AC motor drive system with a two phase power supply

What is claimed is:

1. An AC motor drive system for driving an AC motor having a first winding and a second winding, the first winding having first and second ends and a first winding impedance, the second winding having first and second ends and a second winding impedance that is greater than the first impedance, the drive system comprising:

two-phase power supply means for converting an inputted power supply voltage into a two-phase AC output voltage, the two-phase power supply means having a common terminal, a first phase output terminal, and a second phase output terminal;

the common terminal for connection to the respective first ends of the first and second windings; and

the first phase terminal and the second phase terminal for connection to the second ends of the first and second windings, respectively;

the two-phase power supply means including

power switching means for selectively converting the inputted power supply voltage to the two-phase output voltage, and

voltage control means for generating switching control signals for controlling the operation of the power switching means;

the voltage control means including

oscillator means for generating an oscillating signal having a frequency representative of a desired operating speed of the motor,

waveform signal generating means, responsive to the oscillating signal, for generating a first sinusoidal waveform signal and a second sinusoidal waveform signal, the first and second sinusoidal waveform signals each having a frequency determined by the oscillating signal frequency and having a predetermined phase angle difference therebetween, the waveform signal generating means including direction control means, responsive to a direction control signal, for selectively changing the predetermined phase angle difference between the first sinusoidal waveform signal and the second sinusoidal waveform signal,

means, coupled to the waveform signal generating means to receive the first and second sinusoidal waveform signals, for providing sinusoidally weighted pulse width modulated (PWM) switching signals as the switching control signals to control the operation of the power switching means,

whereby the two-phase power supply means converts the inputted power supply voltage to a two-phase voltage provided on the first, second, and common terminals to selectively drive the motor in either direction.

2. The AC motor drive system of claim 1, wherein the power switching means includes a plurality of semiconductor switches driven to provide the two-phase output voltage, each semiconductor switch including overcurrent sensing means, connected to the voltage control means, for disabling the voltage control means upon sensing an overcurrent condition in the semiconductor switch.

3. The AC motor drive system of claim 1, wherein said means for providing sinusoidally weighted pulse width modulated switching signals includes:

means for generating a fixed frequency comparison signal, the fixed frequency comparison signal having a frequency of approximately 7.5 KHz; and

means for comparing the first and second sinusoidal waveform signals and the fixed frequency comparison signal to provide the sinusoidally weighted pulse width modulated switching signals.

4. The AC motor drive system of claim 1, wherein the waveform signal generating means further comprises:

means, coupled to the oscillator means, for generating successive memory address signals;

memory means, coupled to the address signal generating means, for providing a digital data signal in response to each address signal; and

converting means for converting the digital data signals from the memory means into the first and second sinusoidal waveform signals.

5. The AC motor drive system of claim 4, wherein the direction control means comprises a logic inverting circuit for selectively inverting a signal from the address signal generating means and for supplying the selectively inverted signal to the converting means to drive the motor in a desired direction.

6. The AC motor drive system of claim 1, wherein the direction control means comprises a signal switching circuit for selectively swapping the first and second sinusoidal waveform to drive the motor in a desired direction.

7. An AC motor drive system for driving an AC motor having a first winding and a second winding, the first winding having first and second ends and a first winding impedance, the second winding having first and second ends and a second winding impedance that is greater than the first impedance, the drive system comprising:

two-phase power supply means for converting an inputted power supply voltage into a two-phase AC output voltage, the two-phase power supply means having a common terminal, a first phase output terminal, and a second phase output terminal;

the common terminal for connection to the respective first ends of the first and second windings; and

the first phase terminal and the second phase terminal for connection to the second ends of the first and second windings, respectively;

the two-phase power supply means including

power switching means for selectively converting the inputted power supply voltage to the two-phase output voltage, and

voltage control means for generating switching control signals for controlling the operation of the power switching means;

the voltage control means including

oscillator means for generating an oscillating signal having a frequency representative of a desired operating speed of the motor,

waveform signal generating means, responsive to the oscillating signal, for generating a first, a second, and a third sinusoidal waveform signal, the first, second, and third sinusoidal waveform signals each having a frequency determined by the oscillating signal frequency, the first and second sinusoidal waveform signals having a predetermined phase angle difference therebetween, the waveform signal generating means including direction control means, responsive to a direction control signal, for selectively changing the predetermined phase angle difference between the first sinusoidal waveform signal and the third sinusoidal waveform signal, and

means, coupled to the waveform signal generating means to receive the first, second, and third sinusoidal waveform signals, for providing sinusoidally weighted pulse width modulated (PWM) switching signals as the switching control signals to control the operation of the power switching means,

whereby the two-phase power supply means converts the inputted power supply voltage to a two-phase voltage provided on the first, second, and common terminals to selectively drive the motor in either direction.

