JP7603884B2 - Drive device and refrigeration cycle device - Google Patents

Description

The present disclosure relates to a drive device for driving an electric motor and a refrigeration cycle device.

Conventionally, a driving device is known that includes a connection switching device that switches the stator winding (hereinafter simply referred to as "winding") of an electric motor between a plurality of different connection states, and that is capable of switching the winding connection while the electric motor is rotating. For example, the following Patent Document 1 discloses a technology in which, when a first connection is selected and the electric motor is in a first state, when switching from the first connection to a second connection, the second connection is selected after the electric motor transitions to the second state. The first state is a state in which the output voltage of the DC power supply circuit is a first voltage value, and the rotation speed of the electric motor is a first speed value. The second state is a state in which the output voltage of the DC power supply circuit is higher than the first voltage value, the rotation speed of the electric motor is higher than the first speed value, and the current flowing through the electric motor is equal to or lower than a predetermined threshold value.

Retable No. 2020/021681

The above-mentioned conventional technology has the effect of preventing the size of the equipment from increasing and obtaining a highly reliable drive unit. On the other hand, the above-mentioned conventional technology does not take into consideration the magnitude of the load when the motor connection is switched, i.e., the load state when the connection is switched. For example, if the load is large when the connection is switched, the regenerative current generated when the voltage of the motor exceeds the voltage of the internal circuit of the drive unit after the connection is switched may cause the internal circuit of the drive unit to become overvoltage. In order to prevent failure of the drive unit and extend the life of the drive unit, it is desirable to suppress the overvoltage that may occur in the internal circuit of the drive unit.

The present disclosure has been made in consideration of the above, and aims to obtain a drive device that can suppress overvoltage that may occur in the internal circuit of the drive device, regardless of the load state at the time of wiring switching.

In order to solve the above-mentioned problems and achieve the object, the drive device according to the present disclosure includes a boost converter, an inverter, a connection switching device, and a control unit. The boost converter receives an AC voltage output from an AC power source and outputs a DC voltage with a variable voltage value. The inverter converts the bus voltage output from the boost converter into an AC voltage with a variable voltage and frequency and applies it to the electric motor. The connection switching device switches the windings of the electric motor between a first connection and a second connection. The control unit controls the boost converter, the inverter, and the connection switching device. The control unit performs zero current control to switch the winding connections with the electric motor current set to zero. In addition, when the second connection is selected by the connection switching device, the control unit switches to the first connection if the rotation speed of the electric motor before the zero current control period does not exceed the upper limit of the rotation speed.

The drive device disclosed herein has the effect of suppressing overvoltage that may occur in the internal circuit of the drive device, regardless of the load condition at the time of wiring switching.

FIG. 1 is a diagram showing a configuration example of a drive device according to a first embodiment; FIG. 2 is a detailed diagram showing a connection state between the connection switching device and the electric motor shown in FIG. 1 . FIG. 2 is a diagram showing a detailed configuration of a switch of the connection switching device shown in FIG. 1; FIG. 2 is a diagram showing two connection states that can be switched in the electric motor shown in FIG. FIG. 1 is a diagram showing an example of a detailed configuration of a control unit according to the first embodiment; 1 is a first time chart for explaining a control sequence in the connection switching control according to the first embodiment; 2 is a second time chart for explaining a control sequence in the connection switching control according to the first embodiment; A flowchart showing an example of a process flow for implementing the control sequence of FIG. 7. FIG. 1 is a block diagram showing an example of a hardware configuration for implementing the functions of a control unit according to a first embodiment; FIG. 11 is a block diagram showing another example of a hardware configuration for implementing the functions of the control unit according to the first embodiment; FIG. 1 is a diagram showing a configuration example of a refrigeration cycle device according to a second embodiment;

The drive device and refrigeration cycle device relating to the embodiments of the present disclosure are described in detail below with reference to the attached drawings.

Embodiment 1.
1 is a diagram showing an example of the configuration of a drive device 100 according to embodiment 1. The drive device 100 is connected to an AC power source 2, and drives an electric motor 1 using power from the AC power source 2. An apparatus 5 is connected to the electric motor 1. The drive device 100 has a boost converter 3, an inverter 4, a connection switching device 20, and a control unit 30. The control unit 30 controls the boost converter 3, the inverter 4, and the connection switching device 20.

The boost converter 3 receives the AC voltage output from the AC power source 2 and outputs a DC voltage with a variable voltage value. Hereinafter, this DC voltage will be referred to as the "bus voltage."

The inverter 4 converts the bus voltage output from the boost converter 3 into a variable voltage and frequency AC voltage and applies it to the motor 1. The control unit 30 controls the operation of the inverter 4 based on the current output by the inverter 4.

An example of the motor 1 is a three-phase motor as shown in the figure. In this paper, a three-phase synchronous motor is assumed. The ends of the windings in the motor 1 are drawn out to the outside of the motor 1. The windings can be a star connection (hereinafter, appropriately referred to as "Y connection") which is a first connection, or a delta connection (hereinafter, appropriately referred to as "Δ connection") which is a second connection. In each of the Y connection and the Δ connection, switching to the other connection is performed by the connection switching device 20. The connection switching device 20 has switches 21, 22, and 23 for switching the connection of the windings of the motor 1. The selection of whether the motor 1 is driven in the Y connection or the Δ connection is controlled by the control unit 30.

Figure 2 is a diagram showing in detail the connection between the connection switching device 20 and the electric motor 1 shown in Figure 1. Figure 3 is a diagram showing the detailed configuration of the switches 21, 22, and 23 of the connection switching device 20 shown in Figure 1.

