REFRIGERATION CYCLE APPARATUS

Abstract

A refrigerant circuit includes a compressor, an outdoor heat exchanger, a throttle device, an indoor heat exchanger, and a four-way valve, and it is configured such that refrigerant circulates therethrough. An inverter is configured to control the compressor as being variable in speed. The refrigerant circuit is configured to perform a defrosting operation in which refrigerant discharged from the compressor is introduced into the outdoor heat exchanger. The compressor includes a motor. The inverter has, as an operation mode, a speed control mode in which the motor is controlled such that a rotation speed thereof is closer to a rotation speed corresponding to a command value and an output control mode in which the rotation speed of the motor is controlled such that output from the motor is closer to a target value. The inverter is configured to operate by using the output control mode in the defrosting operation.

Description

TECHNICAL FIELD

The present disclosure relates to a refrigeration cycle apparatus.

BACKGROUND ART

In a refrigeration cycle apparatus, in order to recover refrigeration capacity or air-conditioning capacity lowered due to frosting of a heat exchanger on a heat source side that serves as an evaporator, a defrosting operation to heat the heat exchanger on the heat source side to thaw frost is performed during operations. Since the heat exchanger on the heat source side is heated in the defrosting operation, thermal energy is consumed in the heat exchanger on the heat source side.

During the defrosting operation, a large amount of liquid refrigerant cooled by consumption of thermal energy is produced in the heat exchanger on the heat source side. Some of liquid refrigerant reaches a compressor through an accumulator. Occurrence of a phenomenon called “liquid carry-over (liquid return)” as such in which liquid refrigerant returns to the compressor has been known. PTL 1 discloses a method of heating refrigerant with an electric power conversion apparatus that supplies electric power to a compressor in order to prevent the liquid carry-over phenomenon during the defrosting operation.

CITATION LIST

Patent Literature

• PTL 1: WO2020/008620

SUMMARY OF INVENTION

Technical Problem

In the defrosting operation, since thermal energy is consumed for heating of the heat exchanger on the heat source side, a temperature of a heat exchanger on a use side is lowered. Basically, the refrigeration cycle apparatus functions as a heat pump to lower a temperature of the heat exchanger on the heat source side with this heat exchanger serving as the evaporator and increases a temperature of the heat exchanger on the use side with this heat exchanger serving as a condenser during a heating operation. Therefore, the defrosting operation in which the temperature of the heat exchanger on the use side is lowered temporarily sets back the function as the heat pump, which is a state undesirable for a user. Therefore, the defrosting operation desirably lasts for a time period as short as possible and causes less variation in temperature.

Though many techniques relating to defrosting have been studied so far, few techniques have been linked to increase in amount of heating of refrigerant. For example, the method in PTL 1 is a method of heating an electric power conversion apparatus for preventing such a special environmental condition as a liquid carry-over phenomenon during the defrosting operation, and it is not directed to improvement in performance of the refrigeration cycle apparatus under a general defrosting condition.

The present disclosure was made to solve the problem above, and an object thereof is to provide a refrigeration cycle apparatus capable of achieving decrease in time period during which heating capacity is lowered by defrosting.

Solution to Problem

The present disclosure relates to a refrigeration cycle apparatus. The refrigeration cycle apparatus includes a refrigerant circuit including a compressor, an outdoor heat exchanger, a throttle device, an indoor heat exchanger, and a four-way valve, the refrigerant circuit being configured such that refrigerant circulates therethrough, an inverter to control the compressor as being variable in speed, and a temperature sensor that measures a temperature of the compressor. The refrigerant circuit is configured to perform a defrosting operation in which refrigerant discharged from the compressor is introduced into the outdoor heat exchanger as a result of switching of the four-way valve. The compressor includes a compression unit and a motor to drive the compression unit. The inverter has, as an operating mode, a speed control mode in which the motor is controlled such that a rotation speed thereof is closer to a rotation speed corresponding to a command value and an output control mode in which a current that flows through the motor is detected and the rotation speed of the motor is controlled such that output from the motor is closer to a target value.

Advantageous Effects of Invention

According to the refrigeration cycle apparatus according to the present disclosure, the speed control mode and the output control mode can selectively be used. Therefore, the time period during which heating capacity is lowered by defrosting can be decreased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit configuration diagram of a refrigeration cycle apparatus 100 according to a first embodiment.

FIG. 2 is a diagram showing a direction of a flow of refrigerant during defrosting.

FIG. 3 is a cross-sectional view showing a structure of a compressor 1.

FIG. 4 is a functional block diagram showing an exemplary configuration of an inverter 20.

FIG. 5 is a diagram showing a configuration of a control device 15.

FIG. 6 is a diagram showing a configuration of a control circuit 41.

FIG. 7 is a diagram for illustrating an operation in an output control mode.

FIG. 8 is a flowchart showing a method of controlling an operating mode in a heating operation and a defrosting operation in control device 15.

FIG. 9 is a time chart showing an operation of refrigeration cycle apparatus 100 according to the first embodiment.

FIG. 10 is a functional block diagram showing a modification of the inverter in FIG. 4.

FIG. 11 is a cross-sectional view of an interior magnet motor for a compressor.

FIG. 12 is a diagram showing stress applied to a rotor core small-thickness portion.

FIG. 13 is a diagram showing relation between a rotation speed of a motor of the compressor and stress in a bridge portion.

