DESC:TECHNICAL FIELD
[0001] The present disclosure relates in general to the field of variable frequency drives (VFDs) for alternating current (AC) motors. In particular it pertains to a method of using variable frequency drives for enhancing performance of AC motors.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art
[0003] It is estimated that over 60% of the electricity generated is consumed by one or other type of motor. With Alternating current being the norm for both domestic and industrial applications, all these motors, with the exception of certain special applications, are AC induction motors. By virtue of principle of their working, these motors do not by themselves have any mechanism for speed control and their rpm is controlled by power supply condition i.e. frequency of the Alternating Current and to some extent on load connected to the motor.
[0004] Before the development of semiconductor power electronics, it was difficult to vary the frequency of the input power supply, and therefore AC motors were mainly used in fixed speed applications. Applications requiring variable speeds such as electric overhead cranes used DC drives or wound rotor motors (WRIM) with slip rings for rotor circuit connection to variable external resistance allowing considerable range of speed control. However, resistor losses associated with low speed operation of WRIMs is a major cost disadvantage, especially for constant loads.
[0005] With developments in the field of power electronics, induction motors are now increasingly being used with variable-frequency drives (VFDs) in variable-speed service. Besides providing speed variation VFDs offer especially important energy savings opportunities for induction motors in variable-torque centrifugal fan, pump and compressor load applications. Any application that does not need to be run at full speed can benefit from a variable frequency drive by cutting down on energy costs by controlling the motor with the VFD. VFDs allow matching of the speed of the motor-driven equipment to the load requirement which cannot be accomplished by any other means. Optimizing motor control systems by VFDs can reduce energy consumption by as much as 70%.
[0006] In a motor, current is dependent on load for a particular voltage and frequency. Therefore, the parameters that can be controlled to change speed of the motor are voltage and frequency. In induction motors using VFD, voltage /frequency ratio is kept constant. However, the conventional VFDs are operated in a way to vary voltage supplied to the load up to a certain point only i.e. up to nominal frequency (50 Hz) (also referred to as rated frequency and two terms used interchangeably hereinafter) after which it is kept constant at nominal/rated voltage. This leads to suboptimal operation of the motor beyond that point. For loads where the torque required is proportional to frequency, the current starts going up and this limits the frequency range of operation of the motor.
[0007] There is, therefore, a need of a method and system to provide a wider range of capacity variation while using a motor.
[0008] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0009] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[0010] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0011] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0012] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.

OBJECT OF INVENTION
[0013] An object of the present disclosure is to provide a method and system that can enable a wider range of capacity variation of a load attached to a motor.
[0014] An object of the present disclosure is to use a variable input power source to its fullest potential to drive a load requiring contact torque.
[0015] An object of the present disclosure is to provide a system a method that enables a load requiring constant toque to avoid higher in-rush current above the maximum rated ampere but still operate with wider voltage range.
[0016] An object of the present disclosure is to provide a system and method to control frequency and voltage to module speed of rotation of a motor.
[0017] An object of the present disclosure is to provide a system and method that can enable a compressor to use its maximum capacity when higher input source power is available, without damaging the motor driving the compressor.

SUMMARY
[0018] Aspects of present disclosure relate to induction motors and their corresponding variable frequency drive. In particular it pertains to using existing setup of induction motors and variable frequency drives that can achieve a wider range of frequency of operation which can be beneficial when the power availability or the load varies. In an aspect the disclosed method can be applicable in any system where a high voltage can be achieved as compared to a standard 3-phase system and for constant torque applications such as a refrigeration compressor. It is to be understood that though the embodiments of the present disclosure have been explained with reference to a compressor, they are equally applicable to other applications of induction motors which require constant torque.
[0019] In an embodiment, the disclosed methodology involves keeping the power supply voltage/frequency ratio constant even beyond the nominal/rated frequency. In an aspect, maintaining a constant voltage/frequency ratio even beyond the nominal/rated frequency can prevent motor current from increasing and can allow the motor to run a load where higher motor output can be gainfully utilized such as in a compressor that can deliver very high refrigeration output at higher frequency and resultant higher speed.
