Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

Big resistor?

Re: Cheapest and easiest way to slow down a 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.
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.

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

[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??

Re: Cheapest and easiest way to slow down a motor
 

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

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)

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.

Re: Cheapest and easiest way to slow down a motor
 

Gosh Stu, for a Mechanical guy, you're sharp! :eek: . 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.

Re: Cheapest and easiest way to slow down a motor
 

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?

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

<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.

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

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

Re: Cheapest and easiest way to slow down a motor
 

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. :D

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

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).

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

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

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

So if I wanted the motor to run at 6 Volts and it draws 5 amps (estimate), I'd need a 1.2 Ohm resistor in series with it?

Re: Cheapest and easiest way to slow down a motor
 

Yup.

Edit: By the way, that would be 30W through the resistor. That'd be a very big, very hot resistor. Keep in mind that lower powered soldering irons are 25W.

Re: Cheapest and easiest way to slow down a motor
 

I don't know what makes up a dimmer switch for lights in a house or office but would that work for 12VDC? (forgive me, I'm not really an electrical guy).

Re: Cheapest and easiest way to slow down a motor
 

Apparently, they won't work. I just read an interesting article on Howstuffworks. They take advantage of the AC to turn the bulb off for a little while at the beginning of each half cycle.

I might get my head bitten off for suggesting this dreaded device again, but maybe you can use a 555. I'm pretty sure it will run on 12V. To make a 50% duty cycle oscillator, it would only take the 555, two resistors, and two capacitors. Then, you could just feed that into a MOSFET. The motor only has to go one direction right? If so, you'd just the single transistor, not an entire h-bridge. You won't have to deal with trying to dissipate as much heat as the resistor will be putting out.

Re: Cheapest and easiest way to slow down a motor
 

You mean like this http://www.allelectronics.com/cgi-bi...cgi?item=MSC-6 Seems like a neat little thing that would actually be kind of fun to try out.

Not exactly cheap or easy, but it is kind of neat.

Re: Cheapest and easiest way to slow down a motor
 

Actually, that's exactly what I was talking about, but fancier. The motor is still driven by the single MOSFET, so if it's not beefy enough, you can easily swap it out for one of appropriate beefiness.

Also, the parts you actually need are probably <$3. If you don't want to spend $15 on that thing, I'd be happy to draw up a nice little schematic for you.

Re: Cheapest and easiest way to slow down a motor
 

Cool! I sent you a PM.

Re: Cheapest and easiest way to slow down a motor
 

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.

Re: Cheapest and easiest way to slow down a motor
 

Wow, after I was toying around with a nearly identical idea for this thread, you'd think I would have figured that out. You'd also think that after having done exactly that to control a motor a few weeks ago, I'd figure it out...

Re: Cheapest and easiest way to slow down a motor
 

Al,
That attached diagram looks much simpler than what I was going to do (pwm control with a 555 timer) but not being an electrical guy, I'm going to need some pictures that look like the real thing (not just symbols) to help me understand how to hook it up. Also, does your setup get much hotter than the other more complicated way?

Re: Cheapest and easiest way to slow down a motor
 

There are only three parts, the battery on the left, a pot in the center and a transistor on the right in series with the motor. The transistor will need to be mounted to a heatsink because it will get hot. As you move the pot, a varying amount of current will be supplied to the transistor base thereby controlling the current in the collector/motor. With the wiper all the way up at 12 volts, the motor will be at max speed. Start with a 10K pot but you may need a 100k pot depending on the range you need and the gain of the transistor.
As an alternative, you could use the speed control from a 12 or 14 volt drill too or the motor control from a car blower which is usually three resistors and a switch.

Re: Cheapest and easiest way to slow down a motor
 

You already have a great DC 12 volt speed control in your possession. A cordless drill has a pretty good electronic speed controller. However, the problem is you want to run the motor continuously for many hour. It may be better to let the motor run at full speed and use belts to speed reduce. A 1.5" pulley and a 3.5" adjustable pulley would give a good range. A couple of shaft inserts would get you from a .5 shaft down to the van door motor shaft.
To bad you don't live in the frigid parts of the country. Heat build up wouldn't be a problem.

