# DC Motors

Discussion in 'Electronic Basics' started by Jon Slaughter, May 28, 2007.

1. ### Jon SlaughterGuest

Does anyone know what kinda motors that most drills use? I was thinking it
was a torque motor but not sure.

Also, for DC motors can I get away with changing the voltage to a lower
setting to increase the torque or do I run a huge risk of burning up the
motor?(i.e., are they usually designed for a very narrow operating voltage).
I have this old broken 18v drill and I want to turn it into opening a gate.
I figure that it has enough torque to do it and I can control the speed
quite easily. 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(which the gears have a lot to
do with of course).

Just wondering how much I can play around with the voltage to get what I
want if I need too. Also, I assume that P is pretty much constant for
motors? i.e., if the rated power for a drill is P then P = I*V for a large
range of I and V? So if I know its a "100w" drill at 20V then I can
calculate the current at 15 voltages = 100/20 A = 5A and it should be
approximately what the drill will pull? (obviously the load has a lot to do
with it but just curious as to some simple approximation)

Thanks,
Jon

2. ### ChrisGuest

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:

http://www.cpemma.co.uk/pwm.html

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

3. ### Paul E. SchoenGuest

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

4. ### Phil AllisonGuest

"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

5. ### ChrisGuest

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

Chris

6. ### Jon SlaughterGuest

Why not just use an scr and AC instead?

7. ### Jon SlaughterGuest

Well, a triac of course...

8. ### Phil AllisonGuest

"Jon Slaughter"

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

........ Phil

9. ### JasenGuest

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.

10. ### Jon SlaughterGuest

OMG.. your a fucking moron... calling me a fool yet you have no idea!

who the **** says you can't use rectification?

Now go shut the **** up! You fucking piece of shit!

BTW, if you use just one SCR then it will rectify the wave anyways in a
sorta half wave rectifier.

Now I know you obviously don't know shit and so theres no bother replying to
future posts because you will be ignored as I ignore all idiots who think
there smart but don't know shit.

11. ### Phil AllisonGuest

" John Slaughter = ASD Fucked, fucking TROLL IDIOT "

** YOU DID - **** BRAIN !!

YOUR stupid, fucking 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

12. ### Rich GriseGuest

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

13. ### Jon SlaughterGuest

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

14. ### Charlie SiegristGuest

Circa Mon, 28 May 2007 18:08:57 -0500 recorded as
<YhJ6i.4124\$> looks like "Jon Slaughter"

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
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.

15. ### Jon SlaughterGuest

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

16. ### Charlie SiegristGuest

Circa Mon, 28 May 2007 19:38:04 -0500 recorded as
<vBK6i.5918\$> looks like "Jon Slaughter"
Well, you're simply not making sense. In your original post you said:

"I figure that it has enough torque to do it and I can control the speed
quite easily."

"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.

17. ### Jon SlaughterGuest

speed"... i.e., fucking change the input voltage... I NEVER SAID I NEEDED TO
HAVE A VARIABLE SPEED CONTROL FOR THE MOTOR.

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

I never blamed anyone. I said I couldn't fucking 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 fucking simple questions and
maybe I didn't do a good job of explaining but you have no fucking right to
try and put your ego trip on me.

Anyways, I'll figure it out by myself as usual cause fuckers 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

18. ### Charlie SiegristGuest

Circa Mon, 28 May 2007 21:05:32 -0500 recorded as
<vTL6i.8305\$> looks like "Jon Slaughter"
Thank god for small favors.

19. ### Rich GriseGuest

[crossposted to sci.engr.mech and rec.crafts.metalworking,
but followups-to set to sci.electronics.basics - if you

.
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

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":
http://en.wikipedia.org/wiki/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:
http://www.1911encyclopedia.org/Image:Dynamometer-1.jpg

Let us know what you find out!

Good Luck!
Rich

20. ### Tim WescottGuest

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