FP ok to stall?

Actually, if you start looking at attainable power, thing get slightly more competitive. The CIM hits max power at 67 amps, which is going to trip a breaker or fault a jag eventually. Granted that’s something like 10 seconds for the breaker, but still. If you look at the power at 40 amps, you’re down to 280 W. Meanwhile, the FP can pull full power at about 35 amps. The CIM still wins, but it’s a bit more competitive of a race.

But yes, even factoring in a 4:1 gearbox to get down to similar speeds, the FP blows the CIM out of the water in power density.

Important discovery pertinent to this discussion:
Our Snap Action breakers are only guaranteed to trip at 135% of rated current. This means if you’re protecting a FP with a 40 amp breaker, it may be happily pulling 54 amps indefinitely. Even at 200% overload, the trip time is between 1.5 and 3.9 seconds. And I have a feeling our nice air conditioned environments will trend us toward longer trip times. As such, it’s likely much much wiser to use a 30 amp breaker on FPs instead of the usual 40 amp. With a 30 amp, your FP can only pull 45 amps for up to 3.9 seconds, which should do a much better job of keeping it safe.

We ran the roller last night at 30% and it performed beautifully. As soon as the ball touched the roller, it got pulled in and the motor stalled. We were able to drive forward and reverse at 20% speed keeping the ball with us. The FP thermal never tripped.
This isn’t ideal but it’ll work for this weekend. Later, we will be building a new gear box for it that will have “too much” torque

“too much” being a theoretical threshold that is never truly reached

You may change your mind after this weekend. It seemed entirely too easy last weekend to accidentally eat balls, especially when maneuvering near the bumps. If you go with a theoretically unstallable gearbox, it might increase your tendency to eat balls. I think it’s something to test before you permanently install it on your bot.

We went with a dual roller that grabbed the ball below the CG and gave it backspin. While it wasn’t perfect, it worked very well, and we are very happy with our decision.

It is driven by a FP motor – that never stalls, because nothing ever gets stuck!





On a sidenote:

Our team scrapped our roller, but before doing so, found some amazing stuff to create traction on the ball.

Go to non-stick surface-gripping tape. Buy some of the rayon cloth stuff. It isn’t cheap, but it provided so much friction that our fp would suck the ball under our test chassis with two people sitting on it.

In '08 we winched our catapult down with two FPs. Of course it would stall when we were winched until we hit the fire button. In initial testing, the kids would often keep it tensioned for long periods of time (10 seconds I would guess). Eventually, they smoked a FP and upon looking at it we saw that the plastic fan blades inside the motor housing were actually melted from the heat!

Our solution was to put heat sinks on them from an RC hobby shop (designed to fit around motors that size), and also to mount fans (like the ones on the Victors) that blew continuously over those heatsinks. That kept our FP motors cool even with long stalls and we never blew one the rest of the year through probably 100 matches. Limiting the current helps a ton too. The combination should do the trick. Pick up a non-contact temp probe from Harbor Freight and check your motor temps with various cooling and current systems and you won’t have to guess if you’re okay or not.

Heatsinks like Mr. Crombe was saying work really well with FP motors. This year we’re using them in our applications and 1323 even used them in their drivetrain. They’re the blue heatsinks in this pic. I’ll try to get a link to them, they’re about 14 bucks IIRC. Edit: http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXJHA7&P=FR

Not sure how different these are but here’s what I found while I was waiting for you to post the link:

http://www.rcplanet.com/Motor_Heatsink_s/1247.htm

Unfortunately all the fan designed ones appear to use a fan a little smaller than the one in the kit so they probably can’t be easily replaced, but the non-fan ones at the bottom of the page are almost half the price of the ones from Tower Hobbies

Kevin, could you clarify what you meant here please? When you say “25% speed”, did you mean “the voltage that produces 25% of the rated 12 volt no load speed” ?

In other words, adjust the PWM to the unloaded FP to produce a speed of 25% of the rated 12V no-load speed, then - while keeping the same PWM - add load to stall the motor; and it is OK ?

~

As a general rule of thumb, unless the datasheet or other documentation says otherwise, most DC motors are designed for continuous operation at about 20% of rated stall torque (aka 20% rated stall current). Operation at higher torques (currents) is allowed, but only intermittently. Ambient temperature, heat sinking, and forced airflow affects the allowable length of the intermittent period.

~