400Hz AC is also closer to a straight line DC voltage in terms voltage
fluctuation. Avionics likes the reduced fluctuation better.
>
Well a lot closer than 50/60Hz which will give 100/120Hz ripple in a power
supply.
As a side note, you need to have a source of power when you get a newer MV
started to flash the field in the alternator to start it charging. As an
example: you can push start an M35A2 and it will run fine, but the lights
won't work until you "flash" for a power source.
>
This is absolutely correct for diesel power provided it doesn't need a supply to
hold the stop plunger off or that possibly non-existent vehicle fitted with a
magneto.
Along this line don't be fooled that an alternator gives DC as it actually
produces uni-directional current pulses, between each rectified half-cycle the
battery will try to drop to its discharge voltage under load owing to internal
resistance, batteries do not look like big capacitors. An oscilloscope will
display possibly 2V of ripple across an alternator charging battery under load
and the frequency will be a function of engine revs; the source of a varying
whine on some audio accessories.
The alternator has no residual magnetism unlike the dynamo (DC Generator) and
being electronically voltage regulated cannot be re-polarised to make a positive
or negative ground machine as required.
A tiny battery excitation current is needed to create the rotor magnetic field
and indeed with absolutely 0V applied it will never charge. In some
applications, especially cars but MV's too, this initial excitation is provided
by the ignition warning lamp since this is grounded through the alternator via
one slip-ring, the rotor coil, the other slip-ring and the regulator power
transistor to ground. Having the lamp light before cranking tells you a lot
about the health of the alternator, however should the bulb blow the machine
will never charge. Ever.
If the bulb fails whilst the engine is running charging will continue until the
engine is stopped, then will not re-start. In some instances a resistor of a
few hundred ohms is placed across the bulb to alleviate this danger and many UK
MV's have the warning lamp arranged in an entirely different manner to prevent
the easily missed no-charge scenario with a blown ignition warning lamp.
When running the alternator IGN or WL terminal is supplied from a half-wave
three phase rectifier stack from the main stator outputs and is capable of
supplying several amps, the lamp goes out as there is the same voltage on each
side of it. This feature can be used as a simple and highly effective
split-charge system for an auxiliary battery.
A high power relay, something with 60 - 80 Amp contacts can have its typically
100 - 180 ohm coil (in a 12V system) connected between the alternator IGN/WL
terminal and ground, this hardly affects the initial field current as the rotor
is well under 10 ohms and cannot operate on the few milliamps supplied by the
warning light of which some 9/10ths are going into the rotor.
When the alternator generates current sufficient to extinguish the warning lamp
and charge, the relay will pull in, it drops out on stopping. The relay contacts
are used to connect the vehicle battery to the auxiliary battery automatically
while the engine is running thereby charging it, but the auxiliary can be safely
discharged whilst stopped without also draining the vehicle battery and then
being unable to start again.
Richard
(Southampton UK)
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