Transient ride-through AC inverter

Claim 2

A transient protection circuit for an inverter, comprising: a rectifier capable of supplying direct current (DC) from an alternating current (AC) power source; a DC bus fed by said rectifier; a bus capacitor connected across said DC bus; solid state switches for supplying alternating current to a load; means for sensing the voltage on said DC bus; a resistor; means responsive to said voltage on said DC bus for connecting said resistor in parallel with said bus capacitor if said DC bus voltage exceeds a predetermined voltage; and, means, responsive to said voltage on said DC bus, for turning off said solid state switches if said DC bus voltage exceeds said predetermined voltage, and for turning on said solid state switches and disconnecting said resistor from parallel connection with said bus capacitor if said DC bus voltage is somewhat below said predetermined voltage, whereby said solid state switches are protected from a DC bus overvoltage caused by a transient voltage rise in said AC power source.

Claim 3

The apparatus as in claim 1 further comprising: means for disconnecting said resistor from a parallel connection with said bus caapcitor and also for turning on said solid state switches on the elaspse of a predetermined idle time interval following an excursion of said DC bus voltage above said predetermined voltage.

Claim 4

The apparatus as in claim 2 wherein said idle time interval is approximately one half of an RC time constant value where R is the value of resistance of said resistor and C is the value of capacitance of said bus capacitor.

Claim 5

The apparatus as in claim 3 wherein said value of capacitance of said bus capacitor is approximately 1,620 microfarads, and the value of said resistor is approximately 9 ohms.

Claim 6

The apparatus as in claim 2 further comprising: means for turning off said solid state switches until they are manually reset when said means responsive to said voltage on said DC bus senses a second excursion of said DC bus voltage above said predetermined voltage within a predetermined waiting time interval.

Claim 7

The apparatus as in claim 5 wherein said waiting time interval is one minute.

Claim 8

The apparatus as in claim 1 wherein said means responsive to said voltage on said DC bus for connecting a resistor in parallel with said bus capacitor comprises a solid state switch.

Claim 9

The apparatus as in claim 1 wherein said means responsive to said voltage on said DC bus for turning off said solid state switches comprises a circuit for generating an inhibit signal to turn off a drive circuit for said solid state switches.

Claim 10

An apparatus as in claim 1 wherein said rectifier draws energy from a three phase alternating current circuit.

Claim 11

The apparatus as in claim 1 wherein said solid state switches comprise gate turn off thyristors.

Claim 12

The apparatus as in claim 1 wherein said solid state switches generate a pulse width modulated three phase alternating current for driving a motor.

Claim 13

The apparatus as in claim 1 further comprising an impedance between AC supply power lines and said inverter.

Claim 14

The apparatus as in claim 12 wherein said impedance is a transformer having a primary connected to said AC supply power lines and said inverter connected to a secondary of said transformer.

Claim 15

A transient protection circuit comprising: a fixed voltage rectifier capable of being driven by an alternating current source of electrical energy; a DC bus driven by said rectifier; a DC bus capacitor connected across said DC bus; an alternating current inverter comprising solid state switches driven by said DC bus for driving an alternating current load; a resistor connected so that one end of said resistor is connected to a first side of said DC bus, a second end of said resistor is connected to a first side of a control switch and a second side of said control switch is connected to a second side of said DC bus; a circuit for sensing the voltage on said DC bus; means responsive to said voltage on said DC bus for closing said control switch if said DC bus voltage exceeds a predetermined voltage, means responsive to said voltage on said DC bus for turning off said solid state switches if said DC bus voltage exceeds said predetermined value; means for disconnecting said resistor from across said DC bus and for restarting said solid state switches on the elapse of a predetermined idle time interval; and, means for turning off said solid state switches until a manual restart if said means responsive to said DC voltage on said DC bus senses a second excursion of said DC bus voltage above said predetermined voltage within a predetermined waiting time.

Claim 16

A transient protection circuit comprising: a fixed voltage rectifier capable of being driven by an alternating current source of electrical energy; a DC bus driven by said rectifier; a DC bus capacitor connected across said DC bus; an alternating current inverter comprising solid state switches driven by said DC bus for driving an alternating current load; a resistor connected so that one end of said resistor is connected to a first side of said DC bus, a second end of said resistor is connected to a first side of a control switch and a second side of said control switch is connected to a second side of said DC bus; a circuit for sensing the voltage on said DC bus; means responsive to said voltage on said DC bus for closing said control switch if said DC bus voltage exceeds a predetermined voltage, means responsive to said voltage on said DC bus for turning off said solid state switches if said DC bus voltage exceeds said predetermined value; means for disconnecting said resistor from across said DC bus and for restarting said solid state switches on the elapse of a predetermined idle time interval; and, means for turning off said solid state switches until a manual restart if said means responsive to said DC voltage on said DC bus senses a second excursion of said DC bus voltage above said predetermined voltage within a predetermined waiting time.

Claim 19

An improved inverter for supplying alternating current (AC) to a load having, means for connection to an AC power source; a rectifier capable of drawing power from said AC power source for charging a DC bus; an inverter capable of drawing power from said DC bus and having solid state switches for generating alternating current of a predetermined frequency for driving said load; a bus capacitor connected across said DC bus for smoothing variations in voltage on said DC bus; wherein the improvement comprises: means for detecting a rise in the voltage across said DC bus, said rise in voltage being caused by a transient increase in voltage from said AC power source; means, responsive to said voltage on said DC bus exceeding a predetermined voltage, for disconnecting said inverter from said DC bus in order to protect said solid state switches from an overvoltage on said DC bus, for connecting a resistor in parallel with said bus capacitor to provide a discharge path for energy stored in said capacitor, and for reconnecting said inverter to said DC bus after said DC bus voltage falls below said predetermined voltage to reestablish the flow of power to said load, whereby said solid state switches are protected from said transient rise in voltage in said AC source and current flow to said load is interrupted for only a time sufficient for said capacitor to discharge sufficiently to bring said DC bus voltage below said predetermined voltage.

Claim 20

An improved inverter for supplying alternating current (AC) to a load having, means for connection to an AC power source; a rectifier capable of drawing power from said AC power source for charging a DC bus; an inverter capable of drawing power from said DC bus and having solid state switches for generating alternating current of a predetermined frequency for driving said load; a bus capacitor connected across said DC bus for smoothing variations in voltage on said DC bus; wherein the improvement comprises: means for detecting a rise in the voltage across said DC bus, said rise in voltage being caused by a transient increase in voltage from said AC power source; means, responsive to said voltage on said DC bus exceeding a predetermined voltage, for disconnecting said inverter from said DC bus in order to protect said solid state switches from an overvoltage on said DC bus, for connecting a resistor in parallel with said bus capacitor to provide a discharge path for energy stored in said capacitor, and for reconnecting said inverter to said DC bus after said DC bus voltage falls below said predetermined voltage to reestablish the flow of power to said load, whereby said solid state switches are protected from said transient rise in voltage in said AC source and current flow to said load is interrupted for only a time sufficient for said capacitor to discharge sufficiently to bring said DC bus voltage below said predetermined voltage.