FOR

METHODS AND SYSTEM BASED ON ADVANCED ENERGY SAVING APPLIED TO

FREQUENCY INVERTERS OF INDUCTION MOTORS

FIELD OF THE DISCLOSURE

[1] The wider technical field of the present disclosure is related to arrangements for adjusting, eliminating or compensating reactive power in networks, the arrangements being an integral part of the load, specifically related to the measure and control of power factor, more specifically applied to save energy in frequency inverters of induction motors.

BACKGROUND

[2] Electric motors have been used for over a century ago, and methods for improving their energy efficiency have been extensively researched in recent years due to predictions of global energy shortages. It is estimated that more than 50% of the electricity generated in all countries is used to drive electric motors, mostly induction motors driven or not by a frequency inverter .

[3] Due to the advantages of using a frequency inverter compared to other drive methods, its application continues to increase in replacement of other traditional induction motor drive solutions . [4] Considering the possibility of increasing the efficiency of induction motors driven by a frequency inverter, there is a lot of attention in recent years for the implementation of methods with the purpose of reducing losses in the induction motor, motivated by the need to reduce consumption of global energy, as well as by the reduction of carbon emission, especially in countries whose energy matrix has a large portion of the electric energy generation carried out by thermoelectric plants.

[5] The induction motor is an electrical machine that is designed to achieve high efficiency when driven near its rated operating point or full load. However, its efficiency is significantly reduced when the operating point moves to another different operating point, for example, where an induction motor drives a mechanical load with partial torque and/or at a low rotational speed. This reduction in efficiency occurs due to the unbalance of electrical (ohmic) and magnetic (iron) losses at the different operating points.

[6] An operating point of an induction motor is defined by a pair of values of rotational speed and mechanical torque provided on the axis of the induction motor.

[7] Efficiency is defined as the percentage between the ratio of mechanical power delivered to a mechanical load by the shaft of the induction motor and the total electrical power provided by an electrical energy source.

[8] Throughout the development of induction motors technology, it has always been a point of discussion between manufacturers and users the creation of a methodology for verifying efficiency. The insertion of frequency inverters expanded the discussions aiming at the possibility of variation of speed, torque, frequency, and other variables.

[9] When trying to define and standardize the operating points and procedures for determining losses in an induction motor driven by a frequency inverter, manufacturers and users came together to elaborate the definition of efficiency classes for the induction motor driven by a frequency inverter and the assembly consisting of the frequency inverter and the induction motor.

[10] As a proposal to harmonize and standardize procedures, the IEC 61800-9-1 standard was developed and published, which defines the drive system losses by energy efficiency classes and operating points for induction motors with nominal voltage lower than lkV and rated power up to 1 MW.

[11] The standard defined efficiency classes (IE0, IE1, IE2, etc. ) for induction motors driven by a frequency inverter that define the limits of relative losses at the operating point with 90% of the rotation speed and 100% of torque.

[12] The other seven operating points, defined by a percentage of the rotation speed and a percentage of the torque, are informed in the technical documentation and their results are independent of the definition of the IE class.

[13] To meet standardization and to increase the efficiency of the induction motor driven by a frequency inverter, several loss minimization methods have been proposed, the main ones being technically divided into i) methods based on a mathematical model and ii) methods based on optimal point search based on physical parameters . [14] Both methods have their advantages and disadvantages for each particular application, depending mainly on the type of mechanical load applied and the dynamic regime of torque variation.

[15] The activation regime is evaluated bv the value of the load variability rate defined by the equation:

[16] The load variability p depends on the load torque variability TL at time t, where k is a constant; When p is large, the process has a fast-triggering regime and if p is small, the process has a slow triggering regime.

[17] When evaluating the variability of the load p, care should be taken to evaluate the time interval in which the torque variation was evaluated, so that evaluation errors do not occur, being prudent to evaluate it at longer intervals of time.

[18] If the application has a wide range of torque variation or if the induction motor is oversized and operates at a fraction of its nominal conditions, these are the characteristic conditions to apply some loss reduction technique, being the opportunities for the percentage increase of the operating efficiency higher when the torque demanded by the load is below of the nominal torque of the induction motor.

[19] Most of the loads driven by induction motors have a work cycle with a wide range of torque (e.g. pumps, fans, compressors, crushers, conveyor belts, etc. ) , allowing the application of methods to optimize efficiency at a point of operation.

[20] However, due to the great diversification of use of induction motors in various industrial applications, it is difficult to develop a universal solution to this problem, which means that for each case, the manners to control the frequency inverter and the understanding of causes of losses, as well as ways to reduce them, must be analyzed.