8. The AC motor drive system of claim 7, wherein the power switching means includes a plurality of semiconductor switches driven to provide the two-phase output voltage, each semiconductor switch including overcurrent sensing means, connected to the voltage control means, for disabling the voltage control means upon sensing an overcurrent condition in the semiconductor switch.

9. The AC motor drive system of claim 7, wherein said means for providing sinusoidally weighted pulse width modulated switching signals includes:

means for generating a fixed frequency comparison signal, the fixed frequency comparison signal having a frequency of approximately 7.5 KHz; and

means for comparing the first, second, and third sinusoidal waveform signals and the fixed frequency comparison signal to provide the sinusoidally weighted pulse width modulated switching signals.

10. The AC motor drive system of claim 7, wherein the waveform signal generating means further comprises:

means, coupled to the oscillator means, for generating a successive memory address signals and for generating a DAC select signal;

memory means, coupled to the address signal generating means, for providing a digital data signal in response to each address signal; and

converting means for converting the digital data signals from the memory means into the first, second, and third sinusoidal waveform signals.

11. The AC motor drive system of claim 10, wherein the direction control means comprises a logic inversion circuit for selectively inverting the DAC select signal from the address signal generating means and for supplying the selectively inverted DAC select signal to the converting mean to drive the motor in a desired direction.

12. The AC motor drive system of claim 7, wherein the direction control means comprises a switching circuit for selectively swapping the first and third sinusoidal waveform to drive the motor in a desired direction.

13. An AC motor drive system, for connection to a single-phase power supply having a line conductor and a neutral conductor, for driving an AC motor having a first winding and a second winding, the first winding having first and second ends and a first winding impedance, the second winding having first and second ends and a second winding impedance that is greater than the first impedance, the drive system comprising:

two-phase power supply means, including a line terminal for coupling to the line conductor of the single-phase power supply, for converting the single-phase power supply into a two-phase output voltage, the two-phase power supply means having a first phase output terminal and a second phase output terminal;

a neutral terminal for connection to the neutral conductor of the single-phase power source and for connection to the respective first ends of the first and second windings; and

the first and second phase terminals for connection to the second ends of the first and second windings, respectively,

the two-phase power supply means including

power switching means for selectively converting the inputted power supply voltage to the two-phase output voltage, and

voltage control means for generating switching control signals for controlling the operation of the power switching means;

the voltage control means including

waveform signal generating means for generating a first sinusoidal waveform signal and a second sinusoidal waveform signal to have a predetermined phase angle difference therebetween and a common frequency that corresponds to a desired operating speed of the motor, the waveform signal generating means including direction control means, responsive to a direction control signal, for selectively changing the predetermined phase angle difference between the first sinusoidal waveform signal and the second sinusoidal waveform signal,

first means, coupled to the waveform signal generating means, for providing a first sinusoidally weighted pulse width modulated (PWM) switching signal,

second means, coupled to the waveform signal generating means, for providing a second sinusoidally weighted pulse width modulated (PWM) switching signal, the first and second PWM signals operating as switching control signals to control the operation of the power switching means,

whereby the two-phase power supply means converts power received from the single-phase power supply to a two-phase voltage to selectively drive the motor in either direction.

14. The AC motor drive system of claim 13, wherein the power switching means includes a plurality of semiconductor switches driven to provide the two-phase output voltage, each semiconductor switch including overcurrent sensing means, connected to the voltage control means, for disabling the voltage control means upon sensing an overcurrent condition in the semiconductor switch.

15. The AC motor drive system of claim 13, wherein said means for providing sinusoidally weighted pulse width modulated switching signals includes:

means for generating a fixed frequency comparison signal, the fixed frequency comparison signal having a frequency of approximately 7.5 KHz; and

means for comparing the first and second sinusoidal waveform signals and the fixed frequency comparison signal to provide the sinusoidally weighted pulse width modulated switching signals.

16. The AC motor drive system of claim 13, wherein the direction control means comprises a signal switching circuit for selectively swapping the first and second sinusoidal waveform to drive the motor in a desired direction.

