JP7516284B2 - Motor Drive Unit - Google Patents

Description

The present invention relates to a motor drive device that drives a motor having multiple phase windings that are not connected to each other.

Permanent magnet synchronous motors with multiple phase windings are used as drive motors for compressors installed in refrigeration cycle devices of heat source equipment such as air conditioners. In addition, open-winding motors, which have multiple phase windings that are not connected to each other, are also known.

The motor drive device for driving this open winding motor (abbreviated as motor) includes a first inverter that controls the current supply to one end of each phase winding of the motor, a second inverter that controls the current supply to the other end of each phase winding of the motor, and a switch connected between the other ends of each phase winding. The device selectively sets a star connection mode in which the phase windings are star-connected (also called star-shaped connection) by closing the switch and the first inverter is switched independently by PWM (pulse width modulation), and an open winding mode in which the phase windings are disconnected and the first and second inverters are linked to each other and PWM-switched by opening the switch. The open winding mode setting allows the motor to be driven at a high rotation speed, and the star connection mode setting in the low rotation speed range allows the motor to be driven with high efficiency, making it possible to drive the motor as efficiently as possible over a wide operating range from high to low rotation speeds. This allows both an expansion of the operating range of the motor and an improvement in the efficiency of the motor drive device to be achieved.

In star connection mode, current flows through each phase winding through the above-mentioned switch. To improve efficiency during operation, relays with mechanical open/close contacts are used as the above-mentioned switch to minimize resistance to this current. However, if the open/close contacts of the relay are opened and closed while a voltage is applied to the open/close contacts while the motor is running, this can cause problems such as surge voltages and arc discharges occurring between both ends of the open/close contacts and high-frequency currents flowing, which can have a negative effect on the life of the relay and, in some cases, can even destroy the inverter drive circuit and switch elements.

As a countermeasure, when switching the relay while the motor is running, a pseudo-neutral point operation is performed in which the modulation rate of the PWM switching of the second inverter is changed to 0% to set the line voltage of each phase winding (the voltage between both ends of the relay's open/close contacts) to zero, and the relay's open/close contacts are opened and closed while this pseudo-neutral point operation is being performed.

Patent No. 4804381 JP 2019-62726 A

When switching each inverter, in order to prevent a short circuit caused by the positive and negative switch elements, which are components of each inverter, being turned on at the same time, a so-called dead time is set in which both switch elements are turned off before either element is turned on.

Because this dead time is also executed during the pseudo-neutral point operation described above, even when the pseudo-neutral point operation is being executed, there are very short and sporadic periods during which the line voltage is not actually zero due to the effect of this dead time. According to tests conducted by the inventors, it was found that, although extremely rare, there is a certain probability that the relay contacts will open and close at a timing when the line voltage is not zero during pseudo-neutral point operation. In short, merely executing the pseudo-neutral point operation cannot fully avoid the above-mentioned problems such as surge voltage, arc discharge, and high-frequency current.

The objective of an embodiment of the present invention is to provide a motor drive device that can shorten or eliminate the period during which the line voltage is not zero while performing pseudo-neutral point operation.

The motor drive device of the first embodiment of the present invention is a motor drive device for a motor having a plurality of phase windings that are not connected to each other, comprising: a first inverter including a plurality of series circuits of upper and lower switch elements, where the interconnection points of the upper and lower switch elements in these series circuits are connected to one end of each of the phase windings; a second inverter including a plurality of series circuits of upper and lower switch elements, where the interconnection points of the upper and lower switch elements in these series circuits are connected to the other ends of the phase windings; a switch having a mechanical opening and closing contact connected between the other ends of the phase windings; and a controller that selectively sets an open winding mode in which the other ends of the phase windings are not connected by opening the opening and closing contacts, and the first and second inverters are linked to each other and PWM switched, and a star connection mode in which the other ends of the phase windings are connected to each other by closing the opening and closing contacts, and the first inverter is PWM switched independently. When the switch is operated, the controller performs a pseudo-neutral point operation that sets the modulation rate of the PWM switching of the second inverter to 0% and lowers the carrier frequency of the PWM switching of the first and second inverters.

The motor drive device of the second embodiment of the present invention is a motor drive device for a motor having a plurality of phase windings that are not connected to each other, comprising: a first inverter including a plurality of series circuits of upper and lower switch elements, where the interconnection points of the upper and lower switch elements in these series circuits are connected to one end of each of the phase windings; a second inverter including a plurality of series circuits of upper and lower switch elements, where the interconnection points of the upper and lower switch elements in these series circuits are connected to the other ends of the phase windings; a switch having a mechanical opening and closing contact connected between the other ends of the phase windings; and a controller that selectively sets an open winding mode in which the other ends of the phase windings are disconnected by opening the opening and closing contacts, and the first and second inverters are linked to each other and PWM switched, and a star connection mode in which the other ends of the phase windings are interconnected by closing the opening and closing contacts, and the first inverter is PWM switched independently. When the switch is operated, the controller executes a pseudo-neutral point operation that sets the modulation rate of the PWM switching of the second inverter to 0%, and shifts the on/off duty of the PWM switching of the second inverter to the decreasing or increasing side by a predetermined value.

