Attached is the original motor spec sheet from Mabuchi for the Fisher-Price motors. Notice that Mabuchi also supplied a separate motor. The spec sheet attached is for the motor that is attached to the gearbox. The spec is for the motor only. This is the chart I sent to FIRST in December.
There are two sheets in this file: one for 6 volts and one for 12 volts. This motor was optimized for the 6 volt ride on car, but can be used at 12 volts.
So from this data, it looks like we can’t really use the motor below about 2/3 free speed or it will draw way too much current and burn something (itself, fuse, etc) very quickly.
Whoa… …that’s a lot of power. That means that the F-P motors are easily the most powerful motors in the kit. The Chiaphua’s (a.k.a. CIM’s) are something like 300-350W as I recall.
For those who do not have experience, if you are planning on using anything close to that amount of power, you had better be sure to leave the vents on the front end of the motor open to breath. There is a lot of heat to get out of a small space. Even with the 150W motors from prior years (again from memory not from specs), we used to melt the fans off the armatures. I think we had 200+W motors in 2002 but nothing like this.
Interesting. What was the spec last year – my memory fades.
FYI PEAK POWER in Watts = (FreeSpeed in RPM) * (Stall Torque in N-m) / 38
This is a useful figure of merit for motor power – the 1/38 comes from the fact that the peak power happens at 1/2 the stall torque and 1/2 the free speed and that RPM to Radians/Sec ~ 1/10
On another note, the Dewalt whitepaper (see plug below) that explains how to use the F-P motor in a Dewalt Transmission, does not open up these holes. It is possible, even without a mill to leave these holes open. If you are going to drive with a F-P motor, you should definitely open the holes.*
Joe J.
*they are already open on the standard transmission, I really mean this as a note to those who are planning on making your own transmission for these motors – it make a huge difference the cooling of your motor – don’t be fooled by folks who put fins on the outside of the motor can. While this can be good to improve the amount of power you can put in these motors for long periods of time, FIRST matches are not “long” in heat transfer terms. To give you a perspective: A curling iron is typically 20W. At stall, you are pounding 150Amps at 12V – that is 1800W of power – we are talking blowdryer’s here. There is just NO WAY you are going to conduct that amount of heat way with fins on the outside of a motor can. You need AIR FLOW to carry that heat away. While I am at it, once you get to stall, there is no way you are going to even blow that much heat away because a stalled motor is not blowing any air!
One final thing, don’t tell me that the 40A breaker is going to save you either. This as been discussed 10s fo times on this sight - search “40A breaker trip time” (in fact I just did and there was a lot of talk of burning up F-P motors – checkit out). You will discover that the 40A breaker can conduct 2 and 3 times their rated current for significant periods of time before they open.
I think the mistake is not Pauls or Mabuchi’s but F-P’s
If the motor is supposed to be used on a 6V car, then it is not unreasonable – 100*W’s is about right – still high but not too bad. 400W is INSANE in that size motor.
Joe J.
*I don’t have the spec open right now, I am going by the theoretical idea that peak power grows like the square of the voltage. So, 1/2 the volts implies 1/4 the peak power.
When my contact first told me about it being a six volt motor I was a bit disappointed, but I asked him a few questions. Disregard his question to me in the quote below. The Excel grid I attached in my previous post does have the data for 6 volts, too. That data is on another tab in Excel. I can test one of the motors I have and see what it can do, but I bet limiting the voltage to the FP motor is probably a good idea. The thing that bothers me the most is the engineer from Mabuchi did not jump up and down and say, “No way! Do not use it at 12 volts or you will destroy the motor.” In my experience, however, the motor will fry near stall conditions at 12 volts. I am confident that the information I have received is accurate, but the missing piece is the robustness at 12 volts.
-Paul
Paul,
It was nice speaking with you earlier today and Mabuchi Motor is pleased to
support programs which challenge the young minds of tomorrow’s work force.
I had an applications engineer, Mr. Dustin Ridgeway, pull off the
performance curves from our data base on the
PF # 74550-0642 , motor RS555PF-8021 you requested. Per our discussions,
the motor is basically a 6.0 V nominal motor
as compared to the 12.0 V RS555PF-6534 you are accustom to.
It can be operated upwards to 9.6 V and for short time duration 12.0 V may
be acceptable.
Can you downgrade the battery to 6.0 V ??
If you have any further questions, please feel free to contact us for
assistance.
