JP7696781B2 - MOTOR DRIVE DEVICE AND REFRIGERATOR - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applied. Academic paper: A paper on a method for reducing vibration during stopping of a compressor drive motor. Publication date: December 11, 2020. Publisher: Taisuke Shioi, Takahiro Kumagai, Junichi Ito

This disclosure relates to a motor drive device that drives a brushless DC motor and a refrigerator that uses the same.

Patent Literature 1 discloses a refrigerator that reduces vibrations when the compressor is stopped by determining the timing of applying a reverse torque when the compressor is stopped depending on the speed. This refrigerator includes a reciprocating compressor and a position detection means that detects the rotation period of the compressor, and when the position detection means detects the rotation period of the piston after the operation is stopped and the rotation period of the piston becomes equal to or smaller than a predetermined value, the position detection means starts applying a voltage that generates a reverse torque while switching the energization depending on the magnetic pole position of the motor, and stops the piston position of the compressor just before the top dead center or without managing the stop position.
Patent Document 2 discloses a torque control device for a motor that reduces vibrations when the compressor is stopped by braking after de-energizing the motor when the compressor is stopped. This torque control device for a motor includes a motor that drives a load element and an inverter that supplies AC power to the motor, and the inverter manages the position at which de-energization of the compressor starts, and the inverter brakes the motor to stop it after de-energizing the motor.

JP 2012-167849 A JP 2001-037281 A

This disclosure provides a motor drive device that suppresses piston reversal when a reciprocating compressor is stopped, reducing vibrations generated in the compressor.

A motor drive device in the present disclosure is a motor drive device for driving a reciprocating compressor composed of a cylinder and a piston for compressing gas, and includes a brushless DC motor for causing the compressor to perform a compression operation, a waveform generation unit for converting power from an externally supplied power source and supplying the power to the brushless DC motor, a terminal voltage detection unit for detecting a terminal voltage value of the brushless DC motor, a speed estimation unit for estimating a rotation speed of the brushless DC motor from the terminal voltage value of the brushless DC motor detected by the terminal voltage detection unit, and a compressor state grasping unit for grasping a position of the piston of the compressor, and the waveform generation unit is configured to detect the rotation speed of the brushless DC motor from the terminal voltage value of the brushless DC motor estimated by the speed estimation unit. and when the brushless DC motor is stopped after the rotation speed of the brushless DC motor falls below a speed threshold value, the output to the brushless DC motor is short-circuited so that rotation of the compressor stops while the piston of the compressor rotates forward from top dead center to bottom dead center, a state in which there is no difference between the pressure of the refrigerant in the compressor and the pressure in the cylinder and no force due to the pressure difference acts to move the piston, and the waveform generation unit initiates a short circuit to begin applying the brakes to the brushless DC motor when the piston of the compressor recognized by the compressor state recognition unit reaches a predetermined position, and the compressor state recognition unit recognizes the piston position of the compressor from the terminal voltage of the brushless DC motor detected by the terminal voltage detection unit.

The motor drive device of the present disclosure (action) can stop the compressor during the intake or expansion process among the intake, compression, discharge, and expansion processes. Therefore, (effect) when the speed of the brushless DC motor becomes zero, the pressure inside and outside the compressor cylinder becomes almost equal, so the piston does not reverse and vibration can be reduced.

A block diagram showing a control configuration of a motor drive device and a refrigerator according to a first embodiment. FIG. 1 is a block diagram showing a configuration of a waveform generating unit according to a first embodiment. A flowchart for explaining the operation of determining whether a waveform generating unit is short-circuited in the first embodiment. FIG. 4A is a timing chart of a current flowing when the brushless DC motor in the first embodiment is short-circuited, and FIG. 4B is a timing chart of a short-circuit command output for the brushless DC motor in the first embodiment. FIG. 5(a) is a timing chart of the terminal voltage of the U-phase winding when current is stopped to the stator in the first embodiment. FIG. 5(b) is a timing chart of the terminal voltage of the V-phase winding when current is stopped to the stator in the first embodiment. FIG. 5(c) is a timing chart of the terminal voltage of the W-phase winding when current is stopped to the stator in the first embodiment. FIG. 5(d) is a timing chart of the average value of the terminal voltage when current is stopped to the stator in the first embodiment. Timing chart of the speed when the brushless DC motor in the first embodiment is stopped

(The knowledge and other information that formed the basis of this disclosure)
At the time the inventors came up with the idea of the present disclosure, there was a technology in existence that started applying a voltage that generates a reverse torque while switching the energization depending on the magnetic pole position of the motor when the rotation period of the piston after the compressor stops falling below a predetermined value, thereby stopping the compressor without managing the piston position just before top dead center or the stop position.
There is also a technique in which an inverter that drives a compressor motor manages the position at which power supply to the compressor is stopped, and the rotation of the compressor is stopped by applying a brake after power supply is stopped.

However, in a reciprocating compressor, there is a possibility that the motor speed becomes 0 when the piston is moving from compression to discharge. As a result, a pressure difference occurs between the inside and outside of the piston, causing the piston to reverse, and the inventors discovered a problem in that vibrations during stopping cannot be sufficiently reduced. In order to solve this problem, the inventors have come to constitute the subject of the present disclosure.
Therefore, the present disclosure provides a motor drive device in which the pressure inside and outside the cylinder of the compressor is approximately equal when the speed of the brushless DC motor becomes zero, preventing the piston from reversing and reducing vibration when the motor is stopped.

Hereinafter, the embodiments will be described in detail with reference to the drawings. However, more detailed explanation than necessary may be omitted. For example, detailed explanation of already well-known matters or duplicate explanation of substantially the same configuration may be omitted. This is to avoid the following explanation becoming more redundant than necessary and to facilitate understanding by those skilled in the art.
It should be noted that the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims.

