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Old 07-16-2006, 09:48 PM
ccarey75
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GP1100's and a brushless 400

I have a brushless 400-size (Warp4 2-turn) motor in my micro RS4 and i've had a pretty interesting time of it trying to make it run properly.

motor: Warp4 2-turn BL (400 size)
esc: Mamba-25
cells: 9x GP100
pinion: 24T (largest i have so far tried) / 20T (current)

It all looks sweet but I was buggered if i could get it to run right. A friend of mine (adamc) got it going initially and it seemed to run more-or-less ok but it was a bit glitchy and tended to stutter at low-rpm. Stuttering at high (max) rpm was less pronounced but still prevalent.

Many people have kindly offered advice as to what the issue was likely to be (most pointed to either the esc or the cells being unable to supply enough current). The ESC gets pretty damn hot and very quickly so we considered overheating / overgearing might be the issue. The last possibility was a faulty receiver but by the time i got around to testing this I had my answer.

I was a little reluctant (from a sheer cost perspective) to replace either component until I was sure what the deal was so I started conducting a few tests with the equipment I had available. I'm sure there are more scientific ways of going about this but here's what I found.

All the tests below were conducted with the car stationary (wheels free spinning). Each run lasted 10 seconds (on the netball courts where we run the car is out of sight in less time than that anyway)

HEAT
- a 10 second run at max throttle puts the ESC at 60 degrees C (according to my infra-red thermometer). From what information i've been able to find online that's close to the maximum operating temp for the unit but should be safe. This test was done running the 24T pinion and the wheels spinning freely so it will probably get somewhat hotter under real load (but some air flow may compensate a bit).

VOLTAGE DROP
- measuring the voltage drop at max-rpm was a bit of a surprise. A fully charged pack hot off the charger sits at around 11.8V. At full throttle this drops to around 7.5V. I had thought the GP1100's would stand up a little better than this. It's possible that the pack is faulty but I don't know how to test this beyond measuring each cell voltage (all around 1.3V give or take 0.02V).

Either way the ESC has the cutoff set at 5V - that is it shuts down if the voltage dips below 5V. I know I can disable this setting but it seemed irrelevant if the voltage was above that anyway.

CURRENT
- All the equipment to effectively measure this seemed very expensive (ie more expensive than just buying all new components and hoping for the best). I finally settled on a kit clamp-meter from Jaycar which I'm quite happy with for $30.

This is where it got interesting for me - I had a 3S (11.1V) 1800 LiPo pack hanging around so I figured i'd use this as a comparison against the GP1100 pack.

The best measurement I could get from the GP1100 pack was around 15A. I say "the best measurement" because the stuttering caused the current to fluctuate fairly wildly making it difficult to get an accurate reading. The meter was effectively measuring a series of short bursts rather than a continuous figure.

By comparison the LiPo pack managed a steady 27A. Interestingly enough the stuttering was completely eliminated! The thing is as sooth as silk through the whole throttle range.

RPM was a fair bit higher with the LiPo (measured with one of those venom speed meter gadgets) so I assume voltage drop wasn't as extreme but I didn't get a reading on it (time was getting away from me by this stage)

CONCLUSION
I'm going to replace the battery pack and see how that goes - having had such a nice result from the LiPo I'm leaning toward just ditching the NiMH cells altogether but the chassis is nicely cut out for them so I will continue to agonise over that for a while.

There exists the possibility that the GP1100 pack is just dead and that's why it's not working properly. I've read a bunch of reports of people managing very high current draws out of these cells (and the IB1300 equivalents).

If anyone's got some advice on how I can test for this definitively I'm all ears!

-CC
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