Cheapest and easiest way to slow down a motor

[sanddrag]

What is the cheapest and easiest way to slow down a permanent magnet 12VDC motor (think Van Door motor)? I was thinking of wiring a halogen lightbulb in series.

[phrontist]

Big resistor?

[Alan Anderson]

Originally Posted by sanddrag

What is the cheapest and easiest way to slow down a permanent magnet 12VDC motor (think Van Door motor)?

To "slow down a...motor" could mean several things. Are you talking about applying a brake to a spinning motor? The Van Door motor's gearing probably won't break if you put a high mechanical load on it, but the motor might overheat. If you want to stop a motor quickly, shorting its input does a good job.

Or are you talking about limiting the unloaded speed? Just give it less voltage.

I was thinking of wiring a halogen lightbulb in series.

That's probably going to give you fits trying to characterize the voltage drop across the bulb. Halogen lamps aren't very happy about being run at lower than their rated voltage anyway, are they? I'd just use a simple resistor, or preferably choose a lower supply voltage in the first place.

[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?

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.

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

[billbo911]

Originally Posted by sanddrag

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

I'd put money (only a couple bucks) on it containing a "chopper circuit". By using an SCR, or pair of SCR's, it will pass as little or as much of the full AC signal as you set the dial to. To understand how it operates really requires a little understanding of electronics and maybe a picture or two, but I'll try to describe it.
At full speed, the circuit allows the full positive and negative cycles of the AC to pass. When you turn down the speed dial, the circuit will only pass a equal portion of the positive and negative cycles.
Say you only want to provide the motor ~50% power, the circuit would pass 4.15 ms of the negative cycle as it approaches 0Vac and 4.15ms of the positive cycle as it moves away from 0Vac. This adds up to 8.3ms per cycle, half of the full 16.6ms normal AC cycle.
I really hope that helps. This again is where "A picture paints a thousand words" really is true.

[Stu Bloom]

Originally Posted by billbo911

I'd put money (only a couple bucks) on it containing a "chopper circuit". By using an SCR, or pair of SCR's, it will pass as little or as much of the full AC signal as you set the dial to. To understand how it operates really requires a little understanding of electronics and maybe a picture or two, but I'll try to describe it.
At full speed, the circuit allows the full positive and negative cycles of the AC to pass. When you turn down the speed dial, the circuit will only pass a equal portion of the positive and negative cycles.
Say you only want to provide the motor ~50% power, the circuit would pass 4.15 ms of the negative cycle as it approaches 0Vac and 4.15ms of the positive cycle as it moves away from 0Vac. This adds up to 8.3ms per cycle, half of the full 16.6ms normal AC cycle.
I really hope that helps. This again is where "A picture paints a thousand words" really is true.

[DISCLAIMER - MECHANICAL ENGINEER'S POST]

That sounds analogous to the Pulse Width Modulation we currently use (our speed controllers/victors) to control some of the motors on the robot, except that in your description you are "super-imposing" a duty cycle on an AC sine wave.

[/DISCLAIMER]

Am I even close??

[billbo911]

Originally Posted by Stu Bloom

[DISCLAIMER - MECHANICAL ENGINEER'S POST]

That sounds analogous to the Pulse Width Modulation we currently use (our speed controllers/victors) to control some of the motors on the robot, except that in your description you are "super-imposing" a duty cycle on an AC sine wave.

[/DISCLAIMER]

Am I even close??

Gosh Stu, for a Mechanical guy, you're sharp! . Just kidding!!
As a matter of fact, I almost used the Victor's operation to try to help describe it. There are some subtle differences, but that is what both of these circuits do, modify the duty cycle of the power supplied to the motor.

[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?

[lukevanoort]

<Off topic> Are any of the rest of you wondering what sanddrag is up to? The smoke and now this. </off topic>
Sanddrag, I don't know what wipers you're talking about, but I think that they just switch the motor on and off, I think they're verying wipe frequency, not speed.

[Madison]

Originally Posted by lukevanoort

<Off topic> Are any of the rest of you wondering what sanddrag is up to? The smoke and now this. </off topic>

He's helping to build a parade float for the Tournament of Roses parade on January 2, 2006.

[sanddrag]

Originally Posted by lukevanoort

<Off topic> Are any of the rest of you wondering what sanddrag is up to? The smoke and now this. </off topic>
Sanddrag, I don't know what wipers you're talking about, but I think that they just switch the motor on and off, I think they're verying wipe frequency, not speed.

I am part of the crew building the Cal Poly Universities "Enchanted Reverie" float for the Tournament of Roses Parade. You and a few million other people can look for it on TV January 2nd! As a note on the smoke, I don't think it is going to work out for this time. The deadline is too quickly approaching.

For the wiper control, yes, I don't know what I was talking about either. I guess in southern California I don't use my wipers often enough to remember what they look like when they move.

[greencactus3]

Originally Posted by sanddrag

For the wiper control, yes, I don't know what I was talking about either. I guess in southern California I don't use my wipers often enough to remember what they look like when they move.

no.. for my truck i have a high speed and a low speed and a whats it called.... wipe. pause. wipe. pause.... with variable pauses. so well there are 2 speeds at the least on a cheap truck. so for i unno... say.. wrxs just might have more speeds? at least the rally ones have veryy fast wipes.

[KenWittlief]

another easy way to drop voltage: silicon diodes have a fixed 0.7V drop. If you have diodes that can handle the current, you can put them in series till you get down to the voltage you want.

BTW, the easiest and cheapest way to slow down a motor is to wack it with a sledge hammer,

and the 'right' way to slow down a DC motor is to increase the strength of the magnets. The stronger the field, the more torque it will have and the slower it will run (due to the EMF voltage).

[Jay H 237]

The wipers have resistors for the different speeds. If equipped with intermitent wipers (almost all vehicles these days) then there's also a timer, this is what greencactus was mentioning.

Depending on the vehicle the resistors and timer can be part of the switch, a separate unit somewhere, or part of the wiper motor. My 1987 Chevy Caprice has a circuit board mounted to the wiper motor where all this is.

The blower motor (for heat, defrost, a/c) for a vehicle also uses resistors to get the different speeds. These resistors would be attached to a single board in the heater box. They are in the heater box to help keep them cool with the moving air.

[Al Skierkiewicz]

Sanddrag,
So many questions, so little time. You can go to a junk yard and get a couple of wiper controls. They are big resistors (low ohms but high power dissipation) and they should be easy to wire up for what you need.
The router control device you saw is like a lamp dimmer for AC motors only. It contains a Triac that switches on for only a portion of the AC sine wave cycle.
Did you need to run all motors at the same low speed at the same time? If so you could buy a variable voltage supply. If the motors don't need direction control, a pot and a power transistor will likely do what you need. Make sure that the transistor is attached to a heatsink of some type. 2n3055 should do but anything Radio Shack has in NPN 5-10 amp range should work. Try a 10K pot to start and see what results you get. I think the junk yard is the best bet.