Control system for controlling an SCR network to regulate three-phase A-C power flow
First Claim
1. A control system for controlling the conduction angles of three pairs of SCR'"'"'s, to each of which pairs is applied a respective assigned one of the three alternating sinusoidal phase voltages of a three-phase A-C power supply, in order to regulate the power flow from the power supply to a load circuit, comprising:
- a transformer for deriving the corresponding line-to-neutral voltage for a predetermined one of the alternating phase voltages;
a band-pass filter for attenuating, in the derived line-to-neutral voltage, frequencies above and below the voltage'"'"'s fundamental frequency, while at the same time introducing no phase shift to that fundamental frequency, the output signal from the band-pass filter thereby being sinusoidal shaped and identical to the predetermined phase voltage except that any harmonics, noise, transients or other distortions in the phase voltage are not present in the filter'"'"'s output signal;
means for effectively detecting the zero voltage crossings in the filter'"'"'s sinusoidal output signal and producing a square wave signal which constitutes a reference voltage waveform having the same frequency as said phase voltages and having amplitude excursions which are precisely synchronized to the zero voltage crossings of the predetermined phase voltage;
means for developing, from said reference voltage waveform, three phase-displaced square wave signals which are phase-displaced with respect to each other by 120° and
have the same frequency as said phase voltages, each of the three phase-displaced square wave signals having its amplitude excursions occurring in exact time coincidence with the zero voltage crossings of a respective one of the three line-to-neutral voltages for the three phase voltages;
means responsive to the three phase-displaced square wave signals for developing three phase-displaced, ramp-shaped analog signals each of which corresponds to and is in phase with a respective associated one of the three square wave signals, each ramp-shaped analog signal having a linearly changing ramp-shaped pulse during each half cycle of its associated square wave signal;
means for supplying a d-c analog control voltage representing a desired power flow to the load circuit;
and means for comparing said d-c control voltage to each of the three ramp-shaped analog signals and for producing, from the comparisons, gating signals for triggering the six SCR'"'"'s into conduction at the angles necessary to delivery the desired power flow to the load circuit.
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Accused Products
Abstract
The power delivered from a three-phase A-C power source and through a network of SCR'"'"'s (connected, for example, to form a full wave rectifier bridge or to provide an A-C switch) to a load circuit is adjusted by controlling the conduction angles of the SCR'"'"'s. The control is achieved by developing, from only one of the three alternating phase voltages provided by the A-C power supply, a reference square wave signal of the same frequency and whose amplitude excursions are precisely synchronized or keyed to the zero voltage crossings of the selected phase voltage, even though that phase voltage may include undesired harmonics, noise, transients or other distortion components. Since the influence of the undesired signal components has now been eliminated and since the reference square wave signal also has a fixed phase relationship with the other two phase voltages, logic circuitry may be controlled by the reference square wave signal, and by a d-c control voltage representing a desired power flow to the load circuit, to produce gating signals for triggering the SCR'"'"'s into conduction at the angles required to translate the desired power to the load circuit. With this arrangement, control of all three phases is made immune to undesired signal components in any of the phase voltages.
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Citations
8 Claims
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1. A control system for controlling the conduction angles of three pairs of SCR'"'"'s, to each of which pairs is applied a respective assigned one of the three alternating sinusoidal phase voltages of a three-phase A-C power supply, in order to regulate the power flow from the power supply to a load circuit, comprising:
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a transformer for deriving the corresponding line-to-neutral voltage for a predetermined one of the alternating phase voltages; a band-pass filter for attenuating, in the derived line-to-neutral voltage, frequencies above and below the voltage'"'"'s fundamental frequency, while at the same time introducing no phase shift to that fundamental frequency, the output signal from the band-pass filter thereby being sinusoidal shaped and identical to the predetermined phase voltage except that any harmonics, noise, transients or other distortions in the phase voltage are not present in the filter'"'"'s output signal; means for effectively detecting the zero voltage crossings in the filter'"'"'s sinusoidal output signal and producing a square wave signal which constitutes a reference voltage waveform having the same frequency as said phase voltages and having amplitude excursions which are precisely synchronized to the zero voltage crossings of the predetermined phase voltage; means for developing, from said reference voltage waveform, three phase-displaced square wave signals which are phase-displaced with respect to each other by 120° and
have the same frequency as said phase voltages, each of the three phase-displaced square wave signals having its amplitude excursions occurring in exact time coincidence with the zero voltage crossings of a respective one of the three line-to-neutral voltages for the three phase voltages;means responsive to the three phase-displaced square wave signals for developing three phase-displaced, ramp-shaped analog signals each of which corresponds to and is in phase with a respective associated one of the three square wave signals, each ramp-shaped analog signal having a linearly changing ramp-shaped pulse during each half cycle of its associated square wave signal; means for supplying a d-c analog control voltage representing a desired power flow to the load circuit; and means for comparing said d-c control voltage to each of the three ramp-shaped analog signals and for producing, from the comparisons, gating signals for triggering the six SCR'"'"'s into conduction at the angles necessary to delivery the desired power flow to the load circuit. - View Dependent Claims (2, 3, 4, 5, 6)
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7. A control system for controlling the conduction angles of three pairs of SCR'"'"'s, to each of which pairs is applied a respective assigned one of the three alternating sinusoidal phase voltages of a three-phase A-C power supply, in order to regulate the power flow from the power supply to a load circuit, comprising:
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means for developing, from a predetermined one of the alternating phase voltages, a reference voltage waveform having amplitude excursions which are precisely synchronized to the zero voltage crossings of the predetermined phase voltage, despite the presence of any undesired harmonics, noise, transients or other distortion components in that phase voltage; means for developing, from said reference voltage waveform, three phase-displaced square wave signals each of which is phase-displaced from the other two square wave signals by 120° and
effectively represents a respective assigned one of the three phase voltages, each of said three square wave signals having the same frequency as the three-phase A-C power supply voltage and having its amplitude excursions occurring in exact time coincidence with the zero voltage crossings of the line-to-neutral voltage for its assigned phase voltage,said means for developing said three phase-displaced square wave signals also producing a series of timing pulses periodically recurring at a frequency very high compared to, and a multiple of, the frequency of the three-phase A-C power supply voltage and being precisely synchronized to the zero voltage crossings of the three phase voltages; means for supplying a d-c control voltage representing a desired power flow to the load circuit; a ramp reset logic circuit responsive to said three square wave signals and to said timing pulses for producing three phase-displaced ramp reset signals each of which is associated with a respective one of the three square wave signals and has a series of reset pulses periodically recurring at twice the frequency of the A-C power supply voltage, each reset pulse occurring immediately prior to an amplitude excursion in the associated square wave signal; ramp forming circuitry for developing, from said ramp reset signals, three phase-displaced ramp-shaped signals each of which is associated with a respective one of the three line-to-neutral voltages and has a series of ramp-shaped pulses periodically recurring at twice the frequency of the A-C power supply voltage, each ramp-shaped pulse extending substantially throughout, and occurring in time coincidence with, an entire half cycle of the associated line-to-neutral voltage; and means including a series of three comparators, each of which compares said d-c control voltage with a respective one of said three ramp-shaped signals, for producing gating signals for triggering the six SCR'"'"'s into conduction at the angles necessary to deliver the desired power flow to the load circuit. - View Dependent Claims (8)
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Specification