Cheapest and easiest way to slow down a motor

[Tom Bottiglieri]

Buy a dimmer switch from Home Depot.

[ChuckDickerson]

Originally Posted by sanddrag

I'm wondering if there is anything from an automotive location that would do it. I believe the motors are actually windshield wiper motors, or they could possibly be window motors. How does a car's windshield wiper system get the different speeds? Does it have all different resistors to get the different speeds?

Many, many, many moons ago I did a science project in high school where I needed to dim a car head light like you would a light in your house. I went to the local auto parts store and asked if I could look a some car dimmer switches. This was a long time ago but back a then a lot of GM cars had a single knob that controlled all of the lights in your car. You pulled the knob out half way and your park lights came on. You pulled it out all the way and your headlights came on also. You turned the knob to the left and your interior dome light came on. Now the important part: You turned it to the right and it dimmed the dash lights. If you wanted to hit your high beams you either pulled the turn indicator or mashed a switch on your floor board under your floor mat. This was long before anything like daytime running lights. So anyway, I roamed up and down some of the shelves behind the counter at the auto parts store until I found this type switch. I put it in a little aluminum box and wired the headlight to the interior dimmer terminals and a 12V battery and it worked great. The little aluminum box got a little warm but I had a 12VDC infinitely variable dimmer. I don't see why this wouldn't work just as well for a small 12VDC motor. Now a dash dimmer switch for a 20+ year old GM car may or may not be a stock item at your local AutoZone or Pep Boys I don't know. If not I bet a GM dealer will know what you need or head to an auto salvage yard. The dimmer switch shouldn't be expensive. Remember K.I.S.S. Sometimes you need to think "Old School".

Edit: The technical term for the dimmer switch is a rheostat and it looks like this.

[Alan Anderson]

Originally Posted by sanddrag

I'm wondering if there is anything from an automotive location that would do it. I believe the motors are actually windshield wiper motors, or they could possibly be window motors. How does a car's windshield wiper system get the different speeds? Does it have all different resistors to get the different speeds?

A two-speed wiper motor actually has two separate power inputs for the two speeds. Most also have additional connections to support "intermittent" operation and stopping at a park position. If your motor doesn't have more than two wires or terminals, it probably isn't a wiper motor.

[greencactus3]

Originally Posted by Alan Anderson

A two-speed wiper motor actually has two separate power inputs for the two speeds.

do you know whatthose 2 inputs are? like 12v and a lower voltage? which is through like a resistor or sumthing?

[sanddrag]

If I was to just put a resistor in series with it (provided I could get a resistor rated for that much power), how would I calculate what resistance (in ohms) I would need to take the 12V down to oh, say, 6V? I should know this but I don't remember.

[sciguy125]

Originally Posted by sanddrag

If I was to just put a resistor in series with it (provided I could get a resistor rated for that much power), how would I calculate what resistance (in ohms) I would need to take the 12V down to oh, say, 6V? I should know this but I don't remember.

Kirchoff's Voltage Law: sum of the voltage drops around a loop is equal to the sources

You have a 12V source and two drops. You can get the voltage across the resistor with Ohm's Law: V=IR. Whatever's left is the voltage across the motor.

12 = IR + V (V = motor voltage)

Notice that the voltage will change with the current.

[KenWittlief]

if you put two motors in series they will have half the power (run at 6V). This is only true if they are equally loaded. If one motor is loaded more that the other it will slow way down or stop, then the other motor will run at full speed.

A simple way to make a chopper circuit is to get a 12V SPDT relay, and put the relay coil in series with the normally closed contacts. When you energize the relay the contacts will open, causing the relay to close again... and you end up with a buzzer. The other pair of contacts can then be used to power the motor, chopping the power, very much like a Victor speed controller.

I cant venture a guess whats in the box. If it runs on DC then it cant be SCRs or Triacs, because they depend on AC to be able to unlatch. There is no duty cycle for an SCR or Triac to chop with a DC power source.

The light bulb will work. Try a normal 110V 100 watt lamp. That way you wont have to worry about the heat dissapation: the bulb will glow dimly. If a 100W lamp lets the motor run too fast, switch to 50W or 25.

[BrianBSL]

Originally Posted by sanddrag

I need to electrically slow down the motor. Sorry I wasn't clear on that. I only have a 12VDC supply. The motor will probably pull something like 4 amps continuous its application. It will be running continuously for maybe 5 hours. Wouldn't the resistor get very hot and need lots of cooling?

Not much more so than any lightbulb thats going to give you the voltage drop you want.

Originally Posted by sanddrag

I have two or three of these motors I'll be using in the system. I'm wondering if I could get what I want by just wiring them all in series.

Don't see any reason why not, as long as you arn't hoping for perfectly equal speeds. Just be careful about chassis grounded stuff if they are from automotive applications. (Don't think you usually have to worry about this with motors, but just an FYI)

Originally Posted by sanddrag

Also, would anyone happen to know what is inside this box? Thanks.

I'm guessing a rheostat, but thats purely a guess, assuming its designed for DC. It (the harbor freight link) is a little unclear about if its for DC or AC. A rheostat is going to throw as much heat as a resistor in the same application.

Basically, if you want to do the same voltage drop, and draw the same current, you have to do something with the power you are throwing away across that voltage drop, likely to heat.

I'd just see if I can find some power resistors. If you get one big enough (over-rated enough on power) then you wouldn't have to worry about any type of forced air cooling.

[Bill_Hancoc]

Originally Posted by sanddrag

Also, would anyone happen to know what is inside this box? Thanks.

Those are used to vary speed on say a non-variable dremel tool. They work great but i am fairly certain that they require a 120AC voltage source to work.