Open delta transformer - Z calculation

I'm after some advice regarding a transformer impedance calculation I need to produce for a design in the near future; basically the transformer will
be 75kVA with a 400V open-delta primary and single phase 230V secondary.
I am familiar on how to calculate the effective secondary impedance on delta /star and single phase transformers by reflecting the impedance of the primary circuit when the transformers percentage impedance is known; however I'm unsure how to do this with the open-delta transformer described above (as open-delta is non-linear).
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I'm after some help regarding a transformer impedance calculation I need to produce for a design in the near future; basically the transformer will be 75kVA with a 400V open-delta primary and single phase 230V secondary.
I am familiar on how to calculate the effective secondary impedance on delta /star and single phase transformers by reflecting the impedance of the primary circuit when the transformers percentage impedance is known; however I'm unsure how to do this with the open-delta transformer described above (as open-delta is non-linear).
Regards
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----------------------------

You haven't given details on the connections- specifically what phases of the secondary are being used for the secondary nor the ratio of the transformer which appears to be 400/230 if the 3rd leg was present. It is not non-linear but it is unbalanced so 3 phase balanced analysis wont work
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Don Kelly snipped-for-privacy@shawcross.ca
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Transformer connection have never loomed large in my life. Why would someone even consider an open delta primary unless insufficient single phase capacity were unavailable?
What is meant by "impedance calculation" in such a situation? Are we talking about impedance transformation? That is,the load impedance as seen from the primary side? Is it just the V/I at full load?
This whole inquiry smacks of weirdness.
Bill
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Salmon Egg wrote:

INot to mention the fact that he posted this twice, so our responses are getting disconnected.
I agree, the open delta is really a waste in this case.
When I respopnded to him, I read his mention of transformer impedance percent to mean that he was asking about that. However, now that you mention it I re-read his post, and he might be asking about the load impedance reflected across the windings. He has not been very clear about what he wants to know.
Ben Miller
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----------------------------

-------------- Open Delta connections have been used for some cases where loads (3 phase) are small but expected to grow- then get by with two transformers giving 57% of the capacity of a 3 phase bank until the load justifies adding the third transformer. Single phase loads can be taken between any pair of the 3 terminals. As someone else has pointed out- regulation is poor. This was once done commonly in rural areas and the transformers were mainly low KVA pole pigs that were available. I don't know whether this practice is still common but it was quite common at one time. The only reason is to save a buck or two.
The impedance of concern is the source impedance which, in this case will be unbalanced. The main component of this is generally the transformer bank. This impedance limits fault currents and affects voltage regulation. In this case the impedances are not balanced.
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delta
however
Um, open delta is a way for power companies to provide 3 phase to a customer with only 2 transformers instead of 3. I don't think an "open delta" transformer exists. I doubt a 3 phase to single phase transformer exists either. Of course you could use one leg of the secondary but your service would be unbalanced
The usual method of operation a single phase load from 3 phase service is by employing a motor generator called a rotary converter or some solid state equivilant.
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need
will
above
by
Thinking of 2 single phase transformers connected to the 3 phase supply with the primary windings connected in open delta configuration - 1 phase connection of each commoned to one phase of the supply; and the other phase connections, connected to the other phases. The secondary windings connected in series, for the single phase supply.
Regards
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John McLean wrote:

