JP7258235B2 - Drive system and air conditioner - Google Patents

Description

The present disclosure relates to drive devices and air conditioners. The present disclosure particularly relates to a drive device for driving a motor whose connection state is switchable by an inverter, and an air conditioner provided with this drive device.

Conventionally, in order to expand the rotation speed range (and therefore the output range) of the motor, the connection state of the stator windings (hereinafter also simply referred to as "windings") of the motor has been divided into star connection (hereinafter referred to as Y connection) and delta connection. (hereinafter referred to as delta connection).

At the time of switching, it is important to reduce the shock applied to the motor at the time of switching. For example, whether or not the permanent magnet motor is accelerating or decelerating is determined based on the output of the rotation speed sensor. A technique for reducing the shock caused by switching connections by performing this switching has been disclosed (for example, Patent Document 1).

JP-A-6-225588 (pages 2-3)

In the technique described in Patent Literature 1, there is a problem that the changeover switch of the connection switching device may be damaged if the connection is switched while current is flowing through the connection switching device. To avoid the damage described above, it is desirable to switch while the motor current is controlled to zero. However, there is a problem that the output torque that makes the current of the motor zero becomes zero, the speed of the motor decreases, the motor loses synchronism, and the motor stops.

The present disclosure is a drive device that drives a motor with an inverter that can switch the connection state by a connection switching device, and does not damage the connection switching device and prevents loss of synchronism, stoppage, etc. due to a decrease in the speed of the motor. An object of the present invention is to provide a driving device that can prevent

A driving device according to the present disclosure includes:
A driving device comprising: a connection switching device for switching a connection state of a motor connected to a load; and an inverter connected to a power source for outputting an AC voltage to the motor,
The connection state is switched during a period of zero current control for controlling the current of the motor to zero, and the speed of the motor at the start of the zero current control is equal to or greater than a threshold;
Letting the output current of the inverter be I1,
Let the torque constant of the motor be Kt,
The length of the period of the current zero control is T0,
Jm is the shaft inertia of the motor.
, the threshold ω0 is
ω0≧I1・Kt・T0/Jm
is determined to satisfy

According to the present disclosure, it is possible to prevent loss of synchronism, stoppage, etc. due to a decrease in motor speed without damaging the connection switching device.

1 is a schematic diagram showing a configuration of a driving device according to Embodiment 1; FIG. 2A and 2B are schematic diagrams showing different configuration examples of the power source of FIG. 1; FIG. 2 is a circuit diagram showing the connection switching device of FIG. 1 in more detail; FIG. 2 is a circuit diagram showing a modification of the connection switching device of FIG. 1; FIG. FIG. 2 is a circuit diagram showing details of an inverter in FIG. 1; FIG. 2 is a wiring diagram showing a computer that implements the functions of the control unit of FIG. 1 together with current detection means, an inverter, and a connection switching device; 4 is a time chart showing an example of operation of a switching control sequence in the driving device of Embodiment 1; 4 is a flow chart showing the operation of the control unit when a switching control sequence is performed in the driving device of Embodiment 1; 9 is a flow chart showing the operation of the control unit when the switching control sequence is performed in the driving device of the second embodiment; 10 is a time chart showing an example of the operation of a switching control sequence performed by the driving device of Embodiment 3; FIG. 11 is a flow chart showing the operation of a control unit when a switching control sequence is performed in the driving device of Embodiment 3; FIG. FIG. 11 is a flow chart showing the operation of a control unit when a switching control sequence is performed in the driving device of Embodiment 4; FIG. FIG. 11 is a schematic diagram showing a configuration example of an air conditioner according to Embodiment 5;

Embodiment 1.
FIG. 1 is a circuit diagram showing the configuration of a drive device 1 according to Embodiment 1. As shown in FIG.
The illustrated drive device 1 is for driving a motor 3 connected to a load 2 and has an inverter 4 , a connection switching device 5 , a current detection means 6 and a control section 7 .

The inverter 4 receives power from the power source 8 and outputs to the motor 3 a three-phase AC voltage with variable frequency and variable voltage. The frequency of the AC voltage is controlled so that the speed of the motor 3 has a desired value.
The connection switching device 5 switches the connection state of the motor 3 .

A current detection means 6 detects an output phase current of the inverter 4 . The current detection means 6 is composed of a current sensor such as ACCT or DCCT provided on wiring connecting the inverter 4 and the motor 3, for example. "ACCT" is an abbreviation for Alternating Current Current Transducer, and "DCCT" is an abbreviation for Direct Current Current Transducer.
The control unit 7 controls the inverter 4 and the connection switching device 5 based on the detection result of the current detection means 6 .

A motor 3 is connected to a load 2 via a motor shaft. Load 2 is, for example, a compression element of a compressor of an air conditioner.

The motor 3 is, for example, a three-phase permanent magnet synchronous motor, and both ends of windings 3u, 3v, and 3w of three phases u, v, and w are drawn out of the motor 3, and are Y-connected and Δ-connected. It is possible to switch the connection state between The switching is performed by the connection switching device 5 .

2A and 2B show configuration examples of the power source 8. FIG.
The power source 8 shown in FIG. 2A is an AC/DC power converter 82a having a converter 82aa that converts AC power supplied from a three-phase AC power supply 81a into DC power, a reactor 82ab, and a capacitor 82ac. It is configured.

The power source 8 shown in FIG. 2(b) is an AC/DC power converter having a well-known converter 82ba for converting AC power supplied from a single-phase AC power supply 81b into DC power, a reactor 82bb, and a capacitor 82bc. 82b.

Although not shown in FIGS. 2A and 2B, a booster circuit for boosting the DC voltage may be inserted on the output side of the AC/DC power converters 82a and 82b. The boost circuit may be, for example, a DC-DC converter.

The power source 8 may be a battery that directly supplies DC power instead of the one shown in FIGS. 2(a) and 2(b). Even when the power source 8 is composed of a battery, a booster circuit may be provided on the output side. Note that the booster circuit described above may be configured as a part of the drive device 1 . Furthermore, the AC/DC power converter 82a or 82b or the battery described above may also be configured as part of the driving device 1. FIG.

Output terminals 4u, 4v and 4w of inverter 4 are connected to first ends 3ua, 3va and 3wa of corresponding windings 3u, 3v and 3w, respectively.

The connection switching device 5 has switches 51u, 51v, and 51w. As the switches 51u, 51v, and 51w, for example, electromagnetic contactors that electromagnetically open and close contacts, such as relays and contactors, are used. An electromagnetic contactor is also called a mechanical relay.
For example, each of the switches 51u, 51v, and 51w may be configured by, for example, a c (switching) contact relay having a function of selecting one of two states.

