Electric apparatus

Abstract

To provide a technique capable of preventing a switching operation of a switching element from being stopped due to a regenerative current.SOLUTION: An electric apparatus comprises: a transformer; a first smoothing circuit; a second smoothing circuit; a feedback circuit; a switching element; a switching control circuit; a motor operated by power from the first smoothing circuit; a motor control circuit operated by power from the second smoothing circuit, and outputting a motor command signal indicating a command value of the number of revolutions of the motor; a motor drive circuit for controlling an operation of the motor according to the motor command signal; an absorption resistor for absorbing a regenerative current from the motor; and a current-limiting circuit for limiting a current flowing through the absorption resistor. The current-limiting circuit is constituted so as to relieve limitation of the current flowing through the absorption resistor, when reducing the command value of the number of revolutions of the motor, which is indicated by the motor command signal.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The technology disclosed in this specification relates to electrical equipment. [Background technology]

[0002] Patent Document 1 discloses an electric device. The electric device includes a transformer having a primary winding, a first secondary winding, and a second secondary winding, a first smoothing circuit that smoothes AC power from the first secondary winding to output DC power of a first voltage, a second smoothing circuit that smoothes AC power from the second secondary winding to output DC power of a second voltage, a feedback circuit that outputs a feedback signal indicating the magnitude of the first voltage, a switching element that switches on / off the power supply to the primary winding, a switching control circuit that controls the switching operation of the switching element based on the feedback signal so that the first voltage approaches a target voltage, a motor that operates with power from the first smoothing circuit, a motor control circuit that operates with power from the second smoothing circuit and outputs a motor command signal that indicates a command value for the number of rotations of the motor, and a motor drive circuit that controls the operation of the motor by controlling the power supplied to the motor according to the motor command signal. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 7-332747 Summary of the Invention [Problem to be solved by the invention]

[0004] In the electric device described above, when the rotation speed of the motor decreases, a regenerative current is generated. When this regenerative current flows into the first smoothing circuit, the first voltage increases. At this time, the feedback circuit detects the increase in the first voltage, and the switching control circuit controls the switching operation of the switching element so as to decrease the first voltage. This suppresses the increase in the first voltage, and the first voltage is maintained at the target voltage. However, when the first voltage increases beyond the adjustment capability of the first voltage, the switching control circuit stops the switching operation of the switching element. In this case, not only is the power supply from the first secondary winding stopped and the motor stops, but the power supply from the second smoothing circuit is also stopped and the operation of the motor control circuit stops.

[0005] This specification provides a technique capable of preventing the switching operation of a switching element from being stopped due to a regenerative current. [Means for solving the problem]

[0006] In a first aspect disclosed in the present specification, an electric device may include a transformer having a primary winding, a first secondary winding, and a second secondary winding, a first smoothing circuit that smoothes AC power from the first secondary winding to output DC power of a first voltage, a second smoothing circuit that smoothes AC power from the second secondary winding to output DC power of a second voltage, a feedback circuit that outputs a feedback signal indicating a magnitude of the first voltage, a switching element that switches on / off power supply to the primary winding, a switching control circuit that controls a switching operation of the switching element based on the feedback signal so that the first voltage approaches a target voltage, a motor that operates with power from the first smoothing circuit, a motor control circuit that operates with power from the second smoothing circuit and outputs a motor command signal that indicates a command value for a rotation speed of the motor, a motor drive circuit that controls operation of the motor by controlling power supplied to the motor in accordance with the motor command signal, an absorption resistor that absorbs regenerative current from the motor, and a current limiting circuit that limits a current flowing through the absorption resistor. The current limiting circuit may be configured to relax the limit on the current flowing through the absorbing resistor when the command value of the rotation speed of the motor indicated by the motor command signal decreases.

[0007] According to the above configuration, the regenerative current generated by the reduction in the rotation speed of the motor flows into the absorption resistor and is absorbed. This makes it possible to suppress the regenerative current from flowing into the first smoothing circuit, and suppress the rise in the first voltage. Therefore, it is possible to suppress the switching operation of the switching element from being stopped due to the rise in the first voltage.