(Embodiment 1)
Hereinafter, the first embodiment will be described with reference to FIGS.
[1-1. Configuration]
[1-1-1. Configuration of a refrigerator using a motor drive device]
FIG. 1 is a block diagram showing a control configuration of a motor drive device and a refrigerator in the first embodiment.
As shown in FIG. 1 , a refrigerator 300 includes a motor drive device 100, a compressor 200, and a rectifying and smoothing unit 2.

Motor driving device 100 drives brushless DC motor 210 with a power supply in which an input from power supply 1 is DC-smoothed by rectifying and smoothing unit 2. Brushless DC motor 210 is housed inside compressor 200.
The brushless DC motor 210 is composed of a four-pole rotor 211 having two pole pairs of permanent magnets, and a stator 212 having three-phase windings.
The stator 212 is energized with a waveform generated by the waveform generating unit 120 of the motor driving device 100, and the generated magnetic field causes the rotor 211 to rotate.
The rotor 211 is connected to a crank 201 of a reciprocating compressor 200 , and the rotational motion of the rotor 211 is converted into a linear reciprocating motion of a piston 202 .

When the center of rotation of rotor 211, the connection between crank 201 and rotor 211, and the connection between crank 201 and piston 202 are aligned in a straight line in that order, piston 202 is furthest from rotor 211. This state is the top dead center, and the refrigerant in cylinder 203 is most compressed.
Also, the pistons are closest when they are lined up in a straight line in the order of the connecting part of the crank 201 and the rotor 211, the center of the rotor 211, and the connecting part of the crank 201 and the piston 202. This state is the bottom dead center, where the volume of the refrigerant in the cylinder 203 is larger and the pressure is equal to the pressure of the sucked refrigerant.

The piston 202, by reciprocating motion, draws the refrigerant filling the compressor 200 into the cylinder 203 and compresses it.
Cylinder 203 discharges the refrigerant, which has been made higher in pressure by piston 202 than when it was suctioned. The refrigerant discharged from cylinder 203 flows through condenser 301, pressure reducer 302, and evaporator 303, which are connected by piping, in that order, and is returned to compressor 200. At this time, heat is released in condenser 301, and heat is absorbed in evaporator 303, so that cooling and superheating can be performed. Refrigerator 300 is configured with such a refrigeration cycle.

The power source 1 is a typical commercial power source, which in Japan is an AC power source of 50 Hz or 60 Hz with an effective value of 100 V.
In the rectifying and smoothing unit 2, the power source 1 is input, and the rectifying unit 2a rectifies AC power to DC power, and the smoothing unit 2b is connected to the output side of the rectifying unit 2a and smoothes the output of the rectifying unit 2a. The rectifying unit 2a is composed of four rectifying diodes connected in a bridge connection. The smoothing unit 2b is composed of a capacitor. A capacitor with a large capacity is required to achieve sufficient smoothing. Therefore, in this embodiment, an electrolytic capacitor, which is inexpensive for its capacity, is used.

In this embodiment, an AC power supply is used as the power supply 1 , so the AC is smoothed into DC by the rectifying and smoothing unit 2 and input to the motor drive device 100 .
When the power supply 1 is a DC power supply, it may be directly connected to the motor drive device 100. In this case, the rectifying and smoothing unit 2 is not necessary.
In the motor drive device 100 , the waveform generating section 120 converts the DC input from the rectifying and smoothing section 2 into AC and outputs it to the brushless DC motor 210 .

The motor drive device 100 includes a waveform generating unit 120, a terminal voltage detecting unit 102, a phase estimating unit 103, a speed estimating unit 104, a compressor state grasping unit 105, and a current detecting unit 108.
Terminal voltage detection section 102 detects the voltage of a terminal connected to waveform generation section 120 of brushless DC motor 210. Then, terminal voltage detection section 102 sends the value of the terminal voltage to phase estimation section 103 and speed estimation section 104, respectively.
The phase estimation unit 103 detects the phase of the rotor 211 of the brushless DC motor 210 from the terminal voltage of the brushless DC motor 210 detected by the terminal voltage detection unit 102. Then, the phase estimation unit 103 inputs the estimated phase of the rotor 211 to the waveform generation unit 120.

The speed estimation unit 104 estimates the speed from the voltage value detected by the terminal voltage detection unit 102. The speed estimation unit 104 then sends the estimated speed to the waveform generation unit 120. The rotation speed of the rotor 211 during operation is estimated from the occurrence period of zero crossings of the induced voltage generated by the brushless DC motor 210. The rotation speed of the rotor 211 when no current is being applied, such as when the brushless DC motor 210 is stopped, is detected from the frequency at which the points at which the voltage of the other phase of the brushless DC motor 210 detected by the terminal voltage detection unit 102 crosses the average value of the terminal voltage appear.

The compressor state grasping unit 105 detects the compression state of the compressor 200 when the supply of current to the brushless DC motor 210 is stopped but the rotor 211 is rotating. When the brushless DC motor 210 is not being energized, such as when it is stopped, the compressor state grasping unit 105 determines that the compression process has progressed each time it detects an intersection with the voltage of another phase of the brushless DC motor 210 detected by the terminal voltage detection unit 102 and an intersection with the average value of the terminal voltage.

When the piston 202 moves from the bottom dead center to the top dead center, it compresses and discharges the refrigerant. When the refrigerant is compressed and discharged, the speed of the rotor 211 drops significantly. Therefore, the compressor state grasping unit 105 determines that the compressor 200 is at the top dead center, where the compressor 200 switches from discharge to suction, when the rate of change of the interval at which the intersection is detected changes from a state greater than a predetermined rate of change to a state smaller than the predetermined rate of change. The compressor state grasping unit 105 then grasps the state of the compressor 200 from the number of times the intersection of the terminal voltage of the stator 212 and the comparison voltage is detected, based on the top dead center.

