Re: *Warning* On talons!!!!
 

The snap action breakers trip based on heat. Short spikes of high current still make it through before the breaker heats up to the trip point.

Even with a breaker in place, you can still damage a motor controller with excessive current (e.g. a short circuit). The breakers simply can't react fast enough to save the components in certain situations. That is why "breakers are for preventing fires, not saving the components."

Breakers do play a role in component safety, but they need to be used in conjunction with proper electrical design. When using a motor controller...

• that is wired correctly
• with fans when necessary
• with breakers installed
• with well designed mechanisms (to reduce the chance of stalling a motor)
• etc.

you shouldn't see any failures.

-David

Re: *Warning* On talons!!!!
 

Cory,

David points out that short spikes of current will get through..

Check out the circuit breaker spec here. As you can see, a 40A circuit breaker will allow 500% (200A) to pass for 0.2 to 0.3s before tripping.

-Scott

Re: *Warning* On talons!!!!
 

Does the above statement apply to Jaguars only? And if not, may I ask where you got your information for the Vics and Talons? Thank you.

Re: *Warning* On talons!!!!
 

Are you implying that 200A for 0.3 seconds will damage a Talon (or a Victor)? If so, would you please tell us where you got that info? Thank you.

Re: *Warning* On talons!!!!
 

Emphasis on can damage.

I have personally caused a Jaguar failure by connecting a dead short across the motor terminals. This was in early Jaguar testing 3-4 years ago.

I haven't personally caused a failure with the others.

Scott and I aren't implying that certain exact conditions will or will not damage any of the motor controllers. We're trying stop a common misconception that the breakers instantaneously trip once you go over 40A. They take time, and can't be guaranteed to protect anything but the wiring.

-David

Re: *Warning* On talons!!!!
 

Understood. But there's a flip side to that coin, to wit:

Assuming the datasheet specs for the 40 amp breaker can be trusted, it's possible to design a product around those specs so that the breaker WILL protect it.

Hence the question: what are the chances of damaging, say, a Talon, if it is properly wired and protected by a properly operating in-spec 40 amp FRC breaker?

I wouldn't lose any sleep over it.

Re: *Warning* On talons!!!!
 

Just some random subjective data, but we've been running fanless Talons in our prototype drivetrain for many weeks now stopping to change batteries and the talons are hardly warmer than ambient to the touch. This is with 4 CIMs and 40a breakers and a ~120lb total robot weight incl. battery (so a bit light) on carpet geared fairly aggressively. I would begin to worry about heating the battery cable and CIMs before I worry about warm talons.

A CIM in a drivetrain will pull full stall current immediately at launch. It will taper down to a much lower level very very quickly, how quickly is dependent on gearing, mass to accelerate, and voltage drop.

Use fans if you want. Think of them as speed controller insurance with a 20 gram weight penalty.

Re: *Warning* On talons!!!!
 

what does "geared aggressively" mean in this context?

Re: *Warning* On talons!!!!
 

We gear our prototypes for high speed because we are interested in high-speed behavior and know that low gear behavior will always be better than high-gear behavior.

This particular one was estimated to travel around 13fps after estimated losses. We don't have any sensors on it to test this, so I can't verify, but it is going somewhere in the neighborhood of that speed, with 4x CIM power.

This particular chassis tends to eat a battery every ~7mins. That's not all hard driving, there's some argument over dynamic performance behaviors and some calibration going on at the same time. About 5mins of actually driving. We change the battery when the radio reboots (we're running the radio to the 5v camera supply, not the nice regulated supply).

Re: *Warning* On talons!!!!
 

I have a feeling someone just looked at the motor spec sheet and a talon spec sheet and concluded they would not work together.... which is incorrect.

Generally, electrical components fail in some shape or form because of heat. If you draw too much power through them, things get hot and their physical/electrical construction degrades rendering them non-functional. This sometimes results in all the magic smoke leaking out :rolleyes:

Every time you pass current through a conductor it generates a small amount of heat. When the component is operating within its specifications, it is capable of dissapating said heat as fast or faster than it is generated. When you exceed the current ratings, the component will continue to function until it becomes overwhelmed by the temperature and burns out. So you can indeed exceed the capacity of the device for short periods of time but if you exceed its ability to keep cool, you'll ultimately shorten its life drastically or burn it out. This also translates to why speed controllers with fans are more hearty than those without.

Re: *Warning* On talons!!!!
 

With good wiring layout and 10 AWG, you can get any of the FRC legal motor controllers to deliver momentary peak currents >100A to a room temperature CIM motor. See test results here.

The peaks don't reach 133A, but they get close in some tests -- duration of the peaks in other test conditions will depend on loading and on motor heating. I have not tried to kill speed controllers by stalling motors (yet).

Re: *Warning* On talons!!!!
 

For what it's worth, Robot in 3 days used the Talons exclusively. We have an hour or two run time on them and have had no issues. We've fused them as per the motor rules. We did not use fans. Maybe over the weekend we can take some readings on current draw of the different systems. In a purely subjective sense they've handled whatever we've thrown at them.

Re: *Warning* On talons!!!!
 

No, this is not the implication. The original confusion, I believe, was that the Talon was not provided 100A due to a circuit breaker tripping at a smaller current level. The point I was making (also David) is that the circuit breakers do not trip at levels that would prevent the device from operating at stall current.. circuit breakers take time to trip.

C'est tout.

Re: *Warning* On talons!!!!
 

I would like to add a little caveat here. While any of the devices can run without fans, please think about your design. You may play fine in a northern regional or in Canada, but come April in St. Louis, the ambient temperature and humidity will change, plan accordingly. While the Talon looks great, time will tell for sure. Please when you give feedback, insure that everyone understands the conditions under which your report is made so that other users and the manufacturer can test and duplicate your results if needed.

Re: *Warning* On talons!!!!
 

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.