| nicksanspam@ece.villanova.edu 2007-09-17, 9:25 am |
| Neon John <no@never.com> wrote:
>
>You're mechanic friend is wrong for most automotive DC motors.
He thought the brushes were replaceable, but the bearings usually went bad
by the time new brushes were needed...
>The issue isn't brush slant because most small motors don't use brush slant.
>The issue is brush timing.
>
>The reaction of the field and armature magnetic fields forces the field
>toward the edge of the pole. It is desirable to commutate at a zero
>potential point and that means where the field is weakest. Therefore
>brushes are advanced over the static timing to the new dynamic neutral
>position. If the motor is reversed without re-timing the brushes, there
>will be heavy sparking, rapid commutator and brush wear, high current draw,
>overheating and low power. The timing changes with load but since
>a fan is a more or less constant load, static timing works.
I wonder if some sort of snubber circuit could help. Or a series choke?
>Larger more expensive DC motors frequently have "interpoles", also known as
>"commutation windings" to stabilize the commutation point but small cheap
>automotive motors don't.
Or a different field excitation? Synchronous PWM?
>The old EV warrior electric bicycle used two Ford radiator fan motors for
>propulsion. One on either side of the wheel, shafts pointing toward each
>other with a rubber roller connecting the two. The roller contacted and
>drove the tire. These motors came in left and right-hand rotation versions.
>
>If one attempts to run one backwards it does everything I described above.
>When the company went bankrupt I bought a box full of the motors and have
>used them on a variety of small EV projects so I'm quite familiar with
>the characteristics.
>
>On the web there are various descriptions of techniques people have used
>to reverse these motors.
Would you have an url or some clues how to find them?
>For some reason there were many more of one type than the other on the
>surplus market so there was a demand for reversing techniques. It looked
>like a lot of work to me, for this motor uses a radial commutator and
>the brushes are riveted to the end bell plus the motor is assembled
>with crimped joints.
Sounds like you have to take them apart and move the brushes. We'd like
to run a single motor in both directions by reversing polarity. How long
would the unmodified fans last in reverse? We'd like the equivalent of
3000-4000 hours at 225 F, ie "forever" at 150 F. Nathan measured 60 watts
at full speed for the Mazda fans, but found no increase in heat transfer
above 30, with a PIC-based PWM drive. The fans turned at 1.2 watts. He
decided on 16, for the most part, vs a 400 W furnace blower :-) Perhaps
low power will make them last longer. I wonder why each fan has 4 wires.
Are there temp sensors in there?
A reversing fan would simplify things...
up 2' up
----------------------------- -----------
| r s| | a| radiator |
|sunspace fa room d| | room d| and fan |
| air d air a| | air a| |
| <== ai ==> <== m| 2' | <== m| sdamper | 2'
| a p| | p| |
| nt e| | e| |
| o r| s | r| 4' |
| r-----------| u | |-----------| west
| 2' | n | | |
| | s | | |
| | p | 20' | |
| | a | | |
| | c | s | |
| | e | o | |
| | | u | |
- |s | t | |
- room sunspace |d | h | |
- air air |a | | |
- ==> ==> |m | | |
- |p | | sdamper |
- |e | | |
- |r | | |
----------------------------- -----------
12'
adamper
----------------------------- Drawing not to scale.
- r ^ s|s |
-sunspace fa | d|d |
- air d room a|a |
- <== ai air m|m 4' | 12' south
- a p|p |
- nt e|e |
- o 2' r|r |
----------r------------------
west
Thanks for the definitive answer, John :-)
Nick
|