questions and trying to fix a high speed ac motor (dremel)

Other Hardware Motors, Mechanics, Power and CNC
1

The other night i dropped and broke my dremel, i took it apart and saw a wire on the stator was broke and had a poor connection on the other coil so i decided to rebuild the stator.... or attempt too. I've already bought a new one, but wanted to try to fix it for the challenge, but it's my first motor attempting to repair.

Normally this dremel is rated at 1.15 amps at 5000-33000 RPM. and looks to be powered by a PWM chip and variable slide switch. the stator had 175 windings of 28 gauge wire with a resistance of 5.8 ohms on each side... and the armature had a resistance of 12.6 ohms at brush contacts

so when modifying motors... Is the number of winding more important? or the resistance of the stator? I have some 30 and 24 gauge wire and not sure which would be better.

If i use the 24 gauge. the resistance is lower, so at the same wraps, the amps would be higher, but if i matched resistance instead i'd need more wraps to make up for the lower resistance

If i used the 30 gauge, the resistance is higher, so at the same wraps, the amps would be lower, but if i matched resistance i'd have less wraps

What effects would it have on the armature if i used the 30 gauge with more windings on the stator with less current? or using the 24 gauge with slightly higher current with less windings?

thanks

2

“the stator had 175 windings of 28 gauge wire”

Your asking if thicker or thinner gauge wire should be substituted for the wire the dremel was designed around?

I would go with 30gauge as it will be easier to wind... Not going to bother with how many windings needed for same 5.8 ohms, I would stop around 150 and take a meter to figure out how much more wire is needed.

3

I guess my question was all over the place. I understand i should try to match the resistance to what it used to be so i don't fry the electronics, but i suppose my question was, what is the effect on a motor that has less windings or more windings? I know with electromagnets, the more windings, the stronger the magnet is, but i was unsure what the effect would have on a motor?

thanks

4

I am very curious to know what the repair of a Dremel has to do with this Arduino Forum?

...R

5

It's not just about fixing the dremel. It's about modifying a small AC motor that happens to be a dremel, which is why it's posted in the motor and power section, and figured that someone would have a little knowledge on windings and power output, which i'm refreshing myself's about motors in other places as well.

At the moment i'm working on multiple arduino projects. I'm making a rotary table gear cutter, and x-y-z arduino mill from a drill press. And i plan on stripping the AC motor from the drill press, and installing a DC motor with speed control via my arduino.... which i've been using my dremel to help build all of those projects, and now since i have an extra scrap dremel, i'm trying to figure out how to modify it for future projects that are yet to be determined.

6

well. i got it fixed. but had to experiment a little so i could answer my questions i originally asked. I used the 30 gauge and overwrapped by double and ended up losing about 50% of the lower speeds, which makes sense since the resistance was doubled... so i unwrapped it to where it should be and regained most of the lower speeds, but i can still tell a difference because of the smaller wire. So i'm debating on stripping it and rewrapping it with the 24 gauge wire to give it more power

but i think i got it all figured out.

thanks

7

I admire your persistence - I would have just binned it !

8

wildcat99s:
The other night i dropped and broke my dremel, i took it apart and saw a wire on the stator was broke and had a poor connection on the other coil so i decided to rebuild the stator.... or attempt too. I've already bought a new one, but wanted to try to fix it for the challenge, but it's my first motor attempting to repair.

Normally this dremel is rated at 1.15 amps at 5000-33000 RPM. and looks to be powered by a PWM chip and variable slide switch. the stator had 175 windings of 28 gauge wire with a resistance of 5.8 ohms on each side... and the armature had a resistance of 12.6 ohms at brush contacts

so when modifying motors... Is the number of winding more important? or the resistance of the stator? I have some 30 and 24 gauge wire and not sure which would be better.

The number of turns is absolutely vital, it sets the inductance of the coil which sets the current.
Higher resistance will make the coil run hotter, so don't reduce the thickness of the wire.

Use exactly the same gauge wire - you won't be able to fit thicker wire on the bobbin probably,
and thinner is a no-no as I've explained. Use the same or better inter-layer insulation scheme
when rewinding too.

