Wire size and motor type

[Muskateer]

I have three separate questions.
1) I will be controlling two motors (one of which will be a motor that draws abut 35 amps maximum (I have a 45Amp nominal, 60 A peak Roboclaw). The wire size looks like it should be 10 gauge for 35 amps. The screw connections on the Roboclaw look pretty small. What is the maximum wire size I can use on the 45 Amp Roboclaw?
2) The specs state "keep the battery leads as short as possible," Why?
My plan was to run about 10-11 feet of twin 10 gauge wire from the battery to a terminal block. Then connect from the terminal block to the roboclaw battery terminals.
3) Unrelated question: Is there any problems with controlling two different types of motors (I.E. one DC motor, and one servo motor) ?

[Basicmicro Support]

1. 8 awg will fit and in a pinch you can get 6 awg to fit.

2. Inductance in the wire. If the inductance is high enough it takes a long time(relatively speaking) for current to rise and fall. When that happens you will get wild swings in voltage which will cause excesive current ripple on the large filter caps on the Roboclaw. Excessive inductance will cause the caps to overheat and wear out much faster and in some extreme cases even melt. The longer the wire is the large the gauge needs to be(larger gauge, smaller awg number, has lower inductance, while longer wires have higher inductance).

3. The Roboclaw only runs DC motors. If the servo motor is a DC motor and you've gutted any other driving electronics from it. then you should be able to use it as well.

[Muskateer]

Follow up: In regards to the wire length. Is it the same problem(inductance) with wire length from the Roboclaw Outputs to the motor?
I hope not. I am going to be using this to run a trolling motor (35 amps max roughly. probably 15-18 amps nominally). For weight distribution the heavy battery needs to be up front, and the trolling motor is at the rear of the vessel. So one way or another I need to run about 10-12 feet of wire. Also I need to have access to a shut off switch which will be mounted mid-vessel.
In this scenario is there a way to limit potential damage to the Roboclaw?

[Basicmicro Support]

Motors are already high inductance so inductance in the wires to them usually isnt a problem. In your situtation running the motor wires the full length would be best and keep the battery right next to the controller. As for the cutoff just run a e-stop switch form the S3 pin(set the pin to e-stop mode) to the middle of your boat. You will need to add a 5v pullup to the signal line since the wire is long and the internal pullups are rather weak.

[Muskateer]

That is very helpful information.
I will have to think about the S3 being used as an E-stop.
This is actually a Kayak mounted trolling motor and I am adding some safety and redundancy to the electronics. Such as an emergency kill switch if I were to dump the kayak, and a manual bypass switch in case the motor control unit malfunctions. But your suggestions, have made me change the way I will do some things.
Thanks Again.

[Muskateer]

One more thought on this:
What about twisting the power and ground wires together over the long run? I would think that the currents in the power wire versus the ground wire should be out of phase and tend to cancel or reduce the inductive effects seen by the PWM circuitry.

[Basicmicro Support]

Yes. Twisting the power wires will reduce the inductance of the wire. However long runs of wires will still have a fairly high inductance. I would recommend testing(carefully) to see if the length you need to run will work peroperly.

[Muskateer]

That is very helpful information.
Unfortunately, I'm not certain how to test the effects (good or bad) of twisting the power/ground wiring. It probably doesn't help much that I don't own any test equipment other than a DVM.
I am guessing all I can really do is maybe monitor the voltages the Roboclaw is seeing with the provided software and if the spikes are to great, I will have to look at the alternative solutions.
Again thank you for the support, it stopped me from making a couple mistakes.

[Basicmicro Support]

To test if the inductance is too large on the wire you need an Oscillascope and you need to run two tests. One with short power wires and one with long power wires. You need to watch the voltage on the battery at the robotclaw screw terminal. If the voltage ripple is significantly higher on the long wire you will then need to run a test and watch the temperature of the large caps on the Roboclaw. Under a normal load. You need to check if they start to over heat. For this test it is best to have a temp sensor attachment for your multimeter. Maximum temp on teh caps should be no more than 105C. If inductance is an issue they can get alot hotter even while the rest of the board is relatively cool.

If you cant run the above tests then you will have to estimate it by feel. Run under a normal load for a little while and keep an eye on the temperature of the caps(by touch). If they stay at a temp you can safely touch for at least a short period of time you are probably ok.

[Muskateer]

Thank You, I will run some tests later this week.