[0008] Also, according to the above configuration, the current limiting circuit limits the current flowing through the absorbing resistor. This makes it possible to suppress energy waste caused by the current from the first smoothing circuit flowing through the absorbing resistor. Furthermore, according to the above configuration, when the command value of the motor rotation speed indicated by the motor command signal decreases, the current limiting circuit relaxes the limit on the current flowing through the absorbing resistor. This makes it possible to relax the limit on the current flowing through the absorbing resistor at a timing when the generation of regenerative current is expected, and effectively suppress an increase in the first voltage.

[0009] In a second aspect, in the electric device of the first aspect, the electric device may further include a switch provided between the first smoothing circuit and the absorbing resistor. The motor control circuit may be capable of switching the switch between conductive and non-conductive states. The motor control circuit may be configured to make the switch conductive when rotating the motor and to make the switch non-conductive when stopping the motor.

[0010] According to the above configuration, in a situation where there is no need to supply power from the first smoothing circuit to the motor, such as when the motor is stopped, the motor control circuit switches the switch to a non-conducting state, thereby preventing the power output from the first smoothing circuit from being wasted in the absorption resistor.

[0011] In a third aspect, in the electrical device of the first or second aspect, the current limiting circuit may include a transistor connected in series with the absorption resistor and configured to operate in response to a transistor control signal, and an operational amplifier configured to receive a voltage corresponding to the motor command signal at an inverting input terminal and to output the transistor control signal from an output terminal.

[0012] In the above configuration, the current flowing through the absorbing resistor is limited by a transistor connected in series with the absorbing resistor. In addition, in the above configuration, when a motor command signal to reduce the rotation speed of the motor is output from the motor control circuit, the voltage input to the inverting input terminal of the operational amplifier decreases, so that the voltage of the transistor control signal increases, thereby relaxing the restriction on the current flowing through the absorbing resistor. With the above configuration, with a simple circuit configuration, when a motor command signal to reduce the rotation speed of the motor is output, the restriction on the current flowing through the absorbing resistor can be relaxed.

[0013] In a fourth aspect, in the electric device of the third aspect, the motor command signal may be a PWM signal in which the magnitude of the command value of the rotation speed of the motor is expressed by a duty ratio. The current limiting circuit may further include a third smoothing circuit that smoothes the motor command signal and inputs a voltage to the inverting input terminal.

[0014] When the motor command signal is a PWM signal, the duty ratio of the PWM signal is reduced when the motor rotation speed is reduced. According to the above configuration, the voltage obtained by smoothing the motor command signal in the third smoothing circuit is input to the inverting input terminal, so that when a motor command signal that reduces the motor rotation speed is output, the voltage input to the inverting input terminal drops. This causes the voltage of the transistor control signal to increase, thereby relaxing the restriction on the current flowing through the absorbing resistor. According to the above configuration, even when the motor command signal is a PWM signal, when a motor command signal that reduces the motor rotation speed is output, the restriction on the current flowing through the absorbing resistor can be relaxed.

[0015] In a fifth aspect, in the electric device according to the third or fourth aspect, the transistor may be an NPN bipolar transistor.

[0016] In an NPN bipolar transistor, the collector current increases or decreases as the base current increases or decreases. With the above configuration, the magnitude of the base current can be adjusted using the transistor control signal, thereby adjusting the magnitude of the current flowing through the absorbing resistor.

[0017] In a sixth aspect, in the electrical device of the fifth aspect, the current limiting circuit may further include a first resistor connecting between the collector terminal of the transistor and the non-inverting input terminal of the operational amplifier, and a second resistor connecting between the non-inverting input terminal of the operational amplifier and the emitter terminal of the transistor.

