Patent Description:
In a power conversion device including a rectifier circuit configured to convert an AC voltage into a DC voltage and an inverter circuit configured to convert the DC voltage into an AC voltage, pulsation of a frequency six times as large as a frequency of the AC voltage input to the rectifier circuit occurs in the DC voltage output from the rectifier circuit. Such pulsation is reduced by increasing a capacitance of a capacitor provided at a DC link portion between the rectifier circuit and the inverter circuit. For example, the pulsation is reduced by a large-capacitance electrolytic capacitor. However, the large capacitance of the capacitor leads to increased cost and volume of the capacitor. Therefore, there has been known a power conversion device (hereinafter, also referred to as "electrolytic capacitor-less inverter") in which a small-capacitance film capacitor or ceramic capacitor permitting pulsation is provided at a DC link portion.

When pulsation occurs in the DC voltage, a beat component corresponding to the pulsation frequency can be superimposed on a current output from the inverter circuit. When a load connected to the power conversion device is a motor, vibration or noise is generated in the motor due to the beat component.

In order to suppress the beat component generated in the electrolytic capacitor-less inverter, <CIT> (PTL <NUM>) discloses a control method in which a phase of a resultant voltage vector from a q-axis pulsates depending on a pulsation component of a DC voltage, the resultant voltage vector being a resultant voltage vector of a d-axis voltage vector and a q-axis voltage vector of a motor.

PTL <NUM> relates to a control device for a power converter capable of suppressing a pulsation component of a DC voltage. A power converter converts a DC voltage (Vdc) including a pulsation component (Vdch) into an AC voltage and outputs the AC voltage to a synchronous electrical motor. A control device for controlling the power converter includes a pulsation component detection unit and a control circuit. The pulsation component detection unit detects a pulsation component (Vdch). The control circuit controls the power converter so that a load angle of the synchronous electrical motor is increased in accordance with increase in an instantaneous value of the pulsation component (Vdch).

PTL <NUM> relates to a power conversion device which does not require adaptation work and can suppress a current beat regardless of the operation state of an electric motor. This power conversion device comprises: an inverter for converting a direct-current voltage into an alternating-current voltage to drive an alternating-current electric motor; and beat compensation units for suppressing a current beat component in the output current of the inverter. The beat compensation units include beat extraction units for calculating the beat component in the output current of the inverter, and beat compensation voltage calculation units for estimating a beat compensation voltage from the beat component calculated by the beat extraction units. The current beat component in the output current of the inverter is suppressed on the basis of the beat compensation voltage estimated by the beat compensation voltage calculation units.

NPTL <NUM> relates to beat-less control of an electrolytic capacitor-less air conditioning motor drive system.

In the technology described in PTL <NUM>, it is necessary to obtain the d-axis voltage vector and the q-axis voltage vector of the motor using a position sensor such as a pulse encoder or a resolver, for example. Therefore, cost is increased by the position sensor. Further, when the motor is included in a compressor of an air conditioner, it is difficult to install the position sensor because the compressor is brought into a high-temperature and high-pressure state.

The present disclosure has been made to solve the above-described problem, and has an object to provide a power conversion device that can suppress a beat component superimposed on a current flowing through a motor without increasing cost.

According to the present disclosure, a power conversion device as defined in independent claim <NUM> is provided. Further embodiments of the claimed invention are defined in the dependent claims. Although the claimed invention is only defined by the claims, the below embodiments, examples, and aspects are present for aiding in understanding the background and advantages of the claimed invention.

According to the present disclosure, the first phase of the rotor of the motor is estimated based on the current flowing through the motor. Therefore, no position sensor is required to detect the position of the rotor of the motor, unlike the technology described in PTL <NUM>. Further, the second phase is generated by adjusting the first phase to suppress the beat component superimposed on the current flowing through the motor. Moreover, the inverter circuit is controlled to output, from the inverter circuit, the AC voltage corresponding to the second phase. Thus, the beat component is suppressed. In the manner described above, the beat component superimposed on the current flowing through the motor can be suppressed without increasing cost.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to figures. It should be noted that in the figures, the same or corresponding portions are denoted by the same reference characters, and will not be described repeatedly in principle. In the figures described below, a relation in size between respective components may be different from an actual relation therebetween.

