HK1094487B - Brushless dc motor coupled directly to ac source and electric apparatus using the same motor - Google Patents

Description

Brushless DC motor with direct AC power coupling and electric apparatus using the same

Technical Field

The present invention relates to a brushless Direct Current (DC) motor that drives a fan, and also relates to an electric apparatus using such a motor.

Background

Recently, a brushless DC motor using permanent magnets has been widely used to drive a fan of a ventilator. Fig. 23 shows a circuit schematic of a conventional brushless DC motor directly coupled to an Alternating Current (AC) power source for driving a fan. The circuit uses a two-phase half-wave driving method and includes the following elements:

a rectifier 101 that rectifies a commercial AC power supply;

a filter large-capacity aluminum electrolytic capacitor 109;

two-phase stator coils 103, 104 that drive the magnet rotor 105; and

controls power to the stator coils, includes switching elements 107, 108 and a controller 102 mounted on a printed circuit board 106.

A high voltage DC power supply directly from the rectifier 101 is provided to the designated sub-coils 103, 104 and a reduced low voltage from the high voltage DC power supply is provided to the controller 102. The aforementioned brushless DC motor driven by the two-phase half-wave driving method generates considerable noise and vibration, and requires a filter capacitor of a large capacity. It is an object of the present invention to provide a brushless DC motor directly coupled to an AC power source that overcomes these problems.

Disclosure of Invention

According to a first aspect of the present invention, there is provided a brushless DC motor directly coupled to an AC power source, comprising:

a stator including a stator coil;

a rotor including a rotor magnet;

a magnetic flux sensor that detects a magnetic flux density distribution of the rotor magnet;

an inverter circuit including a plurality of switching elements coupled in a full-wave bridge including an upper arm and a lower arm;

an AC power coupler;

a rectifier for full-wave rectifying the voltage of the AC power source;

a DC voltage converter for converting the rectified voltage supplied from the rectifier into a flat low DC voltage and applying the flat low DC voltage as a power source to the inverter circuit; a controller for controlling the inverter circuit according to a signal provided from the magnetic flux sensor so as to supply a low DC voltage to the designated sub-coil in a full-wave driving method;

a current controller for constantly adjusting an average current value applied to the inverter circuit at a set current;

a set current changer for changing the set current regulated by the current controller;

wherein the AC power coupler includes a weak output terminal and a strong output terminal, and

wherein the set current changer reduces the set current adjusted by the current controller to be lower when the weak output terminal is selected than the set current determined when the strong output terminal is selected.

According to a second aspect of the present invention, there is provided a brushless dc motor directly coupled to an ac power source, the motor comprising:

a stator including a stator coil;

a rotor including a rotor magnet;

a magnetic flux sensor that detects a magnetic flux density distribution of the rotor magnet;

an inverter circuit including a plurality of switching elements coupled in a full-wave bridge including an upper arm and a lower arm;

an alternating current power supply coupler;

a rectifier full-wave rectifying a voltage of an alternating current power supply;

a direct-current voltage converter for converting the rectified voltage supplied from the rectifier into a low direct-current voltage and applying the low direct-current voltage as a power source to the inverter circuit;

a controller for controlling the inverter circuit according to a signal provided from the magnetic flux sensor so as to supply a low direct current voltage to the designated sub-coil in a full-wave driving method; and

current command means for commanding an average current value supplied to the inverter circuit;

a current controller for constantly adjusting an average current value supplied to the inverter circuit at a command value;

an output device for outputting a signal of the rpm of the motor based on the signal provided by the magnetic flux sensor,

wherein the current command means provides the average current value to the inverter circuit in response to the motor rpm command.

Furthermore, the present invention provides an electric apparatus mounting the brushless dc motor according to the first and second aspects.

Brief Description of Drawings

Fig. 1 shows a cross-sectional view of a brushless DC motor according to a first exemplary embodiment of the present invention;

fig. 2 shows a circuit diagram of the brushless DC motor shown in fig. 1;

FIG. 3 illustrates output voltage waveforms of a rectifier of the brushless DC motor shown in FIG. 1;

fig. 4 shows a circuit diagram of a brushless DC motor as shown in fig. 1 using a PWM (pulse width modulation) method;

fig. 5 shows a circuit diagram of a brushless DC motor according to a second exemplary embodiment of the present invention;

fig. 6 shows a revolution per minute (rpm) -torque characteristic of the brushless DC motor shown in fig. 5;

