A magnetic material’s hysteresis curve is a powerful visual tool that can perform a graphic analysis of a transformer’s behavior. Better-equipped laboratories employ a hysteresis graph to view the actual curve. However, unless serious magnetic work is being performed, the high-priced equipment needed for this approach is out of reach for most engineers. Sometimes a simple graphic representation is all that’s required, and the circuit explained here can fit the bill.
The circuit relies on the fact that the magnetomotive force “H” is a direct function of the exciting current, and the flux density “B” is the integral of the applied voltage. A shunt resistor may measure the exciting current, and R1 accomplishes precisely that (Fig. 1). R1’s value should be calculated to create a 150- to 200-mV drop peak-to-peak at the DUT’s rated current.
To integrate the applied voltage, a series-RC circuit consisting of R2, R3, and C1 is employed. Assuming that the capacitive reactance at the operating frequency is about 100 times smaller than the series resistor value, the voltage appearing across the capacitor terminals will be a fairly accurate integration of the applied voltage. The resistor values need to be adjusted slightly depending on the ac-line frequency, as indicated in the schematic. C1 should be a polyester-film type. A 5% tolerance for all components is sufficient.
Rounding up the test set is a Variac with an isolated output, a dual-channel oscilloscope with an X-Y trace capability, and an ac voltmeter to accurately measure the value of the applied voltage. The scope’s X, Y, and common probes are connected to the points indicated in the schematic. The invert function on the scope’s “X” channel should be used if available, otherwise a mirror image of the hysteresis curve will appear. The timebase should be set so that at least a complete power cycle can be seen.
Before closing the main switch, ensure that the Variac is turned completely down to zero. Double check that all of the secondary terminals are disconnected. Then, slowly increase the output voltage until the rated voltage value of the device under test is reached. The scope’s vertical sensitivity controls may need to be adjusted along the way. A curve like the one shown should obtained (Fig. 2).
One word of caution: Potentially dangerous voltages are present in this circuit. Any exposed voltage-bearing surfaces should be properly insulated, and a properly sized primary fuse should always be employed. If your Variac does not provide isolation, a 1:1 isolation transformer will be required.