Abstract
The present invention relates to a speed control system for hermetic compressors of the variable speed type, which uses a PMSM motor, containing a strategy for optimization, or minimization, of the electric current. Once the minimum current has been defined to meet a given demand for torque, it is applied to the hermetic compressor motor, which will operate at the maximum efficiency point for each load point.
Claims
1. A method for increasing the efficiency of hermetic compressors applied in refrigeration and air conditioning, characterized by having an electronic converter, which executes at least one control algorithm to conveniently drive a variable speed hermetic compressor, said compressor being an integral part of a plurality of refrigeration systems, such as refrigerators and freezers for residential or commercial use, window-type or split-type air conditioners.
2. The method for increasing the efficiency of hermetic compressors applied in refrigeration and air conditioners of claim 1, wherein at least one electronic converter, consisting of a power circuit containing at least one rectifier circuit, a DC filter, an inverter circuit and a digital signal processor, which can be implemented by a multiplicity of digital processors.
3. The method for increasing the efficiency of hermetic compressors applied in refrigeration and air conditioners of claim 2, characterized by having a current sensor and a voltage sensor on the DC bus.
4. The method for increasing the efficiency of hermetic compressors applied in refrigeration and air conditioners of claim 2, characterized by having at least one electric motor control algorithm that is performed by the digital signal processor.
5. The method to increase the efficiency of hermetic compressors applied in refrigeration and air conditioners of claim 4, characterized by having an algorithm for controlling the speed of the hermetic compressor's electric motor, being this vector-type algorithm (FOC).
6. The method for increasing the efficiency of hermetic compressors applied in refrigeration and air conditioners of claim 5, characterized by having a second algorithm of the MTPA type, which is responsible for defining the lowest operating current for the compressor hermetic, to supply to a certain conjugate.
7. The method for increasing the efficiency of hermetic compressors applied in refrigeration and air conditioners of claim 6, characterized by having a curve with the trajectory of the best combinations of currents i.sub.d and i.sub.q to meet a given demand for conjugate.
8. The method for increasing the efficiency of hermetic compressors applied in refrigeration and air conditioners of claim 7, characterized by having a multiplicity of curves of the same conjugate that, when intercepted by the MTPA curve, define the lowest currents i.sub.d and i.sub.q to operate the hermetic compressor and thus consume the lowest power, maximizing its efficiency over the entire operating speed range and at all load points within the operating envelope delimited by.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The accompanying drawings illustrate the present invention, in which:
[0032] FIG. 1 is a simplified block diagram of the system;
[0033] FIG. 2 is a block diagram of the electronic converter;
[0034] FIG. 3 is a detailed block diagram of the control system; and
[0035] FIG. 4 is an MTPA operating curve.
[0036] The present invention will now be described in detail through FIGS. 1 to 4, which illustrate preferred, but not mandatory designs, not limited to these, to implement the present invention.
DETAILED DESCRIPTION
[0037] FIG. 1 illustrates the simplified block diagram of the system considered in the present invention, consisting of an electronic converter (1), connected to the AC power distribution network, which, by means of a digital signal processor, performs a control algorithm (2), whose function is to efficiently drive the electric motor, of the PMSM type, which is embedded in the hermetic variable speed compressor (3). Said compressor is part of a plurality of refrigeration systems (5), which can be, but not limited to, refrigerators and freezers for residential or commercial use, “window” type air conditioners (where the evaporator and condenser are in the unit of the equipment) or of the “split” type (where the evaporator and condenser are in different equipment units). In the design illustrated through FIG. 1, the components that make up the cooling system (5) are not shown, such as: evaporator, condenser, filter, and expansion element, among others, as they are not decisive for the purposes of the present invention. However, FIG. 1 highlights an important element of the refrigeration system, which is the thermostat (4), which in this conception is of the electronic type, as opposed to the electromechanical thermostats, being that the referred thermostat (4) is responsible for the temperature control of the cooling system (5), whatever it may be, from a temperature reference indicated by the user. In applications containing hermetic compressors of variable speed, it is necessary to determine a speed reference (ω.sub.r, ref) for compressor operation, so that the temperature of the refrigeration system is maintained in accordance with that desired by the user. Once the speed reference is defined, it is made available to the control algorithm (2) for operation of the compressor motor. It is not an object of the present invention to discuss the determination of the compressor operating speed reference.
