Current limiting control for an electric power system

Claim 2

In an electrical system including power conversion means for supplying electric power to a load, and means operative during the delivery of electric power to the load for developing an electrical signal representative of a selected system operating parameter; control means comprising: at least one overlimit signal producing means having an input coupled to receive the signal representative of the operating parameter and being responsive thereto to produce an overlimit signal only when the operating parameter exceeds a predetermined level, the overlimit signal having a fixed magnitude independent of the amount by which the operating parameter exceeds the predetermined level, first circuit means responsive to input signals including a reference signal for producing a corrected reference signal, said first circuit means including means for preventing the magnitude of the corrected reference signal from exceeding a predetermined maximum level substantially less than the fixed magnitude of an overlimit signal produced by said overlimit signal producing means, second circuit means coupled to said first circuit means and said overlimit signal producing means for receiving corrected reference and overlimit signals therefrom and being responsive to signals supplied thereto to produce a control signal, said second circuit means including integrating means for limiting the maximum rate of change of the control signal in response to abrupt changes in the level of the signals supplied thereto by said first circuit means and said overlimit signal producing means, and means coupled to said second circuit means for receiving the control signal therefrom and vaRying the electric power output of the power conversion means in response to variations in the control signal, the polarity and fixed magnitude of the overlimit signal being such that said second circuit means produces in response thereto a control signal for adjusting the power output of the power conversion means such that the level of the operating parameter is reduced irrespective of independent changes in magnitude and polarity of any and all other input signals including the reference signal to said first circuit means.

Claim 3

In an electrical system including power conversion means for supplying electric power to a load, and means operative during the delivery of electric power to the load for developing an electrical signal representative of a selected system operating parameter; control means comprising: at least one overlimit signal producing means having an input coupled to receive the signal representative of the operating parameter and being responsive thereto to produce an overlimit signal only when the operating parameter exceeds a predetermined level, the overlimit signal having a fixed magnitude independent of the amount by which the operating parameter exceeds the predetermined level, first circuit means responsive to input signals including a reference signal for producing a corrected reference signal, said first circuit means including means for preventing the magnitude of the corrected reference signal from exceeding a predetermined maximum level substantially less than the fixed magnitude of an overlimit signal produced by said overlimit signal producing means, second circuit means coupled to said first circuit means and said overlimit signal producing means for receiving corrected reference and overlimit signals therefrom and being responsive to signals supplied thereto to produce a control signal, said second circuit means including integrating means for limiting the maximum rate of change of the control signal in response to abrupt changes in the level of the signals supplied thereto by said first circuit means and said overlimit signal producing means, and means coupled to said second circuit means for receiving the control signal therefrom and vaRying the electric power output of the power conversion means in response to variations in the control signal, the polarity and fixed magnitude of the overlimit signal being such that said second circuit means produces in response thereto a control signal for adjusting the power output of the power conversion means such that the level of the operating parameter is reduced irrespective of independent changes in magnitude and polarity of any and all other input signals including the reference signal to said first circuit means.

Claim 4

In an electrical system including power conversion means for supplying electric power to a load, and means operative during the delivery of electric power to the load for developing an electrical signal representative of a selected system operating parameter; control means comprising: at least one overlimit signal producing means having an input coupled to receive the signal representative of the operating parameter and being responsive thereto to produce an overlimit signal only when the operating parameter exceeds a predetermined level, the overlimit signal having a fixed magnitude independent of the amount by which the operating parameter exceeds the predetermined level, first circuit means responsive to input signals including a reference signal for producing a corrected reference signal, said first circuit means including means for preventing the magnitude of the corrected reference signal from exceeding a predetermined maximum level substantially less than the fixed magnitude of an overlimit signal produced by said overlimit signal producing means, second circuit means coupled to said first circuit means and said overlimit signal producing means for receiving corrected reference and overlimit signals therefrom and being responsive to signals supplied thereto to produce a control signal, said second circuit means including integrating means for limiting the maximum rate of change of the control signal in response to abrupt changes in the level of the signals supplied thereto by said first circuit means and said overlimit signal producing means, and means coupled to said second circuit means for receiving the control signal therefrom and vaRying the electric power output of the power conversion means in response to variations in the control signal, the polarity and fixed magnitude of the overlimit signal being such that said second circuit means produces in response thereto a control signal for adjusting the power output of the power conversion means such that the level of the operating parameter is reduced irrespective of independent changes in magnitude and polarity of any and all other input signals including the reference signal to said first circuit means.

Claim 5

In an electrical system including power conversion means for supplying electric power to a load, control means responsive to input signals for varying the power output of the power conversion means, and means operative during the delivery of electric power from the power conversion means to the load for developing an electrical signal representative of a selected system operating parameter; means for limiting the level of the selected operating parameter comprising: signal input means for receiving the signal representative of the selected operating parameter, voltage input means including a first voltage input for connection to a voltage source of negative polarity and a second voltage input for connection to a voltage source of positive polarity, circuit means including a first resistive circuit interconnecting said signal input means and said first voltage input and a second resistive circuit interconnecting said signal input means and said second voltage input, overlimit signal producing means including first and second switching means each having conductive and non-conductive states, said first switching means being connected to a predetermined point in said first resistive circuit and assuming a selected one of its states when the potential at said predetermined point in said first resistive circuit exceeds a level corresponding to a predetermined operating parameter level of a first polarity and said second switching means being connected to a predetermined point in said second resistive circuit and assuming a selected one of its states when the potential at said predetermined point in said second resistive circuit exceeds a level corresponding to a predetermined operating parameter level of a second polarity, and means coupling said overlimit signal producing means to the control means for supplying the overlimit signal to the control means as an input signal, the polarity and magnitude of the overlimit signal being such that said control means adjusts the power output of the power conversion means in response thereto to reduce the level of the selected operating parameter.

