FP ok to stall?
 

I know in years past, FP motors loved to produce magic smoke when they stall (now replaced by thermal trip). My dilemma this year is our roller (single fp) isn't geared down enough (2500rpm) and we are having trouble controlling the ball.

Question: If we run the FP motor at 30% power and stall it for no more than 10 seconds, will this still cause thermal trip/magic smoke or is this a "safe" option?

Re: FP ok to stall?
 

The FPs have an external fan attached to their output shaft to keep them cool. Stalling the FP not only causes high current, but also stops this cooling fan. Not a good idea.

I would not suggest running an FP in stall for very long at all (not even 10 seconds).

Just my opinion.

Re: FP ok to stall?
 

I'm going to have to say this is a bad idea. FP motors are super sensitive to stalls, and will go up in smoke in seconds.

-Nick

Re: FP ok to stall?
 

The only thing that makes me consider this is that when I have seen FP motors go to smoke, they have been at 100% power and stalled for at least 2 seconds. By running them at 30%, the motor would be drawing significantly less power.
I'm just wondering if anyone has tried running the FP at less than full power and still fried it?
(If not, I can resort to destructive testing but I rather ask here first)

Re: FP ok to stall?
 

The only way to effectively use fisher price motors and not product magical white smoke is to have enough gear reduction built into the system so it doesn' stall. If you manage to do that, then the motors will produce magic in the form of winning matches :) otherwise, there is no real solution to the problem your team is having. The window motors or cimz would be better suited for the solution you previously mentioned

Re: FP ok to stall?
 

The FP have a thermally activated switch that protects them from getting too hot but it also prevents you from pushing them to the limits of their capability.

I thought it was fine to have intermittent use at 1/2 stall torque, but I have heard war stories that make me believe otherwise. I am planning on having a modification that will allow me to drop my motor current to 1/4 to 1/3 of the stall value during lift.

As I said, I think I should be okay for 4 seconds at 1/2 stall but I don't dare bet the farm on it...

So... ...we make contingency plans and hope we don't need them...


Joe J.

Re: FP ok to stall?
 

Our FP is geared down 108:1, powering a 2" roller. It was stalled for about 10 seconds, there was no smoke, but a strong burning smell came out. Thankfully neither the motor nor the wires were hot, but we have a spare one ready to go just in case.

Our roller and back plate are design such that the ball doesn't get jammed in too far. The material is chosen such that it brushes a ball like getting a car wash at a gas station. The height have been modified a hundred times before it was finalized in one of the later matches at SD. All of this and we are still fine tuning it as time goes on.

Be very gentle to your FP motors... Be very gentle.

Re: FP ok to stall?
 

Fisher price are not a nice motor to use effectively. They are just too fast. The pinion (and gear) is a real pain if you need to change it for whatever reason and they hate being stalled. Personally, I'm not a fan. Can I have my big cim back please?

Re: FP ok to stall?
 

What Daniel meant above is the fan is internal. Frequently teams have loaded the motor significantly which slows the shaft speed to the point at which the fan is inefficient. (or they block the open slots at both ends of the motor) Regardless, stalling the motor is bad even at low throttle values. The thermal device gives no warning, the motor just stops.

Re: FP ok to stall?
 

In 2007 the BeachBots used stalled FP's to hold our ramps closed. We did this by using substantially less than full power and gearboxes to lower the shaft speed. We had a 4:1 reduction after a BaneBots planetary gearbox, I forget if it was 16:1, 32:1 or 64:1. I think we used something like 25% power for retention, more if we actually wanted things to move.

Before we settled on this solution we did a fair amount of testing to verify that doing so would not cause problems. The motors did get slightly warm to the touch during a match, but they were not close to hot. IIRC we used the same set of motors all season.

In this situation all we had to do was prevent movement from starting, in the stowed position the ramps wanted to stay put unless there was some outside event, like a hard hit from the side.

So you can successfully stall an FP without burning it up, but you need to be pretty careful how you do it.

Re: FP ok to stall?
 

We've had great luck using a FP by cutting down the gearbox to whatever state best fits our application. Two years in a row, we have rollers powered by FPs with the third stage on the plastic gearbox removed, and they have been tremendously reliable.

That said, I don't know that *any* motor likes to stall for very long when hooked to a 12V battery...

Re: FP ok to stall?
 

ChrisH,
I'm probably wrong, but I thought you guys used the BaneBot motors on your ramps and the fps on your arm in 2007.

Another option if your still worried about stalling though, is to put some kind of slip clutch into your system so that if the ball jams the fp motor will not stall.

Re: FP ok to stall?
 

I love the FP motors. At least I did before the Thermal protection was added. I think I will end up liking it just as much with the breaker, if not more. They are easily the most power dense motors in the kit. Used with a Dewalt transmission or a Banebot transmission designed to be loaded properly (<1/3 stall torque if planned to be stalled for any duration of time) the motor is a real work horse.