FIG. 14 is a diagram showing relation between a frequency, and a voltage and a current that are outputted from the inverter.

FIG. 15 is a diagram showing a configuration of a speed controller 52 applied to a fourth embodiment.

DESCRIPTION OF EMBODIMENTS

A refrigeration cycle apparatus according to the present embodiment will be described below with reference to the drawings. In the drawings below, relation of a size of each constituent member may be different from actual relation. In the drawings below, identical or corresponding elements have identical reference numerals allotted, which is assumed as being common throughout the specification. A form of a constituent element expressed throughout the specification is merely by way of example and is not limited as described.

First Embodiment

<Configuration of Refrigeration Cycle Apparatus>

FIG. 1 is a circuit configuration diagram of a refrigeration cycle apparatus 100 according to a first embodiment. As shown in FIG. 1, refrigeration cycle apparatus 100 includes an outdoor unit 103 and an indoor unit 104. Outdoor unit 103 and indoor unit 104 are connected to each other through extension pipes 101 and 102.

Refrigeration cycle apparatus 100 includes a refrigerant circuit 105, an inverter 20, a control device 15, and a temperature sensor 30.

Refrigerant circuit 105 includes a compressor 1, an outdoor heat exchanger 2, a throttle device 3, an indoor heat exchanger 4, a four-way valve 5, and an accumulator 6 coupled through a pipe, and it is configured such that refrigerant circulates therethrough.

Compressor 1 suctions and compresses refrigerant to set refrigerant into a high-temperature and high-pressure state, and then discharges refrigerant. Compressor 1 contains a compression mechanism portion 12 and a motor 11. Motor 11 generates motive power for driving compression mechanism portion 12 of compressor 1. Motor 11 is electrically connected to inverter 20. Drive of motor 11 is controlled by inverter 20.

FIG. 3 is a cross-sectional view showing a structure of compressor 1. Compressor 1 includes a housing 13, motor 11, and compression mechanism portion 12.

Compressor 1 suctions refrigerant through a suction pipe 1b, compresses refrigerant to set refrigerant into the high-temperature and high-pressure state, and discharges refrigerant through a discharge pipe 1a. Compression mechanism portion 12 and motor 11 are accommodated in housing 13 of compressor 1.

Motor 11 includes a winding 11a and an iron core 11b that are in contact with refrigerant suctioned through suction pipe 1b. Winding 11a and iron core 11b are each constructed to collect thermal energy from refrigerant.

Description continues with reference again to FIG. 1. Outdoor heat exchanger 2 and indoor heat exchanger 4 have heat exchanged between refrigerant and a heat medium such as air. For example, a fin tube heat exchanger can be used as outdoor heat exchanger 2 and indoor heat exchanger 4.

During a cooling operation of refrigeration cycle apparatus 100, outdoor heat exchanger 2 functions as a condenser. During a heating operation of refrigeration cycle apparatus 100, outdoor heat exchanger 2 functions as an evaporator. Though some refrigeration cycle apparatuses such as a refrigerator are reverse to an air-conditioner in roles of the condenser and the evaporator, an air-conditioner will be described below as a representative example.

Throttle device 3 serves to expand and decompress refrigerant. Throttle device 3 is an apparatus an opening of which is freely controllable, such as a solenoid expansion valve. The opening of throttle device 3 is controlled, for example, by control device 15. Throttle device 3 is connected between outdoor heat exchanger 2 and indoor heat exchanger 4.

Throttle device 3 sets refrigerant that flows out of one heat exchanger that functions as the condenser, of outdoor heat exchanger 2 and indoor heat exchanger 4, into a low-temperature and low-pressure state, and has this refrigerant flow into the other heat exchanger that functions as the evaporator. During the cooling operation of refrigeration cycle apparatus 100, refrigerant that flows out of outdoor heat exchanger 2 flows into throttle device 3 to be set into the low-temperature and low-pressure state, and that refrigerant flows into indoor heat exchanger 4.

Four-way valve 5 performs a function to make switching of a direction of flow of refrigerant between heating and cooling. An operation of four-way valve 5 is controlled, for example, by control device 15. Four-way valve 5 switches a flow channel of refrigerant such that a discharge side of compressor 1 is connected to a heat exchanger to function as the condenser, of outdoor heat exchanger 2 and indoor heat exchanger 4. Accumulator 6 serves as a storage where excessive refrigerant is stored.

FIG. 1 shows the direction of flow of refrigerant during heating, and the refrigerant at the high temperature and the high pressure discharged from compressor 1 flows into indoor heat exchanger 4. During defrosting, the direction of flow of refrigerant is reverse. FIG. 2 is a diagram showing the direction of the flow of refrigerant during defrosting. As four-way valve 5 is switched as shown in FIG. 2, the refrigerant at the high pressure discharged from compressor 1 flows into outdoor heat exchanger 2 and heats outdoor heat exchanger 2.

Control device 15 includes a mode determination unit 22 to determine to which of the speed control mode and the output control mode the operating mode of inverter 20 is to be set based on a current operating state (during normal operations or during defrosting) and a refrigeration cycle controller 23 to generate a speed command value for compressor 1 during the normal operations. Since an operating speed (rps) of compressor 1 is often expressed by a frequency (Hz), the speed command value is also called a frequency command value.

