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Using Arduino => General Electronics => Topic started by: Kallum1664 on Apr 19, 2019, 10:14 pm

Title: Dc motor resistance
Post by: Kallum1664 on Apr 19, 2019, 10:14 pm
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
Title: Re: Dc motor resistance
Post by: MarkT on Apr 19, 2019, 10:21 pm
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.
Title: Re: Dc motor resistance
Post by: Kallum1664 on Apr 19, 2019, 11:02 pm
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
Title: Re: Dc motor resistance
Post by: Paul__B on Apr 20, 2019, 12:40 am
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:
Title: Re: Dc motor resistance
Post by: Kallum1664 on Apr 20, 2019, 08:33 am
Nope no specs for the motor, no data badge, bought it as part of a starter kit. Thanks for your help
Title: Re: Dc motor resistance
Post by: wvmarle on Apr 20, 2019, 08:54 pm
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 :-)
Title: Re: Dc motor resistance
Post by: JCA79B on Apr 21, 2019, 06:48 am
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.
Title: Re: Dc motor resistance
Post by: MarkT on Apr 21, 2019, 02:04 pm
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?