CN114175494A

CN114175494A - Motor driving device and refrigerator using the same

Info

Publication number: CN114175494A
Application number: CN202080053325.XA
Authority: CN
Inventors: 竹冈义典; 堀尾好正
Original assignee: Panasonic Intellectual Property Management Co Ltd
Current assignee: Panasonic Intellectual Property Management Co Ltd
Priority date: 2019-10-29
Filing date: 2020-10-08
Publication date: 2022-03-11
Also published as: WO2021085066A1; JPWO2021085066A1

Abstract

The starting torque and the rotation start position of a brushless DC motor (5) are determined so that vibration at the start of operation of a compressor (17) and vibration caused by reduction of load torque cancel each other out. By adopting this configuration, the motor drive device (30) can reduce the vibration generated from the compressor (17) as compared with a case where the peak value of the vibration amplitude generated at the start of operation and the peak value of the vibration amplitude generated by rapid acceleration due to reduction of the load torque are generated individually. Thus, a motor drive device (30) which can be stably started while suppressing vibration even in a compressor (17) having a large torque variation can be provided.

Description

Motor driving device and refrigerator using the same

Technical Field

The present invention relates to a motor drive device for a brushless DC motor that drives a compressor, and a refrigerator using the motor drive device.

Background

Patent document 1 discloses a conventional motor driving device for a refrigerator that drives a brushless DC motor of a compressor. When a cooling operation of a refrigeration apparatus such as a refrigerator equipped with a compressor driven by a motor drive device is stopped, a refrigeration cycle is periodically separated into a high-pressure side and a low-pressure side, thereby preventing inflow of a refrigerant and achieving energy saving.

However, in the case of the above-described structure, there is a large difference between the suction pressure and the discharge pressure inside the compressor. Therefore, at the start-up of the compressor, a large amount of energy is required in order to bridge the compression step.

Therefore, the conventional motor driving device for driving the compressor moves the position of the piston of the compressor before starting to the vicinity of the top dead center (a predetermined position, which corresponds to a position closest to the top dead center but not the top dead center) from the top dead center to the bottom dead center. Then, the motor drive device is started from a position near the top dead center. Thus, the compressor is started after a compression step by applying a large amount of acceleration and stored energy to the piston of the compressor.

A conventional motor drive device described in patent document 1 will be described below with reference to fig. 6.

Fig. 6 is a block diagram showing a structure of the conventional motor driving device disclosed in patent document 1.

As shown in fig. 6, the conventional motor drive device is configured by a brushless DC motor 201, a compressor 203, a control unit 204, an inverter 205, and the like. The compressor 203 includes a brushless DC motor 201 and a piston 202 coupled to a rotor of the brushless DC motor 201. The control section 204 includes control operations such as an initial positioning stage for moving the motor to the bottom dead center, a forced positioning stage for moving the start position to the vicinity of the top dead center in the suction step, and an acceleration stage for accelerating the rotor of the brushless DC motor 201. Inverter 205 supplies electric power to brushless DC motor 201 based on a drive signal from control unit 204.

In the motor drive device having the above-described configuration, when compressor 203 is stopped, piston 202 is likely to stop immediately before the compression step, and piston 202 is likely to stop near the bottom dead center. Therefore, at the initial positioning stage, the control unit 204 transmits a signal indicating that the piston 202 is at the phase of the bottom dead center to the inverter 205. Then, inverter 205 causes a current to flow through the stator of brushless DC motor 201, and rotates the rotor of brushless DC motor 201. Thereby, the piston 202 moves to the bottom dead center.

Next, the control unit 204 sends a signal to the inverter 205, which is sequentially switched from the phase at the bottom dead center of the piston 202 to the reverse direction, at the forced positioning stage. Thereby, the position of the piston 202 is moved to the vicinity of the top dead center on the suction step side.

Then, the control section 204 starts the brushless DC motor 201 in the acceleration stage, and sends a signal for acceleration to the inverter 205. Thereby, brushless DC motor 201 rotates. That is, in order to accelerate the piston 202 from near the top dead center, the speed in the compression step becomes large. As a result, the motor driving device for driving the compressor can be easily started over the compression step.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 2007-107523

Disclosure of Invention

The invention provides a motor driving device which can be started stably and at low cost while suppressing vibration even in a compressor with large fluctuation of load torque with large difference between suction pressure and discharge pressure.

A motor driving device includes a compressor and a brushless DC motor serving as a driving source for performing a compression operation of the compressor. The motor driving device is configured to determine a starting torque and a rotation start position of the brushless DC motor so that vibration at the start of operation of the compressor and vibration caused by reduction of load torque cancel each other out.

The motor driving device of the present invention is configured to cancel a peak value of vibration amplitude generated at the start of operation and a peak value of vibration amplitude generated by rapid acceleration due to a decrease in load torque. Therefore, the peak value of the vibration amplitude is cancelled and becomes smaller than the case where the peak value of each vibration amplitude is generated alone. Thus, the motor driving device can suppress the vibration generated by the compressor to be small, and can start the compressor stably at low cost.

Drawings

Fig. 1 is a block diagram of a motor drive device according to an embodiment.

Fig. 2A is a schematic view showing a relationship with a rotor at a bottom dead center of a piston of a compressor as a driven body in the present embodiment.

Fig. 2B is a schematic view showing a relationship between the piston and the rotor in a state where the piston of the compressor as the driven body is rotated 90 degrees in the normal rotation direction from the bottom dead center in this embodiment.

Fig. 2C is a schematic view showing a relationship with the rotor at the top dead center of the piston of the compressor as the driven body in this embodiment.

Fig. 2D is a schematic view showing a relationship between the piston and the rotor in a state where the piston of the compressor as the driven body is rotated 90 degrees in the normal rotation direction from the top dead center in this embodiment.

Fig. 3A is a waveform diagram showing the vibration of the compressor as a driven body generated at the start of operation in this embodiment.

Fig. 3B is a waveform diagram showing the vibration of the compressor as the driven body generated when the compression step is first passed after the start of operation in this embodiment.

Fig. 3C is a waveform diagram showing the vibration of the compressor as the driven body, which is a combination of the vibration at the start of operation and the vibration at the time of passing through the compression step in this embodiment.

Fig. 4A is a waveform diagram showing the vibration of the compressor as the driven body generated at the start of operation when the time until the top dead center is reached is shifted by 30 degrees in phase from the period of the vibration at the start of operation in this embodiment.