Temperature sensor 30 detects a temperature of refrigerant discharged from compressor 1. Temperature sensor 30 is attached, for example, to discharge pipe 1a of compressor 1. Information on the temperature measured by temperature sensor 30 is inputted to mode determination unit 22 of control device 15.

A detailed configuration of the inside of inverter 20 will now be described with reference to FIG. 4. FIG. 4 is a functional block diagram showing an exemplary configuration of inverter 20.

Inverter 20 includes a control circuit 41 and an electric power converter 40. Control circuit 41 receives a speed command value ω*₁ from external and higher-order control device 15 and current detection signals Iu and Iw detected by current sensors 42a and 42b, and outputs three-phase voltage command values Vuvw* to electric power converter 40.

Control circuit 41 includes an output controller 21, a selector 61, a dq converter 50, a speed estimator 51, a speed controller 52, and a dq reverse converter 55. In the present embodiment, inverter 20 is provided with output controller 21 to control outputted electric power to be substantially constant.

dq converter 50 generates a d-axis current value Id and a q-axis current value Iq based on current detection signals Iu and Iw and a phase estimation value θ.

Speed estimator 51 generates a speed estimation value ω_est based on a d-axis voltage command value Vd*, a q-axis voltage command value Vq*, a d-axis current value Id, and a q-axis current value Iq.

Speed controller 52 generates d-axis voltage command value Vd* and q-axis voltage command value Vq* based on a speed command value ω*, speed estimation value ω_est, d-axis current value Id, and q-axis current value Iq.

dq reverse converter 55 generates three-phase voltage command values Vuvw* based on d-axis voltage command value Vd* and q-axis voltage command value Vq*, and outputs three-phase voltage command values Vuvw* to electric power converter 40 for PWM control.

FIG. 5 is a diagram showing a configuration of control device 15. Though control device 15 includes mode determination unit 22 and refrigeration cycle controller 23 as the functional blocks in FIGS. 1, 2, and 4, it includes, for example, a microcomputer as an actual hardware component.

Specifically, control device 15 includes a central processing unit (CPU) 151, a memory (a read only memory (ROM) and a random access memory (RAM)) 152, and a not-shown input and output apparatus for input of various signals. CPU 151 develops a program stored in the ROM on the RAM or the like and executes the same. The program stored in the ROM is a program in which a procedure of processing in control device 15 is described. Control device 15 controls the refrigeration cycle apparatus in accordance with the program. Specifically, CPU 151 performs processing corresponding to mode determination unit 22 and refrigeration cycle controller 23 in accordance with the program stored in memory 152. This processing is not limited to processing by software, and processing by dedicated hardware (electronic circuitry) is also applicable.

FIG. 6 is a diagram showing a configuration of control circuit 41. Though control circuit 41 includes output controller 21, selector 61, speed controller 52, speed estimator 51, dq reverse converter 55, and dq converter 50 as functional blocks in FIG. 4, it includes, for example, a microcomputer as an actual hardware component.

Specifically, control circuit 41 includes a CPU 411, a memory (a ROM and a RAM) 412, and a not-shown input and output apparatus for input of various signals. CPU 411 develops a program stored in the ROM on the RAM or the like and executes the same. The program stored in the ROM is a program in which a procedure of processing in control circuit 41 is described. Control circuit 41 carries out PWM control of the inverter in accordance with the program. Specifically, CPU 411 performs processing corresponding to output controller 21, selector 61, speed controller 52, speed estimator 51, dq reverse converter 55, and dq converter 50 in accordance with the program stored in memory 412. This processing is not limited to processing by software, and processing by dedicated hardware (electronic circuitry) is also applicable.

The functional blocks may be implemented by a single controller controlled by the same CPU or by different controllers controlled by respective different CPUs.

Description continues with reference again to FIG. 4. Inverter 20 performs an operation based on an operating mode MODE determined by control device 15. In the speed control mode which is the normal operating mode, selector 61 is set such that speed command value ω*₁ for compressor 1 attains to speed command value ω* to be given to speed controller 52. Inverter 20 thus controls an output frequency based on speed command value ω*₁ and controls three-phase voltage command values Vuvw* to substantially minimize loss in inverter 20 and motor 11. Thus, during the normal operations, efficient operations are performed.

In the output control mode, output controller 21 in inverter 20 successively calculates output (electric power) P from motor 11 and an upper limit value P* of output P, and controls a speed command value ω*₂ such that output P becomes closer to P*. Selector 61 is set such that speed command value ω*₂ attains to speed command value ω* to be given to speed controller 52.

Though P* represents an electric power command value, an example in which a maximum value Pmax is adopted to expand an operating range will be described. Maximum value Pmax is a maximum value of output electric power uniquely determined by a motor constant which represents a characteristic of the motor and a direct-current (DC) voltage Vdc. Output P and speed command value ω*₂ are calculated, for example, in accordance with expressions (1) and (2) below, where k represents a constant, ω represents a rotation speed, and Iq represents a torque current.

P=k×ω×Iq  (1)

ω*₂=P*/Iq/k  (2)

A difference in operation depending on the operating mode with varying load torque will now be described with reference to FIG. 7. FIG. 7 is a diagram for illustrating an operation in the output control mode. In FIG. 7, Tmax [Nm] represents an upper limit value of mechanical torque of compressor 1 and f_max represents a maximum frequency at the upper limit of torque. Areas of two hatched quadrangles having a point B and a point B′ as respective vertices thereof represent magnitude of output of the motor. Great motor output in heating and defrosting operations corresponds to high heating capacity and defrosting capacity.

