Most domestic consumers (in the UK) are provided with a single-phase supply unless exceptionally heavy loading is foreseen. It may, however, occur that a low-voltage three-phase supply is required for experimental purposes and in such cases, the simulator described can prove useful.
The source signal for the phases, R, S, and T are generated with a standard Wien bridge. The sine-wave generator is formed by IC1a. Preset P1 enables the frequency to be set accurately to 50 Hz; the output level (pin 1) is set with P2 to 1 V (peak).
Circuit IC2a provides a constant load impedance for IC1a, which is important for the stability of the generated frequency. It
also raises the signal level to 5.6 V (peak). The peak value of the phases is set to 0-12 V with P3. Series capacitor C9 prevents the offset voltage of IC1a and IC2a adding direct voltage to the outputs of IC2b
The R phase results from inverting the signal at the wiper of P3, that is, shifting it by 180°. Owing to low-pass filter R12–C11, the T output lags the signal at the wiper by 60°, while R9-C10 provide a 60° lead at the
S output. There is, therefore, a 120° phase difference between any pair of phases. Presets P4 and P5 need to be set only once and that in such a manner that the peak
values of the three phases are identical.
Low-current LEDs D5-D7 light only if there is an alternating voltage present at the associated output. An accidentally short-circuited phase is, therefore, detected
The opamps are short-circuited proof and can provide a current of about 10 mA.
When a symmetrical ±15 V supply is used, the quiescent current is ±20 mA.