Fig. 4B is a waveform diagram showing the vibration of the compressor as the driven body generated when the compression step first passes from the start of operation when the time until the top dead center is reached is shifted by 30 degrees in phase from the period of the vibration at the start of operation in this embodiment.

Fig. 4C is a waveform diagram showing combined vibrations generated in the compressor as the driven body at the start of operation and at the time of the transition to the compression step when the time until the top dead center is reached is shifted by 30 degrees in phase from the period of the vibration at the start of operation in this embodiment.

Fig. 5 is a flowchart of searching for bottom dead center by the bottom dead center searching portion in this embodiment.

Fig. 6 is a block diagram showing a conventional motor driving device.

Detailed Description

(findings on the basis of the present invention, etc.)

In order to conceive of the present invention, the inventors have already proposed a motor drive device described in patent document 1. Since the motor drive device has a difference between the suction pressure and the discharge pressure of the compressor, the variation in load torque is large, and it is difficult to stably start the motor drive device while suppressing vibration. That is, in the case of the configuration of patent document 1, although the speed for passing through the compression step can be sufficiently obtained, the speed is rapidly accelerated when passing through the compression step and the suction step, and thus vibration is likely to occur. When the compressor is stopped, the motor drive device applies a voltage to the compressor on the assumption that the piston is stopped near the bottom dead center. Thereby, the piston moves toward the bottom dead center. Therefore, in the case of a configuration in which the number of poles of the brushless DC motor is other than 2, there is a problem as follows: when the piston stops near the top dead center, the piston cannot be smoothly started from an assumed position, and vibration, poor start, and the like occur. The present inventors have found the above problems and have completed the subject matter of the present invention to solve the problems.

That is, the present invention provides a motor drive device capable of stably starting a compressor even if the load torque fluctuates greatly while suppressing vibration.

Hereinafter, an embodiment, here, a motor driving device of a compressor mounted in a refrigerator will be described as an example with reference to the drawings. However, the detailed description may be omitted. For example, detailed descriptions of widely known contents or repeated descriptions of substantially the same structures may be omitted. This is to avoid the following description being too lengthy to enable those skilled in the art to readily understand the present invention.

In addition, the drawings and the following description are provided to enable those skilled in the art to fully understand the present invention, and thus do not limit the scope of the present invention.

(embodiment mode)

The motor drive device of the present embodiment will be described in terms of its components.

[1-1. Structure ]

First, the structure of the motor drive device 30 according to the present embodiment will be described with reference to fig. 1 to 2D.

Fig. 1 is a block diagram of a motor drive device 30 according to the present embodiment. Fig. 2A to 2D are schematic views showing a positional relationship between the piston 17b and the rotor 5a of the compressor 17 as a driven body according to this embodiment.

As shown in fig. 1, the motor drive device 30 of the present embodiment is connected to an ac power supply 1 and drives a brushless DC motor 5. As shown in fig. 2A to 2D, the rotor 5a of the brushless DC motor 5, the crankshaft 17a, the piston 17b, the cylinder 17c, and the like constitute the reciprocating compressor 17. In the present embodiment, the compressor 17 is mounted on the refrigerator 22 and constitutes a part of the refrigeration cycle.

The ac power supply 1 is a general commercial power supply, and is a 50Hz or 60Hz power supply having an effective value of 100V in japan.

Hereinafter, the structure of the motor drive device 30 will be specifically described.

As shown in fig. 1, the motor drive device 30 of the present embodiment includes a rectifier circuit 2, a smoothing unit 3, an inverter 4, a position detection unit 6, a speed detection unit 7, a voltage detection unit 8, a drive unit 9, an output determination unit 10, a bottom dead center search unit 11, a torque determination unit 12, and the like.

The rectifier circuit 2 receives the ac power supply 1 as an input and rectifies the input ac power into dc power. The rectifier circuit 2 is composed of 4 rectifier diodes 2a, 2b, 2c, 2d connected in a bridge manner.

The smoothing unit 3 is connected to the output side of the rectifier circuit 2, and smoothes the output of the rectifier circuit 2. The smoothing section 3 is constituted by a smoothing capacitor 3e, a reactor 3f, and the like. The output from the smoothing section 3 is input to the inverter 4.

The reactor 3f is interposed between the ac power supply 1 and the smoothing capacitor 3e, and therefore may be provided before or after the rectifier diodes 2a to 2 d. When the common mode filter constituting the high-frequency elimination unit is provided in a circuit, the reactor 3f is preferably configured in consideration of a composition component with a reactance component of the high-frequency elimination unit.

The inverter 4 sequentially switches the voltage input from the smoothing unit 3 to dc power including a large Ripple (Ripple) component at a period 2 times the power supply period of the ac power supply 1, and converts the dc power into 3-phase ac power. The inverter 4 is configured by 3-phase bridging of 6

switching elements

4a, 4b, 4c, 4d, 4e, and 4 f. At this time, 6 reflux

current diodes

4g, 4h, 4i, 4j, 4k, and 4l are reversely connected to the switching elements 4a to 4 f.

The brushless DC motor 5 is configured by a rotor 5a having a permanent magnet, a stator 5b having a 3-phase winding, and the like. The 3-phase alternating current produced by the inverter 4 is supplied to the 3-phase winding of the stator 5b of the brushless DC motor 5. Thereby, the rotor 5a of the brushless DC motor 5 rotates.

The position detection unit 6 detects the magnetic pole position of the stator 5b of the brushless DC motor 5 based on the induced voltage generated in the 3-phase winding of the stator 5b, and the current and applied voltage flowing through the 3-phase winding of the stator 5 b.

In the present embodiment, position detecting unit 6 acquires a terminal voltage of brushless DC motor 5, and detects a relative position of a magnetic pole of rotor 5a of brushless DC motor 5. Specifically, the position detector 6 detects the relative rotational position of the rotor 5a based on the induced voltage generated in the 3-phase winding of the stator 5 b. Further, the position detection unit 6 compares the induced voltage with a reference voltage to detect a zero crossing. The voltage used as a reference for the zero crossing (zero crossing) of the induced voltage may be a voltage obtained by creating a virtual midpoint from the 3-phase terminal voltage. Further, the dc bus voltage may be acquired and used as a reference voltage for the zero crossing of the induced voltage. In the present embodiment, the voltage at the virtual midpoint is set to a voltage that is a reference for the zero crossing of the induced voltage.

