Alarm system for warning of unbalance or failure of one or more phases of a multi-phase high-current load

An alarm system for warning of unbalance or failure of one phase of a three-phase load circuit having three power leads running thereto and capable of being used with a high current flow in the load circuit and in which power control means such as mechanical contactors, silicon controlled rectifiers, triacs and saturable reactors may be used to control the flow of electrical power into the load circuit comprising:

• three similar magnetically permeable transformer cores each having an opening therein through which the perspective power lead is passed for defining a magnetically permeable path encircling each power lead,
• three similar secondary transformer windings, one of said windings being positioned on each of said cores and passing through the opening thereof,
• said secondary windings each having the same winding sense with respect to the power lead passing through the core on which the respective secondary winding is located,
• three similar resistors,
• one of said resistors being shunted across the terminals of each of said secondary windings,
• said three transformer shunt resistors being connected together at a common junction forming a first Y resistance pattern defining a phantom neutral point at their common junction,
• a second Y resistance pattern having three similar resistance legs joined at a common junction defining a second neutral point, each of said resistance legs including a potentiometer having an adjustable tap thereon,
• means connecting the outer terminal of each of said resistance legs to a respective one of the phases of the load circuit,
• three similar resistance elements each connected in circuit between one of the outer terminals of said transformer shunt resistors and one of said adjustable taps,
• said resistance elements each being connected in circuit between the outer terminal of the secondary winding of the transformer which is associated with a particular phase of the load circuit and the tap of the potentiometer which is in the resistance leg that is connected to the same phase of the load circuit,
• an unbalance sensing resistance element connected in circuit between said phantom neutral point and said second neutral point,
• said potentiometers being adjustable for producing a null in voltage across said unbalance sensing resistance element when balanced three-phase current is flowing through the load circuit,
• alarm circuit means including alarm means for producing a warning signal when a voltage appears across said unbalance sensing resistance element, and
• said alarm circuit means including time-delay means for rendering said alarm circuit unresponsive to transient voltages appearing across said unbalance sensing resistance element due to uneven operation of said control means.