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Topic: Dc motor resistance  (Read 284 times) previous topic - next topic

Kallum1664

Apr 19, 2019, 10:14 pm Last Edit: Apr 24, 2019, 06:24 pm by Kallum1664 Reason: Photo
Hi, just a quick question I can't seem to figure out, I have a dc motor with only 1.9 ohms resistance, the current it is drawing is 230 milliamps with a supply voltage of 4.95vdc. According to ohms law this can't be possible can it? I've tried a few different multimeters Incase it is faulty and all 3 read the same resistance

MarkT

When a motor is running most of the supply voltage is countered by the back-EMF in the armature
windings (due to their movement through the magnetic fields).

You'll be seeing about 0.4V volts that's pushing current through the winding resistance, ie supply
is 4.95V, back EMF is about 4.55V during normal running.

When you initially apply power, for the brief moments before the motor gets up to speed, the current
through the winding will be much larger (about 2.5A initially).

This is known as the stall current, as this sort of current would also flow if you locked the motor's
spindle so it can't move and applied power (which can burn out a motor, so be careful about that).

Back EMF is porportional to a motors speed of rotation, and is often quoted in the spec's as the motor
constant, perhaps measured in V/rpm.
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Kallum1664

Oh ok, so ohms law is telling me the initial start up current of around 2.5 amps until the motor gets going?

Thanks for the quick response

Paul__B

#3
Apr 20, 2019, 12:40 am Last Edit: Apr 20, 2019, 12:43 am by Paul__B
That's it.  Your power supply and switching device(s) must be rated for the stall current, not the run current.

Back EMF is porportional to a motors speed of rotation, and is often quoted in the spec's as the motor
constant, perhaps measured in V/rpm.
If of course, you ever have such a thing as the motor's specifications!  :smiley-roll:

Kallum1664

Nope no specs for the motor, no data badge, bought it as part of a starter kit. Thanks for your help

wvmarle

Oh ok, so ohms law is telling me the initial start up current of around 2.5 amps until the motor gets going?
Pretty much - that's more commonly called the stall current.

This as you will also see this current when the motor is stalled - i.e. stopped from rotating while powered. Now you also know why tha's when motors overheat and burn :-)
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

JCA79B

Lets say your motor, with + 5V applied, runs clockwise at 10000 RPM. If you disconnect it, put a light load,say, 1000 Ohms across it's leads and spin it with another motor at 10000 RPM clockwise, it would generate close to  -5V across the load resistor. So, when you put + 5V on it's leads and it's running, it's also generating close to -5V, bucking the +5V you are putting in (that's called back EMF). That's why it's only drawing the low current. Now, if you put a heavier load and slow it down, the back EMF will be less, so the difference between applied voltage and back EMF will be greater, causing current to increase. More current means more torque, so the speed stabilizes at a lower value.

MarkT

Nope no specs for the motor, no data badge, bought it as part of a starter kit. Thanks for your help
Well we still have much less information than you do - a photo of the motor perhaps?
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