I looked at your schematic and you are right, the first six are
rectifiers but I suspect they form a synchronous rectifier with phase
control. Their output is pulsed DC and the output voltage is
controlled by the on time. since the supply voltage is 120 degrees per
phase offset, they are pulsed in sequence BUT each should be triggered
with the same time offset from the zero voltage crossing of it's
corresponding phase to prevent current hogging.

I cannot tell if what you are calling "protection coils" are in fact
noise filters, swinging chokes or saturable reactors. The giant MOV
and reverse diode should also be checked.

It looks like there are a triple set of control transformers in the
electronics package. There may also be isolation transformers at each
of the thyristor trigger modules that could simplify your testing of
the upstream components, timing etc at reduced voltage potentials and
unified circuit ground, ie if there is 0 ohms between the low side of
the trigger transformer primaries, that is all are connected to the
circuit ground and that ground is at earth, chassis potential, you can
ground your scope probe at that point and probe the high side of the
trigger transformer primaries to rule out problems in the circuit
board itself.

There is no substitute for an AC/DC current probe in your line of
work. It is a must purchase item. At least an ampclamp meter but
preferably one for use with a wideband scope. Tektronix makes a very
nice one with a 3000A adapter and jaws big enough for your
application. The smaller probe can be used on the gate leads. The
larger on high power portions of the circuit. They are inherently very
safe BUT be aware of the open laminations when clamping the jaws.

Get a captive ball flow meter sight glass and splice it into your
cooling lines to verify flow.

Shorts on the load should be evident and in fact dramatic.

What is the status of the high frequency thyristors? I think they
would be more likely to toast the "protection coil".

None of these ratings you specify for the thyristors have ANY
RELATIONSHIP TO THE WAY YOU ARE MEASURING THEM. What I am trying to
say is, all of your power devices have been subject to overloads way
beyond design values and must be tested for leakage in the off state,
voltage drop under full load, excess heating under load etc. When you
connect a scope you will find peaks of current and voltage far beyond
what your multimeter says. Those values correspond to semiconductor
ratings.

I would MOUNT the scope on this inverter to view both voltage and
current waveforms being sent to the furnace along with the existing
panel meters. You might be able to use existing meter shunts to drive
the scope. This will give you early indication of impending failures
as you are using the system.

Tom


On Dec 11, 10:27 am, rex wrote:
Thanks for your informative reply , I owe you
I am in Greece and waiting for a specialist to come from England,
unfortunatley next week so I had to do something myself ...I
understand the danger and I will follow your instructions step by
step.
Trying to work again today since the voltage -not accurate measures I
took looked "normal" to me, that is instead of 400 V/50 Hz ... 430-460
V due to harmonics I supose.
So after talking to him despit the heating up of the busbars he told
me I can work ... I have a short circuit thyristor more ((
Amperage was not high , according to the instruments steadily at 900 A
- it is designed for 1.300 A, thyristors can take 1800 V - Itav 1400 A
Yes , it is an Inverter ... I take two bus bars from the 6 bridge
rectifier SCR's that is DC ... passing through a secondary protection
coil goes to an Inverter ... 2 pairs of fast , inverter grade
thyristors and they finaly give to the Load high frequency AC at
600-1000 Hz
All started two weeks ago with the secondary coil overheating - just
one widing , it has two , one for each DC phase I supose
I was told that it is short circuit and it went for rewind ... waste
of time and money ...
After having the same overheating at the secondary - protection
coil ... I was told and it was true that I had one water cooled cable
to the load cut ... it was so
After that I was able to work for some melts until the source coming
bus bars started to overheating ... I though it was from bad
thyristors they can;t take too much heat for long , though the system
has everything on it like drops the power from overheating at various
points etc.
Water that cools the cabinet components are highly deionised but I
noticed that in this particular winding , water pressure was low ....
the same cooling line passes from the thyristors so I am thinking of
giving a new one just for the coil and thyristors with enough pressure
to cool them down
Tomorrow I will have one new slow rectifier phase thyristor to test
I am also suspecting short circuit at the load coil ... it is heavily
built up with yokes and supporting woods and isulations between
them ... so I have to exclude any short circuit conditions on the load
side
I exchange two thyristors ... 1-3 and the voltage ... with the
multimeter I did measured it ... seems normal as I told you before .
THanks for the thyristor and triggering instructions I shall try them
also tomorrow !