[0038] FIG. 2 presents a block diagram of the electronic converter (1) illustrating the minimum elements that comprise it, illustratively, and this conception should not be understood as limiting, in any way, within the scope of the present invention. So, in the design illustrated through FIG. 2, a low-pass filter coupled to the grid (6) is used to minimize the emission of electrical noise from the switched circuits that integrate the electronic converter (1) and, thus, contribute to maintaining the quality of the electricity from the electricity grid. A rectifier circuit (7), connected to the filter outlet (6), is responsible for converting electrical energy into alternating current (AC), coming from the electrical network, into pulsed direct current (DC). The rectifier circuit (7) admits a plurality of constructive variants, which can be single-phase or three-phase, uncontrolled or controlled, or of the synchronous type. Since the rectifier output (7) is a pulsating DC signal, a filter stage (8) is necessary to eliminate the low-frequency pulsation and thus create an oscillation-free DC “bus”. The filter output of the DC bus is used for two purposes, the first being to supply the input of the inverter stage (9) and the second to supply the input of the low voltage DC power supply (10). The inverter stage (9) is a three-phase H bridge type structure, typically used as a voltage source type inverter, but not limited to it, implemented by fast semiconductor switches, such as IGBT or MOSFET transistors, but not limited to them. The inverter (9) is controlled by the digital signal processing stage (11) in an appropriate way to transform the DC voltage at its input, into AC voltage at the output. This AC voltage, whose fundamental component is typically sinusoidal, can be adjusted in amplitude, frequency, and phase to drive the electric motor of the hermetic compressor. An electrical level conversion element, not shown in FIG. 2, is necessary for the connection between the inverter (8) and the digital signal processing stage (11). The DC source (10) is an electronic converter of the CC-DC type, which can be implemented by a multiplicity of topologies, which converts the high DC voltage of the DC bus into low amplitude DC voltages necessary for the operation of the digital signal processor and other low-power circuits not shown in FIG. 2. It is worth mentioning that the connection between the inverter stage (9) and digital signal processing (11) is bidirectional, since in addition to the command signals of the semiconductor switches that make up the inverter (9), voltage and current sensors, not shown in FIG. 2, they are necessary for the operation of the control algorithm (2) which will be detailed in FIG. 3. The signals from these sensors are applied to the analog to digital conversion inputs (A/D converters), which make up the digital signal processor (11).
[0039] FIG. 3 presents the detailed block diagram of the control algorithm (2), showing a possible implementation of the present invention, in which a current sensor (13) measures the current of the DC bus (i.sub.cc) and a sensor of voltage (14) measures the DC bus voltage (v.sub.cc), and the outputs of these sensors are converted to digital values in the A/D converters (15) and (16), respectively. The digital word from (15) is applied to an algorithm to reconstruct the phase currents of the motor (17), which estimates the currents at the motor terminals i.sub.a, i.sub.b, and i.sub.c through i.sub.cc, since the reading of the A/D (15) is performed in synchronization with the triggering of the semiconductor switches of the inverter bridge (9), so that it is possible to correlate the instantaneous current icc with the phase currents of the motor (12) of the hermetic compressor (3). The output of the algorithm (17), i′.sub.a, i′.sub.b, and i′.sub.c, has two purposes: the first is to provide the input signals for an algorithm to estimate the speed and angular position of the rotor (18), producing in the its output two pieces of information, with ωr being the velocity estimate and θr being the estimate of the angular position of the rotor. The second purpose of (17) is to provide the input signals for the algorithm that performs the Clarke transform, that is, the transformation of a sinusoidal, three-phase and, time-dependent abc coordinate system, to a sinusoidal coordinate system, two-phase, stationary, and time-dependent, known as αβ (19). Together with the estimate of the angular position of the rotor θr, the output of (19) provides the input signals for the algorithm that performs the Park transform, that is, the transformation of a sinusoidal, biphasic, and time-dependent coordinate system, said ab, for a rotating coordinate system, synchronous with the motor current frequency (12), called dq (20). The speed reference ω.sub.r ref, produced by the thermostat (4), works as an input command for the control algorithm (2), this signal being compared with the speed estimate w coming from (18) through the error detector (21). The speed error signal is applied to a proportional and integral type controller (22), but not limited to it, whose output is the reference of electromagnetic torque T.sub.e, ref, which will be the input of the MTPA algorithm (23). The outputs of (23) are the references for the current controllers i.sub.d, ref and i.sub.q, ref which will be applied to the error detectors (24) and (25) that compare the references with the current feedback signals i.sub.d and i.sub.q, coming from (20). The error signals of the direct axis and quadrature currents will be applied to controllers of the proportional and integral type (26) and (27), but not limited to these, whose outputs are applied to algorithms that perform the Park inverse transforms (28) and Clarke (29). The outputs of (29) are again three-phase sine, used as references for Pulse Width Modulation (PWM (30), which generates the trigger signals for the semiconductor switches of the inverter bridge (9).