The speed command unit 107 sends a command to the waveform generating unit 120 for the speed at which to rotate the rotor 211 of the brushless DC motor 210 depending on the temperature state inside the refrigerator 300 and the like.
The current detection unit 108 detects the current flowing through the brushless DC motor 210. The current flowing through each of the three phases of the brushless DC motor 210 is detected.
The waveform generating unit 120 rotates the rotor 211 of the brushless DC motor 210 by applying an appropriate current according to the phase of the brushless DC motor 210 estimated by the phase estimating unit 103 .
The waveform generating unit 120 changes the magnitude of the voltage applied to the stator 212 based on the difference between the speed command received from the speed command unit 107 and the rotational speed of the rotor 211 estimated by the speed estimation unit 104 from the terminal voltage of the brushless DC motor 210 detected by the terminal voltage detection unit 102.

Furthermore, when the command speed of speed command section 107 becomes 0, waveform generating section 120 stops the supply of current to brushless DC motor 210 and then shorts all the terminals of brushless DC motor 210 inside waveform generating section 120 .
When a short circuit occurs, brushless DC motor 210 becomes a generator and current flows. While the motor is short circuited, a current larger than that during normal operation flows. Therefore, based on the current flowing through stator 212 detected by current detection unit 108, waveform generation unit 120 repeats short circuiting and output stop when the current is centered around a predetermined current threshold. This makes it possible to forcibly stop brushless DC motor 210.

The current flowing through the stator 212 is compared with the current threshold value periodically.
Therefore, the current threshold is determined based on the relationship between the current increase rate and the current detection period so that the maximum current exceeding the current threshold does not cause demagnetization of brushless DC motor 210, keeps the temperature of stator 212 below the usable temperature, and is a current that can be passed by waveform generating unit 120.
When a short circuit occurs, the current flows between brushless DC motor 210 and waveform generating unit 120, and increases to a current value according to the rotation speed of rotor 211. When the output is stopped, the current flows from brushless DC motor 210 through waveform generating unit 120 to smoothing unit 2b of rectifying smoothing unit 2, and the current decreases.

The short-circuiting process is performed when the rotor 211 of the brushless DC motor 210 is rotating in the forward direction. Forward rotation is defined as the direction in which the rotor 211 rotates counterclockwise when viewed from the side where the rotor 211 and the crank 201 are connected. The short-circuiting process is started so that the piston 202 of the compressor 200 stops while moving from top dead center to bottom dead center.

In this embodiment, in order to stop the piston 202 of the compressor 200 while it is moving from top dead center to bottom dead center, the waveform generating unit 120 starts short-circuiting when the speed estimated by the speed estimating unit 104 becomes equal to or lower than the speed threshold value.
The speed threshold is determined by the stroke (position of piston 202) of compressor 200 relative to the refrigerant. When piston 202 moves from bottom dead center to top dead center, there is a possibility that it may stop during the compression stroke, so the speed threshold is higher than when piston 202 moves from top dead center to bottom dead center.

Also, the short circuit can be started by bringing the pressure of the compressor 200 to a predetermined pressure condition by adjusting the refrigerator's refrigeration cycle, etc., and then stopping the power supply, and determining the timing of the short circuit start from the time after the power supply is stopped and the number of compressions. With this method, the timing of the short circuit start does not require complex calculations, and the process can be simplified.

The top dead center can be inexpensively and easily detected by compressor state grasping unit 105 from the terminal voltage of brushless DC motor 210 detected by terminal voltage detection unit 102 .
During normal operation of refrigerator 300, the suction pressure and discharge pressure of compressor 200 are within predetermined values, so that refrigerator 300 can be reliably stopped between top dead center and bottom dead center at a predetermined speed threshold.

However, if an abnormality occurs in the refrigerator 300, the suction pressure and discharge pressure of the compressor 200 become higher than in normal operating conditions. In this case, even if the speed is higher than the predetermined speed threshold for starting the short circuit, there is a possibility that the compression process will stop midway. Therefore, the waveform generating unit 120 once decelerates the speed so that it becomes equal to or lower than the speed threshold for starting the short circuit. By using a short circuit as the method for decelerating, costs can be reduced without introducing a new method.

Whether or not to decelerate before starting the short circuit to stop the rotor 211 is determined by the output determination unit 121 from the deceleration rate when the rotor 211 makes one rotation.
Brushless DC motor 210 is in a free-running state until waveform generating unit 120 stops output to stop the motor and starts short-circuiting. While brushless DC motor 210 is free-running, the discharge pressure and suction pressure of compressor 200 hardly change. Therefore, if the rotation speed and the deceleration rate per rotation are obtained, the load torque of compressor 200 can be obtained, and it is possible to predict how the speed will decelerate and how many rotations it will take to stop the motor.

The waveform generating unit 120 calculates the deceleration rate immediately after the output is stopped, and if it is predicted that the compressor 200 will stop without reaching the speed threshold, it shorts and decelerates the rotor 211 so that the speed of the rotor 211 falls below the speed threshold at the top dead center just before the compressor stops. This makes it possible to stop the piston 202 as it moves from the top dead center to the bottom dead center, even if the pressure of the compressor 200 is abnormal.

While piston 202 moves from top dead center to bottom dead center, cylinder 203 draws in refrigerant. Therefore, there is no difference between the pressure of the refrigerant in compressor 200 and the pressure in cylinder 203, and no force is exerted to move piston 202 due to the pressure difference. In other words, rotor 211 is stopped while piston 202 moves from top dead center to bottom dead center, so there is no reversal of rotation after the speed of rotor 211 becomes 0, and vibrations caused by stopping compressor 200 can be reduced.

