Dremel stopped spinning

[Vicolaships]

Hello

I have a Dremel that spins very slowly or does not spin at all while making weird noises.
The output torque is close to zero (I can stop the shaft with my hand) and the RPM is nowhere near what is is supposed to be.

I have opened the Dremel and done these tests:
- Bearings feels good, no play, turns without resistance.
- Rotor does not show any marks indicating it collapsed into the stator at some point.
- There was a little bit of "dirt" inside the casing so I cleaned all the contacts.
- The commutator brushes looks good and are far from needing replacement.
- A common failure on Dremel are the stator wire that break at the connectors, mine are ok because I can measure 23 Ohm between each connectors of the 2 coils.
- I can measure a constant resistance (18 Ohm +/- 0.5 Ohm) between each adjacent bars of the commutator bars
- When the Dremel is powered on (locked in a vise) I can measure 230 VAC across the commutators terminals (brushes removed)
- I have cleaned the contacts on the small white PCB that allows for speed adjustments. The moving parts with 3 legs is clean and makes contact with the PCB when assembled.













It is my understanding that the Dremel uses a universal motor that is directly powered on 230 VAC.

What else should I test to find what is wrong?
Bye

[coromonadalix]

i had a problem like this in the past, i shorted out the dremel speed controller and used an external scr controlled fan speed box, never failed

Had this setup for 5 years, the dremel head bearing died ... of usage

[smithnerd]

Measure the field windings (your second image). If the two windings are not of a similar resistance (or open), then push the terminals down in their slots to make a better contact with the wires.

Mine come loose from time to time and it behaves weirdly, as you describe.

[Doctorandus_P]

There are an awful lot of connectors in between all the different parts, and none look like "high quality"

I would start by bypassing all electronics and directly apply power to the motor to see if it starts then.

[coromonadalix]

do you have a variac to test the motor ?

[floobydust]

I would connect the motor (directly) to a bench PSU, they will run even with 12VDC.
The commutator is pretty black so I'd give it a light scuff with fine sandpaper.
Otherwise, I would suspect the speed control pcb triac plus diac (is that a burn hole it has?) has failed.
I did a schematic for the Dremel Benchtop Pro MD95 speed controller, it has a lot more parts but still the usual diac+triac arrangement. This speed controller is too cheap, it would overload very easy.

edit: yes I see the little black blob next to triac is burnt

[coromonadalix]

the black rectangle is a resistor for the speed slider do not sand this section

[floobydust]

I'm not saying "sand the potentiometer" lol
Sand the motor's commutator, OP will have to do it anyway- unless he marked the brushes and puts them in the exactly as they were. I put pencil marks on carbon brushes to remember that.

But that mysterious semiconductor? With the black epoxy cover, it's dead. Not sure what it is, the MELF is the diac.
Need a replacement motor controller pcb, or make one and use it externally. Many people are doing that for their Dremel.

[james_s]

Don't sand the commutator, it looks fine. Do clean out the slots between the segments though, they fill up with carbon dust from the brushes which then shunts current away from the armature windings. I had a cordless drill that was shooting sparks and smoke out of the brushes until I used a sharp knife blade to scrape the carbon out of the slots. An ultrasonic cleaner might work well for that too.

[drussell]

The blowed-up black blob could just be a cheap-ass way of doing a resistor.  Since they were laying down carbon anyway for the potentiometer, they probably just made the low-resistance-limit resistor out of the same carbon material and then covered it with epoxy for protection.  That would make sense for the usual simple DIAC-TRIAC-style dimmer circuit.

Try shunting the black blob with a 10k resistor and see if it works.  Or maybe down to 5k or even 3.3k if it doesn't behave quite right with 10k.

There really shouldn't be anything else to that circuit.

(You could also bench-test the TRIAC, I suppose...  5V and an LED w/ resistor in series with T1-T2 should latch on when you momentarily hit the 5V power to the gate.)

What does the black blob currently read with an ohmmeter across it?

[drussell]

Standard dimmer circuit topology:



The TRIAC could have died from inductive spikeyness, especially since there is no snubber network and it's an inductive motor load, but it's a really simple circuit and should be easy to fix.  TRIACS and resistors are cheap. 

[Vicolaships]

Hi guys thanks a lot for all your messages

I watched a few videos to understand how universal motors work when plugged on AC to make some tests.

I removed the control PCB and put a wire instead so that the motor always sees the full sin wave of the 230 VAC:


The motor spins fast/fine (I can't measure the speed but it sounds normal from what I remember) so all the windings are intact.

I have tried to measure the resistance on the black blob but can't get a good reading, the value fluctuates a lot. Maybe its because of the other components around but it is most probably a dead resistor. That would explain the problem: the diac is triggered randomly, thus the triac is triggered randomly, thus the motor only sees short burst of 230 VAC current at random timing.

Do you think cutting the black blob and putting a resistor would be a good idea for a repair? What values would you suggest?

 

EDIT: I did not saw the values drussell suggested. I will try 10k / 5k / 3.3k tomorrow and report my results here!

[Vicolaships]

It now works with a 10k resistor and the dead one cut in half!

Thanks a lot