Power conversion device

Abstract

To enhance an upper limit of controllable rotational speed.SOLUTION: A power conversion device 1 for supplying power to a sensorless type motor comprises a plurality of circuit elements 21, 22, 31, 32, 41, and 42 in each of which a switching element 51 and a diode 52 are connected in parallel to each other. The plurality of circuit elements 21, 22, 31, 32, 41, and 42 are connected so as to be able to output three-phase alternating current. Each of the circuit elements 21, 22, 31, 32, 41, and 42 has a resistor 53 connected in parallel to the switching element 51 and connected in series to the diode 52.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a power conversion device.

Background Art

[0002] In recent years, the use of brushless motors has been increasing in place of brushed motors. A brushed motor switches the direction of the current supplied to the coil by means of a brush. On the other hand, a brushless motor electrically switches the direction of the current flowing through the coil by means of a control circuit. Patent Documents 1 to 5 disclose drive devices for supplying current to a brushless motor.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0004] Basic performances of a motor include torque and rotational speed. In the technical field of motors, it is desired to increase the upper limit of the controllable rotational speed. Therefore, the present disclosure describes a power conversion device capable of increasing the upper limit of the controllable rotational speed.

Means for Solving the Problems

[0005] A power conversion device that supplies power to a sensorless motor according to the present disclosure includes a plurality of circuit elements in which a switching element and a diode are connected in parallel to each other. The plurality of circuit elements are connected so as to be able to output a three-phase alternating current. The circuit element has a resistor that is connected in parallel to the switching element and in series to the diode.

[0006] The power conversion device outputs a three-phase alternating current indicated as the U-phase, V-phase, and W-phase. These outputs alternately repeat an energization period in which power is supplied and a non-energization period in which power is not supplied. When switching from the energization period to the non-energization period, a reflux current flows from the motor to the power conversion device. This reflux current flows through a circuit in which the coil of the motor and the resistor of the power conversion circuit are connected in series. The time constant of the circuit in which the coil and the resistor are connected in series is represented by an expression having the reactance of the coil as the numerator and the resistance value of the resistor as the denominator. Then, due to the presence of the resistor, the time constant becomes smaller. That is, the convergence period of the reflux current becomes shorter. When the convergence period of the reflux current becomes shorter, the induced voltage that had been invisible due to the increase in the voltage caused by the reflux current can be seen earlier. The sensorless motor grasps the position of the rotor using this induced voltage. Then, even when the frequency of the three-phase alternating current provided from the power conversion device to the motor is increased, the induced voltage can be confirmed, so the position of the rotor can be grasped. Being able to grasp the position of the rotor means, in other words, that the motor can be controlled. That is, as a result of the action that the time constant of the RL series circuit becomes smaller due to the arrangement of the resistor in the power conversion device, the convergence of the reflux current is accelerated, so the induced voltage necessary for controlling the motor can be confirmed earlier, and thus the upper limit of the controllable rotation speed can be increased.

[0007] The resistor of the above power conversion device may be arranged between the cathode of the diode and the switching element. Also with this configuration, the upper limit of the controllable rotation speed can be increased.

[0008] The resistor of the above-described power conversion device may be disposed between the anode of the diode and the switching element. Also with this configuration, the upper limit of the controllable rotational speed can be increased.

[0009] The switching element of the above-described power conversion device may be an insulated gate bipolar transistor. Also with this configuration, the upper limit of the controllable rotational speed can be increased.

Advantages of the Invention

[0010] According to the power conversion device of the present disclosure, the upper limit of the controllable rotational speed can be increased.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0012] Hereinafter, the power conversion device of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same reference numerals are assigned to the same elements, and redundant descriptions are omitted.

[0013] As shown in FIG. 1, the power conversion device 1 is disposed between a power source 91 and a motor 92. The power conversion device 1 converts the form of the power received from the power source 91 into the form of the power required by the motor 92. For example, the power source 91 outputs DC power. The motor 92 requires so-called three-phase AC power. Therefore, the power conversion device 1 converts DC power into three-phase AC power. The power conversion device 1 is a so-called DC-AC inverter.

[0014] The motor 92 is of the brushless type. The motor 92 has a rotor and a stator. The rotor includes a magnet that rotates together with the rotation axis. The stator includes, for example, a U-phase coil 92u, a V-phase coil 92v, and a W-phase coil 92w that are star-connected. Note that the stator may be delta-connected.

[0015] Since the motor 92 is of the brushless type, the direction of the current supplied to each coil is electrically switched. The direction of the current needs to correspond to the rotational position of the rotor. Therefore, it is necessary to obtain the rotational position of the rotor. As a means for obtaining the rotational position of the rotor, for example, a magnetic sensor such as a hall sensor may be used. On the other hand, there is also a means for obtaining the rotational position of the rotor without using a magnetic sensor. The means that does not require a magnetic sensor is called sensorless position detection. In sensorless position detection, the induced voltage generated by the motor is utilized.

