7.1.33 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 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.
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 AT3600s internal generator which can provide up to 270V ac at 40W.