"T" Winding transformers. 8

[waross]

A comment on the obsolete two phase systems in another thread reminded me of an issue that has puzzled me for years.
There was a time years ago when we saw a lot of "T" winding lighting transformers. These were typically 25 KVA, 480 Volts to 120/208 Volts.
Many years ago when three phase systems were replacing two phase systems, the Scott connection was able to convert two phase power to three phase power with two suitable transformers.
Around the 60's, someone realized that a similar connection with suitable ratios and taps could also be used to transformer three phase power to three phase power.
One of the advantages was two cores instead of three cores.
I have always wondered what other advantages there were, and also what were the disadvantages that led to these transformers going the way of the Dodo.
Anyone else willing to admit to being old enough to remember these transformers?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

@Hamburgerhelper: I disagree, cost will only go down. Do you have any idea how many new appliances are using triacs in place of switches? Its only time. Both advancements in solid state and the limitations of AC will drive the cost down.


Regarding the new thread- what name and subject? I'll get it rolling.

 

[waross]

Considering Amps-turns and Volts-per-Hertz; a 5 KVA transformer rated at 120:1.2 Volts, 50 Hz will transform
500 KVA at 12000:120 Volts, 5000 Hz.
Given a 100:1 advantage in material usage, there will be a lot of very smart people working on improving efficiency and insulation and finding various ways to mitigate the various disadvantages of high frequency switching as compared to a low frequency sine wave source.
I understand that more and more motor driven appliances are using invertor driven three phase motors.
A DC supply will offset some of the extra losses by eliminating the rectifier section.

How many remember the television series "MASH"?
An oft repeated dialog sequence:
Radar;
"Incoming. Choppers."
Other character;
"I don't hear anything."
Radar;
"Wait for it."

In the TV series the first indication of incoming choppers was the distant whup whup sound of rotors.
In the future, the first indication of a switch to DC distribution may be major appliances having an AC/DC rating.
"Wait for it."

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Mbrooke; How about "AC versus DC".
Those of us who are interested may cut and paste our replies from this thread to the new thread.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Yup, and try 25,000Hz. 500,000Hz. Try air core transformers... The limits are endless. AC is a waste of copper, aluminum, iron, steal, space, resources just to name a few.

 

[Mbrooke]

@Waross: We have lift off!

https://www.eng-tips.com/viewthread.cfm?qid=439398

 

[MatthewDB]

But in rail and furnace application T connection is only on primary and two independent secondaries ( with 90 degree shift between them) are available

I've often pondered what advantages you would get in using Scott T for those applications. I've yet to see it used though. I've always seen rail use 2 of the HV phases feeding a single phase transformer for rail. Usually it is two transformers feeding each direction independently off a different pair of phases. If one transformer is down, it can serve both directions from one transformer for redundancy.

I work on active harmonic & statcom converters. We've sold a number of them for correction of AC single phase rail loads. We inject leading or lagging vars to eliminate the flicker / sag and we inject negative sequence to counteract the single phase loading.

If the system instead used a Scott-T, there would be significantly reduced negative sequence if there was a reasonable balance of loading on the system... but I've never seen this on rail. Usually the loading is highly cyclical. There would be no difference when the phase to phase transformer is loaded. When the phase to midpoint transformer is loaded, you still have the same negative sequence current, just at a modified phase angle that causes two of the phases to be half loaded and one fully loaded instead of two fully loaded phases.

You'd also lose redundancy. If the center tapped transformer burns out, the other one won't work either.