I both agree and disagree.
I work with servos (previously steppers) which are very close to std motors like we talk about here.
A PM (permanent magnet) motor, or AC motor, or servo motor, or any motor, needs to change the commutation of the fields often, very fast.
E.g.
A PM motor might have 10-20 sections, with brushes.
So, at 3000 rpm = 50 times per sec, it need to change the commutation 50 x 10 time per sec, or 500 Hz.
The higher the voltage, the faster this occurs.
The faster it occurs, the more efficient / powerful the motor is.
There is a huge difference between 20V and 48V.
There is a huge difference between 48V and 80V.
There is a huge difference between 80V and 220V.
An excellent 3-phase motor might accelerate to 1500 rpm top speed in .5 seconds. No load.
A typical AC servo motor (220V) will accelerate to 3000 rpm in 20 ms, or 0.02 secs.
You want bigger cables not because they will not melt, but because they are "better".
For exactly the same reason, you always want higher voltage.
Tech/budget/availability/safety minimax in mind.
Cables (everything) have impedance and-or reluctance.
This is like resistance to change, inertia, mass.
A thick cable is like a ping-pong ball, a thin one is like dumbo the elephant, at high speed.
Ideally, motors would run at 2000 volts and 100.000 rpm.
There are reasons we cannot do it commercially, yet; (bearings, magnetic flux saturation, cost of mfct), nor want to (safety).
The typical best minimax is 48V, 80V, 220V, 400V.
Ideally, you would run 400V, but this really needs double shielded cables in a marine environment.
That's a great chart, Scott---thanks for the link.
And it makes it clear why 4 or 2 ga can be too small for a lot of us while on a golf cart it might be just fine. Let's say you're drawing under 100amps continuous and the round-trip distance from your pack to your controller is just 10 feet, the chart shows that 4ga would be fine for a 3% voltage drop. And if one is fine with up to another 3% in efficiency loss, it could be fine.
But let's say we might draw 150amps continuous at speed and the high current path length is more like 15feet round trip between the controller and battery---for that, the chart recommends 0 or 1ga.
Even accepting 3% voltage loss may be too much compromise given it will translate directly into efficiency loss. With my 16+ft main high current path, I use 1/0 cable---it's cheap enough and there were neither space, nor weight concerns that made it the wrong choice. My controller-to-motor cabling also uses 1/0 cable given I could easily see 200amps into the motor for a minute or 2 at least when trying to, e.g., catch a wave with my boat. J
You never want cabling or high resistance with lugs to cause key elements to overheat---not battery posts, not the controller and not the motor. Always be sure your contact resistances are low. Do that by running at full clip for a long enough time to generate heat, then stop and check all connections for heat. With my 1920 Milburn Electric car, I've had bad lug connections cause lead-acid battery posts to instantly turn into a pool of lead with current draw between 100-150amps. I also found a bad connection behind the original Amp/Amp-hour gauge after arriving home one time after a 2 mile jaunt only to find the glass at the front of the gauge was wa-a-a-ay too hot to touch…
Anyway, enough about this---thanks for the link to the great chart.
-Myles
-- -hanermo (cnc designs)
Posted by: Hannu Venermo <gcode.fi@gmail.com>
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