17. An AC motor drive system, comprising:

two-phase power supply means for converting an inputted power supply voltage into a two-phase AC output voltage, the two-phase power supply means having a common terminal, a first phase output terminal, and a second phase output terminal;

an AC motor having a first winding and a second winding;

the first winding having first and second ends and a first winding impedance;

the second winding having first and second ends and a second winding impedance that is greater than the first impedance;

the respective first ends of the first and second windings being connected to the common terminal of the two-phase power supply means; and

the second ends of the first and second windings being respectively connected to the first phase terminal and the second phase terminal of the two-phase power supply means,

the two-phase power supply means including

power switching means for selectively converting the inputted power supply voltage to the two-phase output voltage, and

voltage control means for generating switching control signals for controlling the operation of the power switching means;

the voltage control means including

oscillator means for generating an oscillating signal having a frequency representative of a desired operating speed of the motor,

waveform signal generating means, responsive to the oscillating signal, for generating a first sinusoidal waveform signal and a second sinusoidal waveform signal, the first and second sinusoidal waveform signals each having a frequency determined by the oscillating signal frequency and having a predetermined phase angle therebetween, the waveform signal generating means including direction control means, responsive to a direction control signal, for selectively changing the predetermined phase angle difference between the first sinusoidal waveform signal and the second sinusoidal waveform signal,

means, coupled to the waveform signal generating means to receive the first and second sinusoidal waveform signals, for providing sinusoidally weighted pulse width modulated (PWM) switching signals as the switching control signals to control the operation of the power switching means,

whereby the two-phase power supply means converts the inputted power supply voltage to a two-phase voltage provided on the first, second, and common terminals to selectively drive the motor in either direction.

18. The AC motor drive system of claim 17, wherein the waveform signal generating means further comprises:

means, coupled to the oscillator means, for generating successive memory address signals;

memory means, coupled to the address signal generating means, for providing a digital data signal in response to each address signal; and

converting means for converting the digital data signals from the memory means into the first and second sinusoidal waveform signals.

19. The AC motor drive system of claim 18, wherein the direction control means comprises a logic inverting circuit for selectively inverting a signal from the address signal generating means and for supplying the selectively inverted signal to the converting means to drive the motor in a desired direction.

20. The AC motor drive system of claim 17, wherein the direction control means comprises a signal switching circuit for selectively swapping the first and second sinusoidal waveform to drive the motor in a desired direction.

21. An AC motor drive system, comprising:

two-phase power supply means for converting an inputted power supply voltage into a two-phase AC output voltage, the two-phase power supply means having a common terminal, a first phase output terminal, and a second phase output terminal;

an AC motor having a first winding and a second winding;

the first winding having first and second ends and a first winding impedance;

the second winding having first and second ends and a second winding impedance that is greater than the first impedance;

the respective first ends of the first and second windings being connected to the common terminal of the two-phase power supply means; and

the second ends of the first and second windings being respectively connected to the first phase terminal and the second phase terminal of the two-phase power supply means,

the two-phase power supply means including

power switching means for selectively converting the inputted power supply voltage to the two-phase output voltage, and

voltage control means for generating switching control signals for controlling the operation of the power switching means;

the voltage control means including

oscillator means for generating an oscillating signal having a frequency representative of a desired operating speed of the motor,

waveform signal generating means, responsive to the oscillating signal, for generating a first, a second, and a third sinusoidal waveform signal, the first, second, and third sinusoidal waveform signals each having a frequency determined by the oscillating signal frequency, the waveform signal generating means including direction control means, responsive to a direction control signal, for reversing the direction of the AC motor by changing the predetermined phase angle difference between the first sinusoidal waveform signal and the third sinusoidal waveform signal,

means, coupled to the waveform signal generating means to receive the first, second, and third sinusoidal waveform signals, for providing sinusoidally weighted pulse width modulated (PWM) switching signals as the switching control signals to control the operation of the power switching means,

whereby the two-phase power supply means converts the inputted power supply voltage to a two-phase voltage provided on the first, second, and common terminals to selective drive the motor in either direction.

22. The AC motor drive system of claim 21, wherein the waveform signal generating means further comprises:

means, coupled to the oscillator means, for generating a successive memory address signals and for generating a DAC select signal;

memory means, coupled to the address signal generating means, for providing a digital data signal in response to each address signal; and

converting means for converting the digital data signals from the memory means into the first, second, and third sinusoidal waveform signals.

23. The AC motor drive system of claim 22, wherein the direction control means comprises a logic inversion circuit for selectively inverting the DAC select signal from the address signal generating means and for supplying the selectively inverted DAC select signal to the converting means to drive the motor in a desired direction.

24. The AC motor drive system of claim 21, wherein the direction control means comprises a switching circuit for selectively swapping the first and third sinusoidal waveform to drive the motor in a desired direction.