<FIG> is a diagram showing an exemplary overall configuration of a power conversion device <NUM> according to a first embodiment. As shown in <FIG>, an AC power supply <NUM> and a motor <NUM> serving as a load are connected to power conversion device <NUM>. AC power supply <NUM> is, for example, a three-phase commercial power supply. Motor <NUM> is, for example, a permanent magnet synchronous motor.

Power conversion device <NUM> includes a rectifier circuit <NUM>, a DC link capacitor <NUM>, an inverter circuit <NUM>, a current detector <NUM>, and a switching signal generator <NUM>.

Rectifier circuit <NUM> rectifies an AC voltage input from AC power supply <NUM> to convert it into a DC voltage. The DC voltage rectified by rectifier circuit <NUM> includes a low-order harmonic component (hereinafter, referred to as "pulsation component") that pulsates at a frequency six times as large as a voltage frequency of AC power supply <NUM>. Rectifier circuit <NUM> is, for example, a full bridge circuit including six rectifier diodes. It should be noted that rectifier circuit <NUM> may use a switching element such as a transistor instead of the rectifier diodes.

Inverter circuit <NUM> converts the DC voltage rectified by rectifier circuit <NUM> into an AC voltage, and outputs the converted AC voltage to motor <NUM>. Inverter circuit <NUM> is, for example, a full bridge circuit including six IGBTs (Insulated Gate Bipolar Transistors). A freewheeling diode is connected to each IBGT in antiparallel. Each IGBT is controlled to be brought into one of an ON state and an OFF state independently in accordance with a switching signal output from switching signal generator <NUM>. By this control, inverter circuit <NUM> converts the DC voltage into an AC voltage. It should be noted that inverter circuit <NUM> may use a switching element such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) instead of the IGBTs.

DC link capacitor <NUM> is connected between rectifier circuit <NUM> and inverter circuit <NUM>. A capacitance of DC link capacitor <NUM> is small to such an extent that the pulsation component of the DC voltage output from rectifier circuit <NUM> is not smoothed. It should be noted that the capacitance of DC link capacitor <NUM> is large to such an extent that a high-order harmonic component resulting from a switching operation of inverter circuit <NUM> is smoothed. DC link capacitor <NUM> is, for example, a film capacitor or a ceramic capacitor.

Current detector <NUM> detects a current flowing through motor <NUM>, and outputs current information indicating the detected current. Current detector <NUM> is, for example, a current sensor using a current transformer, called CT, for instruments. It should be noted that current detector <NUM> may detect the current flowing through motor <NUM> by using a single-shunt current detection method or a three-shunt current detection method. The single-shunt current detection method is a method that uses a shunt resistor provided at a negative DC bus of power conversion device <NUM>. The three-shunt current detection method is a method that uses shunt resistors provided in series with switching elements on the lower side of inverter circuit <NUM>.

Switching signal generator <NUM> generates a switching signal for controlling inverter circuit <NUM>, based on an operation command input from outside such as a speed command or a torque command. Switching signal generator <NUM> generates the switching signal to output, from inverter circuit <NUM>, an AC voltage corresponding to a designated phase. The generated switching signal is output to inverter circuit <NUM>.

As a method of controlling a speed or a torque, for example, vector control to feedback-control the current flowing through motor <NUM> using a dq coordinate system can be employed. The current flowing through motor <NUM> is indicated by current information output from current detector <NUM>. Switching signal generator <NUM> calculates a voltage command in the dq coordinate system through the vector control using the current information output from current detector <NUM>. Then, switching signal generator <NUM> uses the designated phase to convert the voltage command calculated in the dq coordinate system into that in a three-phase coordinate system. Thus, an AC voltage corresponding to the designated phase is output from inverter circuit <NUM>.

It should be noted that switching signal generator <NUM> may generate the switching signal using V/f constant control for outputting a voltage proportional to an operation frequency of motor <NUM>, or using direct torque control for controlling magnetic flux and torque of motor <NUM>.