FIG. 1 is a circuit block diagram showing the configuration of a first embodiment and a second embodiment. FIG. 2 is a flowchart showing the control of the first embodiment. FIG. 3 is a time chart showing the operation of each relay and changes in motor current in the first embodiment. FIG. 4 is a time chart showing how the collector-emitter voltage, line voltage, and zero-axis current of each switch element change with a decrease in carrier frequency in the first embodiment. 5 is a waveform diagram showing the relationship between the zero-axis current, the motor current, and the line voltage that occurs with the decrease in the carrier frequency of FIG. 4. FIG. 6 is a waveform diagram showing, for reference, the relationship between the zero-axis current, the motor current, and the line voltage when there is no reduction in the carrier frequency as shown in FIG. FIG. 7 is a flowchart showing the control of the second embodiment. FIG. 8 is a waveform diagram showing the relationship between the modulation factor of PWM switching of each normal inverter, the line voltage, the motor current, and the zero axis current in the second embodiment. 9 is a waveform diagram showing the relationship between the line voltage, the motor current, and the zero axis current that occurs with a decrease in the modulation factor of the PWM switching of the second inverter in FIG. 8. 10 is a waveform diagram showing the relationship between the line voltage, the motor current, and the zero axis current when the modulation rate of the PWM switching of the second inverter in FIG. 9 drops to 0%. 11 is a waveform diagram showing the relationship between the line voltage, the motor current, and the zero axis current when the on/off duty of the PWM switching of the second inverter is shifted to the increasing side in the state of FIG. 10. 12 is a waveform diagram showing the relationship between the line voltage, the motor current, and the zero-axis current when the PWM switching of the second inverter is stopped in the state of FIG. 11. FIG. 13 is a waveform diagram showing a part of FIG. 11 in a time-enlarged manner. FIG. 14 is a waveform diagram showing a part of FIG. 8 or FIG. 9 in a time-enlarged manner.

[1] First embodiment
As shown in FIG. 1, a motor drive circuit 2 is connected to a three-phase commercial AC power supply 1 , and a motor 3 and a controller 4 are connected to the motor drive circuit 2 .

Motor 3 is a three-phase permanent magnet synchronous motor that has multiple phase windings Lu, Lv, and Lw that are not connected to each other and drives a compressor for a refrigeration cycle of a heat source device such as an air conditioner. It is a so-called open-winding motor that has six terminals at both ends of each of the phase windings Lu, Lv, and Lw.

The motor drive circuit 2 includes a converter 10 connected to a three-phase commercial AC power source 1, which converts the three-phase AC voltage into a DC voltage and outputs it, an inverter (first inverter) 20 that controls the current flow from the output end of the converter 10 to three terminals that are one end of each of the phase windings Lu, Lv, and Lw of the open winding motor 1M, and an inverter (second inverter) 30 that controls the current flow from the output end of the converter 10 to three terminals that are the other ends of each of the phase windings Lu, Lv, and Lw of the open winding motor 1M. A DC link common system is adopted in which the converter 10 is used as a common DC power source for the inverters 20 and 30. The converter 10 is a full-wave rectifier, a PWM converter, or the like.

The inverter 20 is a three-phase inverter including a U-phase series circuit in which the upper switch element Tua and the lower switch element Tub are connected in series and to which the output voltage of the converter 10 is applied, a V-phase series circuit in which the upper switch element Tva and the lower switch element Tvb are connected in series and to which the output voltage of the converter 10 is applied, and a W-phase series circuit in which the upper switch element Twa and the lower switch element Twb are connected in series and to which the output voltage of the converter 10 is applied. The interconnection points Xu, Xv, and Xw of the upper and lower switch elements in these series circuits are connected to one end of the phase windings Lu, Lv, and Lw, respectively.

Like inverter 20, inverter 30 is a three-phase inverter including a U-phase series circuit in which upper switch element Tua and lower switch element Tub are connected in series and to which the output voltage of converter 10 is applied, a V-phase series circuit in which upper switch element Tva and lower switch element Tvb are connected in series and to which the output voltage of converter 10 is applied, and a W-phase series circuit in which upper switch element Twa and lower switch element Twb are connected in series and to which the output voltage of converter 10 is applied. The interconnection points Yu, Yv, and Yw of the upper and lower switch elements in the series circuit of inverter 30 are connected to the other ends of the phase windings Lu, Lv, and Lw, respectively.

Each switch element of these inverters 20, 30 is, for example, an IGBT, and includes a freewheel diode (also called a freewheel diode) D connected in inverse parallel to the main body of each switch element. The switch elements are not limited to IGBTs, and other semiconductor switch elements such as MOS-FETs may also be used.

The inverters 20 and 30 are modules that house in a single package a main circuit in which a U-phase series circuit, a V-phase series circuit, and a W-phase series circuit are bridge-connected, and peripheral circuits such as a drive circuit that drives the switch elements of the main circuit and an overcurrent protection circuit that protects the switch elements, called an IPM (Intelligent Power Module). Note that all switch elements and drive circuits may be configured as discrete components. The inverters 20 and 30 are each a three-phase inverter, but each inverter 20 and 30 may be configured using three single-phase inverters.