[21] As for frequency inverters, they also have differences in the type of control used to control the induction motor, which are divided into two types of speed control strategies: scalar control or vector control.

[22] The scalar control uses the ratio between voltage and frequency imposed at the induction motor terminals, which is kept constant throughout the frequency variation range, to keep the induction motor's magnetic flux constant. This type of control is the most used due to its simplicity and the fact that in most applications it does not require high precision in the rotation speed control.

[23] Vector control considers the instantaneous electrical variables, referenced to the flux coupled to the rotor of the induction motor, and its equations are based on the spatial dynamics of the mathematical model of the induction motor, which is seen as a vector and controlled as a direct current motor, with separately controlled torque and magnetic flux. This type of control allows fast responses and a high level of precision in controlling rotational speed and mechanical torque.

[24] For the frequency inverter with scalar control, the most used method to reduce losses is to modify the voltage/f requency ratio, reducing and regulating the voltage imposed on the induction motor. For the frequency inverter with vector control, the most common methods are the reduction and regulation of magnetic flux.

[25] An immediate effect directly linked to the low efficiency of an induction motor is a low power factor. On the other hand, the point of greatest efficiency occurs when the magnetic flux is regulated according to the need of the actuated mechanical load.

[26] In an induction motor working at partial load and the frequency inverter with scalar control, decreasing the voltage applied to the induction motor terminals can increase the power factor, however its slip increases with the reduction of the voltage at the terminals of the induction motor, indicating that the voltage reduction proportionality may not be the same as the power factor increase.

[27] Similarly, in an induction motor working at partial load and the frequency inverter with vector control, when reducing the magnetic flux, the power factor does not vary proportionally, due to the slip variation to maintain the induction motor rotation speed .

[28] With this consideration, just reducing the voltage of the terminals or the magnetic flux of the induction motor does not serve the purpose of increasing efficiency and only when the reduction in this voltage is greater than that of the slip and with the increase in the power factor, induction motor efficiency can be improved.

[29] Additionally, an induction motor driving a mechanical load has different optimized power factor values, and by adjusting the power factor to this optimized power factor value, they cause it to indirectly vary the characteristics of the current and voltage applied to the induction motor, reducing electrical and magnetic losses. [30] This evaluation led to the development of control techniques to keep the power factor constant by varying the triggering angle of thyristors, which despite its apparent simplicity, regulate the power factor by electronic means following a power factor reference. But this technique does not show good results due to the constructive differences and powers of the induction motors, which has different values of optimized power factor and, moreover, for different nominal operating points it has different values of optimized power factor.

[31] From the identification of such a problem, a system that tracks the best value for a power factor reference, according to the needs of the induction motor at different operating points will have its efficiency increased.

[32] In view of that, the present invention presents a method for reducing the total losses of the induction motor driven by a frequency inverter, by including a usable advanced energy saving for both scalar and vector control modes of a frequency inverter.

[33] The advanced energy saving performs the tracking of the best power factor reference value to be followed by scalar control or vector control of a frequency inverter and acts on the indirect regulation of the voltage applied to the stator windings of the motor induction for scalar control or acts as indirect regulation of magnetic flux for vector control.

[34] For scalar control, the voltage versus frequency ratio is regulated to obtain the minimum power consumption point with the highest power factor, allowing the voltage versus frequency ratio to assume values smaller or larger than the nominal voltage frequency ratio. [35] For vector control, the magnetic flux is regulated to obtain the minimum power consumption point with the highest power factor, allowing the magnetic flux to assume values smaller or larger than the nominal magnetic flux.

[36] There are some prior art documents that use power factor regulation to reduce losses, however, there are still some deficiencies in this power factor control.

[37] For example, the document US 4,052, 648, expired in 1994, presents a control system for the regulation of the power factor that, when placed in circuit with the input of energy from an induction motor, effects a reduction in the energy normally supplied to the motor when operated in a condition where the line voltage is greater than normal and/or the motor load is less than a rated load by modifying the current waveform to follow a reference displacement between the voltage waveform and current waveform. This increases the efficiency but is just used for fixed speed .

[38] The document US 4,291,264, expired in 1999, discloses a control system which uses means to measure the power factor and means to control an inverter drive ac induction motor to follow a constant desired power factor, regardless of load variation to improve the efficiency, but the user needs to select by manually adjusting for the desired power factor.

[39] The document US 5, 010,287, expired in 1998, discloses a variable frequency drive to drive the induction motor, including a power factor detecting section to detect the power factor phase angle between the motor current and the inverter voltage output. Some judge means are used to compare the detected power factor phase angle with a predetermined reference angle, in order to provide a control output response with a predeterminate relationship with the frequency for improving motor efficiency, by using the storage/designation of one of voltage per frequency curves, selecting at least one curve to control the voltage per frequency relation in order to improve efficiency, the method needs the input related to previous knowledge of a plurality of voltage per frequency curves together to the different induction motor.

