Re: [DML] Fuse 7 - Engine Control - MELTED
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Re: [DML] Fuse 7 - Engine Control - MELTED

--- In dmcnews@xxxxxxxxxxxxxxx, "John Hervey" <john@...> wrote:

> Maybe one of the EE will explain the difference between resistive
load and
> inductive load. 

Here's kind of a simple explanation...

There are a couple of varieties of "resistance". What is known as DC
resistance is a resisatnce to the flow of current, regardless of
whetehr that current is unfluctating, such as a steady DC current
flowing through a resistor, or changing, as in the case of an AC circuit.

There is also a kind of "resistance" known as reactance. This is a
"resistance" to CHANGES in a current or voltage across certain kinds
of loads. Inductive and capacitive loads present this type of
"resistance" to CHANGING currents and voltages. Changing currents may
be in the form of AC currents, but currents also are changing from 0
amps or no current flow, to some non-zero value when a switch or relay
completes a circuit to a reactive load such as a motor or a capacitor. 

In most cases when we are discussing automotive electrics, we're
talking about inductive loads such as motors. Motors have what is
known as inductive reactance. When a switch is first thrown completing
a circuit to a motor, a very heavy current flows at first. This
current is several times the amount of current that the motor draws
when it is up to speed and running at constant speed and load. So
components such as relays and switches used to control inductive loads
must be rated to carry several times (at least 4 times, in most cases)
than the steady-state current of the motor.

In relays the extra capacity is achieved by a variety of mechanical
and electrical design choices made by the relay designer. These
include contact size, contact pressure, wiping action of the contacts,
and the metallurgy of the contacts. In spite of these design choices,
the relay's contacts are still under considerable stress when dealing
with inductive loads and must be protected using one or more
techniques designed to prevent the contacts from burning, sticking, or
degrading with use.

By the way, it might surprise some to learn that an ordinary
incandescent light bulb can be a much more demanding load in terms of
current inrush than even a motor. The reason is because the filament
has an extremely low resistance when cold. When you throw your wall
switch and turn on an incandescent lamp, the lamp may draw TEN OR
TWELVE TIMES the steady state current it draws after the filament has
heated up! SO when you turn on a 100W incandescent lamp, it may
actually present a 1200 watt load for a very brief period of time.

-Joe Kuchan

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