As described above, the DC voltage rectified by rectifier circuit <NUM> includes the pulsation component that pulsates at the frequency six times as large as the voltage frequency of AC power supply <NUM>. This pulsation component is not smoothed by DC link capacitor <NUM>. Therefore, a beat component resulting from the pulsation component can be superimposed on the current flowing through motor <NUM>. When a difference is small between a frequency (hereinafter, referred to as "pulsation frequency") of the pulsation component and a frequency (hereinafter, referred to as "operation frequency of motor <NUM>") of the AC voltage output from inverter circuit <NUM>, a large beat component is likely to occur. As a configuration for suppressing the beat component, power conversion device <NUM> according to the present embodiment further includes a speed estimator <NUM>, a pulsation detector <NUM>, and a beat suppression controller <NUM>.

Speed estimator <NUM> estimates rotation speed and magnetic pole position of a rotor of motor <NUM> using the current information output from current detector <NUM> and the voltage command calculated by switching signal generator <NUM>. Speed estimator <NUM> estimates rotation speed and magnetic pole position of the rotor of motor <NUM> using a known estimation method. As the estimation method, a method of calculating it in accordance with a speed electromotive force of motor <NUM> is generally employed. For example, a method such as an arctangent method or an adaptive flux observer method can be employed. Speed estimator <NUM> outputs the estimated magnetic pole position, i.e., estimated phase, to beat suppression controller <NUM>.

Pulsation detector <NUM> detects the pulsation frequency in accordance with a DC voltage applied across DC link capacitor <NUM>, and outputs the detection result to beat suppression controller <NUM>. Since DC link capacitor <NUM> has a small capacitance as described above, the DC voltage applied across DC link capacitor <NUM> pulsates at the pulsation frequency about six times as large as the voltage frequency of AC power supply <NUM>. Pulsation detector <NUM> detects this pulsation frequency. For example, pulsation detector <NUM> detects the pulsation frequency by allowing the value of the DC voltage to pass through a band pass filter. Alternatively, pulsation detector <NUM> may detect the pulsation frequency by subtracting, from the value of the original DC voltage, a result obtained by allowing the value of the DC voltage to pass through a notch filter.

Beat suppression controller <NUM> outputs, to switching signal generator <NUM>, an adjusted phase obtained by adjusting the estimated phase output from speed estimator <NUM> so as to suppress the beat component superimposed on the current flowing through motor <NUM>. Switching signal generator <NUM> uses the adjusted phase as the designated phase.

<FIG> is a diagram showing exemplary internal configurations of switching signal generator <NUM> and beat suppression controller <NUM>. As shown in <FIG>, switching signal generator <NUM> includes a converter <NUM>. Converter <NUM> uses a designated phase θ to convert a voltage commands Vd*, Vq* in the dq coordinate system into voltage commands Vu*, Vv*, Vw* in the three-phase coordinate system in accordance with the following conversion formula. Switching signal generator <NUM> generates a switching signal for controlling inverter circuit <NUM> using voltage commands Vu*, Vv*, Vw*.

Beat suppression controller <NUM> includes an amplifier <NUM>, an integrator <NUM>, and an adder <NUM>. Amplifier <NUM> multiplies, by a gain K, the pulsation frequency output from pulsation detector <NUM>. Gain K is determined in accordance with the voltage frequency of AC power supply <NUM> and the magnitude of the DC voltage across DC link capacitor <NUM>. Gain K may be a fixed value determined in advance. Alternatively, gain K may be a variable value determined in accordance with states of AC power supply <NUM> and motor <NUM>.

Integrator <NUM> outputs an integral value of the output of amplifier <NUM>. The integral value indicates a phase (hereinafter, referred to as "pulsation phase") of the pulsation component included in the DC voltage.

Adder <NUM> outputs, as the adjusted phase, a phase obtained by adding the estimated phase output from speed estimator <NUM> and the pulsation phase output from integrator <NUM>. Thus, beat suppression controller <NUM> adjusts the estimated phase using the pulsation phase that is an integral value of the pulsation frequency.

