Meaning
Sub-cycle voltage disturbance refers to the momentary voltage drop that occurs during the commutation of power semiconductors in alternating current power lines. When a thyristor or silicon controlled rectifier switches from one phase to another, a brief short circuit is created that results in a line notch. This localized disturbance can disrupt sensitive electronic timing circuits if the voltage falls too low, creating severe errors in automated manufacturing runs.
Commutation Mechanism
Simultaneous conduction of two diodes or thyristors in a bridge rectifier occurs during phase transition. This overlap draws a massive, short-duration current that causes the line voltage to plunge almost to zero for a fraction of a millisecond. Since this occurs many times per second, the power line develops a distinct notched profile.
Circuit Interference
Zero-crossing detection circuits in controllers can be fooled by these rapid voltage dips, leading to timing errors. These errors cause synchronization issues, triggering false alarms or system resets in industrial computers. The resultant interference can cause erratic behavior in automated production equipment.
Mitigation Reactor
Installing line reactors or isolation transformers between the drive and the supply line represents the most effective solution. These inductive components limit the rate of current rise during commutation, which smooths out the voltage profile and limits the depth of the notch. This inductive smoothing protects neighboring equipment from voltage stress.
