Did I read <R32> Correctly? Six CIMs?

[theprgramerdude]

Originally Posted by Trent B

Not to mention Banebots puts a max torque spec of 85 lb-ft or 115.2 N-m of torque on their P80 gearboxes.

As for the 2 vs 3 CIM Motors, correct me if I am wrong, but based off of this http://content.vexrobotics.com/docs/...otor-specs.pdf

2 Motors at 300oz-in each will draw about 115 amps a piece. 230 amps total.
3 Motors at 200oz-in each will draw about 75 amps a piece for 225 amps total.

Because the curves are linear, total current draw shouldn't matter on number of motors. However, the motors will be rotating faster on the 3 motor setup at ~2200rpm compared to around ~700rpm because each motor has to put out less torque.

The motors will heat up extremely quickly at those current levels, too. I wouldn't be surprised if some team that doesn't think too deeply into current levels runs a dual or even single CIM setup at 115 amps enough to effectively ruin the entire motor (demagnetization, something melting, etc.)

[aldaeron]

2 questions related to the number of motors allowed

1) Do you think the rules would be changed to allow a large capacitor across the inputs to the Power Distribution Board to regulate voltage levels? Look at Figure 4-7 in the rules - the capacitor would be hooked across the wires where the text "Power Distribution Board" appears

2) Do you think in the future that they will allow two 12V batteries or larger 12V batteries (automotive instead of motorcycle) to allow for more powered motors?

Cheers!

-matto-

[Racer26]

Originally Posted by aldaeron

2 questions related to the number of motors allowed

1) Do you think the rules would be changed to allow a large capacitor across the inputs to the Power Distribution Board to regulate voltage levels? Look at Figure 4-7 in the rules - the capacitor would be hooked across the wires where the text "Power Distribution Board" appears

2) Do you think in the future that they will allow two 12V batteries or larger 12V batteries (automotive instead of motorcycle) to allow for more powered motors?

Cheers!

-matto-

Not likely on both cases. 1 12V 18AH battery has been plenty for all FRC applications when designed properly. You don't need more than one.

[AhAhPatel]

how much weight can a CIM pull? just a quick question..

[slijin]

Originally Posted by AhAhPatel

how much weight can a CIM pull? just a quick question..

A better question is how much torque or how much power can it generate. See here.

[ttldomination]

Originally Posted by AhAhPatel

how much weight can a CIM pull? just a quick question..

This is dependent on a number of factors, including but not limited to gear ratios, lever length, and the method of lift.

- Sunny G.

[Trent B]

Originally Posted by theprgramerdude

The motors will heat up extremely quickly at those current levels, too. I wouldn't be surprised if some team that doesn't think too deeply into current levels runs a dual or even single CIM setup at 115 amps enough to effectively ruin the entire motor (demagnetization, something melting, etc.)

Yeah, someone was just asking the difference between 2 and 3. For the same torque output you will have same current consumption but 3 motors will run at higher rpm.

[Donut]

Originally Posted by Trent B

Not to mention Banebots puts a max torque spec of 85 lb-ft or 115.2 N-m of torque on their P80 gearboxes.

As for the 2 vs 3 CIM Motors, correct me if I am wrong, but based off of this http://content.vexrobotics.com/docs/...otor-specs.pdf

2 Motors at 300oz-in each will draw about 115 amps a piece. 230 amps total.
3 Motors at 200oz-in each will draw about 75 amps a piece for 225 amps total.

Because the curves are linear, total current draw shouldn't matter on number of motors. However, the motors will be rotating faster on the 3 motor setup at ~2200rpm compared to around ~700rpm because each motor has to put out less torque.

Although this specific example is not advised (75 amps still blows the breakers!) what you're saying is correct. If you drive the same load at the same speed with 2 or 3 motors, then the total power used will be roughly the same. Each motor individually will draw less power in a 3 motor configuration but you still need the same total power to move the load (power = torque * angular velocity, none of which are motor dependent). There will be a slight difference in total power due to where the motors are running in their efficiency curve but I think it would be negligible for most applications.

Adding extra motors can be a benefit if you need more torque than was available with the previous CIM limit, but as Al mentioned you might not have enough power available to power that many CIMs anyway. Additional CIMs don't change how much power you can draw from the battery at one time.

[PayneTrain]

Also on the subject of reading it correctly, are Fisher Price motors in the doghouse this year?

[z_beeblebrox]

Originally Posted by PayneTrain

Also on the subject of reading it correctly, are Fisher Price motors in the doghouse this year?

Yes, but the smaller CIM derivatives are pretty close to them in power.

[apalrd]

Originally Posted by z_beeblebrox

Yes, but the smaller CIM derivatives are pretty close to them in power.

CIM (real) - 340w
CIM (mini) - 230w
CIM (BAG) - 145w
RS-550 - 250w
RS-775 at 12v - 260w
FP 9015 - 190w
FP 0673 - 290w
Andyprice - 190w
And I think that's all the large motors. The FP 0673 is incredibly power dense. The less dense motors are theoretically much harder to smoke (unless you short several through their cases and the frame).

[AlexH]

anyone thinking 6 cim and 4 shorty cim drivetrain geared for ~15mph?

[Chris is me]

Originally Posted by AlexH

anyone thinking 6 cim and 4 shorty cim drivetrain geared for ~15mph?

A 10 motor drivetrain would destroy your battery and end up probably moving less well than a four or six motor drivetrain.

[Leav]

Originally Posted by Trent B

Yeah, someone was just asking the difference between 2 and 3. For the same torque output you will have same current consumption but 3 motors will run at higher rpm.

Just to slightly elaborate on this:

The current of DC motors is approximatley linearly related to the torque applied to the shaft, and is usually assumed to of the form I=k*T.

where I the the current and T is the applied torque and k is the "motor constant" (that's what we call it in hebrew, I imagine the name is similar in english).

so no matter how you split the torque between any number of motors, the current draw will be roughly the same (if you neglect friction, which isn't always a wise thing to do).

If i'm not mistaken, the motor's speed is also approximately linearly related to the torque, but it reduces, for example: w=3000-a*T

where w is the rotational velocity. so just like Trent said, reducing the torque on each motor would increase their rotation speed.

-Leav

[Al Skierkiewicz]

Nemo,
Here are a few items for your calculations. The internal impedance of the battery is 0.011 ohms fully charged. The #6 wire is 0.0005 ohms per foot, #10 is 0.001 ohms per foot. I am trying to remember, I think Jaguars are 0.004 ohms, Victors are 0.006 ohms. So all you need to do is add up the losses and solve for Ohm's Law. Al's Rule of the Wire Foot (WF) states 100 amps in one foot of #10 or two feet of #6 equals 0.1 volt drop per foot. Starting six CIM motors would max out the current handling of the battery at 600 amps. With a typical FRC robot the wire/foot losses would be 11 (battery resistance) + 2 (four feet of #6)= 13 WF. 13 * 6 (for max battery current) would result in 7.8 volts of drop in the path to the PD. Since the power supply for the Crio drops out at 4.5 volts, this would produce a sufficient drop to reboot the Crio each time the robot started. Predictably, the 120 amp main breaker may or may not trip at this initial start but the temperature in the breaker is certain to rise at that demand. Now all things being equal, the losses in the remainder of the wiring feeding the speed controllers and the motors would prevent maximum stall currents to be reached. However, it is still likely that six CIM motors could under certain circumstance max out the current ability of the battery and main breaker. Teams did report main breaker trips last year with additional motors. My recommendation is to monitor your currents and adjust your design appropriately.