Another electrical question

[craigb]

Well I'm not advocating that anybody should use their cables coiled. I was merely curious as to why the warning label.

It still seems to me that it's mainly asre covering.

Anyway, thanks for the info.

Cheers
Craig

[Barry_White]

Running an electric drill wouldn't be a problem. Run a welder a whole different kettle of fish. I have run a welder high up on the duty cycle with the lead uncoiled and it has got warm.

[doug the slug]

Well I'm not advocating that anybody should use their cables coiled. I was merely curious as to why the warning label.

It still seems to me that it's mainly asre covering.

Anyway, thanks for the info.

Cheers
Craig

yes it is,to an extent, but with high-end usage there is a genuine danger, aned what the sparky's wont tell you that falls into the realm of us amateur radio operators is that it can cause TV and radio interference too. Even if they are coiled up and not even plugged in, a strong radio signal can induce sufficient current into the loop to emiit a signal, albiet a weak one

[q9]

Actually is the magnetic field that adds to the heat produced...Electric motors with lots of tight coils do get quite hot after all - but that is designed into them.

[Schtoo]

Actually is the magnetic field that adds to the heat produced...Electric motors with lots of tight coils do get quite hot after all - but that is designed into them.

Huh?

I don't buy that, not in a little lead.

In standard wiring, the magnetism concern is more about destructive problems than heat. About the only devices that use electromagnetic effect to do anything in regard to heating are induction heaters, and they don't work too well unless you crank up the frequency of the current.

In a motor, the reason it gets warm is mostly because of the resitance in the windings, not because it's a spinning magnet. If the core was a solid chunk of iron, the thing would melt down due to inductive heating, but the cores are laminated to avoid eddy currents building up and melting the motor into slag.

Anyways, a motor might heat up due to magnetism, but it's a miniscule percentage when compared to the resistive heating of the windings and other losses.

Heck, wind resistance is prolly a bigger loss... :eek:

[Cliff Rogers]

I'm with Schtoo on this one, it the resistance & the heat that is the main problem.

A short, small, low freq AC coil that is counterwound (IE 2 conductors side by side with current flowing in oppisite direction cancel each other's magnetic field) has very little inductance.

The problem is about 99.99% resistance.

If I run my big compressor on a straight 5m 10Amp ext cord, the lead is warm by the time the compressor is up to pressure.
If the lead is lying straight out across the concrete floor, it cools down fairly quickly.
If it was wrapped up in a heap, it wouldn't cool so quickly.

Doug, the stuff you are talking about means nothing to sparkys 'cos it's high freq & that is a whole different bag of worms.

[bennylaird]

Remember we are talking air core and high cross sectional area of the coil as well as low frequency. While a small amount of inductance will occur I doubt if you would either generate much in the way of rf or dissipate to much power as heat. Remembering the coil will produce apparent power not true power. True power is purely Resistive as Cliff so rightly put it.

Now if you were to wind the lead tightly around an iron bar then you may have problems. Hence the reel being a problem.

However it pays to keep your nether regions covered which is what the warmning is.

[Andy Mac]

It is something drummed into most tradesman, but I tend to go with it being a normal heat build up that can't dissipate due to the thermal mass. I just put it to a couple of guys here who disagree, one the OH&S officer and the other an ex-RAAF elec. fitter. Both of them are of the opinion its a back EMF buildup that increases the current through the coiled wire, and that exceeds the wire rating. I've also put it to a tradey electrician who spun me some yarn about the reel inside starting to spin due to induced current and friction builds up from all that spinning!! LOL
Whatever the cause, unroll the lead and loosely spread it across the floor.

Cheers,

[Ivan in Oz]

Huh?

I don't buy that, not in a little lead.
Heck, wind resistance is prolly a bigger loss... :eek:

Schtoo,
You mightn't Buy it;

BUT
You WILL pay for it, sooner or later,
normally later, Believe me

[craigb]

O.K. so it's the heat either from resistance or induced magnetic field.

Actually, I generally only use my 15 amp extension when I have to move my TS further than the cord on the saw will allow.

Like I said, it's usually loosely coiled on the floor in around 450 - 500mm diameter loops.

I've never noticed the cord become warm but then I'm generally only making a few cuts before moving the saw back to its normal position.

Every time I do it though, I see that little safety label and feel guilty about the looped cord.

If I ever get a welder, I'll definitely fully uncoil my extension lead.

[bennylaird]

The only way you can get the heat is from resistance, ie True Power. Reactive power is stored and returned and produces no heat. Back EMF if anything will oppose the voltage producing it and therefore reduce the current. Back emf will give you a good kick but you would have to be touching the bare cable which with 240V live I think is beyond the stupidity of most humans, animals as well, fear of pain does it.

Basically a coiled cable can't dissipate the heat, gets hot resistance goes up, current goes down. If it gets too hot it melts the insulation, short occurs, fire starts if breacker doesn't cut out in time . nuff said?

[Cliff Rogers]

Yeap, back EMF is horses hit.


In the end, as Craig says, it is heat we are trying to avoid.

[DavidG]

Resistive heating.
Solid mass unable to dissipate heat.

Don't make me dig out the electronics books and actually calculate the inductance of a 30 meter coil of three core wire, but it will be fairly low.

[bennylaird]

Want the formula?

[Cliff Rogers]

No, the dept to the bottom of the pile of books to where the electronics ones are now.