Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1994-08-02
2001-05-15
Martin, David (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S801000, C318S774000, C318S785000
Reexamination Certificate
active
06232742
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to dc/ac inverter apparatus and, more particularly, to dc/ac inverter apparatus configured to selectively drive either a three-phase motor or a single-phase motor. The invention also relates generally to variable-voltage power systems incorporating a photovoltaic array and the like, for powering loads such as the inverter apparatus and, more particularly, to circuits and techniques for preventing an excessively high input voltage from damaging the load.
Inverter apparatus of this particular kind are commonly used to power ac motors using dc power derived from photovoltaic arrays and other variable-voltage power sources. Both three-phase motors and single-phase motors have been driven using inverters of this kind. Three-phase motors typically include three input terminals and three inductive windings arranged either in a Y configuration or a delta configuration. Single-phase motors, on the other hand, typically include two input terminals and two windings, with a main winding and a supplemental winding. The supplemental winding typically has the same inductance as, but a higher resistance than, the main winding, and it is commonly used with a series capacitor for starting.
Inverter apparatus for driving three-phase motors provide three ac output voltage signals that are phased at 120° relative to each other so as to efficiently drive the motor. This is accomplished using three pairs of switches, typically high-speed power transistors, with each pair being connected in series between an input terminal carrying the dc input voltage and a reference terminal carrying a negative reference voltage, or ground. Reverse-biased diodes are connected across each transistor, for use when switching inductive loads, such as motors. The nodes between the three pairs of switches, or poles, constitute the inverter's three output terminals. Generally, one transistor or the other of each pole is switched ON at any one time, and the duty cycles of the switching are controlled such that each pole simulates an ac voltage having the desired frequency and phase angle. The three-phase motor thereby is driven at a speed proportional to the ac voltage frequency.
Inverter apparatus for driving single-phase ac motors of the kind described above typically provide a single ac voltage signal for coupling through the motor's main winding and via a capacitor through the motor's supplemental winding. A second terminal of the motor couples the node between the two windings to a negative voltage reference, or ground. Inverters of this kind typically have included two pairs of switches or poles, again typically high-speed power transistors, and a controller switches ON just one transistor of each pole at a time. The duty cycle of the switching is controlled so as to simulate an ac voltage waveform having the desired frequency.
The inverter apparatus described briefly above have functioned satisfactorily to drive the three-phase or single-phase motors for which they have been configured. Sometimes, however, it is desirable to provide for the selective use of either a three-phase motor or a single-phase motor. In the past, this generally has required two separate inverters, one configured for three-phase motors and the other for single-phase motors. This adds significantly to the apparatus'expense and complexity. There is a need for an inverter apparatus that can conveniently be used to drive either a three-phase motor or a single-phase motor without requiring any substantial hardware reconfiguration. The present invention fulfills this need.
Inverter apparatus of the general kind described above, as well as other electrical loads, are vulnerable to damage from the application of excessive input voltages when they are powered by a variable dc power source such as a photovoltaic, or solar, array. Such power sources are considered to be “soft,” meaning that their voltage levels can vary over a wide range, depending on several factors, including current loading and temperature.
FIG. 1
depicts a voltage versus current relationship for one typical photovoltaic array. It will be noted that the array's voltage level drops monotonically with increasing current draw and that one particular combination of voltage and current provide maximum power output. Photovoltaic power systems typically are controlled to operate at or near this peak power point. It will be noted that the open-circuit voltage level is substantially greater than the voltage level at the peak power point. In addition, the voltage level varies significantly with variations in the array's temperature, and the current level varies substantially with variations in the incident sunlight, or insolation.
Because the photovoltaic array's open-circuit voltage can be substantially higher than the array's voltage at the peak power point, particularly at cold temperatures, appropriate steps must be taken to prevent this high voltage from damaging the load, e.g., an inverter apparatus. Typically, this is achieved by configuring the load to withstand the application of such a voltage. This can dramatically increase the load's cost and complexity. A need therefore exists for a less costly and less complex means for preventing the application of such high voltages to a load such as an inverter apparatus. The present invention fulfills this need.
SUMMARY OF THE INVENTION
The present invention is embodied in a power inverter apparatus for selectively driving either a three-wire, three-phase ac motor or a three-wire, single-phase ac motor, with minimal increased hardware complexity over that of a power inverter apparatus for driving just one kind of such motors. More particularly, the power inverter apparatus is used with a dc voltage supply, e.g., a photovoltaic array, and it includes first, second and third pairs of electrical switches, or poles, each pole including first and second series-connected switches connected between a terminal carrying a positive dc voltage and a terminal carrying a negative reference voltage, or ground. The nodes between the first and second switches of the three poles form first, second and third output terminals suitable for connection to the three input terminals of the connected motor. Further, a controller selectively switches ON and OFF the switches of the three poles according to a predetermined sequence that provides high-speed pulse-width modulation, such that the apparatus is selectively conditioned to drive either the three-phase motor or the single-phase motor.
More particularly, the controller alternately switches ON the first switch and the second switch in each pole, at duty cycles that vary substantially sinusoidally between 0 and 100% and at a common frequency proportional to the speed at which the connected motor is to be driven. The relative phase angles of the sinusoidally varying duty cycles of the switching of the three switch pairs are selected according to whether a three-phase motor or a single-phase motor is being driven.
In a separate and independent feature of the invention, the inverter apparatus, or any similar load, is protected against the application of a dc input voltage exceeding a predetermined maximum level. This feature of the invention is useful when the load is being driven by a “soft” source such as a photovoltaic array that provides a dc voltage that varies substantially with load current. In accordance with the invention, the power apparatus includes a voltage clamp having an input terminal connected to the dc power source and an output terminal connected to the load, and the clamp is responsive to the voltage level being supplied to the load, for intermittingly shorting out the dc power source, such that the voltage applied to the load never exceeds the predetermined maximum voltage level.
More particularly, the voltage clamp includes a storage capacitor connected between the output terminal and ground, a diode connected between the input terminal and the output terminal for charging the capacitor t
Rippel Wally E.
Wacknov Joel B.
AeroVironment Inc.
Brueggemann James R.
Martin David
Sheppard Mullin Richter & Hampton LLP
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