S-10 - Performance Followup

[craig.dusing]

I've been treating my S-10 with kid gloves since I completed this first conversion in April.  Mostly driving around town, 4-5 miles per day, 35 mph or less.  Took a few 15-20 mile runs. Charged at least 20-25 times and put about 600 miles on the odometer

Last week I took it out for a range test.  Mostly 2 lane rural blacktops.  Rolling countryside, no major hills to climb; a couple of 200 footers with 3% or less grade.  Farm country. Tried to keep motor amps under 200. Speeds from 35-45 mph.  Managed to drive it 48 miles that day.  The last 10 miles were a challenge.  35 mph top speed with some serious voltage sag near the end.  When I was all done, I ended up at 65% DOD.

Well this morning I took it out and "drove it like a hated it".  (old racing term  ). Started fully charged and equalized.  I started from Milledgeville and drove 50-60 mph on IL Rte 40, a 2 lane highway, to Sterling IL.  Spent some time in a shopping mall parking lot checking motor, controller, and cable temps.  Everything was good.  Put it back on the highway and drove it home.  50-60 mph.  Ran about 200 motor amps on level ground, hit 400 amps on hills.  Ran in 3rd gear, 5.55 final drive ratio, about 4000 motor rpm. Truck ran fine, never ran below 50 mph.  28.7 miles total.  30.50% DOD  at end of trip.

Conclusions:  Mechanically, everything is good.  But, as I drive this truck I'm becoming more and more convinced that these T-875 8 volt batteries were the wrong choice.   I used them as I didn't see a way to fit 24 (6) volts in the truck.  The 144V system  provides more than enough speed.  It'll go 70 mph easy, with more left.   They seem to do very well during 5-10 mile runs, but they like their "rest".  Short run, recover. Short run, recover.  When I try to stretch them out on longer runs they seem to experience more voltage sag than I expected.  The voltage sags, the amperage goes up, the speed falls, the voltage falls more, the amperage goes up, the speed falls more, ........and, well, you get the picture.  You better be looking for a place to rest, or the spiral will continue.  Maybe I'm asking for too much, I mean I did just drive about 30 miles continuous at highway speeds, but that would have been about it for a continuous drive.  I've got to believe the (6) volters might have been a better overall choice. 

I built the truck to use as a test bed for our adapters, couplers, battery boxes, and other components we're building, so I'm going to do just that.  I'm thinking about fitting up (12) 12V gel batteries next.  I'm really curious to see how much the range decreases, if any, and what effect it will have on performance.  After what I just described, would that be stupid, and trying to re-invent the wheel?  Should I just go with the 6 volt bats? I feel like I know how the 6 volters would perform, and I'm really interested to see how much effect the weight reduction from using 12 volters would impact performance. I'm also not too keen on the Curtis controller, so I'm looking at other options there too.  George at Netgain has me setup with a WarP9 to replace and compare with the ADC motor, so it looks like it's going to be a major rebuild! .....

Now that I just typed and re-read that last line, I think what I might really need is just another S-10 candidate.  Looks like here we go again!

Craig Dusing
EV-Solutions Inc.

[nathan.stowe]

Craig,

Thanks for the real world data.   It is nice to have real figures, instead of theoretical.  I am considering the T875 and T1275 for my project and was concerned with the range.  I was using an on line calculator for the range estimates and your data shows the calculations to be a bit conservative.  It figures about 14 miles for your truck at 40mph with the T-875s, where as you showed 30 was possible (I googled Milledgeville to Sterling).  Maybe you can calibrate the drag/rolling coefficients to match your tests and then compare to other batteries or motors.

http://evconvert.com/tools/evcalc/?vals=veh=7:mot=2:bat=3:ctl=1:vlt=144:nst=1:dod=80:cwt=20:wtr=600:mwt=500:inc=1:wnd=0:sec=195:asp=60:rim=14:rr=0.015:bs=0.003:kwh=0.09:miles=20:

Nathan.

[pat.mitchell]

I've been treating my S-10 with kid gloves since I completed this first conversion in April.  Mostly driving around town, 4-5 miles per day, 35 mph or less.  Took a few 15-20 mile runs. Charged at least 20-25 times and put about 600 miles on the odometer
       

Last week I took it out for a range test.  Mostly 2 lane rural blacktops.  Rolling countryside, no major hills to climb; a couple of 200 footers with 3% or less grade.  Farm country. Tried to keep motor amps under 200. Speeds from 35-45 mph.  Managed to drive it 48 miles that day.  The last 10 miles were a challenge.  35 mph top speed with some serious voltage sag near the end.  When I was all done, I ended up at 65% DOD.

Well this morning I took it out and "drove it like a hated it".  (old racing term  ). Started fully charged and equalized.  I started from Milledgeville and drove 50-60 mph on IL Rte 40, a 2 lane highway, to Sterling IL.  Spent some time in a shopping mall parking lot checking motor, controller, and cable temps.  Everything was good.  Put it back on the highway and drove it home.  50-60 mph.  Ran about 200 motor amps on level ground, hit 400 amps on hills.  Ran in 3rd gear, 5.55 final drive ratio, about 4000 motor rpm. Truck ran fine, never ran below 50 mph.  28.7 miles total.  30.50% DOD  at end of trip.

