Device for detecting opposite phase and open phase

A device for detecting opposite phases and an open phase in an input three-phase voltage for driving a DC motor, wherein said input three-phase voltage is rectified by a three-phase bridge circuit having a DC output supplied to the armature of said DC motor, wherein said input three-phase voltage is supplied to a .DELTA.-Y transformer and the outputs of said transformer are connected to a gate circuit for generating firing pulses to said three-phase bridge circuit, said device comprising:

• a first phase shifter connected to a first output of said .DELTA.-Y transformer, for producing a first voltage which lags a first phase of said input three-phase voltage by a phase angle of 60.degree.;
• a second phase shifter connected to a second output of said .DELTA.-Y transformer, for producing a second voltage which lags a second phase of said input three-phase voltage by a phase angle of 180.degree., said second phase leading said first phase by a phase angle of 240.degree.;
• a differential amplifier connected to outputs of said first phase shifter and said second phase shifter for producing a difference voltage therebetween;
• comparator means connected to said differential amplifier for comparing the maximum level of said difference voltage with a reference voltage;
• and
• controller means connected between said comparator means and the input side of said three-phase bridge circuit for controlling the application of said input three-phase voltage to said three-phase bridge circuit, such that said input three-phase voltage is applied to said three-phase bridge circuit only when the maximum level of said difference voltage is equal to or smaller than said reference voltage.