Re: Gear ratios for launcher
 

2250 * 3.67 = 8258 motor RPM

0673 @ 12 volts @ 8258 RPM is pulling 68.5 amps and generating 512 watts of waste heat.

This seems like it would either trip a breaker or smoke the 0673.

Are you sure of your numbers?

Re: Gear ratios for launcher
 

I was pretty certain until you put it that way. :) It doesn't appear to run very hot so far. I can certainly check the current today by measuring the voltage drop across a high wattage, low resistance power resistor in series.

I have seen plenty of shooters on CD this year with the same kind of setup - AM gearbox, FP motor, aiming for about 3 to 5,000 RPMs. If your concern is correct aren't we all in the same boat? If not, any idea where my assumptions have gone wrong?
Thanks!

Re: Gear ratios for launcher
 

Yours is the first I've seen on these forums with an 0673 running at 39% speed at full voltage.

If you are sure of the motor part number and the gearing, then I'd suspect either the wheel speed measurement is incorrect, or else the 2250 rpm is not at full motor voltage.

Re: Gear ratios for launcher
 

So we just rechecked wheel speed, and it maxes out at about 3500 RPMs, or 12,991 RPMs motor speed. We measured the voltage across a .01 ohm power resistor and it was about 210 mv. So at max speed it looks like we are drawing about 21 Amps. In-rush current when the wheel starts up peaks out at about 50 Amps for a brief moment.

Now we need to check the motor voltage, maybe that is where the flaw in the applied math is. SW is sending a "1" to the Jaguar, but I guess there is more voltage drop that I expected.

Thanks again for the insight. Everything is working ok, at this point it's just a study of the real world numbers we are collecting compared to the expected data based on the motor curves.

Re: Gear ratios for launcher
 

3500*3.67 is equal to 12845 motor rpm.

According to the motor curves,
An FP-0673 at 11.79 volts (= 12 - 0.210) and 12845 rpm should be drawing about 40 amps.

Since you measured the current to be 21 amps, let's use that data instead:

According to the motor curves,
An FP-0673 drawing 21 amps at 12845 rpm is getting about 9.7 volts. Add the 0.21 drop across the measurement resistor and your Jag is putting out about 9.9 volts. I'd expect it to be a bit higher than that, unless:

- your battery is weak
- you've got excessive resistance in your wiring or connections
- you're running a bunch of other heavy-demand stuff while you're running these tests
- the Jag is not putting out 100% PWM
- the motor curves are wrong or the motor you have differs substantially from them


The math is sound, but the data or assumptions could be wrong. I agree it would be a good idea to check the voltage at the motor, to validate the above engineering calculations.


When the engineering calculations don't jibe with the observed test data, it could be a sign something is not right. I understand this doesn't necessarily apply to FRC, but in many engineering fields when the test results don't agree with the expected results based on calculations or simulations, you stop the presses and figure out why.

Re: Gear ratios for launcher
 

Ok here is a good reason it is good to compare real world measurements to the expected/theoretical numbers. After scratching our heads we asked SW one last time if they were sending '1' to the Jaguar. They said 'Yes. Wait - It's a Jaguar? We have it coded as a Victor'. Opps! Since Jaguars have a larger band of PWM Full/Reverse signal input, that means they really were not sending it a '1'. They changed to Jaguar and now it runs a good amount faster at hight input values and is much closer to the curve.

Thanks for sticking with me on this one Ether! :)

Re: Gear ratios for launcher
 

and thank YOU for following up and not quitting.