<FIG> is a diagram showing waveforms of a DC voltage, a current flowing through a motor, and a pulsation phase added to an estimated phase in a power conversion device including no beat suppression controller <NUM>. <FIG> is a diagram showing waveforms of the DC voltage, the current flowing through the motor, and the pulsation phase added to the estimated phase in power conversion device <NUM> including beat suppression controller <NUM>. In each of <FIG> and <FIG>, the vertical axis of the graph at the upper part represents the DC voltage, the vertical axis of the graph at the intermediate part represents current flowing through motor <NUM>, and the vertical axis of the graph at the lower part represents the pulsation phase. The horizontal axis of each graph represents elapsed time.

As shown in <FIG>, in the case of the power conversion device including no beat suppression controller <NUM>, the pulsation phase added to the estimated phase is zero. In this case, switching signal generator <NUM> converts the voltage command in the dq coordinate system into the voltage command in the three-phase coordinate system using the estimated phase. Therefore, the current flowing through motor <NUM> is influenced by the pulsation component included in the DC voltage, and therefore includes the beat component as shown in the intermediate part of <FIG>. In particular, when the pulsation frequency is close to the operation frequency of motor <NUM>, a large beat component appears.

As shown in <FIG>, in the case of power conversion device <NUM> including beat suppression controller <NUM>, switching signal generator <NUM> converts the voltage command in the dq coordinate system into the voltage command in the three-phase coordinate system using the adjusted phase obtained by adding the pulsation phase shown in the lower part to the estimated phase. Thus, the influence of the pulsation component included in the DC voltage is canceled in the AC voltage output from inverter circuit <NUM>. Therefore, as shown in the intermediate part, no beat component appears in the current flowing through motor <NUM>.

Thus, according to power conversion device <NUM> of the first embodiment, the estimated phase of the rotor of motor <NUM> is estimated based on the current flowing through motor <NUM>. Therefore, no position sensor is required to detect the position of the rotor of motor <NUM>, unlike the technology described in PTL <NUM>. Further, the adjusted phase is generated by adjusting the estimated phase so as to suppress the beat component superimposed on the current flowing through motor <NUM>. Moreover, inverter circuit <NUM> is controlled to output, from inverter circuit <NUM>, the AC voltage corresponding to the adjusted phase. Thus, the beat component is suppressed. In view of the above, the beat component superimposed on the current flowing through motor <NUM> can be suppressed without increasing cost.

Further, the technology disclosed in PTL <NUM> requires phase information of a voltage in the dq coordinate system. The phase information is calculated, for example, from a d-axis voltage Vd and a q-axis voltage Vq using an arctangent function (Arctan). However, the calculation of the arctangent function involves a large computational load, requires a high-performance microcomputer, and leads to increased cost. However, in power conversion device <NUM> according to the first embodiment, the computation load is reduced, thus suppressing increased cost for the microcomputer.

<FIG> is a diagram showing a portion of a configuration of a power conversion device according to a second embodiment. As shown in <FIG>, a power conversion device 100A according to the second embodiment is different from power conversion device <NUM> according to the first embodiment in that a beat suppression controller 10A is provided instead of beat suppression controller <NUM>.

Beat suppression controller 10A is different from beat suppression controller <NUM> in that beat suppression controller 10A includes an integrator 13A instead of integrator <NUM> and includes a switch <NUM>. As with integrator <NUM>, integrator 13A integrates an output of amplifier <NUM> so as to output an integrated value (i.e., pulsation phase). Integrator 13A resets the integral value to zero in response to input of a reset signal.

Switch <NUM> switches between a first mode and a second mode, the first mode being a mode in which the adjusted phase is output to switching signal generator <NUM>, the second mode being a mode in which the estimated phase is output to switching signal generator <NUM>. Switch <NUM> switches from the second mode to the first mode in response to a predetermined operation condition being satisfied, and switches from the first mode to the second mode in response to the operation condition being not satisfied.

