Hello,

I am having problems measuring the voltage of a 600 kW six pulse
bridge rectifier with a simple multimeter ....
it is runing through 3 pairs of thyristors (1800 V, 1500 A 200 ìs
typical time) the AC burs after a Current transformer (???) attached
to each one passes current to thyristors in pair
When I power ON at around 250 kW , it is an induction furnace solid
state inverter at 600 kW - yet I am reaching just the 500 kW for the
moment ... and I measure (multimeter in AC V scale) the AC bars in
one end and the other to DC (upper thyristors blocks) I get
respectively say 1-2 ~ 650 V 1-3 ~ 1000 V(!) and 2-3 1350 V (!!!),
to the lower thyristor block from AC bars again , low voltage like
120-220 Volts
DC+DC- around 1000 Volts
when power of the generator is OFF , normal 400 V AC is coming through
the 3 AC bars ....
measuring the AC burs at the exit of the main switch from my board to
the cabinet I get 500, 730, 840 Volts respectively
the AC bars leading to the thyristor pairs usualy overheats, I suspect
it is a thyristor triggering related problem but cant measure them
correct , though in 200 Ohms none is showing short circuit

thyristors are like ...

1 3 5
2 6 2

and bus bars 1 2 3


I would apreciate if someone could enlighten me on how to measure the
voltages and/or thyristors thanks !

rex wrote in message
...
Hello,

I am having problems measuring the voltage of a 600 kW six pulse
bridge rectifier with a simple multimeter ....
it is runing through 3 pairs of thyristors (1800 V, 1500 A 200 ìs
typical time) the AC burs after a Current transformer (???) attached
to each one passes current to thyristors in pair
When I power ON at around 250 kW , it is an induction furnace solid
state inverter at 600 kW - yet I am reaching just the 500 kW for the
moment ... and I measure (multimeter in AC V scale) the AC bars in
one end and the other to DC (upper thyristors blocks) I get
respectively say 1-2 ~ 650 V 1-3 ~ 1000 V(!) and 2-3 1350 V (!!!),
to the lower thyristor block from AC bars again , low voltage like
120-220 Volts
DC+DC- around 1000 Volts
when power of the generator is OFF , normal 400 V AC is coming through
the 3 AC bars ....
measuring the AC burs at the exit of the main switch from my board to
the cabinet I get 500, 730, 840 Volts respectively
the AC bars leading to the thyristor pairs usualy overheats, I suspect
it is a thyristor triggering related problem but cant measure them
correct , though in 200 Ohms none is showing short circuit

thyristors are like ...

1 3 5
2 6 2

and bus bars 1 2 3

I would apreciate if someone could enlighten me on how to measure the
voltages and/or thyristors thanks !


reply:

It is "electronics" in the title of this NG

rex wrote:

Hello,

I am having problems measuring the voltage of a 600 kW six pulse
bridge rectifier

What's a "six pulse bridge rectifier" ? Do you mean three phase by any
chance ?

Graham

rex wrote:
Hello,

I am having problems measuring the voltage of a 600 kW six pulse
bridge rectifier with a simple multimeter ....
it is runing through 3 pairs of thyristors (1800 V, 1500 A 200 ìs
typical time) the AC burs after a Current transformer (???) attached
to each one passes current to thyristors in pair
When I power ON at around 250 kW , it is an induction furnace solid
state inverter at 600 kW - yet I am reaching just the 500 kW for the
moment ... and I measure (multimeter in AC V scale) the AC bars in
one end and the other to DC (upper thyristors blocks) I get
respectively say 1-2 ~ 650 V 1-3 ~ 1000 V(!) and 2-3 1350 V (!!!),
to the lower thyristor block from AC bars again , low voltage like
120-220 Volts
DC+DC- around 1000 Volts
when power of the generator is OFF , normal 400 V AC is coming through
the 3 AC bars ....
measuring the AC burs at the exit of the main switch from my board to
the cabinet I get 500, 730, 840 Volts respectively
the AC bars leading to the thyristor pairs usualy overheats, I suspect
it is a thyristor triggering related problem but cant measure them
correct , though in 200 Ohms none is showing short circuit

Holy crap! You are playing with 1800 volt 3-phase AC at 600 KW power
levels, and you don't know how to run a multimeter? Where shall I send
the sincere condolences to your wife?