2, the first ends 41a, 42a, 43a of the three phase windings 41, 42, 43 of the motor 1, consisting of the U-phase, V-phase, and W-phase, are connected to external terminals 41c, 42c, 43c, respectively. The second ends 41b, 42b, 43b of the U-phase, V-phase, and W-phase windings 41, 42, 43 are connected to external terminals 41d, 42d, 43d, respectively. The external terminals 41c, 42c, 43c, 41d, 42d, 43d are terminals that can be connected to the outside of the motor 1. The external terminals 41c, 42c, 43c are connected to the U-phase, V-phase, and W-phase output lines 61, 62, 63 of the inverter 4.

As described above, the connection switching device 20 has switches 21, 22, and 23. The switches 21, 22, and 23 are connected to the windings 41, 42, and 43, respectively. The switches 21, 22, and 23 switch the paths of the currents flowing through the windings 41, 42, and 43, respectively. The switches 21, 22, and 23 are electromagnetic contactors whose contacts are electromagnetically opened and closed. Such electromagnetic contactors include relays, contactors, and the like. The switches 21, 22, and 23 are configured, for example, as shown in FIG. 3. In FIG. 3, the contacts of the switches 21, 22, and 23 are configured to be in different connection states when a current is flowing through the excitation coils 211, 221, and 231 and when a current is not flowing through them.

3, the excitation coils 211, 221, and 231 are connected to receive the switching power supply voltage V20 via the semiconductor switch 204. The opening and closing of the semiconductor switch 204 is controlled by the connection selection signal Sc output from the control unit 30. For example, when the connection selection signal Sc has a first value, the semiconductor switch 204 is turned off, and when the connection selection signal Sc has a second value, the semiconductor switch 204 is turned on. The first value is, for example, a logical value of "Low", and the second value is, for example, a logical value of "High". These may be inversely related. Note that, when the connection selection signal Sc is output from a circuit having sufficient current capacity, the current due to the connection selection signal Sc may be configured to flow directly from the circuit to the excitation coils 211, 221, and 231. In that case, the semiconductor switch 204 is not required.

The semiconductor switch 204 is generally formed using a semiconductor element made of a silicon-based material, but is not limited to this. The semiconductor switch 204 may be formed using a semiconductor element made of a wide band gap semiconductor. By using a switching element made of a wide band gap semiconductor, a device with lower loss can be configured.

Returning to FIG. 2, the common contact 21c of the switch 21 is connected to an external terminal 41d via a lead wire 71, the normally closed contact 21b is connected to the neutral node 24, and the normally open contact 21a is connected to the V-phase output line 62 of the inverter 4. The common contact 22c of the switch 22 is connected to an external terminal 42d via a lead wire 72, the normally closed contact 22b is connected to the neutral node 24, and the normally open contact 22a is connected to the W-phase output line 63 of the inverter 4. The common contact 23c of the switch 23 is connected to an external terminal 43d via a lead wire 73, the normally closed contact 23b is connected to the neutral node 24, and the normally open contact 23a is connected to the U-phase output line 61 of the inverter 4.

In Fig. 3, when no current flows through the excitation coils 211, 221, 231, the switches 21, 22, 23 are switched to the normally closed contact side as shown, i.e., the common contacts 21c, 22c, 23c are connected to the normally closed contacts 21b, 22b, 23b, as shown. In this state, the motor 1 is in a Y-connection state. When current flows through the excitation coils 211, 221, 231, the switches 21, 22, 23 are switched to the normally open contact side, as shown, i.e., the common contacts 21c, 22c, 23c are connected to the normally open contacts 21a, 22a, 23a, as shown. In this state, the motor 1 is in a Δ-connection state.

As explained above, when the connection selection signal Sc is a first value, for example Low, the motor 1 is in a Y-connection state. When the connection selection signal Sc is a second value, for example High, the motor 1 is in a Δ-connection state. Hereinafter, the control that switches the windings of the motor 1 between the Y-connection and the Δ-connection will be referred to as "connection switching control" as appropriate.

Next, the advantages of switching the connection state of the electric motor 1 to either the Y connection or the Δ connection will be explained with reference to Fig. 4. Fig. 4 is a diagram showing two connection states that can be switched in the electric motor 1 shown in Fig. 1.

Figure 4(a) shows the connection state when three windings are Y-connected, and Figure 4(b) shows the connection state when three windings are Δ-connected. Let the line voltage in Y-connection be VY and the current flowing in be _{IY ,} and let the line voltage in Δ-connection be _VΔ and the current flowing in be _IΔ , and assume that the voltages applied to the windings of each phase are equal to each other. In this case, the relationship between voltage _VY and voltage _VΔ is expressed by the following equation (1).

Moreover, the relationship between the current _IY and the current _IΔ satisfies the following formula (2).

When the voltage _VY and current _IY in the Y connection and the voltage _VΔ and current _IΔ in the Δ connection have the relationship of the above formulas (1) and (2), the power supplied to the motor 1 in the Y connection and the Δ connection are equal. In other words, when the powers supplied to the motor 1 are equal, the Δ connection requires a larger current to drive it and conversely requires a lower voltage to drive it.

If the motor 1 is a synchronous motor, as the rotation speed increases, that is, as the load increases, the back electromotive force increases and the voltage required for driving increases. As mentioned above, this back electromotive force is larger in the Y connection than in the Δ connection.

When the motor 1 is a permanent magnet motor, it is possible to reduce the magnetic force of the permanent magnet or the number of turns of the windings in order to suppress the back electromotive force at higher speeds. However, doing so increases the current required to obtain the same output torque, which increases the current flowing through the motor 1 and inverter 4 and reduces the efficiency of the device.

It is therefore possible to switch the connection depending on the rotation speed. For example, in high-load areas where high-speed operation is required, the connection state can be set to delta connection. This makes it possible to reduce the voltage required for drive to 1/√3 compared to Y connection. This eliminates the need to reduce the number of turns in the windings, and reduces the decrease in efficiency.