25. An AC motor drive system, comprising:

two-phase power supply means for converting an inputted power supply voltage into a two-phase AC output voltage, the two-phase power supply means having a common terminal, a first phase output terminal, and a second phase output terminal;

an AC motor having a first winding and a second winding;

the first winding having first and second ends and a first winding impedance;

the second winding having first and second ends and a second winding impedance that is greater than the first impedance;

the respective first ends of the first and second windings being connected to the common terminal of the two-phase power supply means; and

the second ends of the first and second windings being respectively connected to the first phase terminal and the second phase terminal of the two-phase power supply means,

the two-phase power supply means including

power switching means for selectively converting the inputted power supply to the two-phase output voltage; and

voltage control means for generating switching control signals for controlling the operation of the power switching means;

the voltage control means including

waveform signal generating means for generating a first sinusoidal waveform signal and a second sinusoidal waveform signal to have a predetermined phase angle difference therebetween and a common frequency that corresponds to a desired operating speed of the motor, the waveform signal generating means including direction control means, responsive to a direction control signal, for reversing the direction of the AC motor by changing the predetermined phase angle difference between the first sinusoidal waveform signal and a second sinusoidal waveform signal,

first means, coupled to the waveform signal generating means, for providing a first sinusoidally weighted pulse width modulated (PWM) switching signal,

second means, coupled to receive the second sinusoidal waveform signal and the fixed frequency signal, for providing a second sinusoidally weighted pulse width modulated (PWM) switching signal, the first and second PWM signals operating as switching control signals to control the operation of the power switching means, and

whereby the two-phase power supply means converts the inputted power supply voltage to a two-phase voltage provided on the first, second, and common terminals to selectively drive the motor in either direction.

26. The AC motor drive system of claim 25, wherein the direction control means comprises a signal switching circuit for selectively swapping the first and second sinusoidal waveform to drive the motor in a desired direction.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

This invention relates in general to induction motor drive systems and, more particularly, to an induction motor drive system for operating a single-phase, two winding induction motor from a two-phase power supply circuit and including a direction reversing capability.

A conventional split-phase capacitor start or capacitor run single phase induction motor, also known in the art and referred to hereinafter as a permanent split capacitor (PSC) motor, has two stator windings, a "main" winding and a "start" winding. FIG. 1 illustrates an exemplary PSC motor 100 that includes a main winding 102 and a start winding 104 that are commonly connected at one end. Main winding 102 and start winding 104 are mounted in the stator (not shown) of motor 100 and spatially separated from each other by an angle related to the rated speed of motor 100, e.g., 90.degree. for a two pole, 3600 RPM motor, as is well known in the art. Such PSC motors are designed to be operated with a run capacitor, such as a run capacitor 106, connected in series with start winding 104. It is a typical practice in the industry for the motor manufacturer to not supply the run capacitor with the motor, but instead to only specify parameters of the capacitor, e.g., capacitance and power rating, sufficient to enable a user to procure and install the capacitor.

In the operation of PSC motor 100, main winding 102 and the series combination of start winding 104 and run capacitor 106 are connected in parallel with each other and directly across a single phase power source 110. Since start winding 104 is energized through capacitor 106, the phase angle of the current flowing through start winding 104 is shifted with respect to the current flowing through main winding 102, such that the phase angle between the respective currents flowing in windings 102 and 104 is approximately 90.degree. while the motor is running. The phase angle between the currents in windings 102 and 104 and the spatial separation of those windings result in the creation of a rotating magnetic field which is inductively coupled to the rotor (not shown) of motor 100, to exert a rotational force on the rotor.

The rotor of motor 100 attempts to rotate in synchronism with the rotating magnetic field but lags the rotating magnetic field by a "slip" factor, resulting in a torque on the rotor which is in part proportional to the amount of slip.

The starting torque exerted on the rotor of motor 100 during a starting period when motor 100 is started and accelerated to rated speed is also proportional to the sine of the phase angle between the currents flowing in windings 102 and 104. Therefore, in order to maximize the starting torque, it is necessary to achieve a phase angle of 90.degree. during starting. However, the starting torque for a single phase PSC motor, such as motor 100, is generally poor because the specified parameters of the run capacitor are only optimized for running conditions, not starting conditions. Thus, the capacitance of run capacitor 106 is specified by the manufacturer based on the impedances of windings 102 and 104 that will be experienced during running of motor 100, rather than during starting. However, as known in the art, the apparent values of motor winding impedances vary during the starting period of a PSC motor and are therefore different during starting than during running. As a result of the capacitance of capacitor 106 being optimized for running and not for starting, its magnitude is too small for starting. This results in the phase angle between the currents flowing in windings 102 and 104 being less than 90.degree. during the starting period and the starting torque being less than a maximum possible starting torque.

