What core magnetic balance test of transformer?
How to do this test in a three phase Star delta (star-LV,
Delta-HV) transformer?
How to measure and where to measure?
thanks and regards
Austin

HI ,
this is rajukumar sripelli from REPUTED TRANSFORMER
MANUFACTURING COMPANY ..testing department ..

AS u all above is not fully correct....

Magnetic balance test is done on TRANSFORMER to see whether
the transformer core is magnetically balanced or
not...AND ALSO WINDING CONDITION....IN FAULT CONDITION DUE
TO HIGH CURRENTS WINDING MAY BE DAMAGE....

for example
we r having delta HV and star winding LV side ...

HV SIDE WE WIL BE HAVING THREE BUSHINGS....THOSE R 1U,1V,1W..
HV SIDE WE WIL BE HAVING FOUR BUSHINGS.....2U,2V,2W,2N...

WE CAN DO THIS MBT (MAGNETIC BALANCE TEST)WITH 1PHASE
SUPPLY(1PHASE AND NEUTRAL(230V)) OR WITH 2-PHASE
SUPPLY(2-PHASES(440V))

1ST STEP OF MBT: GIVE POWER SUPPLY 1-PHASE OR 2 PHASE
BETWEEN 1U AND 1V ,MEASURE THE VOLTAGES BETWEEN 1V-1W AND
1U-1W AND ALSO 2U-2N,2V-2N,2W-2N..

SO HERE IF THE CORE IS CORRECTLY BALANCED THEN WE WILL GET
RESULTS AS FOLLOWS

THAT MEANS HV SIDE : 1U-1V = 1V-1W + 1U-1W
I.E 440V = 300 + 140

AND ALSO LV SIDE : 2U-2N = 2V-2N + 2W-2N

90 = 50 + 40

STEP 2: AFTER DOING THIS ..INTERCHANGE THE CONNECTIONS
BETWEEN 1V-1W AND MEASURE THE VALUES AS ABOVE...

STEP 3: AFTER DOING THIS ..INTER CHANGE THE CONNECTIONS
BETWEEN 1U-1W AND MEASURE THE VALUES AS ABOVE...
IF ANY FAULT IS THERE IN TRANSFORMER WE CAN NOT FIND GOOD
RESULTS....
IF THE WINDING IS DAMAGED .... WE WILL GET ZERO VALUES ....

ex: HV SIDE 1U-1V = 1V-1W + 1U-1W
440V = 440 + 0

AND ALSO LV SIDE : 2U-2N = 2V-2N + 2W-2N

90 = 90 + 0
THIS IS THE ACTUAL APPLICATION OF MANGNETICA BALANCE TEST...

Answer #5 is fine.. but i want to add something more..
normally this test is made in star side of the transformer.
the result should be varified like following:
1) if 2u-2n is fed by 230 volt.
- the nearest phase (2v-2n) will experience the 85% to 90%
of fed voltage at 2u-2n. (i mean 2v-2n = 195.5 to 207
volt). And the distant phase (2w-2n) will experience the
10% to 15% of fed voltage at 2u-2n. (i mean 2w-2n = 23 to
34.5 volt).
2) if 2v-2n is fed by 230 volt.
- then the fed pahase is middle phase, and both other phase
are magnetically in equal distance.. so the measured
voltages in 2u-2n or 2w-2n = 50 % (or 40 to 60 % is also
accaptable). so the 2u-2n or 2w-2n = 92v to 138v is
accaptable.
3) if 2w-2n is fed by 230 volt.
- the nearest phase (2v-2n) will experience the 85% to 90%
of fed voltage at 2w-2n. (i mean 2v-2n = 195.5 to 207
volt). And the distant phase (2u-2n) will experience the
10% to 15% of fed voltage at 2u-2n. (i mean 2w-2n = 23 to
34.5 volt).

*If the results are found beyond these above mentioned
limits then other tests (like open ckt test with rated
voltage) should be performed to be sure about core and
winding.
*You can measure and note down the current of each injected
phase. it will give u a signaficant idea about winding. if
the winding has an inter-tern short then the faulty phase
will draw comapretively large magnetizing current like 3A
or 5A, wheather other phases will draw 60mA to 100mA.
*Remember, the magnetic reluctance path of a core is not
lenier, so the result veries itself. But big differences
are remarkable.
*Sometime due to non-lenier characteristic of magnatic
circuit and the presense of harmonics in the fluxes of un-
excited phases, the algebric sum of induced volage in two
phases may not be equal to the applied voltage.
*if there is a turn to turn fault in any phase winding, the
faulty section will draw comparetively large magnatizing
current which will oppose the magnetic fluxes and will
result a very low induced voltage (tends to zero) in the
faulty phase.
*this test should be performed after demagnaizing the
transformer properly. slightly magnatized transformer (like
after insulation test) can give you an unexpacted and
distorted result of magnetic balance test.

Provide 2-phase supply on the HV side of transformer on two
phases.
Then measure magnetizing current in the supplied phases.
Then measure the voltages on the LV side.
Repeat the same for the other phases and measure current
and LV corresponding voltages.

Hi,
You may use either single-pase on the star side of the
transformer as in Answer #2 or apply two line voltages i.e.
apply 415V on A and B terminals on the HV-side of the
transformer and follow the steps of Rajukumar Sripelli in
Answer #5.

in case of star-delta trafo
if you apply single phase supply on primary 1u-1n then the
voltage should be divided to 1v-1n(70%) and 1w-1n(30%).
this result shows the correct magnetic coupling between the
windings.
example:
apply single phase supply to 1u-1n----100%(230v approx)
measure voltage in 1v-1n and 1w-1n then you will get
70% of 230V in 1v-1n and 30% of 230V in 1w-1n approximately.

apply single phase supply to 1v-1n-----100%(230v approx)
measure voltage in 1u-1n and 1w-1n then you will get 50% in
each.

apply single phase voltage to 1w-1n-----100%9230v approx)
measure voltage in 1u-1n and 1v-1n then you will get 30% of
230v in 1u-1n and 70% in 1v-1n approximately.

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