Low switch count AC-to-AC power converter

A method for controlling operation of an AC load comprising:

• providing a supply voltage from an AC power source to an AC load, the supply voltage having a supply frequency;
• providing a circuit in series between the AC power source and the AC load to condition the supply voltage, the circuit comprising a plurality of switches forming a group of line-side switches connected to supply lines between line terminals of the AC power source and load terminals of the AC load and a group of floating-neutral side switches connected to the supply lines at one end and together at a common connection at another end;
• and
• selectively operating the circuit in an active mode and a free-wheeling mode so as to selectively provide a full phase voltage to the load terminals during the active mode and a zero voltage to the load terminals during the free-wheeling mode of operation;
• wherein operating the circuit further comprises controlling a duty cycle of the circuit so as to control an average voltage to be applied to the AC load, with the duty cycle comprising a ratio of a time of circuit operation in the active mode to a total time of circuit operation;
• and
• wherein the duty cycle of the circuit is controlled by applying a voltage modulating periodic function to the circuit, so as to modulate a frequency of a load voltage present across the load terminals of the AC load and enable variable speed operation of the AC load.

A motor drive to control transmission of voltage and current from an AC power source to an AC motor, the motor drive comprising:

• an input connectable to line terminals of an AC power source so as to receive a supply voltage therefrom, the supply voltage having a supply frequency;
• an output connectable to motor terminals of an AC motor;
• a plurality of supply lines connecting line terminals of the AC power source to motor terminals of the AC motor, each supply line corresponding to a phase in the AC motor;
• a plurality of insulated gate bipolar transistors (IGBTs) selectively switchable between an On condition and an Off condition to control transmission of voltage and current from an AC power source to an AC motor, the plurality of IGBTs including:
• a group of line-side IGBTs connected to the supply lines between the line terminals and the motor terminals, such that each supply line includes a line-side IGBT connected thereto;
• and
• a group of floating-neutral side IGBTs connected to the supply lines at one end and together at a common connection at another end, such that each supply line includes a floating-neutral side IGBT connected thereto;
• and
• a processor programmed to apply a voltage modulating control algorithm to control switching of each of the group of line-side IGBTs and each of the group of floating-neutral side IGBTs between an On condition or an Off condition to selectively operate the circuit in an active mode and a free-wheeling mode, with a full phase voltage being provided to the motor terminals during the active mode and a zero voltage being provided to the motor terminals during the free-wheeling mode;
• wherein, in applying the voltage modulating control algorithm, the processor is further programmed to:
• control a duty cycle of the motor drive so as to control an average motor voltage to be applied to the AC motor, with the duty cycle comprising a ratio of a time of motor drive operation in the active mode to a total time of motor drive operation;
• and
• modulate a frequency of a motor voltage present across the motor terminals of the AC motor, so as to enable variable speed operation of the AC motor.