Motor load controller for AC induction motors

Abstract

This invention is an improved energy-saving method and apparatus for attaining and maintaining optimum operational efficiency in single-phase or three-phase alternating current (AC) induction motors that are operating under varying loads.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] None

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT

[0002] None

REFERENCE TO A MICROFICHE APPENDIX

[0003] None.

BACKGROUND OF THE INVENTION

[0004] The field to which this invention pertains is an electrical energy-saving method and apparatus for controlling power input to an alternating current (AC) three-phase induction motor under varying loads, so that power supplied to the motor is optimally kept as a function of the load currently being imposed on the motor.

[0005] Some of the applicable prior art patents are Nola: U.S. Pat. No. 4,439,718; NOLA: U.S. Pat. No. 4,052,648; and Sugimoto: U.S. Pat. No. 4,379,258.

[0006] It is well-known that alternating current (AC) induction motors normally operate under constant supply voltage regardless of whether or not that motor is actually operating at its rated load. Accordingly, when the motor is operated at a low load, for example, considerable power is wasted, thus unnecessarily resulting in higher operating costs.

[0007] The prior art has attempted to correct this problem by using various methods and devices whose object is to try to control power input to the motor so that it is a function of, and proportional to, the actual load then being imposed upon the motor.

[0008] The Nola patents disclose methods which use a power factor as a reference or indicator of motor efficiency for controlling the flow of power to the motor under varying loads. The problem with Nola's approach is that his power factor is based upon an evaluation of a displacement phase angle (theta) that may exist between the voltage and the current waveforms as they respectively reach zero value while crossing the X axis (abscissa).

[0009] The Nola approach is truly effective only if the respective waveforms are purely sinusoidal. In actuality, the waveforms are infrequently purely sinusoidal.

[0010] The Sugimoto patent discloses an alternative circuit for attempting to control the flow of supply voltage to an AC induction motor that is operating under conditions of variable loads, as a function of load, rather than as a function of maximum rated-load capacity.

[0011] Sugimoto teaches a power control circuit wherein he periodically samples and detects both supply power and feedback power. He then establishes a pre-determined, fixed ratio between supply power and feedback power in relation to attempts to obtain operational efficiency of the motor.

[0012] Sugimoto contends that this reference ratio is used in the control of the supply of voltage across the motor windings so that supply voltage is then a function of a load then being seen by the motor.

[0013] Sugimoto also contends that the Nola apparatus is flawed and disadvantageous. The prior art essentially uses either a phase displacement angle (theta), a pre-set optimum power factor or ratio, or a table of pre-set voltages for a corresponding table of load values being imposed upon the motor.

SUMMARY OF THE INVENTION

[0014] It is well known that in either three-phase or single-phase, alternating current induction motors, that when voltage and current values in the motor, are plotted on a point-by-point basis, with the vertical axis, (Y, also known as the ordinate) representing magnitude, and the horizontal axis (X, also known as the abscissa) representing time, that separate waveforms exist for voltage as well as for current. These waveforms can manifest either a sinusoidal or non-sinusoidal appearance, due in part to switching characteristics inherent in electrical componentry related to voltage control, for example.

[0015] When voltage and current values are taken from their respective waveform paths and then multiplied on a point-by-point basis along the X-axis to get true value, a voltampere (VA) waveform is generated. This waveform is also known as a true power waveform which represents power existing in the electrical system.

[0016] The oscillating appearance of the power waveform when plotted along the X and Y axes will generally have both positive and negative values with reference to the X-axis. The portion of the waveform above the X axis is taken as being positive in value, while the portion below the X axis is taken as having a negative value.

[0017] During operation of these induction motors, the action of the movement of rotors relative to motor windings, affects value of a reactive power also known as (VARs) or “true voltamperes reactive” which is taken as having a negative value.

[0018] The VARs would be visually represented as that portion of the voltampere (VA) waveform that is below the X-axis.

[0019] If a load imposed upon the motor has a value which is equivalent to rated load capacity of the motor, the motor is purportedly then operating at maximum efficiency. However, even then, the motor still produces VARs due to the motor's inherent physical characteristics and relative movement of the rotors (not shown) of the motor to the windings of the motor. These VARs are always present regardless of whether or not the motor is being operated at its rated load. These VARs will be referred to as “inherent VARs”.

[0020] Whenever an induction motor is being operated with a load that is less than its rated capacity, additional VARs or “true voltamperes reactive” now exist in the VA waveform. These VARs will be referred to as “excess VARs”.

[0021] There is a direct proportionality between the (VARs) and the loads that may be imposed upon the motor.

[0022] It is an object of the present invention to maintain both inherent and excess VARs at a minimal value in order to attain optimal operating efficiency of the motor.

[0023] Visually, the VARs or negative portion of the (VA) waveform, would be maintained as close to the negative side of the X-axis as possible.

