RE: voltage light

[Hirsch <Catenary@domain.elided>]

Robert W. Murphy asks:
 =

>> Whenever I start my 90 SPider in the morning or afternoon
(after sitting for several hours) the voltage light remains
lit until I after I drive about 100 feet or so or until I
rev the engine some. Is this normal? <<

This question came up in '97. My answer then applies now
and is repeated below:

[Quote starts...]
This is apparently proper operation. Quoting from Bosch's
"Automotive Electric/Electronic Systems," p. 314:

The circuits of the alternator
- ------------------------------

The following three circuits are standard for the alternator:

- - Pre-excitation circuit (separate excitation using battery
current)
- - Excitation circuit (self-excitation)
- - Generator or main circuit.

Pre-excitation circuit
- ----------------------

When the ignition or driving switch is operated, the battery
current first of all flows through the charge indicator lamp,
through the excitation winding in the stator, and through the
voltage regulator to ground. In the rotor, this battery current
serves to pre-excite the alternator.

Why is pre-excitation necessary?
- --------------------------------

The reason is that in conventional alternators at the instant of
starting, or at very low engine speeds, the residual magnetism
(remanance) in the iron core of the excitation winding is so weak
that it is insufficient to provide the self-excitation required
for building up the magnetic field and for generating the
required voltage.

A power diode and an exciter diode are connected in series for
each phase of the excitation circuit. Self-excitation can only
take place when the alternator voltage exceeds the voltage drop
across the two diodes (2 x 0 7 =3D 1.4 V). This is precisely what
the pre-excitation current does. It generates a magnetic field
in the rotor which results in increasing generator voltage in
the stator and so to speak, helps the self-excitation
to "get going".

When the engine runs up to speed following the start, the
alternator must reach a certain speed in order to ensure that
self-excitation is reliably initiated. THIS SPEED IS HIGHER
THAN THE IDLE SPEED BUT ONCE IT HAS BEEN REACHED THE EXCITER
FIELD IS STRONG ENOUGH FOR THE ALTERNATOR TO DELIVER POWER
EVEN AT IDLE.

[and on p. 333 is defined the "Cutting-in speed"]

The cutting-in speed is defined as that speed at which the
alternator starts to deliver current when the speed is increased
for the first time. It is above the idle speed, and depends upon
the pre-excitation power, the rotor's remanence, the battery
voltage, and the rate of rotational-speed change.
[Quote ends...]

Having said this, Bosch injected Alfas of the '80s had an
auxiliary air valve (AAV) that increased cold idle rpm slightly.
If this device is working properly, you really shouldn't
see a glow before the throttle is blipped. (I know a '90
Spider doesn't have an AAV, but I assume it has an equivalent
device.)
 =

Rich
Editor, i Saluti, St Louis AROC
Website: http://www.catenary.com/victor/isaluti

------------------------------