An automatic PF controller is a 'stepped' PF capacitor bankutilizing canotactors or thyristors to switch PF capacitor groups on and off line according to the PF of the electrical system. Logic monitors the voltage and current to calculate the phase angle displacement to determine how many steps to add or remove from the system. Time of step changes may be 10-30 seconds.
An active harmonic filter (AHF) is quite different. The AHF is a power electronic device that utilizes IGBT (very fast semiconductor switches) to inject reactive current – may be harmonics or phase displaced current. The logic is micorprocessor controlled for ultra rapid speed of analysis and synthesis of a control signal. The control signal directs the IGBT to duplicate the control signal for injection of the reactive current. This device works on a per cycle basis.