[0016] The motor 92 is driven by a rectangular wave drive method called so-called 120° conduction. In the rectangular wave drive method, for example, the U-phase repeats a conduction period of 120° in phase and a non-conduction period of 60° in phase. The same applies to the V-phase and W-phase.

[0017] For example, when the U-phase is in the non-conduction period, the V-phase and W-phase are in the conduction period. A predetermined voltage is generated at the terminals of the U-phase that is in the non-conduction period. This voltage is an induced voltage generated because the magnetic field from the magnet of the rotor rotating in the U-phase coil 92u intersects.

[0018] In sensorless position detection, the rotational position of the rotor is obtained by comparing the induced voltage with a reference voltage. For example, when the reference voltage is set to 0V, the point at which the induced voltage intersects the reference voltage is called a zero cross. In sensorless position detection, the rotational position of the motor is obtained with the zero cross as a reference.

[0019] The power conversion device 1 has a high-side input terminal 11a, a low-side input terminal 11b, a U-phase terminal 12u, a V-phase terminal 12v, and a W-phase terminal 12w. The high-side input terminal 11a is connected to the positive electrode of the power supply 91. The low-side input terminal 11b is connected to the negative electrode of the power supply 91. A capacitor 93 may be connected to the high-side input terminal 11a and the low-side input terminal 11b. The U-phase terminal 12u is connected to the U-phase coil 92u of the motor 92. The V-phase terminal 12v is connected to the V-phase coil 92v of the motor 92. The W-phase terminal 12w is connected to the W-phase coil 92w of the motor 92.

[0020] The power conversion device 1 has six circuit elements. Specifically, the power conversion device 1 has a high-side U-phase circuit element 21, a low-side U-phase circuit element 22, a high-side V-phase circuit element 31, a low-side V-phase circuit element 32, a high-side W-phase circuit element 41, and a low-side W-phase circuit element 42.

[0021] The high-side U-phase circuit element 21 is connected in series to the low-side U-phase circuit element 22. The high-side U-phase circuit element 21 is connected to the high-side input terminal 11a and the U-phase connection point 23. The low-side U-phase circuit element 22 is connected to the low-side input terminal 11b and the U-phase connection point 23. The U-phase connection point 23 where the high-side U-phase circuit element 21 and the low-side U-phase circuit element 22 are connected to each other is connected to the U-phase terminal 12u. The V-phase includes a V-phase connection point 33 and has the same connection configuration as the U-phase. The W-phase also includes a W-phase connection point 43 and has the same connection configuration as the U-phase. Therefore, a detailed description of the connection configurations of the V-phase and the W-phase is omitted.

[0022] The six circuit elements each include the same components and have the same connection configuration. Therefore, the high-side U-phase circuit element 21 will be described in detail as an example.

[0023] The high-side U-phase circuit element 21 includes a switching element 51, a diode 52, and a resistor 53. The switching element 51 switches between conduction and insulation in response to the input of a control signal φ. A transistor may be used as the switching element 51. In the present disclosure, it will be described assuming that the switching element 51 is an insulated-gate bipolar transistor (IGBT). The collector of the switching element 51 is connected to the high-side input terminal 11a. The emitter of the switching element 51 is connected to the U-phase connection point 23. The gate of the switching element 51 receives the control signal φ from the controller 13. According to the control signal φ, it is possible to switch between permitting and stopping the current flowing from the collector to the emitter.

[0024] The diode 52 permits current to flow in one direction and prohibits current from flowing in the other direction. The anode of the diode 52 is connected to the U-phase connection point 23. The cathode of the diode 52 is connected to the high-side input terminal 11a via the resistor 53. In other words, the cathode of the diode 52 is connected to the collector of the switching element 51 via the resistor 53. That is, the diode 52 is connected in parallel with the switching element 51. More specifically, the diode 52 is connected in anti-parallel with the switching element 51.

[0025] The resistor 53 is disposed between the cathode of the diode 52 and the high-side input terminal 11a. In other words, the resistor 53 is connected to the cathode of the diode 52 and the collector of the switching element 51. Then, the resistor 53 is connected in series with the diode 52. Also, the resistor 53 is connected in parallel with the switching element 51.

[0026] According to the connection of such a switching element 51 and diode 52, the current flowing from the U-phase connection point 23 toward the high-side input terminal 11a is always permitted. On the other hand, the current flowing from the high-side input terminal 11a toward the U-phase connection point 23 is permitted or prohibited depending on the state of the switching element 51.

