Encyclopedia of Science

[Tony]

Hi folks this is my first post and I hope you might be able to help me. I use small brushed DC motors for powering my model planes and boats. To get the most out of these small motors (2" long and they draw 10 amps at 8.2 volts when on load) I run the motors in to get a good running fit on the commutator. After proper running in the amount of sparking on the brushes is greatly reduced...we run small capacitors to help elimianate radio interfearance and they might also reduce sparking...not sure with that statement?

My two question are

1. when under heavy load why is it that the positive brush still sparks and not the negative brush...

I assume it is something to do which way the DC electricty flows in a wire from the battery...

2. which way does the DC current flow from a battery?

Thanks Tony

[treelover]

Electricity is a flow of electrons, electrons are negatively charged.
So electricity flows from negative to positive.

As to the dc motor i dont know definatively.

I suspect that the coils producing the magnetic field are negatively charged as is the negative wire leading to the carbon brush connecting to the commutator.
I suspect the real difference in voltage would come at the commutator to positive brush, hence the spark only at the positive brush.

Under power your motor with a 'SMALL' battery. Then....

Try placing a voltmeter black (negative) probe on the negative terminal of your battery, touch the red (positive) probe to the Negative Brush - should show 0 volts.
Then move the black probe to the commutator segment just before the positive brush - preferably where it wont foul the positive brush as it rotates to touch it. Place your red prove on the positive carbon brush. Watching the volt meter as you slowly rotate the stator.

You might see a 'tiny' reading just before it touches the positive carbon brush. If this doesnt work then I suspect the voltage difference across the air gap is too small to be detected by your voltmeter.

If it shows a 'small' voltage increase then an arc will be formed across the air gap with a larger voltage.

I am assuming that the spark occurs on the leading edge of the carbon,
If the spark occurs on the trailing edge then... the transfer of electrons is occuring in the same way as with an arc welder and a mig welder. Because you touch you electrode to the surface to be welded, to start the flow of electricity then you pull the electrode away and an arc is formed.

This is because air... is what they call a dielectric, in other words a non conductor. You can get electricity to flow through air, but with high voltage. lightning is an example of that. Lightning starts at the ground, which is negatively charged. The path of the negative charged current moves up a small way through the air then fails. But that failing changes the quality of the air. The next charge follows the old path and gets higher. By this process which is invisible, a channel of more conductive (or less dielectric) air is formed. when this channel reaches a cloud with massive positive charge, small amounts of negative charge flows from negative to positive. This heats the air which speeds the movement of electrons through the air. To cut a long story short finally the channel of conductive air passes a particular threshold and massive amounts of electricity race upwards getting faster and faster as this electricity reaches near to the positive cloud the air is stripped of its electrons (ionisation). This makes a super conductive upper channel. Now the length of the non super conductive air is reduced (its resistance is reduced!) and more electricity flows and yet more ionisation occurs. the ionisation occurs through multiple channels until a ionised path opens down a channel all thw way to the ground. The ionised gasses emit light as they reform normal gas.

This is how lightening is formed. Which is an arc! Arc welders start with a zero air resistance channel ( touching ) then as the smalles of gaps occur the air ionises across the gap, as the gap increases so does the air channel. The Arcing from you commutator to positive carbon bruch could be like the arc welder....

Sorry I dont now the answer but I hope what I have written helps you.

BoB