Re: Cheapest and easiest way to slow down a motor
 

I've decided to try this out http://www.dprg.org/tutorials/2005-11a/

I have one question though, where is exactly is pin 3 on the 555 supposed to connect to?

Re: Cheapest and easiest way to slow down a motor
 

Wow, that's quite a thread.

1. To reduce the speed of the motor, cut the voltage. R=V/I, if you want 6 volts to the resistor and I = 4 Amps (You measured that?), 1.5 Ohms. A dozen feet of 18 gauge wire will cover that pretty well, just coil it loosely so it can dissipate the heat. BUT all this is OK only if you can accept significantly reduced torque and power.

2. To control the speed to any value, and not lose torque or power, use a PWM motor controller in "Locked Anti-phase" mode. Victors (at leats 2005 version) don't do this, but something like that should be easily findable, try a google search or visit IFI and other robotics sited. PM me if not.
(FYI: Locked Anti-Phase control switches voltage polarity quickly, so the motor actually has full current at all times (max torque) but at, say, 50% duty cycle the net current flow over time is zero, so the motor stands still (hums, actually). At 75% duty cycle, it moves forward around half speed, but again with full torque.)

3. To control the speed with a Victor, just build the circuit I posted near the end of this post and get a victor, or similar. You do lose some torque though, but it seems to work for most FIRST teams...

4. Modern cars use a special motor for the wipers, with two windings, one for low and the other for high speed. Both windings take 12 volts. Resistors haven't been used for decades.

5. The cordless drill idea from Gdeaver is pretty good, though.

Re: Cheapest and easiest way to slow down a motor
 

The schematic near the bottom of the page you linked to gives the answer.
Pin 3 connects to the wiper of the 100K potentiometer. The commentary after the schematic explains why.

Re: Cheapest and easiest way to slow down a motor
 

In Al's drawing of the simple pot and transistor setup, how do I know which way to oreint the transistor so the pns go to the right places. I know where the middle one goes, but how do I know which way the others go, or does it not matter? Does that trapezoidal shape represent the shape of the actual transistor? Does the arrow signify something?

Re: Cheapest and easiest way to slow down a motor
 

Buy a dimmer switch from Home Depot.

Re: Cheapest and easiest way to slow down a motor
 

do they have DC ones at homedepot? they tend not to have anything for 12V....at least from what i can see.
car dimmer switches are pretty cheap on ebay too..

Re: Cheapest and easiest way to slow down a motor
 

I've attached a labeled transistor symbol.

If you've got the 2N3055 that Al suggested, then hold it such that the pins are facing you, then rotate it so that the pins are closer to the left side (see P.4 of the linked datasheet). The lower pin is the base, the upper pin is the emitter, the case is the collector.

If you have the TIP3055 that I mentioned to you, hold it such that you can read the writing on its face and have the pins pointing down. From left to right: base, collector, emitter. The tab is also connected to the collector.

There's also an issue that someone, maybe Al, can resolve. I noticed that the gain with Ic = 4A is only 20-70. That would require a base current of 200-57mA to get up to the 4A. With 12V, your pot would have up to 2.4W going through it. That much power seems a little disconcerning to me. The readily available transistors with higher gains can't handle enough current. Any ideas?

Re: Cheapest and easiest way to slow down a motor
 

Don,
Victors are in "locked antiphase" operation. They are a classic "H" bridge with one input signal representing both magnitude and direction. You could set the input signal to switch direction as rapidly as the PWM allows but the current draw is significant. Since that is not needed in most of our applications, you won't see it very often.

Sandrag,
your 555 circuit is an excellent method, just be sure that the MOSFET can handle the current for you motor. The spec sheet for the device shown says it is good for 50 amps if it is at 25 degrees C. It might need heatsink but any chunk of aluminum should work if it is insulated.