Re: volts vs. amps

On Thu, 13 Jul 2006 16:44:03 -0400, ben carter <carter@xxxxxxxxxxxxxx>
wrote:

Anybody got a clear way to explain to an educationally diverse group of
students how amps and volts are different in arc welding ?

First off, I'm an electronics engineer and not a welder. I also don't
know if the following explanation is any clearer than anyone else's.

But I know that welding requires heat. Heat represents power
dissipation in the metals. This heat has to come from the
electricity.

Power is amps times voltage (P=I*E) in electrical circuits. To
increase the power (heat), you either have to increase either the amps
or the voltage. Either will do, but high voltage can kill you, so
welding is usually done with low voltage and high current (amps).

The reason high voltage kills is that it can overcome the resistance
of the skin and conduct current through your body - enough to stop
your heart maybe. There's a whole lesson here someplace on safety and
watching out for 110/220v from the line. It's ac, but it's high
enough voltage to perhaps cause damage.

Voltage can be used to model water pressure. Think of the high
pressure coming into your house. Resistance to water flow can be
modeled by an electrical resistor or throttling device (hose bib).
This is why having too long a delivery wire to the work can cause
problems. It has too much distributed resistance that will cause a
voltage drop at the weld, much like having a really long hose or a
hose with a kink in it will drop water pressure.

Amperage can be modeled by water flow. A large diameter hose can
carry more water per minute than a small diameter hose can at the same
pressure difference. So amps are essentially gallons per minute in
the analogy. A large diameter wire will have less resistance than a
small diameter wire, so welding cable has a large diameter to reduce
its resistance.

This is probably not as clear as you'd like, but I'm old and it's late
:-)

Regards,
Larry
.

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