[0018] According to the above configuration, a voltage obtained by dividing the collector-emitter voltage of the transistor by the first resistor and the second resistor is input to the non-inverting input terminal of the operational amplifier. According to the above configuration, a voltage corresponding to the voltage applied between the collector-emitter of the transistor can be output as a transistor control signal from the output terminal of the operational amplifier. With this configuration, the behavior of the transistor can be changed with appropriate sensitivity in response to an increase or decrease in the motor command signal.

[0019] In a seventh aspect, in the electrical device of the sixth aspect, the absorbing resistor may be connected between the positive terminal of the motor drive circuit and the collector terminal of the transistor, and the current limiting circuit may further include a third resistor connecting between the positive terminal of the motor drive circuit and the non-inverting input terminal of the operational amplifier.

[0020] According to the above configuration, by using a third resistor having an appropriate resistance value, it is possible to change the behavior of the transistor with appropriate sensitivity in response to an increase or decrease in the motor command signal. [Brief description of the drawings]

[0021] [Figure 1] 2 is a diagram showing a schematic circuit configuration of an electric device 2 according to the embodiment. [Diagram 2]1 is a diagram showing a schematic circuit configuration of an electric device 102 of a comparative example. [Diagram 3] 4 is an example of a timing chart of each signal in an electric device 102 of a comparative example. [Figure 4] 4 is an example of a timing chart of each signal in an electric device 2 of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] (Configuration of Electrical Device 2 of the Example) 1 is a blower that blows air for combustion to a burner of a water heater, etc. The electric device 2 includes a power supply circuit 4, a motor 6, a motor drive circuit 8, a motor control microcomputer 10, an absorption resistor 42, and a current limiting circuit 12.

[0023] The power supply circuit 4 includes a primary smoothing circuit 14, a transformer 16, a switching element 18, a switching control circuit 20, a first secondary smoothing circuit 23, a feedback circuit 30, a second secondary smoothing circuit 27, and a voltage regulator 32. The first secondary smoothing circuit 23 includes a rectifier diode 22 and an electrolytic capacitor 24. The second secondary smoothing circuit 27 includes a rectifier diode 26 and an electrolytic capacitor 28.

[0024] The primary smoothing circuit 14 rectifies and smoothes the AC power supplied from the commercial power source 13, and outputs DC power.

[0025] The transformer 16 includes a core 16a, a primary winding 16b, a first secondary winding 16c, and a second secondary winding 16d. The primary winding 16b, the first secondary winding 16c, and the second secondary winding 16d are wound around the core 16a and are magnetically coupled via the core 16a. The primary winding 16b and the switching element 18 are connected in series to the output of the primary smoothing circuit 14.

[0026] The switching element 18 is, for example, an N-channel MOSFET. When the switching element 18 is turned on, a current flows from the primary smoothing circuit 14 to the primary winding 16b. When the switching element 18 is turned off, the current flowing from the primary smoothing circuit 14 to the primary winding 16b is interrupted.

[0027] The switching control circuit 20 outputs a switching control signal to the switching element 18 to control the switching operation of the switching element 18. Specifically, the switching control circuit 20 repeatedly switches the switching element 18 on and off. This causes a periodic pulse-like current to flow through the primary winding 16b, thereby inducing AC power in each of the first secondary winding 16c and the second secondary winding 16d.

[0028] The AC power induced in the first secondary winding 16c is rectified by the rectifier diode 22 in the first secondary smoothing circuit 23, and smoothed by the electrolytic capacitor 24. As a result, DC power of the first voltage V1 is output to the first positive terminal 4a and the first negative terminal 4b. The feedback circuit 30 monitors the first voltage V1, and transmits a feedback signal indicating the magnitude of the first voltage V1 to the switching control circuit 20 via a photocoupler (not shown). Based on the received feedback signal, the switching control circuit 20 adjusts the ON / OFF duty ratio in the switching operation of the switching element 18 so that the first voltage V1 approaches a target voltage (e.g., 24 V).