[0020] In another embodiment, six terminals of the motor can be connected in delta connection and motor’s nominal voltage reduced to 230 V from 415 V thereby making available a power supply voltage above the nominal voltage. In an aspect, with this arrangement, the V/f ratio can be maintained till the output voltage to the motor becomes 415 V. Thus the methodology can ensure that the motor current remains less than the rated current even at higher frequency thereby allowing increasing range of compressor capacity variation using the same compressor-motor combination.
[0021] In another embodiment, the methodology can be used in solar power applications to achieve maximum capacity of the compressor during day time when solar power generation is maximum. In an aspect, the solar systems are sized such that the maximum capacity of the compressor is achieved at the maximum solar power generation during the day. Therefore, a compressor configured to operate over a wider capacity range, can operate on solar power for a longer duration and hence the methodology can result in better operational efficiency.
[0022] In an aspect, the disclosed methodology can also be used in normal variable refrigerant flow systems where grid is used as a power source to achieve a better capacity variation. Using the methodology same compressor can deliver higher refrigeration output which can lead to better adjustment to variation in heat load resulting in a better operational efficiency. In addition the methodology can result in lower capital costs as lower capacity compressors can be used for same refrigeration output. In another aspect, the methodology can improve the loading of DG sets in case the system is operated by Diesel generator there by improving the efficiency of the DG set
[0023] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components

BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0025] FIG. 1 illustrates a typical prior art set up of an induction motor and its corresponding VSD.
[0026] FIG. 2 illustrates a typical frequency and supply voltage relationship for a conventional VSD.
[0027] FIG. 3A to FIG. 3C illustrate typical curves indicating induction motor characteristics and capabilities.
[0028] FIG. 4 illustrates an exemplary frequency and supply voltage relationship for the disclosed methodology in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION
[0029] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0030] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0031] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0032] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0033] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0034] Embodiments of the present disclosure relate to induction motors used in variable speed configuration along with a variable frequency drive. In particular the disclosure provides a methodology of using existing setup of induction motors and variable frequency drives that can achieve a wider range of frequency of operation which can be beneficial when the power availability or the load varies.
[0035] A system for augmented voltage/frequency control is provided. The system enables a wider range of frequency of operation, which is beneficial when the power availability or the load varies. The system include a motor to drive a load, wherein six terminals of the motor are connected in delta configuration to reduce a predefined nominal voltage associated with the motor to a reduced nominal voltage to enable wider voltage range for operation of the motor, and a variable frequency drive configured to draw power from a source and provide a variable frequency power input based on the reduced nominal voltage to the motor keeping voltage to frequency ratio constant. In an exemplary implementation, the motor can be configured to be run with a contact voltage to frequency ratio above a rated frequency to achieve enable motor to run at higher RPMs.
[0036] In an exemplary implementation, the load can be a compressor that can provide varying compressor capacity. The compressor can be a refrigerator compression. Though the present specification has been described for a compressor, any load having requirement of running at constant torque (i.e. torque remains constant over a cycle of operation) can be driven by the system of present disclosure.
[0037] In an exemplary implementation, the system enable a load to be driven efficiently even by solar power that varies depending on weather conditions.
[0038] A method for augmented voltage/frequency control to drive a motor is provided. The method includes steps of reducing a predefined nominal voltage associated with a motor to a reduced nominal voltage by connect six terminals of the motor in delta configuration to enable wider voltage range for operation of the motor, proving, by a variable frequency drive, variable frequency power input based on reduced nominal voltage to the motor keeping voltage to frequency ratio constant. In an exemplary implementation, the variable frequency drive can be configured to be run the motor with a contact voltage to frequency ratio above a rated frequency to enable the motor to run at higher RPMs.
[0039] In an embodiment, a control system for refrigerator compressor is provided. The control system includes a motor to drive a refrigerator compressor, wherein six terminals of the motor are connected in delta configuration to reduce a predefined nominal voltage associated with the motor to a reduced nominal voltage to enable wider voltage range for operation of the motor, and a controller configured to draw power from a source and provide a variable frequency power input based on the reduced nominal voltage to the motor keeping voltage to frequency ratio constant. In an exemplary implementation, the controller can be configured to run the motor with a contact voltage to frequency ratio above a rated frequency to achieve enable motor to run at higher RPMs.
