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Greg N
04-26-2006, 04:53 PM
Two questions:

Neither of these were an issue before I started racing ISRA cars on a flat track. I have 15 band difalco with a built in choke. Works great and I love it. I'm a very average driver, in way over my head with those hot cars racing on a flat track.

About the heat:
Both with contenders and 12s, while practicing and after the 3 minute heats, the controller transistor and the heat sink get very hot. This is getting better as I get faster, but it's still an issue. How much of a problem is this? Should the transistor be replaced annually? Should I hook up a little computer power supply fan to the heat sink?

About the choke:
I find that both contenders and 12s are easier to drive and very markedly decrease their braid wear with 1 foot of choke. My lap times don't change noticeably. I find this hard to believe, I wouldn't think that 1 foot of choke would do anything much. Is this a common experience?

Thanks!

Greg

JayGee
04-27-2006, 11:31 AM
Greg,

The heatsinks and power transistors used are high-wattage and are designed to dissipate the heat you're seeing.

The reason the transistor heats up more as you are first learning to drive a flat track with a high current draw motor is that you are spending more time in the intermediate power bands. The transistor is partially turned on and is conducting high current at the same time that it's dropping high voltage across it.

The power (voltage drop x current draw) is turned into heat.

As you start to drive faster, you spend more time in the high speed bands which reduces the voltage drop across the transistor. Then, when fully punched, the transistor is bypassed completely by the blast relay and/or full power contact in the controller handle.

Putting some numbers behind this discussion...

I use a transistor rated at 160 watts. (I've got to think Jim uses a similarly rated transistor...I just know the specs for mine). That means with a large enough heatsink, you'd be able to run on a 12V supply, keep the throttle on 1/3 power so the controller drops 8V across the transistor and have the motor draw 20 amps before exceeding the power dissipation capability of the transistor.

Jeff

Greg N
04-27-2006, 01:11 PM
Jeff,

That's just what I wanted to know.

On the main board there's a discussion today about G7s, braid burn and track power. The strong suggestion there is that even on a high power track (which I think the track at Mineral Ridge is), small amounts of choke on medium draw motors should not do much of anything (positive or negative). If I get some time this summer, I'll do some formal testing on this issue. I'd run a GT12 for 10 minutes each on 12', 6', 3', 1', and 0 choke and look at lap times, motor heat, and braid wear.

Greg

JayGee
04-27-2006, 03:42 PM
You'll find that the choke will limit the surge current that a motor pulls while bogged in glue, loaded in the bank or accellerating out of a tight, slow speed corner. It's affect is more pronounced by higher current motors.

Once the motor is "at speed" it draws less current. The choke has a smaller effect, although still noticable for high current GP7s.

The lower the current draw of a motor when fully reved, the less pronounced the effect of the choke will be on top end speed.

Now, running with a choke will help keep your controller cooler because you'll be keeping the trigger up in the higher bands more. The power will be dissipated in the choke wire...not the transistor. Jeff

Greg N
04-28-2006, 06:17 AM
Maybe the dampening of peak current draws is what I think that I'm seeing. As I wrote up above, the cars just seem smoother, easier to drive and with less wear and tear, particularly the braid. I'll test for lap times, braid wear, and motor heat as above during the summer.

At least at the Mineral Ridge UK Black, even for the expert drivers, absolute horsepower is rarely an issue. Quite different than racing on a king or oval.

Concerning the choke, my difalco choke is wire inside of an aluminum box. It never gets even warm. I have an additional, traditional wire coil choke that I clip-on when running 1/32 F1 (hilarious little cars with fancy chassis, no downforce at all, and in-line detuned open motors). The clip-on coil adds 28 feet to whatever more I dial in with the difalco. The coil does get slightly warm, there's next to no braid wear, the motor gets very warm, not really hot, but the transistor does still get very hot, tho I have to say that those cars run most of the time on less than full power.

Greg