[1-1-2. Configuration of Waveform Generation Unit]
Next, the internal structure of the waveform generating section 120 will be described with reference to FIG.
FIG. 2 is a block diagram showing a configuration of the waveform generating unit according to the first embodiment.
2, the waveform generating unit 120 is composed of an output determining unit 121, a drive unit 122, and an inverter 123. The output determining unit 121 receives outputs from the phase estimating unit 103, the speed estimating unit 104, the compressor state grasping unit 105, the current detecting unit 108, and the speed command unit 107.
The inverter 123 converts the DC power from the rectifying and smoothing unit 2 into AC power. The inverter 123 is configured by connecting six switching elements 123a to 123f in a three-phase bridge configuration. In addition, six freewheeling current diodes 123g to 123l are connected in the reverse direction to the switching elements 123a to 123f.

The output determination unit 121 determines and outputs either the conduction rate, the short circuit command, or the current stop command. When the command speed of the speed command unit 107 is greater than 0, the output determination unit 121 outputs the conduction rate. The conduction rate is determined from the difference between the rotation speed of the rotor 211 estimated by the speed estimation unit 104 and the command speed of the speed command unit 107. The determined conduction rate is output to the drive unit 122. Specifically, the waveform generation unit 120 increases the voltage when the speed command is higher, and decreases the voltage when the speed command is lower, and outputs the voltage to the stator 212.

Furthermore, when the command speed of the speed command unit 107 is 0, the output determination unit 121 outputs a short circuit command or a current stop command for turning off all the switching elements 123a to 123f. The output determination unit 121 outputs a current stop command immediately after the speed command of the speed command unit 107 becomes 0.
The start of the short circuit command is determined by the output determination unit 121 based on information from the speed estimation unit 104 and the compressor state grasping unit 105, and the output is switched to the short circuit command. Furthermore, the output determination unit 121 also determines the deceleration before the short circuit to ensure a stop even in an abnormal load state, based on the change in the estimated speed by the speed estimation unit 104.

In addition, the output determination unit 121 also switches between a short circuit command and a current stop command based on the current flowing through the stator 212 when a short circuit occurs.
Based on the position information of the rotor 211 of the brushless DC motor 210 detected by the phase estimation unit 103, the drive unit 122 determines the timing of the power supply that the inverter 123 supplies to the three-phase windings of the stator 212 of the brushless DC motor 210, and outputs a drive signal for PWM control.

Specifically, the drive signal turns on or off the switching elements 123a to 123f of the inverter 123. This applies optimal AC power to the stator 212, causing the rotor 211 to rotate and driving the brushless DC motor 210. The drive waveform can be a square wave or a sine wave, but there is no particular preference. In addition, the drive unit 122 calculates and outputs the PWM duty width based on the conduction rate determined by the output determination unit 121.

When a short circuit command is received, the drive unit 122 turns on the combination of switching elements 123a, 123c, and 123e of the inverter 123, or the combination of switching elements 123b, 123d, and 123f, to make them conductive. Since diodes 123g to 123l are connected in the opposite direction to each of the switching elements 123a to 123f, the drive unit 122 shorts the terminals of the stator 212.

In general, to drive the control side switching elements 123a, 123c, and 123e, it is necessary to charge a power source with a different potential reference from the smoothing unit 2b. Therefore, in order to short-circuit for a long time, the components to be charged (capacitors, etc.) become large. On the other hand, the negative side switching elements 123b, 123d, and 123f can be driven by a power source with a common potential reference to the smoothing unit 2b, so no power source charging is required to drive them. In this embodiment, since there is no need to charge the power source that drives the switching elements when short-circuited, the negative side switching elements 123b, 123d, and 123f are short-circuited. This makes it possible to prevent the circuit from becoming larger and the costs from increasing.

When the drive unit 122 shorts the stator 212, the brushless DC motor 210 functions as a generator, and current flows through the stator 212. The current flowing through the stator 212 passes through one of the switching elements 123b, 123d, and 123f that are turned on or the diodes 123h, 123j, and 123l, and then flows back to the stator 212.

Furthermore, when a stop current command is received, the drive unit 122 turns off all switching elements 123a to 123f. The current that was flowing through the stator 212 continues to flow, so the current that was flowing through diodes 123h, 123j, and 123l continues to flow. On the other hand, the current that was flowing through switching elements 123b, 123d, and 123f is cut off, so it flows through diodes 123g, 123i, and 123k, respectively. As a result, the current in the stator 212 flows to the smoothing unit 2b, which charges it. Then, the current flowing through the stator 212 rapidly decreases.

[1-2. Operation]
The operation of motor drive device 100 configured as above will now be described.
[1-2-1. Short circuit determination operation]
The short circuit determination operation of the output determining unit 121 will be described with reference to FIG.
FIG. 3 is a flowchart for explaining the operation of determining whether or not a short circuit has occurred in the waveform generating unit according to the first embodiment.
First, a short circuit determination for stopping brushless DC motor 210 is not performed until the command speed from speed command unit 107 becomes 0 (S101: No). Then, when the command speed becomes 0 (S101: Yes), output determination unit 121 outputs a power supply stop command (S102). After power supply is stopped and rotor 211 is free running, a speed threshold value is determined from a table created by calculating in advance a command speed according to the state of the compression process of compressor 200 grasped by compressor state grasping unit 105 (S103).