[40] The document US 5,204, 606, expired in 2010, shows the means to detect the power factor, being that, once detected, said power factor is compared with an ideal value of power factor, to obtain an error amount that is determined in accordance with the motor load. An optimum power factor circuit to convert this load variation parameter into an optimum value of power factor is provided, as a relationship between the working power and the compensation. However, this compensation is an approximation, not an optimization, and the power factor is just increased and decreased with the increase and decrease of the working power in a proportional manner.

[41] The document US 5,241,256, expired in 2010, uses the same basic idea from US 5,204, 606, but instead of using an approximation, it uses a set of curves, in a set of tables stored memory, between motor load and optimal power factor and a family of curves related to the power factor compensation and motor supply voltage with different frequencies applied to the induction motor. One drawback of this solution is: if the characteristics of the induction motor change, a new set of tables must be used and stored m memory. [42] In the document US 8,339, 093, the control system inputs a plurality of modified voltage-frequency commands to the ac motor drive and determines the real-time value of the motor parameter corresponding to each of the plurality of modified voltage frequency commands and identifies an optimal value of the motor parameter based on the real-time values of the motor parameter for open-loop motor drive. Thus, this method disturbs the system with different V/Hz ratios and measure parameters to use in open-loop motor drives but cannot be used for vector control or closed-loop applications .

SUMMARY

[43] The present invention provides a method for continuous tracking of the power factor in an induction motor driven by a frequency inverter with escalar or vetorial control, by the use of a system for advanced saving energy, comprising a proportional and integral controller (14) , at least a selection switch (15) and a tracking module (16) . The system tracks the best power factor reference, according to the needs of the induction motor at different operating points and acts on the indirect regulation of the voltage applied to the stator windings of the motor induction for scalar control or acts as indirect regulation of magnetic flux for vector control, increasing in both types of the efficiency of a motor due to the successively adjustments in the power factor reference. The method also can be used in closed-loop regulation for both scalar or vector control and for different mechanical load characteristics either can be linear, quadratic, constant, or another one since that the operation point is a stable operating point (speed and torque) .

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

[44] Figure 1 shows the block diagram of the scalar control of the frequency inverter with the advanced energy saving (4) ;

[45] Figure 2 presents the block diagram of the frequency inverter vector control with the advanced energy saving (4) ;

[46] Figure 3 presents the block diagram of the advanced energy saving (4) ;

[47] Figure 4 presents the flowchart of the ways to store the variable reference power factor FPvar* to use for optimization tools or frequency inverter replacement;

[48] Figure 5 shows the dynamic behavior of the adjusted power factor reference FP* over time and of an X variable to be minimized in relation to the power factor variable reference FPvar;

[49] Figure 6 shows the results applying the method, showing the variation of voltage, current, losses and temperature increase in different operation points of speed and torque.

[50] Figure 7 shows the standardized operation points used to evaluate the method.

[51] Figure 8 shows the application of the advanced energy saving with an induction motor operating at 1/4 of the rated speed with scalar control (V/f) ; [52] Figure 9 shows the application of the advanced energy saving with an induction motor operating at 2/3 of the rated speed with scalar control (V/f) ;

[53] Figure 10 shows the application of the advanced energy saving with an induction motor operating at nominal speed with scalar control (V/f) ;

[54] Figure 11 shows the convergence of the tracking module from the variable reference power factor FPvar to scalar control (V/f) ;

[55] Figure 12 shows the application of the advanced energy saving with an induction motor operating at 1/4 of the nominal speed with vector control;

[56] Figure 13 shows the application of the advanced energy saving with an induction motor operating at 2/3 of the rated speed with vector control;

[57] Figure 14 shows the application of the advanced energy saving with an induction motor operating at nominal speed with vector control;

DETAILED DESCRIPTION

[58] In the following description, for the purpose of explanation, numerous specific details are set forth to provide a thorough understanding of the present disclosure. It will be apparent, however, that embodiments may be practiced without these specific details. Embodiments are disclosed in sections according to the following outline:

[59] The present invention presents a method for reducing the total losses of the induction motor driven by a frequency inverter by including an implementable advanced energy saving either one scalar or vector control modes.

[60] The advanced energy saving performs the tracking of the best power factor reference value to be followed by scalar control or vector control of the frequency inverter and performs indirect regulation of the voltage applied to the stator windings for scalar control or indirect regulation of magnetic flux for vector control.

