Transformer Potential to Ground

[serbaguna]

Hi Guys,

I need your advice.I have an old Dry Type Transformer 5KVA,L-L 460/240 Vac,1 Phase,Isolated Winding Type of Transformer...say this is Transformer A.

Without Load,I measured the Voltage as follows:

Primary Voltage: 460 Vac L-L
Secondary Side;
L1-L2: 240Vac
L1-Ground( Transformer Metal frame or Housing): 80 Vac!
L2-Ground : 240 Vac!

I mentioned the Ground as the Metal Frame of the transformer( There is no electrical connection between the Primary or Secondary line to earth or ground)

In my very limited knowledge about Transformer,I thought I should only get half of the output Voltage which is about 120 Volts for each Line-ground potential, please correct me if I am wrong.

I decided to test the insulation for Primary and Secondary Winding and found no problem at all with insulation,(Megger with 500 VDC is above 10Megaohm result)

I compared it with other identical specs and type of transformer...say this is Transformer B:

Primary:460 Vac L-L
Secondary output is 240 Vac L1-L2
L1-Ground is 80Vac
L2-Ground is 170 Vac.

With load or no load, I've got the same readings for both Transformer.

Please comments for the following questions:

1.Is there something wrong with these 2 transformers?

2.Is failure of Transformer winding about to happen?

3.What may caused this unbalance ground potential?

The Transformer is used to supply a bridge rectifier for DC Motor Field Excitation.

I will appreciate technical explanation or suggestions that may clear my doubt,I apologize if you find my query a very basic one.

Thank you and more power to this site! Topic and knowledge sharing is very impressive on this site.You're doing a good deeds guys, keep it up!


Best regards,

Serb

 

[ScottyUK]

Why do you think a voltage measurement to ground has any real meaning? The winding has no connection to ground, except through whatever leakage resistance and capacitance paths exist by virtue of the transformer design, so you have no reference. It is akin to holding one probe on the grounded metal and holding the other in the air and trying to make sense of whatever reading you get. If you made a solid connection of one end of the winding to ground then you could make a meaningful measurement of the voltage at the other end to ground. With no ground connection the measurement becomes prone to stray currents in leakage paths and capacitance; the current taken by the meter itself is significant acts as a connection to ground which affects the readings.

You would find more meaningful information about transformer insulation condition if you test between the winding and ground using a Megger. In all probability there is nothing wrong with the transformer, but the Megger test will prove the isolation between the windings and between either winding and ground. You should have three test results: primary-ground; secondary-ground; primary-secondary. All should be in the high M[Ω].



----------------------------------

Sometimes I only open my mouth to swap feet...