Joined: 21 June 2013
I was wanting to know if there is anyone who can help me with a little prob.
I opened a control panel and saw a 400vac (3 phase supply) +PE into the panel but no neutral.. Now L1 and L2 were doubled up and wired into the supply of a smallish transformer and the output voltage is 110vac and there is also a neutral supplied now..
How can two phases (brown and black 400vac when tested across) wired into a transformer give an output of 110vac with neutral?
Joined: 14 September 2010
If you have two phases on the secondary, it is a centre-tapped split phase; ratio 400-220
Two phase conductors are connected to either end of the primary winding.
Two phase conductors are connected to either end, and neutral tapped to the centre point.
On the LV you get 110V phase voltage and 220V line voltage. (the angle between the two LV phases is 180°)
This would be slightly unusual for these voltages.
It is quite common to see this on 11kV - 480V transformers (i.e. 240 phase voltage, 480 line voltage), especially on spurs in rural areas feeding a limited number of consumers. A 2-wire split phase HV spur is cheaper to install than 3-wire 3-phase.
If just one phase on the secondary, it is a simple step down, ratio 400-110
Primary winding - as above
Secondary - phase taken from one end of the secondary winding, neutral from the other end.
Edited: 22 June 2013 at 01:26 PM by Zuiko