One solution known in the art to compensate for the insufficient magnitude of capacitor 106 during starting is to connect a starting capacitor 112 across capacitor 106 to increase the total capacitance in series with start winding 104 and thereby increase the current flowing in the start winding, the phase angle and the starting torque of motor 100. Starting capacitor 112 is disconnected, e.g., by means of a centrifugal switch, positive temperature coefficient thermistor (PTC device), or relay, once the motor has reached running speed. Disadvantageously, although operation of starting capacitor 112 generally improves the starting torque of motor 100, its use still does not maximize torque throughout the starting period of motor 100. Ideally, the magnitude of the capacitance in series with start winding 104 would have to be continuously varied during the start period to maintain a desired phase angle while the respective impedances of windings 102 and 104 vary.

Conventional single phase PSC motors are commonly used in heating, ventilating and air-conditioning (HVAC) systems to drive system loads such as fans, pumps and compressors. HVAC systems are subject to widely varying demand cycles due to a variety of factors such as, for example, daily and seasonal fluctuations of ambient temperature, human activity in the controlled environment, and intermittent operation of other equipment in the controlled environment. Accordingly, in order to assure a satisfactory temperature of the controlled environment, the HVAC system must have the heating and/or cooling capacity to accommodate "worst case" conditions. As a result, under less than worst case conditions the HVAC system has a significant over-capacity and is necessarily operated at reduced loading. Since the maximum operating efficiency of a motor, such as a PSC motor, is normally obtained only when the motor is operating at full load, the reduced HVAC system load results in inefficient operation of the motor. Further, to the extent that motors are required to cycle on and off to meet HVAC load requirements less than the capacity of the HVAC system, further significant operating inefficiencies are experienced. Such further inefficiencies include the operating cost of frequently starting motors. A reduction in the useful life of such motors also results from the well known thermal and mechanical stresses existing in this field.

A solution for overcoming the above inefficiencies resulting from the excessive capacity of an HVAC system is to vary the system capacity to meet the demand on the system. One method for varying HVAC system capacity is by varying the speed of the motors driving the HVAC system loads in accordance with the demand. With respect to HVAC system loads driven by single phase motors, such as PSC motors, in order to effect a desired motor speed control, it would be necessary to vary the frequency of the single phase power supplied to the motor. However, with respect to PSC motors, the run capacitor, e.g., capacitor 106 of motor 100 (FIG. 1), is optimized for a particular set of running conditions, including operation at a nominal frequency, e.g., 60 Hz. As a result, operation of a PSC motor at other than the nominal frequency results in production of less than optimal torque and inefficient operation. While some applications may exist in which very limited speed control of a PSC motor is achieved by a small variation of the single phase source frequency, such variation from the nominal frequency results in less efficient operation since the motor is nevertheless designed for optimum performance at the nominal source frequency.

A conventional implementation of varying motor speed to modulate HVAC system capacity typically requires a two- or three-phase motor supplied with two- or three-phase power, respectively. The use of such polyphase motors and power supplies enables variation of motor speed by varying the frequency of the voltage applied to the motor while maintaining a constant volts/frequency (volts/hertz) ratio. Maintenance of a constant volts/hertz ratio corresponds to maintenance of an approximately constant air gap flux and efficient motor operation while delivering rated torque. The use of polyphase motors also offers several other advantages over that of a single phase motor such as, for example, lower locked rotor currents, higher starting torque, lower full load currents and improved reliability due to elimination of the start and/or run capacitor which are required in single phase motors. Disadvantageously, such polyphase motors are more expensive than single phase motors having the same horsepower rating.

Such applications employing polyphase motors generally require provision of variable frequency polyphase power from either a single phase or polyphase line source by means of a power supply circuit, including a polyphase inverter, coupled between the motor and the line source. One drawback to this arrangement occurs in the event that the power supply circuit fails and it is not possible to connect the polyphase motor directly to the line source, such as, for example, when a three-phase motor is driven by an inverter which receives power from a single phase line source. Failure of the power supply circuit therefore results in failure and unavailability of the system utilizing the polyphase motor.

Previous attempts to address the problem of backup power for polyphase motors fed from a single phase power source have required inverter redundancy or additional circuit means for temporarily directly connecting the polyphase motor to the single phase power source. However, the additional circuit means required to "simulate" polyphase power may not provide truly polyphase power and therefore may not drive the polyphase motor at optimum efficiency.

Further, in a few applications it has been desirable to provide a single phase motor which can easily reverse its direction. Direction reversal could be very useful in such applications as dishwashing machines, clothes washing machines, fans, and blowers, for example.

