6.8 Stress Wattage (STRW)
At no load, and with its secondaries open circuited, a transformer will still draw current and consume power as described in the WATT test. If the voltage and frequency applied to the transformer are changed together and in proportion, Faraday’s law shows that the flux density(B) in the core will remain the same.
B V / (f x A x N)
Where N is the number of turns and A is the cross sectional area of the core. Over a limited range, the core loss (in Watts) should also remain the same as voltage and frequency are changed. Should the input power increase dramatically, then a winding fault is indicated.
The stress watts test is used to check for faults in inter turn insulation within a winding. The test may be used on transformers wound with very fine wire (where the wire insulation is more susceptible to damage) such as miniature line frequency and some high frequency transformers. See also the STRX test for higher power measurements using an external AC source.