Then, while the rotation speed of the rotor 211 estimated by the speed estimation unit 104 is greater than the speed threshold (S104: No), the speed threshold is continuously updated (S103). The rotation speed of the rotor 211 gradually decreases in a free-running state. Then, when the rotation speed becomes equal to or less than the speed threshold (S104: Yes), it is determined whether the current flowing through the stator 212 detected by the current detection unit 108 is less than a predetermined current threshold (S105).

If it is determined that the current flowing through the stator 212 is less than the current threshold (S105: Yes), the output determination unit 121 outputs a short circuit command (S106). Since no current flows until the short circuit is started, the short circuit is started first. The determination of whether the current value is less than the current threshold (S105) is repeated until the actual rotation speed of the rotor 211 becomes 0 (S108: No). By repeating the determination periodically, for example by synchronizing it with the PWM period, the processing time can be reduced.

During the short circuit, if the rotation speed of the rotor 211 is constant, the current flowing through the stator 212 gradually increases to a current value corresponding to the rotation speed. When the short circuit starts and the current flowing through the stator 212 increases, and the current value becomes equal to or greater than the current threshold (S105: No), the output determination unit 121 outputs a current stop command (S107). The current decreases as a result of the current being stopped.
Until the rotation speed becomes 0 (S108: No), the current value is compared with the current threshold value (S105), and the current is suppressed to a certain value or less by the current increase due to the output of the short circuit command (S106) and the current decrease due to the output of the de-energization command, so that the rotation speed decreases more quickly than in the deceleration during free run. As the rotation speed decreases, the current value becomes smaller, and when the rotation speed approaches 0, the current value does not exceed the current threshold value (S105: Yes), and the short circuit command is continued.

By reducing the current value, the force slowing down the rotation of rotor 211 becomes smaller, and the rotor stops with a smooth speed change, making it possible to suppress vibrations when the rotor stops.
Furthermore, since the speed threshold is set so that the compressor stops between the top dead center and the bottom dead center, the piston 202 of the compressor 200 is not pushed back by the pressure difference between the inside and outside of the cylinder 203, and no vibration occurs due to the reverse rotation of the rotor 211. When the rotation speed becomes 0 (S108: Yes), the output determination unit 121 ends the short circuit control and outputs a power supply stop command (S109).

Note that, although the short circuit control ends when the rotation speed becomes 0, it may also end when the current flowing through the stator 212 detected by the current detection unit 108 becomes 0. Also, the rotation speed may not be completely 0, but may be a value close to 0, such as 0.1 r/s, and since the rotational energy is small, vibrations during stopping can be suppressed. Similarly, vibrations during stopping can also be suppressed by setting the current value close to 0, such as 0.1 A, as the end condition.

The operation at this time will be explained in more detail using Figures 4(a) and 4(b). Figure 4(a) shows the change in the value of the current flowing through the stator 212 when a short circuit is performed (the time between T106 and T107 in Figure 6). Figure 4(b) shows the timing at which the output decision unit 121 outputs a short circuit command and a current stop instruction in response to the current value of the stator 212 detected by the current detection unit 108. When the short circuit is started (T106 in Figure 4(b)), the current starts to increase (between T106 and T201 in Figure 4(a)). It is periodically determined whether the current value exceeds the current threshold. It starts from a state where the current value is 0 immediately after the short circuit is started.

After the short circuit starts, the current value is less than the current threshold in the first judgment cycle (T201 in FIG. 4(a)) and the next judgment cycle (T202 in FIG. 4(a)), so the short circuit command continues (from T201 to before T203 in FIG. 4(b)). In the next judgment cycle (T203 in FIG. 4(a)), the current value is greater than or equal to the current threshold, so the current stop command is selected and output (T203 in FIG. 4(b)). The current flowing through the stator 212 decreases due to the current stop command.

Then, in the next judgment cycle (T204 in FIG. 4(a)), the current value is again below the current threshold, so a short circuit command is selected and output (T204 in FIG. 4(b)). Then, in the next cycle (T205 in FIG. 4(a)), the current value exceeds the threshold, so a de-energization command is selected and output (T205 in FIG. 4(b)), and in the cycle after that (T206 in FIG. 4(a)), the current value falls below the current threshold, so a short circuit command is selected and output (T206 in FIG. 4(b)). By passing a current through brushless DC motor 210, the rotational speed of rotor 211 decreases.

Therefore, the induced voltage generated in the stator 212 decreases, and the current flowing through the stator 212 during the short circuit decreases. As a result, in the next cycle (T207 in FIG. 4(b)), even if the short circuit command continues, the current does not exceed the current threshold (T207 in FIG. 4(a)). Even if the determination is made periodically thereafter (T208 to T211 in FIGS. 4(a) and (b)), the short circuit command continues while the current flowing through the stator 212 decreases until the rotor 211 stops.

Then, in the cycle where it is determined that the rotor 211 has stopped (T107 in Figures 4(a) and (b)), the process of stopping the rotor 211 due to the short circuit is completed, and a command to stop the current is output. This protects the inverter 123 and the stator 212 from thermal destruction. In addition, the strength of the magnetic field created by the stator 212 is kept below a certain value, preventing demagnetization of the rotor 211. Furthermore, the rotor 211 can be stopped with the largest possible current that protects the inverter 123, the stator 212, and the rotor 211.

[1-2-2. Speed estimation operation when compressor is stopped]
Next, the operation of the speed estimation unit 104 when the inverter 123 stops supplying current from a state in which the command speed is 0 and the rotor 211 is rotating will be described in more detail with reference to Fig. 5(a) to Fig. 5(d). The stator 212 is star-connected. In the star connection, one end of each of the three-phase windings of the stator 212 is all connected. The terminals of the windings of the stator 212 that are not connected to each other are connected to the connections of the switching elements 123a and 123b, the switching elements 123c and 123d, and the switching elements 123e and 123f of the inverter 123, respectively. The winding of the stator 212 connected to the switching element 123a is the U-phase, the winding of the stator 212 connected to the switching element 123c is the V-phase, and the winding of the stator 212 connected to the switching element 123e is the W-phase.