[0029] The AC power induced in the second secondary winding 16d is rectified by the rectifier diode 26 in the second secondary smoothing circuit 27, and smoothed by the electrolytic capacitor 28. As a result, DC power of the second voltage V2 is output to the second positive terminal 4c and the second negative terminal 4d. The second voltage V2 is smaller than the first voltage V1, and for example, when the first voltage V1 is 24 V, the second voltage V2 is about 10 V. The voltage regulator 32 steps down the DC power of the second voltage V2, and outputs DC power of a third voltage V3 (for example, 5 V).

[0030] The motor 6 is, for example, a three-phase motor having a U-phase terminal 6a, a V-phase terminal 6b, and a W-phase terminal 6c. The motor drive circuit 8 includes an inverter circuit 34 connected to the motor 6, and a motor driver 36 connected to the inverter circuit 34. The inverter circuit 34 includes, for example, a positive terminal 34a, a negative terminal 34b, a first switching element 34c, a second switching element 34d, a third switching element 34e, a fourth switching element 34f, a fifth switching element 34g, and a sixth switching element 34h. A DC power of a first voltage V1 is supplied to the positive terminal 34a and the negative terminal 34b via the current limiting circuit 12.

[0031] The first switching element 34c, the second switching element 34d, the third switching element 34e, the fourth switching element 34f, the fifth switching element 34g, and the sixth switching element 34h are, for example, N-channel MOSFETs. The first switching element 34c connects between the positive terminal 34a and the U-phase terminal 6a. The second switching element 34d connects between the U-phase terminal 6a and the negative terminal 34b. The third switching element 34e connects between the positive terminal 34a and the V-phase terminal 6b. The fourth switching element 34f connects between the V-phase terminal 6b and the negative terminal 34b. The fifth switching element 34g connects between the positive terminal 34a and the W-phase terminal 6c. The sixth switching element 34h connects between the W-phase terminal 6c and the negative terminal 34b.

[0032] The motor driver 36 controls the power supplied to the motor 6 by switching on / off the first switching element 34c, the second switching element 34d, the third switching element 34e, the fourth switching element 34f, the fifth switching element 34g, and the sixth switching element 34h of the inverter circuit 34. In this way, the motor driver 36 can rotate the motor 6 at a desired rotation speed or stop the rotation of the motor 6.

[0033] The motor control microcomputer 10 operates on DC power of a third voltage V3 supplied from a voltage regulator 32. The motor control microcomputer 10 generates a motor command signal indicating a command value for the rotation speed of the motor 6, and outputs the signal to a motor driver 36 via a motor command output terminal 10a. The motor command signal is, for example, a PWM signal. In this case, the larger the command value for the rotation speed of the motor 6, the larger the duty ratio of the PWM signal becomes, and the smaller the command value for the rotation speed of the motor 6, the smaller the duty ratio of the PWM signal becomes. The motor driver 36 controls the operation of the motor 6 via an inverter circuit 34 in accordance with the motor command signal from the motor control microcomputer 10.

[0034] The current limiting circuit 12 includes a transistor 38, an operational amplifier 40, resistors 46, 48, 50, 52, 54, 56, and 58, capacitors 60, 62, and 64, and a switch 66.

[0035] The transistor 38 is, for example, an NPN bipolar transistor. The collector terminal of the transistor 38 is connected to the positive terminal 34a of the inverter circuit 34 via an absorption resistor 42. The emitter terminal of the transistor 38 is connected to the negative terminal 34b of the inverter circuit 34.

[0036] The output terminal of the operational amplifier 40 is connected to the base terminal of the transistor 38 via resistors 46 and 48 connected in series with each other. The operational amplifier 40 outputs a transistor control signal from its output terminal. The base terminal of the transistor 38 is connected to the negative terminal 34b of the inverter circuit 34 via a resistor 50. The connection point of the resistor 46 and the resistor 48 is connected to the negative terminal 34b of the inverter circuit 34 via a capacitor 60. The output terminal of the operational amplifier 40 is connected to the inverting input terminal of the operational amplifier 40 via a capacitor 62.