An operating state at a point A (f_max, Tmax) as an initial state is considered. When load torque quickly lowers in the defrosting operation or the like, in the case of the speed control mode, the rotation speed is constant and not varied regardless of load torque. Therefore, motor 11 operates at maximum frequency f_max corresponding to torque Tmax. Therefore, in FIG. 7, an operating point moves from point A to point B′ and motor output lowers in proportion to an amount of lowering in load torque.

In the case of the output control mode, on the other hand, motor 11 is controlled to increase in speed to maintain output Pmax. Therefore, the operating point moves to point B and output from motor 11 is not varied. In other words, the output control mode is an operating mode in which lowering in defrosting capacity or heating capacity due to change such as lowering in temperature, lowering in pressure, or lowering in dryness of refrigerant suctioned by compressor 1 is less likely.

<Operation of Refrigeration Cycle Apparatus>

Operations in the defrosting operation and the heating operation which are characteristics of the present invention will now be described.

A method of setting the operating mode in control device 15 will initially be described with reference to FIG. 8. FIG. 8 is a flowchart showing a method of controlling the operating mode in the heating operation and the defrosting operation in control device 15. A step will be abbreviated as S below. When the current operating state falls under defrosting (YES in S1) and when a certain time period has elapsed since start of defrosting (YES in S2), control device 15 sets the operating mode to the output control mode (S4).

When the current operating state falls under heating (NO in S1) and when a discharge temperature is equal to or lower than a criterion value (YES in S3) as well, the operating mode is similarly set to the output control mode (S4).

When the current operating state falls under defrosting (YES in S1) and when the certain time period has not elapsed since start of defrosting (NO in S2), control device 15 sets the operating mode to the speed control mode (S5).

When the current operating state falls under heating (NO in S1) and when the discharge temperature is higher than the criterion value (NO in S3) as well, similarly, the control device sets the operating mode to the speed control mode (S5).

An operation of the entire apparatus will now be described with reference to FIG. 9. FIG. 9 is a time chart showing an operation of refrigeration cycle apparatus 100 according to the first embodiment. Operations in the present embodiment will be described with reference to waveforms T1 and F1 shown with solid lines.

As shown at time tA, control device 15 switches four-way valve 5 such that a destination of gas discharged from compressor 1 is changed from indoor heat exchanger 4 to outdoor heat exchanger 2 for performing the defrosting operation. Consequently, the direction of flow of refrigerant is changed from the direction shown in FIG. 1 to the direction shown in FIG. 2. Immediately after switching, gas discharged from compressor 1 is at the high temperature and the high pressure. Refrigerant at the high temperature, however, flows into frosted outdoor heat exchanger 2 and it is cooled and decompressed therein. Therefore, as the defrosting operation is continued, the temperature and the pressure of refrigerant suctioned into compressor 1 are also lowered as shown with waveform T1, and accordingly, the discharge temperature lowers as shown at time tB.

At time tB, control device 15 detects lowering in discharge temperature shown with waveform T1 and switches the operating mode of inverter 20 from the speed control mode to the output control mode.

In the output control mode, as shown with waveform F1, inverter 20 has compressor 1 operate at an operation frequency such that output P attains to P=Pmax. At this time, since the temperature and the pressure of refrigerant around compressor 1 are relatively high, load torque of compressor 1 is also high. Therefore, in the output control mode, in inverter 20, speed command value ω*₂ is set to a relatively small value. At this time, refrigerant at the high temperature for defrosting flows into outdoor heat exchanger 2. Since heat is carried away to frost, at an exit of the heat exchanger, the temperature of refrigerant lowers. During the defrosting operation, on the other hand, indoor heat exchanger 4 is controlled not to exchange heat by stop of air blow or the like. Consequently, thermal energy and the pressure in the entire refrigerant circuit 105 connected through a refrigerant pipe are lowered, and with this lowering, load torque for operation of compressor 1 is also lowered.

Output controller 21 detects a q-axis current to recognize this lowering in load torque, and increases speed command value ω*₂ based on the expression (2). With increase in rotation speed, a speed of flow of refrigerant in refrigerant circuit 105 during the defrosting operation increases. A pressure loss in the flow channel that increases with the speed of the flow increases. With the pressure loss, thermal energy, that is, the temperature and the pressure, of refrigerant increases, and defrosting capacity of outdoor heat exchanger 2 increases. By repeating operations above, defrosting can quickly end (time tC in FIG. 9).

When defrosting ends at time tC, control device 15 turns off inverter 20, has four-way valve 5 operate to switch the direction of circulation of refrigerant to thereby start the heating operation, and turns on inverter 20 again. From time tC to time tD, the output control mode continues also after start of heating, and hence an operation at the output upper limit of P=Pmax is performed. Since motor 11 operates at the maximum output at this time, capacity high also as heating capacity is obtained, and early recovery from lowering in temperature of refrigerant and a state of lowering in temperature on an indoor side during defrosting is achieved (time tD in FIG. 9).

