Motor, potentiometer, & power supply.

[Catana]

Hi I'm new to electronics.<p>I want to mount a small motor on metal, turn it on and very the speed of the motor with a control knob.
I know I can buy parts at Digi-key but I don't know just what to buy.
For example:
1. How does the motor plug into the wall?
2. Must I wire this through a potentiometer so I can control the speed of the motor?
3. Am I missing anything? I've never done this before.<p>Thanks for any help.

[dyarker]

"1. How does the motor plug into the wall?"<p>Whoa, slow up partner! Are we talking about a variable speed 115VAC motor here? This can be VERY, VERY dangerous unless you have experience selecting properly rated parts, safely enclosing all live connections, and grounding all exposed metal. And, it sounds like you aren't!<p>If you mean a small DC motor, and the closest you'll get to 115VAC is plugging in a UL rated wall wart, there is lots of help here.<p>Either way, what are the details of the motor and what you want to accomplish?<p>I don't believe anybody on this forum is willing to help you electrocute yourself or a family member. Varying the speed of an AC motor is certain possible. But, it will need more than a potentiometer, and A LOT more explanation to do it safely.<p>Cheers,<p>[ February 07, 2005: Message edited by: Dale Y ]</p>

[Externet]

Hi Catana.
We need to know more about the motor.
If it is AC the size of a hand drill or smaller, you may use a $5 dimmer; some are available as in-line dimmers allowing to plug to the wall.
If it is a DC motor as canibalized from a tape recorder, a suitable power transistor circuit and a wall adapter should work.
If it is a stepper motor, things get more complex.
Miguel

[Catana]

Hi.<p>1. I would like to use a small motor preferably no larger than 3 cubic inches. Maybe a computer hard drive motor would work.
2. I want to very the speed with a knob or some control from about 120rpm to at least 3000rpm.
3. I would rather this motor not slow down when there is a load put on it. I think the word tork is apropriate here.
4. I want to plug this into my kitchen wall socket which I'm pretty sure is 110 vac. I've heared of things like rectifiers, transformers, ac/dc adapters.

I talked to a Digi-Key tech. this morning and was told they have a limited selection. I'm having trouble locating a motor company online.<p>Any suggestions on what to buy, where to buy it and how to put it together would be much appreciated.

[haklesup]

Designing with motors is often an exercise in compromise between size, speed, torque, cost and complexity of control circuits. It all comes down to what you really want to do with it.<p>The first thing you need to understand is that there are many kinds of motors. Some can be speed variable and some do not do so well at that function. You want a motor with good Torque at all speeds. That rules out a lot of motor types.<p>If the motor is AC the speed circuit will modify the waveform of the AC wall voltage. If it is DC motor, you will first need to use a rectifier and transformer to reduce and convert the AC to DC before applying that voltage to your speed control and motor. What voltage and how much current depend on the motor you use and the amount of torque you need to supply.<p>For simplicity you will probably find that modifying a hand drill will be easiest. There are many models both AC and DC, most are variable speed, all have good torque (even with the gear box removed). They may not be small enough for your application though.<p>A stepper motor is likly to give you the flattest torque curves (torque vs. speed) and be compact enough but the circuits to drive them are more complex and expensive.<p>A motor in a disk drive may work but they are intended to run at one speed and may suffer loss of torque at lower speeds as is common with many motors. This will be true of most DC permenant magnet motors. Many of these motors are run at high speeds and geared down to the proper speed to relieve loss of torque.<p>Going back to your original question. A potentiometer may be part of the speed control circuit but if you use it to directly vary the voltage to the motor, it will likly result in loss of torque. <p>Search the web using "types of motors" and you will learn a bit more.

[Engineer1138]

If you want to vary the speed of the motor, and not have it slow down when a load is applied, you'll need some sort of feedback like a servo (not the little RC servos; I mean industrial servomotors).<p>Do a google search on "servo motor controller" and see what comes up. Be prepared to spend $$$. There doesn't seem to be much need for them outside industrial controls, so the ones available tend to be high priced.<p>It's not hard to build a simple controller, but for someone who's new to electronics, I wouldn't recommend it. It's a path ripe with frustrations, trust me

[Catana]

I can be flexable with tork if it will make selecting the right kind of motor easier.

[Engineer1138]

In that case, I'd go with the suggestion to use a cheap electric screwdriver.
Wire a rheostat/high current potentiometer where the speed trigger goes, or use the one that was built in and you should be good to go.

[Catana]

Can an electric screwdriver produce at least 3000rpm? I also need to mount the motor on a metal plate, tighten it down and attach something to the drive shaft.<p>By the way thanks for the "servo motor controller" tip. That may come in handy in the future when I'm more knowledgeable.

[CeaSaR]

OK, we know that you want a variable speed motor (~120 - 3000 rpm) attached to a metal plate that has fairly constant torque across it's rpm range, to which you want to attach something to the output shaft.<p>Perhaps knowing your ultimate goal would give us a bead on what you really need. One thing we should know is what do you intend to attach to the output shaft and what you will do with/to that attachment? Different uses have different needs. Fan blades can be driven by small DC motors with a simple rheostat power control (think TYCO slot car controller). Drive wheels require a heftier motor (AC or DC) and their more complicated control circuitry. Shop tools, such as a table saw or lathe, would require an AC motor (if stationary) and would probably not be of the variable speed type (normally).<p>A 3 cu.in. motor - sounds like a small DC motor. These can be anything from the cheap $0.59 hobby motor to RC motors costing $200 or more.<p>Give us a little more info so we can narrow it down.<p>CeaSaR

[Catana]

I will attach fan blades.
Is a simple rheostat power control two devices in one?
1. converts ac to dc.
2. used to adjust the speed of the fan, kind of like adjusting the speed of the slot cars.