This is really a single-phase situation, not three phase. You have a single-phase transformer with two windings in series and two equal winding voltages 120 degrees apart. The result is a third equal voltage at 120 degrees to the other two.
Since your load is single phase, the line currents will be equal and 180 degrees out of phase on the two secondary lines and on the corresponding two primary phases, and will be zero on the common phase. There is no advantage to the two transformers connected open delta. You can simply use one transformer with appropriate kVA and voltage ratings across two of the phases instead, and get the same line current.
Each transformer presents its rated impedance, reflected to the primary side, across two of the primary phases. Since the secondaries are in series, the total impedance seen by the load will be the sum of the two transformer impedances reflected to the secondary side.
Ben Miller
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secondary.
is
state
with
phase
connected
Please don't think that way.
First read up on what 3 phase is. http://en.wikipedia.org/wiki/Three-phase_electric_power
http://www.answers.com/topic/single-phase-electric-power
http://science.howstuffworks.com/power3.htm
Perhaps if you stated what you wanted to accomplish someone here could suggest a practical way to achieve it.
However, at 75,000 V-A most are probably going to say it's time to consult a local master electrician, licensed engineer, and the local power company.
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It should be possible to take an ordinary two leg core such as can be made from L shaped laminations to make an open delta primary to a single phase secondary.
Although variations are possible, put two primary windings on one leg to be connected in open delta. If both windings are wound in the same direction, connect the delta's apex (V corner) to one end of one winding and to the far end of the other winding. Then connect the two phases to the remaining ends of the windings. Doing this will provide a 60 degree phase shift between the mmf's generated buy the two windings. This will provide about 86.6% or cos(30), of the voltage capability you could get with the two windings in series as a single phase winding.
Now just put a secondary winding on the other leg.
Again, I have no idea why someone would want to do that.
Bill
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----------------------------
wrote:

---------------- Neither would power engineers- that's why they don't bother. Much easier to put a standard single phase transformer between two legs of the supply or between line and neutral of a Y connected system-- as most single phase transformers are connected. For some very large single phase loads the rotary converter can be used but this is something I have not seen and is uncommon. What you suggest is not considered open delta which is a 3 phase configuration with two transformers -essentially a delta connection with one transformer removed.
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wrote:

I am just trying to describe the kind of thing mentioned by the OP. You c oould use separate single phase transformers with the secondaries connected in series to give a zig-zag connection with a 60 shift between the secondary windings' outputs.
I feel somewhat stupid belaboring a stupid transformer design. It would take an amazingly provocative reply for me to respond.
Bill
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one
The rotary converters I have seen have been at remote locations that only have single phase service and are used to operate 3 phase equipment.
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----------------------------

That makes much more sense than going the other way with a rotary converter.
Actually a nice scheme involves use of a 3 phase induction motor which is energised on one phase and started by whatever means is available (rope on a pulley or a small single phase motor) and, as a result, rotor mmf induces voltage in the other phases giving a nearly balanced 3 phase supply to drive smaller motors. I first heard of this from a farmer who did this with a scrounged or surplus machine, and, because of some problems after a move, asked for advice. My first reaction was "huh!" but after some analysis- I sure was impressed with him because this was the first that I had heard of this, and, fortunately, was able to help him. I don't think this hit the textbooks- it should have. A true rotary converter would be more efficient and better all round but in his case as well as many others, this gives a better bang for the buck. I did find that a local utility also applied this in a rural area where the only supply was single phase and this was the cheapest way to provide for a small 3 phase load.
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>

only
a
drive
the
efficient
this
I suspect that the cost + efficiency loss of the rotary converters would be made up over time in less maintenance costs due to a certain amount of surge immunity such a system provides to the load. The converters should laugh off fast transients. That is of course assuming the load is not just more motors.... what equipment doesn't have associated electronics these days?
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----------------------------

------------
But, unless there is a need to go from single to 3 phase (which usually implies motor loads-with or without electronic control), maintenance and capital costs probably be lower be lower with proper solid state surge protection and filtering. Admittedly- getting a DC power supply from a polyphase source does cut down on smoothing filter costs.
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I converted a transmitter from three phase to single phase one time. I ran into a "gotcha". It turns out you should not neglect to increase the resistance of the step start resistors if you wish to avoid the main circuit breaker intermittently opening on turn on. The new power transformer was so heavy that the moving company I hired to deliver it broke the loading ramp in half.
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I cannot think of any advantage for such a conversion unless three-phase power is unavailable.
Bill
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----------------------------

Thank you- fair enough. There are few situations where I can see an advantage in converting from 3 phase to single phase-particularly for electronic power supplies. What was the overriding requirement for this? If it was for some remote location where 3 phase wasn't available, I can understand. What was the transformer rating? Note that I am looking at this from a power system and machine orientation rather than an electronics orientation so I may be speaking a different dialect of EE than you are. :)
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