Bill

On 9 Δεκ, 19:39, Bill Jeffrey wrote:
rex wrote:
Hello,

Â*I am having problems measuring the voltage of a 600 kW six pulse
bridge rectifier with a simple multimeter ....
it is runing through 3 pairs of thyristors (1800 V, 1500 A 200 ìs
typical time) the AC burs after a Current transformer (???) attached
to each one passes current to thyristors in pair
When I power ON at around 250 kW , it is an induction furnace solid
state inverter at 600 kW - yet I am reaching just the 500 kW for the
moment ... and I measure (multimeter in AC V scale) Â*the AC bars in
one end and the other to DC (upper thyristors blocks) I get
respectively say 1-2 ~ 650 V 1-3 ~ 1000 V(!) and 2-3 1350 V (!!!),
to the lower thyristor block from AC bars again , low voltage like
120-220 Volts
DC+DC- around 1000 Volts
when power of the generator is OFF , normal 400 V AC is coming through
the 3 AC bars ....
measuring the AC burs at the exit of the main switch from my board to
the cabinet I get 500, 730, 840 Volts respectively
the AC bars leading to the thyristor pairs usualy overheats, I suspect
it is a thyristor triggering Â*related problem but cant measure them
correct , though in 200 Ohms none is showing short circuit

Holy crap! You are playing with 1800 volt 3-phase AC at 600 KW power
levels, and you don't know how to run a multimeter? Where shall I send
the sincere condolences to your wife?

Bill- ΑπόκÏ�υψη κειμÎ*νου σε παÏ�άθεση -

- Εμφάνιση κειμÎ*νου σε παÏ�άθεση -

lol Bill
she will be happy if I fly away .. it is a malfunction I have to
resolve, service is not very helpfull and I am not an electrician
Though from practice I know some things ... but I get puzzled on the
basics
If you do know how a bridge full wave rectifier works you can tell me
how to be sure that I measure the correct voltage DC or AC ...

On 9 Äåê, 19:21, Eeyore
wrote:
rex wrote:
Hello,

I am having problems measuring the voltage of a 600 kW six pulse
bridge rectifier

What's a "six pulse bridge rectifier" ? Do you mean three phase by any
chance ?

Graham


six pulse bridge rectifier to my knowledge is a device consisting of
two thyristors for each AC phase. 6 in total ... with the correct
triggering they convert the AC to DC one way always ... 3 pairs , one
pair for each AC phase ... current runs one way only throug two of
the thyristors for some mseconds adding DC voltage ...

rex wrote:

Hello,

I am having problems measuring the voltage of a 600 kW six pulse
bridge rectifier with a simple multimeter ....
it is runing through 3 pairs of thyristors (1800 V, 1500 A 200 ìs
typical time) the AC burs after a Current transformer (???) attached
to each one passes current to thyristors in pair
When I power ON at around 250 kW , it is an induction furnace solid
state inverter at 600 kW - yet I am reaching just the 500 kW for the
moment ... and I measure (multimeter in AC V scale) the AC bars in
one end and the other to DC (upper thyristors blocks) I get
respectively say 1-2 ~ 650 V 1-3 ~ 1000 V(!) and 2-3 1350 V (!!!),
to the lower thyristor block from AC bars again , low voltage like
120-220 Volts
DC+DC- around 1000 Volts
when power of the generator is OFF , normal 400 V AC is coming through
the 3 AC bars ....
measuring the AC burs at the exit of the main switch from my board to
the cabinet I get 500, 730, 840 Volts respectively
the AC bars leading to the thyristor pairs usualy overheats, I suspect
it is a thyristor triggering related problem but cant measure them
correct , though in 200 Ohms none is showing short circuit

thyristors are like ...

1 3 5
2 6 2

and bus bars 1 2 3


I would apreciate if someone could enlighten me on how to measure the
voltages and/or thyristors thanks !
you are dealing with a 3 phase, phase firing system. if you need
accurate measuring. You need a voltage isolator+ divider and a
scope to monitor the output..

And for your problem that you're trying to detect, you either have
a miss aligned firing drivers, bad thyristors or shorted turns in the
load that is changing the induction.

Connect the load into a 3 phase Resistor network, take the measurements
from there and get your self a scope!...
It sounds like you need to tailor the firing thresholds..
Using a Reactor may also help out.






--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

Thanx Jammie

I shall check them all ... I suspect a thyristor that is not
triggering right ... propably on the pair I get the low voltage
readings ...or the high ones ...lol... and I willl find me a scope to
see the wave coming out to DC , I think I know the pins to measure it
externaly

The generator is working for two melting furnances so it is evil
coincidence to have a short circuit at both at the same time ... but
you never know

thanks for the advice

You are seriously over your head here and should not be playing
around.