[0037] The inverting input terminal of the operational amplifier 40 is connected to the motor command output terminal 10a of the motor control microcomputer 10 via a resistor 52. The inverting input terminal of the operational amplifier 40 is connected to the negative terminal 34b of the inverter circuit 34 via a capacitor 64. The resistor 52 and the capacitor 64 configure a signal smoothing circuit 63. The negative terminal 34b of the inverter circuit 34 is connected to the first negative terminal 4b of the power supply circuit 4.

[0038] The non-inverting input terminal of the operational amplifier 40 is connected to the collector terminal of the transistor 38 via a resistor 54. The non-inverting input terminal of the operational amplifier 40 is connected to the positive terminal 34a of the inverter circuit 34 via a resistor 56. The non-inverting input terminal of the operational amplifier 40 is connected to the negative terminal 34b of the inverter circuit 34 via a resistor 58.

[0039] The switch 66 is, for example, a P-channel MOSFET. The switch 66 connects between the first positive terminal 4a of the power supply circuit 4 and the positive terminal 34a of the inverter circuit 34. When the switch 66 is turned on, DC power of the first voltage V1 is supplied from the power supply circuit 4 to the current limiting circuit 12 and the inverter circuit 34. When the switch 66 is turned off, the supply of DC power of the first voltage V1 from the power supply circuit 4 to the current limiting circuit 12 and the inverter circuit 34 is cut off. The motor control microcomputer 10 switches the switch 66 on and off. For example, the motor control microcomputer 10 turns on the switch 66 when rotating the motor 6 at a predetermined rotation speed, and turns off the switch 66 when stopping the rotation of the motor 6.

[0040] (Configuration of Electrical Device 2 of Comparative Example) 2 shows, as a comparative example, an electric device 102 that does not include a current limiting circuit 12. The electric device 102 in FIG. 2 has a similar configuration to the electric device 2 in FIG. 1, except that it does not include a current limiting circuit 12. In the electric device 102 in FIG. 2, a first positive terminal 4a of the power supply circuit 4 is connected to a positive terminal 34a of the inverter circuit 34, and a first negative terminal 4b of the power supply circuit 4 is connected to a negative terminal 34b of the inverter circuit 34.

[0041] (Behavior of Electrical Device 102 of Comparative Example When Regenerative Current Occurs) When the motor 6 is rotating, if the number of revolutions of the motor 6 decreases or if the rotation of the motor 6 stops, a regenerative current is generated in the motor 6. In the comparative electric device 102 shown in FIG. 2, when a regenerative current is generated in the motor 6, the regenerative current flows into the first secondary smoothing circuit 23 via the positive terminal 34a and the negative terminal 34b of the inverter circuit 34 and the first positive terminal 4a and the first negative terminal 4b of the power supply circuit 4. This increases the charge stored in the electrolytic capacitor 24, and the first voltage V1 rises. At this time, the feedback circuit 30 detects the rise in the first voltage V1, and the switching control circuit 20 adjusts the on / off duty ratio in the switching operation of the switching element 18 so as to lower the first voltage V1. This suppresses the rise in the first voltage V1, and the first voltage V1 is maintained at a target voltage (for example, 24V), as shown in the period from time T0 to time T1 in FIG. 3. At this time, the switching control circuit 20 adjusts the ON / OFF duty ratio in the switching operation of the switching element 18 so as to drop the first voltage V1, and accordingly the second voltage V2 drops.

[0042] Thereafter, when the regenerative current from the motor 6 further flows into the first secondary smoothing circuit 23 and the first voltage V1 rises beyond the ability of the power supply circuit 4 to regulate the first voltage V1, the switching control circuit 20 stops the switching operation of the switching element 18, as shown at time T1 in Fig. 3. This stops the induction of AC power in the first secondary winding 16c and stops the induction of AC power in the second secondary winding 16d. In this case, the first voltage V1 continues to rise slightly due to the inflow of the regenerative current, but starts to drop as the regenerative current decreases. In addition, the second voltage V2 drops significantly.