[0024] The essence of the invention is to have a computer program instruct a microprocessor to continually search for both excess and inherent VARs or negative values of the true power (VA) waveform. Upon discovery of same, the microprocessor would be instructed to send one or more correcting digital numbers toward a solid state relay (SSR), the latter of which does actual control of input voltage or changing of voltage values to the motor windings, so that the input voltage is a function of, and proportional to, varying loads being imposed upon the motor.

DESCRIPTION OF THE DRAWING

[0025] Sheet 1/1 is a block diagram illustrating conventional electrical components, signal flow and correcting digital numbers flow of a preferred embodiment of the invention relating to a three-phase induction motor.

DESCRIPTION OF THE INVENTION

[0026] The best mode of the invention contemplated by the inventor is as follows: Electrical components of the invention are readily accessible and off-the-shelf. For example, a microprocessor 1 can be any number of conventional microprocessors.

[0027] As shown in Sheet 1/1, a motor 2 is located within an energy-saving circuit 3. During the operation of the motor 2 at less than full load, significant VARs are present in the VA waveform.

[0028] Voltage associated with any two of three motor phase lines (4, 5, or 6), to the motor 2 is being continually sampled by a conventional voltage detector 7, the latter of which is also structurally conformed so that voltages passing through it are maintained or adjusted to a magnitude that is compatible with a voltage input scaler 11. A conventional voltage divider (not shown) within the voltage detector 7, would satisfy this protective function.

[0029] Current associated with any one of two motor phase lines being sampled for their voltage, is also being continually sampled by a conventional current detector 9, as shown in Sheet 1/1. The current detector 9 is also configured so that current passing through it is transformed to a voltage signal that is both representative of input current as well as being made compatible in magnitude with a current input scaler 10. A conventional transformer and burden resistor (both of which are not shown), which are within the current detector 9, would satisfy this protective function.

[0030] The voltage signal, which is representative of current, that comes from the current detector 9, is applied to the conventional current input scaler 10 which maintains the voltage signal's magnitude at a level appropriate for receipt by an analog-to-digital converter 8, in accordance with the rated capacity of the analog-to-digital converter 8.

[0031] The voltage detector 7 continually samples line voltage in, for example, motor phase line 5 and motor phase line 4, and directs those voltages to the voltage input scaler 11 which adjusts those voltages to a value which is also compatible with rated input capacity of the analog-to-digital converter 8, as shown in Sheet 1/1.

[0032] Alternatively, the voltage detector 7 may sample voltage across any two of the motorphase lines 4, 5 or 6.

[0033] The voltage input scaler 11 takes the difference between the two sampled voltages and directs that difference to the analog-to-digital converter 8.

[0034] Up to this point, electrical signals representing current and voltage have been analog in nature.

[0035] The electrical analog signals respectively coming from the current input scaler 10 and the voltage input scaler 11, are then both directed to the analog-to-digital converter 8 which converts those signals into a digital binary number (hereinafter referred to as a digital number).

[0036] Those digital numbers are then directed to a microprocessor 1.

[0037] Upon receipt of the associated voltage and current signals in their digital form, the microprocessor 1 multiplies those digital numbers to obtain a voltampere (VA) value.

[0038] The microprocessor 1 then takes the product of this multiplication and, pursuant to instructions from a program memory 12, determines whether or not that product has a negative value. The product of this multiplication will generally also have a variable value, if the load on the motor 2 is varying.

[0039] If the product has a negative value, this indicates that VARs are present. Pursuant to further instructions from the program memory 12, the microprocessor 1 would then issue one or more digital numbers.

[0040] The digital number(s) issued by the microprocessor 1 indicates whether input voltage to the motor 2 needs to be changed so that input voltage will then be proportional to the load being imposed upon the motor 2.

[0041] As long as the voltampere (VA) value has a negative component (VARs) in it, which the microprocessor 1 could optimally reduce pursuant to instructions from the program memory 12, the microprocessor 1 will continually search for and discover both excess and inherent VARs. Upon discovery of these VARs, the memory 12 instructs the microprocessor 1 to issue and direct the aforesaid digital numbers continually to a digital-to-analog converter 13, where the digital number(s) will be converted to an analog signal which is then directed by the digital-to-analog converter 13 to an operational amplifier 14 where the analog voltage signal is scaled so that it can be no less than zero volts and no greater than five volts. This zero to five volt range is the proper voltage-input range which a solid state relay (SSR 15) is currently configured to accept. In the event that this range is changed to a more optimal value, the operational amplifier 14 will likewise be configured to reflect that change, in the voltage-input range values it is capable of providing.

[0042] As stated earlier, when the motor 2 is operated at less than its full rated load, significant reactive power (VAR) values or excess VARs are present in the (VA) waveform.

[0043] The essence of applicant's invention is to keep whatever (VARs) may exist, to a minimum. Stated another way, the invention takes whatever negative values of the multiplication product may exist, and pursuant to instructions from the programmable memory 12 directed to the microprocessor 1, keeps those negative values at an optimal minimum consistent with the microprocessor 1's inherent capacity to do so.