If i use the 24 gauge. the resistance is lower, so at the same wraps, the amps would be higher

No, the amps will not be higher, the inductance totally dominates.

but if i matched resistance instead i'd need more wraps to make up for the lower resistance

If i used the 30 gauge, the resistance is higher, so at the same wraps, the amps would be lower, but if i matched resistance i'd have less wraps

What effects would it have on the armature if i used the 30 gauge with more windings on the stator with less current? or using the 24 gauge with slightly higher current with less windings?

thanks

Don't mess with the winding scheme, these commercial motors are wound as close as they can get away with
to the thermal limits, rewind identically or risk the thing burning up. If there's space for thicker wire
(which I doubt), you can go with thicker wire, same turns count.

9

Slumpert:
“the stator had 175 windings of 28 gauge wire”

Your asking if thicker or thinner gauge wire should be substituted for the wire the dremel was designed around?

I would go with 30gauge as it will be easier to wind... Not going to bother with how many windings needed for same 5.8 ohms, I would stop around 150 and take a meter to figure out how much more wire is needed.

No, unsafe, do not follow this advice.

10

hammy:
I admire your persistence - I would have just binned it !

Yeah, most people would have. but i have this hobby of attempting to fix whatever is broken first before trashing it. And this week i repaired my computer monitor that fried, the dremel, and tomorrow i'm finishing up an ATX power supply that fried last month which will be used for one of my arduino projects.

About the dremel, i knew overwrapping it with the 30 gauge would have created a bunch of extra heat, and it did. But after i took off the extra wire so it matched the original resistance, it didn't feel any hotter than it did before it broke, just slightly under-powered from the drop in current.. But tonight it's going to be used a lot doing a bunch of metal cutting with my carbide bit so i'll see how it performs, and i can take a power reading to see how much current i lost by using the smaller wire.. But i have a feeling i'm gonna end up stripping it again and rewrapping it with the 24 gauge wire. And yes, there is enough room for it, barely.

In my previous post i mentioned "if i use the 24 gauge. the resistance is lower, so at the same wraps, the amps would be higher" I know the current wouldn't be higher if i used the same amount of wire...from a resistance standpoint; but from a current standpoint, the original coil was 175 turns of 28 gauge wire. And if i made a coil of 175 turns with 24 gauge wire, the resistance would be lower because of the difference in wire size, and as an effect, it should have a higher current? At least that's the way i see it. Lower resistance at a certain voltage means higher current. But i'm still figuring out motors so i'm unsure how much inductance effects the overall current. But was saying bigger wire has higher current capabilities.

I was/am planning on bumping up the current a little so it will put out a little more power... not much, just another .2-.5 amps .... but by doing that i know there's a chance of burning up speed control, but then that gives me the opportunity of rebuild the electronics and/or armature if it comes to that. As for factory specs. I know commercial companies cut corners to save a penny here and there, and they want to make a quality product within reason, but they also want it to break so you can buy their newer product. So as good as something is made, there's usually always improvements to be made on their design and/or to fix their design flaws.

11

You are not listening to me.

The current through the coil is limited by the coil inductance, nothing at all to do with the
wire resistance. The impedance of the coil will be of the order of several kilo-ohms inductive,
the resistance of the coil makes no detectable contribution to this at all.

All the resistance does is convert electrical power into heat.

The current is determined by the inductance, which in turn depends on the square of the turns-count
and on how close the stator laminations are to saturating magnetically.

Monkey with the number of turns and you could lead to magnetic saturation, which would be extremely
bad news.

The armature is a different matter, typically the inductance is a lot less and the motor back-EMF
is responsible for current limiting, not the winding. This is why stalling such a motor can rapidly
overheat it - the armature windings draw high current and cook with no back-EMF.

12

thanks for explaining that a little better, I was listening and get what your saying now, but i still have a lot to learn when it comes to motors and inductance. I can understand the armature would have less inductance because it's constantly rotating, and the different segments of the coil are being energized, and when the motor stalls, the windings on that particular segment on the armature overheats and fries because that segment of coil between the commutator segments can't handle that much energy transference.

So with this motor, i'm dealing with a fixed number of 6 commutator segments. So basically it's the armature's winding inductance that controls the overall power output for a motor? A lower inductance on the armature translates to higher current through the field coils? Am i getting that right?

