micro dc motor wasn't working, now is, how many times do i need to test to have confidence

Thread Starter

walnutcontractors

Joined Mar 24, 2022
1
I have a flood safe laundry solenoid that is activated by a ~5v dc motor. When I received it I tested it and one of the solenoids wasn't working. I did watch it work once but it failed to work at least ten more times that I tested it.

I swapped the cable leads and used my fluke to test voltage on the motors when using the controller to manually open and close the valve. I manually activated the 3/4" ball valve that wasn't working. Eventually I spun the shaft of the dc motor manually and then tested it and now it is working again. Tested it ten more times and all were succcessful.

Initially I read this thread:

https://forum.allaboutcircuits.com/threads/dc-brush-motor-failure.75703/

and it seemed to indicate that maybe one of the phases or windings could have gone open circuit but it predicts a roughly 1 in 3 chance of the motor spinning and stopping in a stop. Testing 10 times should have had a really good chance of that being the case.

How many times should I test to have confidence, maybe the shaft was sticky? Really these motors only activate if there is a leak and that shouldn't happen very often.

The gear has ten teeth and it is approximately 3cm in depth from the surface of the gear to the back of the motor. How hard is it to find a replacement for something like this?
 

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luvv

Joined May 26, 2011
191
Bend the tabs back and pull the armature,have a look at the contact points. If the motor is new then I would suspect a manufacturing flaw,perhaps in dimensions of the contact points.
 

Tonyr1084

Joined Sep 24, 2015
7,956
Bend the tabs back and pull the armature,have a look at the contact points. If the motor is new then I would suspect a manufacturing flaw,perhaps in dimensions of the contact points.
First - pull the plastic gear off the motor shaft. Then once you bend the four locking tabs out you can slip the motor armature and the plastic motor back bearing support out along with the contacts for the brushes. CAREFULLY spread the brushes apart and remove the armature from the brush assembly. The brushes should spring back closed. Check the commutator for any issues. Normally it should be clean and bright with no wear. If that's the case then check the brushes for wear. They will likely be carbon material, which is dark, and they should appear to match the radius of the commutator. If they're worn out then you've found your reliability issue.

Like "Luvv" said, it could be a manufacturing issue if the motor is new. If the motor is used then reliability goes down, deepening on how heavily it was formerly used.

The only other issue affecting reliability would be the power source. If it's faulty the motor would be the most likely point where an issue would be seen.

Many years ago I had a toy with a motor. It stopped working. So I spun the motor by hand and all of a sudden it worked. I don't recall how long it worked for, I just know - as a child would think - I fixed it. Even though I didn't fix anything. Apparently either the commutator was dirty or the brushes of the motor had some issue that cleared itself when spun by hand. Y'know, like when something doesn't work so you hit it and all of a sudden it's working - - - . "/
 

Tonyr1084

Joined Sep 24, 2015
7,956
I think the nature of the question is that the TS had a motor failure that was momentary. TS wants to know if sufficient successful tests will prove out whether the motor is reliable enough to be put into service for whatever project it is intended for.

While some of us have suggested checking different things the bottom line is - if the motor failed once it will fail again. The truth is that everything will fail eventually. It all depends on the use, how frequent the use and how much stress its anticipated functioning will occur during expected lifetime of service.

Murphy's law predicts that the motor will fail at the most inopportune time possible. One example is my neighbor borrowed my electric weed whacker. It failed in his possession. The failure was the trigger switch. No, it wasn't his fault, it was just one of those Murphy moments where it failed in his possession. Had he not borrowed it - it logically would have failed the next time I used it. He felt bad and was worried I'd blame him. Couldn't blame him, the switch just reached the end of its usable lifespan. Sometimes that's just how things go.

If the TS motor is new then it shouldn't be failing. However, we don't know the nature of the failure. It could have been a power source failure just as easily as anything else. Bad switch. Bad contact. Bad solder joint. Defective (new) motor - or worn out previously used motor. And that's just what I can think of. I suppose there could be other modes of failure as well. Without seeing the test rig and how it's hooked up, examining all the components and the condition of them - we can only guess as to how reliable the motor will be. If it's a kids toy I wouldn't worry about it. When it fully fails it can be replaced. However, if it's something intended to keep Grandma alive - - - I'd opt for a much more robust design and foolproof engineering. It all depends on the intended use of the motor. Likely it's not going to be keeping anyone alive. But I don't know that for sure.
 

MisterBill2

Joined Jan 23, 2018
18,991
Befoore you break off the tabs by bending them, get hod of an ohm meter and connect it across the motor terminals. Then sowly rotate the motor whie watching the resistance. If there is a broken wire or a disconnected joint, then you will see some portion of the rotation with a much higher resistance, or an open. That will indicate that the reliability is zero. Then you can open the motor and see if it can be repaired.
 
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