Motor protector for three-phase motors

Claim 2

A motor protector for a three-phase electric motor supplied electric power from a three-phase electric power supply through a motor control circuit, said motor protector comprising: a first sensing step-down transformer having a first primary winding energized by the voltage across a first phase of said power supply and having a first secondary winding in which a first secondary voltage is induced from said first primary winding; a first relay having a first AC coil in a first series circuit with a first variable resistor and said first secondary winding and having a first set of contacts and a second set of contacts; a second sensing step-down transformer having a second primary winding energized by the voltage across a second phase of said power supply through said first set of contacts and having a second secondary winding in which a second secondary voltage is induced from said second primary winding; and a second relay having a second AC coil in a second series circuit with a second variable resistor and said second secondary winding electrically independent of said first series circuit and having a third set of contacts, said second and third sets of contacts arranged for controlling the power to said motor control circuit so that when there is a failure in the power supply that reduces the electric voltage across said first phase below a predetermined amount the electric power is disconnected from said second primary winding and removed from said motor and when there is a failure in the power supply that reduces the electric voltage across said second phase below a predetermined amount the electric power is removed from said motor, each of said first and second AC coils having a threshold voltage a selected amount of voltage less than the normal sensed voltage in the associated secondary winding above which the power is maintained and below which the electric power is removed from said electric motor, each said variable resistor having a maximum relay coil voltage setting, a threshold relay coil voltage setting and a range of intermediate relay coil voltage settings between said threshold and maximum settings, the maximum relay coil voltage setting requiring the greatest amount of sensed phase voltage decrease to actuate the associated relay with less sensed phase voltage decrease necessary as the associated relay coil voltage settings approach the threshold voltage setting so that a lesser reduction in sensed phase voltage is required to actuate the associated relay coil as the settings approach the threshold voltage setting, thereby increasing the sensitivity of the associated relay to decrease in associated phase voltages.

Claim 3

A motor protector as set forth in claim 1 wherein said first phase is a high leg of a secondary of a wye-delta connected transformer of said power supply and said second phase is a low leg of a secondary of a wye-delta connected transformer of said power supply.

Claim 4

A motor protector as set forth in claim 1 wherein said first phase is a first pair of secondary windings of a wye-delta connected line transformer across which there is no connection to ground and said second phase is a second pair of secondary windings of a wye-delta connected line transformer having a ground connection between said second pair of secondary windings.

Claim 5

A motor protector for a three-phase electric motor supplied power from a three-phase power supply through a motor control circuit arranged for selectively connecting electric power to and removing electric power from said motor, said motor control circuit including a normally-open start switch, a normally-closed start switch, and a holding coil for line contactors that couple the power to the motor when energized, and a holding contactor, said holding coil being energized upon actuation of said start switch and held energized through said holding contactor and said stop switch, said motor protector comprising in combination: a first voltage sensing step-down transformer having a first primary winding connected in parallel with a high leg of said power supply, said first transformer having a first secondary winding in which a first secondary voltage is induced from said first primary winding; a first relay having a first AC coil connected in a first series circuit with a first variable resistor and said first secondary winding and having a first set of normally open contacts arranged so that when said high leg is energized at a normal line voltage said first set of contacts is closed; a second voltage sensing step-down transformer having a second primary winding connected across a low leg of said power supply through said first set of contacts for disconnecting said second primary winding from said low leg when there is a failure in said power supply that drops the voltage across said high leg below a predetermined amount, said second transformer having a second secondary winding in which a second secondary voltage is induced from said second primary winding; and a second relay having a second AC coil connected in a second series circuit with a second variable resistor and said second secondary winding electrically independent of said first series circuit with no direct electric connections between said first and second secondary windings, said second relay having a second set of normally-open contacts so that when said low leg is energized at normal line voltage said second set of contacts is closed; said second set of contacts being connected in said motor control circuit in series with said start switch and holding coil for controlling said motor control circuit so that when there is a failure in the power supply that reduces the electric voltage across said high leg below a predetermined amount the electric power is disconnected from said second primary winding and removed from said motor and when there is a failure in the power supply that reduces the electric voltage across said low leg below a predetermined amount the electric power is removed from said motor. each of said first and second AC coils having a threshold voltage a selected amount of voltage less than the normal sensed voltage in the associated secondary winding above which the power is maintained and below which the electric power is removed from said electric motor, each said variable resistor having a maximum relay coil voltage setting, a threshold relay coil voltage setting and a range of intermediate relay coil voltage settings between said threshold and maximum settings, the maximum relay coil voltage setting requiring the greatest amount of sensed phase voltage decrease to actuate the associated relay with less sensed phase voltage decrease necessary as the associated relay coil voltage settings approach the threshold voltage setting so that a lesser reduction in sensed phase voltage is required to actuate the associated relay coil as the settings approach the threshold voltage setting, thereby increasing the sensitivity of the associated relay to decreases in associated phase voltages.

