Abstract:
A method and apparatus are implemented in software to control motor speed as a function of available power in a DC source-inverter-AC motor system, i.e. to perform maximum power tracking of motor speed. An inverter or motor drive converts DC power from a DC source, such as a solar panel, to AC power, to power the motor. The inverter or motor drive is controlled by software, implemented either by programmable features built directly into the inverter or drive or by a separate programmable device connected to the inverter or drive, to track motor power as a function of source power. The software-controlled inverter or drive sets motor speed as a function of source power by sensing only a single parameter, the DC source voltage. The software-controlled inverter or drive samples the source voltage at preset intervals, and changes the frequency of the AC output of the inverter or drive to match or track the available power so that the motor operates at or near its optimum for any source voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the operation of AC motors or similar loads with AC motor drives that convert power from a DC source to AC, and more particularly to operation of the motor at maximum power as the power from the DC source varies. A particular application is to solar powered systems and to water pumps. 
     2. Description of Related 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 (I-V) curve shifts under varying conditions, e.g. amount of sun. Thus the available power will vary. One application of solar power is to operate water pumps, which typically include three phase AC motors. However, the load curve of the AC pump motor can also shift with varying conditions, e.g. water depth. Thus it can be difficult to efficiently operate an AC pump from a solar array. 
     A solar powered water pumping system typically has three primary components: the solar array, made of photovoltaic (PV) modules; a converter (inverter or motor drive) which converts the DC from the PV array to AC; and an AC motor (pump). The motor typically runs at a particular frequency (speed), e.g. 60 Hz. The converter will usually be set to provide AC power at that particular frequency. The motor will run at a speed equal to the AC frequency. 
     In operation, the motor demands power. The motor pumps the most water when it is at the maximum power point. As the solar array output changes, e.g. decreases from a maximum to a lower voltage, the I-V power curve changes, but there is always a maximum power point. However, if the motor continues to run at the same speed, e.g. 60 Hz, then as the voltage drops, the current must increase to meet the power requirements, until the increased current can damage the motor. 
     Thus, controlling motors at fixed frequency is very difficult. 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 and current increase 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 (MPP) on a power curve. However, it is difficult to track power. Power tracking generally requires detecting two parameters, current (I) and voltage (V), and measuring changes in the product (IV). 
     If the motor operates at a reduced frequency, then it requires less power. While this is not as good as operating at full power, the motor can be kept operating at the maximum operating frequency for the existing conditions, without damaging the motor. Therefore, it is desirable to provide a method and apparatus to operate an AC motor from a motor drive by changing the AC frequency and thus the motor speed to correspond to the available power. 
     U.S. Pat. No. 6,275,403 is directed to a bias control circuit connected to a DC to AC converter to control motor frequency of a connected motor by applying a bias voltage to the converter to control the frequency of the AC output of the converter. The bias control circuit is responsive to the DC voltage from a DC source, e.g. solar array, connected to the converter. The system is designed to operate an AC motor or other load from a DC source under varying source and/or load conditions. In a preferred embodiment, the bias control circuit has a multistage configuration and provides bias voltages at a plurality of discrete DC source voltages. Thus the system, while providing significant improvement in motor operation, requires an additional hardware circuit, and operates at a number of discrete levels limited by the number of stages in the circuit. 
     Accordingly it is desirable to provide a simple system for controlling the motor speed to better match the maximum power point without having to measure power. It would also be desirable to provide a system which is implemented in software and eliminates the need for additional hardware circuits. 
     SUMMARY OF THE INVENTION 
     The invention is method and apparatus implemented in software to control motor speed as a function of available power in a DC source-inverter-AC motor system, i.e. to perform maximum power tracking of motor speed. An inverter or motor drive is used to convert DC power from a DC source, such as a solar panel, to AC power, which powers the motor. The inverter or motor drive is controlled by software, implemented either by programmable features built directly into the inverter or drive or by a separate programmable device connected to the inverter or drive, to track motor power as a function of source power. The software-controlled inverter or drive sets motor speed as a function of source power by sensing only a single parameter, the DC source voltage, which is input into the inverter or drive. The software-controlled inverter of the invention samples the source voltage at preset intervals, and changes the frequency of the AC output of the inverter or drive to match or track the available power so that the motor operates at or near its optimum for any source voltage. 
