Digital voltage and phase monitor for AC power line

A circuit for monitoring a three-phase power supply signal and controlling the connection between the power supply signal and a load, the circuit comprising;

• switch means for controlling the connection between the three phase power supply signal and the load (20),
• digital voltage level monitoring means coupled to the three-phase power supply signal for disconnecting the power supply from the AC load when any phase of the power supply signal is outside a predetermined voltage window (138A, 138B, 138C), the digital voltage level monitoring means including conversion means for generating a DC voltage responsive to the AC power supply signal voltage.
• first and second scaling means each coupled to the covnersion means for scaling the DC voltage to form first and second separately controllable digit monitoring signals each responsive to the AC power supply signal, the first digital monitoring signal traversing a first predetermined logic level when the AC power supply line voltage is in an under-voltage condition and the second digital monitoring signal traversing a second predetermined logic level when the AC power supply line voltage in in an over-voltage condition, and
• first and second digital comparator means, each comprising a digital logic gate coupled to receive a corresponding one of said digital monitoring signals and including means for providing separately adjustable hystersis, each logic gate having an internally generated voltage level that is used as a reference to determine the logic state of the corresponding digital monitoring signal, the first and second comparator means generating a fault condition signal when either of said corresponding digital monitoring signals traverses the internal reference voltage of the corresponding digital logic gate;
• and
• digital phase monitoring means coupled to the power supply signal for disconnecting the power supply from the AC load when any two phases of the power supply are within a predetermined phase angle, the phase monitoring means having an internally generated voltage level that is used as a reference to determine a digital logic state for each of said two phases, the logic state of the two phases compared to determine when said phases are within the predetermined phase angle (154, 160, 162, 169).

A method for controlling a power signal connection between an AC power line and AC-powered user equipment, the method comprising the steps of:

• converting the AC power line voltage into a DC monitor voltage;
• scaling the DC monitor voltage to form first and second independently controllable digital logic signals each responsive to the AC power supply signal and capable of generating a different voltage amplitude;
• monitoring the first and second digital logic signals with a digital logic circuit, the first digital signal traversing a first predetermined logic level when the AC power supply line voltage is in an under-voltage condition and the second digital signal traversing a second predetermined logic level when the AC power supply line voltage is in an over-voltage condition (138, 154, 160, 162);
• generating a fault condition signal when one of the first and second digital logic signals indicate either an under-voltage or over-voltage condition, respectively;
• and
• independently controlling hystersis in the digital logic circuit for both the first and second digital logic signals so that each digital logic signal has a first independently controllable fault activation voltage level for activating a fault condition in the digital logic circuit and a second separately controllable fault deactivation voltage level for deactivating the fault condition.