A time chart in the case of control only in the speed control mode is shown with waveforms T2 and F2 drawn with dashed lines in FIG. 9 as a comparative example. Unlike the example shown with the solid lines, in the comparative example, the operation frequency is not increased with lowering in discharge temperature during defrosting. Therefore, a time period for defrosting (time tB to time tC′) is long. Furthermore, recovery of the temperature of refrigerant after switching to heating is also delayed, and hence the discharge temperature does not increase. Therefore, as shown with waveform T2 from time tC′ to time tD′, a state in which an indoor temperature is low lasts for a long time. In other words, in the comparative example, the time period for defrosting of the refrigeration cycle apparatus is long and time required for start-up of heating is long. The comparative example shows that comfort of the user is compromised.

Second Embodiment

In the first embodiment, an example in which switching between the output control mode and the speed control mode is made by control device 15 is shown. In a second embodiment, an example in which switching between the operating modes is made in the inverter will be described. FIG. 10 is a functional block diagram showing a modification of the inverter in FIG. 4.

An inverter 20A shown in FIG. 10 includes a control circuit 41A and electric power converter 40. Control circuit 41A includes a minimum value selector 120 and a speed upper limit calculator 121 instead of output controller 21 and selector 61 in the configuration of control circuit 41 shown in FIG. 4. Since control circuit 41A is otherwise similar in configuration to control circuit 41 shown in FIG. 4, description will not be repeated.

Operations will now be described. Speed upper limit calculator 121 calculates a rotation speed upper limit value ω_max in accordance with expressions (3) and (4) below.

$\left[\mathrm{Expression}1\right]$ $\begin{array}{cc}{T}_e=L_q{I}_q{I}_d+\left({\Phi }_f+L_d{I}_d\right)I_q& \left(3\right)\end{array}$ $\left[\mathrm{Expression}2\right]$ $\begin{array}{cc}{\omega }_{\max }=\frac{P\max }{T_e}& \left(4\right)\end{array}$

Te represents output torque and Pmax represents maximum output torque.

Minimum value selector 120 compares command value ω*₁ in the speed control mode and rotation speed upper limit value ω_max in the output control mode with each other, and outputs a smaller value as speed command value ω* for actual control. Minimum value selector 120 allows automatic switching between the output control mode and the speed control mode, and addition of a control signal from control device 15 is not required. When control device 15 requests the output control mode, a rotation speed sufficiently higher than rotation speed upper limit value ω_max should only be designated. Since speed command value ω* is thus set to rotation speed upper limit value ω_max, the operating mode of inverter 20A is set to the output control mode.

Third Embodiment

A suitable motor specification relating to an acceleration operation during a high-speed operation which is a main operation in the present embodiment will be described below with reference to FIGS. 11 and 12. FIG. 11 is a cross-sectional view of an interior magnet motor for a compressor. FIG. 12 is a diagram showing stress applied to a rotor core small-thickness portion.

As shown in FIG. 11, motor 11 includes a rotor 66 and a stator 67. Rotor 66 of motor 11 includes a plurality of permanent magnets 71 and an iron core (rotor core) 70. Stator 67 includes an iron core (stator core) 73 and a winding 74 of a coil. As shown in FIG. 12, iron core 70 includes a bridge portion 72A that extends in a radial direction on the q axis located between adjacent magnets 71A and 71B of the plurality of permanent magnets 71 and holds positions of adjacent magnets 71A and 71B.

An interior permanent magnet synchronous motor (IPMSM) has widely been adopted as a form of configuration of a compressor motor, because a structure thereof for preventing scattering of a magnet is simplified.

In general, a maximum rotation speed of the compressor is defined by an operation frequency (f_nom) corresponding to a rotation speed at which maximum rated capacity is exhibited. In the first embodiment, a control command value in the output control mode is described as a fixed value (Pmax). Under a condition where load torque during defrosting is low, however, when the operation frequency is controlled to exceed maximum frequency f_nom, defrosting performance is improved.

With a smaller thickness of the rotor core (bridge portion 72A in FIG. 12) at an end of the magnet, the IPMSM achieves less flux leakage in the rotor and high output and high efficiency, whereas stress against centrifugal force lowers. Control to achieve light-load and high-speed operations described in the first and second embodiments is described as particularly being effective in the IPMSM. In the vicinity of the q axis at the end of the magnet, the core inevitably has to be smallest in thickness in order to meet requirements in terms of efficiency. Mechanical strength of the rotor core of the IPMSM is dependent on strength in the vicinity of the q axis at the end of the magnet. Stress applied during rotation to a portion in the vicinity of the q axis is expressed in expressions (5) to (7) below and FIG. 12.

F²=Fr²+Ft²  (5)

Fr=Mω²  (6)

Ft=k₁×T_L  (7)

In the expressions, Fr represents centrifugal force, Ft represents stress in a circumferential direction applied to the core, M represents inertial moment around an outer circumferential portion of the core, ω represents an angular velocity, k₁ represents a proportionality factor, T_L represents load torque, and F represents stress applied to the bridge portion. Though stress applied to the bridge portion has conventionally been designed to withstand centrifugal force, a width of bridge portion 72A in FIG. 12 is designed such that a width in the circumferential direction is smaller in an attempt to achieve higher output of the motor. Consequently, it has been found that force F which is the resultant of stress in the circumferential direction and centrifugal force should be an indicator for design. This is the background of derivation of relation in the expression (5).