In the present embodiment, the position detection unit 6 detects the relative position of the magnetic poles of the stator 5b of the brushless DC motor 5 by the induced voltage as an example, but is not limited to this. For example, the relative position of the magnetic pole may be detected based on the current flowing through the brushless DC motor 5. In this case, the current may also be detected as follows: the voltage generated in the shunt resistor of the dc bus disposed in the inverter 4 is detected, the current flowing in the dc bus is calculated from the resistance value of the shunt resistor, and the current flowing in each phase is separated from the current supply state to the motor and detected. Further, a sensor, a shunt resistor, or the like may be disposed for each of the 3 phases to detect the current individually. However, in comparison of the above-described methods of detecting a current, a method of detecting a current value of a dc bus is a more inexpensive configuration, but a waveform may be distorted to separate currents of respective phases. Therefore, in detecting the relative position of the magnetic poles of the rotor 5a, a method of detecting by an induced voltage is more preferable than a method of estimating the position from a current. This is because the method of detecting the induced voltage is less computationally intensive, has a simple structure, and can be implemented more inexpensively. Further, the method of detecting the currents of the 3 phases individually requires a large number of circuit components and is costly, and the method of detecting the induced voltage can be realized at low cost.

Speed detecting unit 7 calculates the current driving speed of brushless DC motor 5 based on the position information of the magnetic pole detected by position detecting unit 6. Specifically, in the present embodiment, the speed detection unit 7 measures the time from the detection of the zero crossing of the induced voltage, and calculates the current speed from the measured time. Thereby, the speed detection section 7 calculates the driving speed of the brushless DC motor 5.

The voltage detection unit 8 detects a voltage between the dc bus lines of the inverter 4. Normally, first, the dc bus of the inverter 4 is divided by a resistor, and the detected voltage is divided from about 140V to a voltage within a range of 5V or less, which is processed by a microcomputer. Then, the microcomputer performs an inverse operation based on the voltage detected by the voltage division to calculate the original voltage between the dc bus lines. In the present embodiment, for example, a value obtained by dividing a voltage by 1 to 100 is used.

When the target speed inputted from the outside changes from 0 (zero) to a value other than 0 (zero), the bottom dead center searching unit 11 drives the brushless DC motor 5 to search for the bottom dead center of the piston 17b of the compressor 17. Specifically, the bottom dead center searching section 11 outputs a drive waveform of a predetermined pattern (pattern) to the brushless DC motor 5 to search for the bottom dead center, and moves the piston 17b to the vicinity of the bottom dead center. That is, the energization pattern corresponding to the bottom dead center is output to brushless DC motor 5, and piston 17b is moved to the bottom dead center. Then, when piston 17b is moved to the bottom dead center, bottom dead center searching unit 11 outputs a drive pattern for moving piston 17b to the start position to brushless DC motor 5. In the present embodiment, since the position at which the start-up is started is set as the bottom dead center, it is not necessary to particularly provide a step of moving the piston 17b to the start-up position after the bottom dead center movement. Then, the bottom dead center searching section 11 stops the output to the brushless DC motor 5 until the next target speed changes from 0 (zero) to a value other than 0 (zero).

When the target speed input from the outside changes from 0 (zero) to a value other than 0 (zero), the torque determination unit 12 first outputs the torque required when the bottom dead center search unit 11 searches for the bottom dead center of the piston 17b of the compressor 17. Further, the movement in the direction toward the bottom dead center is hardly affected by the load condition because the piston 17b does not perform the compression work. Therefore, the torque required to move the vehicle in the bottom dead center direction is smaller than the torque at the start of operation, and has a constant torque value. Then, torque determining unit 12 outputs a torque larger than the torque for searching for the bottom dead center to brushless DC motor 5 when piston 17b is finally moved to the bottom dead center. Then, after the bottom dead center search, if there is a time for moving piston 17b of compressor 17 to the start position, torque determining unit 12 outputs the torque required for that time period to brushless DC motor 5. That is, in the present embodiment, the motor drive device 30 performs control so that the piston 17b is started from the bottom dead center. Therefore, the torque determination unit 12 determines the torque (corresponding to the starting torque) required for starting the operation from the bottom dead center when the output to the bottom dead center is completed. At this time, the torque determining unit 12 outputs the torque determined so that the time until the piston reaches the top dead center coincides with the cycle of the vibration generated at the time of starting, from the bottom dead center starting piston 17 b.

Further, the torque determination unit 12 determines the torque for performing the normal operation after the time from the stop of the output of the bottom dead center search unit 11 has elapsed. At this time, the torque is determined by comparing the current speed of brushless DC motor 5 input from speed detecting unit 7 with a target speed input from the outside. That is, if the current speed is insufficient with respect to the target speed, the output torque is increased. On the other hand, if the current speed exceeds the target speed, the output torque is decreased. Thereby, the speed of the brushless DC motor 5 is brought to the target speed.

The output determination unit 10 determines the applied voltage based on the torque determined by the torque determination unit 12, the torque constant of the brushless DC motor 5, the induced voltage constant, the resistance value, and the like. Then, the output determination unit 10 calculates the PWM duty of the driving inverter 4 based on the determined applied voltage and the voltage between the dc bus lines detected by the voltage detection unit 8.

Further, the output determination unit 10 determines which phase of the 3-phase brushless DC motor 5 is to be energized based on the information from the position detection unit 6 and the speed detection unit 7 or the output from the bottom dead center search unit 11. At this time, when there is an input signal from the bottom dead center searching unit 11, the output determining unit 10 determines the phase of the current supply using the signal input from the bottom dead center searching unit 11. On the other hand, when there is no input signal from the bottom dead center searching section 11, the output determining section 10 determines a signal to be output based on the position information of the position detecting section 6 and the velocity information of the velocity detecting section 7.

Here, the drive waveform for driving the brushless DC motor 5 includes, for example, a rectangular wave and a sine wave, but is not particularly limited. In the case of a rectangular wave, since the structure is simple and the calculation is simple, it is possible to cope with this by an inexpensive microcomputer. Therefore, the output determination unit 10 can be realized at low cost. In addition, in the case of a sine wave, complicated calculation, current detection, and the like are required, but the position of the motor can be detected more finely. In the present embodiment, the brushless DC motor 5 is driven by rectangular wave driving which can be realized at a lower cost.

Specifically, in the present embodiment, the motor drive device 30 is driven by a 120-degree rectangular wave. Therefore, the

switching elements

4a, 4c, and 4e in the upper arm of the inverter 4 are energized with drive waveforms shifted by 120 degrees, respectively. Similarly, the

switching elements

4b, 4d, and 4f of the lower arm (arm) of the inverter 4 are also energized with drive waveforms shifted by 120 degrees, respectively. Thus, the

switching elements

4a and 4b, 4c and 4d, and 4e and 4f have off periods of 60 degrees each between the energization periods of each other, respectively.