Between the other end of the phase winding Lu and the other end of the phase winding Lv of the motor 1M, a normally open switching contact (referred to as a relay contact) 12a of a switch having a mechanical switching contact, for example, a relay 12 is connected. Between the other end of the phase winding Lv and the other end of the phase winding Lw of the motor 1M, a normally open switching contact (referred to as a relay contact) 13a of a switch having a mechanical switching contact, for example, a relay 13 is connected. The relays 12 and 13 are provided to commonly connect the other ends of the phase windings Lu, Lv, and Lw of the motor 1M to change the motor 1M to a star connection state, and the phase windings that are short-circuited by the relays 12 and 13 may be between the other end of the phase winding Lu and the other end of the phase winding Lw, or between the other end of the phase winding Lu and the other end of the phase winding Lv. These relays 12 and 13 are turned on (energized) by supplying an excitation current and turned off (deenergized) by cutting off the excitation current at mutually synchronized timing by the controller 4. A single relay having two normally open contacts may be used in place of the relays 12 and 13.

When relays 12 and 13 are turned on, relay contacts 12a and 13a are closed, and the other end of phase winding Lu is interconnected with the other end of phase winding Lv, and the other end of phase winding Lv is interconnected with the other end of phase winding Lw, resulting in a star-connected state of phase windings Lu, Lv, and Lw. When relays 12 and 13 are turned off, relay contacts 12a and 13a are opened, and phase windings Lu, Lv, and Lw are in a non-connected state, i.e., an electrically separated open winding state. In this open winding state, the line voltage Euv of phase windings Lu and Lv is applied to relay contact 12a, the line voltage Evw of phase windings Lv and Lw is applied to relay contact 13a, and the line voltage Ewu of phase windings Lw and Lu is applied to the series circuit of relay contacts 12a and 13a.

Current sensors 11u, 11v, and 11w are arranged on three current lines between the inverter 20 and one end of the phase windings Lu, Lv, and Lw, and the output signals of these current sensors are sent to the controller 4. The current sensors 11u, 11v, and 11w detect the currents (called motor currents) Iu, Iv, and Iw that flow through the phase windings Lu, Lv, and Lw.

The controller 4 controls the opening and closing of the relay contacts 12a, 13a and the switching by PWM (pulse width modulation) (called PWM switching) of the inverters 20, 30 so that the rotation speed N of the motor 3 becomes the target rotation speed Nt commanded by the higher-level external controller 50, and includes a main control unit 40, a current detection unit 41, a relay drive unit 42, a timer 43, a PWM signal generation unit 44, and a carrier frequency switching unit 45. The external controller 50 is, for example, a controller for an air conditioner.

The current detection unit 41 detects the instantaneous values of the motor currents Iu, Iv, and Iw detected by the current sensors 11u, 11v, and 11w. The relay drive unit 42 energizes and deenergizes the relays 12 and 13 in response to a command from the main control unit 40. The PWM signal generation unit 44 generates drive signals for U-phase PWM switching (U-phase upper drive signal and U-phase lower drive signal), drive signals for V-phase PWM switching (V-phase upper drive signal and V-phase lower drive signal), and drive signals for W-phase PWM switching (W-phase upper drive signal and W-phase lower drive signal) in response to the target rotation speed Nt commanded by the external controller 50 and commands from the main control unit 40. The carrier frequency switching unit 45 can switch the carrier frequency of the PWM switching of the PWM signal generation unit 44 in response to a command from the main control unit 40.

The main control unit 40 is composed of a microcomputer and its peripheral circuits, and estimates (detects) the rotation speed N of the motor 3 based on the detection result of the current detection unit 41, and drives each switch element of the inverters 20 and 30 on and off according to each of the drive signals generated by the PWM signal generation unit 44.

The main control unit 40 selectively sets the open winding mode in which the other ends of the phase windings Lu, Lv, and Lw are disconnected by opening the relay contacts 12a and 13a, and the inverters 20 and 30 are linked together to perform PWM switching, and the star connection mode in which the other ends of the phase windings Lu, Lv, and Lw are interconnected by closing the relay contacts 12a and 13a, and the inverter 20 is PWM switched independently, depending on the estimated rotation speed N and the mode command from the external controller 50. For example, the star connection mode is set when the motor rotation speed N is low and the motor currents Iu, Iv, and Iw are below a predetermined value under low load, and the open winding mode is set when the motor rotation speed N increases and the motor currents Iu, Iv, and Iw are equal to or greater than a predetermined value under high load. This allows high efficiency to be achieved over the entire operating range of the motor.

When driving the upper and lower switch elements in each series circuit of the inverters 20 and 30 on and off, the main control unit 40 performs a complementary operation in which when one switch element is turned on, the other switch element is turned off. In this case, when changing one of the switch elements from off to on, a dead time td is ensured during which both switch elements are turned off, so that the upper and lower switch elements connected in series are not turned on at the same time.

When relay contacts 12a, 13a are opened or closed, the main control unit 40 changes the modulation rate of the PWM switching of the inverter 30 to 0% so that the line voltages Euv, Evw, Ewu applied to the relay contacts 12a, 13a become 0 before the closing operation, and executes a pseudo-neutral point operation in which each upper switch element and each lower switch element in the inverter 30 are alternately turned on and off with a predetermined on/off duty, for example, an on/off duty of 50%.

