Need help for basic RoboClaw 2x30A wirings.

[avatli]

I'm trying to build an autonomous car using RoboClaw 2x30A. As a simple software developer, I am not very good at hardware work. I designed something myself, but I have to confirm the correctness to not damaging my electronic parts. I think my questions are very simple, please help me..

First, I have 4 motors, each's stall current is 20A, voltage range is 6V - 12V and no load current is ~0.5A. Also I have two 2x30A RoboClaw driver. I though driving two motors with one RoboClaw driver.

1) I'll use 3S, 1750mAh - 25C LiPo battery, is it adequate?
2) I'll use 13 AWG silicone wires (max amps is 35A) to connect a LiPo battery to RoboClaw (+) and (-) inputs. LiPo battery will connect two RoboClaws. Is it possible?
3) I'll use 15 AWG silicone wires (max apms is 28A) to connect RoboClaw M1A/B-m2A/B to motors.
4) Can I use RoboClaw BEC output to power a Raspberry PI?
5) According to RoboClaw Series User Manual, page 62, Packet Serial Wiring, there is not a fuse or diode, but at page 40, Safety Wiring needs a fuse, a pre-charge resistor, and a a high current diode. I'll use a raspberry Pi, so do I need safety, or can I use the packet serial wiring without a concern?

Best Regards.

[Basicmicro Support]

If you are not planning on using encoders you should be able to run all 4 motors from one Roboclaw 2x30. You will connect two motors in parrelel to each motor channel.
1 to 3 Looks fine
4. Yes
5. The different wiring diagrams are simplified. A fuse and diode across the fuse with an optional power switch is recommended.

[avatli]

Ok, got it, since I'll not use encoders I can control 4 motors with on RoboClaw 2x30A.

In this case,

1) Do I need 12 AWG (instead of 13 AWG) wires to connect a LiPo battery to RoboClaw (+) and (-) inputs. My motor's stall current is 20A and two of them can draw 40A at max, so 13 AWG is not enough now, am I right?

2) Still same for 15 AWG silicone wires (max apms is 28A) to connect RoboClaw M1A/B-m2A/B to motors.

3) I know I need a power diode but what rating of power diode do I need?

4) If I connect the four motors to one RoboClaw, what rating of fuse and pre-charge resistor? I'm really dump about hardware, etc..

Best regards

[Basicmicro Support]

1. I do recommend going with 12awg but if the wires is short the 13awg will probably be fine. To tell if there is a problem see if the wire is getting warm. If it gets too warm(eg hot) then you need a larger wire).

2. The motor wire should be fine though I would probably go 14awg. 14awg and 12awg is fairly common. The odd number awg not so much.

3. 1amp to 5amps is enough. It will only be used in the case where the fuse blows(or if you have a switch the switch is opened) while running the motors and the motors are being told to slow down. This is when regen happens. If it is in regen when the battery connection is opened the voltage on the motor controller can rise to damaging levels very quickly. Similarly if you have a big robot and you push it around on its wheels manually this causes regen and can in some cases cause the same kind of voltasge spike which can damage the board. The diode prevents this.

4. Pre-charge resistor has nothing to do with what motors you use. So anything from 1k to 220ohm should be fine. You want around a 1 watt resistor though if you are getting down around 220ohms. As far as fuses go its hard to say. First you need to estimate the maximum current your battery CAN supply(whats its C rating?) then you need to estimate the maximum amps the motors will pull. In most cases it will be no were near the maximum stall current rating of all the motors combined but start with that. Hopefully there will be a fairly large window between the maximum battery current and the maximum motor current so choosing a fuse is easy. Then you use that number to find an approriate fuse. You need to look at the fuses timing charge which will list how fast it will blow at a given amps. You want one that will blow in .1 to 1 second at or near the lower end of the amp window you determined. Note that if the window of amps is small or non-existant you may need to determine the actual real work draw your motors will pull from the battery. This is easier said than done though since the amps at the battery(thew amps that matter as far as the fuse is concerned) is a factor of the motor amps and the PWM duty cycle. A 50% duty and 20amps at the motor means 10amps at the battery for example.

[avatli]

I want to ask the last ridiculous question.

Since each motor's can draw 20A at stall and I have 4 of them, is my fuse around 80A?

[Basicmicro Support]

In the real world it is very unlikely you will run both channels at 100% duty and stalled which could produce the 80a draw you mention.

But you could assume 80a max and then look at you battery spec. That says your battery can supply around 44amps. So what can you do? Obviosly an 80amp fuse would be a bad idea since it would never blow even if you shorted the battery across it. So you need to choose something reasonabe below the battery max current capability and high enough to cover your power needs.

In this case I'd probably go with a fuse that will blow in .1 to 1second at 40amps. That fuse should blow at lower amps(say 30amps) in 10 to 20 seconds as well. This gives you some protection but still gives you access to most of the power you battery can supply for at least short periods.

Because your fuse timings are so specific though, it may be difficult to find a fuse that will work. So in the end you may have to go with a fuse you can find but it may be rated to open at 25 or 20amps.

Note, do not just buy a 30amp fuse. A 30amp fuse will not blow at 30amps ever. Most 30amps fuse will take 10s to 100s of seconds to blow and 40 or even 50amps. You MUST find the fuse timing table to determine what the fuse will really do.