Claim 6

In an electrical system including power conversion means for supplying electric power to a load, control means responsive to input signals for varying the power output of the power conversion means, and means operative during the delivery of electric power from the power conversion means to the load for developing an electrical signal representative of a selected system operating parameter; means for limiting the level of the selected operating parameter comprising: signal input means for receiving the signal representative of the selected operating parameter, voltage input means including a first voltage input for connection to a voltage source of negative polarity and a second voltage input for connection to a voltage source of positive polarity, circuit means including a first resistive circuit interconnecting said signal input means and said first voltage input and a second resistive circuit interconnecting said signal input means and said second voltage input, overlimit signal producing means including first and second switching means each having conductive and non-conductive states, said first switching means being connected to a predetermined point in said first resistive circuit and assuming a selected one of its states when the potential at said predetermined point in said first resistive circuit exceeds a level corresponding to a predetermined operating parameter level of a first polarity and said second switching means being connected to a predetermined point in said second resistive circuit and assuming a selected one of its states when the potential at said predetermined point in said second resistive circuit exceeds a level corresponding to a predetermined operating parameter level of a second polarity, and means coupling said overlimit signal producing means to the control means for supplying the overlimit signal to the control means as an input signal, the polarity and magnitude of the overlimit signal being such that said control means adjusts the power output of the power conversion means in response thereto to reduce the level of the selected operating parameter.

Claim 7

Limiting means as defined by claim 2 in which each of said first and second switching means is a transistor having its base connected to the predetermined point of the respective one of said resistive circuits.

Claim 8

An a-c adjustable speed drive system comprising: a polyphase a-c motor; power conversion means for changing unidirectional electric power to polyphase electric power, said conversion means including a plurality of output phases each connected to a respective phase of said motor; control means responsive to input signals including a reference signal for controlling the power output of said power conversion means; means operative during the delivery of polyphase electric power to said motor by said power conversion means to generate a signal proportional to the output current of said power conversion means which is in phase with the output voltage; and current limiting means comprising: signal input means coupled to said in-phase signal generating means for receiving the in-phase signal therefrom, voltage input means including a first voltage input for connection to a voltage source of negative potential and a second voltage input for connection to a voltage source of positive potential, circuit means including a first resistive circuit interconnecting said signal input means and said fIrst voltage input and a second resistive circuit interconnecting said signal input means and said second voltage input, overcurrent signal producing means including first and second switching means each having conductive and non-conductive states, said first switching means being connected to a predetermined point in said first resistive circuit and assuming a selected one of its states only when the in-phase current signal is of positive polarity and exceeds a predetermined level, and said second switching means being connected to a predetermined point in said second resistive circuit and assuming a selected one of its states only when the in-phase current signal is of negative polarity and exceeds a predetermined level, said overcurrent signal producing means producing an overcurrent signal of a first polarity when its first switching means assumes its selected state and an overcurrent signal of a second polarity when its switching means assumes its selected state, and means coupling said overcurrent signal producing means to said control means for supplying the overcurrent signal to said control means as an input signal, the polarity and magnitude of the overcurrent signal being such that said control means adjusts the frequency of the a-c power output of said power conversion means in response thereto to reduce the level of in-phase output current.

Claim 9

An a-c adjustable speed drive system comprising: a polyphase a-c motor; power conversion means for changing unidirectional electric power to polyphase electric power, said conversion means including a plurality of output phases each connected to a respective phase of said motor; control means responsive to input signals including a reference signal for controlling the power output of said power conversion means; means operative during the delivery of polyphase electric power to said motor by said power conversion means to generate a signal proportional to the output current of said power conversion means which is in phase with the output voltage; and current limiting means comprising: signal input means coupled to said in-phase signal generating means for receiving the in-phase signal therefrom, voltage input means including a first voltage input for connection to a voltage source of negative potential and a second voltage input for connection to a voltage source of positive potential, circuit means including a first resistive circuit interconnecting said signal input means and said fIrst voltage input and a second resistive circuit interconnecting said signal input means and said second voltage input, overcurrent signal producing means including first and second switching means each having conductive and non-conductive states, said first switching means being connected to a predetermined point in said first resistive circuit and assuming a selected one of its states only when the in-phase current signal is of positive polarity and exceeds a predetermined level, and said second switching means being connected to a predetermined point in said second resistive circuit and assuming a selected one of its states only when the in-phase current signal is of negative polarity and exceeds a predetermined level, said overcurrent signal producing means producing an overcurrent signal of a first polarity when its first switching means assumes its selected state and an overcurrent signal of a second polarity when its switching means assumes its selected state, and means coupling said overcurrent signal producing means to said control means for supplying the overcurrent signal to said control means as an input signal, the polarity and magnitude of the overcurrent signal being such that said control means adjusts the frequency of the a-c power output of said power conversion means in response thereto to reduce the level of in-phase output current.

Claim 10

A drive system as defined by claim 4 in which each of said first and second switching means is a transistor having its base connected to the predetermined point of the respective one of said first and second resistive circuits.