I’m from team 1251 and we are trying to make prototypes on drive trains to learn about them and come up with a good design during building season. We haven’t work with the FP motors and I will like to hear from some people with experience. What are the good things and bad things of usings this motors? How are the more efficient? And so on, any advise and experiences will help.
Thanks for all your help.
David Guzman
Team 1251
2004 All Star Rookie Team Award at UCF Regional
Dave,
Beware of breaking teeth off the final two stages if using the FP transmissions.
a single lesson learned from 2004.
Do not use FP motors in your drive train unless you are comfortable basking in the aroma of sizzling plastic.
I remember that smell @ the Jersey regional. I thought the stands were on fire or something.
I guess I was upwind of it.
Seriously…
Try to keep these motors from EVER running at stall while drawing 12V. This will minimize their strain over the course of a match, and prevent them from smoking.
If you EVER stall these motors during a pushing match…
… hope you have some spares ready.
:wiffs the air: Smells like… fisher Price, :ahh:
The FP motors are really pretty strong motors but like any of the motors we use there are some flaws. FP’s need to be run at the RPM they were designed for. Low RPM on these motors spells disaster as the fan that pulls colling air through the frame is very inefficient. At low speeds the heat from the motor will melt the fan (which is just pressed on the armature shaft) and it will stop working altogether. Remember that this motor is designed to power a drive around kids car in almost continuous duty until the batery dies, without failure. Keep the RPM up, and the motor ends unobstructed to allow cooling air to flow through and you should have no problems. Stall current is above 90 amps, so more than 100 watts can be dissipated in that little package.
Thank You every one for your input. From what i have heard, it isnt very reliable to use them in the drive train. What has your team used them for? How did it go?
Dave,
I should add that we have used FPs for many years in our drives including our 2003 Championship winning design. They are as reliable as anything in the KOP if used as intended.
I will not be as harsh on the motors as others in this thread. They are powerful and compact actuators. We have used the FP motors in quite a few key applications over the years (we have avoided using the FP gearbox since the first year). I don’t think that we have ever failed a motor.
Prolonged stalling of any DC motor is something we try to avoid by design. Beside heating effects, the larger motors (drill, CIM, van door) will trip circuit breakers and the smaller (window, seat) will rapidly slow (internal PTC resistor). All of these are undesirable in a competition robot…
Another note: I usually inspect the FP motor wires where they attach to the motor rather carefully. I have found the integrity of the connection at that point to be lacking…
Ok now im confused because firt John V says that the motors will burn before the 30 amp breaker trips. However Al says that their stall current is over 90 amps. I don’t know if im understanding correctly but if the motor burns before it trips the 30 amp circut breaker then that would mean that it would burn before it gets to its stall current?
Dave,
Please refer to the chart labeled “time vs. current” on page 2 of this document Snap Action Specification then read on…
Any fuse or circuit breaker uses heat to trip. Heat is related to current by ohms law. When you plot the time versus current required to trip the device, you get a characteristic curve called an inverse time curve.
All current interrupting devices are made to pass the specified current (what is called 100% rated current) forever. Therefore, a 30 amp circuit breaker will not trip at 30A.
Referring to the curve for Snap Action, at 90 amps (300% rated current), the device will take between 1.5 and 4 seconds to trip. The range of values is due to variations in manufacturing.
Two factors further mitigate this operation point. (1) You never get to 90A because of voltage drops in the speed controller and wiring. (2) The internal resistance on the Exide battery will drop your voltage under load. As such, the voltage seen at the motor is closer to 8 or 9 volts during a stall.
It is likely that it could take 5 to 10 seconds for a FP motor st stall conditions to trip the breaker.
Now let’s give the system a second or two to cool, the breaker resets and it starts again… During a two minute match, this can happen many times… The thermal mass of the FP is large compared to the breaker. Heat builds up in the motor and, after a while, the motor wires smoke.
The breaker tried to do it’s job. If we had used a fuse or a non-self resetting circuit breaker, the motor would have been stopped after the first stall and the interrupting device would have protected the motor.
