7.1.37 VOCX - O/C Voltage (External Source)
The AT3600 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 (MAGI or MAGX) in your program, to give confidence that the absolute number of turns is correct as well as the ratio.
This version of the VOC test uses an external source to provide the test signal which must be coupled to the tester via a Voltech AC Source Interface (contact your supplier for details). See the AC Source Interface user manual or the Editor help system for details of how to configure external ac sources for use with an AT3600.
Open circuit voltage is measured by applying an ac test voltage (supplied by an external source) 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 (VOCX) 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 (MAGX) test which has the same energised winding and the same test voltage and frequency, place this immediately before the first open circuit voltage test.
The test signal can have a frequency in the range 20Hz to 5KHz, and an amplitude from 5V to 600V depending on the type of external source used. The current rating of a single AC Source Interface is 10A rms. See also the MAGI test which uses the AT3600s internal generator which can provide up to 270V ac at 40W.
SPECIFYING THE TEST LIMITS
The AT3600 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.
While the AT3600 is able to test transformers fitted with a rectifying diode, a capacitor must not be fitted as part of the rectifying circuit.
Any charge remaining in the capacitor could damage the AT3600's relay matrix at the end of the test.
To give the best accuracy, the dc measurement is averaged over the period of the energization frequency.
In addition, the VOCX test can be used to test the polarity (or phasing) of the windings, if this is required. This is not available when some types of external source.