Testing of Transformer, Polarity test of Transformer, Phasing out test of Transformer

(A) Transformer Polarity Test

• The H.V. and L.V. terminals of a single phase and three phase transformer should always be marked as shown in Figure (IS 2026-1962).

• For a 3 phase transformer, viewed from H.V. side the arrangement of both sets of terminals shall be in alphabetical order from left to right.
• If neutral is provided , it should be on the extreme left end. As per IS 2026, 1977 (Revised I.S.) the phase marking u, v, w, is adopted in place of a, b, c in line with the international practice.
• In case of 10 transformers the terminal subscript numbers should be arranged in descending order from left to right.
• Capital letters are used to indicate H.V. winding small letters are used to indicate L.V. winding. 
• The identical suffixes on H.V. and L.V. side represent identical polarity. i.e. at any instant in a cycle if A1 is positive then a1 must be positive and at the same instant A2 and a2 both must be negative.
• To find polarity correctly, following test is performed.
• The connections are made as shown in Figure (a) i.e. two windings are connected in series assuming that upper pair and lower pair have like polarities.
• A.C. voltage is applied A1 – A2 terminals of Transformer . The voltage across terminals A1 and a2 is measured by voltmeter (V). If V > E1 it clearly indicates that both the emfs induced E1 and E2 are additive. So assumed polarities are correct.
• If in case it is not so then polarity marking is not correct. So marking of any one side are to be changed. Test is again carried out by making connections as shown in Figure (b) to confirm the correct polarity. In this case V should be less than E1 (i.e. V < E1).

(B) Transformer Phasing Out Test
Q. With the help of neat figure explain phasing out test carried out on transformer.

• This test is carried out only on 3 phase transformers to identify primary and secondary winding belonging to the same phase.
• As shown in Figure 2 all the phases of a 3 phase transformer are short circuited except a primary and a supposed secondary.
• A low voltage d.c. supply is given through a battery to the primary winding.
• The galvanometer is connected to the terminals of secondary winding which is not shorted.
• The key ‘K’ is connected as shown. Now the key is pressed and deflection of galvanometer is observed carefully.

• Similarly galvanometer is connected to the other secondaries and procedure is repeated.
• The winding across which maximum deflection occurs is the secondary phase winding that corresponds to primary winding to which source is connected.
• Repeat the same procedure for other primary windings and find the corresponding winding on secondary side.