DC Motors

Hi, Jon. My Milwaukee rechargeble drill uses a DC permanent magnet motor.

You can reduce the DC voltage applied to the cordless drill without burning up the motor. It will just reduce the speed. It's overvoltage that will fry the motor. And current is proportional to the voltage applied. That is, if you have an 18V drill, it will require only half the current at 9V. The thing is, it will then only be using 1/4 the power (P = I^2 * R), which means your output power will decline accordingly, too.

Reducing the power supply voltage will reduce the torque. You'll have to look at your project to see what's best, but your work output from the motor will decrease a lot if you run it at less than 6V. I'd suggest using some additional gearing if you need to run it at lower speeds.

Also, most rechargeables require several amps of current. You might want to look at using PWM control if you need to have an adjustable power source. Here are some good circuits:

The "A Practical PWM Circuit" toward the top of the page will work well for a cordless motor requiring several amps DC.

Good luck Chris

You might be able to get a higher torque at a slower speed by using a low frequency PWM, which will provide a full power pulse long enough to cause the mechanical movement you need, and then waiting a little while before the next pulse. This might work well if there is some springiness in the mechanical drive, so you can essentially wind up a spring and then wait until the load moves, and then apply more pulses as required. It would be much like a hammer drill, which works well to overcome initial friction and inertia to get the load moving.

Otherwise, probably starting with full power and then throttling back once it is moving might be a good option.

Paul

"Chris"

** It will need two additional things to do that job:

1. A diode across the motor, rated for 7 amps or more, preferably a Schottky type.
2. A heatsink for the MOSFET and the diode.

A higher current MOSFET should be used - one with an on resistance less than 20 milliohms.

...... Phil

Hi, Phil -- you're right. Thanks.

Chris

Why not just use an scr and AC instead?

Well, a triac of course...

"Jon Slaughter"

** Cos it is a DC motor - you PITA bloody fool.

....... Phil

a farrly conventional permanent magnet motor, Jacobs is a popular brand.

that won't happen.

don't stall them for too long. or run them at low speed under load.

if you want slow use a gearbox.

the biggest problem is heat, which is the difference Energy in - Work out.

--

Bye.
   Jasen

" John Slaughter = ASD Fucked, f****ng TROLL IDIOT "

** YOU DID - CUNT BRAIN !!

YOUR stupid, f****ng post above says " AC " !!!!!!!!!

Two minutes later you posted this to confirm it:

" Why not just use an scr and AC instead?

Well, a triac of course... "

Go EAT SHIT - MUTHERFUCKER !!!!!!!!!!!

...... Phil

If you already have speed control, don't mess with the voltage. What's broken about the 18V drill? Right after saying it's broken, you say it has torque and speed control.

Run it at 18 V and either figure out a trigger setting, or hack the controller to do with what you want.

Good Luck! Rich

Its broken at the shaft. The motor seems to run fine but overall the drill is broken(you know, there are other parts to a drill). The speed control is through the trigger and I need to automate it... The "control" is encapsulated so not much I can do with it. I didn't say it had speed control though... I said I can control the speed quite easily(by suppling a lower voltage).

Since it runs on dc either I can use some dc method of power control such as pwm or AC method such as triacs and rectification(both of which probably result in essentially the same result). But seems to me using dc and just lowering the voltage is the easiest thing? I really don't understand what the point of pwm is as it would ultimate seem to still average out things to an equivilent voltage supply? (i.e., why not just just lower the voltage in the first place if I can do that(remember, I don't need to automate the speed of the motor but just easily be able to find the right speed which makes the gate open fast enough).

i.e. the issue is not to automate the speed of the drill but to know how the torque and speed are related to the current and voltage so that when I guesstimate the right settings needed for the drill to run properly. On wiki it seems to say that the torque is inversely related to the speed and that the torque is proportional to current while the speed is proportional to the voltage. This is very confusing to me and I can't explain why it happens that way(except that maybe the motor sets up a reverse MMF that counters the current or something like that) but if its the case then it seems that running the drill motor slower will give me more torque.... but might be to slow.

