Our team is trying to learn some electrical basics. Up to this point, we’ve just been trying stuff and hoping they work, but we want to know some basic ways of electrical work.
-best methods for wiring; to reduce circuit resistance, loose connections, portability, shorting, damage to wires and connectors, etc
-methods for testing electrical components - motors, controllers, switches, etc.$@# to be used during build and troubleshooting
-battery management - how do we test batteries for how fast they draw down under load (there are some complex methods and equipment that apply a 100 A load for 30 seconds that tell you a lot about a battery that simply looking at as charged voltage does not).$@# A new battery is not necessarily the best battery.
-ability to measure circuit variables (voltage, amperage, resistance).$@# Measuring the resistance of our drivetrain is very difficult and requires special methods that I don’t understand (resistance < 0.05 ohms)
-explanation of why we get certain results or don’t get them.$@# I still don’t understand why we see lower amperage on the competition drivetrain than what is predicted.$@# And I don’t even know how to figure out why other than just start replacing things.
-methods to combat tripping our current limits through the various breakers and controllers
Definitely a broad set of good questions, so here we go:
Reducing resistance usually amounts to increasing wire gauge. >= 6AWG for battery/breaker, >= 12AWG for high current motors (CIMs). We’re going to 4AWG for our breaker connection and 10AWG for our CIMs this year since we found wires getting warm last year. Also, reduce wire length as much as possible by placing connected components near each other.
Creating solid connections: it’s overkill, but soldering on crimped terminals is the best way to ensure that they stay on place. We strip off the plastic housing on these terminals, slide on some heat shrink, crimp the terminal, solder over the crimp and then heat shrink. We didn’t have a single terminal come loose all season.
Make sure there’s electrical tape or (preferably) heat shrink tubing over any bare connections. This reduces chance for shorting and makes things look cleaner.
It’s often overlooked, but put thought in to where you position the components. Minimize wire length from battery->breaker->PD board (within reason, allowing for ease of battery removal). Keep your motor controllers close to your motors, PD board and cRIO. Keep cRIO near to the digital breakout and motor controllers. Before drilling any holes, set out all the components as a test on a similar sized board to make sure you have an optimal arrangement.
For testing, provided you guys have enough components to recycle, create a test board with all the electrical fixtures on it. This is a great off-season activity and will save you lots of time during build season when you’re not having to kludge together something to test components.
Get a battery beak for battery testing. It’s a one-time cost and has saved us more times than you’d think. http://www.andymark.com/product-p/am-0995.htm. If you want to get fancy, try putting them through a real world loading test with a test bot and five minutes of drive time after a full charge. Measure the battery’s discharge level and keep record of your “best” batteries.
The new PD board will have current measuring capabilities over the CAN bus, so you can use that for your measurement needs.
You’ve learned a very important lesson about engineering: theory doesn’t always match experimentation – and that’s okay! There are so many variables in one of these systems: battery internal resistance/charge level, motor parameters, electrical parasitics (R/C/L). If you’re not getting the current you’re expecting, it’s most likely that either your computations didn’t take all the variables into account or your experimentation is violating some assumption. Oftentimes, replacing components with “known good” ones is the best way to debug a fault.
As we found out in elims at IRI, once you’ve tripped a breaker, it is more likely to trip again. For a competition bot, you’ll want to replace it just to be paranoid. Keep that breaker for a practice bot.
Make sure you have your gear ratios such that you will not pull too much current under the inevitable pushing matches. If you have yourself geared for 20fps with 6 CIMs and you get into a pushing match with a robot (or a wall), you’re likely going to pop a breaker. If you’ve already built the system and there’s no time left to fix it mechanically, you can also implement a software limit on how much duty cycle you’ll put on the motors. This way, the motor controllers will never actually deliver 100% of the rated power to your motors and it should save your breaker.
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.
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.
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 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.
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=IV, 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=IR for a conductor resistance R, the power rating is P=IV=I^2R, 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.
While we are on the subject…
There are various current ratings for wiring so you need to know when looking at charts and breaker sizes. The current robot rules are written using the NEC guide for conductors in an open space (as opposed to conduit and bundles). This table takes into account the resistance of the wire and the expected temperature rise caused by continuous current in the conductor for general insulation. It also adds a little fudge factor by calculating the voltage drop in a typical house wiring run and making sure that at maximum current, there is still sufficient voltage at the load. The insulation used on the CIM motor wire is (or at least was in the past) a higher temperature rating and is therefore rated for higher current then you would expect for it’s size.
Another set of current tables is established by the aircraft industry. That current table is based on the minimum allowed voltage drop to instruments in the cockpit. Adjustments to those tables account for length (both conductors please), load current, bundled conductors and altitude. You can find more from the FAA here… http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99c827db9baac81b86256b4500596c4e/$FILE/Chapter%2011.pdf
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.
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.
There is absolutely no need to both crimp and solder every electrical connection. Crimping is just fine when it’s done properly. I would strongly encourage you to do rudimentary tension testing on a couple crimped fittings. They should be able to sustain far more force than one can every apply by tugging on the connection by hand. Ratcheting crimpers are key to ensuring consistent and strong crimped connections.