When the main control unit 40 executes this pseudo neutral point operation, it controls the carrier frequency switching unit 45 to lower the carrier frequency of the PWM switching of the inverters 20, 30 to a value (first predetermined value) that is lower than the normal value (second predetermined value), for example 5 KHz, for example 4 KHz.

Next, the main controls executed by the main control unit 40 will be explained with reference to the flowchart in FIG. 2, the time charts in FIG. 3 and FIG. 4, and the waveform diagrams in FIG. 5 and FIG. 6. Steps S1, S2, etc. in the flowchart will be abbreviated simply as S1, S2, etc.

While the motor 3 is being driven, the main control unit 40 checks whether it is necessary to switch from the open winding mode to the star connection mode (S1), and checks whether it is necessary to switch from the star connection mode to the open winding mode (S10).

If it is necessary to switch from the open winding mode to the star connection mode (YES in S1), the main control unit 40 gradually increases the modulation rate of the PWM switching of the inverter 20 so that the output voltage of the inverter 20 becomes twice as high as before, and gradually changes the modulation rate of the PWM switching of the inverter 30 to 0% so that the output voltage of the inverter 30 becomes 0 (S2), and confirms that these changes are complete (S3).

By changing the modulation rate of the PWM switching of the inverter 30 to 0%, a pseudo-neutral point operation is performed in which each upper switch element and each lower switch element in the inverter 30 are alternately turned on and off with an on/off duty of 50%. This causes the line voltages Euv, Evw, and Ewu to approach 0.

When the above change in modulation rate is complete (YES in S3), the main control unit 40 determines that pseudo-neutral point operation has been entered and reduces the carrier frequency of the PWM switching of the inverters 20, 30 to 4 KHz, lower than the normal 5 KHz (S4).

After this reduction in carrier frequency, the main control unit 40 turns on (energizes) the relays 12 and 13 (S5). Then, after a certain time t1, which is longer than the time required from when the relays 12 and 13 are turned on until the relay contacts 12a and 13a actually close, has elapsed as measured by the timer 43 (YES in S6), the main control unit 40 controls the carrier frequency switching unit 45 to return the carrier frequency of the PWM switching of the inverters 20 and 30 from the above 4 KHz to the normal 5 KHz (S7). Next, the main control unit 40 stops the switching of the inverter 30 (S8) and starts the independent PWM switching of the inverter 20 (S9), thereby transitioning to the star connection mode. After this transition, the main control unit 40 returns to the judgments of S1 and S10 above.

During the execution of the pseudo-neutral point operation, for example, as shown by the solid arrows in FIG. 1, at the timing when motor currents Iu and Iv flow from phase windings Lu and Lv to interconnection points Yu and Yv of inverter 30 and motor current Iw flows from interconnection point Yw of inverter 30 to phase winding Lw, if the carrier frequency of the PWM switching of inverters 20 and 30 remains at the normal 5 KHz (carrier period T is 200 μsec), the line voltage Euv applied to relay contact 12a will be 0, but the line voltage Evw applied to relay contact 13a will not be 0 during the dead time td when both the upper and lower switch elements are off, as shown in FIG. 4.

The reason for this is that the flow direction of the motor currents Iu and Iv is different from the flow direction of the motor current Iw, and a current path is formed through the freewheeling diode D of either the upper switch element Tua, Tva, Twa or the lower switch element Tub, Tvb, Twb, so that the potential of the interconnection point Yw, which is determined by the voltage (collector-emitter voltage) Vwa across the upper switch element Twa and the voltage (collector-emitter voltage) Vwb across the lower switch element Twb, and the potential of the interconnection points Yu and Yv, which are determined by the voltages (collector-emitter voltages Vua and Vva) across the upper switch elements Tua and Tva and the voltages (collector-emitter voltages) Vub and Vvb across the lower switch elements Tub and Tvb, respectively, have different values only during the dead time td.

If the timing when the line voltage Evw is not 0 coincides with the timing when the relay contact 13a is closed, the line voltage Evw will suddenly drop to 0V, causing a discharge between both ends of the relay contact 13a, or a steep high-frequency current will flow through the small capacitance component between the relay contact 13a. That is, the output capacitance (output capacitance) of the IGBTs, which are the upper switch element Tva and the lower switch element Twb in the inverter 30, i.e., the collector-emitter capacitance, is charged, and even though the upper switch element Tva and the lower switch element Twb are off, the voltages at both ends of the upper switch element Tva and the lower switch element Twb rapidly increase several times faster than during normal switching operation. At this time, a high-frequency current flows through the collector-gate parasitic capacitance of the upper switch element Tva and the lower switch element Twb.

This high-frequency current flows through the parasitic capacitance between the gate and emitter of each of the upper switch element Tva and the lower switch element Twb to the emitter side. At that time, high-frequency noise is superimposed on the gate-emitter voltage of each of the upper switch element Tva and the lower switch element Twb. In the lower switch element Tvb and the upper switch element Twa, the voltage across each of them suddenly decreases several times faster than normal switching, and high-frequency noise is superimposed on the gate of each switch element by the same principle as above. If the level of this high-frequency noise is high or the frequency of occurrence of the high-frequency noise is high, it may cause malfunction of the drive circuit that drives each switch element, or lead to thermal destruction due to oscillation of the drive circuit or overvoltage destruction of the gate. The above was explained using the example of the line voltage Evw, but this phenomenon occurs between any phases.