Conventionally, direction reversal is most easily achieved in DC motors by simply reversing the current flow in either the stator coil or armature coil. However, in single phase AC motors, direction reversal is not so simple. In the conventional single phase motor 100 shown in FIG. 1, run capacitor 106 and start capacitor 112 cause a negative phase differential between main winding 102 and start winding 104. In other words, the phase angle of main winding 102 "lags" that of start winding 104. As is well known, the phase relationship between the main and start windings determines the direction of rotation of the motor. Motor direction may be reversed by changing the phase relationship between the main and start windings from a lagging to a leading relationship, or vice versa.

In conventional motor 100, a leading phase relationship may be achieved by connecting the run capacitor 106, and during starting start capacitor 112, in series with the main winding 102, rather than in series with start winding 104. This will achieve the desired direction reversal. However, this approach suffers from many difficulties. For example, the main winding and start winding often have different characteristics, such as current carrying capability and impedance in many conventional motors. In such motors, moving the run and start capacitors to the main winding will cause inappropriate voltages to be applied to and currents to flow through the main and start windings, thereby reducing efficiency and performance. Furthermore, due to the typically high currents flowing through main winding 102 and start winding 104 during motor operation, an automatic switching system (i.e., a relay) would necessarily be undesirably large and expensive. Changing the connection of these capacitors from start winding 104 to main winding 106 or altering the internal connections of the motor is often impracticable in most applications and will result in impaired performance for many types of motors.

Finally, conventional motor 100 may suffer from the drawback of being acoustically noisy or of producing too much vibration for a desired application. Such noise and vibrations are primarily caused by the rotation of the rotor and by the fluctuating magnetic fields created by the windings of the motor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an AC motor drive system which overcomes the aforementioned problems and disadvantages of conventional drive systems. To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is directed to an AC motor drive system for driving an AC motor having a first winding and a second winding, the first winding having first and second ends and a first winding impedance and the second winding having first and second ends and a second winding impedance that is greater than the first impedance. The drive system comprises two-phase power supply means for converting an inputted power supply voltage into a two-phase AC output voltage. The two-phase power supply means has a common terminal, a first phase output terminal, and a second phase output terminal. The common terminal of the two-phase power supply means is adapted for connection to the respective first ends of the first and second windings. The first and second phase terminals of the two-phase power supply means are adapted for connection to the second ends of the first and second windings, respectively. The two-phase power supply means includes power switching means for selectively converting the inputted power supply voltage to the two-phase output voltage, and voltage control means for generating switching control signals for controlling the operation of the power switching means. The voltage control means includes oscillator means for generating an oscillating signal having a frequency representative of a desired operating speed of the motor. The voltage control means further includes waveform signal generating means, responsive to the oscillating signal, for generating a first sinusoidal waveform signal and a second sinusoidal waveform signal. The first and second sinusoidal waveform signals each have a frequency determined by the oscillating signal frequency and have a predetermined phase angle difference therebetween. The waveform signal generating means may include direction control means, responsive to a direction control signal, for selectively changing the predetermined phase angle difference between the first sinusoidal waveform signal and the second sinusoidal waveform signal. The voltage control means also includes means, coupled to the waveform signal generating means to receive the first and second sinusoidal waveform signals, for providing sinusoidally weighted pulse width modulated (PWM) switching signals as the switching control signals to control the operation of the power switching means. The two-phase power supply means converts the inputted power supply voltage to a two-phase voltage provided on the first, second, and common terminals to selectively drive the motor in either direction.

In accordance with another embodiment of the invention similar in some respects to the first embodiment, the voltage control means of the AC motor drive system includes oscillator means for generating an oscillating signal having a frequency representative of a desired operating speed of the motor. The voltage control means further includes waveform signal generating means, responsive to the oscillating signal, for generating a first, a second, and a third sinusoidal waveform signal. The first, second, and third sinusoidal waveform signals each have a frequency determined by the oscillating signal frequency, and the first and second sinusoidal waveform signals have a predetermined phase angle difference therebetween. The waveform signal generating means include direction control means, responsive to a direction control signal, for selectively changing the predetermined phase angle difference between the first sinusoidal waveform signal and the third sinusoidal waveform signal. The voltage control means also includes means, coupled to the waveform signal generating means to receive the first, second, and third sinusoidal waveform signals, for providing sinusoidally weighted pulse width modulated (PWM) switching signals as the switching control signals to control the operation of the power switching means. The two-phase power supply means converts the inputted power supply voltage to a two-phase voltage provided on the first, second, and common terminals to selectively drive the motor in either direction.

