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From: John De Armond
Subject: Re: Attn : Nick Pine - got a live one for ya ! Re: Too much humidity 
	from my swamp cooler
Date: Sun, 16 Sep 2007 17:14:34 -0400
Message-ID: <>

On 16 Sep 2007 15:34:25 -0400, wrote:

>>>12V fans have brushes that are slanted in the normal rotation direction
>>>in a way that lets the commutators quickly grind them up in reverse?
>My mechanic friend says no. Reversing should be OK.

You're mechanic friend is wrong for most automotive DC motors.  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.

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.

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.  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.


From: John De Armond
Subject: Re: Attn : Nick Pine - got a live one for ya ! Re: Too much humidity 
	from my swamp cooler
Date: Mon, 17 Sep 2007 23:08:47 -0400
Message-ID: <>

On Mon, 17 Sep 2007 17:46:14 -0400, "daestrom" <>

>> I spoke with a motor guy who didn't think brush timing would be an issue.
>Nah...  It will just burn the edge of the commutator bars and the brushes
>will wear out faster.  Once the bars are burned you'll have to dress them up
>each time you replace the brushes, otherwise the pitting will mechanically
>wear the brushes more and more.  But other than that, you shouldn't have too
>much trouble :-/

Yup.  "Motor guys" scare me with what they don't know.  I moonlight in a friend's
motor repair shop, in part, to help keep him out of trouble :-)

Automotive radiator fan motors don't permit changing the brushes, at least not
easily.  All the connections including the brush pigtail leads are spot-welded and
the can is either pressed or welded together.  The motors are effectively disposable.

Here's a page on changing the timing of the EV Warrior (Ford radiator fan motor)
timing, as an example of what is involved:

At 140,000 miles I recently had to replace the radiator fan motor on my Caprice. It
quit running due to brush wear.  Being an engineer and a motor guy, I decided that
I'd rebuild the old one and put it on the shelf as a spare.  This motor is pressed
together so I pressed it apart on my arbor press.

The destruction was impressive.  The brushes were worn to nubs.  The commutator bars
were worn to within a few thousandths of breaking through to the underlying plastic.
The windings were spot-welded to the armature bars, making replacement fairly
impractical.  There were grooves worn in the shaft under both bearings. Whomever did
the engineering on this motor nailed the lifecycle, causing everything to wear out
together.  It went into the garbage.

>How you plan on reversing these anyway, open 'em up and reverse the polarity
>going to the field or to the armature? (hint: don't reverse both or you get
>nowhere).  Reversing the applied voltage to the motor leads won't work
>unless it's a PM motor.

Almost all these motors are permanent magnet field so reversing is just a matter of
reversing the polarity.  If I were going to try to reverse one of these then I'd open
it up and time the brushes mechanically neutral.  The sparking and other bad behavior
will be about the same in both directions but it will be less than if a properly
timed motor is run against the timing.

If you have reversible drill and can see the commutator then you can observe a
neutrally-timed motor.  The sparking is fairly heavy under load in either direction.
Since a drill motor's lifetime is typically measured in hundreds of hours, it doesn't
matter much.

There are other problems to deal with.  Many radiator fan blades are held onto the
shaft by simple threads.  Reverse the rotation and the fan unscrews from the shaft.
Also few fan blades are bi-directional.  They blow air efficiently in one direction
only.  In the reverse direction they primarily make noise.  A quick peek at the blade
on most any radiator fan will reveal just how asymmetrical they are.  Shallow attack
angle quickly ramping up to a sharp discharge angle.  They just don't work in


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