Just one man's opinion.

Joe J.

Re: FP ok to stall?
 

We're using 2 FP (one at each end of the roller) running at 30% power. In another thread I read running FP's above about 50% under load will lead to the motor failing. I didn't verify the claim. But our trick has been to drive the roller with rubber o-rings. At first, we used special pulleys machined to press onto the FP shaft. The motors didn't stall, but the roller would resulting in burned and broken o-rings. Then, by accident we noticed during additional testing that one o-ring had slipped off the pulley and was running just on the motor shaft. We noticed that seemed to work by allowing the o-ring to slip on the shaft when the roller stalled. We made it through the entire Bayou regional last week, including elimination rounds, without losing an o-ring! Give it a try.

MJ
Mentor, Team 1421

Re: FP ok to stall?
 

We're using 2 FP (one at each end of the roller) running at 30% power. In another thread I read running FP's above about 50% under load will lead to the motor failing. I didn't verify the claim. But our trick has been to drive the roller with rubber o-rings. At first, we used special pulleys machined to press onto the FP shaft. The motors didn't stall, but the roller would resulting in burned and broken o-rings. Then, by accident we noticed during additional testing that one o-ring had slipped off the pulley and was running just on the motor shaft. We noticed that seemed to work by allowing the o-ring to slip on the shaft when the roller stalled. We made it through the entire Bayou regional last week, including elimination rounds, without losing an o-ring! Give it a try.

MJ
Mentor, Team 1421

Re: FP ok to stall?
 

I can attest to friction driving FPs to prevent stalls. While not yet used in this configuration, I would give it a try if you could.

Re: FP ok to stall?
 

Seconded! You can't beat them for sheer power. Especially because you actually have the option of running them at 1/2 stall, even if it's not the best idea. You won't run a CIM at 1/2 stall for more than a few seconds before you trip a breaker.

That said, we're using an FP to hold up a ramp this year. We've currently settled on 25% speed as a semi-safe value. We tried 30%, but after a long session of unintentional idling in the pits, the motor was producing some distressing odors. In the interests of my sanity, the students backed it down to 25% and promised to not leave it idling in the pits.

Re: FP ok to stall?
 

We learned this the hard way this year.

We tried to use a FP to run a centrifigal fan out of a dewalt vaccuum. It functioned o.k. 1:1 but didn't give us the speed we needed. We geared it up 1:2 and after running 30 seconds wide opened the Fish smoked a bit and thermal-tripped. We thought we had a bad motor since there wasn't that much load on it (it was spinning at a REALLY good clip) so we wired up another and prompty toasted that. :o

That's how we learned that you don't run an FP at 100% of rated power if you can help it - even if it is not stalled and is still turning pretty darn fast.

Re: FP ok to stall?
 

I love data. Nice.

From the above two posts we learn:

1. The breaker is good, but when you have good airflow (due to high RPM of the motor) it is sort of a marginal protection device.
2. 1/3 to 1/4 full stall torque looks to be about the upper limit of high duty cycle applications.

One final clarification. From Kevin: "You can't beat them for sheer power." This is not exactly correct. The CIMs do actually beat them for sheer power (300+ W peak power vs. 200- peak power), but the CIMs are much heavier and much larger (even after you include an extra gear stage to slow down the speedy FPs).

What I intended say is that the FP motors are easily the top motor in the KOPs based on both power per unit volume and power per unit weight measurements.

Regards,
Joe J.

Re: FP ok to stall?
 

Getting back to the original design point, of using a motor to spin roller fast, but possibly allowing it to stall.

In one iteration of 'ball magnet" roller development, we used a polycord loop around a grooved hub on the roller running under light/no tension. We provided a tensioner, but also lubricated the polycord with lithium grease. This allowed us to "tune" the slipping of the polycord on the hub, and allowed stalling the roller without stalling the motor, although the motor would still be "loaded". That part worked great, but we found other ball control approaches that worked better than stalling the roller on the ball. It does allow you to spin the roller fast, but allow it to stall and then recover roller speed quickly.

I think some teams also use pool noodle on PVC tube, taking advantage of the slipping of the noodle on the PVC to keep from stalling the motor, while stalling the roller.

Re: 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.

Re: FP ok to stall?
 

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

Re: FP ok to stall?
 

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.

Re: FP ok to stall?
 

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!

Re: FP ok to stall?
 

On a sidenote:

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

http://www.mcmaster.com/#high-performance-tape/=65eqsn

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.

Re: FP ok to stall?
 

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.

Re: FP ok to stall?
 

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...&I=LXJHA7&P=FR

Re: FP ok to stall?
 

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

Re: FP ok to stall?
 

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 ?


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Re: FP ok to stall?
 

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.


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