I’ve searched the forum looking for a reason for rule C6 (below, for reference). I am puzzled by the need for the 10AWG wire to connect the controller to the device, regardless of the fact that the device may well have a smaller gauge wire to connect to (Globe motor comes immediately to mind). Why? Surely manufacturers of products that use these motors don’t run excessively large conductors from the power source to the device. Or do they?
“C6 You must use 10 AWG or larger diameter wire for connections to and from the Speed Controllers if they are used with the Drill, CIM, Fisher-Price, Globe or Van Door motors. Even though the wires that are factory attached to these motors may be of a smaller size, the connection from the motor wires to the speed controllers must be as stated above.”
We actualy melted the wire that was on the drill motor.
Literaly melted the solder and the electrical tape around it to insulate it.
Now granted that our drive system was having alot of problems and we were horribly inificant, fixed now, The wire was melted to…we ended up taking the old wire off and following first recommendation of not soldering to the leads, we just head shrinked a wire onto the motor that was 10 guage to try and prevent this from happening again…
This rule is pretty strange and doesn’t make sense…alot like many other rules this year.
The amount of current a wire can take is a function of the diameter (AWG) and the type/thickness of insulation. The type of insulation determines the temperature at which the wire will fail. The thickness (among other factors) determines the voltage at which the wire will suffer a dielectric breakdown and arc over.
The next thing to understand is that the wire generates heat when a current passes though it. With a “perfect” insulating jacket, the heat would never escape and the temperature would rise to infinity. All things being equal, the thicker the insulating jacket, the higher the temperature of the conductor will become before thermal equilibrium is established.
Now… Most all wire commonly available is either 300V or 600V wire. This includes the 10 AWG wire FIRST provides. The insulation is thick and heat builds up rapidly. Whereas the voltage rating of the wire on the motors is much less (it is custom built for the motor manufacturer) and less heat is built up.
With special, low voltage, Teflon (high temperature) wire, the motor leads could be made as small as 18 AWG or so (but this is expensive wire…).
Bottom line: FIRST has tried to be conservative and safe and still allow you to use readily available off the shelf wire. Please do not try and “second guess” without doing the engineering calculations which are verrrry messy (been there and done that).
Use the 10 AWG wire and move on to other problems you might have…
Mike Betts
Bobcat Robotics
Postscript to Mr. Dwarf:
I’m not sure what you have done but be sure to have an electrical engineer check it out. If you don’t have an EE on your team, seek help from another team at your regional… I’m serious… It sounds like you have heat shrink tubing holding a 10 AWG wire on the motor terminal and that sure fire is a recipe for a failure.
*Originally posted by Mike Betts *
**CBP:
The amount of current a wire can take is a function of the diameter (AWG) and the type/thickness of insulation. The type of insulation determines the temperature at which the wire will fail. The thickness (among other factors) determines the voltage at which the wire will suffer a dielectric breakdown and arc over.
**
Thanks Mike,
the type of insulation affects the current capability as interpreted by the electrical safety councils and the National Electrical Code (which in turn is driven by the insurance companies) and is mostly determined by it’s melting point or the point at which the temperature of the wire is sufficient to melt the insulation and contact another conductor. The safety devices (breakers) must therefore open the circuit before the wire reaches that temperature based on known current heating, wire size and whether the conductors are in conduit or in free air. Since all wiring to motors on our robots are protected by 30 amp circuit delayed action circuit breakers, it follows that wiring should be capable of all characteristics of that type of protection. Using #16 wire to feed a Globe seems like an OK thing to do unless you consider that a moving part, pinching the wires might cause a short in the wire. It is conceivable that a #16 wire would set itself on fire before the breaker would open. Hence the rule.
Now many of you are going to bring up that the drill motor has #16 wires supplied and it has 129 amps at stall. Well, check the small printing on the wire and you will see that the insulation has a melting point of 105 C.
Many teams are reporting heating of the drill motors. As pointed out in these forums on many occasions and in the Robot docs, the drill motors are not designed to get rid of heat at low RPM. If you design your drive system such that maximum current is drawn at low RPM or near stall, the wire and the motor will reach temperatures at which the wiring, the brush assy, the soldered connections and/or the motor will fail. You must get the operating RPM on the motor into a more efficient range.
It is standard practice (National Electrical Code) to size the conductor based on the amperage rating of the overcurrent device (circuit breaker or fuse) used to protect the circuit, not the load connected to the circuit. The reason being that should the conductor short circuit, the overcurrent device will open before the conductor fuses or the insulation fails, possibly starting a fire. If you used a #16 AWG conductor and there was a short circuit, you could end up with 30 Amperes going through a conductor rated for
Typically, you will use #10 AWG on a 30 Amp circuit, #12 on a 20 Amp circuit and #14 on a 15 Amp circuit. Obviously, there are exceptions, such as the #18 AWG lamp cord that you plug into a 15 or 20 Amp receptacle.
At Arizona some team’s robot caught on fire (a few people thought it was part of the show). That’s why you need to have a good electrical system. We have 8 guage finely stranded to everything that supplies current to the drive motors and 4 guage welding cable on our battery and to the breaker. We also have a huge ground terminal block for the drive motors and we use a connector rated for 200 Amps on our battery.
Yah, that would me my team’s robot that caught fire… Turns out that the serial cable that we left on the teather port (stupid idea) shorted out when we tipped over (well, the tip caused something to cut the cable, so when I released the estop button, electricity came down the wire shorting it out.) surprisingly, the only thing that was damaged was that serial cable and some velcro was blackened. (I admit, that was a VERY VERY VERY stupid idea leaving the serial cable on there…)
Fire on a robot causes great excitement and cheering from the crowd.
