2.31 VOC - Open Circuit Voltage.

Open circuit voltage is the voltage appearing across a secondary winding when the primary is energized at a specified voltage and frequency, with the secondary at no-load.

The voltage is dependent not only on the turns ratio of the transformer, but also on the voltage drop in the primary winding due to the magnetizing current.

Example: -

Where Used

The AT offers two basic alternative ways to confirm that the transformer has been assembled properly, with the appropriate number of primary and secondary turns.

Open circuit voltage measurements are the preferred tests for line frequency transformers, designed to operate over the full extent of the B-H curve, including the non-linear regions. (For other transformers, such as pulse transformers and those used in switched mode power supplies, a measurement of turns ratio is the preferred test.)

Clearly an open circuit voltage test cannot tell you the actual number of turns on a winding, only the ratio between one winding and the next. You should therefore also include a magnetising current test in your program, to give confidence that the absolute number of turns is correct as well as the ratio.

Measurement Conditions

Open circuit voltage is measured by applying an ac test voltage to a selected winding (usually a primary winding), and measuring the resulting voltage produced on another winding.

If, there are several windings to be tested, the program will execute more quickly if the following points are observed: -

Place all the open circuit voltage (VOC) tests consecutively at the same point in the program.

Use the same energised winding, with the same test voltage and frequency for each test.

If there is a magnetising current (MAGI) test which has the same energised winding and the same test voltage and frequency, place this immediately before the first open circuit voltage test.

Specifying the Test Limits

The AT offers you three ways to specify the test limits: -
Using a normal AC (rms) measurement.
Using a rectified (mean) measurement.
Using a DC (mean) measurement.
Generally, the AC (rms) value would be used, but you could use the rectified (mean) or DC (mean) measurements if, for example, you are testing transformers fitted with a rectifying diode.