On the other hand, in the low-load range where operation at low speeds is possible, the wiring is in a Y-connection. This makes it possible to reduce the current value required for driving to 1/√3 compared to a Δ-connection. Also, since high-speed operation is no longer required in the Y-connection state, it is possible to design the windings in the Y-connection state so that they are suitable for driving at low speeds. This makes it possible to reduce the current value compared to when the Y-connection is used across the entire speed range. This makes it possible to reduce losses in the inverter 4 and increase the efficiency of the device.

From the above, by switching the wiring of motor 1 depending on the load conditions, it is possible to improve efficiency at low loads while also achieving high output at high loads.

The control unit 30 controls the boost converter 3 to change the bus voltage output from the boost converter 3. The control unit 30 also controls the inverter 4 to change the frequency and voltage value of the voltage applied to the motor 1. Furthermore, the control unit 30 controls the connection switching device 20 to switch the connection state of the motor 1 between Y connection and Δ connection.

In this document, the Y connection is an example of the first connection, and the Delta connection is an example of the second connection. As long as the relationship in formula (1) above holds between the voltages between the terminals of the first and second connections, and the relationship in formula (2) above holds between the currents flowing through the windings, connections other than the Y connection and Delta connection are also acceptable.

Figure 5 is a diagram showing an example of the detailed configuration of the control unit 30 according to embodiment 1. As shown in Figure 5, the control unit 30 can be configured to include an operation command unit 31, a bus voltage control unit 32, and an inverter control unit 33.

5 also shows a voltage detection unit 6 and current detection units 7 and 8. The voltage detection unit 6 detects the bus voltage Vdc output by the boost converter 3 and outputs the detected value to the bus voltage control unit 32. The current detection unit 7 detects the bus current Idc flowing between the boost converter 3 and the inverter 4 and outputs the detected value to the inverter control unit 33. The current detection unit 8 detects the motor current Im flowing through the motor 1 and outputs the detected value to the operation command unit 31.

The operation command unit 31 calculates a bus voltage command value Vdc ^* , a frequency command value ω ^* , a zero selection signal Sz, and the above-mentioned connection selection signal Sc. The bus voltage command value Vdc ^* is output to a bus voltage control unit 32, and the frequency command value ω ^* and the zero selection signal Sz are output to an inverter control unit 33. The operation command unit 31 refers to information on the motor current Im whenever necessary.

As described above, when the Y connection is selected, the connection selection signal Sc is controlled to a first value (e.g., Low), and when the Δ connection is selected, the connection selection signal Sc is controlled to a second value (e.g., High).

The zero selection signal Sz is normally controlled to a first value (e.g., Low) and is controlled to a second value (e.g., High) during the zero current control period described below.

The operation command unit 31 determines, for example, whether the windings of the motor 1 are to be Y-connected or Δ-connected, and determines the target rotation speed, and outputs a connection selection signal Sc and a frequency command value ω ^* generated based on this determination to the connection switching device 20 and the inverter control unit 33, respectively.

Here, consider a case where the device 5 connected to the motor 1 is, for example, an air conditioner. In the air conditioner, the control unit 30 determines to use a Δ connection when the difference between the room temperature and the set temperature is large, and sets the connection selection signal Sc to a second value. The operation command unit 31 sets the target rotation speed to a relatively high value, and generates a frequency command value ω ^* that gradually increases the frequency to the set target rotation speed after startup. When the frequency corresponding to the target rotation speed is reached, the operation command unit 31 maintains that state until the room temperature approaches the set temperature, and when the room temperature approaches the set temperature, sets the connection selection signal Sc to a first value and switches to a Y connection. After that, the operation command unit 31 performs control to maintain the room temperature close to the set temperature. Note that these controls include frequency adjustment, stopping and restarting the motor 1, etc.

In addition, in order to switch from one of the Y connection and the Δ connection to the other, the operation command unit 31 changes the value of the connection selection signal Sc, and temporarily changes the values of the frequency command value ω ^* and the zero selection signal Sz during the switching operation.

For example, when switching the connection state, the operation command unit 31 temporarily sets the bus voltage command value Vdc ^* and the frequency command value ω ^* to larger values. Then, during a period in which the bus voltage command value Vdc ^* and the frequency command value ω ^* are set to the larger values, the zero selection signal Sz is temporarily set to a second value (e.g., High) from a first value (e.g., Low). Then, during a period in which the zero selection signal Sz is at the second value, the connection selection signal Sc is switched from the second value to the first value, or from the first value to the second value.

The bus voltage control unit 32 generates a drive signal Xn based on the bus voltage command value Vdc ^* and the detection value of the bus voltage Vdc, and outputs the drive signal Xn to the boost converter 3. The boost converter 3 operates a switching element (not shown) in the boost converter 3 in accordance with the drive signal Xn so that the bus voltage Vdc coincides with the bus voltage command value Vdc ^* .

The inverter control unit 33 generates a pulse width modulation (PWM) signal Sm based on the detected values of the bus voltage Vdc and the bus current Idc, and outputs the signal to the inverter 4. The inverter 4 drives the motor 1 by maintaining or changing the frequency and voltage value of the output voltage according to the PWM signal Sm.

Next, the operation of the drive device 100 when the connection switching device 20 is operated while the motor 1 is running will be described. This operation includes zero current control, which is a characteristic control in embodiment 1. Zero current control is a control that switches the winding connections while the current of the motor 1 is set to zero.