Stress F can have a maximum value Fmax specific to equipment based on mechanical strength of the rotor. Considering the condition of F=Fmax, it can be seen based on the relational expressions (5) to (7) between angular velocity co and load torque T_L that large angular velocity ω can be set when load torque T_L is low. Maximum rated torque is then defined as T_Lmax, and an angular velocity under conditions of T_L=T_Lmax and stress F=Fmax, that is, the angular velocity at rated load, is defined as a maximum angular velocity ωnom. FIG. 13 shows a diagram of relation between a rotation speed of the motor of the compressor and stress in the bridge portion.

On the other hand, for the compressor, load torque T_L is a function of a pressure of refrigerant gas; as the pressure is lower, load torque is lower. In this case, load torque during defrosting is defined as T_Ldef and a maximum angular velocity is defined as ωdef. Initially, relation of T_Lmax>T_Ldef is satisfied, and furthermore, stress Ft in the circumferential direction decreases by an amount comparable to decrease in torque. Based on the expressions (5) to (7), even when stress Fr in a direction of centrifugal force is increased by an amount comparable to decrease in stress Ft, total stress F can be the same.

By making use of the fact that output torque becomes lower than rated torque during the defrosting operation, the upper limit frequency can be increased without increase in rotor core strength against torque. In the first and second embodiments, the high-speed operation is performed during the defrosting operation which is the operating state in which load torque is lowered, and therefore, stress applied to the rotor core clearly does not excessively increase. In a third embodiment, the compressor on which less load is imposed in terms of strength against break due to increase in centrifugal force while a time period for defrosting is decreased by the high-speed operation can be provided.

Fourth Embodiment

During defrosting, such a phenomenon (which is referred to as liquid carry-over below) that refrigerant cooled and liquefied in outdoor heat exchanger 2 reaches the compressor may occur. Liquid carry-over causes frothing of lubricating oil in the compressor and resultant poor lubrication. When the high-speed operation is performed during defrosting as in the first to third embodiments, however, there is a concern about more likeliness of liquid carry-over. A method of addressing liquid carry-over will be described below with reference to FIG. 14.

FIG. 14 is a diagram showing relation between a frequency, and a voltage and a current that are outputted from the inverter. In FIG. 14, a solid line represents an output voltage and a chain dotted line represents a current. A maximum rated frequency is denoted as f_max, a maximum value of the output voltage is denoted as Vmax, and a maximum frequency at which an operation at highest efficiency can be performed is denoted as f_nom. When the motor operates at a rotation speed comparable to a frequency exceeding maximum frequency f_nom, in general, an underexcitation operation is performed. From a point of view of energy saving, however, an efficient operation is generally performed. Therefore, the operation is performed with a voltage at maximum value Vmax being maintained. In other words, the motor operates at a voltage of Va and a current Ia shown in FIG. 14.

Considering the operation of the motor at a rotation speed corresponding to a higher frequency (f_max+Δf), usually, the operation is performed with maximum value Vmax of the voltage being maintained, so long as there is no particular problem. Possibility of a mechanical damage due to liquid carry-over, however, should be lowered in the defrosting operation, and hence in a fourth embodiment, the inverter is intentionally controlled to output a voltage (Vmax−ΔV) lower than maximum value Vmax. By doing so, the current increases from Ib to Ic, and with increase in current, the motor generates heat. Since refrigerant that flows into a compression mechanism of the compressor can thus be heated, resistance to liquid carry-over can be improved.

FIG. 15 is a diagram showing a configuration of speed controller 52 applied to the fourth embodiment. In the third embodiment, speed controller 52 shown in FIG. 15 is used as speed controller 52 in FIG. 4 or 10. Speed controller 52 includes a q-axis current command calculator 110, a d-axis current command calculator 111, a voltage command calculator 112, a phase calculator 113, and subtractors 114 to 116. Control device 15 includes a heating determination unit 117 to control ON/OFF of heating control based on a compressor discharge temperature Td and operating mode MODE. HEAT represents a heating control signal given from heating determination unit 117 in control device 15.

Heating determination unit 117 monitors compressor discharge temperature Td and operating mode MODE. When the operating mode is set to the output control mode and discharge temperature Td is equal to or lower than a criterion temperature, it is determined that the compressor should be heated, and heating determination unit 117 sets the heating control signal to ON. Otherwise, heating determination unit 117 sets the heating control signal to OFF.

When externally given heating control signal HEAT is OFF, the d-axis current command calculator outputs d-axis current command Id* in accordance with normal d-axis current control. In this case, d-axis current command Id* is determined in accordance with expressions (8) and (9) below.

When relation of V<Vmax is satisfied, Id* is determined as

Id*=0  (8).

When relation of V=Vmax is satisfied, Vmax is determined as below.

[Expression 3]

Vmax=√{square root over ((RI_d*−ωL_qI_q*)²+(RI_q*−ωL_dI_d*+ωΦ_f)²)}  (9)

R represents a phase resistance, Ld represents a d-axis inductance, Lq represents a q-axis inductance, Φf represents an induced voltage constant, ω represents an electrical angular velocity, Id* represents a d-axis current command, and Iq* represents a q-axis current command.

When heating control signal HEAT is ON, Id* is determined in accordance with an expression (10) below.

[Expression 4]

I_d*=−√{square root over (I_max²−I_q*²)}  (10)

Imax represents a maximum current rating.

