# DC motor

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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 ?

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This is the motor I am using

Anything I can test to see what the problem is ?

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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.

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What value of resistance did you measure?

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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.

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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.

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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

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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

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