# DC motor

Discussion in 'Electronic Basics' started by aman, Apr 19, 2005.

1. ### amanGuest

I can detect a short across the terminals of a DC motor(when I do a
continuity test) before I even connect it. Can anyone tell me what the
problem can be. Do you guys think the motor burnt away or what ?

2. ### Lord GarthGuest

Has the commentator ring slipped or are you using brushes that are too
large?

Pull the brushes and test the coils directly at the commentator. You could
have some failed insulation in one of the coils.

4. ### Lord GarthGuest

Looks sealed! So no. Since they tested this gear motor at 19 volts to
measure a stall current of 1.76 A. I see no reason you can't connect
it to 12 volt supply and limit the current too a few mA. Slowly
increase the current while observing the voltage. If the motor is
shorted, the voltage will only indicate 0 no matter the current limit.

6. ### JamieGuest

i assume if its much less than 10.8 ohms ! then you must
really have a short.
because the data that shows does specify 10.5 ohms as the
norm.

7. ### Rich GriseGuest

Are you sure it's a "short", or are you just measuring a very low
resistance? They do have a very low DC resistance, you know - the
current flow is limited by what's called "back EMF", which is a
voltage created by the motor's generator action while it's running,
that opposes the applied voltage. You haven't said how big the motor
is, or its ratings, or anything else.

Just for perspective, I've never seen a motor fail short. If it's
busted, it'll fail open or will have already burned up.

How big is the motor, and on what resistance range were you? Did you
zero the meter?

Good Luck!
Rich

8. ### Rich GriseGuest

Get your hands on a 12V power supply and a 10 ohm, 25 watt
resistor.

Put the resistor in series with the power supply and motor:

10 ohms, 25 watts
+12 ----/\/\/----. A
|
[Motor]
|
12V ret. --------' B

Apply power, and see if it runs. The resistor will limit
the current to 1.2A if it's short, which is somewhat less
than the motor's rated stall current (1.76A, from the
datasheet), so even if the motor is a dead short, it will
not burn.

If the motor runs, albeit slowly, then you've got a good
motor.

If not, measure the voltage at point A (the + side of the
motor) relative to point B (the - of the motor). If it's
zero volts, you have a shorted motor. If it's more, I'd
say it's safe to try the 19V supply - first with the
resistor, and if it runs lamely, then leave out the
resistor, because you've most likely got a working motor.

Good Luck!
Rich