VOLTECH AT3600 USER MANUAL TRANSFORMER TESTS
TESTS AND TEST CONDITIONS VPN 98 - 024 PAGE 7.1.55.
1.33. STRX – STRESS WATTS (EXTERNAL SOURCE)
WHERE USED
The Stress Watts (External Source) test tests the transformer for breakdown of
winding insulation by measurement of the energising power of a winding, usually the
primary, with the remaining windings open circuit. This test applies the voltage from
an external ac source (usually at twice the normal operating voltage and frequency),
and measures the power drawn.
This 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.
This test is suitable for large line frequency bobbin wound and some HF
transformers. It checks the integrity of inter-turn insulation, the magnetic material and
joints at higher than the normal operating voltage.
MEASUREMENTS CONDITIONS
A constant voltage (supplied by an external source) is applied to the winding under
test. Both the rms voltage and the power are measured. If necessary (and depending
on the source type used), the voltage can be trimmed to the user specified value, and
the measurement repeated. The measurement is repeated many times until the dwell
time has elapsed. The result given is the measured power.
The test can detect winding insulation defects in the following way:
1. The winding insulation is stressed by the use of the increased voltage (increased
volts per turn).
2. Increasing the frequency in proportion to the voltage increase ensures that the
magnetic core of the transformer is exercised over the same area as it would be at
the normal operating voltage and frequency. - The core losses remain the same.
3. A significant increase in the power drawn when tested at increased voltage and
frequency indicates a failure of the winding insulation that would not be detected
at normal operating conditions.
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 STRW test which uses the
AT3600s internal generator which can provide up to 270V ac at 40W.
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