The switches 51u, 51v, and 51w are controlled by a switching control signal Sc output from the control section 7. FIG. The switching control signal Sc is, for example, a signal that takes either a first state or a second state. When another connection state, eg, the Δ connection state, should be selected, the second state, eg, the H state, is selected.

The switches 51u, 51v, and 51w, which are c (switching) contact relays, have three terminals, a common terminal COM, a normally closed terminal NC, and a normally open terminal NO, as shown in FIG.
Switches 51u, 51v, and 51w are provided corresponding to the windings 3u, 3v, and 3w, respectively, and the common terminal COM of each switch is connected to the second end of the corresponding winding, and normally closed. Terminal NC is connected to neutral node 52, and normally open terminal NO is connected to the first end of the next phase winding of the corresponding winding. Here, it is assumed that the phase order is u, v, and w, and for example, the phases following the u, v, and w phases are the v, w, and u phases, respectively.

When the switches 51u, 51v, 51w are in the OFF state, that is, when the normally closed terminal NC is connected to the common terminal COM, the second ends 3ub, 3vb, 3wb of the windings 3u, 3v, 3w are neutral. They are connected at a point node 52, and the motor 3 is in a Y-connected state.

When the switches 51u, 51v, and 51w are in the ON state, that is, when the normally open terminal NO is connected to the common terminal COM, the windings 3u, 3v, and 3w are connected to the first ends 3ua, 3va, and 3wa, respectively. are connected to the corresponding phase output terminals 4u, 4v, 4w of the inverter 4, and the second ends 3ub, 3vb, 3wb are connected to the following phase output terminals 4v, 4w, 4u of the inverter 4 corresponding phases. and the motor 3 is in the Δ connection state.

In the above example, the normally open terminal NO of each switch is connected to the first end of the next phase winding of the corresponding winding, and the normally closed terminal NC is connected to the neutral point node 52. However, the configuration is such that the normally open terminal NO of each switch is connected to the neutral point node 52 and the normally closed terminal NC is connected to the first end of the next phase winding of the corresponding winding. Also good.

In the case of the c (switching) contact relay, excitation loss occurs when the normally open terminal NO is connected to the common terminal COM in the ON state. In order to reduce the excitation loss, the connection configuration of the connection switching device 5 may be determined based on which is longer, the time of operation in the Y connection state or the time of operation in the Δ connection state. For example, if the operation time in the Y-connected state is longer, it is preferable to set the motor 3 in the Y-connected state when the connection switching device 5 is in the OFF state.

In the configurations of FIGS. 1 and 3, a switch is used as each of the switches 51u, 51v, and 51w of the connection switching device 5. FIG. Alternatively, each switch may be configured by a combination of a normally closed switch and a normally open switch. FIG. 4 shows a configuration example of the connection switching device in that case.

In the configuration of FIG. 4, a combination of a normally open switch 51ua and a normally closed switch 51ub is used as a switch 51u, a combination of a normally open switch 51va and a normally closed switch 51vb is used as a switch 51v, and a switch 51w A combination of a normally open switch 51wa and a normally closed switch 51wb is used.
4, one terminals of the normally closed switches 51ub, 51vb, and 51wb are connected to the neutral point node 52. As shown in FIG.

As shown in the figure, the motor 3 is in the Y connection state when the normally closed switches 51ub, 51vb, and 51wb are closed (on) and the normally open switches 51ua, 51va, and 51wa are open (off). Contrary to the illustration, the motor 3 is in the delta connection state when the normally closed switches 51ub, 51vb and 51wb are open and the normally open switches 51ua, 51va and 51wa are closed.

One terminal of the normally open switches 51ua, 51va, and 51wa is connected to the neutral point node 52, the normally closed switches 51ub, 51vb, and 51wb are closed, and the normally open switches 51ua, 51va, and 51wa are open. The wiring may be such that the motor 3 is in the delta connection state, the normally closed switches 51ub, 51vb, and 51wb are open, and the normally open switches 51ua, 51va, and 51wa are closed, and the motor 3 is in the Y connection state.

As shown in FIG. 4, even when each switch is composed of a combination of a normally open switch and a normally closed switch, an electromagnetic contactor can be used as each switch. An electromagnetic contactor is preferable because it has a small conduction loss when it is turned on.

As shown in FIG. 4, when each switch is configured by a combination of a normally open switch and a normally closed switch, a semiconductor switch may be used as each switch. The semiconductor switch may be composed of a wide bandgap semiconductor (WBG semiconductor). Wide bandgap semiconductors (WBG semiconductors) may be silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga ₂ O ₃ ), diamond, and the like. A semiconductor switch made of a wide bandgap semiconductor (WBG semiconductor) has a low on-resistance, a low loss, and little element heat generation. They are also capable of rapid switching operations.

By using the connection switching device as described above, it is possible to switch the connection state of the motor 3 between the Y connection and the Δ connection.

Here, the advantage of using the motor 3 that can be switched between Y-connection and Δ-connection will be described.

The line-to-line voltage during Y-connection is _VY , the current flowing into the winding is _IY , the line-to-line voltage during Δ-connection is _VΔ , the current flowing into the winding _{is IΔ} , and the voltage applied to each phase winding. are equal to each other, there is a relationship of the following equations (1) and (2).
V _Δ =V _Y /√3 (1)
I _Δ =√3×I _Y (2)

When _the voltage VY and the current _IY in the Y connection and the voltage _VΔ and the current _IΔ in the Δ connection have the relationship of the formulas (1) and (2), the motor 3 are equal to each other. That is, when the electric power supplied to the motor 3 is equal to each other, the delta connection requires a larger current and a lower voltage for driving. It is conceivable to use this property to select the connection state according to load conditions and the like. For example, when the load is low, the Y-connection is used for low-speed operation, and when the load is high, the Δ-connection is used for high-speed operation. By doing so, it is possible to improve the efficiency under low load and increase the output under high load.

Three-phase permanent magnet synchronous motors are widely used, particularly when the motor 3 is used to drive a compressor of an air conditioner. In recent air conditioners, when the difference between the room temperature and the set temperature is large, the motor 3 is operated at high speed to quickly bring the room temperature closer to the set temperature, and when the room temperature is close to the set temperature, the motor 3 is operated at low speed. to maintain room temperature. When controlling in this way, the ratio of the time of low-speed operation to the total operation time is large.