As described above, when switching the connection state of the motor 1, the switches 21, 22, and 23 of the connection switching device 20 are operated. In this operation, the switches 21, 22, and 23 switch the connection of the common contacts 21c, 22c, and 23c between the normally closed contacts 21b, 22b, and 23b and the normally open contacts 21a, 22a, and 23a. If this switching operation is performed while the motor 1 is in operation, i.e., while power is being supplied from the inverter 4 to the motor 1, an arc discharge will occur between the contacts of the switches 21, 22, and 23, which may cause a failure such as contact welding.

To avoid such a failure, it is possible to stop the power supply from the inverter 4 to the motor 1 before operating the wiring switching device 20, and to perform the switching when the rotational speed of the motor 1 is zero.

However, if the rotation speed of the motor 1 is set to zero, the torque required for restarting increases, which may increase the current required at startup or make it impossible to restart. For example, if the device 5 driven by the motor 1 is an air conditioner, the torque required for restarting increases immediately after the rotation speed is set to zero to drive the compressor, because the state of the refrigerant is not stable. It is also possible to restart the motor 1 after the rotation speed is set to zero and the time required for the state of the refrigerant to be sufficiently stabilized has elapsed. In that case, the compressor will no longer be able to pressurize the refrigerant, and there is a risk that the room temperature will deviate significantly from the desired temperature due to a decrease in cooling and heating capacity.

Therefore, the current flowing through the connection switching device 20 is controlled to be zero, and the connection switching device 20 is caused to perform switching operation in this state. This control is called zero current control. By using zero current control, it is possible to prevent arc discharge from occurring between the contacts of the switches 21, 22, and 23 when switching the connection state. Furthermore, by using zero current control, it is no longer necessary to set the rotation speed of the motor 1 to zero each time the connection state is switched.

To make the current flowing through the connection switching device 20 zero, the current flowing through the motor 1 is detected and the inverter 4 is controlled to switch to zero by the switching operation. Alternatively, the current is interrupted by stopping the switching operation of the inverter 4. This can also be achieved by using both of these methods in combination.

However, when implementing the above-mentioned zero current control, the following points must be noted. First, when switching the winding connections of the motor 1, it is necessary to maintain the zero current state for a certain amount of time. During the zero current control period, the output torque of the motor 1 is zero, and the rotation speed decreases in proportion to the magnitude of the load torque applied to the motor 1. Also, the longer the period of zero current control and the greater the load torque, the greater the decrease in rotation speed. Furthermore, if zero current control is started at low speed, the rotation speed may decrease to near zero, causing the motor 1 to step out.

Regarding the decrease in the rotational speed of electric motor 1, in order to prevent the rotational speed from dropping to near zero, it is possible to simply increase the rotational speed of electric motor 1 and perform zero current control in that state.

However, as the rotation speed of the motor 1 increases, the back electromotive force generated in the motor 1 increases, and a voltage equal to or greater than the back electromotive force must be output from the inverter 4. On the other hand, the voltage that can be output from the inverter 4 is restricted by the bus voltage, which is the output voltage of the boost converter 3. The region where the output voltage from the inverter 4 exceeds the upper limit limited by the bus voltage and becomes saturated is called the "overmodulation region."

In order to suppress the back electromotive force of the motor 1 in the overmodulation region, it is necessary to perform the well-known flux-weakening control, which reduces the magnetic flux generated by the motor 1, which is the source of the back electromotive force. On the other hand, in the flux-weakening control, a negative d-axis current must be passed. Therefore, flux-weakening control and zero current control cannot be achieved at the same time. In addition, when the switching operation of the inverter 4 is stopped, the voltage of the motor 1 increases due to the back electromotive force, and when the voltage of the motor 1 exceeds the bus voltage, a regenerative current is generated from the motor 1 to the inverter 4, causing the inverter 4 to enter an overvoltage state.

Therefore, in embodiment 1, before performing zero current control, boost control is performed to increase the bus voltage using the boost converter 3. This boost control makes the bus voltage higher than the voltage of the motor 1 due to the back electromotive force. Zero current control is then performed in this state to switch the connections. Then, once the connection switching is complete, the bus voltage is returned to its original state. These controls make it possible to switch the connections smoothly, even in high load areas.

Figure 6 is a first time chart for explaining the control sequence in the connection switching control of embodiment 1. In Figure 6, switching from Y connection to Δ connection is assumed. In order to avoid complicating the explanation, Figure 6 does not show a control sequence for avoiding transition to the overmodulation region. The control sequence for avoiding transition to the overmodulation region will be explained with reference to Figures 7 and 8 described later.

Fig. 6(a) shows the current flowing through the connection switching device 20. Fig. 6(b) shows the change in the zero selection signal Sz. Fig. 6(c) shows the change in the connection selection signal Sc. Fig. 6(d) shows the change in the bus voltage command value Vdc ^* . Fig. 6(e) shows the change in the frequency command value ω ^* .

As shown in Fig. 6, before the connection is switched, the bus voltage command value Vdc ^* and the frequency command value ω* are temporarily increased, the zero current control is performed while the bus voltage command value Vdc* and the frequency command value ω ^* are being increased, and the connection is switched while the zero current control is being performed. This will be described in more detail below.

First, before the start of the switching process, it is assumed that the electric motor 1 is driven in a state in which the bus voltage command value Vdc ^* is the first voltage value Vdc ^* (0) and the frequency command value ω ^* is the first frequency value ω ^* (0).

After the switching process starts, in the period from time ta1 to time ta2, the bus voltage command value Vdc ^* is set to a second voltage value Vdc ^* (1) that is greater than the first voltage value Vdc ^* (0) (FIG. 6(d)). This causes the boost converter 3 to increase the bus voltage Vdc.

At time ta2, when the increase in the bus voltage Vdc is completed, the frequency command value ω ^* is set to the second frequency value ω ^* (1) greater than the first frequency value ω ^* (0) during the period from time tb1 to time tb2 ( FIG. 6( e )). This causes the frequency ω to increase.