I'm just a little confused about that aspect ultimately I can hard wire the circuit for the speed as it won't need to be changed(and the motor will not run much so it doesn't have to be efficient).

Thanks, Jon

Circa Mon, 28 May 2007 18:08:57 -0500 recorded as looks like "Jon Slaughter" sounds like:

I had similar questions. The first question I had of the original post was what was meant by the term "torque motor." Seems a redundancy.

If the shaft is bent, then I would not count on being able to use the motor for any other purpose. For the motor shaft to be broken yet...

...then I must assume that there is a clean break somewhere, and that you intend to mechanically splice a new shaft on what's left of the old?

So, figure out what the trigger does to regulate the speed, and mimic it via automation. The same goes for the polarity control. Reverse-engineer the operation and you will know what to do.

I will assume that the input to the "control" is a constant 18V DC. In which case all you need do is measure the output voltage and polarity as you fiddle with the trigger speed control and the switched direction control. Gather the empirical data, and if you need more help, post again.

no. The shaft uses several gears and at some point the gears are not turning the shaft... the motor itself works fine as far as I can tell. I didn't investigate it much but it could easily be fixed but tha is not the goal. I'm not trying to repair the drill but use the motor.

Thats not the issue... you guys keep thinking I'm trying to control the speed of the motor.. I'm not... i'm trying to find the effects that changing the voltage has on the motor's properties... I guess I should have asked this question in a physics or mechanics NG instead...

look, I don't know how else to say it. The drill runs on 18V and it has the ability to control the speed using the trigger which my guess is simply a pot. It also has a switch to run it in two speeds which is also probably just a switched in resistor.

The issue is not the electronics that control the motor for varous speeds but how the motor works w.r.t to those speeds. I will not be using need to automate the speed of the drill(which I never said but you all have assumed that). When you are opening a gate you do not need to have it at more than one speed... theres no need to open it fast in some cases and slow in others.

The issue is, I need to find the right voltage to run the motor at so that it opens the gate at the speed I want. Surely I can accomplish this to some degree with gears but I want to be able to adjust the speed manually instead of buying a new gear just cause its to fast or slow. Say at 18V it turns the gate to fast... if I decrease the voltage to 10 voltages it will slow it down.. but now maybe the torque is not enough to turn the gate... or is it? How do motors work? At 15V will it draw more current than 18 and produce more torque at the lower speed(this is what I got from wiki but doesn't make sense to me). I do not need to automate the speed but only find out how the motor works w.r.t to different voltages. I can only do so much with the gears. If I need 25V to get the speed and torque then it could burn up the motor... but maybe it would be ok if it only runs for a few seconds at a time(With enough delay to cool off).

The way I see it is that when I run the motor at a higher voltage it will increase not only the speed but the torque... not that they are necessary directly related but that more voltage means more current by ohms law. I do know that it is not necessarily true but not sure. I can't make sense of wiki as to me it implies that increase teh voltage decreases the current...

Jon

Circa Mon, 28 May 2007 19:38:04 -0500 recorded as looks like "Jon Slaughter" sounds like:

"I plan on using some gears so I can open and close it with a change of polarity."

"I'm a little worried that it might have to much torque or that it might be to fast/slow for the gate...."

And now you say you are not trying to control the speed. Well, which is it? Have you not found that changing the voltage changes the speed? Can you not see that experimenting with the existing controls will help you understand how those controls affect the motor operation? Do you understand that the motor's properties are inherent, and changing the voltage does not change the motor's properties but instead changes its operational performance?

Have you done any research on the subject? Do you understand the relationship between rotational speed and torque? Are you making an effort to? Are you converting the motor to a practical project (a motorized gate), or are you just dinking around? Make up your bloody mind what you want to do, present it in an intelligible fashion, and quit blaming your lack of ability to understand the help you are getting on the people that are helping you. Crikey!! Jung n guvpx phag.

Ok... your on my ignore list... you want to bitch at me about this shit yet your the one adding your own assumptions.. I said "Easily controlt he speed"... i.e., f****ng change the input voltage... I NEVER SAID I NEEDED TO HAVE A VARIABLE SPEED CONTROL FOR THE MOTOR.

