Electrical basics

[Richard Wallace]

Step 1: locate any post by Al Skierkiewicz

Step 2: click on his name in the post heading and select "Find all posts by ..."

(Warning: he has over 9000 posts, and the majority of them are about wiring safety.)

Happy reading! 😊

[Al Skierkiewicz]

To help you in your search,
Center the PD in the robot, this will keep the wiring short. There is no significant advantage to moving to #4 from #6. If you keep it short, the difference is millivolts. #6 is .0005 ohms per foot and #4 is .0003 ohms per foot. #10 is much better than #12.
Uninsulated terminals are available from the same sources and cost less. Use the right crimper. Those meant for insulated terminals will not give you the same crimp force.
Breakers that have tripped will not degrade. If they are repeatedly abused over a long period (5-10 matches) the internal contacts will pit and raise the internal resistance. Under those conditions, replace them. However, if they are warm to begin with, they will trip at a lower current than at ambient temperature.
For a little more than the cost of the Battery Beak, you can get a West Mountain CBA IV USB analyzer. This will draw current for a much longer time and give you a display that you can save to disk, and overlay in the future. This allows you to track the health of the same battery over years. It also will match the curves used by the manufacturer and calculate amp hour ratings. It will show battery cells that do not match. The Beak is great to put in your pocket and know if the battery you put in the robot is charged.
Use the "wire foot" analogy. At 100 amps, 1 foot of wire has a voltage drop that is predictable. #6 will drop 0.05 volts/ft, #10 will drop 0.1 volts/ft and #12 will drop 0.2 volts/ft. The stall current of a single CIM motor is 131 amps under test at 12 volts. On a typical FRC robot this more like 116 amps. That is the current it will draw when starting and anytime you are applying full throttle and the robot is not moving. Yes, 6 CIM drives have the ability to draw over 600 amps from the battery. The internal resistance of the battery is 0.011 ohms/11 wire feet. So at 600 amps, that is 6.6 volts dropped in just the battery. Yes the main breaker can withstand that for short periods but not forever. Loose connections (bad crimps, bad solder jobs, loose battery terminals, loose PD connections, etc.) can amount to several wire feet of loss per connection and can also raise the temperature of the device they are attached to. A loose terminal on the main breaker can raise the internal temperature above 100 degrees.

[jojoguy10]

Thanks for all of the great advice! Right now, we are without an electrical engineering mentor to help us out with this, so we are struggling a bit in this area.

If I have any questions, I will post them here.

Thanks again!

-Joe

[M.Kong-Sivert]

I've been trying for years to convince my team to solder the wires or use heatshrink or both, since our connections kept coming loose, but, while we were talking to some people at a conference, someone gave us a few of these: https://www.wagobox.com/shop/wago-22...412-20pcs.html

We were a little skeptical, but we tried replacing some of our connections with them, and they worked perfectly. If you're not careful, they can pinch your fingers, but they're easy to use, they don't come loose, and they're reusable. We bought a box of them and used them on our robot, and I haven't brought up soldering or heatshrink to my team since then.

If you must crimp, though, make sure the people know how to do it properly. Too many of our connections have come loose because someone didn't crimp the right way.

[Richard Wallace]

Originally Posted by M.Kong-Sivert

...someone gave us a few of these: ...

AndyMark has them. (link)
They work well.

[Al Skierkiewicz]

They are only rated for 32 amps.

[Michael Hill]

Originally Posted by Al Skierkiewicz

They are only rated for 32 amps.

Hey, could you shed some light on current/power ratings for me? Something that's always confused me are ratings at high voltages. These say they're rated for 32 Amps @ 400 Volts. Are these the maximum for both current and voltage? That is, you should neither operate above 32 Amps OR 400 Volts? Or is it it a power rating? That is, it's rated for 32 Amps * 400 Volts = 12.8 kW. Or is it a P=I^2*R power? In that case, the current would be the max current no matter the voltage

[Al Skierkiewicz]

Micahel,
The insulation is rated to withstand 400 volts without arcing through the material. If you were to use this for line voltage it would not cause a safety hazard if handled or attached to metal. (line voltage of 120 volts is the RMS rating not the peak.) The current rating is based on temperature rise using continuous current (usually for 24 hours) If you exceed the current, the device will warm and the max voltage rating will likely fall. Once you exceed the melting point of the plastic, it will deform bringing the conducting parts closer to the surface. While CIM motors in an efficient design won't run at 32 amps continuous, under certain conditions, you may exceed that for several seconds to a minute. In that case, high temperature will result. In addition, the temperature will likely also effect the holding tension device and may just release the wire. Your mileage might vary.

[Aren Siekmeier]

Originally Posted by Michael Hill

Hey, could you shed some light on current/power ratings for me? Something that's always confused me are ratings at high voltages. These say they're rated for 32 Amps @ 400 Volts. Are these the maximum for both current and voltage? That is, you should neither operate above 32 Amps OR 400 Volts? Or is it it a power rating? That is, it's rated for 32 Amps * 400 Volts = 12.8 kW. Or is it a P=I^2*R power? In that case, the current would be the max current no matter the voltage

The current rating of a conductor is itself a power rating, basically how much power can it dissipate as heat. The power dissipated as heat is indeed P=I*V, but this V is the voltage drop across the conductor, not across the load the conductor leads to. The voltage drop across the conductor is given by V=I*R for a conductor resistance R, the power rating is P=I*V=I^2*R, and depends only on the current and the physical characteristics of the conductor (size, resistivity, length...).

The voltage rating of the device is the max voltage before the insulation between conductors or between a conductor and the outside breaks down and allows a current. This is entirely a function of the various dielectric and insulating materials being used between conductors and the electric fields they can withstand before ionizing and carrying a current. The current carried by the insulating conductors has no effect.

So these two ratings are unrelated. You should not exceed either of them, even if you are way under the other one.

[Richard Wallace]

Originally Posted by Al Skierkiewicz

They are only rated for 32 amps.

Or 20A, as stated on the bag -- I think the figure depends on which agency basis is used.

Also the WAGO lever nuts are only sized for 12 AWG and smaller wire -- not really big enough for the 10 AWG many teams (like mine) prefer for drivetrain circuits. We will probably stick with 45A APP connectors for the drivetrain.

[Al Skierkiewicz]

When possible we connect the motors directly to the speed controller by placing the controllers near the motors they drive. This eliminates another connector and therefore another possible failure point. We use screw mount push on connectors on Victors and Jaguars.

[Ken Streeter]

Originally Posted by Al Skierkiewicz

We use screw mount push on connectors on Victors and Jaguars.

These sound neat. I don't think I've seen these before. Can you provide a pic and/or a supplier/part number?

Thanks!

[Al Skierkiewicz]

http://www.digikey.com/product-detai...7866-ND/293277
We buy one hundred at a time. When needed we use the APP 35 and 45 amp contacts. Crimper is available from West Mountain Radio (outside Milwaukee) as well as other places.

[RonnieS]

Will you still be doing this with the limited length of leads on the new speed controllers? Or splicing in on the PD board side?
-Ronnie

[Al Skierkiewicz]

We will likely add APP connectors to the new speed controller output wiring. I don't like that but we will adapt. I hate adding another point of failure.