Motor Brush Advance
 

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...

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.

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

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:

From this post in the FS build thread: http://ecomodder.com/forum/hreshowth...3-post801.html

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

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

FYI, I have read that running a motor under heavy load with the brushes retarded can damage the comm. See: http://ecomodder.com/forum/527-post189.html 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.

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

Hopefully tomorrow!

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

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.

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!

Hey Tom, I have been thinking a bit about brush timing lately.

Any new info about your advancement, thoughts now that you have been driving it a bit?

After a couple of weeks I put it back to where it was. Turns out (at least with this motor / voltage / controller / vehicle weight configuration) that advancing it made it somewhat slower (7.5 seconds 0-25 compared to 7.0 standard) and less efficient (about 300 wh/mile compared to ~250 standard).

There may be a happy medium somewhere, but without drilling a few more holes (or constructing a variable ring) where it was seems to be the best position. Eventually, I may drill holes at 5 degrees advance, and 5 degrees retard to see how those positions may change power/efficiency.

On a side note, it appears that this motor may be maxed out for this 1250 lb car.

Today I put 13” tires on the Citicar to see how it would affect speed and acceleration. For baseline, my Citicar has a top speed of 32 / 36 (normal / field weakened) and accelerates from 0-25 in an average of 7.0 seconds with 145R12 tires. (R in this configuration implies an 82 aspect ratio.)

I have a pair of tires from a 1981 Comutacar with 13” rims. They have 135R13 tires with the same bolt pattern as my 1976 Citicar. For the test, I put the 135R13 tires on the back of the Citicar.

Using a GPS, I determined that the speedometer was spot on with the 12” tires. With 13” tires, the speedometer registered just under 24 MPH when the Citicar was actually moving 25 MPH.

Acceleration from 0 – 25 (actual) remained the same with the 13” tires as with 12” tires – 7.0 seconds average (5 tests).

More interestingly, the actual (GPS measured) top speed was exactly the same (32/36) with 12” and with 13” tires, although motor RPM was lower with 13” tires. This leads me to believe the main limitation with my Citicar is not in gear ratio, maximum RPM of the motor, voltage, or amps available, but in the ultimate power of the motor. Last year, I put an extra battery in series (for 54 volts) and the top speed was also 32/36.

For reference, at 3000 RPM / 6.8:1 the Citicar is going 27.74 MPH, and with 13’ tires it is going 29.05 MPH.

Looks like I'm running out of ideas to improve speed and acceleration of this motor. The best / most successful experiment so far was field weakening... (other experiments were with increased voltage, increased amperage - direct contactor - increased tire size and advanced timing. All others experiments did not shown much/any gain. But again, this is with this particular motor... If your motor is not severely overloaded as this one appears to be, you may have better success...

If you just want a higher top speed, my money would be on field weakening, and a second step would be to adjust timing a few degrees positive or negative to see where your particular setup stands.

Tom

Thanks for all the numbers.

I have always thought it would be fun to put a bigger motor on a Citicar, although a Warp13 may be overkill....

Tom: boat tail! :) I suspect that even at 30 mph, more than half your power is being consumed by aerodynamic drag.

It would be interesting to see a Citi/Comuta car owner do a complete "rebody" job with aero in mind.

Ok, so it's off topic of brush timing, but YES, I would LOVE to see somebody do a full new body of a C-Car!

Many of them have very beat up bodies, and it would be fairly easy to make a new one from scratch!

I am planning on going to the C-Car World Get Together next month. I will make sure to take lots of photos, and I'm sure I will see a body mod or two out there.

If field weakening has an effect, a larger tire or different gearing or different voltage should have an effect.

Trouble is your WEIGHT and rolling resistance must also stay the same another battery must have added weight.

Was your amp draw the same with bigger tires? Its possible they had more rolling resistance.

My recommendation to you is to locate some geo metro 105x13" tires for $5 from a salvage yard (be very carefull mounting them), fill to 60psi (sidewall rating) and test.

What PSI do you run your tires at now? Remember their "rating" is at speeds in excess of normal highway speeds, the slower and lighter the vehicle the more air you can safely put in, if you aren't already test at a much higher air pressure, sure ride sucks but hey it does anyway? I would say 50psi-60psi, this is how I got my miles zx40 up to 30mph from 24mph.

My c-cars top speed is effected 5mph if I change from 60psi space savers to normal 40psi radials and my amp draw increases, I am sure you would see a different if you increase pressure.

Good Luck

I thought that larger tires would increase the speed as well, but with field weakening both on and off, the speed was the same with it on/off w/145r12 tires. Probably the 3.5 motor is just at the end of its capability, and wind drag, etc. combine to defeat it going any faster than 36.

The tires are run at 40 psi, mostly because I can't stand the road noise and bumpiness - even at 42 psi it gets bad. I may try 60 psi for a test to see how much of a difference it makes. If it'll do 45 mph at 45 psi, I may just learn to tolerate a bit more noise. 8-/

I did a cd calculation, and the Citicar ended up being around .68 if memory serves. Not quite a brick, but close... If I can figure out a way to mount a temporary support structure to the body without making holes in the plastic, I may try a boat tail and see what change happens.

I should be at the c-car meeting in mid-May as well - see you there...

I always thought an interesting aerodynamics test would be to bolt two citicars together - back end to back end.

Put the back one in reverse and the front one in forward and go. It would be a slightly more aerodynamic four-wheel drive vehicle!

It would also look suspiciously like a STAR TREK shuttle craft....

... or the Star Tours shuttle craft at Disneyland. Maybe that's the mod needed - plasma drive - just as long as the plasma doesn't come out of the 3.5 :eek:

I took the motor out and drilled two more holes at 5 degrees advance. Hard to say if there has been an improvement, but there are a couple of interesting data points -

Over the past 56 miles I have been testing accel times and in general driving faster than normal. Even with the 'non optimal' driving, wh/mile is at 243. This is about the wh/mile I get by trying to be efficient.

During a longer than normal drive today (16 miles) the top non field-weakened speed crept up to 34+ MPH. The normal max is 32 on that stretch of road. And that was after about 11 of the total 16 miles, so the batteries were slightly tired.

Acceleration time 0-25 now ranges from 6.0 to 6.6 seconds. Normally, it would be 7.0.

Other than the weather being somewhat warmer these days the only change I am aware of is the 5 degree timing advance. I'll keep it this way for a while to see how it works out. Perhaps 10 degrees was too much, and zero is OK for the stock motor at its rated 36v, with five being a better fit for 48v...