Figure 5(a) shows the waveform of the U-phase terminal voltage of the stator 212 when the rotor 211 is stopped from rotating while the current is being applied. Figure 5(b) shows the waveform of the V-phase terminal voltage of the stator 212 when the rotor 211 is stopped from rotating while the current is being applied. Figure 5(c) shows the waveform of the W-phase terminal voltage of the stator 212 when the rotor 211 is stopped from rotating while the current is being applied. Figure 5(d) shows the average voltage waveform of the terminal voltage of the stator 212. The reference voltage for Figures 5(a) to 5(d) is the negative side of the smoothing section 2b.

First, switching elements 123a to 123f are turned off from the operating state, and the current to stator 212 is stopped (T300). When the current is stopped, the induced voltage of rotor 211 can be observed in all phases, U phase, V phase, and W phase. The phase with the lowest terminal voltage of the three phases has a potential almost equal to that of smoothing section 2b. Here, the terminal voltage at the start of stopping is W phase > U phase > V phase. As rotor 211 rotates, the self-sufficiency of stator 212 changes, and the induced voltage of each phase changes. Here, the voltage of V phase remains the lowest and does not change (between T300 and T301 in Figure 5 (b)).

However, the U-phase voltage gradually increases (between T300 and T301 in Figure 5(a)), and the W-phase voltage gradually decreases (between T300 and T301 in Figure 5(c)). Then, the U-phase and W-phase terminal voltages match (T301 in Figures 5(a) and 5(c)). After that, the magnitude of the terminal voltages becomes U-phase > W-phase > V-phase (between T301 and T303 in Figures 5(a) to 5(c)). During this time, the W-phase terminal voltage gradually decreases, and the W-phase and V-phase match (T303 in Figures 5(b) and 5(c)).

As the rotor 211 rotates, the terminal voltage state changes as follows: U phase > V phase > W phase (between T303 and T305 in Figures 5(a) to 5(c)), V phase = U phase > W phase (T305 in Figures 5(a) and 5(b)), V phase > U phase > W phase (between T305 and T307 in Figures 5(a) to 5(c)), V phase > U phase = W phase (between T305 and T307 in Figures 5(a) and 5(c)), 5(c)), V phase > W phase > U phase (between T307 and T309 in Figures 5(a) to 5(c)), V phase = W phase > U phase (T309 in Figures 5(b) and 5(c)), W phase > V phase > U phase (between T309 and T311 in Figures 5(a) to 5(c)), W phase > V phase = U phase (T311 in Figures 5(a) and 5(b)).

Then, W phase>U phase>V phase (between T311 and T312 in FIG. 5(a) to FIG. 5(c)), the state is the same as when the current is stopped again, and the brushless DC motor 210 makes one rotation in electrical angle. On the other hand, in this embodiment, the rotor 211 has four poles, so two electrical cycles equal one mechanical cycle. In other words, the rotor 211 makes one rotation and the piston 202 reciprocates once in two electrical cycles. The state in which two of the three-phase terminal voltages coincide occurs every 60 degrees in electrical angle, so it occurs six times in one electrical cycle and 12 times in one mechanical cycle. The speed estimation unit 104 detects the timing at which the magnitude relationship between the terminal voltages of the three phases changes, and the reciprocal of 12 times the occurrence cycle is the rotation speed of the rotor 211.

The timing at which the average of the three-phase terminal voltage values matches the terminal voltage of each phase may also be detected. The points of match between the terminal voltage of each phase and the average of the three-phase terminal voltages appear in the middle of the electrical angle at which the terminal voltages of two phases match (T302, T304, T306, T308, T310, and T312 in Figures 5(a) to 5(d)). In other words, this occurs six times in one electrical angle cycle, and in this embodiment, it occurs twelve times in one mechanical angle cycle. The rotation speed of rotor 211 is determined as the reciprocal of 12 times the cycle at which the terminal voltage of each phase matches the average of the three-phase terminal voltages.

The timing when the terminal voltage of each phase matches the average of the terminal voltages of the three phases, and the timing when two of the three terminal voltage phases match occur a total of 24 times every 15 degrees of mechanical angle. By detecting the timing when the terminal voltage of each phase matches the average of the terminal voltages of the three phases, and the timing when two of the three terminal voltage phases match in combination, it is possible to estimate the speed when current is stopped from passing through the stator 212 more accurately than if it were done individually.

[1-2-1. Compressor status detection operation]
The compressor state grasping unit 105 detects a change in the magnitude relationship of each of the three-phase terminal voltages from a state in which the command speed is 0 and the rotor 211 is rotating, in the same manner as the speed estimating unit 104 when the inverter 123 is deenergized. Each time such a change occurs, the rotor 211 rotates by a certain angle, the piston 202 moves, and the process changes. The period in which the magnitude relationship of the terminal voltages changes becomes large when the piston 202 compresses and discharges the refrigerant moving from the bottom dead center to the top dead center.

The period in which the magnitude relationship of the terminal voltages changes becomes smaller when the motor moves from top dead center to bottom dead center and sucks in the refrigerant. This period of change is compared with a predetermined change period threshold, and the point at which the period of change changes from a state in which it is greater than the change period threshold to a state in which it is smaller than the change period threshold is detected as the top dead center. The compressor state grasping unit 105 grasps the state of the compressor 200 from the number of times the magnitude relationship of the terminal voltages changes, based on this top dead center. In this embodiment, since the motor is a three-phase, four-pole brushless DC motor 210, for example, the bottom dead center is reached when the number of times the magnitude relationship changes from the top dead center is six. The top dead center is reached when the number of times the magnitude relationship changes from the top dead center is twelve.
Furthermore, similarly to the speed estimation unit 104, by utilizing the timing at which the terminal voltage of each phase coincides with the average of the terminal voltages of the three phases, the state of the compressor 200 can be grasped in detail.

