Armature rewind with larger wire on 2 pole brushed DC motor

Thread Starter

maarkus

Joined Jul 23, 2020
14
I know I know, not a good idea but this is from a cheap and free broken coffee grinder that I just had to troubleshoot and I'm planning on getting a 'real' grinder for espresso at some point anyway. This grinder was returned to my friend's store after only a few uses and when I removed the motor I could see the fan ring had come loose and hula hooped around thereby cutting through several parts of the 38 ga wire.

I worked with 42 ga wire when rewinding guitar pickups but that was with a homemade winding machine and I don't have one for armatures so I recklessly chose 29 gauge wire just to see if I had the patience and skill to make a working motor. I hooked it up to my 15v dc power supply since I knew it would supernova at 100v and it spins at what I judge to be a decent grinding speed. If that's the end of my journey I can accept it but I'd sure like to grind some coffee at least a few times.

I read much on what affects magnetic field/torque and although there was much contradictory information I understand I may have reduced the torque due to the large reduction in windings. Is that true?

This is a 12 slot 12 bar armature and the original adjacent bar to bar resistance was 27 ohms. Correct me if I'm wrong but I understand the working resistance is brush to brush which would be half of the total winding resistance, so 6 x 27 = 162 ohms. The output of the bridge rectifier was measured at 100vdc so that gives me 0.617 amps theoretically (100v/162ohms) which I was never able to measure as the motor only ran a few seconds and sporadically as it only had brush to brush continuity on a few pairs. The schematic leaves out the two limit switches for simplicity.

Can I simply use a power resistor in series with the rectifier's positive output to solve this (100w ~150 ohms)? Considering this is a once a day 15 second usage I'm not too concerned about the power wasted as heat through the resistor which I'm sure would otherwise be a bad idea for a constant or daily cycling motor.

Since my circuit design skills are near zero I'm assuming there's a more intelligent and less wasteful method using a few other components I could solder in without creating a pcb which you can see this grinder doesn't even have.

Thanks for any help.
 

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KeithWalker

Joined Jul 10, 2017
3,137
Try running it with half wave rectified 120VAC using just a single diode in series instead of the bridge rectifier. Then you will not need the series resistor. It will take more current than the original so I don't know how long your switch will last.
 

Thread Starter

maarkus

Joined Jul 23, 2020
14
Try running it with half wave rectified 120VAC using just a single diode in series instead of the bridge rectifier. Then you will not need the series resistor. It will take more current than the original so I don't know how long your switch will last.

Thanks for your quick reply, you must have already had coffee.

I infer you're telling me to ditch the BR (four terminals) and replace it with a diode (two terminals) so I'm going to

1) Run SW/2 to positive diode terminal and then to motor pos
2) Run motor neg to ground/neutral junction

After wondering how this would reduce current I read an explanation of the half wave rectifier and found out the output is Vdc = Vp/π so it's about a third of the supply voltage, very interesting. So that's how I get the reduced current but still roughly twice what I want which is what you implied. Also it's pulsed so they mention using a smoothing capacitor in parallel.

Thanks in advance, I'll do some more reading on smoothing caps and the formulas and check in tomorrow.
 

BobTPH

Joined Jun 5, 2013
9,124
After wondering how this would reduce current
A half wave rectifier, without a capacitor, supplies current inly half the time.

The RMS voltage of a full bridge is Vpeak / sqrt(2). The half wave is half of that.

Even though today is Pi day (3/14) you get no pie.
 

Thread Starter

maarkus

Joined Jul 23, 2020
14
A half wave rectifier, without a capacitor, supplies current inly half the time.

The RMS voltage of a full bridge is Vpeak / sqrt(2). The half wave is half of that.

Even though today is Pi day (3/14) you get no pie.

What are the odds I'd get schooled on pi day? I didn't even pay attention to it all these years. Good thing I know how to make my own pi(e), I'm going to carve the symbol into the dough this time.

Thanks for the correction, I need to find an on online course with circuit simulation for troubleshooting and learning to pick up from where I started in junior college forty years ago. I only occasionally read articles and get overwhelmed with the mental invasion without physical/practical application.
 

Thread Starter

maarkus

Joined Jul 23, 2020
14
It should work without a smoothing capacitor.

Thanks, I'll give it a try after reading several articles on choosing diodes.

Last question:

I'm calculating around 40 amps of current with the half wave rectifier due to my armature resistance of only 1.5 ohms, is that in the ballpark? That is so far above the 1.2a rating of 29 gauge wire so besides the switch issue that seems like a quick fry job of the armature and overheating the 18 gauge chassis wiring.

I must have goofed somewhere or maybe I should use a combo of the diode and resistor.
 

BobTPH

Joined Jun 5, 2013
9,124
I'm calculating around 40 amps of current with the half wave rectifier due to my armature resistance of only 1.5 ohms, is that in the ballpark?
That world be the stall current when run from pure DC. The inductance swamps the DC resistance with a half rectified voltage.

Also, when motor is turning, it sees a much lower voltage due to back EMF.
 

Thread Starter

maarkus

Joined Jul 23, 2020
14
That world be the stall current when run from pure DC. The inductance swamps the DC resistance with a half rectified voltage.

Also, when motor is turning, it sees a much lower voltage due to back EMF.

Thanks again for your help, I've read about inrush current and back EMF before and just read about stall current so I actually understand your answer to some extent.
 

MisterBill2

Joined Jan 23, 2018
18,934
If you have enough turns of wire then it should run fairly well with the half-wave rectifier arrangement. Certainly there will be a reduction in the torque, no telling how much, though.
 

Thread Starter

maarkus

Joined Jul 23, 2020
14
If you have enough turns of wire then it should run fairly well with the half-wave rectifier arrangement. Certainly there will be a reduction in the torque, no telling how much, though.

Thanks for your reply. I originally did a trial with a few slots using 30 loops but although it looked like I could finish with that many (accounting for doubling per slot once I was done) I decided to go with 20. Unfortunately the slots don't look as full as they probably should be but once I test it under load I'll know if it's adequate or not.
 

MisterBill2

Joined Jan 23, 2018
18,934
There may also be excessive sparking and too much heat. But for really short bursts in a coffee grinder like mine, probably like yours, heating for just a few seconds is not a problem. Just allow a bit of cooling time.
 
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