Multiple motor operation using solar power

Abstract

A solar powered system controls multiple motors directly from a single solar power source with a single variable frequency drive (VFD). A control system selectively connects and disconnects individual motors as solar power varies so that the available power can be used effectively and some of the motors can be kept operating rather than shutting down the entire system. The control system includes a detection circuit for detecting the solar array voltage, and a selection circuit for providing shutoff control signals to individual motors based on the solar array voltage and preselected motor shutoff priority criteria.

Description

RELATED APPLICATIONS

This application claims priority of Provisional Application Ser. No. 60/723,268 filed Oct. 3, 2005.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to solar powered systems, and more particularly to the operation of multiple motors using solar power.

2. Description of Related Art

Solar power is being increasingly used as an alternate energy source. One application is to provide power to operate motors. Most motors are AC motors. The DC output of a solar panel array is converted to AC power by a variable frequency drive (VFD). In many applications, it is desirable to drive an array of multiple motors from a single VFD connected to a solar power source.

Controlling multiple motors from a single variable frequency drive (VFD) has been done but always from an AC grid voltage which is a stable power source capable of supplying the required power. When used with solar power, caution must be taken as the power supply is not stable but a varying power supply. Multiple motor operation under these conditions can result in starved power to the motors, resulting in motor damage. Under these conditions, there may be sufficient power to operate only some of the motors. Thus a system is desired in which motors can be selectively switched in and out of operation, depending on the available power, so that the most essential motors keep operating, instead of shutting down the entire system.

It is important for solar powered motor systems to alter or control the power to the motors to prevent starving, commonly referred to as “brownout conditions”. The present invention covers the requirement of varying power to the motors as a function of solar voltage or power.

An important aspect of these solar powered motor systems is motor inrush which can result in shutdown of the entire VFD drive if not controlled. These inrush currents last for approximately 2-3 cycles, but are an overload current. One way to avoid shutdown is to stagger the turn on of each motor so that the VFD does not reach an overload condition. Another way is to size a drive capable of handing the inrush currents for all motors. The latter is most expensive and dependent on motor inrush currents. This is true for AC grid operated designs as well. Thus it is desirable to provide a system to control the turn on and turn off of each of the motors to prevent an overload shutdown condition of the entire system.

SUMMARY OF THE INVENTION

This invention is a solar powered system for controlling multiple motors directly from a single solar power source with a single variable frequency drive (VFD). A control system selectively connects and disconnects individual motors as solar power varies so that the available power can be used effectively. The invention detects overload currents and staggers motor operation in the event of an overload condition. Thus some of the motors can be kept operating rather than shutting down the entire system. The invention also detects solar array voltage and automatically adjusts motor frequency for optimum power performance.

In one aspect of the invention, the control system includes a detection circuit for detecting the solar array voltage, and a selection circuit for providing shutoff control signals to individual motors based on the solar array voltage and preselected motor shutoff priority criteria.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram of a solar powered multiple motor system with a control system of the invention.

FIG. 2 is a block diagram showing the connection of the control system to individual motors.

FIG. 3 is a schematic diagram of a control logic circuit of the invention.

FIG. 4 is a block diagram of the control system of the invention.

DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a solar powered multiple motor system 10 of the invention utilizes a solar array 12 as a source of power. The DC output of solar array 12 is input into a variable frequency drive (VFD) 14 to convert the DC power to AC power. The AC output of VFD 14 drives motor array 16, which comprises a plurality (N) of individual motors. Control system 18 is connected between solar array 12 and motor array 16 to control the individual motors therein on the basis of the power produced by solar array 12.

As shown in FIG. 2, a plurality of motors 20, 21, 22, 23 (Motor 1 . . . Motor N) of the motor array 16 are connected in parallel to bus 24 of VFD 14. Control system 18 is connected to each of the motors 20, 21, 22, 23. Control system 18 selectively turns the individual motors 20, 21, 22, 23 off and on depending on the available solar power, as determined from the solar array voltage.

In accordance with the invention, a control system is provided which monitors the solar array voltage. With the solar array at maximum power, all motors are powered by the solar driven VFD. As the solar array power varies, the solar voltage varies with the variation in solar power. For decreasing solar voltage, the solar power is decreasing and the motor frequency is adjusted for the change in solar power. There is a condition where the solar power is not capable of supporting all the motors and continued use will result in an automatic shutdown. This is not a fault that can result in motor damage but a controlled shutdown for all motors, which may have significant adverse consequences.

If selected motors are required to run under lower solar power which cannot sustain operation of all the motors, the control system selectively shuts down nonessential motors so that the remaining motors can stay in operation. This can be done by shutting down individual motors or a bank of motors. As the power increases, additional motors can be turned back on. The flexibility of the logic circuit allows for this operation. For example, there may be 10 motors connected to a specific VFD drive all in parallel. The logic allows selection of one motor or any number of motors for controlled shutdown. The user can select which motors are the most important and which are the least important, and at what solar voltage (power) level each motor drops out or comes back on.

FIG. 3 shows a logic control circuit 30 which is used to select or sequence an orderly shutdown or control of selected motors. A solar array voltage sensing resistor network 32 is provided to detect changes in solar voltage, which is representative of changes in solar power. Network 32 is made up of resistors R1, R2, R3, R4, R5 connected in series between solar array positive line 33 and solar array negative line 34. The voltage drops across each of the resistors provide different voltage level inputs V1, V2, V3, V4 to the positive inputs of corresponding comparators 35, 36, 37, 38 whose outputs are connected to a corresponding motor 20, 21, 22, 23. A reference voltage Vref1, Vref2, Vref3, Vref4 is input into the corresponding negative inputs of the comparators 35, 36, 37, 38.