The drive unit 9 outputs a drive signal to each switching element of the inverter 4 based on the on ratio determined by the output determination unit 10, the power supply timing of the brushless DC motor 5, and a predetermined PWM period.

The drive signal turns on or off the switching elements 4a to 4f of the inverter 4. Thereby, an optimal ac power is applied to stator 5b of brushless DC motor 5. As a result, rotor 5a of brushless DC motor 5 rotates, and piston 17b is driven.

The motor drive device 30 is constructed in the above manner.

Next, a refrigerator 22 using the motor drive device 30 of the present embodiment will be described with reference to fig. 1 to 2D. In the following description, the refrigerator 22 will be described as an example, but the same applies to the refrigerating apparatus.

The refrigerator 22 is loaded with the compressor 17. The compressor 17 has a reciprocating structure. That is, the compressor 17 is constituted by a compression mechanism including the brushless DC motor 5, a crankshaft 17a, a piston 17b, a cylinder 17c, and the like. The rotational motion of the rotor 5a of the brushless DC motor 5 is converted into a reciprocating motion by the crankshaft 17 a. A piston 17b connected to the crankshaft 17a reciprocates in a cylinder 17 c. By this reciprocating operation, the refrigerant is sucked into the cylinder 17c, and the sucked refrigerant is compressed.

In the reciprocating compressor 17, torque fluctuation is large and speed and current value fluctuation is large in the suction step and the compression step.

The refrigerant compressed by the compressor 17 flows through the refrigeration cycle in which the refrigerant passes through the condenser 19, the two-way valve 18, the decompressor 20, and the evaporator 21 in this order and returns to the compressor 17 again. At this time, heat is radiated from the condenser 19 and absorbed into the evaporator 21. This enables cooling and heating in refrigerator 22. That is, the refrigerator 22 is configured to be loaded with the compressor 17 that realizes the above-described refrigeration cycle.

The two-way valve 18 is a solenoid valve or the like that can be opened and closed by energization. The two-way valve 18 is opened during operation of the compressor 17, and communicates the condenser 19 with the decompressor 20 to allow the refrigerant to flow. On the other hand, while the compressor 17 is stopped, the two-way valve 18 is closed, and the space between the condenser 19 and the decompressor 20 is closed, so that the refrigerant does not flow.

Refrigerator 22 using motor drive device 30 is configured as described above.

[1-2. actions ]

The operation of the motor drive device 30 mounted in the refrigerator 22 configured as described above will be described with reference to fig. 2A to 4C.

In fig. 3A to 4C, the horizontal axis represents time, and the vertical axis represents vibration amplitude in a direction perpendicular to the reciprocating direction of piston 17b and perpendicular to the rotation axis of brushless DC motor 5.

When rotor 5a of brushless DC motor 5 rotates in the normal direction (clockwise) from the position where piston 17B is at the bottom dead center as shown in fig. 2A, the volume of cylinder 17c decreases as piston 17B moves upward as shown in fig. 2B. Thereby, the refrigerant sucked into the cylinder 17c is compressed. Therefore, when the rotor 5a further rotates, the pressure of the refrigerant in the cylinder 17c rises to the condenser-side pressure. Then, when the pressure of the refrigerant rises to the pressure on the condenser side, as shown in fig. 2C, the refrigerant is discharged and the piston 17b reaches the top dead center. Thereby, the discharge of the compressed refrigerant of the cylinder 17c is completed. Thereafter, when the rotor 5a further rotates, the piston 17b moves in the bottom dead center direction as shown in fig. 2D. This increases the volume in the cylinder 17c, and the low-pressure refrigerant is sucked from the evaporator 21. Then, when the rotor 5a further rotates, the piston 17b reaches the bottom dead center shown in fig. 2A again, and the refrigerant in the cylinder 17c starts to be compressed. Through the above process, the operation of discharging and sucking the refrigerant is repeatedly performed.

On the other hand, when the direction is reversed (counterclockwise) from the state of bottom dead center shown in fig. 2A to the state shown in fig. 2D, the volume of the cylinder 17c is decreased and the refrigerant is compressed, similarly to the clockwise direction. Then, before piston 17b reaches the top dead center shown in fig. 2C, compression of the refrigerant is performed. When the piston 17B is reversed from the state shown in fig. 2C, the state shown in fig. 2B is obtained. Thereby, the refrigerant is sucked into the cylinder 17C from the state shown in fig. 2C in which the refrigerant is completely discharged. After that, when the reverse rotation of the piston 17b is further continued, the refrigerant continues to be sucked into the cylinder 17c until the bottom dead center shown in fig. 2A. When the piston 17b moves from the bottom dead center to the top dead center, the refrigerant is compressed.

That is, as the reciprocating compressor 17, the piston 17b reciprocates in the cylinder 17c in the same manner regardless of whether the rotor 5a rotates normally or reversely. Therefore, when the piston 17b rotates in the normal rotation or reverse rotation direction from the state of the bottom dead center, the refrigerant compression-discharge step is similarly performed. When piston 17b rotates in the normal rotation or reverse rotation direction from the top dead center state, the suction step of sucking the refrigerant into piston 17b is similarly performed.

That is, in a state where there is a pressure difference between the evaporator 21 and the condenser 19, a large torque is required to move the piston 17b in the top dead center direction. On the other hand, when the piston 17b is moved in the bottom dead center direction, it can be operated with a slight torque.

Therefore, in the motor drive device 30 of the present embodiment, taking advantage of the above-described characteristics, the bottom dead center searching section 11 outputs a drive pattern that is rotated 180 degrees in the normal rotation direction from the top dead center to the bottom dead center to the brushless DC motor 5. Then, the bottom dead center searching section 11 outputs a drive pattern that rotates 180 degrees from the top dead center to the bottom dead center in the same manner in the reverse direction to the brushless DC motor 5. On the other hand, the torque determination unit 12 determines in advance the torque at the time of performing the bottom dead center search by the bottom dead center search unit 11. That is, the torque determining unit 12 determines that the rotor 5a of the piston 17b rotates less than 60 degrees from the bottom dead center in the compression direction when the compressor stops or starts under the pressure condition when the refrigerator 22 stops in a state in which the interior thereof is sufficiently cooled.

