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TomEV 12-07-2008 11:21 PM

Motor Brush Advance
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Anyone ever changed the brush advance on their series motor? Did it change performance or effeciency?

The other day I was inspecting the motor brushes on my Citicar (3.5HP GE) and noticed that the brushes appeared to be set 'after' the stator center line. From what I've read, the brushes are factory set to zero for a bi-directional motor, but can be set advanced for a motor that doesn't see high speed in reverse.

After I took the motor out, measurements confirmed that the brushes were indeed set after the center line. Oddly enough, the brush block assembly was angled 'into' the normal forward rotation - they should be angled away.

Third oddity - when the motor is wired normally (A1 and S1 same polarity, A2 and S2 the other polarity) the motor turns in reverse. Not a big deal (just swap S1 and S2) but still strange.

All together, makes me believe that this motor was designed to turn in the other direction, and is less than optimal when operated in 'reverse' to make the Citicar (with a Terrell axle) go forward. :confused:

Not much I can do about the brush block angle (without changing a bunch of rivets...) so I drilled and threaded two additional holes in the motor center case to change the brush advance. The brushes were retarded 5 or so degrees before the change, and they are now advanced about 5 degrees.

The motor still moves the Citicar, but I'll have to wait a few days to see how it runs... hopefully better! Front brakes are currently awaiting a new set of shoes...

Ryland 12-08-2008 10:03 AM

off topic, but with your brakes, apparently you should install the drums with lug nuts only hand tight, apply the brakes, turn the wheel 90 degrees apply them again and then tighten, there is alot of slop in the brake drum stud holes.
I'll have to look in my book on electrical engenering on brushes to get a good answer on that one.

TomEV 12-08-2008 06:57 PM

I'll try that when I put the front drums back on - there is a bit of slop there.

MetroMPG 12-08-2008 07:57 PM

I agree - sounds like your motor is running "backwards" to how it was originally intended.

The brushes on the motor in the ForkenSwift were originally advanced pretty far:


I was reading up on the pile of EVDL messages ripening in my inbox, and came across a message from motor guru Husted about low voltage systems & brush advance. Noted that he said "5 degrees for mild" voltages. Apparently the common "10 degrees for street use" applies to higher voltages (say 120+). So I went out and measured... and the brushes were advanced (stock) almost 17 degrees!!

Whoops. For 48v he suggested putting them to neutral.

I now have them near 2 or 3 degrees advanced. According to Jim, the closer to neutral timing = more low end torque and less top end RPM. The motor should run more efficiently too. After making the adjustment (drilled a new set of adjustment holes in the end cap ... with the motor still in the car), I can vouch for the first 2 statements. The car goes up hills better (relatively speaking), but I can no longer wind it out to 4000 rpm before shifting - it just runs out of steam.

And it still won't spin the tires on gravel, before or after the adjustment. I tried.
From this post in the FS build thread:

I can't comment on how it may have affected efficiency though. I'm not sure I could test it in a controlled way.

TomEV 12-09-2008 07:14 PM

Hopefully a mild advance (5 degrees) won't ruin the torque - not much there to begin with :rolleyes: I wonder how much the 5 degree -retard- affected efficiency and speed, it didn't seem too bad, considering the 3.5 HP rating.

I have a fairly complete record of how fast it goes, how many wh/mile, accel amps, and even a movie of the brushes under load. I'll compare the old data with the new setup.

Here are the numbers I hope to improve -

* Acceleration (25 MPH @ 7.5 seconds)
* Top speed (33/36 MPH) (36 field weakening active)
* Power use (250 wh/mi)

Best outcome - (more efficient, faster)
< 7.5 sec; > 33/36 MPH; < 250wh/mi

Worst outcome - (less efficient, slower)
> 7.5 sec; < 33/36 MPH; > 250 wh/mi

Acceptable outcome - (as predicted by some - somewhat faster, no efficiency change)
~ 7.5 sec; > 33/36 MPH; = 250 wh/mile

MetroMPG 12-09-2008 10:23 PM

FYI, I have read that running a motor under heavy load with the brushes retarded can damage the comm. See: Obviously that'll depend on the degree... When I was playing with brush timing, I mistakenly set them retarded and could hear the arcing on the comm before I even got out of the driveway. It did scorch the edges of the comm bars.

I predict you're going to notice a difference in performance (speed) with the brushes advanced.

The motor in the ForkenSwift was spinning the "wrong" way in its previous life as a hydraulic pump motor. After we reversed the field connections internally to reverse its rotation, the brush timing of course changed from 17 degrees advanced to 17 degrees retarded.

Result (no load): 1480 RPM @ 12 volts retarded, vs. 2275 RPM @ 12 volts advanced.

TomEV 12-10-2008 10:23 PM

Brakes not here yet... Shipping companies are a bit stacked up with all the holiday packages!

Hopefully tomorrow!

TomEV 12-12-2008 12:11 AM

Well, the new brakes are in! Good news is the Citicar stops faster...

The change to the brush advance is inconclusive. Acceleration 0-25 is the same. Top speed is about the same, but may be a bit off because of cold batteries (not used for a week, and the temperature outside is about 10 degrees colder than last week when it was tested). Could be the front brakes are dragging a bit since they haven't 'worn in' yet. Adjusters are fully closed, only thing that could be done is to re-arc the new shoes...

At any rate, this experiment did not produce the speed increase I was looking for.

I'll leave the brushes advanced for a while to see if it is an improvement or not. Not hard to put back if it ends up being less efficient.

Tom - 1976 Citicar - Alameda, CA

MetroMPG 12-12-2008 08:27 PM

Tom, what about a simple, repeatable test: jack up the car's rear end and compare motor speed in the 2 brush positions? I forget what you use for speed control (contactors or PWM), but to simplify (and avoid the risk of overspeeding) just do a direct 12v test.

TomEV 12-13-2008 02:24 AM

I'll see what it does in comparison sometime next week. So far, it is looking like it needs to be set back - wh/mile is around 270 averaging the last two days (was about 250). Torque may have dropped off a bit as well, because climbing the bridge (both ways) was two MPH slower than before.

Oh well, some experiments work, some don't!

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