     An aspect of the invention is an apparatus for converting DC power from a DC source to AC power to drive an AC motor, formed of a software-controlled inverter which produces an AC output from a DC input, wherein the software-controlled inverter carries out an algorithm for varying the AC output frequency in response to changes in the DC voltage from the DC source so that the speed of an AC motor driven by the inverter tracks the maximum power available from the DC source. 
     Another aspect of the invention is a system including a DC source; a software-controlled inverter connected to the DC source to produce an AC output from a DC input; and an AC motor connected to the AC output from the inverter; wherein the software-controlled inverter carries out an algorithm for varying the AC output frequency in response to changes in the DC voltage from the DC source so that the speed of the AC motor tracks the maximum power available from the DC source. 
     A further aspect of the invention is a method for powering an AC motor from a DC source by obtaining DC power from a DC source; converting the DC power to AC power; powering the AC motor with the AC power; and varying the AC frequency in response to changes in the DC voltage from the DC source so that the speed of the AC motor tracks the maximum power available from the DC source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1A  is a block diagram of a DC source-software controlled inverter-AC motor system of the invention, with a separate controller. 
         FIG. 1B  is a block diagram of an alternate embodiment of the software controlled inverter, with an internal controller. 
         FIG. 2  is a series of current (I) vs. voltage (V) curves for a PV solar array with the maximum power point (MPP) and associated power (P) vs. voltage (V) curves also shown. 
         FIG. 3A  is a graph of measured I-V for changes in motor frequency. 
         FIG. 3B  is a graph of power consumption vs. frequency. 
         FIG. 4  is a flow chart of a maximum power point algorithm used by the invention. 
         FIG. 5  is a flow chart of some specific steps in an algorithm for maximum power tracking. 
         FIG. 6  is a maximum power tracking timing diagram for the specific steps of the algorithm of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1A , a DC source-software controlled inverter-AC motor system  10  according to the invention comprises a DC source  12 , an inverter  14  connected to the DC source  12 , a programmed controller  16  connected to the inverter  14 , and an AC motor  18  connected to the inverter  14 . DC source  12  is preferably a solar array made up of conventional silicon solar cells or panels, but may be another type of DC source. The DC source will generally be a source whose output voltage and power vary. The AC motor is typically a three phase motor, and may drive a water pump  20  (or other device), which may be combined with motor  18  into a single integral unit  19 . The invention may also be applied to other loads that have load characteristics similar to motor  18 . 
     Inverter  14  is a conventional DC to AC converter, also commonly known as a motor drive or variable speed drive (VFD). Controller  16  is programmed to carry out an algorithm which produces maximum power point tracking by varying the AC output frequency from the inverter  14  as a function of the DC source voltage. In an alternate embodiment of the invention, inverter  14  and controller  16  are replaced by inverter  15  with an internal controller  17 , as shown in  FIG. 1B , i.e. the inverter is itself programmable and does not need an external controller. Controller  17  is programmed to carry out an algorithm which produces maximum power point tracking by varying the AC output frequency from the inverter  15  as a function of the DC source voltage. In either embodiment, the DC to AC converter is software-controlled and carries out an algorithm to vary the AC frequency so that the motor is operated at the maximum power that is at that moment available from the DC source. The motor speed changes as the available power from the DC source changes. The invention includes the software-controlled inverter and the DC source-software controlled inverter-AC motor system. 