For example, if the stop position of the piston 17b is before the top dead center, the torque determined by the torque determination unit 12 is in the compression direction, and therefore cannot be rotated in the normal rotation direction. However, in the case of the next 180 degrees rotation in the reverse direction, since it is a suction step, torque is hardly required. Therefore, even with a small torque determined by the torque determination unit 12, the piston 17b can be rotated in the bottom dead center direction and moved to the bottom dead center.

On the other hand, when the piston 17b stops beyond the top dead center, the piston 17b can rotate in the intake step when it is rotated 180 degrees from the top dead center in the normal rotation direction. Therefore, the piston 17b moves to the bottom dead center. However, in the reverse rotation direction, the piston 17b is rotated from the bottom dead center to a position of 60 degrees. Finally, the output determination unit 10 outputs a phase corresponding to the bottom dead center. Thereby, the rotor 5a located near the bottom dead center rotates to a position where the piston 17b moves to the bottom dead center. That is, when the output determination unit 10 outputs the phase corresponding to the bottom dead center, the piston 17b rotates in the direction approaching the bottom dead center. For example, as in the present embodiment, in the case of the brushless DC motor 5 having the 4-pole structure, the phases for moving to the top dead center and the bottom dead center are the same phase. However, since the piston 17b is located at a position close to the bottom dead center, it moves toward the bottom dead center. In the case of the brushless DC motor 5 having the 6-pole structure, the same phase as the bottom dead center is a position rotated by 120 degrees from the bottom dead center toward the top dead center. Then, the rotor 5a rotates by less than 60 degrees, and the piston 17b moves in the bottom dead center direction. Further, in the case of the brushless DC motor 5 having the 8-pole structure, the brushless DC motor can be moved in the bottom dead center direction by applying a torque that makes the rotatable range from the bottom dead center smaller than 45 degrees. The large number of poles is set to a value obtained by dividing 360 by the number of poles, which is lower than the range of rotation of the rotor 5 a. Accordingly, the piston 17b can move in the direction of the bottom dead center.

Here, the compressor 17 has vibration generated at the start of operation as a factor of generating vibration. Further, after the start of operation, the compressor 17 completes discharge from compression, and in the operation of switching to suction, acceleration may occur due to rapid decrease in torque, and vibration may occur. The kinetic energy of the rotor 5a of the brushless DC motor 5 is proportional to the angular velocity to the power of 2. Therefore, the lower the speed of the rotor 5a, the greater the speed decrease and increase, and the greater the vibration generated.

On the other hand, the compressor 17 applies a large torque in the start-up in which there is a pressure difference between suction and discharge. Therefore, by increasing the speed of the rotor 5a to increase the kinetic energy, the vibration generated from the start of the operation to the speed at which the normal operation is performed can be suppressed to some extent. However, when the top dead center is first passed from the start of operation of the compressor 17, the speed of the rotor 5a becomes the lowest, and therefore vibration is most likely to occur.

Further, the rotor 5a is coupled to the crankshaft 17 a. Therefore, when piston 17b starts rotating from the bottom dead center, vibration occurs in a direction perpendicular to the reciprocating direction of piston 17b and perpendicular to the rotation shaft of brushless DC motor 5 due to inertia. In addition, when the piston 17b goes beyond the top dead center, rapid acceleration occurs. Therefore, due to inertia, vibration is generated in a direction perpendicular to the reciprocating direction of piston 17b and perpendicular to the rotation shaft of brushless DC motor 5. In this case, similarly to the start from the bottom dead center, the vibration generated from the bottom dead center and the top dead center of the piston 17b in the rotation direction of the rotor 5a is in the same direction due to the vibration generated by the acceleration. However, since the crankshaft 17a is located symmetrically about the rotation axis of the rotor 5a when viewed from the compressor 17, the generated vibration is in the opposite direction.

The generation of the vibration will be described below with reference to fig. 3A and 3B.

Fig. 3A is a diagram showing vibrations generated when the piston 17b starts operating from the bottom dead center. Fig. 3B is a diagram showing vibration generated when the piston 17B passes the top dead center. That is, as shown in fig. 3A and 3B, the direction in which vibration starts to be generated is in opposite phase in the rotation of piston 17B from the bottom dead center and the rotation of piston 17B from the top dead center.

Therefore, in the torque determining unit 12, the torque is determined so as to go beyond the top dead center at the timing of the cycle of the vibration when starting from the bottom dead center of the piston 17 b. At this time, the period of the vibration of the piston 17b starting from the bottom dead center is determined by the composite natural vibration of the components constituting the compressor 17. In addition, the natural vibration is obtained in advance by, for example, a hammer test or the like.

The time until arrival is calculated based on the compression work of the compressor 17, the torque generated in the brushless DC motor 5, and the inertia of the rotor 5a and the member coupled to the rotor 5 a. At this time, the inertia is fixed (constant), and the compression work of the compressor 17 changes. On the other hand, the torque generated by brushless DC motor 5 can be determined by selecting the load condition at the center in the normal operating range in which the inside of the cabinet of refrigerator 22 is sufficiently cooled. Specifically, the torque applied to the brushless DC motor 5 under the selected load condition is calculated and held in advance. This reduces the load on the control system that calculates the torque in real time. Therefore, even a microcomputer with low performance can easily calculate the time until arrival. As a result, the cost associated with the control system can be reduced.

The waveform of fig. 3A, which is the vibration when the piston 17B starts from the bottom dead center, and the waveform of fig. 3B, which is the vibration when it passes over the top dead center, are made to coincide with each other at the start position of the vibration cycle and to be in opposite phases by the torque predetermined by the above-described method, as shown by the time (i). Therefore, the vibration of fig. 3A and the vibration of fig. 3B cancel each other out. As a result, as shown in fig. 3C, the combined vibration of the vibration at the start of operation and the vibration when the vibration passes the top dead center is suppressed to about 50% as compared with the peak value of the vibration at the start shown in fig. 3A.

When the state of the refrigerator 22 changes, the vibration cycle does not match at a constant torque. However, the load in a state in which the refrigerator is sufficiently cooled changes less than when the power is turned on, and the vibration phase changes within ± 30 degrees.

The above state will be described with reference to fig. 4A to 4C.

Fig. 4A is a graph showing the vibration amplitude when the piston 17b is started from the bottom dead center, as in fig. 3A. Fig. 4B is a diagram showing vibration when the piston 17B goes beyond the top dead center. Fig. 4B shows a waveform in which the phase is delayed by +30 degrees from the period of the vibration at the start of operation. Fig. 4C is a diagram showing a combined vibration waveform of fig. 4A and 4B.

