# Student question on DC motors

Discussion in 'Electrical Engineering' started by qwerty, May 8, 2007.

1. ### qwertyGuest

I'm reading a book on DC motors design.

It shows the schematic of a motor and it follows that a brush short-
circuits 2 wires of the armature, those that go on the same part of
the commutator. The book says that if those wires were under voltage
then a large current would flow through the brush and this would wear
out the brushes and waste a lot of energy.

So it says that the brushes should be positioned in such a way to
always short-circuit wires that are under zero voltage, those that
are on the neutral zones of the magnetic field.

My question is, if the brushes always make contact with zero voltage
wires, then how can we have voltage in the output of the generator?

And another point. As the commutator rotates the brush will make
contact with two parts of the commutator which means it will short
circuit four wires, which complicates matter even more. Isn't this
taken into account?

Please excuse some terminology errors as the book is not in English
and I'm translating.

2. ### Don KellyGuest

----------------------------
The coil that is shorted is only one of a series of coils which make up a
winding. Don't look at just this coil but at the armature winding as a
whole. If you look at the winding, you will see half the conductors moving
in the stator field. If you simply had long leads attached to the windings,
in addition to the problem of them tangling up, the voltage seen at the
terminals would be an approximate square wave AC. Moving contacts are needed
and as DC is wanted, rectification is alos needed. The commutator does both.
Think of a continuous winding which is half under the N pole and half under
the S pole of the field. Each coil in the winding produces a voltage but as
each coil passes through the neutral axis, its voltage will reverse
polarity. If the contacts to the coil are on the neutral axis, as the coil
reverses, its connection to the outside world also reverses. The coil being
shorted by the brushes, is ideally at the point where the voltage reverses
and shorting it will not change the total voltage due to the other windings.
If you have a winding with 40 coils, only 2 will be shorted at any instant,
leaving two current paths with 19 voltage generating coils in each path at
any instant.
This is hard to describe in words but look up "Gramme Ring" for an obsolete

Don Kelly

3. ### qwertyGuest

Maybe I'm dumb, really, but I still can't understand anything.

Aren't the coils shorted by the commutator anyway?

4. ### Don KellyGuest

The commutator segments are connected between coils in the winding.
in the diagram below 0 is a coil and "_ is the connection between coils
N pole of field
--> movement of coils
_0_0_0_0_0_
_ | |_
--|_ 0 0_ |-- brush |
|_0_0_0_0_0_|
<-----
S pole of field
Here there are 12 coils - 6 per side so at any instant of time 5 coils are
moving under each pole and 2 are under the brushes (shorted) so that the top
and bottom windings each generate 5E and as they are in parallel, the total
voltage is 5E with 2 current paths (say both to the right and right brush
+ ) between brushes. As the rotor turns, each coil will pass, sequentially
from the generating position, to the shorted position, where the current
reverses in that coil before it passes under the next pole and generates
again.
In effect the winding axis stays stationary perpendicular to the field but
voltage is being generated in the coils under the poles.
Any text dealing with DC machines should deal with commutation in detail
with better pictures than those I can produce here.

Don Kelly

5. ### qwertyGuest

Here is a common type of winding, the lap winding:

http://img513.imageshack.us/img513/614/mtdfig14uq2.gif

At the moment that the picture is showing, can you tell me which
conductors carry what voltage? and which conductors are shorted?

6. ### qwertyGuest

snip

Thanks for the great reply. Saved for future reference.