Then, in the period from time tc to time te, the zero selection signal Sz is set to High (FIG. 6(b)), thereby performing zero current control by selecting zero as the current command value (FIG. 6(a)). Also, at time td during the zero current control period, the connection selection signal Sc is changed from Low to High (FIG. 6(c)), thereby switching the contacts of the connection switching device 20.

When the switching of the contacts of the connection switching device 20 is completed, the frequency command value ω ^* is returned to the original value, the first frequency ω ^* (0), during the period from time tf1 to time tf2 (FIG. 6(e)). Then, when the rotation speed decreases, the bus voltage command value Vdc ^* is returned to the original value, the first voltage value Vdc ^* (0), during the period from time tg1 to time tg2 (FIG. 6(d)).

In the above description, it is assumed that there is no delay in the bus voltage Vdc relative to the bus voltage command value Vdc ^* , and no delay in the frequency ω relative to the frequency command value ω ^* . When it is necessary to take into account the delay in operation, the connection switching device 20 may be switched during a period in which the bus voltage Vdc and the frequency ω are greater than one another. During the zero current control period, the motor 1 generates no torque, and the speed of the motor 1 decreases in proportion to the magnitude of the load torque applied to the motor 1. For this reason, in anticipation of this speed decrease, the frequency command value ω ^* at the time te is set to a frequency value lower than the second frequency value ω ^* (1). Depending on the magnitude of the load torque, it is also assumed that the frequency command value ω ^* at the time te may be smaller than the first frequency value ω ^* (0). In such a case, a frequency command value ω ^* is given that accelerates the motor to the first frequency value ω ^* (0).

In Figure 6, switching from a Y connection to a Δ connection is assumed, but switching from a Δ connection to a Y connection can also be performed using a similar sequence. However, when switching from a Δ connection to a Y connection, the connection selection signal Sc in Figure 6(c) switches from High to Low, not from Low to High.

Note that the sequence in Figure 6 is just an example, and sequences other than that in Figure 6 are also acceptable. However, when performing zero current control in the high speed rotation range, it is necessary to increase the bus voltage Vdc to increase the voltage that can be output from the inverter 4 before driving the motor 1.

Next, a description will be given of the set value of the bus voltage command value Vdc ^* for the boost converter 3 during the above-mentioned zero current control. The motor 1 may be any type of motor, but a permanent magnet synchronous motor will be taken as an example for description here.

First, the voltage equations on the dq coordinate axes of a permanent magnet synchronous motor are expressed by the following equations (3) and (4).

In the above equations (3) and (4), Vd and Vq represent the dq-axis components of the armature voltage, and id and iq represent the dq-axis components of the armature current. Ld and Lq represent the dq-axis inductances, and Ra represents the armature winding resistance. Φa represents the armature flux linkage, and p represents the differential operator. ω is the rotational speed expressed in electrical angles.

Here, in the above equations (3) and (4), id = iq = 0 due to the aforementioned zero current control. Also, as mentioned above, the zero current control continues from time tc to time te, so p·id = p·iq = 0. Therefore, the following equation (5) holds from the above equation (4).

As mentioned above, when performing zero current control, it is necessary to boost the bus voltage Vdc. The condition of the bus voltage Vdc can be expressed by the following equation (6) based on the above equation (5).

The value of the armature flux linkage Φa varies depending on the connection state, but it is necessary to use the larger of the values before and after the connection switching. For example, the value of the armature flux linkage Φa in the Y connection is √3 times the value of the armature flux linkage Φa in the Δ connection. Here, for example, when switching from the Δ connection to the Y connection, consider setting the bus voltage Vdc using the value of the Δ connection as the armature flux linkage Φa in the above formula (6). After setting in this way, if the speed reduction of the motor 1 during the zero current control period is small, it is assumed that the armature voltage due to the back electromotive force of the motor 1 will be larger than the set value of the bus voltage Vdc. Therefore, immediately after switching from the Δ connection to the Y connection, the armature voltage due to the back electromotive force of the motor 1 will exceed the voltage that can be output from the inverter 4, and there is a risk of an overvoltage occurring in the inverter 4.

Therefore, when the connection state of the motor 1 is switched between the Y connection and the Δ connection, the value of the armature interlinkage magnetic flux Φa in the Y connection is used. Also, in order to suppress overvoltage, the frequency command value ω ^* when switching the connection is selected so that the voltage of the motor 1 after the connection switching is smaller than the maximum output voltage of the boost converter 3. Also, in order to suppress overvoltage, an upper limit is set on the rotation speed when switching the connection of the motor 1 based on the bus voltage Vdc.

Next, the connection switching control that takes into account the load state at the time of connection switching will be described with reference to Figures 7 and 8. Figure 7 is a second time chart for explaining the control sequence in the connection switching control of embodiment 1. Figure 8 is a flowchart showing an example of a processing flow for implementing the control sequence of Figure 7. The connection switching control that takes into account the load state here is a control that switches the connection state of the motor 1 without entering the overmodulation region when switching the connection state of the motor 1. As mentioned above, flux weakening control and zero current control in the overmodulation region cannot be achieved at the same time. By using the control sequence of Figure 7 and the flowchart of Figure 8, it is possible to switch the connection state of the motor 1 without entering the overmodulation region.

Fig. 7 shows the switching operation from the Δ connection to the Y connection. Specifically, Fig. 7(a) shows the change in the rotation speed of the motor 1. Fig. 7(b) shows the change in the armature voltage together with the bus voltage Vdc. Fig. 7(c) shows the change in the connection selection signal Sc. Fig. 7(d) shows the change in the bus voltage command value Vdc ^* .