13

Yeah, most people would have. but i have this hobby of attempting to fix whatever is broken first before trashing it.

Are you my dad reincarnated? He used to fix things that other people would throw away. Long after he died I managed to cut through the mains cable of his circular saw. I took the mains plug off the damaged cable to discover he'd repaired the plug. One of the terminals for the wire had broken somehow and he'd managed to repair it. Anyone else would have used a new plug.

14

i don't know. People fix things for different reasons. mine is just learning new skills so i can fix or hack virtually anything.... except software.... you know.... some skills that would come in handy if the world goes down hill.

I've been watching a bunch of videos on inductance and magnetism and see where i was wrong with some of my motor assumptions above. It wouldn't have worked like i hoped because i was only tinkering with one aspect of the motor and wasn't considering the others. But there's still a few final things i need to understand about the workings of the motor before i attempt to work on it again. But it's gonna get an entire overhaul when that time comes

15

well if an ac dremell motor has ever been made i'll eat my hat,all the ones ive seen have dc motors runing from an ac to dc psu!.

16

as far as i know, dremel makes both an AC and DC version of their rotary tool. i've never dissected a DC version yet, but from what i can figure out, the AC unit uses a triac and a pot for their speed control. my unit is about 14 years old, and the chip they used has either been discontinued, or they used a generic version from a random company, so finding a spec sheet on it has been impossible, but i can find info on the newer units and imagine they are similar or the same.

i've also read that you can completely bypass the switch and it will run at full speed. And don't believe they have enough components on the board for the AC/DC conversion and speed control since i can count the components on the board with one hand.

But i could be wrong. The next time i have it apart i'll hook it up to my oscilloscope to see what kind of activity is actually taking place at the coils and on the board.

17

wildcat99s:
thanks for explaining that a little better, I was listening and get what your saying now, but i still have a lot to learn when it comes to motors and inductance. I can understand the armature would have less inductance because it's constantly rotating, and the different segments of the coil are being energized, and when the motor stalls, the windings on that particular segment on the armature overheats and fries because that segment of coil between the commutator segments can't handle that much energy transference.

So with this motor, i'm dealing with a fixed number of 6 commutator segments. So basically it's the armature's winding inductance that controls the overall power output for a motor? A lower inductance on the armature translates to higher current through the field coils? Am i getting that right?

The power output is determined by the load - more mechanical load (ie torque), and the armature slows down,
so less back-EMF from the motion is present, so the current increases (which will generate more torque
to balance out with the load). The difference between supply voltage and back-EMF is whats left to drive
current through the armature winding's inductance and resistance. Normally that will be a small fraction
of the supply voltage as only back EMF represents useful mechanical work.

With a universal motor like this running off AC the details are complicated as the voltages are changing
all the time, as is the magnetic field from the stator, and there are various phase shifts going on, so the paragraph
above is really an oversimplification, but your intuition about the armature inductance isn't quite there
yet.

Field coils use their inductance to limit current, armature windings need less inductance as the back-EMF does
most of this job. In fact you want the armature inductance a lot less as current is being switched
from segment-to-segment by the commutator rapidly and the current must change rapidly enough or
the motor's top speed will be limited. Universal motors can spin at 15,000rpm or so, the armature
segments see higher rotational frequencies than the mains frequency.

In both cases you want the resistance of the windings to be low to reduce wasted power in the windings.

Ultimately the heat in the windings will limit the continuous power rating of most motors. Universal motors
tend to be shunt-wound, so the field coils are independent of the armature current. The stall current
in the armature is set by the mains impedance of the winding (at stall there is no commutation, so the
effective frequency is that of the mains). It may be either resistance or inductance that dominate for
the armature winding at stall. At high speed the armature inductance will be dominant and be a limiting
factor for the speed (along with the supply voltage).

In designing a motor the interaction between winding resistance, inductance and mass has to be taken
into account. Its quite a balancing act, and commercial motors will have everything pretty much optimized
for efficiency, cost, temperary overload handling and max temperature rise.

Something like a Dremel will probably be on its 2nd or 3rd iteration of detailed motor design too - its likely
to be closely tailored to the specific needs of the tool.

18

Thanks for the info MarkT. i've been studying up on motors and electromagnetism for the past week or so and that made a lot more since when i reread that this morning.