When a three-phase permanent magnet synchronous motor is used, the back electromotive force increases as the rotation speed increases, and the voltage value required for driving increases. This back electromotive force is higher in the Y-connection than in the Δ-connection, as described above. In order to suppress the back electromotive force at high speed, it is conceivable to reduce the magnetic force of the permanent magnet or reduce the number of turns of the stator winding. However, if this is done, the current required to obtain the same output torque increases, so the current flowing through the motor 3 and the inverter 4 increases, resulting in a decrease in efficiency.

Therefore, it is conceivable to switch the connection state according to the rotation speed. For example, when high-speed operation is required, the Δ connection state is set. In this case, the voltage required for driving can be reduced to 1/√3 compared to the Y connection state, there is no need to reduce the number of winding turns, and the motor voltage rise is suppressed in the three-phase permanent magnet synchronous motor. It eliminates the need to use the well-known flux-weakening control for

On the other hand, at low speed rotation, the current value can be reduced to 1/√3 compared to the Δ connection by setting the Y connection state. Furthermore, it becomes possible to design the windings in the Y-connection state so as to be suitable for low-speed driving, and it becomes possible to reduce the current value compared to the case where the Y-connection is used over the entire speed range. . As a result, the loss of the inverter 4 can be reduced and the efficiency can be improved.

As described above, switching the connection state according to the load condition is effective, and in the present embodiment, the connection switching device 5 switches the connection state of the motor 3 .

The inverter 4 has an inverter main circuit 41 and a drive circuit 45, as shown in FIG. 8b.

The inverter main circuit 41 has six arms each including switching elements 411-416. Rectifying elements 421 to 426 for freewheeling are connected in antiparallel to the switching elements 411 to 416, respectively.

The drive circuit 45 generates drive signals Sr1 to Sr6 based on the PWM signals Sm1 to Sm6 output from the control unit 7, and controls the switching elements 411 to 416 to be turned on and off by the drive signals Sr1 to Sr6. , a three-phase AC voltage with a variable frequency and a variable voltage is applied to the motor 3 from the output terminals 4u, 4v, and 4w.

The switching elements 411 to 416 and the freewheeling rectifying elements 421 to 426 are silicon carbide SiC, gallium nitride GaN, and diamond, which are wide bandgap semiconductors capable of high withstand voltage and high temperature operation even if they are made of silicon Si. etc. may be used.

The control unit 7 can be partially or wholly configured with a processing circuit.
For example, various functions of the control unit 7 may be realized by separate processing circuits, or a plurality of functions may be collectively realized by one processing circuit.
The processing circuitry may be configured in hardware or software, ie, a programmed computer.
Some of the functions of the control unit 7 may be realized by hardware, and other parts may be realized by software.

FIG. 6 shows a computer 9 that realizes all the functions of the control section 7 together with the current detection means 6 , the inverter 4 and the connection switching device 5 .

In the illustrated example, the computer 9 has a processor 91 and a memory 92 .
The memory 92 stores programs for realizing various functions of the control unit 7 .

The processor 91 uses, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), a microprocessor, a microcontroller, or a DSP (Digital Signal Processor).

The memory 92 is, for example, a RAM (Random Access Memory), a ROM (Read Only Memory), a flash memory, an EPROM (Erasable Programmable Read Only Memory) or an EEPROM (Electrically Erasable Programmable Read Only Memory), a magnetic disk, a semiconductor memory such as a magnetic disk, and a semiconductor memory. Alternatively, a magneto-optical disk or the like is used.

The processor 91 implements the functions of the control unit 7 by executing programs stored in the memory 92 .
Functions of the control unit 7 include a function of controlling the inverter 4 and the connection switching device 5 based on the detection result of the current detection means 6 .

Although the computer of FIG. 6 includes a single processor, it may include two or more processors.

The control unit 7 calculates the voltage to be applied to the motor 3, generates a voltage command value, generates PWM signals Sm1 to Sm6 based on this voltage command value, and turns on/off each switching element of the inverter 4. to control.

The control unit 7 also generates a switching control signal Sc for selecting the connection state of the motor 3, and controls the ON/OFF operation of the switches 51u, 51v, and 51w of the connection switching device 5. , Y-connection and Δ-connection.

Connection switching is performed in response to a connection switching request. The connection switching request may be generated as a result of internal processing of the control unit 7 or may be generated by an external host control unit (not shown) and transmitted to the control unit 7 . Connection switching is performed when it is determined that switching is possible in the switching control sequence described below.

In the switching control sequence of the present embodiment, it is determined whether or not the speed of the motor 3 is equal to or higher than the threshold, and if the speed of the motor 3 is equal to or higher than the threshold, connection switching is executed. If the speed of the motor 3 is less than the threshold, connection switching is not performed.
Note that the threshold is a positive value, and determining whether the speed of the motor 3 is equal to or higher than the threshold means determining whether the absolute value of the speed is equal to or higher than the threshold.

Connection switching is performed in a state where current zero control is performed.
The current zero control is control to make the current flowing between the inverter 4 and the motor 3 zero. Here, zero is not limited to the state of being exactly zero, but also includes the state of being so close to zero that it can be regarded as being substantially zero.

In order to perform zero current control, for example, in the control unit 7, a current control system is inserted before the voltage command value generation unit so that zero can be selected as the current command value. Zero should be selected as the current command value. If zero is selected as the current command value for the current control system, a voltage command value for matching the current of the motor 3 to zero is generated. PWM signals Sm1-Sm6 are generated based on this voltage command value, and the current of the motor 3 can be converged to zero by controlling the ON/OFF operations of the switching elements 411-416 of the inverter 4. FIG.
The current control system described above can be realized by, for example, a well-known PI (proportional integral) control system.

The reason why current zero control is performed when switching the connection state is as follows.
During the switching operation of the connection switching device 5, the connection of the common terminal COM of the switches 51u, 51v, and 51w switches from the normally open terminal NO to the normally closed terminal NC or from the normally closed terminal NC to the normally open terminal NO.
When this switching operation is performed in a state in which a current is flowing between the inverter 4 and the motor 3, that is, in a state in which a current is flowing to the switches 51u, 51v, and 51w, the contacts of the switches 51u, 51v, and 51w An arc discharge may occur between them, which may cause failures such as contact welding.

In order to avoid such a failure, the current zero control is performed only for a certain period (current zero control period) T0 to keep the current flowing between the inverter 4 and the motor 3 at zero. Device 5 is switched. By doing so, switching can be performed without arc discharge occurring between the contacts of the switches 51u, 51v, and 51w.