[61] This invention provides a method for continuous adjustment of the power factor in an induction motor driven by a frequency inverter, comprising the steps of: a) to select a power factor variable reference FPvar* by a selection switch (15) for tracking at an operation point; b) to increase or decrease with steps of AFP the current power factor FP of the induction motor (7) during a time interval At using the tracking module (16) defining an adjusted power factor reference signal FP*; c) to input the adjusted power factor reference signal FP* in the proportional and integral controller (14) ; d) to provide a control sign df _P * to be added to the current power factor FP using the proportional and integral controller (14) ; e) to repeat for countless times the steps (a) to (d) while the frequency inverter is driving the induction motor.

[62] Figure 1 presents the block diagram of the scalar control of a frequency inverter with the inclusion of the advanced energy saving (4) proposed in this invention. [63] The said method for continuous adjustment of the power factor in an induction motor driven by a frequency inverter when used with scalar control, further comprises the steps: d.l) provide a module of advanced energy saving, wherein the voltage reference Us, represented by the value of the voltage vector initially imposed on the induction motor, is added with the control sign df _P *, outputting the adjusted quadrature voltage reference Usq* d.2) to provide an integrator which defines, from the angular speed reference a _e * , the synchronous angle 9 _e * to the structure of the scalar control; d.3) to provide an activation sequence of the power stage switches of the induction motor from the calculation of an inverse Park transform, having as parameters the synchronous angle 9 _e * , the calculated adjusted quadrature voltage reference Usq* and a null direct voltage reference Usd*; d.4) to estimate the current power factor FP constantly from the current signal provided by a current sensor and the Usq* adjusted quadrature voltage reference; d.5) to fed back the current power factor signal FP to the block, wherein the output varies the amplitude of the voltage imposed on the induction motor stator (7) ; d.6) to track and determine the operation point with the highest power factor and the lowest voltage amplitude imposed on the induction motor stator, from the disturbance caused by the variation in the amplitude of the voltage; d.7) to regulate the voltage versus frequency ratio obtaining the lowest power consumption point with the highest power factor; d.8) to allow the voltage versus frequency ratio to assume values smaller or larger than the nominal voltage frequency ratio.

[64] The speed reference signal (1) provides the frequency inverter with an acceleration ramp with predefined acceleration and deceleration limits for driving the induction motor (7) . The rotational speed at which the induction motor (7) will be accelerated is defined by the signal a _e * given by (1) .

[65] The voltage reference signal (2) provides the frequency inverter which voltage is to be imposed on the induction motor (7) along with the speed acceleration ramp required for scalar control.

[66] The voltage reference provided by the frequency inverter is defined by variable Us, provided by the voltage reference signal (2) and represents the vector value of the voltage imposed on the induction motor (7) .

[67] The integrator (3) provides, from the signal a _e * , the synchronous angle 0 _e * for the scalar control structure, used as an input signal for the Inverse Park Transform block (5) .

[68] The advanced energy saving (4) represents the control function that is applied in the frequency inverter to reduce the energy consumption of the induction motor (7) and said advanced energy saving (4) is detailed in Figure 3 and Figure 4.

[69] The control signal df _P * is summed with the Us signal to generate the output of the reference voltage signal Usq* and represents the voltage vector value imposed on the induction motor (7) adjusted by the advanced energy saving (4) .

[70] The control signal df _P * represents the control action of the advanced energy saving (4) and the signal Usd* represents a null voltage vector. [71] With the signals Usd*, Usq* and 9e*, the Inverse Park Transform (5) is obtained, which provides the activation sequence of the power stage switches (6) for the activation of the induction motor ( 7 ) .

[72] The current sensor (12) provides a signal that represents the current passing through the induction motor stator windings (7) which together with the Usq* reference voltage is estimated the current power factor FP (13) .

[73] The current power factor signal FP represents the power factor signal which is one of the advanced energy saving inputs (4) .

[74] The scalar control mode works as follows: When applying a speed reference (1) , block (2) generates a voltage reference Us proportional to the speed reference signal a _e * .

[75] Block (5) applying the Inverse Park Transform generates a three-phase voltage signal for the power stage (6) that feeds the induction motor (7) .

[76] The frequency inverter constantly acquires the currents from the stator windings of the induction motor (7) to estimate the current power factor signal FP of the induction motor (7) .

[77] The current power factor signal FP is fed back to the block (4) , whose output varies the amplitude of the voltage imposed on the induction motor stator (7) .