The operation condition is, for example, a condition that motor <NUM> is not undergoing acceleration/deceleration. Alternatively, the operation condition may be a condition that the magnitude (amplitude) of the pulsation included in the DC voltage across DC link capacitor <NUM> is equal to or more than a reference value. Alternatively, the operation condition may include a plurality of conditions. When the operation condition includes such a plurality of conditions, it may be determined that the operation condition is satisfied when all of the plurality of conditions are satisfied, or it may be determined that the operation condition is satisfied when at least one of the plurality of conditions is satisfied.

Beat suppression controller 10A inputs, to integrator 13A, a reset signal for resetting the pulsation phase to zero, before switching from the second mode to the first mode by switch <NUM>. Specifically, beat suppression controller 10A inputs the reset signal to integrator 13A while the second mode is selected by switch <NUM>.

<FIG> is a flowchart showing a flow of a beat suppression control process in power conversion device 100A according to the second embodiment. Steps S1 to S6 shown in <FIG> are repeatedly performed.

In step S1, beat suppression controller 10A calculates the pulsation phase by integrating the value obtained by multiplying the pulsation frequency by gain K. Next, in step S2, beat suppression controller 10A adds the pulsation phase to the estimated phase, thereby generating the adjusted phase.

Next, in step S3, switch <NUM> determines whether or not the operation condition is satisfied. When the operation condition is satisfied (YES in step S3), switch <NUM> selects the first mode and outputs the adjusted phase to switching signal generator <NUM> (step S4). After step S4, power conversion device 100A ends the beat suppression control process.

When the operation condition is not satisfied (NO in step S3), beat suppression controller 10A inputs the reset signal to integrator 13A (step S5). Switch <NUM> outputs the estimated phase to switching signal generator <NUM> (step S6). After step S6, power conversion device 100A ends the beat suppression control process.

When motor <NUM> is undergoing acceleration/deceleration, the output value of speed estimator <NUM> is not stable. Therefore, when switching signal generator <NUM> performs the conversion using the adjusted phase, the effect of suppressing the beat component may not be sufficiently exhibited or diverging may be resulted in the control. Since the operation condition includes the condition that motor <NUM> is not undergoing acceleration/deceleration, the estimated phase is output to switching signal generator <NUM> when motor <NUM> is undergoing acceleration/deceleration. As a result, diverging in the control can be prevented.

When the pulsation component included in the DC voltage is small, the beat component is less likely to be superimposed on the current flowing through motor <NUM>, so that it is not necessary to suppress the beat component using the adjusted phase. Therefore, since the operation condition includes the condition that the magnitude of the pulsation included in the DC voltage across DC link capacitor <NUM> is equal to or more than the reference value, the suppression of the beat component using the adjusted phase is not performed when the pulsation component included in the DC voltage is small.

When the value of the phase input to switching signal generator <NUM> is greatly changed upon switching from the second mode to the first mode, loss of synchronization may occur in motor <NUM>. The loss of synchronization is such a phenomenon that motor <NUM> cannot follow a pulse signal and stops rotation.

When the reset signal is input to integrator 13A in step S5 as described above, the pulsation phase output from integrator 13A is reset to zero in the second mode. That is, the pulsation phase is reset to zero before switching from the second mode to the first mode. Therefore, when switching from the second mode to the first mode, an amount of change in the value of the phase input to switching signal generator <NUM> is suppressed. Thus, when switching from the second mode to the first mode, occurrence of the loss of synchronization in motor <NUM> is suppressed, with the result that the beat suppressing effect is gradually exhibited.

<FIG> is a schematic diagram showing an air conditioner <NUM> according to a third embodiment. Air conditioner <NUM> includes a refrigeration cycle apparatus <NUM> and a blower <NUM>. Refrigeration cycle apparatus <NUM> includes a refrigerant compressing apparatus <NUM>, a condenser <NUM>, an expansion valve <NUM>, and an evaporator <NUM>. Refrigerant compressing apparatus <NUM> has a compressor <NUM> and power conversion device <NUM> described above.

As shown in <FIG>, compressor <NUM> and condenser <NUM> are connected together by a tube. Similarly, condenser <NUM> and expansion valve <NUM> are connected together by a tube, expansion valve <NUM> and evaporator <NUM> are connected together by a tube, and evaporator <NUM> and compressor <NUM> are connected together by a tube. Thus, refrigerant circulates through compressor <NUM>, condenser <NUM>, expansion valve <NUM>, and evaporator <NUM>.

