STRW (stress watts) testing
The primary role of stress watts testing is to indicate a fault in the inter-turn insulation of a winding. This test can be used on windings with very thin wire. At no load and with the secondary open circuit a transformer will still draw current this current is proportional to the core losses (eddy currents and hysteresis). Hysteresis is the energy used up by changing the magnetic state of the core during each cycle and eddy currents are currents induced in the core by time varying fluxes. Faraday's law suggests that providing the voltage and frequency is increased proportionally core loss should remain the same. Therefore if a dramatic power increase was measured it would indicate that a winding fault was present.
Stress watts testing allows a line frequency transformer of 110V @ 60Hz to be tested at 220V @120Hz according to Faraday's law as discussed above the core losses should not change from 110V @ 60Hz to 220V @ 120Hz allowing double the voltage stress testing on the windings.
Figure 4 shows the parameters required for STRW on the secondary winding of the sample transformer.
The voltage and frequency levels have been selected as double the induced voltage and frequency with the maximum power set at 2 watts for a dwell period of 1 second.
Core losses are caused by the magnetising of the core and are always present. The way they are measured is by running full primary voltage with no load on the transformer.