Abstract: A method for controlling an inverter to improve the ability of the inverter to support power factor corrected (PFC) loads under transient conditions is provided. Changes in the load being provided power by an inverter may cause the RMS output voltage level of the inverter to drop. A PFC load may respond to such drops in the applied voltage level by increasing its current demand. This may cause the inverter current limit to be reached, forcing the inverter to be turned off, and causing the load to be dropped. This problem is avoided by responding to such load changes by immediately increasing the RMS output voltage level of the inverter to a level sufficient to minimize the voltage drop and prevent excessive current demands from the PFC load. The output voltage level of the inverter is then gradually reduced from the high voltage level to prevent the occurrence of oscillations due to interactions between the PFC load and the inverter controller.

Abstract: An uninterruptible power system having a main power path between input and output terminals monitors the phase of the voltages at the input and output terminals while power is applied to the input terminals from an AC power source. The phase difference between the voltages at the input and output terminals is determined and, upon occurrence of a fault in the power supplied to the input terminals, an inverter is turned on to provide power to the load at a frequency which matches the frequency of the input voltage before the fault and at a phase which is the phase of the voltage at the input terminals before the fault corrected in accordance with the phase difference determined before the fault. Prior to transfer of the supply of power back to the main AC power source when the fault ends, the phase and frequency of the voltage at the input terminals is again measured.

Abstract: A backup power system has power supply lines extending from input terminals, connectable to a main AC power system, and output terminals, connectable to a load, with a relay connected in a power supply line. The AC input power is monitored and when a fault in the power is detected, the relay is opened and an inverter is simultaneously turned on to provide backup power from a battery to the output terminals at a phase and frequency synchronized with the AC input voltage before the fault occurs. When the AC power system power is restored, the inverter power is again synchronized with the AC input power at the input terminals, the relay switch is closed and the inverter is turned off to allow the main power system to provide power to the load. The inverter may be providing power to the power lines before the relay contacts are opened to minimize the disruption of power supplied to the load during transfers.

Abstract: Phase and frequency locking of a controlled signal to a reference signal is obtained in a digital phase locked loop using efficient computational procedures which are carried out rapidly with minimal computational burden on a digital processor. During each cycle of the controlled signal, the time difference between the periods and the phases of the reference signal and the controlled signal are measured. These measured variables determine the predicted changes of the period of the controlled signal to bring the controlled signal into phase and frequency lock with the reference. The change allowed in the period between each discrete cycle is constrained to be less than or equal to a selected maximum amount of time.