[0040] In an exemplary implementation, the variable frequency drive of present system and method can be configured to keep the voltage/frequency ratio constant when the motor runs above its nominal frequency (50Hz/60 Hz). This prevents the motor current from increasing and allows the compressor to deliver very high refrigeration output at higher frequency. The system and method ensures that the current drawn by the motor is less than rated amperes even at higher frequency, thereby allows increasing the range of compressor capacity variation using the same compressor. The system, method and controller of present disclosure can be useful especially when the input source is solar power. Solar system sizing is done to achieve maximum capacity of the compressor at maximum solar power generation during the day. The system, method and controller of present disclosure can enable a compressor to operate over a wider capacity range, for a longer duration on the solar power and hence provide better operational efficiency.
[0041] The system, method and controller of present disclosure can be used to operate any normal variable refrigerant flow systems where grid is used as an input power source to lead to a better capacity variation. The same compressor would be able to deliver higher refrigeration output.
[0042] FIG. 1 illustrates a typical prior art set up for a VFD along with a motor winding wherein the VFD 100 receives grid input 102 of 415 V. the grid input 102 is rectified and then inverted to a Variable Frequency output 104 having a nominal voltage of 415 V and is fed to stator windings 106 of the corresponding induction motor. As shown in the FIG. 1 the stator windings 106 are typically connected in a star configuration with nominal voltage being 415 V. In an aspect, they can belong to a refrigeration compressor motor that is required to work under a constant torque application.
[0043] In typical prior art set ups, the power supply voltage/frequency ratio (V/f) is kept constant up to rated frequency of the input power. For example if the rated voltage is 415 V and the rated frequency is 50 Hz, the frequency and voltage is varied in direct proportion to vary the motor speed. However, once the frequency reaches the rated value (also referred to as critical frequency), the voltage is not increased beyond the rated voltage. Increase in motor speed beyond this point is achieved by increasing the frequency only. This results in reduction of rated output torque of the motor beyond rated frequency.
[0044] FIG. 2 illustrates a typical frequency and supply voltage relationship for a conventional VSD. As shown therein, initially the voltage increases with frequency to maintain V/f ratio till a frequency of say 50 Hz (critical frequency) during which voltage also reaches rated voltage (say 415V). Thereafter, the voltage remains constant at the rated voltage (say 415V) while the frequency continues to go up to drive the motor at higher speed.
[0045] FIG. 3A to FIG. 3C illustrate typical curves indicating induction motor characteristics and capabilities. Shown therein is the output of VFD drive that increases voltage linearly upto 415 V corresponding to 50Hz after which it only increases the frequency keeping the voltage constant (352). Hence as a result of this the torque which is constant until the V/f was constant, decreases beyond 50Hz (302). In most cases voltage can’t go above 415 V because the power source available is only of 415 V. Accordingly, most variable frequency drives do not accept voltages above that. Under the situation if load demands more torque, the current may cross the motor rated current. Therefor it is not possible to run the motor over the critical frequency.
[0046] In an embodiment, the system of present disclosure can involve connecting six terminals of the motor in delta connection/configuration to a pre-defined nominal voltage to a reduced voltage, thereby increasing the available power supply voltage above the nominal voltage. For example, the delta configuration connection can reduce the nominal voltage to 230 V from 415 V, thereby increasing the available power supply voltage above the nominal voltage to a value of 585V (square root 2 x 415V – 415 V being voltage difference between two input phases). Reduction in nominal voltage from 415 V to 230V can reduce the V/f ratio from 415/50(8.3) to 230/50(4.6) leaving enough room for voltage to increase beyond critical frequency.