That is, when the refrigerator 22 is warmed and the load state is heavy, the time until the piston 17B reaches the top dead center is delayed and the phase is shifted as shown in fig. 4B. On the other hand, when refrigerator 22 is cooled and the load state becomes light, the time until piston 17b reaches the top dead center becomes fast. Therefore, the phase shifts to the negative side.

For example, as shown at time (ii) of fig. 4A, the start of vibration when the piston 17b passes the top dead center is shifted by 30 degrees. However, even if the phase deviation is 30 degrees, as shown in fig. 4C, the peak of the vibration of fig. 4A is about 6, which is a vibration peak. Therefore, even if a phase shift occurs, a sufficient vibration suppression effect can be obtained.

Next, the control of finding the bottom dead center by the bottom dead center searching section 11 of the motor drive device 30 will be described in detail with reference to fig. 5.

Fig. 5 is a flowchart of the bottom dead center searching section 11 finding the bottom dead center.

As shown in fig. 5, first, the bottom dead center searching section 11 checks whether or not the target speed input from the outside when the processing is entered the previous time is 0 (zero) (step 201). At this time, if the target speed is 0 (zero) (yes in step 201), the process proceeds to step 202, and if the target speed is a value other than 0 (zero) (no in step 201), the process proceeds to step 203. Here, the previous target speed is set to 0 (zero), and the process proceeds to step 202.

Next, it is checked whether or not the current target speed inputted from the outside is a value other than 0 (step 202). At this time, if the target speed is a value other than 0 (yes in step 202), the process proceeds to step204, and if the target speed is 0 (no in step 201), the process proceeds to step 203. That is, it is determined whether or not the target speed has changed so as to start from the stopped state between the previous processing and the current processing. Here, the current target speed is set to a value other than 0, and the process proceeds to step 204.

Next, the bottom dead center searching section 11 sets the phase change amount for recording how much the rotor 5a is rotated to perform the bottom dead center search to 0 (zero), and initializes the current output phase to the top dead center (step 204).

Next, the bottom dead center searching section 11 determines whether or not the phase change amount, which is the amount of rotation of the rotor 5a for performing the bottom dead center search, is less than 180 (step 205). At this time, if the phase change amount is less than 180 (yes in step 205), the process proceeds to step206, and if the phase change amount is not less than 180 (no in step 205), the process proceeds to step 207.

Next, a phase rotated 30 degrees in the normal direction from the current output phase is output as a new output phase (step 206). Further, 30 equal to the rotated angle is added to the phase change amount recorded in the bottom dead center searching section 11 by how much the rotor 5a is rotated (step 206). Then, after waiting for 100ms, the process proceeds to step 205. The standby time of 100ms is a time required to wait for the rotor 5a to reliably rotate. The standby time is actually a predetermined value while the operation is confirmed.

Then, step 205 and step206 are performed 6 times, respectively. Thereafter, when the process returns to step 205, the phase change amount is 180 (no in step 205), and the process proceeds from step 205 to step 207.

In addition, before STEP204 to STEP206 are executed and the process proceeds to STEP207, a phase rotated by 180 degrees from the top dead center is output. At this time, when the connection portion between the crankshaft 17a and the rotor 5a is between the top dead center and the bottom dead center in the normal rotation direction, the normal rotation corresponds to the intake step. Therefore, even with a small torque, the rotor 5a can be rotated in the normal rotation direction. Thereby, the piston 17b moves to the bottom dead center. On the other hand, the compression-discharge step is performed when the connection portion between the crankshaft 17a and the rotor 5a is between the bottom dead center and the top dead center in the normal rotation direction. Therefore, the piston 17b can hardly rotate in the normal rotation direction except when it is near the bottom dead center. Thereby, the piston 17b stays between the bottom dead center and the top dead center.

In the present embodiment, the brushless DC motor 5 is configured to have 4 poles and be driven by 120-degree rectangular waves, and therefore the amount of change in the output phase is rotated by 30 degrees. This corresponds to a rectangular wave electrical characteristic of 120 degrees, and the phase is changed by 60 degrees each time, and the rotation is performed by 1 cycle in 6 output modes. Further, since the brushless DC motor 5 has a 4-pole structure, 2 cycles in which 360 degrees of the electrical characteristic is equal to the number of pole pairs are output in 1 rotation of the rotor. That is, the output phase of the 120-degree rectangular wave is changed to 30 degrees when the angle of the rotor is expressed. On the other hand, in the 6-pole configuration, 3 cycles in which 360 degrees of the electrical characteristics are equal to the number of pole pairs are output in order to rotate the rotor by 1 turn. Therefore, the rotor rotates 20 degrees with respect to 1 change of the output phase. That is, the change amount of STEP206 shown in fig. 5 is a result of dividing 360 by the number of pole pairs of the motor and the number of commutation patterns 6.

Next, in order to record how much the rotor 5a is rotated in the reverse direction for the bottom dead center search by the bottom dead center search unit 11, the phase change amount is set to 0, and the current output phase is set to the top dead center, and initialization is performed (step 207).

Next, it is determined whether or not the phase change amount, which records how much the bottom dead center searching section 11 reversely rotates the rotor 5a to perform the bottom dead center search, is less than 180 (step 208). If the phase change amount is less than 180 (yes in step 208), the routine proceeds to step 209, and if the phase change amount is not less than 180 (no in step 208), the routine proceeds to step 210. Here, since the phase change amount is just initialized to 0 in step207, the process proceeds to step 209.

Next, a phase rotated by 30 degrees in the reverse direction from the current output phase is output as a new output phase (step 209). Further, 30 equal to the rotated angle is added to the phase change amount recorded in the bottom dead center searching section 11 by how much the rotor 5a is rotated in the reverse direction (step 209). Then, after waiting for 100ms, the process proceeds to step 208.

Then, step 208 and step 209 are performed 6 times, respectively. Thereafter, when the process returns to step 205, the phase change amount is 180 (no in step 208), and the process proceeds from step 208 to step 210.