[rshayes]

A permanent magnet DC motor is probably the easiest type to use for variable speeds. The speed is roughly proportional to the voltage applied to the motor, with some reduction in speed as the mechanical load on the motor is increased.<p>The torque of a DC motor is proportional to the armature current. The motor will have a maximum current rating, and this will determine the maximum torque that the motor can deliver.With a fan as a load, the torque required will be maximum at the maximum fan speed.<p>A turning motor generates a voltage directly proportional to speed. This voltage opposes the applied voltage, and is sometimes referred to as the "back EMF". (EMF is Electromotive force) An additional voltage equal to the armature current times the armature resistance is added to this. The sum of these voltages is equal to the applied voltage. When the motor is starting, the back EMF will be zero, and the current will be equal to the applied voltage divided by the armature resistance. This will produce a high current, which results in a high torque, and the motor will accellerate rapidly. As the motor speeds up, the back EMF subtracts from the applied voltage and the torque decreases. The motor accellerates until the torque being delivered equals the torque required by the load at the speed that the motor is running.<p>In effect, you need to apply a voltage that determines the speed of the motor (the back EMF for that speed) plus an additional voltage to compensate for the voltage drop in the armature resistance due to the current required to maintain that speed.<p>Since you will be varying the speed by reducing the motor voltage, you probably want a motor that operates at a little over 3000 RPM with 12 volts applied. The voltage required to run the motor at 150 RPM would be about .6 volts in this case.<p>Controlling the motor voltage directly with a rheostat is not very satisfactory. The motor currnet may be an amp or two, depending on the torque required, and a rheostat capable of handling this type of current is not a common item.<p>A potentiometer can be used to choose the desired motor voltage. The voltage can then be amplified with a linear power amplifier to drive the motor. The amplifier can be an op amp with a power stage added to increase the current capacity of the amplifier. It might also be possible to use an integrated circuit meant as an audio amplifier. Some of these are effectively high current op amps. A third alternative is to build an amplifier from discrete components. This would allow single supply operation.<p>The power supply will need a transformer, rectifier, and filter capacitor. A 25 volt center tapped secondary would allow using a bridge rectifier for a positive and negative supply of about 15 volts each. Some ripple on these supplies can be tolerated, and they do not have to be regulated.<p>A small three terminal regulator can be used to supply the potentiometer used to set the motor speed. This can be a 5 or 6 volt regulator. The output of the potentiometer will be 0 to 5 or 6 volts. This will require a gain of about 2 to 2.5 in the power amplifier used to drive the motor.<p>The power amplifier will also need a heat sink, since it may have to dissipate a substantial amount of power. The worst power dissipation will probably be at settings of about half maximum speed.<p>First, you have to find a suitable motor. The rest of it will have to be disigned around the motor characteristics.

[CeaSaR]

Took a quick look through Nuts and Volts and found that Windsor Distributors Company has a motor (PN 0123850R) that might work for you. It runs a bit faster than you might want, but at least they have it. The next thought was to use a standard LM317 adjustable voltage regulator circuit to control the fan speed. Here are 2 sites that will help you out: http://ourworld.compuserve.com/homepage ... page12.htm and http://www.national.com/ds/LM/LM117.pdf . Lastly, I would find a DC wallwart that could supply 3 volts more than what you would think of using (LM317 needs this overhead) at a current rating of 1 amp minimum. Should do you just fine.<p>If this motor won't do, just peruse the advertisers on the main Nuts and Volts site (scroll to the bottom of the page and click Nuts and Volts Magazine) to see what is available.<p>Good luck!<p>CeaSaR

[Catana]

This is a little more complicated than I originally thought.
But I’m learning.<p>It sounds like I should just order a 12 vdc permanent magnet motor of at least 3000rpm and work from there. I will look for the (PN 0123850R) from Windsor Distributors Company you mentioned and see what else they have.<p>I hear a lot of other components mentioned for example:
linear power amplifier
transformer, rectifier, and filter capacitor
LM317 adjustable voltage regulator circuit
DC wallwart<p>I’m guesing a DC wallwart converts ac to dc. Maybe it’s another word for transformer. Any way, after I have the exact specifications for the motor I order, I will need help figuring the rest out.
Thanks for all of your help.

[Engineer1138]

Catana:
It's not really that bad. Just that for someone with no electronics knowledge, going from zero to motor control can be rather overwhelming.<p>I never asked you what the motor/speed control combo was for, if you can tell us, perhaps we can make better recommendations.<p>As far as a simple speed control, I wrote an article for Nuts&Volts years ago that showed basic PWM control of a DC motor that could be built with Radio Shack parts.
I just looked on the back issues page. It's September 1996. Having moved a few times since then, I have no idea where my original schematics are, but N&V can sell you a back issue of the magazine and you can ask me any questions you have about it.... as much as I can remember 9 years later, anyway