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How to Sync Phases on an AC Circuit

Synchronous generators on AC circuits supply power with three phases. To connect such a circuit to the power grid you have to synchronize the three phases with the three phases of the power grid to avoid large compensating currents that may cause damage or trip circuit breakers. You first have to match the frequency of the two systems by changing the speed of the generator slightly. Changing the generator field voltage adjusts the voltage to match that of the grid. When the voltage differences between the AC circuit and the grid are close to zero for each phase, the systems are synchronized and you can close the connecting circuit breaker.

Things You'll Need

Speed switch
Voltage controller
2 frequency meters
2 voltmeters
3-phase circuit breaker
3 pilot lights

Instructions

- 1
  Switch off all power. Switch the circuit breaker into the open position. Connect the three phases from the AC circuit to the line side of the circuit breaker. Connect the three phases of the power grid to the load side. Connect one voltmeter and one frequency meter across two of the phases of the AC circuit. Connect the other voltmeter and frequency meter across the same phases on the power grid side of the circuit breaker. Connect the pilot lights across the poles of the circuit breaker, from phase A of the AC circuit to phase A of the power grid, and similarly for phases B and C.
- 2
  Check each component to make sure the voltage and current ratings match the expected values. Switch on the power on both sides of the circuit breaker. Read the voltmeters and frequency meters to determine the differences between the voltages and frequencies of the two systems. Use the generator speed switch to speed up or slow down the generator, making the frequency of the AC circuit match that of the power grid. Use the generator voltage controller to change the voltage of the AC circuit to match the voltage of the power grid. Keep making adjustments until both the frequency meters and the voltage meters of the two systems show the same values.
- 3
  Use the pilot lights to make the final, more precise adjustments by making slight changes to the AC system voltage and frequency until the lights go dark, indicating that there is no voltage across the circuit breaker and the systems are synchronized. If the lights are blinking, adjust the frequency until they are steady at their lowest brightness. Adjust the voltage until they go dark. If they start blinking slowly again after the voltage adjustment, make another slight frequency correction. When the lights are dark, close the circuit breaker to complete the synchronization of the phases of the two systems.

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