[0047] FIG. 4 illustrates an exemplary frequency and supply voltage relationship for the disclosed methodology for VFD in accordance with embodiments of the present disclosure. The disclosed system and method enables increase in voltage even beyond the critical frequency which coupled with earlier embodiment that makes a higher voltage available to the stator of the motor, can help in getting a constant torque even beyond the critical frequency. In another aspect the methodology can ensure that the motor current remains less than the rated current even at higher frequency thereby allowing increased range of compressor capacity variation using the same compressor-motor combination and can allow the motor to run the compressor to deliver very high refrigeration output at higher frequency and resultant higher speed.
[0048] In an embodiment, the disclosed method can be applied in solar powered system where a high voltage can be achieved as compared to a standard 3-phase system and for constant torque applications such as a refrigeration compressor. The methodology can enable achieving maximum capacity of the compressor during day time when solar power generation is at maximum. In an aspect, the solar systems are sized such that the maximum capacity of the compressor is achieved at the maximum solar power generation during the day. Therefore, a compressor configured to operate over a wider capacity range, can operate on solar power for a longer duration and hence the methodology can result in better operational efficiency.
[0049] In an aspect, the disclosed methodology can also be used in normal variable refrigerant flow systems where grid is used as a power source to achieve a better capacity variation. Using the methodology same compressor can deliver higher refrigeration output which can lead to better adjustment to variation in heat load resulting in a better operational efficiency. In addition the methodology can result in lower capital costs as lower capacity compressors can be used for same refrigeration output.
[0050] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF INVENTION
[0051] The present disclosure provides a method and system that can enable a wider range of capacity variation of a load attached to a motor.
[0052] The present disclosure enable use of a variable input power source to its fullest potential to drive a load requiring contact torque.
[0053] The present disclosure provides a system a method that enables a load requiring constant toque to avoid higher in-rush current above the maximum rated ampere but still operate with wider voltage range.
[0054] The present disclosure provides a system and method to control frequency and voltage to module speed of rotation of a motor.
[0055] The present disclosure provides a system and method that can enable a compressor to use its maximum capacity when higher input source power is available, without damaging the motor driving the compressor.
,CLAIMS:1. A system comprising:
a motor to drive a load, wherein six terminals of the motor are connected in delta configuration to reduce a predefined nominal voltage associated with the motor to a reduced nominal voltage to enable wider voltage range for operation of the motor; and
a variable frequency drive configured to draw power from a power source and provide a variable frequency power input based on the reduced nominal voltage to the motor so as to keep voltage to frequency ratio constant;
and wherein the motor is run with the constant voltage to frequency ratio above a rated frequency to enable the motor to run at higher RPMs.
2. The system of claim 1, wherein the load is a compressor that provides varying compressor capacities.
3. The system of claim 2, wherein the compressor is a refrigerator compressor.
4. The system of claim 1, wherein the power source is a solar power source.
5. The system of claim 1, wherein the predefined nominal voltage is 415 V, and the reduced nominal voltage is 230 V.
6. The system of claim 1, wherein the rated frequency is 50 Hz or 60 Hz.
7. A method for driving a motor comprising steps of
reducing a predefined nominal voltage associated with a motor to a reduced nominal voltage by connecting six terminals of the motor in delta configuration so as to enable wider voltage range for operation of the motor; and
providing, by a variable frequency drive, variable frequency power input based on the reduced nominal voltage to the motor so as to keep voltage to frequency ratio constant;
and wherein the motor is run with the contact voltage to frequency ratio above a rated frequency to enable the motor to run at higher RPMs.
8. The method of claim 7, wherein the load is a compressor that provides varying compressor capacities.
9. The system of claim 8, wherein the compressor is a refrigerator compressor.
10. A control system for refrigerator compressor comprising:
a motor to drive a refrigerator compressor, wherein six terminals of the motor are connected in delta configuration to reduce a predefined nominal voltage associated with the motor to a reduced nominal voltage to enable wider voltage range for operation of the motor; and
a controller configured to draw power from a source, and provide a variable frequency power input based on the reduced nominal voltage to the motor so as to keep voltage to frequency ratio constant;
wherein the motor is run with the contact voltage to frequency ratio above a rated frequency to enable the motor to run at higher RPMs.