AC-to-DC converter and variable-frequency drive

An alternating current (AC) to direct current (DC) converter adapted to receive an AC input voltage, comprising:

• an energy storage element, which is one of a supercapacitor and a lithium-ion capacitor;
• a plurality of silicon controlled rectifiers (SCRs) electrically connected to each other to form a full-bridge rectifier that is disposed to receive the AC input voltage, and electrically connected to said energy storage element, said SCRs being configured to receive a control signal, and to be controlled to switch between a conducting state and a non-conducting state based on the control signal so as to convert the AC input voltage into a DC output voltage that is outputted to said energy storage element;
• and
• a first control unit configured to detect zero-crossing points of the AC input voltage and a magnitude of the DC output voltage, and to generate the control signal based on the zero-crossing points and the magnitude of the DC output voltage in a manner that causes the DC output voltage to be at a predetermined voltage magnitude,
• wherein when a logic value of the control signal is equal to a first logical value, said SCRs are in the non-conducting state,
• wherein when the logic value of the control signal is equal to a second logical value, said SCRs are in the conducting state,
• wherein said first control unit is configured to make each change of the logic value of the control signal from the first logical value to the second logical value occur at one of the zero-crossing points of the AC input voltage, and the control signal is a periodic signal having a period that corresponds to a period of the AC input voltage,
• wherein said first control unit is further configured to reduce a duty cycle of the control signal when said first control unit determines that the magnitude of the DC output voltage is greater than the predetermined voltage magnitude, where the duty cycle of the control signal is a ratio of a duration where the logic value of the control signal is equal to the second logical value to the period of the control signal, and
• wherein said first control unit is further configured to increase the duty cycle of the control signal when said first control unit determines that the magnitude of the DC output voltage is smaller than the predetermined voltage magnitude.

A variable-frequency drive (VFD) adapted to an alternating current power source and an inductive load, comprising:

• an alternating current (AC) to direct current (DC) converter including
• an energy storage element, which is one of a supercapacitor and a lithium-ion capacitor,
• a plurality of silicon controlled rectifiers (SCRs) electrically connected to each other to form a full-bridge rectifier that is disposed to receive an AC input voltage from the AC power source, and electrically connected to said energy storage element, said SCRs being configured to receive a control signal, and to be controlled to switch between a conducting state and a non-conducting state based on the control signal so as to convert the AC input voltage into a DC output voltage that is outputted to said energy storage element, and
• a first control unit configured to detect zero-crossing points of the AC input voltage and a magnitude of the DC output voltage, and to generate the control signal based on the zero-crossing points and the magnitude of the DC output voltage in a manner that causes the DC output voltage to be at a predetermined voltage magnitude,
• an inverter unit electrically connected to said AC-to-DC converter so as to receive the DC output voltage, configured to be controlled to convert the DC output voltage to an AC output voltage, and to be electrically connected to the inductive load so as to output the AC output voltage to the inductive load;
• and
• a second control unit electrically connected to said inverter unit, and configured to control said inverter unit to generate the AC output voltage, and to control one of a frequency and an amplitude of the AC output voltage.