The purpose of this lab is to design and build a variable phase
shifter and use it to simulate a three-phase power system.
Before coming to lab design a circuit using op--amps and a
constant amplitude phase shifter to simulate a 3-phase generator.
There should be one input for a sine wave. Choose a frequency greater
than 1 kHz for your model. There should be three low impedance
outputs $120^\circ$ apart. Each output should have $V_{RMS}$ of
1.2~V. The ground or common of the input can serve as the common or
neutral of the outputs (Figure~1). You should be able to design the
circuit using three op--amps, two capacitors, two pots, and several
resistors.
Have your instructor check your design before building it. You
should have a neat schematic with pin numbers. Carefully construct
the circuit, and check wiring carefully before turning on power.
Connect a 1~k$\Omega$ load to all outputs. Make adjustments to
the produce the proper phases and amplitudes. Measure and record the
line-to-line and line-to-neutral voltages using a voltmeter. Compare
these to the expected ones.
Connect your 3-phase generator to a balanced Y-load, each of a
1~k$\Omega$ resistor. Use 100$\Omega$ in each line to simulate a long
line. Measure the voltage from n to N, and adjust the phases to make
this voltage as close to zero as possible. Measure the current in
each line and in the neutral.
Unbalance the load by replacing one of the 1~k$\Omega$ resistors
with a 2~k$\Omega$. Remeasure the currents. In particular, note the
change in current in the neutral.