Field testing small brushed motor

[Bicky22]

Hi all,

I am looking for some advice.

With a small 24VDC 5 brushed DC motor is there a way to do a simple field test to determine whether the motor “good” or not? The only thing that is accessible, is the motor leads.

I put the word good in quotations because obviously this is a relative term and without specs or a test, I currently do not have a way of defining “good”. So as of right now, I am just looking for a potential test that could theoretically be performed.

I also underlined the word simple because I am trying to determine something that a field technician could perform without significant knowledge of brushed motors and only typical tools that a field technician would have, such as a multi-meter.

My ideal scenario would if the technician could determine whether the motor is good or bad just by measuring the resistance across the terminals. Obviously, with brushed motors the terminal resistance varies based the commutator position. A thought is that I might be able to determine a range to consider acceptable based on testing.

Does anyone have any thoughts or suggestions?

Thank you all

 

[waross]

Turn it on. See if it runs without noise or excess sparking.
As for resistance:
If the resistance is too low, the motor will overheat.
If the resistance is too high the motor may be weak or may not run.
Brush problems will often cause a higher resistance but not always enough difference to measure.
The brushes will have a junction voltage drop which will cause a resistance measurement error that will depend in part on the test voltage applied by the multi-meter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Bicky22]

Thank you for the response Bill.

"Turn it on. See if it runs without noise or excess sparking."
I agree but I cant assume a technician would be able to determine what is normal noise plus the motor is connected to a gear train, shaft, etc.
The motor is in-cased, so there is no visibility to see sparks.

I would love to be able to come up with a spec and say "If resistance between terminals measures between X ohms and Y ohms. Motor is good"
But my concern is in a brush motor there are too many variables as you touched on in your last two statements.

"Brush problems will often cause a higher resistance but not always enough difference to measure."
"The brushes will have a junction voltage drop which will cause a resistance measurement error that will depend in part on the test voltage applied by the multi-meter."

The first, just because the resistance is ok, doesn't mean the motor is "good" and the second, error in measurements due to measurement devices, worn brushes, temperature, etc.

 

[Skogsgurra]

If the motor is fed from "clean" DC and not from rectified AC, then it is usually possible to see problems in the rotor and commutator by checking the DC current and see if there is excessive ripple.

A bad commutator bar, a bad winding in the rotor or shorted commutator bars will all cause current variations that is best seen on an oscilloscope but that can also be observed with a clamp meter on AC range. The ripple in the current shows as AC and if you measure a perfectly good motor under normal load, you will get a reference value for "good". Then, you may assume that twice that value is still good, but that four times that value is bad.

You can get better results by doing some homework/research:
If you have access to a machine that you can abuse, then you can short two rotor bars and get a definitive "bad" value. Then you can open (cut) a rotor winding and get another bad value. And so on.

There are lots of little oscilloscopes (DMM:s with scope screens) that can be of immense help. They don't cost much and can usually help in many other situations, too.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[sreid]

I would

1. Smell the motor. It should not smell like burned magnet wire.

2. Turn the motor shaft while holding the motor to your ear. It should turn and not make "bad" noises.

3. Connect the motor to a battery (1.5 V D Cell). The motor should turn.

 

[Bicky22]

Thank you for your responses

Skogsgurra,

I do see a ripple between a good and a bad motor when measuring DC current. I have not tried the clamp meter I will do this when I have a chance. Either way, I don't believe this will solve my problem.

The motor is still attached to a gear train and has a slight load attached. The motor also has a short "on" time (1 second) before the component shuts it off. So there will be a slight fluctuation in current based on the load, and with only a run time of 1 second, I don't believe a technician would be able to make a decisive yes or no answer to whether the motor is good or bad. Also I can’t trust that everybody has a clean DC source and a little oscilloscope.

The only way I believe I can make this work if there is a static method to measure resistance/capacitance/etc across the leads and have a tolerance where I am confident the windings and brushes are good. I don’t know if there is a way to determine what is good or bad based on all the variables involved and that is what I am really looking for here.