First let me say hello to all.

Know I have an issue with a 480 step down transformer. 115 230 115 secondary. I have 480 across the primary and 277 on each leg to ground on the scondary I have 110 and 110 the problem is the 230. Across the legs I have 230 to ground I have 200 on one leg and 60 on the another. The 230 is powering 3heaters in parallel. Can someone help me with the why and theory behind the cause?

Thank You
Shaun

I cannot make sense of your data. Punctuation would help I think.

Shaun:

First, Welcome to ECN Forums from one of the 'Jersey Guys'!

Secondly, in order for the members here to help you, re-post your questions with more details.

Something like this......

480 volt transformer, single or three phase??
Primary voltage is 480 volts
Secondary voltage is 120/240 (nominal voltage) single or three phase??

Your secondary voltages are....(phase to phase) and (phase to neutral)??

Try that please

Ok lets try this. the primary is single phase 480
secondary is 115/230/115. The 115 reads good phase to neutral/ground. The 230 has no neutral. phase to phase I get 230 volts Phase to ground I get 200 volts on one leg and 60 volts on the other. My questions are how is this possible? What causes this?

Some back ground on me I am maintenance technician. I am by no means an electrician. I enjoy working with electricity and respect it. When I get stumped I have to know the answer reason to the problem so it does not happen again. My company is perfectly happy with a just is answer however I need to know.

I wonder if that is a 3 phase transformer, or maybe an auto transformer.

I think you "Hit The Target" on this issue, as it _appears_ to be a Tapped Secondary Winding... however..., the following statement has me Corn-Fused :



shaunkad;

I have some Questions regarding the readings you have posted.

1: Is this Transformer a Typical Single Phase Isolation Transformer - with:

1a: a Single Winding Primary Coil (H1 & H2); and Split-Coil Secondary Windings - both Secondary Coils are connected in Series (X1''''''X2--X3''''''X4); (120V/240V 1 Phase 3 Wire)

OR...

1b: Does the Primary Side have a "Jumper" from One Terminal - H1 for example, to a Terminal on the Secondary Side - X4 for example;

OR...

1c: Is the Secondary Side comprised of (3) Separate Windings, such as:

X1''''''X2 (120V), X3''''''X4 (240V), X5''''''X6 (120V)

2: Is the Secondary Side's "Neutral" Conductor Physically Bonded to the same Earth Reference as the Primary Circuit is? Is the Secondary Neutral Grounded at all? Is the only Equipment Grounding / Bonding Conductor the one from the Primary Circuit?

3a: If this Transformer is a "Center Tapped 3 Wire Secondary", are you measuring 115V between X1 and X2/X3, along with measuring 115V between X2/X3 and X4?

3b: Are you measuring 230V between X1 and X4, then measuring 200V between X1 and "Ground", along with measuring 60V between X4 and "Ground"?

4: Are you using a High Input Impedance (HI Z) Voltmeter, set on the 0-250VAC Selection?

5: Is there any Load on the Transformer when these Tests are performed?

6: Are there any issues with Load(s) connected to the Secondary, of which prompted you to perform voltage Tests?

7: Do you know who/m Setup this Transformer?

First thoughts would be you have an Auto Transformer - maybe unintentionally (i.e.: incorrectly hooked up).

Other reasons for the odd measurements would be from the Secondary "Neutral" is not Ground Bonded, the Split-Coils are connected improperly, using a High Z Voltmeter across an "Open Neutral" connection point, or the Transformer's Secondary Coil has an additional Tap - a "25%/75%" Tap, along with the "50%" Center Tap (Common Neutral Point).

If there are no connected Loads on the 240V output (L-L), connect a 240V Load which draws at least 1 Amp.
Leave any 120V (L-N) loads off for now.
Measure the L-L, L-N and L-G Voltage with either a Low Z Voltmeter, or Shunt the Leads of the HI Z Voltmeter with an appropriately sized Resistor.

After performing the above Test, connect (2) 120V 60W Incandescent A19 Lamps on "Both" L-N Connections. Now measure Voltage.
If anything changes, make note of the variations (Voltage and Light output).

Lastly, record the Nameplate Information on this Transformer, and post it with your next reply.

If the Nameplate also includes a Schematic, does it look like any of the following?:


FIG. 1: 480V x 120/240V 1 Phase 3 Wire Isolated Transformer
---------------------------------------------------------


FIG. 2: 480V x 240V 1 Phase 2 Wire Isolated Transformer
----------------------------------------------------------


FIG. 3: 1 Phase Isolated Transformer; Two 2-Wire Secondaries
----------------------------------------------------------


FIG. 4: 1 Phase Isolated Transformer; Multiple 2-Wire Secondaries
----------------------------------------------------------


FIG. 5: 1 Phase Isolated Transformer; Multiple Tapped Secondary Windings.
----------------------------------------------------------

-- Scott

With a floating secondary, it becomes a voltage divider problem involving your meter's input impedance and any leakages and/or references to ground in the connected equipment. Any ground detection has to reference to ground in order to detect a ground. It is not unusual for the sum of the 2 voltages to ground to be more than the leg to leg voltage.
Joe

Please give me till Sunday I think the best thing to do is for me to take some pictures draw my readings on the picture. I may be using the wrong terms. they say a picture is worth a thousand words.

what program did you use for the transformer drawings

Shaun:

Take your time, most of us are around all the time.

Scott35 is our resident 'guru', he has tons of useful schematics here in the forum.

*** EL-BUMP'O ***

Forgot about this one!

Still curious as to the Transformer involved.

-- Scott