[0027] The operation and effects of the power conversion device 1 will be described below. It has already been stated that the motor 92 is of the sensorless type. Fig. 2(a) shows the terminal voltage at the U-phase terminal 12u. The graph Dt shows the terminal voltage. The graph Dn shows the induced voltage. As shown in Fig. 2(a), in the case of the sensorless type, the rotational position of the rotor of the motor 92 is obtained based on the zero crossing C of the induced voltage (graph Dn). That is, in order to control the motor 92, it is a prerequisite that the zero crossing C can be detected.

[0028] When explaining the motor 92, it was explained that the induced voltage (graph Dn) can be confirmed during the non-energization period S1 of the coil. However, the induced voltage (graph Dn) cannot be confirmed throughout the entire non-energization period S1. Now, assume that current is being supplied to the U-phase coil 92u and the W-phase coil 92w (that is, the energization period). The circuit in this case is shown in Fig. 3. At this time, the current flows through a circuit including the power supply 91, the high-side W-phase circuit element 41, the W-phase terminal 12w, the W-phase coil 92w, the U-phase coil 92u, the U-phase terminal 12u, the low-side U-phase circuit element 22, and the low-side input terminal 11b.

[0029] Next, the switching is made from the energization period S2 to the non-energization period S1. The circuit in this case is shown in Fig. 4. Specifically, the switching element 51 of the low-side U-phase circuit element 22 is switched to the non-conducting state. This circuit includes the high-side U-phase circuit element 21, the high-side W-phase circuit element 41, the W-phase terminal 12w, the W-phase coil 92w, the U-phase coil 92u, and the U-phase terminal 12u. This circuit does not include the power supply 91. However, the U-phase coil 92u and the W-phase coil 92w, to which current has been supplied until just before, are in a state where energy is accumulated. As a result, the energy accumulated in the U-phase coil 92u and the W-phase coil 92w is released as current. This current is referred to as the reflux current. The period during which the reflux current flows is also referred to as the commutation period S1a.

[0030] As shown in the graph Dt of Fig. 2(a), a non-conduction period S1 of 60° and a conduction period S2 of 120° are repeated. The non-conduction period S1 includes a commutation period S1a and a non-commutation period S1b. During the commutation period S1a, the reverse current flows through the diode 52 of the high-side U-phase circuit element 21. At this time, since the cathode of the diode 52 of the high-side U-phase circuit element 21 is the voltage of the power supply 91, the period during which the commutation period S1a flows rises to the anode potential of the diode 52. That is, the terminal voltage of the U-phase coil 92u rises to the voltage of the power supply 91. This voltage is larger than the induced voltage (graph Dn). Therefore, the commutation period S1a cannot confirm the induced voltage (graph Dn).

[0031] The commutation period S1a is determined by the characteristics of the coil of the motor 92. That is, it has nothing to do with the rotational speed of the motor 92. For example, assume that the rotational speed of the motor 92 is low. The rotational speed of the motor 92 is based on the frequency of the current output by the power conversion device 1. When operating the motor 92 at a low rotational speed, the power conversion device 1 outputs a current with a low frequency. At this time, since the non-conduction period S1 is sufficiently long, a zero-crossing C of the induced voltage (graph Dn) appears in the non-commutation period S1b. That is, the sensorless position detection operation is possible.

[0032] Next, as shown in Fig. 2(b), assume that the rotational speed of the motor 92 is high. As the frequency increases, the non-conduction period S1 becomes shorter. As described above, since the commutation period S1a is not affected by the rotational speed or the frequency, even if the non-conduction period S1 becomes shorter, the time of the commutation period S1a does not change, so the time of the non-commutation period S1b becomes shorter. As a result, it may also happen that the timing at which the zero-crossing C of the induced voltage (graph Dn) occurs is included in the commutation period S1a. In this case, the zero-crossing C cannot be detected. That is, the sensorless position detection operation is impossible. In short, when the rotational speed of the motor 92 is increased, the induced voltage (graph Dn) is buried in the voltage caused by the reverse current, so the zero-crossing C cannot be detected.

[0033] Therefore, the power conversion device 1 widens the frequency band in which the zero cross C can be detected by shortening the commutation period S1a. In the circuit shown in FIG. 4, attention is paid to the resistor 53 of the U-phase coil 92u, the W-phase coil 92w, and the high-side U-phase circuit element 21. These connection configurations are the connection configurations of a so-called RL series circuit. The time constant (t) of the RL series circuit is represented by the following formula including the reactance (L) of the coil and the resistance value (R) of the resistor. t = L / R ··· (1)

[0034] Then, it can be seen that the presence of the resistor 53 reduces the time constant. As shown in FIG. 2(c), the reflux current converges earlier. In other words, the commutation period S1a is shortened. As a result, since the non-commutation period S1b in the non-energization period S1 becomes longer, even when the frequency is increased, the zero cross C is not buried in the commutation period S1a. Therefore, the upper limit of the controllable rotation speed can be expanded.

