Length of wires

It is common knowledge in the RC world that one should have as short wires as possible
from the battery to the controller and longer to the motor if necessary. The thought being that the controller
takes electrons in pulses and long wires can cause a shock to the system(like water hammer).

On the other hand, phase current is often higher than battery current, so short motor wires will lower losses
from the higher current wires, and longer battery wires will not have the same losses.

Any thoughts?

Phase currents are generally greater, but it's usually easy to increase phase wire size to cope. Doing this with the battery wires would cause the same problem as making them longer: increased inductance, which causes voltage spikes that can destroy controller components.

These spikes should be safely absorbed by the input capacitors, but on RC controllers there is little space for these, so they are vulnerable to long battery wires. Industrial/EV/ebike controllers have more installed capacitance so extended the wires is less problematic.

I did not realise that larger wires increase inductance. Is there a way to for me to figure out the ideal gauge of
wire to minimise inductance and resistance in the battery and phase wires?

Common motors can take a certain amount of abuse. The voltage spikes that can occur between the battery and controller can damage the sensitive electronics in the controller.

It's good for all conductors from the battery to the motor be short and fat. The cost of fatter wire is small, and my time is valuable. How fat should they be? Too fat.

If you can't mount the battery, controller, and motor close to each other, it's better for the battery to be as close to the controller as possible, and the motor farther away.

It might be useful to new readers to chat about inductance and voltage spikes, but...the bottom line is that...every possible configuration has been tried, and you should copy what works.

DaveDog said:

I did not realise that larger wires increase inductance.

Click to expand...

AFAIK, they don't.

Here's a page on factors that affect inductance:

https://www.allaboutcircuits.com/textbook/direct-current/chpt-15/factors-affecting-inductance/


Here's some threads about wire lengths in different applications:

https://endless-sphere.com/forums/search.php?keywords=wir*+length*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

https://endless-sphere.com/forums/search.php?keywords=wir*+long*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

There's more that don't directly have the terms in their titles if the above isn't enough.

Sorry, brain fart on my part: fatter wires = greater capacitance, not inductance.

Spacing between the wires increases inductance so keep the wires close together. But I would size them to be simply to be plenty sufficient for the current.

One more question. Do the different type of controllers trap, sine and FOC, turn on and off the fet's at a different speed or way that would change the magnitude of the impedence created in the battery wires?

Not much difference. They all turn on/off at the PWM frequency so any issues from inductance or resistance would be the same.

If the controller is adequately rated, the relative length of the battery wires vs. the phase wires won't really matter. Phase wires will have more current, so will have more loss (argument for short phase wires). But in real life, controllers are not overbuilt or are being used at the design limit. In this case, having shorter battery wires and longer phase wires will minimize the voltage/current spikes on the FETs and possibly help prevent them from blowing up.

Thanks for the great answers everyone, I have a clearer image in my mind now.

And where shorter battery wires are not possible, then fatter wires and adding lots of low-ESR capacitance right at the controller's input can help.