For each comparator, its output will be high as long as Vn>Vrefn. When Vn drops to Vrefn or less, the output of the comparator will go to zero (low). The output of each comparator controls the associated motor. When the comparator output is high, the motor is turned or kept on; when the comparator output is low, the motor is turned or kept off.

The input voltages V1, V2, V3, V4 are proportional to the solar array voltage (and power). For example, the resistors may be chosen so that at full array voltage, e.g. 300 V, the input voltages are 10 V, 8 V, 6 V, and 4 V. When the array voltage drops a certain percent, e.g. 10%, these voltages will also drop the same percent, e.g. 10%. The reference voltages can be chosen so that each comparator switches from high to low at a particular array voltage level. The comparator connected to a motor that is deemed more essential and should be kept on as long as possible will have a reference voltage that is much lower than the maximum input voltage (Vnmax). Then that motor will not be shut off until the array voltage drops very low, and will come back on sooner. The comparator connected to a motor that is deemed less essential and can be shut off sooner will have a reference voltage that is much closer to the maximum input voltage. Then that motor will shut off sooner, and will come back on later. For example, if M1 is very important and M2 is not very important, then Vref1 can be set at 50% of V1max while Vref2 can be set to 90% of V2max. Then when the solar power drops 10%, M2 will be shut off, leaving more power for the rest of the motors, while M1 will not be shut off until the array power drops 50%. If the whole motor array is shut off, M1 will come back on when the power reaches 50% while M2 will not come on until array power reaches 90%.

Thus levels can be set such that a sequence of motors can begin to be shut off when predetermined threshold voltages are reached. The control system can be set to turn off various motors, highest power rating or the lowest, based on whatever selection criteria the user chooses, to preserve the array power for the balance of motors to run. When the solar array voltage reaches a minimum level, no more solar power is available for any motor, and the control logic will shut down all the motors.

Logic circuit 30 is a particular embodiment of control system 18. However, control system 18 can be implemented in any embodiment based on the principles of the invention. For example, the comparators shown in FIG. 3 can be replaced with low logic relays for isolation if needed.

FIG. 4 shows a more general control system 40 for carrying out the invention. Control system 40 has a sensing circuit 42 coupled to a selection circuit 44. Sensing circuit 42 senses the solar array voltage. Selection circuit 44 is connected to the sensing circuit 42 and provides shutoff control signals to individual motors based on changes in the solar array voltage and preselected motor shutoff criteria.

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.

Claims

1. A control system for operating a motor array from a solar array, comprising:

a sensing circuit for sensing the solar array voltage;

a selection circuit connected from the sensing circuit to the motor array, the selection circuit providing shutoff control signals to individual motors in the motor array based on changes in the solar array voltage and preselected motor shutoff priority criteria.

2. The control system of claim 1 wherein the sensing circuit comprises a resistor network.

3. The control system of claim 2 wherein the resistor network comprises a plurality of resistors in series, each resistor producing a voltage drop proportional to the solar array voltage.

4. The control system of claim 1 wherein the selection circuit comprises a plurality of comparators, each comparator having one input connected to the sensing circuit for receiving a signal based on the solar array voltage and a second input connected to a reference voltage determined by the preselected motor shutoff priority criteria, the output of each comparator being connected to an associated motor.

5. The control system of claim 3 wherein the selection circuit comprises a plurality of comparators, each comparator having one input connected to an associated voltage drop in the resistor network and a second input connected to a reference voltage determined by the preselected motor shutoff priority criteria, the output of each comparator being connected to an associated motor.

6. Apparatus comprising:

a solar array for producing DC power;

a variable frequency drive (VFD) connected to the solar array for converting the DC power to AC power;

a motor array comprising a plurality of individual motors connected to the VFD;

a control system according to claim 1 connected between the solar array and the individual motors of the motor array for selectively shutting off individual motors as the solar voltage changes according to the preselected motor shutoff priority criteria.

7. The apparatus of claim 6 wherein the sensing circuit comprises a resistor network.

8. The apparatus of claim 7 wherein the resistor network comprises a plurality of resistors in series, each resistor producing a voltage drop proportional to the solar array voltage.

9. The apparatus of claim 6 wherein the selection circuit comprises a plurality of comparators, each comparator having one input connected to the sensing circuit for receiving a signal based on the solar array voltage and a second input connected to a reference voltage determined by the preselected motor shutoff priority criteria, the output of each comparator being connected to an associated motor.

10. The apparatus of claim 8 wherein the selection circuit comprises a plurality of comparators, each comparator having one input connected to an associated voltage drop in the resistor network and a second input connected to a reference voltage determined by the preselected motor shutoff priority criteria, the output of each comparator being connected to an associated motor.

11. A method for controlling the individual motors of a motor array operated from a solar array, comprising:

sensing the solar array voltage;

selecting individual motors to shut off based on changes in the solar array voltage and preselected motor shutoff priority criteria.

12. The method of claim 11 wherein selecting individual motors to shut off is performed by comparing a sensed voltage proportional to the array voltage to a reference voltage.

13. The method of claim 12 wherein the reference voltage is determined to shut off an individual motor when the solar voltage reaches a preselected level.

14. The method of claim 11 further comprising shutting off enough individual motors so that available solar power is sufficient to operate the remaining motors.