[0040] An important feature of the present invention is the use of the MTPA algorithm in conjunction with the vectorial control of the motor speed which, according to the teachings already described in the present invention and, as known from the state of the art, it is recognized that the conjugate produced by a PMSM is a function of the number of pairs of poles of the motor (P), the magnetic flux (φ.sub.M) produced by the rotor magnets and concatenated in the stator windings, in addition to depending on the currents i.sub.d and i.sub.q and the inductances of the stator in the synchronous referential, (L.sub.d) and (L.sub.q) respectively, and the conjugate can be represented by the expression (E.1):
T.sub.e=3/2.Math.P.Math.[ϕ.sub.M.Math.i.sub.q+(L.sub.d−L.sub.q).Math.i.sub.d.Math.i.sub.q] (E.1)
the first portion being (E.1) called the magnetic conjugate, produced by the magnets and the second portion is called the reluctance conjugate, produced by the protrusions of the rotor, which is a function of its constructive shape. In rotors with magnets inserted in the rotor steel package, as is the case of motors used in variable speed compressors applied in air conditioners, the inductance in the direct axis is less than that of the quadrature axis, analyzing the expression (E.1) this means that a negative id must be entered to maximize the generation of the conjugate. However, the expression (E.1) can be used in another way, that is, the introduction of negative i.sub.d reduces the need for i.sub.q for the same conjugate. This is the aspect explored in the present invention, that is, the smallest i.sub.q necessary to generate the conjugate required by the charge, obtained by introducing a negative i.sub.d.
[0041] The MTPA concept can be better understood through FIG. 4, which shows a graph of i.sub.q×i.sub.d in a preferred design, with only the negative portion of the direct axis being represented, since a positive i.sub.d is never wanted. The MTPA curve (31) shows the trajectory of the pairs i.sub.d and i.sub.q that produce the desired conjugate with the smallest possible magnitude of these currents, this curve intercepts constant conjugate curves exemplified by (32a), (32b) and (32c), but not limited to these, so that any pair of i.sub.d and i.sub.q, on these curves, produces the same conjugate. However, according to the teachings already described in the present invention, there is a unique and particular pair i.sub.d and i.sub.q that produces the desired conjugate with the smallest possible magnitude of these currents. This pair is given by crossing the curve (31) with the constant conjugate curves, exemplified by the intersections (33a), (33b) and (33c); therefore, it is always preferable to operate the engine as close as possible to the crossing points. Curve (34) exemplifies the maximum current limit for a particular motor, the intersection point (35) being between curves (31) and (34) the maximum point combined with the best efficiency
[0042] In a preferential but not mandatory design, the MTPA technique used in this invention is based on the angle (b) of the motor phase current, defined by the expression (E.2):
[00001]
where is the stator current defined by the expression (E.3), with T.sub.e, ref calculated by (22):
[00002]
[0043] Once the angle of the MTPA algorithm (23) has been calculated, it will define the optimal references for the pair of currents i.sub.d and i.sub.q according to the expressions (E.4) and (E.5):
i.sub.d,ref=−i.sub.s.Math.sen(β) (E.4)
i.sub.d,ref=−i.sub.s.Math.COS(β) (E.5)
[0044] The present invention proposes a technique to optimize the efficiency of hermetic compressors of variable speed, by means of a vector control technique coupled to the MTPA algorithm, in order to find the lowest excitation current of the motor for the development of the required torque, unlike other inventions that use the MTPA technique to maximize the availability of engine torque; thus, the present invention implies a competitive advantage for the aforementioned electronic converter (1) operating with the control algorithm (2).
[0045] The examples and descriptions mentioned in the current invention are merely illustrative and are not to be construed as limiting in any way, within the scope of the invention, according to the claims.
