Re: DC Motors

On Mon, 28 May 2007 18:08:57 -0500, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:


"Rich Grise" <rich@xxxxxxxxxxx> wrote in message
news:pan.2007.05.28.22.56.03.213552@xxxxxxxxxxxxxx
On Sun, 27 May 2007 18:52:19 -0500, Jon Slaughter wrote:

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.

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.


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.

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.
.

Relevant Pages

  • Re: DC Motors
    ... was a torque motor but not sure. ... for DC motors can I get away with changing the voltage to a lower ... If you already have speed control, ... The motor seems to run fine but overall the drill ...
    (sci.electronics.basics)
  • Re: Notching-up scenarios
    ... motors in series, and then reconfigure as two parallel strings of ... Notch 1 was for shunting, and held the applied voltage at a low ... For DC units using either contactors or camshaft resistance control, ...
    (uk.railway)
  • Re: how to make a stepper motor step gradually?
    ... in dc motors the torque is proportional to current, speed is proportional to voltage. ... if you reduce the torque enough you should be able to slow the angular acceleration down at each step. ... an object on the platform. ...
    (comp.robotics.misc)
  • Re: source of largish gears?
    ... torque" one at Electronic Goldmine, and draw a lot (or in the Jameco ... a LOT) more current at similar voltage and RPM. ... and most likely these other two motors ...
    (comp.robotics.misc)
  • Re: Variable torque motor for belt grinder ?
    ... 3Phase motors have true rating on the plate. ... After getting VFD, think about what else, in your shop, you can control ... About torque: most VFDs will give you area of constant torque. ... RPMs, the torque will be lowered as well, but you'd need lower speeds ...
    (rec.crafts.metalworking)