Tranformer Testing

[dkelect]

Drawing a blank !
What is the calculation needed to test for correct resistance on a tranformer without load?
I.E. 480 volt 150 VA single phase control transfomer with a 120 volt secondary.

[bennie]

ANSI/IEEE C-57 publication has the testing method for resistance measuring.

Basically it is with a Wheatstone bridge, or a battery, and voltmeter/ammeter configuration.

The standard is C57.12.91.1979

[bennie]

I am curious as to why you feel the correct resistance is necessary to know?

[charlie b]

If you are asking about theory, then the most common formula that is related to this question is that power equals (voltage squared) divided by resistance. That means that resistance will equal (voltage squared) divided by power.

R = (480 x 480) / (150) = 1536 ohms (as seen by the primary)
R = (120 x 120) / (150) = 96 ohms (as seen by the secondary)

Please note that the actual numerical values are approximate, simply because the 150VA, 480V, and 120V are “nominal values.” In order to determine the actual value of resistance for a specific coil, you need to use the test equipment that Bennie has described.

[rickcham]

You could also try a TTR test, Transformer Turns Ratio to determine if you have the correct winding ratio. This test equipment is easily rented
Only a thought

[bennie]

Dkelect:

No.#1. Is there other transformers available for comparing the readings?

No.#2. Check with the manufacturer, or go to a distributor and measure a new transformer.

No.#3. Turn this one on and perform a smoke test :D

[bennie]

I measured a 150 va, 480/120 volt transformer windings. Any resistance less than 27 ohms plus or minus 3, will indicate shorted turns. There will not be a higher reading except infinity. The lower resistance should produce smoke.

[ July 08, 2003, 07:59 PM: Message edited by: bennie ]

[bennie]

Shorted turns in a transformer are usually caused by a low side surge. The magnitude of the surge causes a layer to layer failure, involving many turns.

The surge can pass through the windings by capacity coupling and can affect the primary and secondary.

[ July 08, 2003, 07:47 PM: Message edited by: bennie ]

[dkelect]

Actual Transformer Testing Results:

We service a sewage treatment plant.
After a lightning strike that went into the MCC, we had (2) identical control transformers side by side with the fuses blown.
When testing the resistance on the primary side of these transformers (480/120V 150VA)one transformer measured 13ohms the other 27 ohms. We were trying to determine what the proper reading should be.

We did go ahead and energize the transformers.
the transformer with the 27 ohm reading appears to be working ok. The transformer with the 13 ohm reading blew the control fuse. We pulled this transformer and tested it on our bench at the shop.
After a period of testing (with higher amp fuses) the transformer coil opened up. (smoked)