Claim 6

A motor protector for a three-phase electric motor supplied power from a three-phase power supply through a motor control circuit arranged for selectively connecting electric power to and removing electric power from said motor, said motor control circuit including a normally-open start switch, a normally-closed start switch, and a holding coil for line contactors that couple the power to the motor when energized, and a holding contactor, said holding coil being energized upon actuation of said start switch and held energized through said holding contactor and said stop switch, said motor protector comprising in combination: a first voltage sensing step-down transformer having a first primary winding connected in parallel with a high leg of said power supply, said first transformer having a first secondary winding in which a first secondary voltage is induced from said first primary winding; a first relay having a first AC coil connected in a first series circuit with a first variable resistor and said first secondary winding and having a first set of normally open contacts arranged so that when said high leg is energized at a normal line voltage said first set of contacts is closed; a second voltage sensing step-down transformer having a second primary winding connected across a low leg of said power supply through said first set of contacts for disconnecting said second primary winding from said low leg when there is a failure in said power supply that drops the voltage across said high leg below a predetermined amount, said second transformer having a second secondary winding in which a second secondary voltage is induced from said second primary winding; and a second relay having a second AC coil connected in a second series circuit with a second variable resistor and said second secondary winding electrically independent of said first series circuit with no direct electric connections between said first and second secondary windings, said second relay having a second set of normally-open contacts so that when said low leg is energized at normal line voltage said second set of contacts is closed; said second set of contacts being connected in said motor control circuit in series with said start switch and holding coil for controlling said motor control circuit so that when there is a failure in the power supply that reduces the electric voltage across said high leg below a predetermined amount the electric power is disconnected from said second primary winding and removed from said motor and when there is a failure in the power supply that reduces the electric voltage across said low leg below a predetermined amount the electric power is removed from said motor. each of said first and second AC coils having a threshold voltage a selected amount of voltage less than the normal sensed voltage in the associated secondary winding above which the power is maintained and below which the electric power is removed from said electric motor, each said variable resistor having a maximum relay coil voltage setting, a threshold relay coil voltage setting and a range of intermediate relay coil voltage settings between said threshold and maximum settings, the maximum relay coil voltage setting requiring the greatest amount of sensed phase voltage decrease to actuate the associated relay with less sensed phase voltage decrease necessary as the associated relay coil voltage settings approach the threshold voltage setting so that a lesser reduction in sensed phase voltage is required to actuate the associated relay coil as the settings approach the threshold voltage setting, thereby increasing the sensitivity of the associated relay to decreases in associated phase voltages.

Claim 7

A motor protector as set forth in claim 4 wherein said motor control circuit includes a fused disconnect and a fuse in each power line from said power supply to the electric motor, said first and second primary windings being connected electrically below an associated fused disconnect and fuse.

Claim 8

A motor protector as set forth in claim 4 wherein said first relay has another set of normally-open contacts arranged for controlling said motor control circuit to directly de-energize said holding coil when the voltage across said first phase reduces to below a predetermined amount.