However, in this embodiment, prior to starting the zero current control, it is determined whether the speed of the motor 3 is equal to or higher than the threshold, and if the speed of the motor 3 is equal to or higher than the threshold, zero current control and connection switching are executed. However, if the speed of the motor 3 is less than the threshold, zero current control and connection switching are not executed. Hereinafter, the speed of the motor 3 may be simply referred to as motor speed.

An example of the operation of the switching control sequence will be described below with reference to FIG.
Here, when the motor 3 is in normal operation, that is, in a state in which the motor 3 is controlled to a desired speed and the output torque of the motor 3 is balanced with the load torque of the load 2, time It is assumed that a switching request is generated at t1 and a switching control sequence is performed by the control unit 7 in response to this request. In FIG. 7, it is also assumed that the motor 3 normally operates at a constant speed until time t1 when the switching request is generated, and the motor speed ω1 at time t1 is equal to or higher than the threshold ω0. The period of normal operation is indicated by Tn.

In this case, at the start of the switching control sequence (time t1), it is confirmed that the motor speed ω1 is equal to or greater than the threshold ω0, the current zero control is started at time t2, the current converges to zero at time t3, and the current converges to zero at time t4. , the connection switching device 5 is instructed to switch by the switching control signal Sc (the state in which the switching control signal Sc specifies one connection state changes to the state in which the other connection state is specified), and the connection switching device 5 instructs switching. , the switching is completed at time t5, the current zero control ends at time t6, and the state of normal operation is restored.
A period from the start (t2) to the end (t6) of the zero current control is the zero current control period T0.

FIG. 7 also shows that the speed decreases from .omega.1 to .omega.n during the current zero control period T0, and that .omega.n is used as the initial value for restarting to return to normal operation.

Strictly speaking, there is a time difference (t2-t1) between the time t1 when the switching control sequence is started and the time t2 when the zero current control is started. is so small that it can be regarded as non-existent. That is, it may be considered that t1≈t2. Moreover, the speed change during this period may also be regarded as unchanged. That is, it may be considered that ω1≈ω2.

FIG. 7 shows the case where the motor speed ω1 at the start of the switching control sequence is greater than or equal to the threshold ω0. And connection switching is not performed.
The reason for restricting the motor speed ω1 in this way is to prevent loss of synchronism, stoppage, or the like due to a decrease in the motor speed during the period T0 of zero current control.

The length of the current zero control period T0 is the time from when the controller 7 issues an instruction to start the current zero control to when the current between the inverter 4 and the motor 3 actually converges to zero. (Current convergence time) Ta and the time (switching operation time) Tb from when the switching of the connection is instructed by the switching control signal Sc to when the switching operation of the switches 51u, 51v, and 51w is completed is set to be equal to or greater than the sum. There is a need.

The current convergence time Ta depends on the responsiveness of the control system of the controller 7 . In general, the control system is designed so that the current convergence time is about several hundred micro[s] to several milli[s].
A predetermined length of time may be used as the current convergence time Ta. That is, it may be considered that the current has converged to zero after a predetermined time has elapsed since the zero current control was started.

The switching operation (instruction to switch by the switching control signal Sc) is performed after the current converges to zero, for example, after time t1, by adding a predetermined margin Tc to the current convergence time Ta ( =Ta+Tc) has elapsed. Specifically, the switching instruction by the switching control signal Sc is performed when the above time Te has passed from time t1.

The switching operation time Tb varies depending on the types of the switches 51u, 51v, and 51w.
When the switches 51u, 51v, and 51w are composed of mechanical relays, the time required for switching is several hundred milliseconds, and when the switches 51u, 51v, and 51w are composed of semiconductor switches, the time required for switching is several millimeters [s].
A predetermined length of time may be used as the switching operation time Tb.
The zero current control is ended after the connection switching is completed, for example, after time t4, when a time Tf (=Tb+Td) obtained by adding a predetermined margin Td to the switching operation time Tb has elapsed. will be

The length of the current zero control period T0 may be set to the sum (Te+Tf) of the time Te obtained by adding the allowance Tc to the current convergence time Ta and the time Tf obtained by adding the allowance Td to the switching operation time Tb. .
When predetermined values are used for the current convergence time Ta and switching operation time Tb, the length of the current zero control period T0 can also be set to a predetermined value.

When the current flowing through the motor 3 becomes zero or nearly zero, the torque generated by the motor 3 also becomes zero or nearly zero, and the load torque of the load 2 connected to the motor 3 gradually reduces the motor speed. If the motor speed drops to zero or a value close to it, the motor 3 will step out or stop.
When returning to normal operation from a step-out or stopped state, there is a possibility that the restart cannot be performed smoothly. For example, when the load 2 applied to the motor 3 is a compression element of a compressor that constitutes a refrigeration cycle of an air conditioner, stopping the motor 3 causes the state of the refrigerant to become unstable. , the torque required for restarting becomes extremely large, and restarting may not be possible.

In such a situation, it is necessary to restart the motor 3 after waiting for the state of the refrigerant to be sufficiently stabilized. In that case, the refrigerant cannot be pressurized by the compressor until restart. For this reason, in the case of a compressor that constitutes an air conditioner, cooling and heating operations are interrupted for a long period of time.

Therefore, in the present embodiment, by restricting the motor speed at the start of zero current control, control is performed so as to suppress loss of synchronism, stoppage, etc. due to a decrease in motor speed.

Specifically, even if the motor speed decreases due to current zero control, the motor speed is prevented from decreasing to zero or a value close to it. Therefore, the speed reduction amount Δω during the zero current control period is estimated, and the value obtained by adding the margin ω _α to the estimated speed reduction amount (estimated value of the reduction amount) Δω is set as the threshold value ω0, and the switching control sequence is started. After confirming that the motor speed ω1 of is equal to or higher than the threshold value ω0, current zero control is started.

As described above, it can be considered that the motor speed ω1 at the start of the switching control sequence and the motor speed ω2 at the start of the zero current control are equal. Therefore, when starting the zero current control after confirming that the motor speed ω1 at the start of the switching control sequence is equal to or higher than the threshold ω0, it can be said that the motor speed ω2 at the start of the zero current control is equal to or higher than the threshold ω0.

When the motor speed ω1 at the start of the switching control sequence is smaller than the threshold value ω0, zero current control is not performed, and therefore switching of the connection switching device is not performed.
It can be said that performing such control gives priority to operation continuity.

The motor speed can be regarded as being equal to the speed command value ω ^* used in speed control of the motor 3 .
As such a speed command value, a value calculated inside the control unit 7 or a value given from an external upper control unit (not shown) may be used.

