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# Re: Single phase loads on Open Delta

phil-news-nospam@ipal.net
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 02-19-2008, 02:22 PM
On Tue, 19 Feb 2008 00:51:13 -0800 (PST) (E-Mail Removed) wrote:
| Hi everyone,
|
| Im an electrical engineer in europe,
| But now I have to design an Electrical installation where they have a
| so called "Open Delta" connection.
|
| I believe it is the same as our 3-phase delta system, exept that one
| leg of the transormer is 'missing".
| Theoretical I understand the principle, but how to use it in practice?
|
| In the installation there are several pumps (3-phase electrical
| motors) wich should not be any problem,
|
| But there are also a lot of single phase consumers
| Computers 100% duty cycle,
| 24VDC generators 100% duty cycle
| Heaters (On/Off switching depending of temp demand)
| coils
| eetc.
|
| How should I devide these consumers between the 3 phases.
| a) equaly over all 3 phases
| or
| b) so that all single phase loads are wired the the phisical windings
| (not using the missing leg of the open delta connection)
|
| Can onyone help me with this situation,

The open delta does have one side of the delta triangle missing. What that
means for loading is that any loads connected across the open side will use
current through both of the other two secondary windings that are present.

For a single phase load connected on the open end, you have to derate the
capacity of the transformer to one half because each winding can have only
that original current maximum, but you are only getting the regular voltage
while using two windings in "series".

For a three phase balanced load, you have to calculate what the current is
through the windings based on the vector sum of the three phases of load.

Normally, you would connect the single phase loads to only the sides that
have windings, and not to the open side. Three phase loads connect as if
the configuration were a closed delta. Now you just need to add up all of
the currents, using the correct vector formulas, for the two windings that
are present. Once you have the current in each winding, select the one
that is larger and double it to use two identical windings, and multiply
by the voltage (230 or 400 or whatever it is you will be using). Maybe
you will want to go back and rebalance the single phase loads to reduce
the current on the winding with the most.

You will also need to calculate the current on the common corner wire of
the system, which would be the vector difference of the current in the
two windings (which, because the vector angles are quite different, will
be a lot). With all three phase loads it will be the same as the other
two wires. With all single phase loads in balance (and not connected
across the open end), it will be twice as much as each end wire. The mix
of single phase and three phase loads will be somewhere in between.

Connecting single phase loads across the open end would normally not be
done. But if you have more transformer capacity and less neutral/common
wire capacity, connecting single phase loads across the open end would
be a way to lessen the loading on that wire, at the expense of excess

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-02-19-(E-Mail Removed) |
|------------------------------------/-------------------------------------|

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