# transformer star 3ph to open delta single

Discussion in 'General Electronics Chat' started by rays, May 3, 2013.

1. ### rays Thread Starter New Member

Mar 11, 2009
4
0
i bought a transformer to bump from 240v to 415v so I can run my vfd and supply my work shop with 3 phase power at 415v!
from what I can determin it has three primary windings origonally conected in star the imput was 3phase 415v simpl enough although there was a newtral conected to the star point and from the colour of the wires it looks as if there was a lot of power going through it?
but the seckondary windings are an open delta arangment
the center primary winding has no seckondary winding on it .
can some one tell me weather it will work in reverse enabling 240 single ph in 415v single phase out and what is the purpose of the center primary winding is?

2. ### Ramussons Active Member

May 3, 2013
562
92
NO. You cannot convert a Single Phase to Three Phase (or vice versa) WITHOUT active components (Rectifiers, Thyristors, etc). However, you can do so between 2 Phase and 3 Phase - the Scott Connection.
Normally, each Phase has it's own Core and the Cores are interlinked - like '8' of a 7 segment display; something like

---
| |
---
| |
---

where the --- are the Cores.
Each Core must have 2 windings, Primary and Sec.

240 V refers Phase to Nuetral and 415 refers Phase to Phase. In your case, since you have only a Single Phase Supply, you cannot derive 415 V Single Phase or Three Phase.

3. ### rays Thread Starter New Member

Mar 11, 2009
4
0
the vfd does the converting to 3 phase I am only trying to convert single phase 240v to a higher voltage 415v single phase as the vfd has a 3hase input that is directed to a rectifer bridge it is dc that is supplying the vfd and that is converted to 3 phase abit a square 3 phase
so long as the electronics are driven at the corect voltage the rest will operate at prity much any voltage
I curently am operating 3 other vfd on 240volt single phase to operate other 3 phase equipment
but on this vfd I want to get max torque from my delta wound 415v motors
the dont have the tappings to operate in star!
further info on the star delta trany
apon further investigation
it was operating on primary side three phase in star returning through the center newtral tap evedent from the discoloration of the copper in the newtral wire
it wood appear it was used to convert 3phase 415v(240v as phase return via newtral)into single phase 240v
so on the imput side there are 3 coils in a row the center coil having twice the windings as the 2 outer coils
the 2 outer coils have a seckondary winding on them conected in an open delta arangment providing 240v single phase
what would the wave look like on a scope?????
whare I am up to
if i conect the 2 seckondary windings in parralle
the 2 outer windings (primary )in series
use the seckondary side as the primary side
i should get 2 outputs one from the center winding @ 415v
and one from the 2 outer windings @415v
both on the same phase
then the 2 outer coils can be conected to the inner coil in parrelle
and hope fully not alter the kva rating much
any one any thoughts?

ps

sorry for the grammer spelling dont have much interest in it

4. ### Ramussons Active Member

May 3, 2013
562
92
From what I understand,

* You have a 3 Phase Bridge Rectifier feeding your vfd.
* Since you have only a Single Phase Input, you are constrained to a lower power
input to the vfd.
* You intend to increase the power input to the vfd by increasing the input voltage to
the bridge rectifier to 415 V from 230 V.

You cannot do this with what you have. The transformer you have cannot increase the voltage to 415 V from 230 V.

Ramesh

Last edited: May 6, 2013
5. ### rays Thread Starter New Member

Mar 11, 2009
4
0
well after trying many combinations I have reconfigured the wiring of the tranny it is now operating as an auto transformer 240v in 380v out close enough I hope