Mabuchi motors that we have used have also fried near stall conditions. We have not limited the voltage on them, in years past. They definitely don’t like to be stalled when full power is being asked for. They tend to give up and let out their white smoke.
So… what we will probably do is run this thing at a maximum of 9.6 volts, as the engineer from Mabuchi suggested. At this stepped-down voltage, we will see these numbers:
2005 Fisher-Price at 9.6 volts
free speed: 19,200 rpm
max power: 325 Watts
stall torque: .518 N-m
Looking at these numbers, at this lower voltage, this motor is less powerful than the 2003-04 drill motor (free speed 19,670, 448 Watts, 0.87 Nm stall torque). It is strikingly similar to the 2002 drill motor (20,000 rpm, 340 watts, 0.65 Nm)
I think there is a better way than to arbitrarily limit the voltage to the motor.
If Mabuchi says 12V is okay, I believe what they are saying is that the armature can stand the 24,000 RPM without “bird caging”*
Given that, I think what is really needed is a way to prevent the build up of heat inside the motor. This can be done more elegantly than simply to limit the voltage of the motor (which is to restrictive at times and not restrictive enough at others).
Here is what I propose to those who want to get the most out of this motor:
measure or infer current (you know voltage in from PWM and if you are driving with it, you know the RPM – that gives you a good method to calculate current).
given current and voltage, you can calculate electrical power input and the efficiency of the motor at that speed/load/voltage point
given power input and effeciency you can determine how much heat you are generating at that moment in time
given the heat input at any moment in time, you know how much energy is put into the system and (assuming initial conditions and the heat out is proportial to the temp of the motor) you can do a basic energy balance calculation temp over time.
if you want to get fancy you can even incorporate a motor speed based cooling factor for the motor (faster speed -> more cooling).
after the temp rises above a certain number, THEN and only then, I would limit POWER INPUT to the amount that I can dissipate at the current speed
Note that #6 in effect is done by limiting voltage, but the difference is important. You are only limiting voltage when needed and then only by the amount needed to keep the temperature from further.
In theory, this is a pretty easy thing to do. In fact, because temperature is a slow process I think you can do this calculation 3 or 5 times per second and still get good data.
Has anyone implemented a current limit based on such ideas? To be honest we have talked about it for 2-3 years now. We never implemented it mostly because we haven’t used the F-P for a drive system in a while.
Any volunteers to share their experience or to take on this subject in a whitepaper?
Joe J.
*where the windings cannot stand the centrifuge effect and they mechanically disconnect from the armature, forming a kind of bird cage.
what is the reduction with the gearbox. i don’t have one to look, is it similar to boxes we’ve had previously? does anyone know for sure what it is? thanks.
It’s a 124:1 reduction. Basic info on the motors in the KOP this year can be found in guidelines_c.pdf off of the FIRST website (look under documents and updates and you should find it. It should be in the robot section if my memory serves me correctly).
I’d take your gearboxes apart and count the number of teeth on each stage. If we are really getting varied pinnion sizes between teams there may be other variations as well.
Woburn has a box of F-P gearboxes dating back to the late 90s. There are at least four different kinds in there, and (except for colour–some are white, most are black), they all look identical from the outside. With the issues with differently-sized F-P pinions this year, you should definitely disassemble them and count teeth, if ratio is critical.
I like your solution better than Dr. Joe’s. :ahh: But please tell: Are you proposing a EE solution that steps down the voltage outboard of the Victor, or a software solution where the PWM signal is limited to a precalculated value, so that the effective voltage at the motor is 9.6v? (Or something else? Will the Victors work properly if you limit their input voltage?)
Do you object to the complexity of the algorithm or to proposed operation? I think several teams did something very like this to stay right on the edge of the 120 Amp breaker from tripping in the 2002 Season. I don’t think it is that complicated and I think the performance of the motor would be significantly better than the performance via limiting to 9.6V. 9.6V would still very easily fry this motor under some conditions while at other times (at the beginning of the match for example) it would limit the output needlessly.
As to whether Andy is proposing putting 9.6V on the input side of the Victor or limiting it in software (via max. & min PWM outputs), I don’t know how you could actually put 9.6V on the input side of the Victor within the rules of FIRST.
Limiting the PWM in software is just as effective as limiting the input power to the Victor as far as the motor is concerned. It has the added benefit of being within the rules as well. A PWM limit would be my mitigation choice.