In addition, before step 209 is executed from step207 and the process proceeds to step 210, a phase rotated by 180 degrees in the reverse direction from the top dead center is output. At this time, when the connection portion between the crankshaft 17a and the rotor 5a is between the top dead center and the bottom dead center in the direction of reverse rotation, the reverse rotation corresponds to the intake step. That is, in the compression-discharge step in the normal rotation direction, the reverse rotation corresponds to the intake step when the piston does not move to the bottom dead center in steps 204 to 206. Therefore, even with a small torque, the rotor 5a can be rotated in the normal rotation direction. Thereby, the piston 17b moves to the bottom dead center. On the other hand, when the connecting portion between the crankshaft 17a and the rotor 5a has moved to the bottom dead center, the compression-discharge step is performed. Therefore, the piston 17b can hardly rotate in the reverse direction. Thereby, the piston 17b stays near the bottom dead center.

Next, a phase corresponding to the bottom dead center is output to the piston 17b located near the bottom dead center (step 210). Thereby, the piston 17b reliably moves toward the bottom dead center.

Next, the current target speed is recorded as the previous target speed, and the process is terminated (step 211).

When the processing of the flow of fig. 5 is restarted after the flow is ended, the previous target speed in step201 is not 0 (zero) (no in step 201) and the process proceeds to step 203.

Then, in step203, no output is performed, the process proceeds to step211, and in step211, the process is terminated after the target speed is updated.

When the refrigerator 22 is sufficiently cooled in the interior, the refrigeration cycle is stopped. Therefore, the target speed is 0 (zero). In this state, when the processing shown in fig. 5 is executed again, since the previous target speed is not 0 (zero), the processes shown in fig. 5, STEP201, STEP203, and STEP211, and the same processes as the flow during operation are executed. Therefore, in step211, when the target speed of the previous time is updated, the target speed is recorded as 0 (zero).

Then, when the processing of the flow shown in fig. 5 is executed again, the previous target speed is recorded as 0 (zero) in step201, and the process proceeds to step 202.

Further, in step 202, since the current target speed is 0 (zero) (no in step 202), the process proceeds to step211 after step 203. That is, in step203, no output is performed, and in step211, the previous target speed is updated to 0 (zero) again.

As described above, by periodically calling and processing the flow processing shown in fig. 5, it is possible to move and start piston 17b to the bottom dead center in the synchronous operation in which the current and the induced voltage flowing through brushless DC motor 5 are not detected. Thus, even if there is a difference in the suction and discharge pressures of the compressor 17, the peak of the vibration generated by the compressor 17 can be suppressed, and the vibration can be reduced.

Next, a case where motor drive device 30 according to the present embodiment is used for compressor 17 and mounted in refrigerator 22 will be described with reference to fig. 1.

First, at the same time as the start of the compressor 17, the two-way valve 18 is opened, and the decompressor 20 and the condenser 19 are communicated with each other. In addition, although the above description has been given of the example in which the two-way valve 18 is opened simultaneously with the start of the compressor 17, a slight difference in time does not cause a problem.

When the compressor 17 continues to be driven, the condenser 19 becomes high pressure. On the other hand, the evaporator 21 becomes low pressure due to the decompression by the decompressor 20. At this time, the discharge side connected to the condenser 19 of the compressor 17 becomes high pressure, and the suction side connected to the evaporator 21 becomes low pressure.

Here, a situation is assumed in which the interior temperature of the refrigerator 22 decreases and the compressor 17 is stopped. In this case, the two-way valve 18 is maintained in the open state, and the pressures of the condenser 19 and the evaporator 21 are gradually equalized. In this case, although there is a difference depending on the system configuration of the refrigerator 22, it takes about 10 minutes until the pressure difference between the suction side and the discharge side of the compressor 17 becomes 0.05MPa or less, that is, a state of equilibrium is established.

On the other hand, when the two-way valve 18 is switched from the open state to the closed state while the compressor 17 is stopped, the pressure difference between the condenser 19 and the evaporator 21 is substantially maintained, and thus a pressure difference is left between the suction side and the discharge side of the compressor 17.

Here, when the interior temperature of the refrigerator 22 rises and the compressor 17 is restarted, the state where the two-way valve 18 is closed and the pressure difference is maintained while the compressor 17 is stopped and the state where the compressor is started from the pressure balanced state are compared. At this time, when the compressor 17 is activated with the two-way valve 18 closed and the pressure difference maintained, the electric power for providing the pressure difference again between the condenser 19 and the evaporator 21 is reduced. Therefore, energy saving of the refrigerator 22 can be achieved.

Further, a case where the two-way valve 18 is kept open even while the compressor 17 is stopped, and an operation of a refrigerator not provided with the two-way valve 18 are examined.

Specifically, the temperature in the refrigerator is examined to increase before about 10 minutes elapses from the stop of the compressor 17 to the pressure equilibrium. In this case, with the conventional configuration, the motor drive device 30 can be started only when the pressure difference between the suction side and the discharge side of the compressor 17 is 0.05MPa or less. Therefore, in the above state, the motor drive device 30 needs to be on standby until 10 minutes elapses.

However, in the refrigerator 22 of the present embodiment, the motor drive device 30 can be started even with a differential pressure greater than 0.05 MPa. Therefore, when the temperature in the interior of the refrigerator rises, the motor drive device 30 can be started at a timing when the operation of the compressor 17 is required. Accordingly, as compared with the case of starting the compressor 17 in a state where the pressures on the suction side and the discharge side are balanced, the electric power for providing the pressure difference between the condenser 19 and the evaporator 21 can be reduced. Therefore, energy saving of the refrigerator 22 can be further achieved.

The two-way valve 18 can simplify the system configuration of the refrigerator and the like, compared to a three-way valve or a four-way valve. In addition, the two-way valve 18 can more reliably maintain the pressure difference between the suction side and the discharge side of the compressor 17.

In the refrigerator 22 of the present embodiment, the compressor 17 may be disposed above the refrigerator 22. In this case, the dead space in the upper portion, which is difficult for the hand to reach, is small, so that it is easy to use, and the volume in the warehouse can be enlarged. On the other hand, the compressor 17 is disposed upward, and the compressor 17 as the vibration source is disposed at the farthest position in the refrigerator 22. Therefore, refrigerator 22 is easy to transmit the vibration of compressor 17 by using the lever principle with the floor as a fulcrum. However, the refrigerator 22 of the present embodiment can effectively suppress the peak of the vibration of the compressor 17. Therefore, even if the compressor 17 is disposed at the upper portion, vibration and noise generated from the refrigerator 22 can be reduced.

