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Topic Title: 420vac to 110vac transformer
Topic Summary: 2 phases (400vac) supply and output 110vac
Created On: 21 June 2013 08:59 PM
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 21 June 2013 08:59 PM
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Joined: 21 June 2013

Hi Everyone,

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?
 22 June 2013 01:32 AM
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Joined: 14 September 2010

If you have two phases on the secondary, it is a centre-tapped split phase; ratio 400-220

Wired thus
Primary winding
Two phase conductors are connected to either end of the primary winding.

Secondary 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
 22 June 2013 02:24 PM
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Joined: 22 November 2007

Is it a neutral (ie referenced to earth) or just the otherside of the winding ??

400-110v transformers are very common in control panels as they allow the control circuits to be at a lower voltage without the need for a supply neutral. These are often found in pump control panels along with 400-24v transformers to provide low voltages to float switches and the like whilst maintaining a 3 phase (400v) supply to the pump motors.


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