When the controller 7 controls the speed of the motor 3, the motor speed is controlled to follow the speed command value ω ^* . That is, a voltage command value is generated using well-known PI (proportional integral) control or the like so that the motor speed follows the speed command value ω ^* , and the PWM signals Sm1 to Sm6 are generated based on this voltage command value. , and controls the ON/OFF operation of the switching elements 411 to 416 of the inverter 4 . Therefore, it can be seen that the motor speed coincides with the speed command value ω ^* during stable operation.

When the controller 7 performs torque control (instead of speed control) of the motor 3, the motor speed itself is not directly controlled, so in such a case, the speed command value ω ^* cannot be used. In this case, the estimated value ω^ of the motor speed may be used. The estimated speed value ω̂ is obtained based on the voltage command value or the current detected by the current detection means 6, for example.

The speed reduction amount Δω during the zero current control period T0 is obtained by estimation. For example, at the start of zero current control, the motor 3 is controlled at a desired speed, the output torque of the motor 3 is balanced with the load torque of the load 2, and the load 2 during the zero current control period T0 Assuming that the change in the load torque T _L [Nm] is sufficiently small, the following formula (3) holds.
Δω= _TL・T0/Jm (3)

In formula (3),
T0 represents the length of the current zero control period T0.
Jm represents the shaft inertia (moment of inertia) [kg·m ² ] of the motor 3 . The value of the shaft inertia Jm of the motor 3 is known in advance by actual measurement or the like.
Since the value of the load torque _TL varies depending on the state of the load 2 and is difficult to measure, the load torque at the start of the switching control sequence is assumed to be in balance with the output torque of the motor 3. Estimated by equation (4).
T _L ≈I1·Kt (4)

In formula (4),
I1 is the output current of the inverter 4;
Kt is a torque constant Kt [Nm/A].

As the output current I1[A] of the inverter 4, a value calculated based on the output phase current of the inverter 4 detected by the current detection means 6 can be used.

The output current I1 of the inverter 4 corresponds to √3 times the effective value Irms [A] of the output phase current of the inverter 4, and is the motor torque current (q axis current) Iq [A].

The torque constant Kt differs depending on whether the connection state is Y connection or Δ connection. A value corresponding to the state of connection at the time of calculation is used for the calculation (calculation represented by the formula (4) or the later-described formula obtained from the formula (4)). Since the calculation is performed before switching the connection, the torque constant in the connection state before switching is used. The value of the torque constant Kt is generally known and stored in the controller 7 in advance.

If the motor 3 is a three-phase permanent magnet synchronous motor that obtains rotational force mainly from magnet torque, the torque constant Kt is given by corresponds to the product of The above magnetic flux Φm corresponds to √(2/3) times the maximum value of the armature interlinkage magnetic flux per phase.

By substituting the formula (4) into the formula (3), the following formula (5) is obtained.
Δω=I1・Kt・T0/Jm (5)

The threshold value ω0 is set to a value equal to or greater than the speed reduction amount Δω obtained by Equation (5). Namely
ω0≧I1·Kt·T0/Jm (6)
is determined to satisfy

For example, a value obtained by adding a predetermined margin ω _α to the speed reduction amount Δω is used as the threshold ω0. That is, the threshold ω0 is given by the following equation (7).
ω0=Δω+ _ωα (7)

In calculating the above equation (5), it is desirable to use a value acquired or estimated at the timing of the calculation or at a timing as close as possible to the calculation as the output current I1.
For example, when calculating the threshold value ω0 using the equation (5), the output current I1 at the time of the calculation is used as the output current I1. Since the above calculation is performed immediately after starting the switching control sequence, it can be said that the output current I1 at the time of the above calculation is the output current I1 at the time of starting the switching control sequence.

By performing the zero current control on the condition that the motor speed ω2 at the start of the zero current control is equal to or greater than the above threshold value ω0, step-out and stoppage of the motor 3 due to the zero current control can be suppressed.

When the connection state switching (execution of switching) is completed as described above and the current zero control is completed, the switching control sequence is completed. After the switching control sequence ends, the motor 3 is returned to normal operation.

When returning the motor 3 to normal operation, it is first necessary to restart the motor 3 .
In restarting, information representing the motor speed is required when generating the voltage command value.
When the motor 3 is a three-phase permanent magnet synchronous motor, if the magnetic pole position information is also obtained, it is possible to generate a voltage command value with a phase suitable for the magnetic pole position, and to restart more preferably.

When the load 2 applied to the motor 3 is, for example, the compression element of the compressor of an air conditioner, the internal temperature of the compressor becomes high, making it difficult to install a sensor for detecting the motor speed or the magnetic pole position. is. Therefore, there is a need to obtain information indicative of motor speed at restart by means other than using sensors.

In the present embodiment, the motor speed ω2 at the start of zero current control (t2) is considered to have decreased by the speed decrease amount Δω during the zero current control period T0, and a value obtained by subtracting Δω from ω2 is obtained at time tn. is assumed to be the estimated speed ω̂n, and restart is performed with ω̂n as the initial speed. Furthermore, by also calculating the amount of change in the magnetic pole position (phase) of the motor 3 that changes according to the speed reduction amount Δω, the magnetic pole position at the start of restarting can be estimated.

The operation of the control unit 7 when the above switching control sequence is performed will be described below with reference to FIG.

FIG. 8 shows the procedure of processing.
The processing in FIG. 8 is performed when the motor 3 is in a normal operating state.
In step ST1, the control unit 7 determines whether or not it is necessary to start the switching control sequence.
It is when a switching request occurs that the switching control sequence needs to be started.

If it is determined in step ST1 that the switching control sequence needs to be started (if YES), the control section 7 starts the switching control sequence in step ST2. In the following description, this start time is assumed to be t1. The control unit 7 also obtains the motor speed at time t1 when the switching control sequence starts. For example, at time t1, the speed command value ω ^* used in speed control of the motor 3 is obtained as the motor speed ω1 at the start of the switching control sequence.

If it is determined in step ST1 that the switching control sequence does not need to be started (if NO), the controller 7 keeps the motor 3 in the normal operating state. That is, the process of step ST1 is repeated.

After starting the switching control sequence, the controller 7 first determines the threshold value ω0 in step ST3. The threshold value ω0 serves as a criterion for determining whether or not zero current control should be started. The threshold value ω0 is set to a value larger than the speed reduction amount Δω obtained by the calculation of the equation (5) by a predetermined margin _ωα , that is, a value represented by the equation (7).

