2.33 STRX - Stress Watts (External Source)
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
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 AT.
This test is suitable for large line frequency bobbin wound and some HF
It checks the integrity of inter-turn insulation, the magnetic material
and joints at higher than the normal operating voltage.
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: The
winding insulation is stressed by the use of the increased voltage
(increased volts per turn).
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.
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 ATs internal generator, which can
provide up to 270V ac at 25W.