[0043] Thereafter, as shown at time T2 in Fig. 3, the operation of the voltage regulator 32 stops as the second voltage V2 drops, and further, as shown at time T3 in Fig. 3, the operation of the motor control microcomputer 10 stops as the third voltage V3 drops.

[0044] 3, when the first voltage V1 drops sufficiently, the switching control circuit 20 resumes the on / off switching operation of the switching element 18. This resumes the induction of AC power in the first secondary winding 16c, and the power supply from the first secondary smoothing circuit 23 is resumed. Also, the induction of AC power in the second secondary winding 16d is resumed, and the power supply from the second secondary smoothing circuit 27 is resumed. This causes the voltage regulator 32 and the motor control microcomputer 10 to resume their operations.

[0045] (Behavior of Electrical Device 2 in the Example When Regenerative Current Occurs) In the electric device 2 of the embodiment shown in Fig. 1, the motor command signal output from the motor control microcomputer 10 to the motor driver 36 is smoothed by the signal smoothing circuit 63 and input to the inverting input terminal of the operational amplifier 40. Since the motor command signal is a PWM signal, when the motor control microcomputer 10 outputs a motor command signal that reduces the rotation speed of the motor 6, the input voltage of the non-inverting input terminal of the operational amplifier 40 drops. This causes the voltage of the output terminal of the operational amplifier 40 to rise, and the base current of the transistor 38 to increase. In response to this, the collector current of the transistor 38 increases, and the current flowing through the absorbing resistor 42 increases.

[0046] Therefore, the regenerative current generated by the reduction in the rotation speed of the motor 6 flows into the absorption resistor 42 and is absorbed. This makes it possible to suppress the regenerative current from flowing into the first secondary smoothing circuit 23 of the power supply circuit 4, and suppress the rise in the first voltage V1 (see the period after time T0 in FIG. 4). Since the regenerative current flowing into the first secondary smoothing circuit 23 cannot be reduced to zero, the first voltage V1 rises slightly, and the on / off duty ratio in the switching operation of the switching element 18 is adjusted accordingly, so that the first voltage V1 is maintained at a target voltage (e.g., 24V) and the second voltage V2 drops. However, in the electric device 2 of the embodiment, the first voltage V1 does not rise beyond the adjustment ability of the power supply circuit 4 for the first voltage V1, so the switching control circuit 20 does not stop the switching operation of the switching element 18. Therefore, it is possible to prevent the ON / OFF switching operation of the switching element 18 from being stopped due to an increase in the first voltage V1, and to prevent the resulting stop of the power supply from the first secondary smoothing circuit 23 and the second secondary smoothing circuit 27. It is possible to prevent the voltage regulator 32 and the motor control microcomputer 10 from stopping their operations.

[0047] (Correspondence) The first secondary smoothing circuit 23 is an example of a first smoothing circuit, the second secondary smoothing circuit 27 is an example of a second smoothing circuit, the motor control microcontroller 10 is an example of a motor control circuit, the signal smoothing circuit 63 is an example of a third smoothing circuit, resistor 54 is an example of a first resistor, resistor 58 is an example of a second resistor, and resistor 56 is an example of a third resistor.

[0048] (Modification) The electric device 2 may be another type of electric device. In addition, the specific configurations of the first smoothing circuit, the second smoothing circuit, the feedback circuit, the switching control circuit, the motor control circuit, the motor drive circuit, the current limiting circuit, the third smoothing circuit, and the like are not limited to the configurations described in the embodiments, and may be replaced with other configurations that exhibit similar functions.