In step ST3, the output current I1 at the time of calculating the threshold ω0 in step ST3 is used as the output current I1 in calculating the equations (5) and (7) to obtain the threshold ω0. Since the processing of step ST3 is performed immediately after the switching control sequence is started in step ST2, the output current I1 at the time of performing the processing of step ST3 is the output current I1 at the time of starting the switching control sequence. I can say

When the threshold value ω0 is determined, in step ST4, the control unit 7 compares the motor speed ω1 obtained in step ST2 with the threshold value ω0 determined in step ST3, and determines whether or not ω1≧ω0. .

If it is determined that ω1≧ω0 (if YES), the controller 7 starts current zero control in step ST5. The control unit 7 also obtains the time at this time as the start time t2 of the zero current control, and obtains the motor speed at this time as the motor speed ω2 at the start of the zero current control (t2).

If it is determined that ω1<ω0 (if NO in ST4), the process returns to step ST1. That is, the control unit 7 does not switch the connection state of the motor 3 and maintains the normal operation state of the motor 3 .

If NO in step ST4, the process may return to step ST2 as indicated by the dotted line in FIG. That is, when it is determined that ω1<ω0, acquisition of the motor speed ω1, determination of the threshold value ω0, and determination of whether ω1≧ω0 (ST2, ST3 and ST4) may be repeated.

If YES in step ST4, the time difference between the start time t1 of the switching control sequence in the immediately preceding step ST2 and the time t2 of the start of the current zero control in step ST5 is very small, so there is no such difference. You can consider Therefore, in step ST5, it may be considered that t2≈t1. Also, the speed change during this period is very small and can be ignored. That is, it may be considered that ω2≈ω1. Therefore, ω1 obtained in the processing of step ST2 immediately before YES in step ST4 may be used as ω2 as it is.

After step ST5, the control unit 7 waits for a predetermined time Te (=Ta+Tc) to elapse from the start of current zero control (t2) (ST6), and when the time Te elapses (YES in ST6). , go to step ST7.

At step ST7, the control section 7 instructs the switching operation by the switching control signal Sc. The switching operation is performed by operating the switches 51u, 51v, and 51w of the connection switching device 5. FIG. The control unit 7 also acquires the time t4 of the switching operation instruction.

After step ST7, the control unit 7 waits for a predetermined time Tf (=Tb+Td) to elapse from the switching operation instruction (t4) (ST8), and when the time Tf elapses (YES in ST8), Go to step ST9.
In the present embodiment, since Te (=Ta+Tc) is fixed, waiting for time Tf (=Tb+Td) to elapse from time t4 and waiting for time period T0 to elapse from time t2 are are substantially the same.
That is, in step ST8, it is possible to wait for the period T0 to elapse from time t2.

At step ST9, the controller 7 terminates the zero current control and estimates the motor speed at the time of termination of the zero current control. Denote the estimated value by ω^n. For example, from the motor speed ω2 at the start of the zero current control (ST5: time t2), it is assumed that the speed has decreased during the zero current control by the speed decrease amount Δω during the period T0, and the value obtained by subtracting Δω from ω2 is obtained at time tn. may be the estimated speed (assumed to be ω^n).

A series of operations of the switching control sequence is completed as described above.
In step ST10, the control unit 7 returns to normal operation with the estimated value ω̂n as an initial value.

According to the first embodiment, when the motor 3 whose connection state is switched by the connection switching device 5 is driven by the inverter 4, the life or reliability of the connection switching device 5 can be improved, and step-out due to a decrease in motor speed can be prevented. And there is an effect that can prevent stoppage.

Further, when the normal operation of the motor 3 is resumed from the zero current control, the value obtained by subtracting the speed reduction amount Δω during the zero current control period from the motor speed ω2 at the start of the zero current control is the motor speed when restarting. Since the initial speed is 3, there is an effect that it can be returned more reliably and quickly.

In the above embodiment, the current detection means 6 is a current sensor such as ACCT or DCCT provided on the wiring connecting the inverter 4 and the motor 3 in FIG. Alternatively, well-known shunt resistors for phase current detection inserted in series with the switching elements 414, 415, and 416 on the negative side of the inverter 4 may be used. Even when these current detecting means 6 are used, the current flowing from the inverter 4 to the motor 3 can be obtained by a well-known method.

In the above embodiment, in the current zero control, the current of the motor 3 is converged to zero by controlling the ON/OFF operations of the switching elements 411 to 416 of the inverter 4 .
As another method of zero current control, all the switching elements 411 to 416 of the inverter 4 may be turned off. If all the switching elements 411 to 416 of the inverter 4 are turned off, the current between the motor 3 and the inverter 4 is interrupted and the current of the motor 3 converges to zero.

Embodiment 2.
In Embodiment 1, when a predetermined length of time has elapsed since the start of zero current control, it is assumed that the current has converged to zero, and the switching operation is started.
Alternatively, the current detection means 6 may detect the current flowing between the inverter 4 and the motor 3, confirm that the detected current has converged to zero, and start the switching operation after confirmation. The processing procedure in this case is as shown in FIG.

The processing procedure in FIG. 9 is generally the same as the processing procedure in FIG. 8, but step ST11 is provided instead of step ST6.

At step ST11, it waits until the detected current converges to zero. "Zero" here is not limited to the state of being exactly zero, but includes the state of being so close to zero that it can be regarded as substantially zero. Current detection can be performed by the current detection means 6, for example.
If YES in step ST11, the process proceeds to step ST7.

The same effects as in the first embodiment can be obtained in the second embodiment. Furthermore, since the timing of execution of switching (operation of the switching device) is determined based on the detected current, damage to the connection switching device can be avoided more reliably.

Embodiment 3.
In the first embodiment, at the start of the switching control sequence (t1), after confirming that the motor speed ω1 is equal to or higher than the threshold value ω0, the connection is switched. At (t1), when the motor speed ω1 is less than the threshold value ω0, switching of the connection is not executed, and the motor speed ω1 is set to be equal to or higher than the threshold value.

On the other hand, in the third embodiment, when the motor speed ω1 is equal to or greater than the threshold value ω0 at the start of the switching control sequence (t1), the connection switching is immediately executed as in the first embodiment. When the motor speed ω1 is less than the threshold value ω0, the motor 3 is accelerated to increase the speed to the threshold value ω0 or more, and then the connection is switched.

Acceleration of the motor 3 takes place as follows.
For example, when the control unit 7 controls the speed of the motor 3, acceleration can be performed by changing the speed command value ω ^* .
The speed command value ω ^* may be switched to a value equal to or greater than the threshold value ω0 all at once, that is, stepwise.

