GB1524475A - Apparatuses for detecting phase shifts - Google Patents

Abstract

1524475 Detecting phase angle changes; monitoring motors SCANIAINVENTOR AB 4 June 1976 [6 June 1975] 23223/76 Heading H2K In a circuit for detecting changes in the phase angle between current and voltage on a line 3, 4, 5 a constant-amplitude A.C. signal in phase with the current is derived by oppositely-poled parallel diodes Dl, D2 in the current path, and a similar signal in phase with the voltage is derived by oppositely-poled parallel diodes D3, D4 in a path between two of the line conductors or between one line conductor and a reference (e.g. artificial neutral) point. The current and voltage signals are detected (e.g. by switching amplifiers F1, F2) and supplied to an exclusive- OR gate G, the pulse train output of which has a mark-space ratio dependent on phase. A first, short time constant, smoothing circuit R8, C1, R9, C2 derives a D.C. voltage a representing the phase and able to follow changes thereof quickly, whilst a further, long time constant, smoothing circuit R10, C3 derives a voltage b which follows changes in phase more slowly. Amplifier F3 compares the voltages a, b from the two smoothing circuits and provides a first output level when a<b and a second level when a>b, each transition from one level to another thus corresponding to a change in phase. The circuit may be used to detect whether several changes of phase occur in rapid succession, e.g. due to a fan 1 going in and out of stall repetivively. Each transistion from the first to second level adds an increment of charge to a capacitor, and if a number of such transitions occur close together the capacitor voltage will exceed a reference (P), thus switching on a transistor TR to operate a protective relay RL and l.e.d. LD.