"Andy Wade" wrote in message
...
George wrote:

It was manufactured by Mawdsley in the early 80s and when I shut it down,
I disconnected the AVR too and kept this and the spare I held. I have
checked with the Company and there is no way that they can help me with
the excitation to fire it up.

Not even some documentation, connection diagrams, etc.?

Before I go to all the trouble of connecting it up to a diesel engine
and spin it up to 1500 rpm, can anyone please tell me a few things
about it.

[...]

How does this set up actually work.

Well it might or might not be something like the set I used to look after
a few years ago. This was of similar rating to yours (100 kVA) and your
description sounds somewhat familiar. AFAICR, 'mine' worked as follows:

The electronic AVR, of which more later, provides a variable DC output
current to the stator (field) of the exciter machine - this will be the
"Exc. volts 16.5, amps 1.8" in your description.

The exciter's rotor produces a polyphase AC output which is immediately
rectified by diodes built into the combined rotor assemblies of the two
machines. The rectified output provides the DC excitation for the rotor
of the main alternator - this is is the "Excitation volts 71. amps 18" in
your description. It's an elegant arrangement since there are no slip
rings or brushes anywhere.

The main 3-phase AC output comes from the stator windings of the main
machine, which will be star-connected so as to provide a neutral point.

The AVR is essential to the operation of the whole set. It's a fairly
simple bit of circuitry, powered from one phase of the AC output via a
small mains transformer, bridge rectifier and reservoir capacitor. The AC
output voltage is sampled, rectified and compared to a Zener reference,
the difference being used to drive the exciter stator winding via a
switching power transistor (PWM control). A 'freewheel' diode and the
self inductance of the winding turn the hard-switched voltage drive to the
winding into a fairly steady DC excitation current.

A negative feedback loop is now in place - if the main AC output voltage
falls the AVR increases the exciter's stator current which increases the
output of the exciter which increases the rotor current in the main
alternator, thus tending to increase the AC output voltage. A pre-set pot
on the AVR module allows fine adjustment of the AC output voltage.

I hope this helps a little. For test purposes you could feed the exciter
stator from a car battery or an appropriately rated DC PSU. Connect a
shunt (reversed biased) diode firmly across the winding terminals to
prevent arcs, sparks and the risk of serious shocks and/or insulation
damage occurring when you disconnected the excitation. Beware that, if you
over-excite, the main output phase voltage may rise to 300 V or more -
enough to damage some loads (DAMHIKT).

It's also essential to observe correct polarity of the excitation current
and direction of rotation. The whole thing relies on residual magnetism
to boot itself up and you don't want to reverse this.


If the unit is as you say a brushless unit it's unlikely to be self exciting
and therefore unlikely that any residual magnetism will be sufficient to for
the rotor voltage to overcome the forward diode drops. If it's brushed then
it is more likely since brushes have very little forward drop.

I would have thought the AVR would have terminals for a battery for the
initial excitation? Given that this is a 1980's alternator I doubt the
electronics at the time would have stretched to supplying 71V and 18A. I
may well be wrong but I would have thought a mechanical vibrating regulator
would be more likely.