TRANSFORMER 240 to N

[Erick D]

Hello,

I have a system that requires 480 VAC and 230 VAC for certain components. The transformer that came with the machine went bad and I am trying to replace it (MURR ELEKTRIK 86147). I got a a different brand replacement. I am hooking up the 480 to the transformer and I am measuring 256 VAC output. The question where I am not sure is that the output it supposed to be 240 to N. When I measure one output of the transformer lets say X4 to ground (earth) I measure 120, and when I measure the other leg I also measure 120. So leg to leg I measure 240. The input to the transformer is 3 phase 480 (not sure if it is Y or Delta) and we grabbed to legs of the 480 to power the transformer. Is this correct? The schematic for the equipment I bought states that the output of the transformer should be 240 to N. If I decide to hook it up does it matter where I put the white cable (neutral) and the red cable (hot)?

Thank you very much in advance.

 

[itsmoked]

Your question is a bit muddled and we don't have the schematic to look at so we can't say for sure.

Generally if you need 240V and have 480V you apply the 480 across the primary which is wired:
480L1 to H1
H2 jumped to H3
480VL2 to H4

You then use the secondary:
240L1 to X1
X2 jumped to X3
240L2 to X4

The secondary of this control transformer should be tied (Bonded) on one side X1 or X4 to earth ground. This is because you've created a "separately derived source" which means a new isolated power source. Either or both legs of it while always being 240V between them could actually rise to 480V from ground via various "leakage scenarios". This means downstream electrical apparatus could fail seeing this 480V to earth difference. (including 300V rated wire)

This bonding can be done to either X1 or X4 in your case. Which ever one you choose (X1 suggested) will become the Neutral of this new separately derived source.

One caution.. You need to be very careful you don't "bond" the other side of the secondary if it's already bonded elsewhere in the system.

I advise with no power you disconnect both primary leads X1 and X4 and measure the flying leads to ground with an ohm meter to see if one side is already bonded to ground/earth somewhere else in the system.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Did you measure the 480 Volts or did you just ASS U ME that it was 480 Volts exactly?
A 240 Volt transformer output with 6.7% regulation will drop to 240 Volts at full load.
You are most likely seeing a combination of no load regulation and slightly high voltage on the 480 Volt side.
Follow itsmoked's advice.
Keith has given instructions based on a 240/480:120/240 Volt transformer.
If yours is different let us know and we will give instructions.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Erick D]

Great!! Thank you very much for the help. It is working great. I just did not know understand why the output of the 240 side was tied to neutral.

Really appreciated it.

 

[7anoter4]

The transformer 480/240 V it has to be single phase with secondary middle point grounded.
The supply line is 3*480 V but this transformer primary is feed from two phases of 480 [between] as you already said. Two phases it is actually a single phase.
According to ANSI C84.1-2016 voltage range A it has to be from 0.9*240=216V up to 1.05*240=252 V.
So 256 it is more than permissible even for range B.
I don't know for sure but some transformers are provided with tapped winding which may be used in order to reduce the secondary voltage.
However ,check the 480 V-supply voltage- if it in the range A [Not more than 1.05*480=504 V].
In this case the Utility has to fix it as soon as possible.

 

[waross]

The white, neutral wire, is always connected to the grounded terminal (X1 suggested). The red wire is connected to the other terminal (X4 suggested).

Bill
--------------------
"Why not the best?"
Jimmy Carter