       FIG. 2  shows several current (I) vs. voltage (V) curves for a PV solar array, ranging between high sun and low sun conditions. The maximum power points (MPPs) and some associated power (P) vs. voltage (V) curves are also shown. The MPP is the point on a particular I-V curve where P(=I×V) is a maximum. The motor being powered from the PV array can do the most work when it is at the MPP. 
     As the solar array output changes, and the associated I-V curve changes, the MPP changes. To optimize motor performance, it is necessary to adjust to the change in MPP. The invention provides a way for the motor to track the MPP. This is accomplished by measuring the DC voltage, and changing the AC frequency (and thus motor speed) in response thereto. 
       FIG. 3A  shows a solar I-V curve for changes in motor frequency. Tests were run at different frequencies and the power requirements, i.e. maximum IV, were logged at each frequency. The curve ranges from zero frequency, where the solar voltage is the open circuit voltage Voc and the solar current is zero, to the maximum frequency. At the other limit the solar voltage is zero and the solar current is the short circuit current Isc (but a motor would stall before reaching that point). The graph shows that the motor can be controlled for maximum power available from a solar source (or other variable DC source). 
     In accordance with the invention, the motor is allowed to operate at a frequency compatible with source power, but this is done without actually sampling the source power. Instead, only the source voltage is sampled, and on the basis of changes in the source voltage the motor speed is decreased or increased to track lower or higher power availability. 
       FIG. 3B  shows a power consumption curve as a function of frequency. Motors in the U.S. are designed to operate at 60 Hz AC frequency at rated power. If the motor power available is less than the power required at 60 Hz, the motor will try to maintain constant power by increased current consumption to compensate for the reduction in source voltage. This will add to excessive power losses and eventual motor damage. To correct this problem, motor speed must be reduced. As shown in  FIG. 3B , at full power the motor can operate at full speed (60 Hz) but at 80% power the motor speed must be reduced to about 55 Hz and at 60% power the motor speed must be reduced to about 50 Hz. The invention provides a simple method and apparatus for adjusting motor speed to track available source power. 
       FIG. 4  presents a flow chart of an algorithm which is implemented by the software controlled inverter of the invention to carry out maximum power point tracking. As a preliminary step  30 , a sampling interval (Δt) is set. The sampling interval should be relatively short so that the motor speed closely follows the available power but cannot be so short that the motor operation becomes unstable because of very rapid fluctuations in power or that the motor cannot respond because of motor inertia. A suitable Δt is in the range of about 1 to 5 sec. The sampling interval can be reset as desired. 
     In step  30 , the array voltage (AV) is sampled. Sampling is done at the sampling interval set in step  30 . In step  34 , the present value of the array voltage is compared to the previously sampled value, i.e. the difference ΔAV=AV(n)−AV(n−1) is computed. (On the initial AV sample when the system is first turned on, there is no previous value of AV to compare so the difference is zero.) 
     In step  36 , a decision as to whether a change in frequency is required is made, based on the comparison made in step  34 . A comparison is made as to whether the measured ΔAV is greater than or equal to a preset threshold value ΔAV(threshold). The value ΔAV(threshold) represents the minimum change in voltage (and power) for which the motor speed should be changed. It should be relatively low so that the motor speed closely follows the available power but cannot be so small that the system tries to respond to insignificant changes in voltage (power). A suitable value is in the range of about 10 to 25 volts. 
     If the measured ΔAV is less than ΔAV(threshold), then no change in AC frequency or motor speed is required, and the algorithm returns to step  32 , takes the next voltage sample, and continues on through step  34  to step  36  again. If the measured ΔAV is greater than or equal to ΔAV(threshold), then a change in AC frequency and motor speed is required. 
     In response to a Yes decision in step  36 , a control signal is produced in step  38 . The control signal may be generated internal to the inverter, as in  FIG. 1B , or may be generated in a separate controller, as in  FIG. 1A . In response to the control signal, the inverter changes the AC frequency of its output, in step  40 . The change in AC frequency changes the motor speed, step  42 , so that the motor speed tracks the maximum power available from the source. After the AC frequency is changed in step  40 , the algorithm returns to step  32  and goes through another cycle. The general process of the algorithm shown in  FIG. 4  can be carried out in many different specific software implementations. 