Conclusions:  Mechanically, everything is good.  But, as I drive this truck I'm becoming more and more convinced that these T-875 8 volt batteries were the wrong choice.   I used them as I didn't see a way to fit 24 (6) volts in the truck.  The 144V system  provides more than enough speed.  It'll go 70 mph easy, with more left.   They seem to do very well during 5-10 mile runs, but they like their "rest".  Short run, recover. Short run, recover.  When I try to stretch them out on longer runs they seem to experience more voltage sag than I expected.  The voltage sags, the amperage goes up, the speed falls, the voltage falls more, the amperage goes up, the speed falls more, ........and, well, you get the picture.  You better be looking for a place to rest, or the spiral will continue.  Maybe I'm asking for too much, I mean I did just drive about 30 miles continuous at highway speeds, but that would have been about it for a continuous drive.  I've got to believe the (6) volters might have been a better overall choice. 

I built the truck to use as a test bed for our adapters, couplers, battery boxes, and other components we're building, so I'm going to do just that.  I'm thinking about fitting up (12) 12V gel batteries next.  I'm really curious to see how much the range decreases, if any, and what effect it will have on performance.  After what I just described, would that be stupid, and trying to re-invent the wheel?  Should I just go with the 6 volt bats? I feel like I know how the 6 volters would perform, and I'm really interested to see how much effect the weight reduction from using 12 volters would impact performance. I'm also not too keen on the Curtis controller, so I'm looking at other options there too.  George at Netgain has me setup with a WarP9 to replace and compare with the ADC motor, so it looks like it's going to be a major rebuild! .....

Now that I just typed and re-read that last line, I think what I might really need is just another S-10 candidate.  Looks like here we go again!
       
       Here is a description of a new 'precision DC motor speed controller' IC, offered by Texas Instuments and can be seen at http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=6417:

Craig Dusing
EV-Solutions Inc.

[[Ideas For Design]
Precision DC motor speed controller

W. Stephen Woodward  |   ED Online ID #6417  |   March 17, 1997




 Optical tachometers that produce a frequency proportional to RPM are popular feedback sources for precision analog motor speed control. This usually involves a frequency-to-voltage converter (FVC) to convert the tachometer output to a voltage that’s then input to a conventional servo. Though it typically works fine, it’s unnecessarily complicated and requires a tachometer with a relatively high pulse/revolution characteristic to allow for both a reasonably fast loop response and adequate ripple filtering in the FVC.


The circuit shown circumvents those problems by replacing the usual FVC with period-to-voltage converter A2. The motor speed setpoint is adjusted with R1 as follows: The voltage on integrator A2 (Vr) is a negative-going sawtooth with slope given by Ir/C1. Ir = (V3 − V1)R1/(R2R3), so if 1/Fm is the period of the tachometer pulses, the peak-to-peak swing of Vr = (V3 − V1)R1/(FmR2R3C1).


During the positive half-cycles of the tachometer square wave, S3 connects C3 to A2 so that the voltage on C3 tracks the sawtooth. On every negative square-wave transition, C3 is connected to the summing point of A4. This happens in such a way that if Vr < (V3 − V1), then a positive increment of charge is deposited on the summing point. This tends to drive A4 negative and thus reduce the voltage applied to the motor, slowing it down.


By contrast, if Vr > (V3 − V1), then a negative increment is dumped onto C4 and the motor voltage and speed are increased. The net result is for A4 to converge on the single motor voltage that makes Fm = R1/(R2R3C1) because only then is Vr = (V3 − V1). Consequently, the equilibrium tachometer frequency and therefore motor speed is directly proportional to R1 and independent of the absolute values of V1 and V3, removing any requirement for precision voltage references. Only the stability of the ratios of R1, R2, R3, and C1 contribute significantly to the error budget of the controller. Therefore, Vs supply voltages from 20 V to 35 V give the same accuracy.


Dc stability of the controller is assured by the relationships derived previously and by the error integration performed by A4. But, as in all servos, dynamic convergence is another matter. Damping of the system response and stable, over-shoot-free response to perturbation are provided by the C4/C5 ratio and R4 x C5 time constant. The values for these parts, shown in the figure, worked well in the prototype. Different motors connected to different loads might require tweaking of these values. In general, the greater the C4/C5 ratio and R4 x C5 time constant, the greater net damping and tolerance of high load inertia.


One additional consideration is what happens if the motor stops and Fm = 0. Because the feedback path depends on charge transfers between C3 and C4, and since these transfers cease if Fm = 0, some means is needed to “jump-start” the servo. Comparator A3 does this by slewing A4 positive whenever Vr is allowed to ramp below V4. Accurate charge transfers in normal operation are promoted by the crisp square wave produced by Schmitt trigger S1.