First, as a preparation operation for connection switching, the bus voltage command value Vdc ^* is changed to the bus voltage command maximum value _{Vdc_max} ^* corresponding to the maximum output voltage of the boost converter 3, and the bus voltage Vdc is boosted (FIG. 7(d)).

After the boost of the bus voltage Vdc is completed, the control unit 30 judges whether or not it is in the overmodulation region (FIG. 8: step S500). If it is in the overmodulation region at this point, it is necessary to perform flux-weakening control, and the current of the motor 1 cannot be controlled to zero. In addition, when the switching operation of the inverter 4 is stopped, a regenerative current is generated from the motor 1 to the inverter 4, resulting in an overvoltage state.

Therefore, if the overmodulation region has been entered (FIG. 8: step S500, No), the control unit 30 performs control to lower the frequency command value ω ^* (FIG. 8: step S501). By lowering the frequency command value ω ^*, the armature voltage of the motor 1 is lowered, and thereafter, it is determined again whether the overmodulation region has been entered (FIG. 8: step S500).

When it is determined that the motor is not in the overmodulation region (FIG. 8: step S500, Yes), the control unit 30 acquires information on the rotation speed and the load torque of the motor 1 before the zero current control period (FIG. 8: step S502). The load torque information can be acquired, for example, by estimating the rotation speed by using the frequency command value ω ^* assuming that the rotation speed follows the frequency command value ω ^* , or by using a well-known speed sensorless technique. Specifically, the load torque Tload of the motor 1 can be calculated using the following formula (7).

In the above equation (7), Pn represents the number of pole pairs of motor 1.

The control unit 30 calculates the amount of rotational speed drop during the zero current control period (FIG. 8: step S503). The amount of rotational speed drop is the amount of rotational speed drop during the zero current control period, and can be obtained from the calculated value of the load torque Tload before the zero current control period. The control unit 30 determines whether the rotational speed before the zero current control period exceeds the upper limit of the rotational speed (FIG. 8: step S504).

The amount of rotational speed drop during the zero current control period due to the load torque Tload is represented by Δω. Note that in this paper, the rotational speed may also be represented by the symbol ω.

The upper limit of the rotation speed taking into consideration the amount of decrease in the rotation speed Δω is represented as β _Tmax . This upper limit of the rotation speed β _Tmax is represented by the following formula (8).

In the above formula (8), _{Vdc_max} represents the maximum value of the output voltage of the boost converter 3, i.e., the maximum value of the bus voltage Vdc, and Φy represents the armature flux linkage in the Y-connection. The rotation speed drop amount Δω depends on the load torque Tload. Therefore, the upper limit value _βTmax of the rotation speed varies depending on the load condition of the motor 1.

During the zero current control period, the most severe condition occurs when no load torque is applied to the motor 1. The condition in this case can be expressed by the following equation (9).

In the above formula (9), β _max is the upper limit of the rotation speed when the rotation speed drop amount Δω is not taken into consideration.

If the rotation speed before the zero current control period exceeds the upper limit of the rotation speed (FIG. 8: step S504, No), the control unit 30 lowers the frequency command value ω ^* (FIG. 8: step S505), and again acquires information on the rotation speed and the load torque Tload before the zero current control period (FIG. 8: step S502).Then, the control unit 30 calculates the rotation speed drop amount during the zero current control period (FIG. 8: step S503), and again performs the determination process of step S504.

On the other hand, if the rotation speed before the zero current control period does not exceed the upper limit of the rotation speed (Figure 8: step S504, Yes), the control unit 30 switches the wiring (Figure 8: step S506).

As described above, the processes of steps S502 to S505 are repeated until the judgment process of step S504 becomes "Yes". Through these processes, as shown in FIG. 7(a), the rotation speed immediately before and after the zero current control period and during the zero current control period does not exceed the upper limit of the rotation speed. This prevents the motor from entering the overmodulation region when the wiring is switched.

As described above, the rotation speed drop amount Δω depends on the load torque Tload, so if the control unit 30 stores a table of the rotation speed drop amount Δω corresponding to the load torque Tload, the upper limit value _βTmax of the rotation speed can be easily calculated.

By using the above sequence to switch wiring, it is possible to switch wiring with the current at zero without entering the overmodulation region.

In the process flow of Fig. 8, the upper limit value _βTmax of the rotation speed is set as a constraint condition. On the other hand, if the connection state is switched when the rotation speed is low, the rotation speed may decrease during the zero current control period, and the rotation speed may drop to near zero, causing the motor 1 to step out. In addition, if the rotation speed of the motor 1 drops to near zero, it may be necessary to restart the motor 1. For this reason, the rotation speed is also restricted on the lower limit side.

Therefore, in the first embodiment, β _min shown in the following formula (10) is set as the lower limit of the rotation speed.

In the above formula (10), T _mmax represents the maximum load torque, J _m represents the inertia around the rotating shaft of the motor 1, and T ₀ [sec] represents the length of the zero current control period, i.e., the zero current control time. In practice, it is preferable to add a minimum guaranteed rotation speed ω _min as a margin to the lower limit value β _min of the rotation speed to ensure that the motor 1 does not lose synchronism. The formula in this case is expressed by the following formula (11).

In the control sequence of FIG. 7, the bus voltage command value Vdc ^* is set to the bus voltage command maximum value _{Vdc_max} ^* corresponding to the maximum output voltage of the boost converter 3, and the bus voltage Vdc is boosted to perform the connection switching. Instead of this, a sequence may be inserted in which the boost amount of the boost converter 3 is gradually increased so that the bus voltage command value Vdc ^* becomes a value according to the rotation speed. When performing this process, it is preferable to add a minimum guaranteed rotation speed _ωmin as a margin to the lower limit value _βmin of the rotation speed to provide an operation guarantee to the extent that the motor 1 does not lose synchronism. The formula in this case is expressed by the above formula (11). The boost amount of the boost converter 3 is increased so that the upper limit value _βmax of the above formula (9) does not fall below the lower limit value _βmin of the above formula (11). Furthermore, if the upper limit value β _max in the above equation (9) does not exceed the lower limit value β _min in the above equation (11) even when the output voltage of the boost converter 3 is increased to its maximum, the connection switching may be performed in a sequence to lower the rotation speed.

