How is the Surge Test waveform generated?
1. Inside the AT3600, a 3nF capacitor bank is charged up to the specified voltage.
2. A high high-voltage switch is then closed to discharge the capacitor into the winding under test, via a 45uH inductor. (The inductor protects the AT3600 from damage caused by very high currents should the output be a short circuit).
3. The resulting waveform (see attached file called “Functional schematic for SURG test” Fig 1) is that of a tuned circuit consisting of the capacitor, the 45uH and the inductance and capacitance of the winding under test. The waveform is damped by losses in the internal and connecting circuits, mostly determined by the resistance of the winding under test.
Because the waveform is determined in part by 'stray' impedances it is difficult to predict theoretically, and the best way to implement the SURG test is to set test limits based on a known good part as described in the user manual.
There is no way to control the duration of the impulse, since in this type of surge test (which is very common in the wound component industry), the waveform is determined largely by the part under test.
Please see the attached “Functional schematic for SURG test" Fig 2
The voltage waveform is measured using a high voltage attenuator and sampled at 10MHz. The measured SURG result is the integral of the voltage waveform over the entire impulse duration (the area under the voltage waveform - computed by multiplying the voltage at each sample by the sample time, and adding them all together).
SURGE test caution
There is always a danger of the transformer being connected the wrong way around during surge testing.
If this is the case, voltages in excess of the safe working voltage of the AT3600 (7kV) may be produced by transformer action.
A transformer with a 100V primary and a 10V secondary is to be SURG tested at 1000V on the primary.
The turn ratio is 100 / 10 = 10
The operator connects the secondary to the primary terminals on the fixture by mistake.
1000V is applied to the 10V winding, which results in 1000 x 10 = 10000V.
This voltage will damage the tester if applied repeatedly.
To avoid this problem, always use a simple test first to check that the transformer is correctly connected.
With 'Stop on Fail' enabled in the program options, the AT will then stop if the transformer is misconnected.
A TR test is ideal, but a suitably tolerance single R test would be OK, and very quick.