Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
1998-12-31
2001-08-14
Patel, Rajnikant D. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S098000
Reexamination Certificate
active
06275403
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to AC motor drives which convert DC to AC, and more specifically to the drive for an AC motor which will drive a pump or other active loads. A particular application is to solar water pumping systems; however, this invention is not restricted to solar systems but to any varying DC power source for the purpose of controlling AC motor systems or other loads.
2. Description of the Prior Art
An AC load can be powered from a DC source by using a converter to change DC to AC. However, because of changes in both the source and the load, it can be difficult to meet the power requirements of the load. For example, a photovoltaic solar cell array is a DC source. However, the current-voltage curve shifts under varying conditions, e.g. amount of sun. Thus at a constant voltage, the amount of current that can be drawn will vary. One application of solar power is to operate water pumps. Standard water pumps are three phase AC pumps. However, the load curve of the AC pump motor also shifts with varying conditions, e.g. water depth. Thus it can be difficult to efficiently operate an AC pump from a solar array.
Many systems that control motor drives operate on a fixed frequency and attempt to maintain a fixed DC voltage. However, attempting to operate at fixed frequency requires a constant power to the motor drive. This can take form in an elaborate DC constant voltage system or some type of DC storage systems such as a battery. For a fixed frequency at constant power the system requires a constant current-voltage relationship. This system can be quite elaborate and often times expensive for an efficiently controlled system.
Controlling motors at a fixed frequency is very demanding on the DC source voltage. If the power is to remain constant at a given frequency, then a change in DC voltage must be accompanied by a change in DC current. If the voltage decreases, the current must increase which results in a further voltage decrease until a point is reached where a shutdown must occur to prevent motor damage or increased heat or other related damage.
In general, it is desirable to operate at the maximum power point on a power curve. However, it is difficult to track power. Therefore, it is desirable to find a simpler method and apparatus to control a variable speed converter or motor drive so that an AC motor or other load may be operated from a DC source under varying source and/or load conditions.
SUMMARY OF THE INVENTION
Accordingly it is an object of the invention to provide a method and apparatus to control a variable speed converter or motor drive so that an AC motor or other load may be operated from a DC source under varying source and/or load conditions.
It is another object of the invention to provide systems which utilize the new method and apparatus to control the converter/motor drive, including solar powered water pumping systems.
The invention is a control apparatus and method for a DC to AC converter, in which a DC bias voltage control is related to AC frequency by a predetermined relationship. For variable speed AC motor drives, a low voltage bias control circuit is effectively used to control motor frequency by applying a bias voltage to the converter so that the load requirements are optimally met by the available DC source voltage. The invention also includes the DC source—bias controlled converter—AC load systems formed with the bias controlled drive, for example a solar powered water pump in which a DC photovoltaic panel is used to power and AC pump motor.
A DC source is connected through a DC to AC converter or motor drive to an AC load. The converter/motor drive may be a conventional programmable variable speed drive, and includes a pulse width modulator (PWM) for changing the frequency of the AC output. A bias control circuit according to the invention operates on the DC source voltage and is connected to the PWM of the motor drive to apply a bias voltage. An optional shut off circuit which also operates on the DC source voltage is also connected to the PWM. In a particular embodiment, the DC source is a solar array and the AC load is a water pump motor.
Basically the invention operates on a fixed frequency slope which is the predetermined relationship of bias voltage to frequency. The position on the curve is a function of the source voltage so that the system allows for changing source voltage. The system operates on a source voltage bandwidth (discrete voltage steps) and adjusts the frequency of the AC output as a function of that voltage bandwidth. This results in reduced frequency hunting for a given voltage change, adjusting the voltage from the source to maintain a more constant source power. The creation of voltage steps allows a different function of frequency vs. source voltage for different voltage levels, searching for a constant and maximum power for the drive. Although the invention is not based on a maximum power point strategy, the point that it seeks is a maximum power point.
REFERENCES:
patent: 4872100 (1989-10-01), Diaz
patent: 4939633 (1990-07-01), Rhodes et al.
patent: 5798633 (1998-08-01), Larsen et al.
McNulty Thomas C.
Williams Douglas B.
Patel Rajnikant D.
Sartorio Henry P.
WorldWater Corporation
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