In this way, the frequency command value ω*(1) at the start time tc of the zero current control is set based on the armature interlinkage magnetic flux ^Φy in the Y connection and the upper limit value β _Tmax of the rotation speed and the lower limit value β _min of the rotation speed, which are determined by the load torque Tload of the motor 1 (see FIG. 6(e)). However, if the rotation speed drop amount Δω due to the maximum load torque Tmmax exceeds the difference between the upper limit value β _Tmax of the rotation speed and the lower limit value β _min of the rotation speed during the zero current control period, it is necessary to limit the load torque condition range at the time of switching the connection. Note that the example shown in FIG. 8(a) is an example in which the rotation speed drop amount Δω due to the maximum load torque Tmmax falls within the range of the difference between the upper limit value β _Tmax of the rotation speed and the lower limit value β _min of the rotation speed.

As described above, by using the connection switching control according to the first embodiment, it is possible to reliably perform the zero current control required for suppressing arcs between the contacts of the switches 21, 22, and 23 when switching the connections of the electric motor 1. Furthermore, by using the connection switching control according to the first embodiment, it is possible to reliably suppress overvoltage that may occur in the internal circuit of the drive device 100, regardless of the load state at the time of the connection switching.

The above-mentioned functions of the control unit 30 can be realized by the hardware configuration shown in Fig. 9 or Fig. 10. Fig. 9 is a block diagram showing an example of a hardware configuration that realizes the functions of the control unit 30 according to embodiment 1. Fig. 10 is a block diagram showing another example of a hardware configuration that realizes the functions of the control unit 30 according to embodiment 1.

When realizing some or all of the functions of the control unit 30 in embodiment 1, the configuration can include a processor 300 that performs calculations, a memory 302 in which programs read by the processor 300 are stored, and an interface 304 that inputs and outputs signals, as shown in Figure 9.

The processor 300 is an example of a computing means called a computing device, a microprocessor, a microcomputer, a CPU (Central Processing Unit), or a DSP (Digital Signal Processor). Examples of the memory 302 include a RAM (Random Access Memory), a ROM (Read Only Memory), a flash memory, an EPROM (Erasable Programmable ROM), an EEPROM (Electrically EPROM), or other non-volatile or volatile semiconductor memory, a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, and a DVD (Digital Versatile Disc).

The memory 302 stores a program that executes the functions of the control unit 30 according to the first embodiment. The processor 300 exchanges necessary information via the interface 304, executes the program stored in the memory 302, and refers to the table stored in the memory 302, thereby performing the above-mentioned processing. The results of calculations performed by the processor 300 can be stored in the memory 302.

In addition, when realizing part of the functions of the control unit 30 according to embodiment 1, the processing circuit 305 shown in FIG. 10 can be used. The processing circuit 305 can be a single circuit, a composite circuit, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination of these. Information input to the processing circuit 305 and information output from the processing circuit 305 can be obtained via the interface 304.

In addition, some of the processing in the control unit 30 may be performed by the processing circuit 305, and processing that is not performed by the processing circuit 305 may be performed by the processor 300 and memory 302.

As described above, the drive device according to the first embodiment includes a boost converter, an inverter, a connection switching device that switches the windings of the motor between the first connection and the second connection, and a control unit that controls the boost converter, the inverter, and the connection switching device. The control unit performs zero current control that switches the winding connections with the motor current at zero. In addition, when the second connection is selected by the connection switching device, the control unit switches to the first connection if the rotation speed of the motor before the zero current control period does not exceed the upper limit of the rotation speed. This makes it possible to switch the winding connections even while the motor is rotating. In addition, it is possible to suppress overvoltage that may occur in the internal circuit of the drive device regardless of the load state at the time of connection switching.

In addition, in the drive device according to embodiment 1, when the first connection is selected by the connection switching device and the state transitions from a first state in which the bus voltage is a first voltage value and the rotation speed of the motor is a first speed value to a second state in which the bus voltage is higher than the first voltage value and the current flowing through the motor is equal to or lower than a predetermined threshold, the control unit switches from the first connection to the second connection. This makes it possible to avoid enlarging the size of the device and obtain a highly reliable drive device.

Embodiment 2.
Fig. 11 is a diagram showing a configuration example of a refrigeration cycle apparatus 900 according to embodiment 2. The refrigeration cycle apparatus 900 according to embodiment 2 includes the drive device 100 described in embodiment 1. The refrigeration cycle apparatus 900 according to embodiment 2 can be applied to products including a refrigeration cycle, such as air conditioners, refrigerators, freezers, and heat pump water heaters. In Fig. 11, components having the same functions as those in embodiment 1 are denoted by the same reference numerals as those in embodiment 1.

The refrigeration cycle device 900 includes a compressor 901 incorporating the electric motor 1 in embodiment 1, a four-way valve 902, an indoor heat exchanger 906, an expansion valve 908, and an outdoor heat exchanger 910, which are attached via refrigerant piping 912.

Inside the compressor 901, there is a compression mechanism 904 that compresses the refrigerant, and an electric motor 1 that operates the compression mechanism 904.

The refrigeration cycle device 900 can perform heating or cooling operation by switching the four-way valve 902. The compression mechanism 904 is driven by a variable speed controlled electric motor 1.