In the open winding mode, a zero-axis voltage generated by the difference between the in-phase (common mode) voltages of inverters 20 and 30 is applied to motor 3, causing a zero-axis current Io, which is a component that flows in phase (common mode) in each phase. This zero-axis current Io is superimposed on each phase of motor currents Iu, Iv, and Iw in the same direction and with the same value. The lower the carrier frequency of the PWM switching of inverters 20 and 30, the larger the value of this zero-axis current Io.

Therefore, during pseudo-neutral point operation, if the carrier frequency of the PWM switching of the inverters 20 and 30 is 4 KHz (carrier period T is 250 μsec), lower than the normal 5 KHz, at the timing when the motor currents Iu, Iv, and Iw flow in the direction of the solid arrows in Figure 1 as described above, as shown by the dashed arrows in Figure 1, the ripple component of the zero-axis current Io flowing from the interconnection points Yu, Yv, and Yw of the inverter 30 to the phase windings Lu, Lv, and Lw increases compared to the normal 5 KHz. At this time, the zero-axis current Io flowing toward the phase winding Lw has the same flow direction as the motor current Iw shown by the solid arrow, and the zero-axis currents Io and Io flowing toward the phase windings Lu and Lv have the opposite flow direction to the motor currents Iu and Iv shown by the solid arrows, and are larger than the value of the motor currents Iu and Iv by the increase in the ripple component.

Figure 5 shows the relationship between the zero-axis current Io, motor currents Iu, Iv, Iw, and line voltage Evw at this point, and the waveform of the line voltage Evw expanded over time. In other words, by lowering the carrier frequency, the zero-axis current Io, which is larger than the motor currents Iu, Iv, Iw, flows dominantly relative to the motor currents Iu, Iv, Iw. When the ripple component of this zero-axis current Io is larger than the fundamental component of the motor currents Iu, Iv, Iw, the flow directions of the motor currents Iu, Iv, Iw are the same, and the occurrence of a period in which the line voltages Euv, Evw, Ewu applied to the relay contacts 12a, 13a are not zero during the execution of the pseudo-neutral point operation can be prevented, or the period in which the line voltages Euv, Evw, Ewu are not zero can be shortened.

By shortening the period during which the line voltages Euv, Evw, and Ewu are not zero, it is possible to reduce the probability that the period during which the line voltages Euv, Evw, and Ewu are not zero will overlap with the timing of closing the relay contacts 12a and 13a. This reduces the possibility of problems such as the occurrence of surge voltages or arc discharges between both ends of the relay contacts 12a and 13a, or the flow of high-frequency current. This also reduces the possibility of the switch elements of the inverters 20 and 30 being destroyed.

Incidentally, if the carrier frequency is not reduced, as shown in Figure 6, the zero-axis current Io does not become very large, so there is a long period of time during which the line voltages Euv and Evw applied to the relay contacts 12a and 13a do not become zero.

In addition to reducing the carrier frequency, if a configuration is adopted in which the wiring length of the relay contacts 12a, 13a is made longer than a predetermined length to give the current path of the relay contacts 12a, 13a a predetermined impedance component, it is possible to eliminate problems such as surge voltages and arc discharges occurring between both ends of the relay contacts 12a, 13a and high-frequency currents flowing, even if the period during which the line voltages Euv, Evw, and Ewu do not become zero overlaps with the timing of closing the relay contacts 12a, 13a.

On the other hand, if it is necessary to switch from star connection mode to open winding mode (NO in S1, YES in S10), the main control unit 40 gradually decreases the modulation rate of the PWM switching of the inverter 20 so that the output voltage of the inverter 20 becomes 1/2 of what it was before, starts the PWM switching of the inverter 30 at a modulation rate of 0% so that the output voltage of the inverter 30 becomes 0 (S11), and confirms the completion of the change in the modulation rate (S12). By starting the PWM switching of the inverter 30 at a modulation rate of 0%, a pseudo-neutral point operation with an on/off duty of 50% is started in the inverter 30.

When the change in modulation rate is complete (YES in S3), the main control unit 40 determines that pseudo-neutral point operation has been entered and reduces the carrier frequency of the PWM switching of the inverters 20, 30 from the normal 5 KHz to the above 4 KHz (S13).

After this carrier frequency is lowered, the main control unit 40 turns off the relays 12 and 13 (S14). Then, after a certain time t2, which is longer than the time required from turning off the relays 12 and 13 until the relay contacts 12a and 13a actually open, has elapsed as measured by the timer 43 (YES in S15), the main control unit 40 returns the carrier frequency of the PWM switching of the inverters 20 and 30 from the above 4 KHz to the above 5 KHz (S16). Next, the main control unit 40 gradually changes the modulation rate of the PWM switching of the inverters 20 and 30 so that the output voltage of the inverters 20 and 30 becomes the same (1x) as the output voltage of the inverter 20 before the change in S11 (S17), and confirms the completion of the change (S18). When the change is completed (YES in S18), the main control unit 40, upon determining that the pseudo neutral point operation has ended, starts the PWM switching of the inverters 20 and 30 in conjunction (S19), thereby transitioning to the open winding mode. After this transition, the main control unit 40 returns to the judgments in S1 and S10 above.