Alternatively, the speed command value ω ^* may be gradually increased over time. When increasing gradually, it may be increased continuously or stepwise. When increasing in steps, for example, it is possible to repeatedly add a constant increment.

As in the first embodiment, the speed command value ω ^* or the estimated value ω̂ may be used to determine whether the motor speed during acceleration is equal to or greater than the threshold value ω0.

For example, when the speed command value ω ^* is changed slowly, it can be seen that the motor speed during acceleration matches the speed command value ω ^* . Therefore, it is possible to judge whether the motor speed is equal to or greater than the threshold value ω0 by considering that the motor speed is equal to the speed command value ω ^* .
If the motor speed changes rapidly during acceleration and cannot be seen to match the speed command value ω ^* , the estimated value ω̂ of the motor speed may be used. The estimated speed value ω̂ is obtained based on the voltage command value or the current detected by the current detection means 6, as described in the first embodiment.

When the motor speed becomes equal to or greater than the threshold value ω0, acceleration is terminated, zero current control is started, and switching is performed during the zero current control period. The processing after the start of zero current control is the same as in the first embodiment.

FIG. 10 shows an example of the operation when performing the switching control sequence in the driving device of the third embodiment.
In FIG. 10, it is assumed that the motor 3 normally operates at a constant speed until time t1 when the switching request is generated, and the motor speed ω1 at time t1 is less than the threshold value ω0.

In this case, at the start of the switching control sequence (time t1), it is confirmed that the motor speed ω1 is less than the threshold ω0, the acceleration of the motor 3 is started, and at time t2b it is confirmed that the motor speed exceeds the threshold ω0. Once confirmed, acceleration ends.
In FIG. 10, the period from the start of acceleration to the end of acceleration (acceleration period) is indicated by Th, and the motor speed at time t2b when acceleration is finished is indicated by ω2.

Current zero control is started as soon as the acceleration ends.
The time point at which the current zero control is started coincides with the time point t2b at which the acceleration ends.
The processing after the start of zero current control is the same as in the first embodiment. That is, when a predetermined time Te (=Ta+Tc) elapses from the start t2b of zero current control, switching is instructed (time t4), and a predetermined time Tf (=Tb+Td) is given from the switching instruction (t4). (time t6), the current zero control ends, and processing for returning to normal operation is performed.

The current converges to zero after a lapse of time Ta (t3) from the start of current zero control (t2b), and the switching operation ends (t5) after a lapse of time Tb from the switching instruction (t4).

FIG. 10 shows that the speed decreases from ω2 to ωn during the zero current control period T0, and that ωn is used as the initial value for restarting to return to the normal operation state.

FIG. 11 shows the procedure of processing in the third embodiment.
The procedure shown in FIG. 11 is generally the same as in FIG.
However, steps ST5 and ST6 are replaced with steps ST5b and ST6b, and steps ST21 and ST22 are added.

If NO in step ST4, that is, if ω1≧ω0 is not satisfied, the process proceeds to step ST21.
At step ST21, the control section 7 performs control for accelerating the motor 3. FIG. The controller 7 also acquires the motor speed ω1′ during acceleration.
The acceleration of the motor 3 may be performed by switching the speed command value ω ^* to a value equal to or greater than the threshold value ω0 at once, or by gradually increasing the speed command value ω ^* as time elapses.

When the speed command value ω ^* is gradually increased, the amount of increase in one processing of step ST21 may be determined in advance.
When the speed command value ω ^* is changed slowly, the motor speed ω1′ during acceleration can be regarded as equal to the speed command value ω ^* . If they cannot be considered equal, an estimated value ω^ of the motor speed is obtained, and this estimated value is used as the motor speed ω1′.

At step ST22, the control unit 7 determines whether or not the motor speed ω1' during acceleration is equal to or greater than the threshold value ω0.

If YES in step ST22, the process proceeds to step ST5b.
If NO in step ST22, the process returns to step ST21 to repeat the acceleration in step ST21 and the determination in step ST22.

In step ST5b, the control unit 7 starts the zero current control, obtains the time at this time as the start time t2b of the zero current control, and obtains the motor speed at this time as the motor speed ω2 at the start of the zero current control. .

In step ST6b, the control unit 7 waits for the time Te to elapse from time t2b, and when the time Te elapses, the process proceeds to step ST7.
The processing after step ST7 is the same as in the first embodiment.

When performing the processing as described above, strictly speaking, between the time t21 when the processing of step ST21 immediately before step ST22 where the determination is YES and the time t2b when the zero current control is started, There is a time difference (t2b-t21) for the time required for the calculation processing (step ST22) of the processing and control unit 7, but since this time difference is minute, it may be assumed that there is no speed change during this time. That is, it may be considered that ω1′≈ω2. Therefore, when starting the zero current control after confirming that the motor speed ω1′ obtained in step ST21 is equal to or higher than the threshold ω0, it can be said that the motor speed ω2 at the start of the zero current control is equal to or higher than the threshold ω0.

In Embodiment 3, it is possible to switch the connection state of the motor 3 regardless of the speed of the motor. Therefore, it is possible to reliably obtain the effect of switching the connection state according to the load condition.

Embodiment 4.
In the third embodiment, the threshold ω0 determined in step ST3 is also used in step ST22. Alternatively, the threshold value ω0 may be re-determined after the acceleration in step ST21. In this case, the same calculation as in step ST3 is performed using the output current I1 after acceleration in step ST21.

FIG. 12 shows the procedure of processing in the fourth embodiment.
The processing procedure of FIG. 12 is generally the same as the processing procedure of FIG. 11, but step ST23 is added.

In step ST23, the controller 7 re-determines the threshold ω0. When re-determining, formulas (5) and (7) are calculated. However, as the output current I1, the output current I1 in the state after acceleration in step ST22 immediately before is used.

By using the post-acceleration output current I1, a more appropriate value can be obtained as the threshold value ω0, and it is possible to more appropriately determine whether or not the post-acceleration motor speed ω1′ is equal to or greater than the threshold value ω0.

Embodiment 5.
FIG. 13 shows an air conditioner 100 according to Embodiment 5 together with a power source 8 . The illustrated air conditioner has an outdoor unit 101 , an indoor unit 102 and an air conditioning controller 103 .
The outdoor unit 101 has a drive device 1 and a compressor 110 . Compressor 110 has motor 3 and compression element 111 as load 2 of motor 3 . The outdoor unit 101 further has a four-way valve 121 , a heat source side heat exchanger 122 and a heat source side expansion valve 123 . The driving device 1 and the motor 3 may be those described in the first to fourth embodiments.

