Transformer in Star/ Delta connections

[Trey]

In my AHU panel I have a transformer connected into the star/delta circuit (First time I've seen this) the original transformer was a 37kw with 415v 70% 60% 40% and star
tappings, this transformer is now faulty so I have ordered a new one which arrived today.

The problem is, on the old transformer the star points were connected seperately (one for each phase) which then went to the star contactor but the new transformer has the star connections connected together, is this correct? does this now give the circuit two star points? does this make any difference at all? do I have to do some modifications in the wiring?

Thanks for your help

 

[Knobhead]

Post some drawings or photo’s. I can’t make head nor tail of what you describe. You give no information regarding voltage. Is this a star delta motor? Nothing makes sense.

The manufacturer would be the best to ask.

 

[Specialist]

Aah, not just me being thick then Tony.

 

[benji]

only transfomer iv,e seen in a ahu panel is the control transformer

 

[Knobhead]

Aah, not just me being thick then Tony.

Just try going on to international engineering sites and make sense of Joe from the corner pub in Jakarta wanting to wire a 275/33KV 150MVA transformer in his back yard in to the grid. You have to have a sense of humour.

 

[Trey]

Brain shut down last night, all sorted now sorry to waste everyones time

 

[Specialist]

Just try going on to international engineering sites and make sense of Joe from the corner pub in Jakarta wanting to wire a 275/33KV 150MVA transformer in his back yard in to the grid. You have to have a sense of humour.

Thanks Matey but way my head is, I think i'll leave that to you. :crazy:

 

[silva.foxx]

Brain shut down last night, all sorted now sorry to waste everyones time

Great! Thanks for sharing your solution. When the next guy comes for similar help we'll know which expert to send him to now!

:50:

 

[Trey]

Great! Thanks for sharing your solution. When the next guy comes for similar help we'll know which expert to send him to now!

:50:

I'm not very clued up when it comes to Star/Delta so I thought the safest option was to give the job to a more senior electrician. I deal more with the domestic side, I don't know what the out come was as I'm only back to work in two weeks.

Anyway what I was asking was this is the drawing of the Star/Delta circuit with the old transformer



And this is what it would have looked like with new transformer



Would this work? Doesn't seem right to me, just curious!:49::49:

 

[bumcrack]

Seems to me the star link outs on top of the star contactor are duplicated at the tranny.

 

[Trey]

Could this be a Direct On Line connection and they are using the transformer instead of a Drive? Its a very old panel!

 

[Knobhead]

All is clear now. The top one is a Korndorfer closed transition autotransformer starter http://www.lmphotonics.com/m_start.htm#AutoTransformer

As for the bottom one, I hope you haven’t connected it like that. It will burn the transformer out. There is a back feed in to the transformer from the run contactor.

I can see where your confusion came from. The drawings are marked wrong:
Star = Start
Delta = Run

 

[Trey]

Thank you very much for your help Tony I had never heard of this before.

No I didn't connect anything (Past experience has taught me if you don't know what you doing leave it!) As I was not sure at all what I was dealing I asked the other electrician who has more experience with three phase to take over.

Once again thank you for the link

 

[Darkwood]

Seems to me the star link outs on top of the star contactor are duplicated at the tranny.

Made me chuckle when i saw your forum name as have missed it up to press but reading this post i think you'd be a tad wise to call it a transformer instead of tranny, not normally a issue but with your tag name its food for thought .......(no offence intended, top name choice lol)

 

[Knobhead]

Just had a 2[SUP]nd[/SUP] look at the first drawing, that’s wrong as well. The main contactor and O/L’s are in the wrong place.

I’ll do a new drawing and a description of the sequence in the morning. (With my insomnia, read about 4am).

 

[Knobhead]

Sod it, I can’t get to sleep!

View attachment 9100

There are two types of autotransformer starters and to add to the confusion they are wired the same. The sequence and the transformer construction are different.

The standard starter is open transition where at the change over from start to run there is a brief moment when no power is supplied to the motor. This can cause mechanical stress to the motor and load.

The Korndorfer starter is closed transition, so no loss of power at change over. Because of this the transformer has to be a bit more “beefy”. But because there is no break in the supply to the motor there is no mechanical stress.

Standard starter, open transition:
contactors 1 and 3 close. Giving a reduced voltage to the motor
time delay
contactors 1 and 3 open
small time delay (no power to motor)
contactor 2 closes. Giving full power to the motor

Korndorfer starter, closed transition:
contactor 3 closes
small time delay
contactor 3 closes. Giving a reduced voltage to the motor
time delay
contactor 3 opens. The transformer is now acting as a choke
small time delay
contactor 2 closes. The transformer is now shorted between it’s incoming and % tappings. But as it's the same potential bewteen the two points there is no current flow
small time delay
contactor 1 opens.

As can you see the sequence for the Korndorfer puts stress on the transformer where it has to withstand acting as a choke. So for small motors it’s uneconomical.

 

[Knobhead]

Correction
small time delay
contactor 1 closes. Giving a reduced voltage to the motor