Question about currents and common mode chokes

1

I have a possibly dumb question about currents in general and common mode chokes in particular.

Whenever I ask the advice of people who work with industrial servos, they always seem taken aback at how much current the little hobbyist BLDC motors are rated for. Their world seems to involve heavier motors with chunkier cables with lots more insulation, shielding, etc all round.

This seems completely at odds with what people who are building electric skateboards, e-bikes, drones, etc. seem to be playing with. Yet they don’t seem to suffer from problems related to heat, EMI, etc etc as much as the industrial servo guys would predict they do.

For example, I’m using a motor which states 80A as its max current. The wires coming out of it are 3.5mm dia, and that’s including the insulation.

But this guide implies that for 80A I need AWG 2, which this table says means a 6.5mm dia cable.

The motor manufacturer chose to use the 3.5mm dia cables and I’m sure they know what they’re doing. I am just confused by the apparent contradiction.

This has come to a head recently when I started looking for a three phase common mode choke.

An 80A motor implies 80A through the coils, which means I need a choke rated for at least 80A. Unfortunately, they don’t seem to come that high, at least through the obvious channels. The highest I could find was only 46A and it cost £40.

Meanwhile, I made a homemade one with a <ÂŁ5 ferrite core and a few twists of 3.5mm dia wire, and it seems to work fine.

I would like to source a proper component, rather than keep making homemade things if I can. But more generally - why don’t these two pictures match? If my homemade thing works, why aren’t they out there on the market? If it doesn’t work, then why does it seem to work?

What’s going on here?

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Anyone have any thoughts on this? It’s actually bugging me more the more I think about it.

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For the AWG, that’s the residential rating for in-wall installations. For chassis wiring on vehicles, robots, etc, you need to go by temperature rise not those charts.

Why not use a choke like the ones we sell on the shop? You use your own wires there :slight_smile:

4

From the page you linked:

When they say “Very, very conservative” they mean they are picking a method that deliberately results in much fatter wire gauges than they need, 1) because that method is designed for wiring huge bundles of cables inside ships, nuclear power plants etc. which may have high ambient temperature, and the ability to cool a conductor in the middle of a large bundle is much reduced. and 2) it was decided by a consortium of wire manufacturers …

You should be able to get away with the same gauge wire that the motor uses, or even slightly thinner (your wires are in air, the motor wiring is in a bundle) but I normally go with one or two gauges down (thicker) than what the motor uses, for efficiency, depending on how long my wires are.

Just work out how much power will be lost (I^2 R) and calculate the efficiency, and choose a value that you are happy with.

5

Hi jbombastor,

I am actually designing industrial equipment for a living, so I try to convey a bit of my view on these topic ;).

I my experience the hobbyist ratings are in many cases very optimistic while the industry stuff is rated pretty pessimistic in many cases. One of the main reasons is the expected use case: an industrial motor is normally made to withstand its maximum ratings 24/7 at the maximum allowed ambient temperature while the hobby motor is rated for short power peaks in an environment that provides decent cooling. And also the usual customer plays a big role as the average tinkerer will not sue the motor (or other hardware) supplier to the bitter end should their product not achieve the stated performance.

And then there is the thing with certification: having a system that works does not say that it adheres to standards. EMI problems have to be pretty immense in most systems to reliably make them not work while a minor error can be enough to make you equipment fail in the lab. The same goes for the current ratings, the regulations are (at least in Europe) pretty stringent as overloaded conductors are a fire hazard. And this does not even tough the topic of reliability, something working on a bench can still be horribly unreliably when pushed hard ;).

On the other hand standard industry equipment is in quite some cases at least a bit outdated and built more for compatibility than for pure performance. So in many cases there is indeed improvement potential in my opinion - but it is not that easy to achieve as the hobbyist realm would suggest.

About the choke: using a few turns in a ferrite is normally not referred to as a choke as far as I know, chokes in most cases work in way lower frequencies and thus also are a lot sturdier. But using a simple ferrite bead or a snap-on ferrite is not unusual in industrial electronics to manage EMI.

3 Likes
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I just realised I never went back and replied to this post, which is remiss of me because I found it very useful and enlightening. Thanks for your insight, guys!

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One thing I want to note: the ferrite rings are common mode, so the current rating they need is much much less. They only need to kill the capacitively coupled energy, which is a function of the voltage and the parasitic capacitance: motor current is not really related, at least not primarily.

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Or rather what you mean is, the current in the motor phase wires always sums to zero except for a tiny amount accounting for RF emissions. There is no net charge that passes through the wire inside the ferrite ring.