[78] This variation in voltage amplitude causes disturbance in the system to track and determine the point of greatest efficiency, in this case, with the highest power factor and the smallest voltage amplitude imposed on the induction motor stator (7) . [79] For scalar control, the voltage versus frequency ratio is regulated to obtain the minimum power consumption point with the highest power factor, allowing the voltage versus frequency ratio to assume values smaller or larger than the nominal voltage frequency ratio.

[80] Figure 2 presents the vector control block diagram of a frequency inverter with the inclusion of the advanced energy saving (4) proposed in this invention.

[81] The addition of the advanced energy saving (4) for frequency inverter with vector control does not imply any changes in its operation compared to that presented for scalar control.

[82] The said method for continuous adjustment of the power factor in an induction motor driven by a frequency inverter when used with vector control, further comprises the steps: a) to provide a module of advanced energy saving, wherein the magnetic flux reference Ad, represented by the value of the magnetic flux initially imposed on the induction motor, is added with the control sign df _P * , outputting the adjusted magnetic flux signal Ad*; b) to provide an integrator (1) which defines the angular speed reference m _e * and a block state observer (11) which defines the synchronous angle 9 _e *; c) to provide an activation sequence of the power stage switches of the induction motor from the calculation of an inverse Park transform, having as parameters the synchronous angle 9 _e * , the calculated adjusted quadrature voltage reference Usq* and a null direct voltage reference Usd*; d) to estimate the current power factor FP constantly from the current signal provided by a current sensor and the Usq* adjusted quadrature voltage reference; e) to feedback the power factor signal FP to the block (4) , wherein the output varies the amplitude of the magnetic flux imposed on the induction motor stator (7) ; f) to track and determine the operation point with the highest power factor and the lowest magnetic flux amplitude imposed on the induction motor stator, from the disturbance caused by the variation in the amplitude of the magnetic flux; g) to regulate the magnetic flux obtaining the lowest power consumption point with the highest power factor.

[83] The speed reference (1) generates the signal a _e * which is used as one of the inputs to the speed and flux controller (9) , the other input receives the adjusted magnetic flux signal Ad* obtained from the sum of the flux reference signalAd (10) and of the control signal df _P * which represents the control action of the advanced energy saving (4) .

[84] With the flux reference signals Ad (10) and df _P * , the speed and flux controller (9) generate the current control signals Isd,q which are used as input to the current controller (8) , which provides the signal Usd,q as an input to the Inverse Park Transform block (5) .

[85] The synchronous angle signal 0 _e * is generated by the state observer block (11) and is used as input to the Park transform inversion block (5) .

[86] A state observer is used to estimate state variables based on measures of output variables and control variables regardless of whether these variables are accessible, since the observability condition is satisfied.

[87] In the same way as in scalar control, with the signals Usd*, Usq* and Q _e * , the Inverse Park Transform (5) is obtained, which provides the activation sequence of the switches from the power stage (6) to the induction motor drive (7) .

[88] The current sensor (12) provides a signal representing the current passing through the induction motor stator windings (7) , with which, together with the voltage with the signals Usd*, Usq*, the current power factor FP (13) is estimated, which is one of the advanced energy saving inputs (4) ;

[89] The determination of the inverse of the Park transform (5) , of the power stage (6) , of the induction motor (7) , of the current sensor reading (12) and of the power factor estimator (13) have the same structure and function for scalar control, so the descriptions previously made are also valid for the vector control.

[90] The vector control mode works as follows: When a three- phase switched voltage signal is applied by the frequency inverter to fed an induction motor, according to the vector control structure in Figure 2, the power factor signal is fed back into block ( 4 ) .

[91] The control signal df _P * from the block (4) summing with the reference magnetic flux d (10) in order to change the amplitude of the adjusted magnetic flux Ad* imposed on the air gap of the induction motor (7) .

[92] This change allows the system to be disturbed and thus the advanced energy saving (4) tracking the point of greatest efficiency, as explained for scalar control, in this case, with the highest power factor and the smallest amplitude of the adjusted magnetic flux Ad*imposed on the air gap of the induction motor (7) .

[93] For vector control, the adjusted magnetic flux Ad*is regulated to obtain the minimum power consumption point with the highest power factor, allowing the magnetic flux to assume values smaller or larger than the nominal magnetic flux.

[94] Figure 3 shows the block diagram of the internal structure of the system of advanced energy saving (4) , object of the present invention, in which the signal source for the regulation of the adjusted power factor reference FP* , selected among a fixed reference of power factor FPcte* or a variable reference of power factor FPvaF to be set at an operating point.