A multimeter is not the correct tool for this job!

You need

3-4 channel scope with triggering capability
4pcs. X10 probes
4pcs 4kv probes
3pcs 3000A Dc/high frequency current probes.

480V to 24 V control transformer

4pcs. 1500v wideband isolation probes

In any case, you are not even measuring the voltage correctly.

The incoming AC is measured:

buss to buss A,B ie Black - Red B,C ie Red - Blue C,A ie Blue
- Black (DELTA or WYE)

or buss to neutral. A,N ie Black - White B,N ie Red - White
C,N ie Blue - White (WYE only)

or buss to ground A,G ie Black - Green B,G ie Red - Green C,G
ie Blue - Green (WYE only)

The main 3 phase thyristor bridge you describe should not be making
DC, it should be making high frequency AC, hence the description
inverter! I would assume, the multimeter measurements would be
consistent from pair to pair but totally inaccurate- you will have no
idea what the true voltage or waveform really is.

Now there are 3 input AC busses, each connected to a pair of
thyristors, one positive, one negative. On the output of the thyristor
set, are there just 2 output busses or are there 6 and do they connect
directly to the load? Is the load connected to ground or neutral in
anyway, perhaps in the center of the coils?

This whole thing sounds really dangerous at best. The AC supply side
should be stable +/- 15% with the thing on or off and in no case
should a 440vac phase start reading 600v or 1000v to ground at the AC
Distribution (Circuit Breaker or Fuse) Box. If it does, you've got big
problems and they might be utility related. An installation of this
size will usually have a company supplied step-down transformer from
say 13.5KV to 440 made up of 3 interconnected transformers one for
each phase. If the junction that connects the three secondaries
becomes weak, that is, fails under load, the system goes up for grabs.
That is in a WYE connected system. In a Delta connected system it is
yet....Well here you read about it:

http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

Since the secondary of the Delta is not ground referenced,
measurements that are ground referenced or neutral referenced have no
meaning and can vary wildly. You must measure Buss to Buss, Phase to
Phase. I am guessing you have a Delta Connected distribution
transformer for this inverter.

With the inverter off at the input side:

Trigger the scope with a 440v to 24v control transformer. Connect the
440v side from phase A to B and ground one leg of the 24v secondary.
Connect the X10 probe to the other 24v leg and the probes ground where
you grounded the other side of the secondary. Connect the probe to the
trigger input. Set the scope for external triggering and adjust for a
stable trigger at a 60hz rate. ( You can cheat here and use the LINE
TRIGGER setting instead)

Using the 4kv X100 probes, Connect both probe grounds to the ground
used above. Set the Ch1 and CH2 scope inputs to add. Press the invert
button on CH2.

1) Connect one to A and one to B.

Set both CH1 and CH2 to identical attenuator settings in the CAL
position so the trace fills the screen vertically (about 1-2 V per
division). Adjust the sweep so you can see 2 complete sine waves.

2) Move the probe on phase A to C and measure the B - C waveform

3) Then move the probe on phase B to A and measure the A - C Waveform

All three should be about the same

Connect your current probe to CH3 and set the attenuator for about
1500A full screen using the probe factor, DC coupled .

Start the inverter

4) Clamp it around each of the 6 thyristors or it's output lead, one
at a time noting the CH3 waveform

5) Clamp it around each of the 3 AC input busses and note the CH3
waveform

Repeat 1) 2) and 3) with the inverter still running.

Repeat 1) 2) and 3) with the inverter still running but this time
connect to the output side of the thyristors.

Be careful, setup each measurement, use protective gloves, a
switchgear mat and have an assistant standby with an 8' 2X4 to pry you
off if you get grabbed. You are working at massive power levels. The
fact that Busses are overheating indicates a high current fault that
could fry your face at any moment. Death by electrocution is very
painful. Death by molten copper plasma is excruciating.

At this point the faulty section should be obvious. You could take a
chance and replace the offending thyristor.

The next test now that you have narrowed it down to a single device
requires you to measure the trigger voltage and this can be tricky
since it is .6v (actually it could be 20) referenced to one side of
the thyristor in a really noisy environment. Measuring it wrong can
trigger the thyristor at the wrong time and then you're in a world of
****. Use 4KV probes on the three terminals of the thyristor. The
gate terminal will be pulsed a few volts above one of the other two
terminals.

