# [EVDL] small DC permag -- bad motor? misadjusted? overloaded? cursed?



## EVDL List (Jul 27, 2007)

Hopefully I can toss this out here, maybe some folks with more experience
than I have can tell me if any of this makes sense. (Crap, this has gotten
huge, main questions at the bottom.)

I'm doing an air conditioning install for an EV we're building for a
customer. We're experimenting with a permanent magnet motor and so far
I've had a very discouraging experience with it, ending in what I think is
probably some damage to the motor. I think it may be primarily a problem
with the motor we're using, but I'd like to check my assumptions from
which I've reached this conclusion.

Though I'm not at all good at it, I'll try to be as brief as I can manage
(heh, um, yeah) but trying to include the relevant clues I can think of. 
We're converting a 2002 Saturn SL and we're trying to run its seemingly
small Zexel scroll compressor. Based on some external constraints, we're
trying to get an inexpensive Surplus Center motor to work. Their server
has been down for several hours and I don't recall the URL or part number,
but I can post a link to it when they're back up. I'm guessing that a
whole lot of folks here have seen or purchased these matte black,
base-mounted OFC motors, ostensibly made by a company called Image (I've
found more information about that, but won't include it here). It's one of
the ones with that annoyingly massive 15 pound cast iron
flywheel/fan/pulley thing, that I've been hoping I can lathe down to a
minimal mass but still keep the fan.

For the time being, the basic details are that it's a 2.5hp motor, I'm
guessing it's about 20 pounds. It's rated at 2.5hp, 130V, 18.something
amps and as I recall 3250rpm at no-load. (I wish I had the motor here with
me; hopefully the SurplusCenter site will come back up soon and we can get
the nameplate values from there). Given that my target for the compressor
for effective cooling was 3krpm, this seemed like a perfect match for an
inline shaft connection. I was figuring somewhere around 4hp or so to
drive the compressor, which I think this motor should be able to handle
for a short period of time.

I tried running it with no load with about 133V, which was the level of
our battery string at the time. Even without the flywheel, I don't think
it would have been easy to hold the motor on startup. I was stepping on it
at the time, and glad I did so. It was unexpectedly loud -- lots of brush
and windage noise. Then with the flywheel/fan thing attached, the startup
torque was incredible and I had to step on it pretty hard to keep it from
leaping up and chasing me around the car. At speed the flow of air pulled
through the motor was equally intense and the noise was something like a
jet engine (actually not appropriate *at all* in an EV -- folks, you don't
really want this motor). Otherwise the motor seemed to be doing fine --
very minor arcing at the brushes, a light, acceptable ozone odor and
almost no arcing at the clips I was using to complete the circuit at make
or break. Running it at 156V was not much different, just a little faster.

For what it's worth, running it backward by reversing the red and black
connections (only for a second or two) at 133V was NOT the same; I'm
guessing it has advanced brushes though I don't really know enough to say
how much. Significant but not huge arcing at the brushes, and definitely
arcing and slight melting at the clips during make and break. Despite SC's
ad description, this is NOT a reversible motor without modification.
Briefly checked the comm area, and from what little I could see it didn't
look damaged.

So, I finally got my shaft adapter machined and my test bracket built. We
connected the motor and compressor through an aluminum/urethane lovejoy
coupler and bolted everything in place. We checked for runout, were able
to adjust the lovejoy and compressor position to an acceptable level, and
figured it was time to test. We strapped down the bracket, turned on the
A/C blower, winced a little bit, and connected the clips.

Of course against no backpressure, the motor started right up despite the
flywheel and achieved what sounded really close to no-load rpm (i.e.
"cleared for takeoff"). As pressure built it didn't really slow down much
at all (like a good little permag motor), but I'm sure it was drawing a
lot more current. The motor seemed really stout and for just a moment I
thought we were in luck. I don't think we ever achieved full running
pressure however, because after about 20-30 seconds we panicked and had to
cut the power. We had started to see *huge* flashing at the brushes, at a
pretty scary level. The smell got bad, and as the motor stopped, some of
its smoke leaked out in a puff while the backpressure spun the shaft in
reverse a couple revolutions. It all happened pretty quickly so I hope my
memory of events is accurate. Unfortunately I decided to put off measuring
voltage sag and current until a later run, and now I'm not willing to test
again with a motor that might be damaged.

We cut off the blue paper band, took out the brushes and looked at them
and the comm. The contact surface of the brushes didn't look right, with
what looked like scorching on one edge. At least the matte appearance to
one side of the surface and the distinctly different shiny appearance at
the other edge didn't look right. The comm has a pretty dark black band on
it now but I don't have enough knowledge of what to look for to know the
extent of the damage. It doesn't look like we had a total flashover; the
brush holder, tiny brush springs and connecting wires were fine. Nothing
visibly amiss besides the brush and commutator surfaces, no significant
damage that I can tell at the comm bar edges.

Here's where I think I need some re-education. I had assumed that a
properly adjusted DC (wound or permag) motor responds to overloading by
faster temperature rise, such that you can use it but not continuously
unless enough additional cooling is possible. More overload = faster
heating. Kinda like 2 little motors pushing a record breaking drag
motorcycle for a few seconds. I did not expect that a motor that runs
apparently fine with no load would arc disastrously with a load -- even
way too much of a load. In summary, these were the failure modes I'd
assumed:

* Excessive load: eventual overheat of coils or brushes, insulation failure
* Excessive voltage: commutator flashover/fireball
* Incorrect brush timing: arcing and/or reduced speed or performance

Is that right? Did I do something wrong? Would brush adjustment help here,
and so would incorrect advance not be detectable at full speed and no
load?

Also, what the heck are those two extra blue wires for? They're the same
gauge as the red and black power wires. No voltage on them when the motor
is running, and they seem to have an inductance between them so they seem
to go to a coil in there somewhere. Are they supposed to go in series with
the power circuit (1 male and 1 female fast-on), and if so, what do they
do?

Thanks for slogging through...

--chris


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## EVDL List (Jul 27, 2007)

If you are looking at this thread to help out, here are the nameplate
values from the motor: (It is sitting in my garage...)

IMAGE variable speed dc motor
Part No: 22373000
MODEL: 4640D-100
HP: 2.50
RPM: 3250
V. ARM: 130
A.ARM: 18.0
Insulation class: H

Cheers,

Aaron Choate




> Christopher Robison <[email protected]> wrote:
> > Hopefully I can toss this out here, maybe some folks with more experience
> > than I have can tell me if any of this makes sense. (Crap, this has gotten
> > huge, main questions at the bottom.)
> ...


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