     The invention includes a method for powering an AC motor from a DC source, e.g. solar panel, by obtaining DC power from the DC source; converting the DC power to AC power; powering the AC motor with the AC power; and varying the AC frequency in response to changes in the DC voltage from the DC source so that the speed of the AC motor tracks the maximum power available from the DC source. The method may be carried out with an algorithm made up of a series of instructions for sequentially sampling the DC source voltage at a preset sampling interval, comparing the present sampled value of the DC voltage to the prior sampled value, determining whether a change of AC frequency is required based on the comparison of the present to the prior sampled DC voltages, producing a control signal if a change in AC frequency is required, changing the AC frequency in response to the control signal, and continuously repeating the series of instructions. 
     A specific sequence of steps illustrating a portion of a particular algorithm for maximum power point tracking is shown in  FIG. 5 , and an illustrative associated wave form and timing diagram is shown in  FIG. 6 . This sequence starts with an initial array voltage V 0  (the maximum voltage), step  50 , at t 0 . A first voltage sample V 1  is taken at time t 1 , step  51 . A first comparison is made, “is V 1 =V 0 ”, step  52 . If the answer to step  52  is Yes (V 1 =V 0 ), then the voltage is still at its initial value, so return to step  51 , and take sample V 1  again. If the answer to step  52  is No, then perform a second comparison, “is V 1 &lt;V 0 ”, step  53 . If the answer is Yes, then the voltage has decreased from the initial (rated) value and the available power is less, so the motor speed should decrease. Signal A to drive input  58  will change the AC frequency of the drive. Also return to step  51  and start a new cycle. If the answer is No, then a second voltage sample V 2  is taken, step  54 , at t 2 . (The test “is V 1 &gt;V 0 ” is not necessary since V 0  is the maximum voltage. The comparisons may actually involve some thresholds as discussed with  FIG. 4 , but for simplicity to illustrate the basic logic of the process, they are not included.) 
     The second voltage sample now goes through a sequence of comparisons. Step  55 , “is V 2 =V 0 ”. If Yes, then the voltage has returned to the initial maximum voltage V 0  so the speed must be increased back to its initial speed. Signal B to drive input  58  will increase the AC frequency, back to the initial frequency. Also return to step  51  and start a new cycle. If No, then “is V 2 =V”, step  56 . If Yes, then the voltage has not changed from the prior value, so return to step  51  and begin a new cycle. If No, then “is V 2 &lt;V 1 ”, step  57 . If Yes, then the array voltage has decreased again, and the available power is even less, so the motor speed should be decreased further. Signal A to drive input  58  results in a further decrease in motor speed. Also return to step  51  and start a new cycle. If No, then V 2 &gt;V 1 , the voltage has increased since the last voltage sample (but not to V 0 ) so the speed should be increased, using signal B. Again return to step  51  and start a new cycle. 
       FIG. 6  shows illustrative Voltage (V), Speed (S) and Power (P) wave forms for the process illustrated in  FIG. 5 . At the initial time t 0 , V is at its maximum value V 0  so S and P are at their maximums S 0  and P 0 . The voltage sampling and speed adjustment is done at a sequence of times t 1 , t 2 , t 3  . . . t(n−1), t(n) defined by a sampling interval. At sample time t 1 , V has decreased to V 1  and P to P 1  so the speed must be reduced to S 1 . At sample time t 2 , the V and P have decreased further to V 2  and P 2  so the speed must be further reduced to S 2 . V, S, and P then remain constant up to sample time t(n−1). But at sample time t(n), V and P have increased back to their maximum values V 0 , P 0  so S must be increased back to S 0 . The method of  FIG. 5  will allow S to track P using V. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.