Even when transitioning to this open winding mode, by performing pseudo-neutral point operation and lowering the carrier frequency, the zero-axis current Io, which is larger than the motor currents Iu, Iv, and Iw, flows predominantly relative to the motor currents Iu, Iv, and Iw. This makes it possible to prevent or shorten the occurrence of periods during which the line voltages Euv and Evw applied to the relay contacts 12a and 13a do not become zero while the pseudo-neutral point operation is being performed.

The period during which the line voltages Euv, Evw, and Ewu are not zero can be shortened, reducing the probability that the period during which the line voltages Euv, Evw, and Ewu are not zero will overlap with the timing of opening the relay contacts 12a and 13a. This reduces the possibility of problems such as surge voltages or arc discharges occurring between both ends of the relay contacts 12a and 13a or high-frequency currents flowing, and therefore reduces the possibility of each switch element of the inverters 20 and 30 being destroyed.

The above-mentioned fixed times t1 and t2 may be the same, and it is desirable to make them as short as possible. In the case of mechanical relays 12 and 13, there is a delay of 10 to 30 msec from the energization and deenergization by the excitation current until the relay contacts 12a and 13a actually open and close. Taking this into consideration, it is desirable for the fixed times t1 and t2 to be around 50 msec to 100 msec.

If a semiconductor switch element is used instead of the mechanical relay contacts 12a, 13a, it is possible to control the ON timing of the semiconductor switch element in units of μsec. Therefore, the above problem does not occur, but the semiconductor switch element has a larger ON resistance than a relay, and since a current always flows through the semiconductor switch element during star connection, there are problems such as large losses and the need to take measures against heat dissipation.
In this embodiment, since the mechanical relay contacts 12a, 13a having extremely low resistance are used, almost no loss occurs and no heat dissipation measures are required.

[2] Second embodiment
In the second embodiment, the function of the main control unit 40 is slightly different from that of the first embodiment, but the other configurations are the same as those of the first embodiment shown in Fig. 1. A description of the same parts will be omitted.
When the relay contacts 12a, 13a are opened and closed, the main control unit 40 changes the modulation rate of the PWM switching of the inverter 30 to 0% so that the line voltages Euv, Evw, Ewu applied to the relay contacts 12a, 13a become 0, thereby executing a pseudo-neutral point operation in which each upper switch element and each lower switch element in the inverter 30 are alternately turned on and off with a predetermined on/off duty, for example, 50%.

When the line voltages Euv, Evw, and Ewu approach 0 due to the execution of the pseudo neutral point operation, the main control unit 40 shifts the on/off duty of the inverter 30 from the normal 50% to the decreasing (or increasing) side by a predetermined value, for example, 3%. Furthermore, the main control unit 40 controls the carrier frequency switching unit 45 to increase the carrier frequency of the PWM switching of the inverters 20 and 30 to a third predetermined value, for example, 6 KHz, which is higher than the normal second predetermined value, for example, 5 KHz.

The main control executed by the main control unit 40 will be explained with reference to the flowchart in FIG. 7 and the waveform diagrams in FIG. 8 to FIG. 13. In the flowchart, the same parts as those in FIG. 2 of the first embodiment are given the same reference numerals.

In the open winding mode, the relationship between the modulation rate of the PWM switching of the inverters 20 and 30, the line voltages Euv, Evw, and Ewu, the motor currents Iu, Iv, and Iw, and the zero-axis current Io is as shown in FIG. 8. Although only one zero-axis current Io is shown, in reality this zero-axis current Io flows through each phase winding Lu, Lv, and Lw.

When it becomes necessary to switch from the open winding mode to the star connection mode (YES in S1), in order to roughly maintain the rotation speed of the motor 3, the main control unit 40 gradually increases the modulation rate of the PWM switching of the inverter 20 so that the output voltage of the inverter 20 becomes twice as high as before, and gradually changes the modulation rate of the PWM switching of the inverter 30 to 0% so that the output voltage of the inverter 30 becomes 0 (S2), and confirms that these changes are complete (S3).

Figure 9 shows the relationship between the modulation rate of the PWM switching of inverters 20 and 30, line voltages Euv, Evw, Ewu, motor currents Iu, Iv, Iw, and zero-axis current Io during the process in which these modulation rates change. The amplitudes of motor currents Iu, Iv, Iw centered on the 0 level and the amplitude of zero-axis current Io centered on the 0 level are increased compared to the case of Figure 8.

When the above modulation rate change is complete (YES in S3), the main control unit 40 determines that pseudo-neutral point operation has been entered and shifts the on/off duty of the inverter 30 from 50% to 47%, while raising the carrier frequency of the inverter 30's PWM switching to 6 KHz, higher than the normal 5 KHz (S4'). Even if the on/off duty is shifted, the pseudo-neutral point operation to make the line voltage zero continues.