[95] The presente invention provides a system for advanced energy saving comprising: a proportional and integral controller (14) ; at least a selection switch (15) ; a tracking module (16) ; wherein a power factor variable reference FPvar* is initially selected by a selection switch (15) , making the tracking module (16) increasing or decreasing the power factor variable reference FFva *with steps of AFP, at each time interval At, to establish an adjusted power factor reference FP* signal to be added to the current power factor FP of the induction motor (7) which is currently inputed in the proportional and integral controller (14) to provide a control sign (df _P *) .

[96] A select switch (15) selects the adjusted power factor reference FP* signal source, which can be selected between fixed power factor reference FPcte* or power factor variable reference

FPvar* values according to a user-configured parameter.

[97] The fixed power factor reference FPcte* represents a constant power factor reference, which can be used to drive the induction motor (7) , selectable by a parameter.

[98] In this condition where the adjusted power factor reference FP* will be a constant value, that is FPcte* = FP* , the frequency inverter, depending on the used type of control scalar or vector, regulates the drive parameters for regulation of the current power factor FP of the induction motor (7) at the fixed power factor reference FPcte*.

[99] A lower value than required for FPcte may demand from the frequency inverter a high voltage value to be supplied to the induction motor stator (7) which may not be supplied, once the DC link may not have enough voltage amplitude to fed that high voltage value, causing pulsating torque and instability in the regulation of rotating speed.

[100] A value greater than required for FPcte, on the contrary, may demand from the frequency inverter a reduced voltage value to be supplied to the induction motor stator (7) which may be insufficient to maintain the rotor movement and cause it to stall.

[101] In view of these usual limitations and undesirable effects, this invention proposes to vary the adjusted power factor reference FP* to a value optimized by tracking module (16) increasing or decreasing the power factor variable reference FPvar* with steps of AFP, at each time interval At, to establish an adjusted power factor reference signal FP* , to avoid those instable operating point of the induction motor (7) due to unoptimized power factor reference.

[102] When selecting by the selection switch (15) , the variable power factor reference FPvar* to the adjusted power factor reference FP*, where the variable power factor reference FPvar* represents the variable power factor reference to be adjusted by the tracking algorithm according to an operating point of the induction motor (7) .

[103] When the induction motor (7) reaches a stable operating point, the tracking algorithm, implemented in the tracking module (16) , starts the increasing or decreasing of variable power factor reference FPvar* to the adjusted power factor reference FP* and evaluation of the current power factor FP of the induction motor (7) and the stability conditions of the control to find the optimized value of the variable power factor reference FPvar* for an operating point.

[104] It is considered that the induction motor (7) reached a stable operating point when the rotation speed of the rotor of the induction motor (7) reached the reference speed a _e * .

[105] The initial value used to start tracking the variable reference power factor FPvar* is the nominal power factor value of the induction motor FP _n usually available on the induction motor nameplate ( 7 ) .

[106] Different initial values of the nominal power factor FP _n can be stored in memory and the values already optimized power factor FPoptm for an induction motor operating point (7) can be stored too, aiming to decrease the convergence time, as shown in figure 4. [107] Each optimized value of current power factor FP shall be stamped in memory with optimized power factor FP _op tm related to an operation point identified by its speed and torque;

[108] Storage can be local, in a non-volatile memory, or it can be stored remotely by some communication medium for later access by the frequency inverter.

[109] This functionality allows that in an environment of applications related to industry 4.0, such as the use of machine learning tools and/or artificial intelligence, these optimizations can be used for the self-configuration of the frequency inverter when in a similar application or in the replacement in case of failure, without any loss of the learning realized on the optimization of the power factor with the induction motor and operating point.

[110] The tracking algorithm automatically looks for the optimized value of the power factor variable reference FPvar* , varying FPvar* with steps of AFP, increasing or decreasing depending on the evaluation of the current power factor FP of the induction motor (7) during the time interval At.

[111] The value AFP is setup calculated as a function of the nominal power factor FP _n value of the induction motor (7) and kept constant throughout the tracking process.

[112] The value of AFP is much smaller than the nominal power factor FPn value of the induction motor (7) to avoid pulsing rotor torque during the tracking process.

[113] The value of time interval At is calculated at the start of the tracking module (16) , from the induction motor nameplate data that is provided by the user during the drive and induction motor configuration process.

[114] The value of time interval At is much greater than the switching period of the power stage (6) of the frequency inverter and is proportional to the rotor time constant of the induction motor and kept constant throughout the tracking process.

[115] The motor rotor time constant is estimated from the rated slip frequency and the rated power of the induction motor.

[116] The proportional and integral controller (14) , whose input is the difference of the adjusted power factor reference signal FP* and the estimated signal of the current power factor FP, is used for the regulation of the FP* and FP variables providing the df _P * variable.