Use your f****ng brain and maybe it would make more sense instead of trying to make it blame it all on me. How f****ng hard is it to conceptualize the problem? open a f****ng gate with a motor... what the f*ck would I need variable speed control for that? who's the f****ng moron.

I never blamed anyone. I said I couldn't f****ng understand it... theres a difference... your the one who started the blame game and I'll be damned if I'm going to take that shit from you. I asked f****ng simple questions and maybe I didn't do a good job of explaining but you have no f****ng right to try and put your ego trip on me.

Anyways, I'll figure it out by myself as usual cause f****rs like you and phil who don't know shit who end up being wrong or misunderstand me(doesn't matter if its my fault cause you have no right to try and make it worse) and then have to put your ego in when I tell you that i not what I needed.

I guess you get to join the ignore list with Phil...

Note for all the other assholes who are going jump in and bitch at me for bitching at him... just to f****ng bad... save your breath.

Circa Mon, 28 May 2007 21:05:32 -0500 recorded as looks like "Jon Slaughter" sounds like:

Thank god for small favors.

[crossposted to sci.engr.mech and rec.crafts.metalworking, but followups-to set to sci.electronics.basics - if you want to chime in and think your group might benefit, please add a followup-to of your home group.]

On Mon, 28 May 2007 19:38:04 -0500, Jon Slaughter wrote: ...

When you decrease the DC voltage, the motor slows down, yes, but the torque drops dramatically. With any decent load, as soon as you start to decrease the voltage, the speed (under load) decreases twice as fast as you thought it would - it's not proportional to the voltage, but the current. And, because the motor has DC resistance, when you decrease the voltage, you decrease the current, which lowers _both_ the torque and the speed.

It might be instructive to go ahead and hack into the gears until you get to the useful one, and put some kind of load on it, and do your speed control experiments.

A good load might be a V-pulley, with a cut V-belt dangling over it with a weight on either end. When the pulley turns, it wants to drag the belt with it, like a capstan. Well, you put a weight on the end that it wants to lift, and adjust the weight until it just holds position. Or, maybe just a fish scale (upside down, of course, fastened to the floor), so it will reach equilibrium. At that point, the weight times the diameter of the pulley will give you the torque at that speed and voltage. This is a "Prony brake" or, according to Wiki, a "de Prony brake":

But their diagram shows a slipping clamp on a shaft - what I described has the pulley slipping inside a loop, something like:

Let us know what you find out! :-)

Good Luck! Rich

It's most likely a permanent magnet DC motor. A PM DC motor, by itself, isn't a 'torque' motor or other type of motor. You can _use_ a permanent magnet DC motor to give a controlled torque if you drive it with a set current instead of a set voltage, but you don't change the motor construction to do that.

Lowering the voltage to the motor isn't going to make it deliver more torque. It will lower the stall torque (because the stall torque depends on the stall current, which depends on the terminal voltage). If you need to drive the motor at the most torque you can get without burning it up, then you need a controlled-current supply, and you need to be ready for some interesting accelerations when whatever you're driving breaks loose.

Mostly, if you want more torque you should either get a bigger motor or gear it down.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

For permanent magnet motors:

No-load speed is proportional to voltage. Torque is always proportional to current.

Under load, the EMF that the armature sees is reduced by the IR drop in the armature winding and speed will then be proportional to this reduced EMF.

The amount of current that the motor can draw is also related to speed, because a counter EMF proportional to speed is induced in the armature. The current is then the (applied voltage - EMF) / armature_resistance. So, at stall the current is V/R (V being applied voltage), torque is maximum (stall torque) and speed (hence EMF) is zero by definition.

Maximum power is produced at 1/2 speed, because that's where the speed

• torque product (and EMF * current product) is greatest. Some motors can't tolerate this much current without overheating, others can handle stall current indefinitely.

--
Some drills use series-wound motors, in which a wound field is in
series with the armature.  Most corded drills are made this way.
Behavior of these motors is more complicated.  However, most cordless
drills use permanent-magnet motors.