In accordance with yet another embodiment of the invention, there is provided an AC motor drive system for connection to a single-phase AC power supply having a line conductor and a neutral conductor and for driving an AC motor having a first winding and a second winding. The first winding has first and second ends and a first winding impedance, and the second winding has first and second ends and a second winding impedance that is greater than the first impedance. The drive system comprises two-phase power supply means, including a line terminal for coupling to the line conductor of the single-phase power supply, for converting the single-phase power supply into a two-phase output voltage. The two-phase power supply means has a first phase output terminal and a second phase output terminal. The drive system comprises a neutral terminal for connection to the neutral conductor of the single-phase power source and for connection to the respective first ends of the first and second windings. The first and second phase terminals are for connection to the second ends of the first and second windings, respectively. The two-phase power supply means includes power switching means for selectively converting the inputted power supply voltage to the two-phase output voltage, and voltage control means for generating switching control signals for controlling the operation of the power switching means. The voltage control means includes waveform signal generating means for generating a first sinusoidal waveform signal and a second sinusoidal waveform signal to have a predetermined phase angle difference therebetween and a common frequency that corresponds to a desired operating speed of the motor. The waveform signal generating means includes direction control means, responsive to a direction control signal, for selectively changing the predetermined phase angle difference between the first sinusoidal waveform signal and the second sinusoidal waveform signal. The two-phase power supply means further includes first means, coupled to the waveform signal generating means, for providing a first sinusoidally weighted pulse width modulated (PWM) switching signal, and second means, coupled to the waveform signal generating means, for providing a second sinusoidally weighted pulse width modulated (PWM) switching signal. The first and second PWM signals operate as switching control signals to control the operation of the power switching means. The two-phase power supply means converts power received from the single-phase power supply to a two-phase voltage to selectively drive the motor in either direction.

Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.

FIG. 1 is a schematic diagram showing a conventional PSC motor coupled to a single-phase power supply;

FIG. 2 is a schematic diagram illustrating a motor coupled to a two-phase power supply in accordance with an embodiment of the present invention;

FIG. 3 illustrates a motor drive system constructed in accordance with an embodiment of the present invention;

FIG. 4(A) illustrates a power switching device included in an inverter circuit shown in FIG. 3;

FIG. 4(B) illustrates an alternate power switching device included in the inverter circuit shown in FIG. 3.

FIGS. 5(A) and 5(B) are phasor diagrams illustrating voltages produced by the inverter circuit of FIG. 3;

FIG. 5(C) is a phasor diagram illustrating voltages produced by the circuit shown in FIGS. 9(A) and 9(B).

FIG. 6(A) is a block diagram of a first embodiment of a circuit for controlling the operation of the inverter circuit of FIG. 3;

FIG. 6(B) is a block diagram of a second embodiment of a circuit for controlling the operation of the inverter circuit of FIG. 3;

FIG. 7 is a graphical illustration of voltage waveforms produced during operation of the circuits shown in FIGS. 3 and 6(A) and 6(B);

FIGS. 8 (A) and 8(B) are graphical illustrations of voltage waveforms produced during operation of the inverter circuit shown in FIG. 3;

FIG. 9(A) is a block diagram of a third embodiment of a circuit for controlling the operation of the inverter circuit of FIG. 3;

FIG. 9(B) is a block diagram of a fourth embodiment of a circuit for controlling the operation of the inverter circuit of FIG. 3.;

FIG. 10 is a graphical illustration of voltage waveforms produced during operation of the circuit shown in FIG. 9;

FIG. 11 illustrates a motor drive system constructed in accordance with another embodiment of the present invention; and

FIG. 12 is a block diagram of a control circuit for use in the system shown in FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

In accordance with illustrated embodiments of the present invention, an induction motor drive system is provided in which a two winding, single-phase induction motor, such as a PSC motor, is coupled to a two-phase power supply circuit such that the two motor windings are respectively connected to the two-phases of the power supply circuit. The power supply circuit is configured and operated to provide voltages that result in achieving an optimum phase angle between the motor winding currents during starting and running of the motor. The power supply circuit is further configured and operated to provide variable speed operation of the single-phase motor while maintaining a constant volts/hertz ratio at all operating speeds. The power supply circuit is additionally configured and operated to enable operation of the motor in either direction of rotation.

Referring now to the drawings, FIG. 2 diagrammatically illustrates a two winding, single-phase induction motor 150 coupled to a two-phase power source provided in accordance with an embodiment of the present invention. Motor 150 includes two stator windings, i.e., a main winding 152 and a start winding 154 that are commonly connected at a common winding node 156. Windings 152 and 154 of motor 150 are connected across a first phase voltage V.sub.P1 and a second phase voltage V.sub.P2 of a two-phase power source. The two-phase power source is preferably configured and operated to provide phase voltages V.sub.P1 and V.sub.P2 with a desired phase angle therebetween, for example +90.degree., that results in optimum performance of motor 150.