[1-3. Speed change when compressor is stopped]
Next, the speed change of rotor 211 of compressor 200 will be described with reference to Fig. 6. Fig. 6 shows the change in the rotation speed of rotor 211 when inverter 123 stops supplying current to stator 212 from a state in which rotor 211 is rotating due to the command speed being 0. When waveform generating unit 120 stops supplying current to stator 212 (T101) due to the command speed from speed command unit 107 becoming 0, rotor 211 becomes free-running.

Here, it is assumed that the position of the piston 202 was at bottom dead center when the current was stopped. As the piston 202 moves from bottom dead center to top dead center, the refrigerant is compressed and discharged (between T101 and T102). During this time, the speed of the rotor 211 drops significantly. When the piston 202 reaches top dead center (T102), there is almost no change in speed while it moves to bottom dead center (between T102 and T103).

In this way, the speed of the rotor 211 gradually decreases as the piston 202 moves from bottom dead center to top dead center (between T101 and T102, between T103 and T104, and between T105 and T106) and then slightly decreases when moving from top dead center to bottom dead center (between T102 and T103, between T104 and T105, and between T106 and T107).

While the speed of rotor 211 is above the speed threshold (between T101 and T106), waveform generating section 120 stops outputting.
Then, when the speed of the rotor 211 becomes equal to or lower than the speed threshold (T106), the waveform generating unit 120 starts short-circuiting. While the short-circuiting is in progress (between T106 and T107), the speed can be decelerated even during the intake stroke in which the piston 202 moves from the top dead center to the bottom dead center. When the rotation speed of the rotor 211 becomes equal to or lower than the speed threshold, the short-circuiting of the stator 212 is started, so that the piston 202 stops while moving from the top dead center to the bottom dead center. In this state, there is no pressure difference between the inside and outside of the cylinder 203, and the piston 202 does not bounce, so even if the waveform generating unit 120 stops the energization, the speed of the rotor 211 remains at 0.

The speed of the rotor 211 is likely to significantly decelerate before the top dead center and cross the speed threshold. In addition, since the speed threshold increases only at the bottom dead center, if the speed does not change, the speed will only fall below the speed threshold at the bottom dead center. In other words, the determination of whether the rotation speed of the rotor 211 is lower than the speed threshold is performed only at the top dead center and the bottom dead center, and two speed thresholds may be prepared, one for the bottom dead center and the other for the top dead center, with the speed threshold at the bottom dead center set higher than the speed threshold at the top dead center.
Furthermore, control becomes easier by fixing the position where the waveform generating unit 120 starts the short circuit to the top dead center. Since the piston 202 is stopped while it is transitioning from the top dead center to the bottom dead center, it may stop anywhere between the start of the short circuit and the bottom dead center. Furthermore, the time from the start of the short circuit to the bottom dead center can be secured as long as possible, so the threshold value can be set high, and speed control becomes easier.

[1-4. Effects, etc.]
As described above, in this embodiment, motor drive device 100 is a motor drive device for driving reciprocating compressor 200 composed of cylinder 203 and piston 202 for compressing gas, and includes brushless DC motor 210 for compressor 200 performing a compression operation, and waveform generating unit 120 for converting power from power supply 1 supplied from an external source and supplying the power to brushless DC motor 210. When stopping brushless DC motor 210, waveform generating unit 120 shorts the output to brushless DC motor 210 so that the rotation of compressor 200 stops while piston 202 of compressor 200 rotates forward from top dead center to bottom dead center.
This makes it possible to make the pressure inside and outside cylinder 203 of compressor 200 approximately equal when the speed of brushless DC motor 210 becomes 0. Therefore, by reducing the force with which the gas in cylinder 203 pushes away piston 202, it is possible to suppress reverse rotation of piston 202 and reduce vibration when stopped.

Furthermore, this embodiment may include a current detection unit 108 that detects a current flowing through brushless DC motor 210. Waveform generation unit 120 reduces the current when the current value of brushless DC motor 210 detected by current detection unit 108 exceeds a current threshold value.
This makes it possible to prevent a current so large that it would degrade the functions of the waveform generating section 120 and the brushless DC motor 210. Therefore, the brushless DC motor 210 can be stopped while vibrations are safely suppressed.

In this embodiment, the current that waveform generating section 120 reduces so that the current value does not exceed the current threshold is the current that flows through smoothing section 2 b, which is the current of brushless DC motor 210 .
This causes a swift reduction in the current flowing through brushless DC motor 210. Therefore, the time during which the current exceeds the threshold value can be shortened.

This embodiment also includes a terminal voltage detection unit 102 that detects a terminal voltage value of brushless DC motor 210, and a speed estimation unit 104 that estimates the rotation speed of brushless DC motor 210 from the terminal voltage value of brushless DC motor 210 detected by terminal voltage detection unit 102. Waveform generation unit 120 starts short-circuiting after the rotation speed of brushless DC motor 210 estimated by speed estimation unit 104 becomes equal to or lower than a speed threshold value.
As a result, when the compressor 200 is stopped due to a short circuit, the rotation of the rotor 211 of the brushless DC motor 210 can be stopped by a simple decision while the piston 202 moves from the top dead center to the bottom dead center. Therefore, the vibration of the compressor 200 when the compressor 200 is stopped can be suppressed by a low-cost microcomputer or the like.

