2.35 VOCX - O/C Voltage (External Source)
The AT Tester 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 AT tester.
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 ATs internal generator, which can provide up
to 270V ac at 25W.
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.