coffee grinder universal motor troubleshooting and repair - help needed

[dOOb]

Hello EEVblog community!

I need help with my coffee grinder, please. It stopped working a few days ago in the following manner: At first it wouldn't start and the blade got stuck, it was impossible to turn it with my fingers. Before that it was very easy to manually turn the blade. I thought it might be due to the build-up of coffee/grains/sugar etc. underneath the blade, so I rinsed it with hot water a few times and dried it with a piece of cloth. This did the trick, the blade started spinning again (though not as freely as before), the motor started working and I managed to get one more grind out of it. But the next time I wanted to use it (a few days later) it would not start again. I could turn the blade but the motor would not start. So I took it apart and started troubleshooting. My observations so far:

• The mains socket, the lead cable, the switch are all working OK.
• There is continuity from the connector all the way to the tip of the brush (point #1 on the photo below).
• The blue element, which I believe to be a safety rated capacitor for electromagnetic noise suppression, does not have continuity (#3), but that's expected for a capacitor.
• The black heatshrink sleave (#4) is most likely a thermal fuse. From my research it seems like they are commonly the cause of coffee grinder motor failures, but I would imagine I can rule this out in this case because there is continuity past that element (see point b).
• I've taken the rotor out and it looks OK visually, no loose wirings visible.
• The brushes look good too, they are still about 1.5 cm in length each. The ends are slightly slanted, but not sure if that's of any relevance.
• The resistance between all the adjacent bars of the commutator is the same.
• I've removed a bit of rust from the rotor shaft and lubricated it before reassembling the motor so that now it spins freely again. Yet powered by a 9V battery it does not turn at all.

Any ideas what's wrong here? What else should I check?

[andy3055]

You cannot run a 230 volt motor with a 9 volt battery!

[thm_w]

Carefully power the motor directly with 230V. Preferably with an isolation transformer or maybe good low current GFCI. If you are unsure how to do it safely, then don't.
Usually either the motor dies and seizes up, or the capacitive dropper circuitry that switches the motor on and off dies.

https://www.eevblog.com/forum/repair/coffee-grinder-failing-motor-ok-but-electronics/?topicseen

[IanB]

If you are trying to diagnose a problem it is always good to start with the symptoms. If the blade seems stuck and hard to turn, then most likely the problem is mechanical--a sticky or jammed bearing. If the motor hums when you turn on the grinder, but the bade doesn't turn, this would reinforce the stuck bearing hypothesis and suggest the problem is not electrical.

What happens if you apply 230 V power to the motor after cleaning and lubricating the bearings so it turns freely?

[floobydust]

What make/model is this coffee grinder? It sounds like a bad connection with the brushes perhaps. Sometimes they get stuck (spring) or a bad connection in the holders. The commutator is worn so a little sandpaper would I would polish it up. The motors are made in china and the armature windings seem to be low quality.

You can run a universal motor off a bench power supply say 12-24VDC 1A if you want to play with it. Don't use mains, it will overspeed with no load!

If the control board is using a triac, the problem could be there.

[dOOb]

Thanks all for your help so far, I really appreciate it. There is progress!!!
So after your comments, I looked up commutators and realized that mine wasn't in great shape so I gave it a bit of polishing with fine sandpaper and a piece of felt.



I then reassembled the motor, mounted it in my makeshift "vice" and, Voila!, it works!!!
I have one remaining question - here's a short video showing the motor running (Warning, the audio is quite loud) - https://vimeo.com/manage/videos/699828994
Is this level of sparking OK or should I try to reduce it?

[IanB]

Some commutator sparking is pretty normal on universal motors. Also from your video it only seems visible when the motor is just starting. If you leave the motor running the sparking seems to drop quite a bit?

[dOOb]

Yep, it only sparks when just starting, and when running it's almost gone.

[themadhippy]

You cannot run a 230 volt motor with a 9 volt battery!

wanna bet? although at the very least you will need another motor and a transformer

[CatalinaWOW]

One other thing that may have happened is worth thinking about.  When the motor was mechanically jammed it may have been drawing excessive current.  In some motors the current path is through the brush springs and they can de-temper and dramatically reduce the spring force.  The only really good fix for this is new springs (usually as cheap and much easier to replace the whole motor), but I have sometimes had success stretching the springs a bit.  If they haven't lost all temper this works, and you can sometimes work harden them enough to do some good.

[amyk]

You cannot run a 230 volt motor with a 9 volt battery!

wanna bet? although at the very least you will need another motor and a transformer

Indeed, a good-quality universal motor can start rotating at a tiny fraction of its rated voltage, but I think the cheap ones common today have far too much frictional losses to do so.