Filter for polyphase rectifier

A polyphase rectifier for receiving at least three-phase polyphase input voltages for rectification into a positive voltage at a first terminal, and a negative voltage at a second terminal, wherein it is known to include a pair of filter and smoothing capacitors connected in series between said first and second terminals, the common connection between said pair of capacitors being connected to a neutral terminal for connection to a point of reference potential, wherein typically said capacitors supply power during a substantial portion of the cycle for each input voltage waveform, to a reactive load connected between said first and second terminals, said reactive load also having a common connection to said third terminal, said pair of capacitors being recharged during a relatively short portion of alternate half-cycles, respectively, of and about the peak amplitudes of the phase-to-phase voltages of said polyphase input voltages, causing substantially large current flow from the source of said polyphase input voltages to said rectifier, and from said rectifier to said load, at such times of recharging said capacitors, it being desired to substantially reduce the peak current demands upon said rectifier and source of said polyphase voltage, wherein the improvement in filtering comprises:

• first means connected between said first terminal and one of said pair of capacitors, for providing a relatively low-impedance current path for reactive currents to flow from said first terminal into said one capacitor, and a relatively high-impedance current path for reactive current flow in the opposite direction, the time constant for the high-impedance current path and said one capacitor being made about four times greater than the period between successive peaks of a ripple voltage appearing at said first terminal;
• and
• second means connected between said second terminal and the other of said pair of capacitors, for providing a relatively low-impedance current path for reactive currents to flow from said other capacitor into said second terminal, and a relatively high-impedance current path for reactive current flow in the opposite direction, the time constant for the high-impedance current path and said other capacitor being made about four times greater than the period between successive peaks of a ripple voltage appearing at said second terminal;
• said first and second means further serving to both substantially eliminate the requirement that said capacitors supply non-reactive current to said load, and to block the flow of ripple current from said first and second terminals to said pair of capacitors, respectively, by permitting said capacitors to charge to a level where the voltage thereacross is greater than the peak voltage and dc level of said ripple voltages, allowing a significant reduction in the size of said capacitors and substantially reducing the magnitude of the peak current requirements upon said rectifier, thereby permitting the application of a relatively lower power rectifier, better utilization of the source of polyphase input voltages, and the use of said pair of capacitors each as high frequency filter capacitors.

In at least a three-phase polyphase rectifier system having a first terminal for providing a positive polarity dc voltage to a reactive load, a second terminal for providing a negative polarity dc voltage to said load, and a third terminal for providing a point of reference potential for said positive and negative dc voltages, and for connection to a common terminal of said load, a first filter capacitor being coupled between said first and third terminals, and a second filter capacitor being coupled between said second and third terminals, and a second filter capacitor being coupled between said second and third terminals, a method of filtering for substantially eliminating the relatively high peak magnitude short duration current demands upon said rectifier for recharging said first and second capacitors during alternate half-cycles, respectively, of the output voltages of the polyphase voltage source supplying power to said rectifier, said first and second capacitors discharging into a reactive load at times that the level of voltage thereacross is greater than the level of voltage at said first and second terminals, respectively, comprising the steps of:

• initially rapidly charging said first and second capacitors to the maximum level and polarity of voltage available at said first and second terminals, respectively, and
• substantially preventing the discharging of said first and second capacitors into said load, concurrent with permitting any reactive load currents to flow bidirectionally between each one of said first and second capacitors and said reactive load, respectively.

In a polyphase rectifier system for receiving at least three-phase polyphase input voltages for rectification into a positive voltage at a first terminal, and a negative voltage at a second terminal, each with reference to a point of reference potential at a third terminal, a first filter capacitor being typically connected between said first and third terminals, a second filter capacitor being typically connected between said second and third terminals, and a reactive load being connected between said first and second terminals, said reactive load having a common connection to said third terminal, a method of filtering substantially reducing the peak magnitude of current demanded from said rectifier over a relatively short period of time about the peak amplitudes of the voltages of said polyphase input voltage, increasing the period of time over each cycle of the voltage phases of said polyphase input voltages that said rectifier delivers the reduced peak magnitude of current to said load, and substantially blocking the flow of ripple current in said first and second capacitors, thereby improving the utilization of the source of said polyphase input voltages, reducing conducted noise, and reducing the power requirements of both said rectifier and said first and second capacitors, said method comprising the steps of:

• providing a relatively low-impedance unidirectional current path for the flow of reactive load current and initial charging current from said first terminal to said first capacitor, for permitting said capacitor to charge to a level of voltage greater than the peak positive plus dc level of ripple voltage at said first terminal;
• providing a relatively high-impedance current path for the flow of reactive current and discharge current from said first capacitor to said first terminal;
• providing a relatively low-impedance unidirectional current path for the flow of reactive load current and initial charging current from said second capacitor to said second terminal;
• and
• providing a relatively high-impedance current path for the flow of reactive current and discharge current from said second terminal to said second capacitor.

In a polyphase rectifier system for receiving at least three-phase polyphase input voltages for rectification into a dc voltage at a first terminal, with reference to a point of reference potential at a second terminal, a filter capacitor being typically connected between said first and second terminals, and a reactive load being connected between said first and second terminals, a method of filtering substantially reducing the peak magnitude of current demanded from said rectifier over a relatively short period of time about the peak amplitudes of the voltages of said polyphase input voltage, increasing the period of time over each cycle of the voltage phases of said polyphase input voltages that said rectifier delivers the reduced peak magnitude of current to said load, and substantially blocking the flow of ripple current in said capacitor, thereby improving the utilization of the source of said polyphase input voltages, reducing conducted noise, and reducing the power requirements of both said rectifier and said capacitor, said method comprising the steps of:

• providing a relatively low-impedance unidirectional current path for the flow of reactive load current and initial charging current from said first terminal to said capacitor, for permitting said capacitor to charge to a level of voltage greater than the peak positive plus dc level of ripple voltage at said first terminal;
• and
• providing a relatively high-impedance current path for the flow of reactive current from said capacitor to said first terminal.