[0035] In short, the power conversion device 1 that supplies power to the sensorless brushless motor includes a plurality of circuit elements in which the switching element 51 and the diode 52 are connected in parallel to each other. The plurality of circuit elements are connected so as to be able to output three-phase alternating current. The circuit element has a resistor 53 that is connected in parallel to the switching element 51 and in series to the diode 52.

[0036] The power conversion device 1 outputs three-phase alternating current indicated as the U-phase, V-phase, and W-phase. These outputs alternately repeat an energization period S2 in which power is supplied and a non-energization period S1 in which power is not supplied. When switching from the energization period S2 to the non-energization period S1, a reflux current flows from the motor 92 to the power conversion device 1. This reflux current flows through a circuit in which any one of the coils 92u, 92v, 92w of the motor 92 and any one of the resistors 53 of the power conversion device 1 are connected in series. The time constant of the circuit in which any one of the coils 92u, 92v, 92w and the resistor 53 are connected in series is expressed by a formula having the reactance of the coils 92u, 92v, 92w as the numerator and the resistance value of the resistor 53 as the denominator. Then, due to the presence of the resistor 53, the time constant becomes smaller. That is, the convergence period (commutation period S1a) of the reflux current becomes shorter. When the commutation period S1a becomes shorter, the induced voltage (graph Dn) that had been made invisible due to the voltage increase caused by the reflux current can be seen earlier. The sensorless motor 92 grasps the position of the rotor using this induced voltage (graph Dn). Then, even when the frequency of the three-phase alternating current provided from the power conversion device 1 to the motor 92 is increased, the zero crossing C of the induced voltage (graph Dn) can be confirmed, so the position of the rotor can be grasped. Being able to grasp the position of the rotor means, in other words, that the motor 92 is controllable. That is, as a result of the action that the time constant of the RL series circuit becomes smaller due to the arrangement of the resistor 53 in the power conversion device 1, the commutation period S1a becomes shorter, so the zero crossing C of the induced voltage (graph Dn) necessary for controlling the motor 92 can be confirmed earlier, and thus the upper limit of the controllable rotation speed can be increased.

[0037] The resistor 53 is arranged between the cathode of the diode 52 and the switching element 51. Also with this configuration, the upper limit of the controllable rotation speed can be increased.

[0038] The switching element 51 of the power conversion device 1 is an insulated gate bipolar transistor. According to this configuration, the circuit elements 21, 22, 31, 32, 41, 42 including the resistor 53 can be configured.

[0039] The power conversion device of the present disclosure has been described in detail based on its embodiments. However, the power conversion device of the present disclosure is not limited to the above content, and various modifications are possible without departing from the gist thereof.

[0040] For example, the resistor 53 of the power conversion device 1 may be disposed between the anode of the diode 52 and the switching element 51. Even with this configuration, the upper limit of the controllable rotational speed can be increased.

Explanation of Reference Numerals

[0041] 1 Power conversion device 11a High-side input terminal 11b Low-side input terminal 12u U-phase terminal 12v V-phase terminal 12w W-phase terminal 13 Controller 21 High-side U-phase circuit element 22 Low-side U-phase circuit element 23 U-phase connection point 31 High-side V-phase circuit element 32 Low-side V-phase circuit element 33 V-phase connection point 41 High-side W-phase circuit element 42 Low-side W-phase circuit element 43 W-phase connection point 51 Switching element 52 Diode 53 Resistor 91 Power supply 92 Motor 92u U-phase coil 92v V-phase coil 92w W-phase coil 93 Capacitor C Zero-cross S1 Non-conduction period S1a Commutation period S1b Non-commutation period S2 Conduction period φ Control signal

Claims

1. A power conversion device that supplies power from a power source to a sensorless motor, comprising: a plurality of circuit elements in which a switching element and a diode are connected in parallel with each other; the plurality of circuit elements are connected so as to be able to output three-phase alternating current; the circuit element has a resistor that is connected in parallel with the switching element and in series with the diode; the circuit element repeats an energization period in which the power source is energized by connecting the power source to the motor and a non-energization period in which the power source is disconnected from the motor and is not energized; the non-energization period includes a commutation period in which the energy stored in the coil constituting the motor flows as a current and a non-commutation period in which the energy stored in the coil constituting the motor does not flow as a current; the power conversion device, wherein a resistance value of the resistor is such that a zero cross at which a current flowing through the circuit element becomes zero is included in the non-commutation period by shortening a time constant defined by the resistance value of the resistor and a reactance of the coil.

2. The power conversion device according to claim 1, wherein the resistor is disposed between a cathode of the diode and the switching element.

3. The power conversion device according to claim 1, wherein the resistor is disposed between an anode of the diode and the switching element.

4. The power conversion device according to any one of claims 1 to 3, wherein the switching element is an insulated gate bipolar transistor.

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