[1-3. Effect, etc. ]

As described above, the motor drive device 30 of the present embodiment includes the compressor 17 and the brushless DC motor 5 for performing the compression operation by the compressor 17. The motor drive device is configured to determine the starting torque and the rotation start position of the brushless DC motor 5 so that the vibration at the start of operation of the compressor 17 and the vibration caused by the reduction of the load torque cancel each other out. According to this configuration, motor drive device 30 is configured to cancel the peak value of the vibration amplitude generated at the start of operation and the peak value of the vibration amplitude generated by rapid acceleration due to a decrease in load torque. Therefore, the peak value of the vibration amplitude becomes smaller than that in the case where the peak values of the respective vibration amplitudes are generated individually. This can suppress the vibration generated from the compressor 17 to a small level.

Further, motor drive device 30 of the present embodiment is configured to have bottom dead center searching section 11 for searching and moving the bottom dead center of piston 17b of compressor 17, and to move brushless DC motor 5 to the rotation start position after piston 17b is moved to the bottom dead center by bottom dead center searching section 11. With this configuration, the motor drive device 30 can grasp the position of the piston 17b of the compressor 17, and can secure an acceleration period in which a sufficient speed for passing through the compression step is obtained. This enables stable start-up of the compressor 17.

The motor drive device 30 according to the present embodiment includes the torque determination unit 12, and the torque determination unit 12 determines the start torque with the rotation start position determined by the bottom dead center search unit 11 as the bottom dead center. With this configuration, the direction of vibration generated at the start of operation of the motor drive device 30 is opposite to the direction of vibration generated at the top dead center due to a reduction in load torque. Therefore, the vibrations are cancelled out. This can more effectively suppress the vibration generated in the compressor 17.

Further, according to the above configuration, the motor drive device 30 does not need to move further to the operation start position after moving the piston 17b to the bottom dead center. This makes it possible to simplify the structure of the motor drive device 30. Therefore, a configuration can be realized in which calculation is performed by an inexpensive microcomputer having low capability, and thus the cost of the product can be reduced.

Further, according to the above configuration, the start position of the piston 17b of the motor drive device 30 is fixed. Therefore, the calculation for determining the starting torque becomes simple. Thus, when the starting torque is determined in advance, the development period can be shortened. On the other hand, even when the starting torque is determined in real time, the calculation can be performed in a short time by an inexpensive microcomputer.

In the motor drive device 30 of the present embodiment, when the compressor 17 is observed with reference to the top dead center, both the normal rotation and the reverse rotation include the non-compression step, and when the compressor is observed with reference to the bottom dead center, both the normal rotation and the reverse rotation include the compression step. The bottom dead center searching section 11 is configured to have a step of rotating the brushless DC motor 5 forward and backward. With this configuration, motor drive device 30 does not need to detect the position of low-speed brushless DC motor 5, which is difficult to detect, from the current, the induced voltage, and the like. That is, motor drive device 30 can move piston 17b to the bottom dead center by rotating brushless DC motor 5 in the synchronous operation. This enables the position of the piston 17b of the compressor 17 to be reliably grasped by an inexpensive microcomputer.

The refrigerator 22 of the present embodiment is configured to include the motor drive device 30. With this configuration, the compressor 17 can be started from a state in which the load torque greatly fluctuates. Therefore, a waiting time until the suction pressure and the discharge pressure of the compressor 17 are balanced is not required until the refrigerator 22 is stopped and operation is resumed. This enables immediate resumption of cooling in the refrigerator 22 when the supply of electric power is stopped during a power failure, movement of the refrigerator 22, or the like.

In addition, the refrigerator 22 of the present embodiment is configured such that the compressor 17 is provided in the upper portion of the casing. With this configuration, even if vibration generated at the start of operation of the compressor provided at the upper portion of the casing in the refrigerator 22 becomes large due to the lever principle, transmission of vibration to the casing can be suppressed by driving the motor drive device 30. This can provide refrigerator 22 with high quietness. Further, a compressor is disposed in a portion of the upper portion of the casing, which is likely to become a dead space. Therefore, the storage volume in the storage that can be actually used is increased. This makes it possible to provide a refrigerator having a large effective storage volume and being easy to use.

The technique of the present invention has been described above using the above embodiments, but the above embodiments are for illustrating the technique of the present invention, and therefore, various changes, substitutions, additions, omissions, and the like can be made within the scope of the present invention and its equivalents.

Industrial applicability of the invention

The present invention can be used for a motor drive device for starting a compressor with large fluctuation of load torque. Therefore, the present invention can be applied to a refrigerator, a freezer, a showcase, and various other refrigeration apparatuses using a compressor that is started by a motor drive device.

Description of the reference numerals

1 AC power supply (Power supply)

2 rectification circuit

2a, 2b, 2c, 2d rectifier diodes

3 smooth part

3e smoothing capacitor

3f reactor

4. 205 inverter

4a, 4b, 4c, 4d, 4e, 4f switching elements

4g, 4h, 4i, 4j, 4k, 4l diode for reflux current

5. 201 brushless DC motor

5a rotor

5b stator

6 position detecting part

7 speed detection part

8 Voltage detection part

9 drive part

10 output determination unit

11 bottom dead center searching part

12 torque determination unit

17. 203 compressor

17a crankshaft

17b, 202 piston

17c cylinder

18 two-way valve

19 condenser

20 pressure reducer

21 evaporator

22 refrigerator

30 motor driving device

204.

Claims

1. A motor drive device characterized by comprising:

a compressor; and

a brushless DC motor as a driving source for the compressor to perform a compression operation,

the starting torque and the rotation start position of the brushless DC motor are determined so that the vibration at the start of the operation of the compressor and the vibration caused by the reduction of the load torque cancel each other out.

2. The motor drive device according to claim 1, wherein:

a lower dead point searching part for searching the lower dead point of the piston of the compressor and moving the piston,

the brushless DC motor is moved to a rotation start position after the piston is moved to a bottom dead center by the bottom dead center searching section.

3. The motor drive device according to claim 1 or 2, characterized in that:

comprises a torque determining part and a torque determining part,

the torque determining unit determines the starting torque with the rotation start position determined by the bottom dead center searching unit as a bottom dead center.

4. A motor drive device according to any one of claims 1 to 3, wherein:

the compressor is characterized in that under the condition that a top dead center is taken as a reference, both forward rotation and reverse rotation are non-compression steps, and under the condition that the bottom dead center is taken as a reference, both the forward rotation and reverse rotation comprise compression steps,

the bottom dead center searching section includes a step of rotating the brushless DC motor forward and backward.

5. A refrigerator characterized in that:

having a motor drive as claimed in any one of claims 1 to 4.

6. The refrigerator of claim 5, wherein:

the compressor is arranged on the upper part of the shell.