The relationship between the modulation rate of the PWM switching of inverters 20 and 30, the line voltages Euv, Evw, and Ewu, the motor currents Iu, Iv, and Iw, and the zero-axis current Io at this point is shown in Figure 10. By shifting the on/off duty of inverter 30 to 47%, zero-axis currents Io, Io, and Io flow from phase windings Lu, Lv, and Lw to interconnection points Yu, Yv, and Yw of inverter 30, and the zero-axis currents Io, Io, and Io increase in amplitude, with the entire amplitude gradually shifting from the initial 0 level to the positive level side.

As the amplitude of the zero-axis current Io shifts, the motor currents Iu, Iv, and Iw also increase in amplitude, gradually shifting from the initial 0 level to the positive level. As the motor currents Iu, Iv, and Iw shift, the line voltages Euv, Evw, and Ewu approach 0. Then, as shown in FIG. 11, when the entire amplitude of the zero-axis current Io is sufficiently far from the 0 level, the entire amplitudes of the motor currents Iu, Iv, and Iw are also sufficiently far from the 0 level, which makes it possible to prevent the generation of the line voltages Euv, Evw, and Ewu, or to reduce their generation period to almost zero.

When the line voltages Euv, Evw, and Ewu become approximately 0, the main control unit 40 turns on relays 12 and 13 (S5). The timing of this process in S5 is indicated by the arrow in Figure 11.

After a certain time t1, which is longer than the time required from when relays 12, 13 are turned on until relay contacts 12a, 13a actually close, has elapsed as measured by timer 43 (YES in S6), the main control unit 40 cancels the shift to 47% on/off duty and returns the on/off duty to the normal 50%, and returns the carrier frequency of the PWM switching of inverter 30 from the above 6 KHz to the normal 5 KHz (S7'). Next, the main control unit 40 stops switching of inverter 30 (S8) and starts independent PWM switching of inverter 20 (S9), thereby transitioning to star connection mode.

The relationship between the modulation rate of the PWM switching of the inverters 20 and 30, the line voltages Euv, Evw, and Ewu, the motor currents Iu, Iv, and Iw, and the zero-axis current Io at this point is shown in Figure 12. The timing of the processing in S8 above is indicated by an arrow in Figure 12.

As described above, by shifting the on/off duty of the inverter 30 from the normal 50% to 47% during pseudo-neutral point operation, the period during which the line voltages Euv, Evw, and Ewu applied to the relay contacts 12a and 13a do not become zero can be shortened or eliminated.

The period during which the line voltages Euv, Evw, and Ewu are not zero can be shortened or eliminated, reducing the probability that the relay contacts 12a and 13a will close before the line voltages Euv, Evw, and Ewu are not zero. This reduces the possibility of problems such as surge voltages or arc discharges occurring between both ends of the relay contacts 12a and 13a or high-frequency currents flowing, and therefore reduces the possibility of each switch element of the inverters 20 and 30 being destroyed.

If the inverters 20, 30 are IPMs with built-in overcurrent protection circuits, there is a possibility that the overcurrent protection circuits will operate when the amplitude of the zero-axis current Io increases due to a shift in the on/off duty. To prevent this unnecessary operation, in the process of S4' above, the carrier frequency of the PWM switching of the inverter 30 is increased to 6 KHz, higher than the normal 5 KHz, as the on/off duty is shifted. By increasing the carrier frequency to 6 KHz, higher than the normal 5 KHz, the ripple component of the zero-axis current Io is reduced. This makes it possible to avoid unnecessary operation of the overcurrent protection circuits.

As shown in Figure 11, the entire amplitude of the zero-axis current Io is sufficiently far from the 0 level, and the entire amplitude of the motor currents Iu, Iv, and Iw is also sufficiently far from the 0 level. As a result, Figure 13 shows an enlarged portion of the waveform where the line voltages Euv, Evw, and Ewu are 0, and there is no period during which line voltages are generated. In contrast, Figure 14 shows an enlarged portion of the waveform when there is no on-off duty shift, as in Figures 8 and 9, and there is a period during which line voltages are generated.

In addition to shifting the on/off duty, if a configuration is adopted in which the wiring length of the relay contacts 12a, 13a is made longer than a predetermined length to give the current path of the relay contacts 12a, 13a a predetermined impedance component, even if the period in which the line voltages Euv, Evw, and Ewu are not zero overlaps with the timing of closing the relay contacts 12a and 13a, it is possible to reliably eliminate the possibility of problems such as the generation of surge voltages or arc discharges between both ends of the relay contacts 12a and 13a or the flow of high-frequency current.

On the other hand, if it is necessary to switch from star connection mode to open winding mode (NO in S1, YES in S10), the main control unit 40 gradually decreases the modulation rate of the PWM switching of the inverter 20 so that the output voltage of the inverter 20 becomes 1/2 of what it was before, starts the PWM switching of the inverter 30 at a modulation rate of 0% so that the output voltage of the inverter 30 becomes 0 (S11), and confirms the completion of the change in the modulation rate (S12). By starting the PWM switching of the inverter 30 at a modulation rate of 0%, a pseudo-neutral point operation with an on/off duty of 50% is started in the inverter 30.