[117] The variable df _P * represents the control action of the proportional and integral controller (14) used to regulate the current power factor FP, which acts on the stator voltage and/or magnetic flux of the induction motor (7) to search the lowest power consumption point for an operating point.

[118] Figure 5 (a) presents the behavior of the power factor variable reference FPvar* over time and Figure 5 (b) represents the minimization curve of the variable to be minimized represented by the value Xmi _n .

[119] The variable to be minimized can be selected by the user from the parameter setting to perform the minimization of a variable to be minimized.

[120] The variable to be minimized can represent, for example, the current signal passing through the stator windings Is or active power P _e consumed by the induction motor (7) . [121] In the search condition for the best power factor variable reference FPvar* , the algorithm automatically searches for the best power factor variable reference FPvar* with the smallest value of variable to be minimized.

[122] By selecting variable to be minimized, the variable to be minimized is read continuously, and the algorithm is updated at each time interval At.

[123] If the result is negative, that is, the variable to be minimized is smaller than the previous sample, checks whether the variable to be minimized has been incremented or decremented previously, and, if this condition is satisfied, it decrements at a step of AFP the variable power factor reference FPvar* or if this condition is not satisfied, it increases at a step of AFP the variable power factor reference FPvar* .

[124] If the result is positive, that is, the variable to be minimized is greater than the previous sample, checks whether the variable has been incremented or decremented previously, if this condition is satisfied, it increments in a step of AFP the power factor variable reference FPvar* or if this condition is not satisfied, decrements by a step of AFP the power factor variable reference FP _vai *.

[125] This process remains indefinitely while the frequency inverter is driving the induction motor (7) , whether using scalar control or vector control.

[126] The power factor variable reference signal FPvar* will continuously apply a variation of a step of AFP at each time interval At in order to find and maintain the minimum point of the chosen variable. [127] The effect of applying the adjusted power factor reference FP* tracking module is not exclusively to increase the current FP power factor, but rather to seek a current FP power factor value so that there is an increase in the efficiency of the induction motor.

[128] Some data were obtained, when applying the advanced energy saving (4) , object of the present invention, in a frequency inverter with scalar control and performing tests at relevant operating points for verification.

[129] Figure 6 shows the standardized operation points, related to speed and torque, used to evaluate the method. The operation points 2, 3, and 8, are at the 100% of the relative torque e different values of the relative speed: 50%, 25% and 10%, respectively. A decreasing in the torque for 50% is evaluated at the points for 4 and 5, with 90% and 50% of the speed. The points 6 and 7, are evaluated in the conditions of 25% of the torque and 50% and 25% of the speed.

[130] Figure 7 shows the results during some tests applied in an induction motor using a frequency inverter with the proposed method and compared to the same without the method in percentage. With 100% of the relative torque, at points 2, 3, and 8, the voltage increases and current, losses and temperature variation decrease and obviously the efficiency of the induction motor decrease. With 50% of the relative torque, at points 4 and 5, a small increase in the voltage and losses is verified, with the reduction in the current and temperature variation. With 25% of the relative torque, the voltage, current, losses and temperature variation also decrease. [131] The results show that using the method, instead to increase or decrease the voltage applied, it is possible to increase the efficiency.

[132] Figures 8, 9 and 10 show the control variables with scalar control of the stator winding current Is, of the current power factor FP, and of the adjusted power factor reference FP* for 1/4 of the rated speed, 2/3 of the rated speed and rated speed, respectively, for an induction motor driven by a frequency inverter operating with no-load torque in figures 8 (a) , 9 (a) and 10 (a) and with rated torque in figures 8 (b) , 9 (b) and 10 (b) .

[133] In the tests with the induction motor operating points with no-load torque, shown in figures 8 (a) , 9 (a) and 10 (a) , when applying the advanced energy saving (4) , the control action signal df _P * starts acting by setting quadrature voltage reference Usq* , which decreases stator current Is and increases current power factor FP to follow set power factor reference FP* .

[134] The current power factor FP initially does not reach the set power factor reference value FP* due to the application of the voltage reduction limitation imposed on the induction motor stator windings, to prevent stalling from occurring of the rotor due to insufficient torque.

[135] Even with the limitation of voltage reduction V _s imposed on the stator windings, it is possible to increase the current power factor FP and decrease the current of the stator windings Is in this case, despite the adjusted power factor reference FP* it has not yet approached the proper FP current power factor for the operating point in the no-load induction motor test time period, the tracking module will trend this value. [136] The value of this adjusted power factor reference FP* at this operating point or other operating points can be stored in a non-volatile memory or stored and made available for remote access (cloud) by a communication means present in the frequency inverter, to decrease the convergence time of the FP* adjusted power factor reference with FP.

