24VDC brushed motor, varisitor 3

[Bicky22]

Hi all,

I have 2 versions of the same small 24VDC brushed motor. (Same commutator, same brushes, same wire).

The only difference is that one version has a ring varisitor, one version does not.

The version without the varisitor consistently has a performance life of more than 10x as long as the version with the varisitor.

The failure mode of the version with the varisitor is that is loses its performance speed and slows more than double what normal speed is, where the non-varisitor version continues to perform as expected. (Obviously under exact same conditions.)

Looking at the commutator, the failed varisitor-version appears to have a large about of carbon in between the commutator segments.

So my question is that has anyone come across a varistor that significantly reduces performance life of a DC brush motor? Any Ideas on what could be going on here?

Thank you

Brendan.

 

[itsmoked]

Haven't seen that before.

Thoughts: Capacitance could cause extra arcing. Switching caps with switches frequently toasts the switches. Of course commutators are just a bunch of switches switching reallllly frequently. Varistors all have capacitance.

And under 'what is a varistor capable of doing', they conduct when their design trip voltage is exceeded. When they do they heat up, they physically change inside due to the heating affect. When they cool they physically change back...almost.... all the way but not quite. This means they wear out. Varitors can only cycle so many times before they fail. They can fail by triggering at a lower voltage. It could be the varistor is robbing the motor of voltage it needs and hence you're seeing them run slower.

I'd dump the varistor as they are a bad idea in the first place unless their trip voltage is safely higher than any normal operation could ever cause. Varistors are only applicable for unusual events because of their wear-out issue.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[Bicky22]

That theory make a lot of sense as I expect that the varistor is seeing is occasional voltage spikes higher than its designed trip voltage.

I would like to follow this theory further however I have found limited sources for good information on ring varistors. Is there any source or literature you can recommend for reading up on this type of varistors? I would like to understand the actual physics behind why the varistors fail mode would cause it to trip at lower voltages. I.e. what in the varistor’s material breaks down and causes this effect.

I also would like to figure out a way I can test your theory that the varisor is drawing current or robbing voltage or even test that the varisor has failed.
I agree it should be dumped.

Thank you, I appreciate the information.

 

[Skogsgurra]

Carbon bridges between commutator bars are burnt away in their "infancy" if spikes are allowed to develop freely.
If you select a varistor with too low a knee voltage, there will not be enough energy to kill the young carbon bridges. So they develop into fully grown adult bridges.

I would take the varistor voltage up a great deal. Knee voltage 100 - 150 V. Or run without them. If arcing interferes with other equipment, which I don't think that a motor with carbon brushes will do, then put a snubber with low capacitance across the brushes instead.

I would run without any suppression at all.

Gunnar Englund

http://www.gke.org

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

 

[Bicky22]

Skogsgurra,

Thank you for the suggestion.

The high voltage is partly interfering with other equipment and the magnitude of voltage (100+ VDC on a 24 VDC motor) I am seeing is undesirable, this is why I wished to suppressed the high voltage spikes. I did not design suppression or the motor… I am just trying to figure out what is wrong.

The varistor’s knee voltage is 28 VDC. Measuring with an oscilloscope, even with the varistor on, I can still see spikes of 50+ VDC spikes (without varistor, I can see 100+ VDC spikes). I would think these spikes would be plenty to burn off any young carbon bridges. Typically with similar type motors I measure in the same manor I won’t see any significant spikes.

I don’t know what is causing the high voltage, it is very undesirable, but ultimately the motor functions well for a significant life span. When a varistor was used to lower the voltage spikes, the life span was significantly reduced to where the motor would no longer provide a consistent RPM at a given torque load.

Is there any feasible way I could test your theory?

I appreciate the input. Thank you.

 

[itsmoked]

Have you tried just a capacitor? Start small at say 0.1uF and work up.

Is this motor rapidly controlled? Or tightly controlled? If not have you tried a large capacitor? Hundreds or thousands of uF? By definition the voltage across a capacitor cannot change instantaneously. That means all your high voltage spikes can not occur thru a capacitor.

Remember what I said about varisors, they wear out! They are a lousy solution that will haunt you since they will go away on their own by either shorting or opening eventually. That's no solution at all. They're for something like incoming power protection where lifetime events are expected to be around a couple of thousand NOT a couple of thousand an hour.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

That sounds like more of a drive problem than a motor problem. Have you tried a free-wheeling diode?

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

 

[Bicky22]

The issue was not related to to the circuit. The commutator on the motors with the circuit was not round causing a high arc which lead to early ware.

Thanks Everybody

 

[itsmoked]

And thank you for getting us always nice feedback! Glad you found the culprit.

Keith Cress
kcress -

http://www.flaminsystems.com