When a synchronous motor is connected to 3-phase power lines in parallel with other AC motors with lagging power factor, the synchronous motor will ? the power factor of the circuit.

• A. worsen
• B. improve
• C. No effect
• D. Drop

Answer: (B)

The key idea is that a synchronous motor can regulate reactive power. When it is overexcited, it acts as a source of reactive power, meaning its current leads the voltage and it supplies vars to the electrical bus. In a system where other motors have lagging power factor, supplying reactive power improves the overall power factor toward unity. So, operated with the right excitation, the synchronous motor will improve the circuit’s power factor. If it were underexcited, it would draw reactive power and worsen the pf, which is why the intended action to correct lagging pf is to overexcite the motor.