Motor <NUM> shown in <FIG> is provided in compressor <NUM> of air conditioner <NUM>, and is subject to variable speed control by power conversion device <NUM> in order to compress refrigerant gas into high-pressure gas. In refrigeration cycle apparatus <NUM>, processes of evaporation, compression, condensation, and expansion of the refrigerant are repeatedly performed. Further, the refrigerant is changed from liquid to gas and is further changed from gas to liquid, thereby performing heat exchange between the refrigerant and outside air. Therefore, air conditioner <NUM> is constructed by combining refrigeration cycle apparatus <NUM> with blower <NUM> configured to circulate the outside air.

An air conditioner in recent years is required to attain not only comfortability but also high efficiency due to tightened regulation for energy saving. Moreover, a demand for air conditioners is being increased in emerging countries. Therefore, it is important to provide an inexpensive air conditioner that uses a power conversion device for variable speed control of a motor. Since power conversion device <NUM> includes inexpensive small-capacitance DC link capacitor <NUM>, these requirements can be satisfied.

When the beat component appears in the current flowing through motor <NUM> while the operation frequency of motor <NUM> and the pulsation frequency of the DC voltage are close to each other, vibration and noise may be generated from compressor <NUM> or the tube connected to compressor <NUM>. As a result, comfortability of the user is compromised. Further, since the pulsation is superimposed on a work performed by motor <NUM>, efficiency of compression of the refrigerant gas is also decreased. Moreover, when an operation is performed while avoiding the operation frequency at which the beat component is generated, an optimum operation of the refrigeration cycle apparatus cannot be performed, thus resulting in decreased cycle efficiency. However, by using power conversion device <NUM> including beat suppression controller <NUM>, occurrence of the beat component is suppressed. As a result, these problems are solved.

As described above, air conditioner <NUM> includes power conversion device <NUM> including small-capacitance DC link capacitor <NUM> and beat suppression controller <NUM>. Thus, inexpensive air conditioner <NUM> allowing for comfortability and high efficiency is provided. It should be noted that air conditioner <NUM> may include power conversion device 100A instead of power conversion device <NUM>. Also in this case, inexpensive air conditioner <NUM> allowing for comfortability and high efficiency is provided.

In the description above, air conditioner <NUM> has been described as an example to which each of power conversion devices <NUM>, 100A is applied; however, each of power conversion devices <NUM>, 100A can also be used for other devices. For example, each of power conversion devices <NUM>, 100A may be applied to a mechanical device such as a fan or a pump.

The embodiments disclosed herein are illustrative and non-restrictive in any respect. The scope of the present disclosure is defined by the terms of the claims, rather than the embodiments described above.

Claim 1:
A power conversion device (<NUM>, 100A) comprising:
a rectifier circuit (<NUM>) configured to rectify an AC voltage to a DC voltage;
an inverter circuit (<NUM>) configured to convert, into an AC voltage, the DC voltage rectified by the rectifier circuit (<NUM>) and output the converted AC voltage to a motor (<NUM>);
a DC link capacitor (<NUM>) connected between the rectifier circuit (<NUM>) and the inverter circuit (<NUM>);
a generator (<NUM>) configured to generate a signal for controlling the inverter circuit (<NUM>) to output, from the inverter circuit (<NUM>), an AC voltage corresponding to a designated phase;
an estimator (<NUM>) configured to estimate a first phase of a rotor of the motor (<NUM>) based on a current flowing through the motor (<NUM>); and
a beat suppression controller (<NUM>, 10A) configured to output a second phase to the generator (<NUM>) as the designated phase so as to suppress a beat component superimposed on the current flowing through the motor (<NUM>), the second phase being obtained by adjusting the first phase, characterized in that
the beat suppression controller (10A) includes a switch (<NUM>) configured to switch between a first mode and a second mode, the first mode being a mode in which the second phase is output to the generator (<NUM>) as the designated phase, the second mode being a mode in which the first phase is output to the generator (<NUM>) as the designated phase.