Power Transducers
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Power transducers are based on sensing the voltage and current and providing a product of them in order to obtain the measured power. In AC systems, the power factor and the sine of the power factor angle are taken into account for measuring the real and reactive powers, respectively. The multiplication is performed in analog or digital circuits. Convenient devices for this purpose are the Hall-effect elements. A multiplier can be built by putting a semiconductor crystal in the gap of an electromagnet such that an alternating current passing through the electromagnet's winding generates the flux, and AC voltage produces the control current. In this case, the DC Hall voltage is proportional to the true power measured in watts (voltage times current, times power factor). Besides this, a double-frequency AC voltage component is developed at the output, and it is proportional to the volt-amperes in the circuit. Measuring the power in a three-phase system requires two Hall-effect elements.
Power transducer. Il = load current, UL = load voltage, Uo = dc output voltage proportional to product IL, UL, and power factor, 1 = semiconductor crystal, 2 = core, 3 = current coil, 4 = voltage transformer, 5 = series resistance.
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