8 USD, so it's not breaking the bank.

If you've already got the mechanical system set up, you can test the speed vs. torque issue by playing with the dial while the motor is running. If it's a low torque application, you may be able to get by with just the pot. If it works, it can also give you a ballpark idea of what size resistor to get for hardwiring the setup, i.e. figure out which end of the dial it's closer to and buy a few different packs of resistors in that general range. Definitly cheaper than engineering gears and pulleys in the short run, but potentially (pardon the pun) an unworkable solution due to torque issues in the long run.

 

The common 1/4W carbon pot is only designed to carry a few mA (a 1k will safely carry 16mA) so test in a well-ventilated area.

 

Right, he'll need a more beefy rheostat.

 

hmm, sounds like a trip to mouser.com or digikey.com ! Might be a bit costly to get something beefier though...

 

With such slow speeds, a variable resistor really isn't going to do the job. The motor will just simply refuse to spin at those currents. Even with PWM there's going to be problems getting them spinning at those speeds unless you want to visibly see each pulse...

Surely it's not going to be too difficult to just hook up a small belt to step down the speed or buy a small gearbox for a few quid? Even something as small as 10:1 would help.