When the above modulation rate change is complete (YES in S3), the main control unit 40 determines that pseudo-neutral point operation has been entered and shifts the on/off duty of the inverter 30 from 50% to 47%, and increases the carrier frequency of the PWM switching of the inverter 30 to 6 KHz, higher than the normal 5 KHz (S13').

After this carrier frequency drop, the main control unit 40 turns off the relays 12 and 13 (S14). Then, after a certain time t2 has elapsed as measured by the timer 43, which is longer than the time required from when the relays 12 and 13 are turned off until the relay contacts 12a and 13a actually open (YES in S15), the main control unit 40 cancels the shift of the on/off duty to 47% and returns the on/off duty to the normal 50%, and returns the carrier frequency of the PWM switching of the inverter 30 from the above 6 KHz to the normal 5 KHz (S16').

Then, the main control unit 40 gradually changes the modulation rate of the PWM switching of the inverters 20, 30 so that the output voltage of the inverters 20, 30 becomes the same (1x) as the output voltage of the inverter 20 before the change in S11 (S17), and confirms the completion of the change (S18). When the change is completed (YES in S18), the main control unit 40, upon determining that the pseudo neutral point operation has ended, starts PWM switching of the inverters 20, 30 in conjunction (S19), thereby transitioning to the open winding mode. After this transition, the main control unit 40 returns to the determinations in S1 and S10 above.

Even when switching to this open winding mode, the on/off duty of the inverter 30 is shifted from the normal 50% to 47% while the pseudo-neutral point operation is being performed, shortening or eliminating the period during which the line voltages Euv, Evw, and Ewu applied to the relay contacts 12a and 13a do not become zero.

Although the on-off duty of the inverter 30 has been shifted to the decreasing side, it may be shifted to the increasing side, for example, to 53%. When the on-off duty is shifted to 53%, the zero-axis current Io flows from the interconnection points Yu, Yv, Yw of the inverter 30 to the phase windings Lu, Lv, Lw, and while the zero-axis current Io increases in amplitude, the entire amplitude gradually shifts from the initial 0 level center to the negative level. As the amplitude of the zero-axis current Io shifts, the motor currents Iu, Iv, Iw also increase in amplitude and gradually shift from the initial 0 level center to the negative level. As the shift of the motor currents Iu, Iv, Iw progresses, the line voltages Euv, Evw, Ewu approach 0.

The above embodiments are presented as examples and are not intended to limit the scope of the invention. The new embodiments and modifications may be implemented in various other forms, and various omissions, rewrites, and modifications may be made without departing from the spirit of the invention. These embodiments and modifications are within the spirit of the invention and are included in the scope of the claims and their equivalents.

2...Drive circuit, 3...Open winding motor, Lu, Lv, Lw...Phase windings, 4...Controller, 12, 13...Relay (switch), 12a, 13a...Relay contact (switch contact), 20...Inverter (first inverter), 30...Inverter (second inverter), 40...Main control unit

Claims (4)

A motor drive device for a motor having a plurality of phase windings that are not connected to each other,
a first inverter including a plurality of series circuits each including an upper switch element and a lower switch element, the interconnection points between the upper switch elements and the lower switch elements in each of the series circuits being connected to one end of each of the phase windings;
a second inverter including a plurality of series circuits each including an upper switch element and a lower switch element, the interconnection points between the upper switch elements and the lower switch elements in the series circuits being connected to the other end of each of the phase windings;
a switch having a mechanical switching contact connected between the other ends of the phase windings;
a controller that selectively sets an open winding mode in which the other ends of the phase windings are disconnected by opening the switching contacts and the first and second inverters are linked to each other and PWM switched, and a star connection mode in which the other ends of the phase windings are connected to each other by closing the switching contacts and the first inverter is independently PWM switched;
Equipped with
When the switch is operated, the controller executes a pseudo-neutral point operation in which a modulation rate of the PWM switching of the second inverter is set to 0%, and reduces a carrier frequency of the PWM switching of the first and second inverters.
Motor drive device. A motor drive device for a motor having a plurality of phase windings that are not connected to each other,
a first inverter including a plurality of series circuits each including an upper switch element and a lower switch element, the interconnection points between the upper switch elements and the lower switch elements in each of the series circuits being connected to one end of each of the phase windings;
a second inverter including a plurality of series circuits each including an upper switch element and a lower switch element, the interconnection points between the upper switch elements and the lower switch elements in the series circuits being connected to the other end of each of the phase windings;
a switch having a mechanical switching contact connected between the other ends of the phase windings;
a controller that selectively sets an open winding mode in which the other ends of the phase windings are disconnected by opening the switching contacts and the first and second inverters are linked to each other and PWM switched, and a star connection mode in which the other ends of the phase windings are connected to each other by closing the switching contacts and the first inverter is independently PWM switched;
Equipped with
When the switch is operated, the controller executes a pseudo neutral point operation in which the modulation rate of the PWM switching of the second inverter is set to 0%, and shifts the on/off duty of the PWM switching of the second inverter to a decreasing or increasing side by a predetermined value.
Motor drive device. The controller increases a carrier frequency of PWM switching of the first and second inverters in accordance with the shift.
The motor drive device according to claim 2 . The motor is a motor that drives a compressor for a refrigeration cycle of a heat source device.
The motor drive device according to any one of claims 1 to 3.

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