A maximum current rated value Imax is a value specific to the motor which is determined by a limit of a demagnetizing current. In other words, the current is controlled in accordance with maximum current rated value Imax which is a maximum current value allowable by the motor. Thus, a loss in the motor can increase and heat generation can increase. Though Id* has a negative value in the expression (10), it is set as such in consideration of the fact that the d-axis current command cannot be increased at the voltage upper limit of the inverter, and when the voltage upper limit of the inverter is sufficiently high, heating can be done even when Id* is positive.

The current limit is determined by the limit of the demagnetizing current in the example above. When the current rating of the inverter or a limit of synchronization of the motor is lower than the limit of the demagnetizing current, however, the motor may be controlled with the current rating or the limit of synchronization being set as the upper limit.

According to the refrigeration cycle apparatus according to the present disclosure, under the control for increasing a frequency of the compressor, a pressure loss in the refrigerant circuit and a mechanical loss, an iron loss, and a copper loss in the compressor increase, which promotes increase in temperature or pressure of refrigerant. Therefore, the defrosting operation in the outdoor heat exchanger can be completed early, and a time period during which heating capacity is low in the indoor heat exchanger after the defrosting operation can be shorter.

Decrease in time period for defrosting is effective not only for elimination of feeling of cold by the user during the heating operation but also for improvement in average heating capacity.

SUMMARY

Finally, the present embodiment will be summarized with reference again to the drawings. Refrigeration cycle apparatus 100 in the present embodiment shown in FIG. 1 includes refrigerant circuit 105 and inverter 20. Refrigerant circuit 105 includes compressor 1, outdoor heat exchanger 2, throttle device 3, indoor heat exchanger 4, and four-way valve 5, and it is configured such that refrigerant circulates therethrough. Inverter 20 is configured to control compressor 1 as being variable in speed. Refrigerant circuit 105 is configured to perform a defrosting operation in which refrigerant discharged from compressor 1 is introduced into outdoor heat exchanger 2 as a result of switching of four-way valve 5 as shown in FIG. 2. Compressor 1 includes compression mechanism portion 12 and motor 11 to drive compression mechanism portion 12. Inverter 20 has, as an operating mode, a speed control mode in which motor 11 is controlled such that a rotation speed of the motor is closer to a rotation speed corresponding to a command value and an output control mode in which a current that flows through motor 11 is detected and the rotation speed of motor 11 is controlled such that output from motor 11 is closer to a target value.

According to such a configuration, the speed control mode and the output control mode can selectively be used as the operating mode of inverter 20 in accordance with a state of refrigerant that changes with switching of four-way valve 5. Therefore, a refrigeration cycle apparatus capable of achieving improvement in capacity, in which the rotation speed can automatically follow the state of refrigerant even when the state changes in a short period of time, can be provided.

Inverter 20 is configured to operate by using the output control mode in the defrosting operation. Thus, a refrigeration cycle apparatus capable of achieving decrease in time period for defrosting, the time period being a shortcoming of a heat pump, can be provided.

In the example shown in FIG. 10, the output control mode is used when a value indicated by externally given command value ω*₁ becomes equal to or larger than rotation speed upper limit value ω_max determined by a DC voltage of inverter 20, a characteristic value of motor 11, and a current in motor 11. Therefore, since the output control mode is applied only based on externally given command value ω*₁, a refrigeration cycle apparatus excellent in viability, with few points of change in interface, can be provided.

As shown in FIG. 8, in the defrosting operation (YES in S1), the operating mode is set to the speed control mode (S5) at the time of start of defrosting, and after a certain time period has elapsed since start of defrosting (YES in S2), switching from the speed control mode to the output control mode is made (S4).

Refrigeration cycle apparatus 100 shown in FIG. 1 further includes temperature sensor 30 to measure discharge temperature Td of refrigerant discharged from compressor 1. As shown in FIG. 8, when discharge temperature Td of compressor 1 is higher than a criterion value (NO in S3), the operating mode is set to the speed control mode (S5), and when discharge temperature Td of compressor 1 is lower than the criterion value (YES in S3), the operating mode is set to the output control mode (S4).

As shown in FIG. 11, rotor 66 of motor 11 includes a plurality of permanent magnets 71 and iron core 70. As shown in FIG. 12, iron core 70 includes bridge portion 72A that extends in a radial direction on the q axis located between adjacent magnets 71A and 71B of the plurality of permanent magnets 71 and holds positions of adjacent magnets 71A and 71B. According to such a configuration, a refrigeration cycle apparatus implemented by a combination of the motor and the inverter that less affects rigidity of the motor involved with a higher speed of rotation and is excellent in viability can be provided.

As shown in FIG. 15, inverter 20 includes d-axis current command calculator 111 to control an amplitude and a phase of a current in motor 11. d-axis current command calculator 111 controls current command value Id* such that the current in motor 11 attains to maximum rated value Imax during the output control mode. According to such a configuration, concern about liquid carry-over involved with a higher speed of rotation can be eliminated by increase in amount of heat generation by the motor, and hence a more reliable refrigeration cycle apparatus can be provided.