Using the 4kv X100 probes, Connect probe grounds to the ground used
above. Set the Ch1 and CH2 scope inputs to add. Press the invert
button on CH2.

a) Connect one probe to the GATE and the other to each of the large
terminals of the thyristor.

Set both CH1 and CH2 to identical attenuator settings in the CAL
position so the trace fills the screen vertically for each
arrangement. The correct orientation will require a lot of gain around
1-10mv/Div . Start at 10v/div and reduce 1 click ch1 then 1 click ch2
etc.

Repeat this test on a known working thyristor for reference.

http://www.littelfuse.com/data/en/Pr...ationNotes.pdf

Where are you located? Thought of hiring someone to do this for you?

Tom






On Dec 10, 12:55 am, rex wrote:
Thanx Jammie

I shall check them all ... I suspect a thyristor that is not
triggering right ... propably on the pair I get the low voltage
readings ...or the high ones ...lol... and I willl find me a scope to
see the wave coming out to DC , I think I know the pins to measure it
externaly

The generator is working for two melting furnances so it is evil
coincidence to have a short circuit at both at the same time ... but
you never know

thanks for the advice

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 !

"rex" wrote in message
...
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 !

Anybody else out there following this thread with absolute horror? I
have been in electronics for 40 years and I am picturing all sorts in
my minds eye. One thing is for sure, I wouldnt go within 100 metres of
it when its fired up, faulty or not!
Gordon

In article ,
"Gordon Bennett" wrote:

"rex" wrote in message
...
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 !

Anybody else out there following this thread with absolute horror? I
have been in electronics for 40 years and I am picturing all sorts in
my minds eye. One thing is for sure, I wouldnt go within 100 metres of
it when its fired up, faulty or not!
Gordon

I agree that the OP isn't qualified. He said himself he's not an
electrician. Man, does he have *any* idea how far that kind of voltage
can leap through air when he introduces any kind of test probe to the
area? The most surprising thing to me is that he isn't dead already.

Gordon Bennett wrote:

"rex" wrote in message
...

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 !


Anybody else out there following this thread with absolute horror? I
have been in electronics for 40 years and I am picturing all sorts in
my minds eye. One thing is for sure, I wouldnt go within 100 metres of
it when its fired up, faulty or not!
Gordon


Ha, don't let that bother you, I've had 2500 Amp Scr's explode from old
reliance drives right in front of me!, ceramic chips everywhere!

Some time's they don't die silently!

--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

On 12 Äåê, 02:02, Jamie
t wrote:
Gordon Bennett wrote:
"rex" wrote in message
...

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 !

Anybody else out there following this thread with absolute horror? I
have been in electronics for 40 years and I am picturing all sorts in
my minds eye. One thing is for sure, I wouldnt go within 100 metres of
it when its fired up, faulty or not!
Gordon

Ha, don't let that bother you, I've had 2500 Amp Scr's explode from old
reliance drives right in front of me!, ceramic chips everywhere!

Some time's they don't die silently!

--
"I'd rather have a bottle in front of me than a frontal lobotomy"http://webpages.charter.net/jamie_5- Áðüêñõøç êåéìÝíïõ óå ðáñÜèåóç -

- ÅìöÜíéóç êåéìÝíïõ óå ðáñÜèåóç -

OMG Jamie ! ... I saw one once getting red and thats all ... now you
really scared me

Gordon Bennett wrote in message
...

"rex" wrote in message
...
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 !

Anybody else out there following this thread with absolute horror? I
have been in electronics for 40 years and I am picturing all sorts in
my minds eye. One thing is for sure, I wouldnt go within 100 metres of
it when its fired up, faulty or not!
Gordon



Likewise
I went to a regional power station open day one day.
It was necessary to kick in one generator set while we were there.
I couldn't believe my eyes when someone had to open up a door on the side of
the casing, reach in , and do something, I don't know what, and there was an
almighty roof rattling bang as bus-bars contacted or something.
The public , including me, was 50 yards away.
Anyone ever visited the National Grid pylon and insulator test facility at
Leatherhead, Surrey , sci-fi kit there.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

