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??

[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.

[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?

[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.

[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.

[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.

[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.