Furthermore, this embodiment includes a compressor state grasping unit 105 that grasps the position of piston 202 of compressor 200. When piston 202 of compressor 200 grasped by compressor state grasping unit 105 reaches a predetermined position, waveform generating unit 120 starts applying the brakes to brushless DC motor 210.
This eliminates the need to simplify the storage device for storing multiple speed thresholds for stopping the compressor 200, and eliminates the need for complex calculations for determining the speed thresholds, thereby reducing the cost of a microcomputer and the like for implementing the waveform generating unit 120.

In this embodiment, compressor state determining unit 105 determines the position of piston 202 of compressor 200 from the terminal voltage of brushless DC motor 210 detected by terminal voltage detection unit 102 .
This allows the compressor state grasping unit 105 to share the speed estimation unit 104 and the terminal voltage detection unit 102. Therefore, it is possible to grasp the state of the compressor 200 while suppressing the addition of parts, and it is possible to suppress costs.

Furthermore, in this embodiment, if the speed of brushless DC motor 210 estimated by speed estimation unit 104 when piston 202 of compressor 200 starts to stop from a predetermined position is higher than the speed threshold value, waveform generation unit 120 decelerates the speed of brushless DC motor 210 to a value equal to or lower than the speed threshold value and greater than zero before brushless DC motor 210 starts to stop.
As a result, even if the pressure is outside the expected range, such as in an abnormal state, the rotation of the rotor 211 can be stopped while the piston 202 moves from the top dead center to the bottom dead center. Therefore, in both the normal state and the abnormal state of the compressor 200, the vibration when the compressor 200 is stopped can be suppressed.

In the present embodiment, the refrigerator 300 includes the motor drive device 100 and the compressor 200.
As a result, the interior of refrigerator 300 becomes sufficiently cold and motor drive device 100 can stop compressor 200 with small vibration. Therefore, refrigerator 300 is installed indoors and noise is often a concern, but a quiet refrigerator 300 can be provided.

In the present embodiment, the compressor 200 is provided at the upper part of the housing of the refrigerator 300.
This makes it possible to suppress noise caused by vibrations when compressor 200 is stopped, even when the force point of vibration generation by compressor 200 is at the top of the housing and the fulcrum is separated from the housing by the floor on which the compressor is placed. Therefore, it is possible to provide refrigerator 300 that is both easy to use and quiet, by narrowing the top of the housing, which is the hardest to reach and use, inside refrigerator 300, and widening the lower part of the interior, which is easier to use.

Other Embodiments
As described above, the first embodiment has been described as an example of the technology disclosed in the present application. However, the technology in the present disclosure is not limited to this, and can be applied to embodiments in which modifications, substitutions, additions, omissions, etc. are made. In addition, it is also possible to combine the components described in the first embodiment to create a new embodiment.

As described above, the motor drive device according to the present invention can be suitably used as a motor drive device that can reduce vibrations when a brushless DC motor stops.

REFERENCE SIGNS LIST 1 power supply 2 rectification smoothing section 2a rectification section 2b smoothing section 100 motor drive device 102 terminal voltage detection section 103 phase estimation section 104 speed estimation section 105 compressor state grasping section 107 speed command section 108 current detection section 120 waveform generation section 121 output determination section 122 drive section 123 inverter 123a to 123f switching elements 123g to 123l diodes 200 compressor 201 crank 202 piston 203 cylinder 210 brushless DC motor 211 rotor 212 stator 300 refrigerator 301 condenser 302 pressure reducer 303 evaporator

Claims (6)

A motor drive device for driving a reciprocating compressor composed of a cylinder and a piston for compressing gas,
A brushless DC motor for causing the compressor to perform a compression operation;
a waveform generating unit for converting electric power from an external power source and supplying the converted electric power to the brushless DC motor;
a terminal voltage detection unit for detecting a terminal voltage value of the brushless DC motor;
a speed estimation unit that estimates a rotation speed of the brushless DC motor from a terminal voltage value of the brushless DC motor detected by the terminal voltage detection unit;
a compressor state grasping unit that grasps a position of a piston of the compressor ,
when stopping the brushless DC motor after the rotation speed of the brushless DC motor estimated by the speed estimation unit becomes equal to or lower than a speed threshold value, the waveform generation unit shorts an output to the brushless DC motor so that the rotation of the compressor stops while the piston of the compressor rotates forward from top dead center to bottom dead center, a state in which there is no difference between the pressure of the refrigerant in the compressor and the pressure in the cylinder and no force acting to move the piston due to the pressure difference is applied;
the waveform generating unit initiates a short circuit to start braking the brushless DC motor when the piston of the compressor recognized by the compressor state recognition unit reaches a predetermined position,
The motor drive device according to claim 1, wherein the compressor state determining unit determines a piston position of the compressor from a terminal voltage of the brushless DC motor detected by the terminal voltage detecting unit . a current detection unit that detects a current flowing through the brushless DC motor,
2. The motor drive device according to claim 1, wherein the waveform generating section reduces the current so that the current value of the brushless DC motor detected by the current detecting section does not exceed a current threshold value. The motor drive device according to claim 2, characterized in that the current that the waveform generating unit reduces so that the current value does not exceed the current threshold is a current that causes the brushless DC motor current to flow to the power supply. 2. The motor drive device according to claim 1, wherein, when the speed of the brushless DC motor estimated by the speed estimator when the piston of the compressor starts to stop from a predetermined position is higher than a speed threshold, the waveform generating unit decelerates the speed of the brushless DC motor to a value equal to or lower than the speed threshold and higher than 0 before the brushless DC motor starts to stop. A refrigerator comprising the motor drive device according to any one of claims 1 to 4 and the compressor. 6. The refrigerator according to claim 5, wherein the compressor is provided in an upper portion of the housing.

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