# Measuring DC motor resistance

#### mikeoz

Joined Sep 25, 2017
86
Hi,

I'm trying to find the resistance of a 12v brushed DC motor so I can calculate the stall current. I intend to run the motor from a 12v lead acid battery. As I understand it, calculating the resistance is possible by applying a small current to the motor and measuring the voltage with a multimeter. However, I've been unable to find a detailed description of how this is done. Would someone be so kind as to walk me through this process?

Many thanks

Michael

Last edited:

#### Alec_t

Joined Sep 17, 2013
14,389
Welcome to AAC!
As I understand it, calculating the resistance is possible by applying a small current to the motor and measuring the voltage with a multimeter.
That's exactly what a digital multi-meter does when on an Ohms range. Just use a DMM and measure with the motor unpowered and the armature in various positions.

#### Sensacell

Joined Jun 19, 2012
3,480
Depending on the motor, the armature resistance might be too low for any accurate VOM readings.
Better to run an amp of DC current through it and read the voltage over the windings, with the armature locked.

Find a nice power resistor and a variable power supply- adjust the voltage so it gives an amp through the series connected resistor and motor.
Measure the voltage right at the terminals to avoid wiring voltage drop errors.

Joined Jul 18, 2013
28,782
Seconded:
The best way is to lock the rotor and apply a small known DC supply and measure the current and extrapolate from there, take a reading a few points and take the lowest reading.
As long as you do not exceed the continuous current rating of the motor, you can apply a voltage direct. A Variac is ideal for this.
Max.

#### mikeoz

Joined Sep 25, 2017
86
Thanks everyone for the replies.

I'm still struggling with the simplest way to supply an amp to the motor in the absence of a variable power supply. Is there something else I could use, perhaps the 12v SLA battery I intend to run the motor off with a power resistor?

In terms of measuring the motor, there are no terminals as such, it just has 12" long positive and negative cables coming straight out of the casing. Will this mean I won't get a good reading from the end of these leads?

And finally, is a 'locked armature' just the same as clamping the shaft of the motor to recreate a stall condition?

Thanks again,

Michael

Joined Jul 18, 2013
28,782
Yes, a locked rotor is clamping the shaft somehow.
Depending on the size of the motor, you could use a fixed supply such as a low voltage battery, the resistor method is second best way.
Do you know the maximum motor current rating? Or the continuous current rating of the motor?
Max.

#### mikeoz

Joined Sep 25, 2017
86
I'm afraid not Max, it just states 12vdc. Here's a photo. I think the cables are 14 awg if that helps at all

Joined Jul 18, 2013
28,782
Why do you need to find the stall current?
The maximum current at stall is the continuous current rating, as opposed to the peak current, which can only be experience momentarily without burning the motor out.
Normally these specs are supplied by the motor manuf.
Max.

#### mikeoz

Joined Sep 25, 2017
86
I was hoping to find the stall current so I can determine what spec driver to buy for it. I've had no luck finding a datasheet for the motor.

Michael

Joined Jul 18, 2013
28,782
What type of driver? off the shelf? Build one?
If variable speed drive, you are usually not going to experience full on voltage at zero rpm, and even if you did, many have current limit adjust.
Max.

#### mikeoz

Joined Sep 25, 2017
86
I'm looking at using the BTS7960 43A H bridge motor driver (if it is within limits) controlled with an Arduino. There will be the occasional possibility that the motor may stall in its application, hence the wish to find its stall current.

Michael

Joined Jul 18, 2013
28,782
If the motor stalls in the application and you have full voltage, you may need a spare supply of motors!
I can't see that motor drawing anywhere near 43a.
Do the resistance test to confirm.
Are you detecting the stall condition via feedback to the Arduino?
Max.

#### mikeoz

Joined Sep 25, 2017
86

I was hoping to protect the motor and driver using a fuse or circuit breaker next to the battery before it reached stall.

Back to the test then, could you give me an idea of what spec battery/power supply would be suitable to do the test with - as a guide?

Also, will the power cables coming from the motor (see posted photo for reference) be a problem when it comes to measuring it?

Michael

Joined Jul 18, 2013
28,782
The motor leads should not present a problem for the current you will be using, if you don't have any adjustable supply then you are generally limited to a DC supply and resistor, measuring resistance with an ohm meter will get you in the ball park for the resistor value.
Pass 2 0r 3 amps should be OK for that motor.
Any DC source that can supply this should work.
If you intend stalling it purposely at full voltage you may get quite a bit of nuisance tripping.
Max.

#### mikeoz

Joined Sep 25, 2017
86
Thanks Max,

Ok, so I need a dc battery which puts out 2 or 3 amps. Should the voltage of the battery be a fraction of the rated motor voltage? If so, how many volts should I be using for the test to avoid damaging the motor when it's stalled?

Apologies for all the questions,

Michael

Joined Jul 18, 2013
28,782
Find a battery capable, it can be anything up to 12vdc, measure the resistance across the leads with a meter to get an approximate reading and then calculate the resistor value that would give you ~ 3 amps.
Max.

#### mikeoz

Joined Sep 25, 2017
86
Would it be sufficient to use the 12v SLA battery I wish to power the motor from with say a 12 Ohm resistor to supply 1 amp. Or do you suggest 3 amps because the motor might not turn below that?

Thanks

Michael

Joined Jul 18, 2013
28,782
You have to apply the voltage with Locked rotor.
Did you measure the armature resistance with a meter Yet?
Max.

#### mikeoz

Joined Sep 25, 2017
86
Not as yet Max, my last post was a question regarding the test. I'm trying to make sure I understand before I do anything that risks damaging the motor!
When you say 'Find a battery capable, it can be anything up to 12vdc' does this mean a 12v battery supplying 3 amps is safe to lock the armature and test the resistance? Or would a lower voltage power supply at 3 amps be safer for the motor in the test?

Michael