A feature of the present invention is the ability to conveniently and inexpensively provide for motor direction reversal. As further explained with regard to the several disclosed embodiments, the present invention achieves this feature by reversing the phase relationship between first phase voltage V.sub.P1 and second phase voltage V.sub.P2. For example, changing the phase angle between V.sub.P1 and V.sub.P2 from +90.degree. (a leading relationship) to -90.degree. (a lagging relationship), or vice versa, will desirably reverse the motor direction. Of course, phase angles other than .+-.90.degree. could be considered depending upon optimum performance and efficiency conditions.

It is preferred that this phase reversal between V.sub.P1 and V.sub.P2 be performed after the rotor has ceased rotating to prevent unnecessarily high voltage, currents, and electromagnetic fields from being created in the motor, and to reduce the mechanical stresses caused by direction reversal.

Motor 150 is preferably provided as a conventional PSC motor that does not include a run capacitor. As a result, in accordance with the features of conventional PSC motors, winding 154 is provided with a smaller conductor size and has a greater number of winding turns than winding 152, so that the impedance of winding 154 is greater than that of winding 152. Another characteristic of conventional PSC motors is that during operation from an AC source, a larger voltage is applied across the start winding than across the main winding when the rotor rotates in its conventionally intended direction. This difference in winding voltages results, in part, from the connection of the run capacitor in series with the start winding. For example, with respect to a conventional 230 VAC PSC motor connected through a run capacitor to a single-phase 230 VAC, 60 Hz source, the voltage applied across the main winding would be approximately 230 V, while the voltage applied across the start winding could be on the order of 20% higher or 276 V.

In view of this difference between winding voltages, it is a feature of the illustrated embodiments of the present invention that the two-phase power source, to which motor 150 is connected, be configured and operated to provide V.sub.P2 >V.sub.P1. The relative magnitudes of phase voltages V.sub.P1 and V.sub.P2 are preferably selected to duplicate the voltages that would otherwise be experienced by the main and start windings of motor 150 if it was operated at its rated voltage and frequency as a conventional PSC motor including a run capacitor in series with the start winding. However, during variable speed operation of motor 150, the drive system of the invention varies the absolute magnitudes of voltages V.sub.P1 and V.sub.P2 to maintain a substantially constant ratio of volts/hertz for each winding while the relative magnitudes of the voltages expressed as a ratio between those voltages, i.e., V.sub.P2 /V.sub.P1, is maintained substantially constant, as more fully described below.

In accordance with the illustrated embodiments of the invention, the phase angle difference between the currents respectively flowing in windings 152 and 154, including the leading or lagging relationship, is controlled by operating the two-phase power source to generate the two-phase voltages V.sub.P1 and V.sub.P2 with the desired phase angle therebetween, rather than as a result of installing a run capacitor in series with the start winding of the conventional PSC motor.

A benefit obtained by such operation of motor 150 is the capability to maintain a selected phase shift between the winding voltages independent of motor rotational speed. This allows the motor to deliver a normally specified range of torque at any speed, including at zero speed corresponding to starting, so long as the air gap flux is held substantially constant. Since the voltages applied to the respective windings of motor 150 have different magnitudes, a different volts/hertz ratio is maintained for each of windings 152 and 154. As more fully described below, the power supply circuit for providing the two-phase power source is preferably configured to maintain a constant volts/hertz ratio for each motor winding for the full range of motor shaft speed.

A two-phase power source for supplying phase voltages V.sub.P1 and V.sub.P2 in accordance with the present invention can be provided in several different forms. For example, the power source can be provided as a two-phase alternator or two, single-phase alternators driven by a common shaft to provide an adjustable phase shift. Output voltage adjustment of the alternator(s) is accomplished by adjustment of the alternator excitation voltage. In the case of two single-phase alternators, the phase angle can be adjusted by angular adjustment of the respective alternator rotors on the common shaft.

The power source can also be provided as a "Scott connected" transformer configured to change three-phase power into two-phase power. Phase angle adjustment of the Scott transformer output is effected by changing winding taps. Frequency adjustment is effected by varying the frequency of the input voltage. However, these embodiments are not preferred for providing the direction reversing feature of the present invention.

It is preferred herein that the power source be provided as a power supply circuit, including an inverter, for electronically generating the two-phase power. FIG. 3 illustrates an induction motor drive system 200 constructed in accordance with an embodiment of the invention. System 200 is preferably constructed to include motor 150. System 200 includes line terminals 202 and 204 for connection to a line conductor and a re