[0049] Although each embodiment has been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or drawings can achieve multiple objectives simultaneously, and achieving one of these objectives is itself technically useful. [Explanation of symbols]

[0050] 2: Electrical equipment 4: Power supply circuit 4a: First positive terminal 4b: 1st negative terminal 4c: Second positive terminal 4d: 2nd negative terminal 6: Motor 6a: U phase terminal 6b:V phase terminal 6c:W phase terminal 8: Motor drive circuit 10: Motor control microcomputer 10a: Motor command output terminal 12: Current limiting circuit 13:Commercial power supply 14:Primary smoothing circuit 16: Trans 16a: Core 16b: Primary winding 16c: 1st secondary winding 16d: Secondary winding 18: Switching element 20: Switching control circuit 22: Rectifier diode 23: 1st secondary smoothing circuit 24: Electrolytic capacitor 26: Rectifier diode 27:Second secondary smoothing circuit 28: Electrolytic capacitor 30: Feedback circuit 32: Voltage regulator 34: Inverter circuit 34a: Positive terminal 34b: Negative terminal 34c: First switching element 34d: Second switching element 34e: Third switching element 34f: Fourth switching element 34g: 5th switching element 34h: 6th switching element 36: Motor driver 38: Transistor 40: Operational amplifier 42: Absorption resistor 46:Resistor 48:Resistor 50:Resistor 52:Resistor 54:Resistor 56:Resistor 58:Resistor 60: Capacitor 62: Capacitor 63: Signal smoothing circuit 64: Capacitor 66: Switching element 102: Electrical equipment

Claims

1. An electrical device comprising: a transformer including a primary winding, a first secondary winding, and a second secondary winding; a first smoothing circuit that smoothes the AC power from the first secondary winding and outputs DC power of a first voltage; a second smoothing circuit that smoothes the AC power from the second secondary winding and outputs DC power having a second voltage; a feedback circuit that outputs a feedback signal indicative of the magnitude of the first voltage; a switching element that switches on / off the current supply to the primary winding; a switching control circuit that controls a switching operation of the switching element based on the feedback signal so that the first voltage approaches a target voltage; a motor operated by power from the first smoothing circuit; a motor control circuit that operates using power from the second smoothing circuit and outputs a motor command signal that indicates a command value for the number of rotations of the motor; a motor drive circuit that controls the operation of the motor by controlling power supplied to the motor in accordance with the motor command signal; an absorption resistor for absorbing a regenerative current from the motor; a current limiting circuit for limiting a current flowing through the absorption resistor; the current limiting circuit is configured to relax the limit on the current flowing through the absorption resistor when the command value of the rotation speed of the motor indicated by the motor command signal decreases.

2. The inverter further includes a switch provided between the first smoothing circuit and the absorbing resistor, the motor control circuit is capable of switching the switch between conductive and non-conductive states; 2. The electric device of claim 1, wherein the motor control circuit is configured to make the switch conductive when the motor is rotated and to make the switch non-conductive when the motor is stopped.

3. The current limiting circuit is a transistor connected in series with the absorbing resistor and configured to operate in response to a transistor control signal; 2. The electric device according to claim 1, further comprising an operational amplifier configured to receive a voltage corresponding to the motor command signal at an inverting input terminal and to output the transistor control signal from an output terminal.

4. the motor command signal is a PWM signal in which the magnitude of the command value of the rotation speed of the motor is expressed by a duty ratio, 4. The electric device according to claim 3, wherein the current limiting circuit further comprises a third smoothing circuit that smoothes the motor command signal and inputs the smoothed voltage to the inverting input terminal.

5. 4. The electric device according to claim 3, wherein the transistor is an NPN type bipolar transistor.

6. The current limiting circuit is a first resistor connected between the collector terminal of the transistor and the non-inverting input terminal of the operational amplifier; 6. The electrical device of claim 5, further comprising a second resistor connected between the non-inverting input terminal of the operational amplifier and the emitter terminal of the transistor.

7. the absorption resistor is connected between the positive terminal of the motor drive circuit and the collector terminal of the transistor; 7. The electric device of claim 6, wherein the current limiting circuit further comprises a third resistor connected between the positive terminal of the motor drive circuit and the non-inverting input terminal of the operational amplifier.

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