Note here that the FP is not unique. I have seen many smoking drill motors in my 10+ years of doing this…
Engineering is all about trade offs. Most teams would prefer a self resetting breaker which allows you to continue playing the match. The down side is that you run the risk of frying a motor.
You can try and educate your drive team to avoid motor stalls. Even better, you can try and design systems which do not stall…
Bottom line… Read and understand the spec sheets.
Peace.
Mike is absolutely right.
I was in a goofy mood, and probably overly harsh on the FP motor. We have had some bad luck using them, despite my best efforts as a mechanical designer to minimize the loads they see.
I still recommend investigating another motor for drivetrain, and possibly saving the FP (and it’s premade, 147:1 gearbox) for an arm or other mechanism.
Good Luck in whatever you do,
John
We used these in our winch last year with great success. The key has been stated above, use as intended, run near full speed for cooling.
If you do a search through the archives you will find a drawing for replacing the final gear with a metal one. Bill Beatty was kind enough to share his design and it is still available at http://www.beattymachine.com/gear.htm.
David,
Mike refers to the current trip point in the breaker but we also must add into the variable mix the fact that your speed controller is not at full up all the time. That means that there are current pulses being fed to the motor that could be at full stall current but the average is actually less. What makes the situation worse is the fast rise times at the switch points do produce some significant back EMF when the motor is heavily loaded. These add to the heat in the motor due to eddy currents and arcing brushes that the circuit breaker will not react to. As to the circuit breakers, they are usually out in the open getting cooled off by moving air and the large diamter wire will draw heat away from the breakers as well. However, when these self resetting breakers start to trip under heavy loads they do get extremely hot and reset times can be under a second, so fast that the breakers actually buzz. When this occurs, do not allow anyone to get near the breaker panels until they have had time to cool. The temperatures are beyond the point of producing second cegree burns.
Other things that are beyond our control that affect whether a motor winding burns are the uniformity of the varnish coating and bending and nicking of the wire as it is wound on the armature. A wire that is bent and/or pulled tightly around a sharp corner will become the weak point in the winding, having the highest resistance and therefore the highest heat. Add these variables to the mix and winding failure is inevitable at the weak point(s).
Again, running this motor at normal design speeds will let the fan move air through the motor minimizing the heating factors.
F P Caution
We have used the FP motor and gearbox every year since it has been in the KOP. Not the most efficient, but easy and economical to use.
Our gear mod will not work on the newer FP gearboxes. The final gear set is much heavier than before. The weakest link now appears to be the gear cases. Don’t have a fix for it yet, but if we come up with something, we will post it.
The BeachBots have used the Fisher Price motors with great success ever since they were introduced. But we have never used them as a drive motor. when combined with the supplied gearbox, they make a pretty good winch. You can even use the supplied wheel adapter for your winch drum. Two plates with a hole in the right spot and a couple of scraps of aluminum angle and you have a really powerful winch. Our winch for pulling us on the bar last year was one of the fastest around, and we had enough pull to lift a second robot, in case somebody should have accidentally gotten hooked on us. It used two FP motors and gearboxes with a custom drum. The only problem we have had was a failed limit switch. It allowed the winch keep pulling beyond its normal travel and bend our lift. Fortunately it was during a demo and not the competition season
As Al S mentioned earlier, it is important to keep them operating at the correct speed. We always design our systems so that the motors are running at 75% of free speed and 25% max torque under a normal load. Those rules pretty much apply to all motors. Follow them and your motors will live long and you will prosper.
ChrisH
Chris,
I saw a few similar setups to yours, but I wonder if you’d be willing to share any pics/drawings? We’ve never been really good at using the FP Gearbox, I’d be VERY interested in seeing how 330 does it.
Thanks,
John
In 2003 we used the FP’s and their transmissions (with addition 5:1 reduction) to operate a very heavy 7 ’ arm (~40 pounds) and we’re fairly successful, but when practicing we kept smelling something awful - our FP’s were smoking but not tripping the breakers - we put two muffin fans on each motor - and never smelled again!!!
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