The drive device 1 is connected to a power source 8 . The power source 8 may supply DC power as shown in FIG. 2(a) or (b), or may supply AC power. If the power source 8 supplies AC power, the driving device 1 may be provided with an AC/DC power converter.

The indoor unit 102 has a load side expansion valve 131 and a load side heat exchanger 132 .
The four-way valve 121 and the heat source side expansion valve 123 are controlled by the air conditioning controller 103 .

The compression element 111 of the compressor 110 includes a four-way valve 121, a heat source side heat exchanger 122, a heat source side expansion valve 123, a load side expansion valve 131, and a load side heat exchanger 132. A refrigerant circuit is connected to each other by a refrigerant pipe 141. , and a refrigeration cycle is established by the refrigerant flowing through the refrigerant circuit.

When the cooling operation is performed, the four-way valve 121 directs the refrigerant discharged from the compressor 110 toward the heat source side heat exchanger 122 and the refrigerant discharged from the load side heat exchanger 132 toward the compressor 110. The flow path can be switched.

By driving the motor 3 by the driving device 1, the compression element 111 connected to the motor 3 compresses the refrigerant and discharges the high-temperature and high-pressure refrigerant. The high-temperature, high-pressure refrigerant discharged from the compressor 110 passes through the four-way valve 121 and flows into the heat source side heat exchanger 122, where it exchanges heat with the outside air to radiate heat. The refrigerant flowing out of the heat source side heat exchanger 122 is expanded and decompressed in the heat source side expansion valve 123 to become a low-temperature, low-pressure gas-liquid two-phase refrigerant. It flows into the side heat exchanger 132 , heat-exchanges with the air in the air-conditioned space, evaporates, becomes low-temperature, low-pressure refrigerant, and flows out of the load-side heat exchanger 132 . The refrigerant that has flowed out of the load-side heat exchanger 132 passes through the four-way valve 121, is sucked into the compressor 110, and is compressed again.

The above operations are repeated.
When performing the heating operation, the four-way valve 121 is switched to reverse the flow of the refrigerant.

In the air conditioner described above, when the compressor 110 is once stopped, it is common to wait for the refrigerant pressure to equalize from the viewpoint of preventing failure of the compressor 110 before restarting the compressor 110. The comfort of the user could be compromised during the pressure equalization time. By using the drive device 1 according to Embodiments 1 to 4 in the air conditioner, it becomes possible to switch the connection of the motor 3 without stopping the compressor 110, and the air conditioning operation can be continued. Therefore, user comfort is improved.

Although one example of the air conditioner has been described above, the air conditioner is not limited to the above example.
For example, an accumulator for storing excessive refrigerant may be provided on the suction side of the compression element 111 of the compressor 110 .

For the purpose of cooling the driving device 1, particularly the inverter 4 thereof, a cooling plate may be brought into contact with the power module, which is a component of the inverter 4, and the cooling plate may be further brought into contact with the refrigerant pipe 141 described above. By doing so, the heat generated in the inverter 4 can be absorbed by the refrigerant flowing through the refrigerant pipe 141, and the temperature rise of the inverter 4 can be efficiently suppressed.

In the configuration of FIG. 13, the indoor unit 102 and the outdoor unit 101 are provided with expansion valves 123 and 131, respectively. With such a configuration, the cooling capacity of the air conditioner can be independently controlled by the two expansion valves 123 and 131, so the refrigerant can be finely and efficiently controlled. However, one of the expansion valves 123 and 131 may be omitted. That is, the expansion valves 123 and 131 may be provided on either the indoor unit side or the outdoor unit side.

As described above, by using the drive device 1 described in Embodiments 1 to 4 in the air conditioner, it becomes possible to switch the connection of the motor 3 without stopping the compressor 110, and the air conditioning operation can be continued. This has the effect of improving the user's comfort.

In Embodiment 5, an example in which the drive device 1 according to Embodiments 1 to 4 described above is used in an air conditioner is shown. In addition, it can be used in equipment having a refrigerating cycle such as a heat pump device and a refrigerating device.

Various modifications can be made to the above embodiment.
For example, although Embodiment 2 has been described as a modification of Embodiment 1, similar modifications can be applied to Embodiments 3 and 4 as well. Moreover, although various modifications have been described in the description of the first embodiment, similar modifications can also be applied to the second, third and fourth embodiments.

1 drive device 2 load 3 motor 4 inverter 4a, 4b DC bus 4u, 4v, 4w output terminal 5 connection switching device 6 current detection means 7 control unit 8 power source 41 inverter main circuit 45 drive circuit 51u, 51v, 51w switch 52 neutral point node 81a, 81b single-phase AC power supply 82a, 82b AC/DC power converter 100 air conditioner 101 outdoor unit 102 indoor unit 103 air conditioning control vessel, 110 compressor, 111 compression element, 121 four-way valve, 122 heat source side exchanger, 123 heat source side expansion valve, 131 load side expansion valve, 132 load side exchanger, 141 refrigerant piping, 411 to 416 switching elements.

Claims (6)

A driving device comprising: a connection switching device for switching a connection state of a motor connected to a load; and an inverter connected to a power source for outputting an AC voltage to the motor,
The connection state is switched during a period of zero current control for controlling the current of the motor to zero, and the speed of the motor at the start of the zero current control is equal to or greater than a threshold;
Letting the output current of the inverter be I1,
Let the torque constant of the motor be Kt,
The length of the period of the current zero control is T0,
Jm is the shaft inertia of the motor.
, the threshold ω0 is
ω0≧I1・Kt・T0/Jm
A drive that is defined to satisfy 2. The driving device according to claim 1 , wherein when the speed of the motor is less than the threshold, the zero current control is started after performing acceleration control so that the speed of the motor becomes equal to or higher than the threshold. After the end of the current zero control,
3. A value obtained by subtracting an estimated decrease in speed of the motor during the period of the zero current control from the speed of the motor at the start of the zero current control is used as an initial speed to return to the normal operation control. The drive device according to . 4. The driving device according to any one of claims 1 to 3 , wherein in the current zero control, the output current of the inverter is controlled to zero by turning on and off a switching element of the inverter. 5. The driving device according to claim 1, wherein all switching elements of said inverter are turned off in said zero current control. An air conditioner comprising the drive device according to any one of claims 1 to 5 , the motor, and a compression element driven by the motor, wherein the compression element compresses refrigerant in a refrigeration cycle.

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