*Warning* On talons!!!!

[Mike Copioli]

Ok,

You will not be able to get 133 amps out of a CIM using an FRC battery, in series with an FRC PD in series with 10 gauge wire and a breaker.

The stall current of the CIM is based on a constant 12 volt supply. The power supply on an FRC robot is anything but constant. Factoring in ALL of the components of the robots power supply, the impedance path to the CIM Thevenizes to about 40 milli-ohms. The CIM motor itself has a stall impedance of about 90 milliohms (12/133).

Using Ohm's law, the voltage at the CIM will equal about 8 volts for a 100 amp load assuming a 12 volt source, 9 volts assuming a 13 volt source. So your stall current is really 8/.09 = 88 amps or 9/.09 = 100 amps.

Another way to look at this is as a simple voltage divider.

Vmotor = (Rmotor/Rtotal)*Vbat = .090/(.040 +.090) = 8.3 volts
Imotor = Vmotor/Rmotor = 92.2 amps.

This is pure DC math and is the absolute maximum that a CIM motor can draw not factoring in the positive temperature coefficient of all that copper inside the CIM or any AC components that add to the path.

Another point that I have not seen discussed; Beside the copper in the power path, the circuit breaker has a positive temperature coefficient as well. This causes the breakers impedance to rise exponentially with temperature thus reducing the motor current even further, long before reaching the trip threshold.

Bottom line:

You will not draw 133 amps with your CIM motor in FRC, not even close. BTW the spec sheet on the Talon reads 60 Amps continuous and 100 peak. The FET's in side the Talon are rated at 100 amps continuous. We de-rate from the 100 amps to be conservative. The reason is because of the heat that is generated at continuous currents higher than 60 amps.

[pfreivald]

Originally Posted by Mike Copioli

Bottom line:

You will not draw 133 amps with your CIM motor in FRC, not even close. BTW the spec sheet on the Talon reads 60 Amps continuous and 100 peak. The FET's in side the Talon are rated at 100 amps continuous. We de-rate from the 100 amps to be conservative. The reason is because of the heat that is generated at continuous currents higher than 60 amps.

Hooray, math!

Thanks for taking the time to outline the design and the rating decision, Mike.