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 Posted: Mon Aug 1st, 2011 11:17 pm
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dapenguin
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It takes a little rocket science. :brill:

It has to do with wattage. 20 amps at 120v is 240 watts, 100 amps at 7 volts is yes 700 watts but it is DC so no skin effect. AC only uses the skin of the wire, DC uses all the conductor. Notive most things rated 120V AC are rated 600Vs DC. 15 amps at 600V is 900 wattsDC.
TCC
former apprentice :old dude:

I also see from reading ads for ESC's that some of them are rated for up to 90, 100 or even 110 amps, continuous. That's a lot of current to be going through a number 12 wire, which is what they appear to be equipped with. Since 12 gauge wire is only rated for 20 amps, I would like to know how the units are rated for 90 or 100 amps.   Has anyone ever asked a manufacturer that question?



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 Posted: Tue Aug 2nd, 2011 02:26 am
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Arthur P. Bloom
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Thank you for reminding me about skin effect. I learned it once, in a century, long, long ago.

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 Posted: Tue Aug 2nd, 2011 06:01 pm
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Huw Griffiths
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dapenguin wrote: I also see from reading ads for ESC's that some of them are rated for up to 90, 100 or even 110 amps, continuous. That's a lot of current to be going through a number 12 wire, which is what they appear to be equipped with. Since 12 gauge wire is only rated for 20 amps, I would like to know how the units are rated for 90 or 100 amps.   Has anyone ever asked a manufacturer that question?
I wonder how long these wires actually need to carry these currents.

This might sound like a crazy comment - I don't think it is - and here's why:


The current carrying ability of wires are often determined by (1) the voltage drop across them - which is a function of their resistance (Ohm's Law ...) - and (2) how much they heat up - due to the current flowing through them.

If the heat gets excessive, you could sometimes (in extreme cases) have problems with insulation melting, or other damage. This probably wouldn't be an issue here - but I suspect this might have something to do with some motors having different ratings, dependant on how long they're expected to work at a given load.


Another example of this concerns resistors used in some electronic gadgets that only get used for a couple of seconds at a time. Calculations might tell you that you need a 1W rated resistor - yet the gadget might actually be using resistors rated at a tiny fraction of this power. Anything bigger might get in the way, or be an expensive overkill.


Most people have encountered fuses (and probably thought little of it). In theory, these are little more than a wire that's supposed to melt (and break a connection) when the current through them exceeds a safe limit. A lot of domestic fuse boxes are based on this (in the UK, lots of places still sell cards of "fuse wire"). Saying that, modern UK installations are much more likely to use circuit breakers (there are good reasons for this - but this is not the place to discuss them).

Many people think fuses are just rated for current - that a 10A fuse will allow current to pass unimpeded at any current up to 10A, but go "pop" the moment the current reaches 10.001A. I wouldn't be so sure.

You might have heard of antisurge fuses, which allow brief pulses of high currents to pass (say when a motor's being started up) but go "pop" if the current stays too high for an extended period. In other words, the current isn't the only factor - time also matters.

As well as long term current ratings, a lot of industrial fuses also have what are known as I2t ratings. It's a number of years since I last had to deal with these (like my industrial training, when I was involved with the design and commissioning of power stations) - so, to try and remind myself what it's all about, I had a look at this article about fuse ratings, on the Acuguard website.


I'll admit that I haven't had dealings with these guys - and some of this stuff wouldn't interest a number of people - but I hope all this stuff goes some way towards explaining why some ESCs (and indeed a lot of DCC wiring for those who haven't gone down the radio control route) have what appear to be ridiculously thin wires.


Regards,

Huw.

Last edited on Tue Aug 2nd, 2011 06:05 pm by Huw Griffiths

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 Posted: Tue Aug 2nd, 2011 09:12 pm
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Toeffelholm
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Motors in RC cars and boats, and especially in racing vehicles need sometimes a very high current when starting and accelerating. But as Hugh already described, this is only a short time peak current and it is a matter of heat developing in the wire.

But the current an car or boat esc is made for doesn't bother us at all. It is only a potential value, not the current that really flows. Our locos motors have a current demand of a few hundred mA. Maybe up to 2 or 3 A when you consider Gscale garden Locos.

My 1:22.5 locos on 32mm and 16mm track don't take more then 400 mA (under load).

Juergen



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