[0044] The invention thus focuses upon whatever negative portion of the power waveform (VA) may exist, and in order to have the motor 2 operate at conditions of optimum efficiency, keeps that negative portion at the optimal minimum, such optimal minimum being the smallest possible value that the microprocessor 1 is physically capable of obtaining, pursuant to programmable instructions from the program memory 12. Visually, if the power waveform (VA) were plotted, we would see either the negative amplitude of the (VA) waveform kept as close to the negative side of the X axis (abscissa) as possible, or the negative area that is bounded by the X-axis and the negative path of the power waveform (VA) kept at a minimum.

[0045] The microprocessor 1 is thus programmed by the memory 12 to not only continually search for and detect both excess VARs and inherent VARs, but also when the microprocessor 1 detects any VARs, to issue one or more incremental digital numbers to the digital-to-analog converter 13, to a point where the microprocessor 1 no longer detects any VAR value, regardless of whether that VAR value represents excess VARs or inherent VARs.

[0046] When the microprocessor 1 no longer detects any VARs, the microprocessor 1 is instructed by the memory 12 to stop issuing incremental digital numbers, and instead, is instructed to immediately start issuing decremental digital numbers to a point where stalling of the motor is avoided.

[0047] In response to the voltage signals coming from the operational amplifier 14, the SSR 15 does the actual work in adjusting and controlling the amount of supply voltage to motor windings (not shown) associated with motor phase lines 4, 5 and 6 which are in series with the SSR 15.

[0048] In this fashion, input voltage to the motor 2 is continually adjusted and controlled so that input voltage is a function of, and proportional to, varying loads being imposed upon the motor 2, thus resulting in significant energy savings and reduction of operating expenses.

[0049] Motor phase lines 4, 5 and 6 are each coming from a conventional three-phase power supply 16. The energy-saving circuit 3 and motor 2 are conventionally grounded. In the interest of simplicity, the ground is not shown in the drawings as such provisions would be obvious to those having ordinary skill in the art.

[0050] Programming the microprocessor 1 to perform the steps described herein, is also obvious to those having ordinary skill in the arts of computer programming, microprocessors, as well as technology associated with electrical induction AC motors.

[0051] Accordingly, the program listing and code will not be set out; however, pursuant to the Manual of Patent Examining Procedures, Section 2106.01, as well as the case of Fonar Corp. v. General Electric Co., 107 F3d 1543, 1549 (Fed. Cir. 1997), the specific functions of software associated with the program memory 12 and microprocessor 1, in connection with the threephase motor 2, are as follows:

[0052] As stated hereinabove, the specific functions of software provided by the program memory 12 are to (1) instruct the microprocessor 1 to receive and multiply the respective digital numbers associated with each other, coming from the analog-to-digital converter 8. (2) Instruct the microprocessor 1 to continually search for and detect negative values in the product of the multiplication described in (1). (3) Instruct the microprocessor 1 that if negative values (VARs) are observed, that it is to issue one or more digital numbers to the digital-to-analog converter 13, such digital numbers being continually issued as long as the voltampere (VA) value has any negative component (VARs) in it which the microprocessor 1 could optimally reduce. (4) Instruct the microprocessor 1 to recognize whether the load on the motor 2 is increasing or decreasing. (5) Instruct the microprocessor 1 to issue the digital numbers continuously in either incremental or decremental fashion to the SSR 15, until the optimal condition of minimal negative value in the voltampere (VA) waveform is achieved.

[0053] The value of the varying digital numbers is proportional to the zero-to-five volt range associated with the operational amplifier 14.

[0054] Essentially, as long as the microprocessor 1 detects (VARs), it will continue to issue the corrective digital numbers to the digital-to-analog converter 13, for ultimate transference of an analog signal then coming from the digital-to-analog converter 13 to the three-phase SSR 15, where the input voltage of the motor 2 is automatically adjusted proportional to and according to the value of the analog signal between zero and five volts that it receives from the operational amplifier 14.

[0055] The invention in the preferred embodiment described herein, which is positioned between the conventional three-phase power source 16 and the three-phase motor 2, can also be applied to a single phase motor (not shown) by using single-phase SSRs as long as those single-phase SSRs were each configured with conventional linear proportional controllers (not shown).

[0056] Although the invention herein has been described in detail with respect to only one exemplary embodiment shown herein, those having ordinary skill in the arts of microprocessors, computer programming and electric motors will recognize that variations and modifications of the invention can readily be made.

Claims

1. An improvement in an energy-saving method for optimizing operational efficiency of single and three-phase alternating current induction motors when such motors are operating under varying loads such improvement in combination comprising the steps of:

continually monitoring and detecting true voltampere reactive power; and

continually issuing signals in order to adjust and control input line voltage as long as true voltampere reactive power is detected so that input line voltage is now optimally a function of load.

2. An improvement in an energy-saving apparatus for optimizing operational efficiency of single and three-phase alternating current induction motors when such motors are operating under varying loads such improvement in combination comprising:

means for monitoring and detecting true voltampere reactive power; and

means for continually issuing signals in order to adjust and control input line voltage as long as true voltampere reactive power is detected so that input line voltage is now optimally a function of load.