During heating operation, as indicated by the solid arrows, the refrigerant is pressurized by the compression mechanism 904 and sent out, and passes through the four-way valve 902, the indoor heat exchanger 906, the expansion valve 908, the outdoor heat exchanger 910 and the four-way valve 902 before returning to the compression mechanism 904.

During cooling operation, as indicated by the dashed arrow, the refrigerant is pressurized by the compression mechanism 904 and sent out, and passes through the four-way valve 902, the outdoor heat exchanger 910, the expansion valve 908, the indoor heat exchanger 906 and the four-way valve 902 before returning to the compression mechanism 904.

During heating operation, the indoor heat exchanger 906 acts as a condenser to release heat, and the outdoor heat exchanger 910 acts as an evaporator to absorb heat. During cooling operation, the outdoor heat exchanger 910 acts as a condenser to release heat, and the indoor heat exchanger 906 acts as an evaporator to absorb heat. The expansion valve 908 reduces the pressure of the refrigerant to expand it.

The refrigeration cycle device 900 of embodiment 2 is equipped with the drive device 100 of embodiment 1, and therefore can enjoy the effects obtained in embodiment 1.

Note that the configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, and parts of the configurations may be omitted or modified without departing from the spirit of the invention.

1 Electric motor, 2 AC power source, 3 Boost converter, 4 Inverter, 5 Equipment, 6 Voltage detection unit, 7, 8 Current detection unit, 20 Connection switching device, 21, 22, 23 Switch, 21a, 22a, 23a Normally open contact, 21b, 22b, 23b Normally closed contact, 21c, 22c, 23c Common contact, 24 Neutral node, 30 Control unit, 31 Operation command unit, 32 Bus voltage control unit, 33 Inverter control unit, 41, 42, 43 Winding, 41a, 42a, 43a First end, 41b, 42b, 43b Second end, 41c, 42c, 43c, 41d, 42d, 43d External terminal, 61, 62, 63 Output line, 71, 72, 73 Lead wire, 100 Drive device, 204 semiconductor switch, 211, 221, 231 excitation coil, 300 processor, 302 memory, 304 interface, 305 processing circuit, 900 refrigeration cycle device, 901 compressor, 902 four-way valve, 904 compression mechanism, 906 indoor heat exchanger, 908 expansion valve, 910 outdoor heat exchanger, 912 refrigerant piping.

Claims (11)

a boost converter that receives an AC voltage output from an AC power supply and outputs a DC voltage having a variable voltage value;
an inverter that converts the bus voltage output from the boost converter into an AC voltage having a variable voltage and a variable frequency and applies the AC voltage to an electric motor;
a connection switching device that switches the windings of the motor between a first connection and a second connection;
a control unit that controls the boost converter, the inverter, and the connection switching device;
Equipped with
the control unit performs zero current control to switch the connections of the windings while keeping the current of the motor at zero,
a drive device which switches to the first connection when the rotation speed of the motor before a zero current control period does not exceed an upper limit of the rotation speed in a state in which the second connection is selected by the connection switching device. The drive device according to claim 1 , wherein the winding connections are switched with an upper limit set on the bus voltage. 2. The drive device according to claim 1, wherein, when the first connection is selected by the connection switching device, the control unit switches from the first connection to the second connection when a transition is made from a first state in which the bus voltage is a first voltage value and a rotational speed of the motor is a first speed value to a second state in which the bus voltage is higher than the first voltage value and a current flowing through the motor is equal to or lower than a predetermined threshold value. The drive device according to claim 3 , wherein the first connection is a star connection and the second connection is a delta connection. A detection unit is provided for detecting a motor current flowing through the motor,
The drive device according to claim 3 , wherein the control unit estimates a rotation speed and also estimates a load torque applied to the motor based on a detected value of the motor current. the motor is a permanent magnet motor,
The maximum value of the bus voltage when the switching is performed is _{Vdc_max} ,
The rotation speed of the motor when the switching is performed is denoted by ω,
When the armature interlinkage magnetic flux of the permanent magnet motor when the switching is performed is denoted by Φy,
The drive device according to claim 5 , wherein the following formula (1) is satisfied before the transition to the second state:

the motor is a permanent magnet motor,
The maximum value of the bus voltage when the switching is performed is _{Vdc_max} ,
The rotation speed of the motor when the switching is performed is denoted by ω,
The armature flux linkage of the permanent magnet motor when the switching is performed is defined as Φy,
When the rotation speed drop amount when the switching is performed is _Δω ,
The drive device according to claim 5 , wherein the following formula (2) is satisfied before the transition to the second state:

The minimum guaranteed operating speed when the switching is performed is defined as ω _min ,
The inertia around the rotating shaft of the motor when the switching is performed is defined as _Jm ,
The maximum load torque when the switching is performed is T _mmax ,
When the zero current control period when the switching is performed is _T0 ,
The drive device according to claim 6 , wherein the following formula (3) is satisfied before the transition to the second state:

The bus voltage is Vdc,
The maximum value of the bus voltage when the switching is performed is _{Vdc_max} ,
The rotation speed of the motor when the switching is performed is denoted by ω,
The armature flux linkage of the permanent magnet motor when the switching is performed is defined as Φy,
When the rotation speed drop amount when the switching is performed is _Δω ,
When the rotation speed ω in the first state does not satisfy the following formula (4), the boost converter boosts the bus voltage Vdc,
The drive device according to claim 6 , wherein the rotation speed is reduced when the following formula (4) is not satisfied even when the bus voltage Vdc is boosted up to a maximum value _{Vdc_max} of the bus voltage.

The drive device according to claim 7 , wherein the rotation speed drop amount is calculated based on the load torque before the zero current control period. A refrigeration cycle device equipped with a drive device according to any one of claims 1 to 10.

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