Pitting Switch Contacts

[rjenkinsgb]

I assumed that with brushed motors, when the field collapses (creating reverse induction) the brushes have already moved on from the energized coil and set of contacts on the commutator and had moved on to the next set. By this time isn't the electrical connection broken, preventing the induced current from heading back through the communtator & brushes

The individual armature coils are connected such that in effect the whole thing is one big endless coil; the commutator switches "taps" rather than disconnecting anything.

You could try a bridge rectifier as a "clamp", the AC side to the motor and the DC side to your DC supply, before the relay contacts.
That's not quite a flywheel diode, but any voltage spikes greater than supply voltage get absorbed by the main supply; justy make sure that is reasonably low impedance itself - eg. add a capacitor across it, at the bridge DC output.

 

[gophert]

By this time isn't the electrical connection broken, preventing the induced current from heading back through the communtator & brushes

Yes, so only release the switch after the voltage spike (and before the next) and you won't have any pitting problems.

 

[dr pepper]

I still have 'my first 'scope', its even more basic, yours looks Ok for this kind of thing.
Take care though if you stuff full back emf from a large coil up the input you might blow the Y amp.

 

[BlueSmoke]

The individual armature coils are connected such that in effect the whole thing is one big endless coil; the commutator switches "taps" rather than disconnecting anything.

Oops, I had always thought when dealing with low voltage DC motors, they were wound with several individual sets of Lap winding's. And while the Lap winding's themselves were ‘center’ tapped, they were still independent of others living on the same armature and not all connected to one another. Sort of like the picture below, just a whole lot of them strategically placed / arranged around the armature.

Now I’m even more confused then before, and that’s saying a lot.

If the armature is basically made up of one large coil, then each time a brush moves from one contact on the commutator to the next, its the polarity of the coil that’s being ‘switched’ instead of energizing & d-energizing. And if that is what’s happening, an electric motor seems to be pure kaos.

My heads starting to hurt, I mean right at the moment of the polarity change, electrons would theoretically come to a ‘stop’ before heading off in the other direction, magnetic fields collapsing, induction taking place and you’d have reverse currents flowing pretty much everywhere.

Ahhhh! That's it, I’m convinced electric motors don't and can't ever work.

Yes, so only release the switch after the voltage spike (and before the next) and you won't have any pitting problems.

Lol - Yea right, I'm not quick enough to move my fingers away from a soldering iron before getting burned.

I still have 'my first 'scope', its even more basic.

Kind of curious, any chance you've got a picture of it?




So I've brought the battery voltage down to just over 13 volts, stopped the induction with diodes and added the Cap & Resister, and I'm no longer being blinded by sparks. Hard to say exactly, but maybe 50% improvement, possibly more. I was also able to use an 8 amp fuse without it blowing, the 6 amp still popped.

 

[rjenkinsgb]

Oops, I had always thought when dealing with low voltage DC motors, they were wound with several individual sets of Lap winding's. And while the Lap winding's themselves were ‘center’ tapped, they were still independent of others living on the same armature and not all connected to one another. Sort of like the picture below, just a whole lot of them strategically placed / arranged around the armature.

You are pretty close in essence with your winding diagram - just keep extending it to the number of commutation segments & armature slots, eg.

https://www.researchgate.net/profile/Henrik_Grop/publication/242220008/figure/fig4/AS:393097180663816@1470733079869/The-configuration-of-the-coils-in-the-armature-winding-with-connections-to-the.png

As each commutator coil section passes a brush, it's polarity changes from attracting the field pole it's passing to repelling it, or vice versa.
Electrically, the whole thing forms an endless set of coils.

 

[BlueSmoke]

Struggling trying to work out the whole motor thing in my head and I’m not winning the battle.

A continuous winding or effectively having a single coil, going from North – South to South – North, then the induced current would be AC, a chopped or square wave, guessing square, right?. It’s got me wondering if this is one of the reasons motors are so electrically noisy.

Sadly this stuff keeps me up at night, what happens to, and or what are the effects of this fly-back AC current – is it the main cause of the sparking I’m seeing?

It all drives me nuts, this AC induced current would be like a really low frequency, or so I think. Taking a stab at things, just because I can’t help but think about it, a motor with like 20 contacts / segments at a 1,000 RPM would be something like

Frequency = ( half the number of commutator segments * the motor’s RPM ) / 60 seconds = 166.66

So what is the 166Hz doing to the overall circuit?

 

[dr pepper]

These are still for sale and they look at lot like yours:

Student Oscilloscope - Single Channel, 20kHz

The Student Oscilloscope from Unilab is an inexpensive, single channel oscilloscope with simplified control layout for student operation and permanent triggering feature It is a great introductory unit to teach the basics of oscilloscope use with a useful set of features e.g. 8 switched gains...

www.philipharris.co.uk

Its a philps harris, mine is a unilab, they became philips harris I think.

old unilab oscilloscope - Google Search

:
Reminds me of all the 555 and 741 circuits I used to blow up back in the old days not knowing what I was doing, nothings changed just the price of the parts.
No bnc's, just bannana's, and the knobs are not switches just pots, there is no calibration just twist & tune.
The 'scope has developed a weird thing where the trace moves up & down a little and will not x shift all the way, power supply electrolytics no doubt.

 

[BlueSmoke]

It does look a lot like mine, even the color scheme matches.

Yea know I've thought a lot about getting a new oscilloscope and actually would love to have one, but wonder how much I'd actually use it.

I've also checked out some of the PC vesions that use a dedicated piece of hardward to capture the signals and then displays the result on the monitor. Don't know if they are any good or not, but they sure look cool.

https://www.ebay.com/itm/USB-2CH-20...323067&hash=item340e59a8d1:g:9FgAAOSw4ztdB0er

 

[dr pepper]

I have on eof those, mines a 60mc version with built in sig gen.
Its Ok for the money.
But if you dont need dont get.

 

[gophert]

But if you dont need dont get

True for many things in electronics and life in general.

 

[shortbus=]

True for many things in electronics and life in general.

Now you tell me!