Testing a Potential Transformer

Thank you Fredski !

I was only given the transformers to test , they were brought from the site after the complain .. I am not much aware of the rest of the circuit , i gave my boss all this data and he told me that though your report has lots of data but you haven't said anything about , the transformers are good or bad , i am begginer and i have not much experience in transformer testing so i wanted to collect more and more data before reaching any conclusion ...

I suspect you just need higher rated primary fuses to accommodate the inrush current. Also measure both the primary and secondary amps when the transformer is loaded.

Past experience states; You need to test the transformer at its full load rated capacity and measure the temperature rise, say over a 24 hr. period to validate its functionality. You may use the following setup;

1) Using an incandescent lamp rated at 120V, 150 W, as your load, with an ammeter in series to measure the secondary current of the transformer.

2) You will also need a properly rated variac, (variable transformer) connected in the primary side. You need the ability to vary the input voltage to primary side of the transformer during this test.

3) using the variac slowly adjust the primary voltage until the secondary ammeter reading indicates the equivalent rated current capacity of the transformer.. at this point you may note that the primary input voltage will be lower than 480V rating of your device. This is normal since you are using a 50 Hertz supply!

4) At this point using the the ambient temp as reference you can start measuring the temperature rise of the transformer core. It is still normal if the transformer register a temperature rise of about 80 degrees centigrade over the duration of this test.. Note that operating at 50 Hertz will also contribute to additional heat rise!

Good luck!

Mr. Tyagi, There is no problem with your transformer. Only use Delayed type HRC fuses or glass fuses. You find these in three ratings, slow or delayed, Fast and Ultrafast. I remember we used to get DZ type delayed fuses from Siemens. You can refer to catalogue of any fuse manufacturer and select the delayed type - hopefully your problem will be solved.

There will be a problem if you keep trying to run a 60Hz transformer on a 50Hz supply. Notice how much lighter the 60Hz transformer is. Wonder why? Consider what this might do to the transformer - in particular the current ...........

Problem is with 0.5A fuse which may be a fast acting fuse and can blow off while in-rush current is applied in short period of 10ms-100ms. If slow blow fuse is used then this problem may not come. Perhaps load is a switching power supply with may be consuming much higher inrush current than what you have stated here. Ask the user to use PTC or resettable fuse of 500mA

The purpose of the fuse is to protect the wiring. It is not there to protect the transformer!

The purpose of the fuse depends on where it is located within the circuit.

If the fuse is on/in the primary side of a transformer it's protection function includes the transformer as well as the wire.

I often come across the same type of issue as listed in the OP.

I quite regularly get calls because a fault occurs, the equipment that caused the fault may or may not be removed and/or replaced, and then the technician either does not know the difference between fuse types or does not have the correct fuse on hand so they substitute a quick acting fuse where a delayed fuse is required or vice-versa and the mystery saga begins.

Many times the original technician is not sent back to the site and instead a second technician replaces the fuse with whatever was/is existing when they accessed the site so the problem takes a long time to actually get properly addressed.

Often I am called to try and put the smoke back into the transformer and/or other ambient equipment because at some point a technician in their frustration installed a fuse that is considerably too large and therfore does not protect anything in the downstream circuitry.

I am a firm believer that especially in today's workplace of hastily trained technicians that instead of depending on memory we need to start intensive labeling of all equipment so that these errors are minimized and hopefully eliminated.

Dear Mr.Mukultyagi,

Your Tst Results indicate - NOTHING WRONG IN THE TRANSFORMER.

Problem is on the Load side, and that circuit is to be checked. Perhaps it may be close to dead short - Circuit Continuity test may indicate the problem.

Pl. post the information, how the problem was solved.

DHAYANANDHAN.S

The primary winding fuse is mainly to isolate a faulty VT from the system.

From your data & tests this VT is a Single Phase line to line 480/120v 150VA Class 1(+/-1% ratio error permitted).

Using ohms law it can be determined from the above that a internal fault on the VT would probably be of the order of 700Amps IE:Z=VD^2/VA & If=V/Z.

The HV fuse should be about 20Amp rated to operate in +/- 10ms for 700Amps.

A far cry from the 0.5 amp presently rupturing.

The secondary fuse can be 10A to allow grading for sub secondary circuits..

For a 480/120 transformer of 150VA, the rated primary current is 0.3125A, and the rated secondary current is 1.25A.

Since the rated primary current <2A, the NEC (U.S. National Electrical Code) allows the primary fuses to be 300% or the next standard size up, which would be 1A.

The secondary fuse may be 125%, which would be 1.5625A. A 1.6A fuse would be a normal choice.

I concur with Mr. Tornado's analysis..

Either that or the OP may use a "Slow Blow" or a "Time Delay" type fuse rated at 0.5A fuse at the primary.. whichever is available..

vsar