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Unread 23-11-2004, 07:43
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Re: Advices on FP Motor

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?
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Unread 23-11-2004, 07:50
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Re: Advices on FP Motor

Quote:
Originally Posted by David Guzman
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.
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Unread 23-11-2004, 08:16
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Re: Advices on FP Motor

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...
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Unread 23-11-2004, 08:16
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Exclamation Re: Advices on FP Motor

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?
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Unread 23-11-2004, 09:15
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Re: Advices on FP Motor

Quote:
Originally Posted by David Guzman
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.
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As easy as 355/113...
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Unread 23-11-2004, 09:32
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Re: Advices on FP Motor

Quote:
Originally Posted by Mike Betts
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.
  • It is a good motor.
  • 3rd most powerful motor in our kit. (Following Drills then Chiaphuas)
  • Many teams have used them with great success. (My favorite usage has been by team 60. They use 2 of these to power their 2-jointed arm.)
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
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Unread 23-11-2004, 10:10
Andy Brockway Andy Brockway is offline
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Re: Advices on FP Motor

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.
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Unread 23-11-2004, 10:19
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Al Skierkiewicz Al Skierkiewicz is offline
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Re: Advices on FP Motor

Quote:
Originally Posted by David Guzman
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?
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.
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Last edited by Al Skierkiewicz : 23-11-2004 at 10:21.
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Unread 23-11-2004, 11:03
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Re: Advices on FP Motor

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.
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Unread 23-11-2004, 11:30
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Re: Advices on FP Motor

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.

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Unread 23-11-2004, 11:48
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Re: Advices on FP Motor

Quote:
Originally Posted by ChrisH
...Two plates with a hole in the right spot and a couple of scraps of aluminum angle and you have a really powerful winch.
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
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Last edited by JVN : 23-11-2004 at 11:52.
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Unread 23-11-2004, 19:59
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Re: Advices on FP Motor

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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Unread 09-12-2004, 14:34
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Re: Advices on FP Motor

Beware of the plastic gear boxes.. they dont like changing directions much.. if you keep going in one direction, i believe you will be fine.. if you change directions (like in skid steer or similar drives) the plastic gear boxes will strip out very easily
JP
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