Single phase to three phase converter capable of variable speed motor operation

Apparatus for converting a single phase AC input to a three phase AC output which is provided to a three phase induction motor comprising:

• (a) a pair of single phase AC input terminals connected to a pair of input lines;
• (b) three output lines connectable to the three phase motor load;
• (c) an inverter circuit connected to the input lines to receive single phase AC power therefrom and having an output node connected to one of the output lines, the inverter circuit including a pair of DC bus lines, a pair of capacitors connected together at a node and connected across the DC bus lines, the node between the capacitors connected to one of the input lines, a pair of rectifying diodes connected together at a node and connected across the DC bus lines, the node between the diodes connected to the other of the input lines, and two gate controlled switching devices connected together at the output node and connected across the DC bus lines;
• (d) a bi-directional switch responsive to a control signal connected between one of the input lines and a second of the output lines, the third output line connected to the other of the input lines;
• (e) control means, connected to monitor the voltage on the three output lines and the output current through the bi-directional switch and from the inverter circuit, for providing control signals to the bi-directional switch and the gate controlled switching devices of the inverter circuit to provide output currents from the inverter circuit and through the bi-directional switch to the motor which have fundamental frequency components lower than the frequency of the AC input voltage on the input lines and at different phases and at increasing frequency as the speed of the motor increases to provide controlled start-up of motor connected to the output lines, wherein after start-up of the motor, the control means turns off the bi-directional switch and controls the switching devices of the inverter circuit to provide single phase power of increasing frequency to the motor as the motor increases in speed until the speed of the motor is near its rated speed, and thereafter controls the bi-directional switch to conduct constantly and controls the switching devices of the inverter circuit to provide balanced three phase power to the motor at the rated frequency of the motor.

Apparatus for converting a single phase AC input to a three phase AC output which is provided to a three phase induction motor, comprising:

• (a) a pair of single phase AC input terminals connected to a pair of input lines;
• (b) three output lines connectable to a three phase motor load;
• (c) an inverter circuit connected to the input lines to receive single phase AC power therefrom and having an output node connected to one of the output lines, the inverter circuit responsive to control signals to provide an AC output voltage at its output node;
• (d) a bi-directional switch responsive to a control signal connected between one of the input lines and a second of the output lines, the third output line connected to the other of the input lines;
• (e) control means, connected to monitor the voltage on the three output lines and the output current through the bi-directional switch and from the inverter circuit, for providing control signals to the bi-directional switch and the inverter circuit when the motor has reached at or near its rated speed to operate the inverter circuit to provide a selected voltage level at the rated frequency to the motor and to switch the bi-directional switch to provide a selected phase delay of conduction and a selected voltage level on the output line connected to the bi-directional switch at the rated frequency, such that the voltage applied to the motor at the three output lines at the rated frequency of the motor may be varied to control the motor speed and flux level.

Apparatus for converting a single phase AC input to a three phase AC output comprising:

• (a) a pair of single phase AC input terminals connected to a pair of input lines;
• (b) three output lines connectable to a three phase motor load;
• (c) an inverter circuit connected to the input lines to receive single phase AC power therefrom and having an output node connected to one of the output lines, the inverter circuit including a pair of DC bus lines, a pair of capacitors connected together at a node and connected across the DC bus lines, the node between the capacitors connected to one of the input lines, a pair of rectifying diodes connected together at a node and connected across the DC bus lines, the node between the diodes connected to the other of the input lines, and two gate controlled switching devices connected together at the output node and connected across the DC bus lines;
• (d) a bi-directional switch responsive to a control signal connected between one of the input lines and a second of the output lines, the third output line connected to the other of the input lines;
• (e) control means, connected to monitor the voltage on the three output lines and the output current through the bi-directional switch and from the inverter circuit, for providing control signals to the bi-directional switch and the inverter circuit switching devices to provide output currents from the inverter circuit and through the bi-directional switch to the motor which have fundamental frequency components lower than the frequency of the AC input voltage on the input lines and at different phases whereby controlled start-up of a three phase AC motor connected to the output lines can be obtained.

A method of converting a single phase AC input to a three phase AC output to drive a three phase AC induction motor and to start the motor, comprising the steps of:

• (a) providing an inverter circuit connected to the AC input lines which has switching devices therein which can be controlled to provide a selected AC output from the inverter circuit, and providing a bi-directional switch connected to one of the single phase AC input lines, the output of the bi-directional switch, the other of the AC input lines, and the output of the inverter providing output voltages connected to the motor;
• (b) on start-up of the motor, switching the bi-directional switch on and off in one direction of conduction only during a plurality of cycles of the single phase AC input voltage, and then switching the bi-directional switch to conduct only in the other direction for a plurality of cycles of the single phase AC input voltage, and repeating the switching of the bi-directional switch to provide a fundamental component of current through the bi-directional switch which is lower than the frequency of the single phase AC input voltage;
• (c) switching the switching devices of the inverter circuit to provide an output current from the inverter circuit which has a fundamental current component which is at the same frequency as the fundamental component of current passed through the bi-directional switch but at a phase angle with respect thereto.