Testing a potential transformer

My boss gave me two numbers ptq and pt2 , 480/120 volts potential transformer of 150 VA , 60 Hertz to test because it was blowing fuses .

I used 50 HZ supply for all my testing as 60 HZ is not available in the workshop .

i did the following tests , i did the ratio test and it gave results very close to the ratio is is designed for i.e 4:1

Sr.No APPLIED PRIMARY (VOLTS ) SECONDARY VOLTAGE (VOLTS ) VOLTAGE RATIO
1 50 12.56 3.98:1
2 100 25.23 3.96:1
3 150 37.78 3.97:1
4 200 50.36 3.97:1
5 256 64.4 3.97:1
I tested the DC Resistance of the primary winding and the secondary windings and it came out to be

Primary winding resistance (H1, H2) - 16.2 Ω

Secondary winding Resistance (X1, X2) - 1.5 Ω

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Conducted Insulation test @ 1000 volts and obtained the below mentioned values:

a) Between primary and secondary : 2.2 GΩ

b) Between primary and ground : 2.2 G Ω

c) Between secondary and the ground : 2.2 G Ω

also i measured the inductance of the both primary and the secondary winding , the winding not being tested was shorted while measuring the inducatance

i have forgotten the exact value but there was a difference of 1.9 mHenry between the primary of pt 1 and pt2 . the inductance of the secondary windings is the same .

i have also measured the inrush current for the pt primary keeping the secondary open , it came out to be 1.28 A for pt1 and 1.31 A for pt2 , i beleive tha fuse being used is of .5 ampere

Please suggest me what other tests i can do .

are these pt good ?

and while doing the inrush current test i applied 380 volts @ 50 Hz , how do i determine the correct voltage while working on 50 Hz , because the transformer is designed for 60 HZ, what care should i take to avoide damage to the equipment

Stationary input current without load would be more interesting than "inrush current" (how did you measure it excatly?).

A transformer can show a short (e.g. 100 ms) inrush current due to temporary saturation.
If fused at rated current, the fuse must be slow blow type to avoid inadvertent tripping.

One problem which may be hard to measure directly is a shorted turn.

The link below has several webpages which discuss transformers.

http://sound.westhost.com/xfmr2.htm

It states:

"If any two turns of a winding short to each other, the current flow is limited only by the DC resistance of the shorted section of the winding. The current flow is enormous, and with even one shorted turn, the transformer is no longer serviceable and must be discarded or rewound."

From the link above:

"A transformer rated for 50Hz can be used anywhere in the world - it will work perfectly at 60Hz. However, the converse is not true. A transformer designed specifically for 60Hz will overheat at 50Hz, even if the voltage is correct!"

Testing a transformer like you did should include two levels: 1. Testing without load, and 2. Testing with a nominal load.

As 1. connect the nominal primary voltage and monitor the primary current and winding temperature. A 60 Hz transformer may overheat even if not loaded. Set a limit at 60 deg.C; if the device without load survives 6...12 hours under 60 deg.C, OK.

Test the nominal loading distributed to the secondaries according to wire gauge. If running a 60 Hz transformer on 50 Hz line, reduce the load by 20-25% and gain monitor the primary current and temperature. Under a full load the temperature may rise to 70-80 deg.C but do not allow more. If the temperature rises slowly, try a fan cooling.

A good 150 VA transformer takes 10-20 W for loss, mostly in the core if unloaded. With the load, the loss may rise to 25-30 W. Take care not to overcurrent the secondary wires, and check the primary current (1.0 - 1.6 A at 120 VAC). Monitor primary current changes with rising temperature. If it rises abruptly, disconnect and discard the transformer. Such rise indicates a short in a winding, only repairable by rewinding.

The cause of blowing fuses may be in a poor fuse rating. Use slow-blow fuses, 150% above the nominal current under load.