[137] In the tests with nominal torque shown in figures 8 (b) , 9 (b) and 10 (b) , the current power factor FP follows the adjusted power factor reference FP*, since the proximity to the rated power factor FPn, set in the drive's initial setup, is permanently set to the best set power factor reference FP* for the operating point.

[138] When applying the advanced energy saving for tracking the best adjusted power factor reference FP*, the actual power factor FP has an increment, and the stator winding current Is having a small reduction due to the point of operation is close to the rated operating point at which the induction motor is designed.

[139] The adjusted power factor reference FP* , in the context of figures 8, 9 and 10, is adjusted to reduce the active electrical power vector P _e , this effect being achieved by increasing the voltage in the stator windings V _s , which for its realization, it is necessary that the voltage on the DC Vbdc of the frequency inverter has a voltage level sufficient to enable the voltage increase in the stator windings Vs.

[140] At these operating points, the possibilities for increasing efficiency are limited to the adjustment of the adjusted power factor reference value FP* and the nominal power factor FP _n of the induction motor used at the beginning of the process, but with faster convergence. [141] During the performance of the advanced energy saving (4) , in addition to monitoring the current power factor FP, the current of the stator windings Is is also monitored, and when an increase or decrease in the current Is is noticed, in the next step of adjusted power factor reference update FP*, a decrease or increase in steps of AFP will occur in the FP adjusted power factor reference FP* .

[142] This statement is shown in Figure 11, which presents the results of the convergence of the advanced energy saving in tracking the best adjusted power factor reference FP* to the smallest current vector Is of the induction motor with nominal load and at 1/4 of the nominal rotation speed.

[143] Likewise, the advanced energy saving, object of the invention, was applied in a frequency inverter with vectorial control, performing the tests at operating points relevant to the verification .

[144] Figures 12, 13, and 14 show the control variables with vetorial control of the stator winding current I _s , the current power factor FP, and the adjusted power factor reference FP* for 1/4 of the rated speed, 2/3 of the rated speed and the rated speed, respectively, for an induction motor driven by a frequency inverter operating with no load in figures 12 (a) , 13 (a) and 14 (a) and with rated torque in figures 12 (b) , 13 (b) and 14 (b) .

[145] In the tests with the induction motor operating points with no-load torque, shown in figures 12 (a) , 13 (a) and 14 (a) , when applying the advanced energy saving, the action signal control switch df _P * starts acting by adjusting the magnetic flux reference Ad*, which decreases the stator current Is and increases the power factor FP to follow the adjusted power factor reference FP* .

[146] The actual power factor FP, as in scalar control, initially does not reach the set power factor setpoint FP* due to the application of the magnetic flux reduction limitation imposed on the induction motor, to prevent the rotor stall due to insufficient torque.

[147] Even with the limitation of the magnetic flux reduction imposed on the induction motor, it is possible to increase the current power factor FP and decrease the current of the stator windings Is, in this case, despite the adjusted power factor reference FP* it has not yet approached the proper current power factor FP for the operating point in the no-load induction motor test time period, the tracking module will tend to this value.

[148] The value of this adjusted power factor reference FP* at this operating point or other operating points can be stored in a memory or stored and made available for remote access by a communication means present in the frequency inverter, to decrease the convergence time of adjusted power factor reference FP* with actual power factor FP.

[149] In the tests with nominal torque shown in figures 12 (b) , 13 (b) and 14 (b) , the adjusted power factor reference FP* follows the current power factor FP, since the proximity to the rated power factor FPn, set at initial drive setup and being permanently set to the best set power factor reference FP* for the operating point.

[150] When applying the advanced energy saving for tracking the best adjusted power factor reference FP*, the actual power factor FP was incremented, and the stator winding current Is had a small reduction due to the set point located close to the rated operating point at which the induction motor was designed.

[151] The adjusted power factor reference FP* , in the context of figures 12, 13 and 14, is adjusted to reduce the active electrical power vector P _e , this effect being achieved by increasing the magnetic flux reference Ad* by the action of control df _P * of the advanced energy saving by increasing the voltage in the stator windings V _s , which for its realization, it is necessary that the voltage in the Vbdc direct current busbar is sufficient to enable the increase of the magnetic flux imposed on the induction motor by the frequency inverter.

[152] As with scalar control, at these operating points, the possibilities for increasing efficiency are limited to setting the adjusted power factor reference value FP* compared to the nominal power factor FP _n of the induction motor used at the start of the process, however the fastest convergence.

[153] The dynamic behavior of the reference power factor search function, whose operating points are the same as those used for scalar control, was no change in the structure of the reference power factor search function when applied in vector control.