On Dec 9, 9:44 am, rex wrote:
Hello,

I am having problems measuring the voltage of a 600 kW six pulse
bridge rectifier with a simple multimeter ....
it is runing through 3 pairs of thyristors (1800 V, 1500 A 200 ìs
typical time) the AC burs after a Current transformer (???) attached
to each one passes current to thyristors in pair
When I power ON at around 250 kW , it is an induction furnace solid
state inverter at 600 kW - yet I am reaching just the 500 kW for the
moment ... and I measure (multimeter in AC V scale) the AC bars in
one end and the other to DC (upper thyristors blocks) I get
respectively say 1-2 ~ 650 V 1-3 ~ 1000 V(!) and 2-3 1350 V (!!!),
to the lower thyristor block from AC bars again , low voltage like
120-220 Volts
DC+DC- around 1000 Volts
when power of the generator is OFF , normal 400 V AC is coming through
the 3 AC bars ....
measuring the AC burs at the exit of the main switch from my board to
the cabinet I get 500, 730, 840 Volts respectively
the AC bars leading to the thyristor pairs usualy overheats, I suspect
it is a thyristor triggering related problem but cant measure them
correct , though in 200 Ohms none is showing short circuit

thyristors are like ...

1 3 5
2 6 2

and bus bars 1 2 3

I would apreciate if someone could enlighten me on how to measure the
voltages and/or thyristors thanks !

If you have to come to a group of strangers for help, you may be
asking for it...not the help...an electrical mishap of epic
proportions! I'll keep my eyes on the news for Greece being mentioned!

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 !

Tom

I had a qualified electronic repairman yesterdays ... we did measuree
the voltages with a descent multimeter and was all normal ... 390 V
phase to phase AC , around 400 VDC all phases equal ...so the
indications i took with the cheap multimeter was all wrong.
We also been able to measure pulses and seems that all were working,
triggering the tyristors
Also the wayform of the output AC was acording to the shape of the
manufacturer at 1,5 V scale as expected
Still the secondary coil ... DC choke actually overheats 2 timesa
during a 1 hour melt ... I never power the load at maximum , I work at
60% maximum , takes some time more but works , I also lower the
cooling water maximum allowable temperature so not to heat up the
thyristors a lot
You say that high frequency thyristors ... I meant fast thyristors 60
microseconds instead of the 200 of the rectifier grade thyristors ...
may involved and I suspect the same along with load short circuit
The power cabinet is connected to two furnaces ... one is working at a
time ... and the two have different behavior ... 500 kgs start at 500
A and the 1000 kgs ..with the same metal charge starts at 900 A! so it
must have something to do with the load shortcircuit too
One expert from Bulgaria is expected today and maybe next week one
from England .. .we will see
Thanks for the suport my friend, got to go to work now I will study
your texts later
Rex

As for the cooling system .. it is very sophisticated and complete of
floaters , sight eye glass .. a total of 18 pieces .. 6+6 for loads
and 6 for the power cabinet itself , the power cabinet floaters have
thermoseters so to cut power off due to high temperature , as I told
you I have adjust this setting to the lowest possible so to cut off
earlier and not overheat too much the power thyristors
I have to get me one pair of fast , inverter grade thyristors , the
one pair is 1 year old and it was an alternative choice of a russian
manufacturer, Westcode has long delivery times .. they are faster at
14 microseconds but a design parameter is kept low so the clamping
plates of copper may short circuit around the thyristor and to the
extend of the cathode or anode surface ... i saw some silver bubbles
there
so they may have overheated due to this fact and who knows what is
happening inside them ... but the load short circuit is still what
concerns me ... furnace used to work before with considerable
fluctations in all the iondications .. KW, A, V or even frequency ..
now it goes steadily increased as if it only one turn to read and
adjust ...
I mean by this that adding metal charge, filling the furnace does not
affect its performance as it did before .. it is said that electronic
board adjust to the sload so to have full power always ... this is not
happening now I simply raise the power by the power regulator

Snip

Anybody else out there following this thread with absolute horror? I
have been in electronics for 40 years and I am picturing all sorts in
my minds eye. One thing is for sure, I wouldnt go within 100 metres of
it when its fired up, faulty or not!
Gordon

I agree that the OP isn't qualified. He said himself he's not an
electrician. Man, does he have *any* idea how far that kind of voltage
can leap through air when he introduces any kind of test probe to the
area? The most surprising thing to me is that he isn't dead already.

This is just scary. This guy doesn't even own a decent multimeter or even
know how to use one and he is playing with a 3 phase HV inverter! I wouldn't
go near this thing either. I hope this guy doesn't end up killing himself.
I'd say he should invest in a good life insurance policy.