It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present disclosure is defined by the terms of the claims rather than the description of the embodiments above and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1 compressor; 1a discharge pipe; 1b suction pipe; 2 outdoor heat exchanger; 4 indoor heat exchanger; 3 throttle device; 5 four-way valve; 6 accumulator; 11 motor; 11a, 74 winding; 11b, 70 iron core; 12 compression mechanism portion; 13 housing; 15 control device; 20, 20A inverter; 21 output controller; 22 mode determination unit; 23 refrigeration cycle controller; 30 temperature sensor; 40 electric power converter; 41, 41A control circuit; 42a, 42b current sensor; 50 dq converter; 51 speed estimator; 52 speed controller; 55 dq reverse converter; 61 selector; 66 rotor; 67 stator; 71 permanent magnet; 71A, 71B magnet; 72A bridge portion; 100 refrigeration cycle apparatus; 101, 102 extension pipe; 103 outdoor unit; 104 indoor unit; 105 refrigerant circuit; 110, 111 axis current command calculator; 112 voltage command calculator; 113 phase calculator; 120 minimum value selector; 121 speed upper limit calculator; 152, 412 memory

Claims

1. (canceled)

2. A refrigeration cycle apparatus comprising:

a refrigerant circuit comprising a compressor, an outdoor heat exchanger, a throttle device, an indoor heat exchanger, and a four-way valve, refrigerant circulating through the refrigerant circuit; and

an inverter to control the compressor as being variable in speed, wherein

the refrigerant circuit is configured to perform a defrosting operation in which refrigerant discharged from the compressor is introduced into the outdoor heat exchanger as a result of switching of the four-way valve,

the compressor comprises a compression mechanism portion and a motor to drive the compression mechanism portion,

the inverter has, as an operating mode, a speed control mode and an output control mode, in the speed control mode, the motor being controlled such that a rotation speed of the motor is closer to a rotation speed corresponding to a command value, in the output control mode, a current flowing through the motor being detected and the rotation speed of the motor being controlled such that output from the motor is closer to a target value,

the inverter is configured to operate by using the output control mode in the defrosting operation, and

the output control mode is selected when a value indicated by an externally provided command value becomes equal to or larger than a rotation speed upper limit value determined by a DC voltage of the inverter, a characteristic value of the motor, and a current in the motor.

3. A refrigeration cycle apparatus comprising:

a refrigerant circuit comprising a compressor, an outdoor heat exchanger, a throttle device, an indoor heat exchanger, and a four-way valve, refrigerant circulating through the refrigerant circuit; and

an inverter to control the compressor as being variable in speed, wherein

the refrigerant circuit is configured to perform a defrosting operation in which refrigerant discharged from the compressor is introduced into the outdoor heat exchanger as a result of switching of the four-way valve,

the compressor comprises a compression mechanism portion and a motor to drive the compression mechanism portion,

the inverter has, as an operating mode, a speed control mode and an output control mode, in the speed control mode, the motor being controlled such that a rotation speed of the motor is closer to a rotation speed corresponding to a command value, in the output control mode, a current flowing through the motor being detected and the rotation speed of the motor being controlled such that output from the motor is closer to a target value,

the inverter is configured to operate by using the output control mode in the defrosting operation, and

in the defrosting operation, at time of start of defrosting, the speed control mode is selected as the operating mode, and after a certain time period has elapsed since start of defrosting, the operating mode is switched from the speed control mode to the output control mode.

4. A refrigeration cycle apparatus comprising:

a refrigerant circuit comprising a compressor, an outdoor heat exchanger, a throttle device, an indoor heat exchanger, and a four-way valve, refrigerant circulating through the refrigerant circuit; and

an inverter to control the compressor as being variable in speed, wherein

the refrigerant circuit is configured to perform a defrosting operation in which refrigerant discharged from the compressor is introduced into the outdoor heat exchanger as a result of switching of the four-way valve,

the compressor comprises a compression mechanism portion and a motor to drive the compression mechanism portion,

the inverter has, as an operating mode, a speed control mode and an output control mode, in the speed control mode, the motor being controlled such that a rotation speed of the motor is closer to a rotation speed corresponding to a command value, in the output control mode, a current flowing through the motor being detected and the rotation speed of the motor being controlled such that output from the motor is closer to a target value,

the inverter is configured to operate by using the output control mode in the defrosting operation, and

the refrigeration cycle apparatus further comprising a temperature sensor to measure a discharge temperature of refrigerant discharged by the compressor, wherein

when the discharge temperature is higher than a criterion value, the speed control mode is selected as the operating mode, and when the discharge temperature is lower than the criterion value, the output control mode is selected as the operating mode.

5. The refrigeration cycle apparatus according to claim 4, wherein

the motor comprises a rotor, the rotor comprising a plurality of permanent magnets and an iron core, and

the iron core includes a bridge portion, the bridge portion extending in a radial direction on a q axis located between adjacent magnets of the plurality of permanent magnets and holding positions of the adjacent magnets.

6. The refrigeration cycle apparatus according to claim 4, wherein

the inverter comprises a d-axis current command calculator to control an amplitude and a phase of a current in the motor, and

the d-axis current command calculator controls a current command value to set the current in the motor to a maximum rated value during the output control mode.

7. The refrigeration cycle apparatus according to claim 4, wherein where ω*2 represents the speed command value for the motor, P* represents an upper limit value of output from the motor, Iq represents a q-axis current, and k represents a constant.

the inverter detects a q-axis current in the output control mode, recognizes lowering in load torque of the compressor, and increases a speed command value for the motor based on an expression (2): ω*2=P*/Iq/k  (2),