FIG. 7 shows a circuit diagram of the brushless DC motor shown in FIG. 5 utilizing a pulse width modulation method;

fig. 8 shows a circuit diagram of a brushless DC motor according to a third exemplary embodiment of the present invention;

fig. 9 shows rpm-current characteristics of the brushless DC motor shown in fig. 8;

fig. 10 shows rpm-torque characteristics of the brushless DC motor shown in fig. 8;

fig. 11 shows air volume-static pressure characteristics of a ventilator using the brushless DC motor shown in fig. 8;

FIG. 12 shows a circuit diagram of the brushless DC motor shown in FIG. 8 utilizing a pulse width modulation method;

fig. 13 shows a circuit diagram of a brushless DC motor according to a fourth exemplary embodiment of the present invention;

fig. 14 shows rpm-torque characteristics of the brushless DC motor shown in fig. 13;

fig. 15 shows the air volume-static pressure characteristics of a ventilator using the brushless DC motor shown in fig. 13;

fig. 16 shows a circuit diagram of the brushless DC motor shown in fig. 13 using a pulse width modulation method;

fig. 17 shows a circuit diagram of a brushless DC motor according to a fifth exemplary embodiment of the present invention;

fig. 18 is a circuit diagram showing the brushless DC motor shown in fig. 17 using a pulse width modulation method;

fig. 19 shows a circuit diagram of a brushless DC motor according to a sixth exemplary embodiment of the present invention;

fig. 20 shows a circuit diagram of a brushless DC motor according to a seventh exemplary embodiment of the present invention;

fig. 21 is a circuit diagram showing the brushless DC motor shown in fig. 20 using a pulse width modulation method;

fig. 22 shows a side view, a front view, and a top view of a ventilator using the brushless DC motor of the present invention; and

fig. 23 shows a circuit diagram of a conventional brushless DC motor.

Best mode for carrying out the invention

Exemplary embodiments of the present invention are described below with reference to the accompanying drawings.

1 st exemplary embodiment

Fig. 1 shows a cross-sectional view of a brushless DC motor according to a first exemplary embodiment. The stator 10 is formed by molding a thermosetting resin 17 such as unsaturated polyester. The stator core 10a including a plurality of slots is wound with the stator coil 2 via an insulating material 11. The bracket 18 fixes the bearing 19, and the magnet rotor 3 rotatably faces the stator 10.

The magnet rotor 3 is formed by injection molding magnets in a body with a rotating shaft 20, and the magnet rotor 3 is provided with a polar orientation at the time of molding. Therefore, the magnet rotor magnet 3a is a magnet having magnetic pole anisotropy.

The Hall (Hall) element 4 serves as a magnetic flux sensor that detects the magnetic flux density distribution of the motor pole 3 a. The space between the hall element 4 and the magnet rotor magnet 3a is provided so that the waveform of the magnetic flux density distribution detected by the hall element 4 becomes a pattern similar to the waveform of the voltage induced in the stator coil 2 by the motor magnetic pole 3 a.

Fig. 2 shows a circuit of the brushless DC motor of the present embodiment. The commercial AC power source is coupled with an AC power coupler 15, and the AC power coupler 15 has two terminals, i.e., a strong output terminal 15a and a weak output terminal 15b, for obtaining a two-step rpm-torque characteristic. More line ends will produce a more step characteristic. The wire end 15c serves as a common end. The external switch 24 selects one of the line terminals 15a and 15 b.

The rectifier 9 full-wave rectifies the commercial AC power source. The DC voltage converter 8 formed of a step-down chopper circuit steps down the high full-wave rectified voltage supplied from the rectifier 9 to a low DC voltage of not more than 45V and outputs the low DC voltage. This low DC voltage is applied as a power source to the inverter voltage 6.

A filter capacitor 13 formed of a polymer capacitor and having a small capacity is located between the rectifier 9 and the DC voltage converter 8 so that the capacitor 13 supplements the voltage in a relatively short period, for example, about 2.1 milliseconds, in the case where the AC 100V of 50Hz full-wave rectification is reduced to not more than 45V (refer to fig. 3).

The connection sensor 16 detects whether the terminal 15a or 15b is selected by the switch 24. In the case where the weak output terminal 15b is selected, the DC voltage changer 14 lowers the low DC voltage supplied from the converter 8 to be lower than that in the case where the strong output terminal 15a is selected. In fig. 2, a circuit enclosed by an alternate long and short dash line forms a single chip IC 21 except for the filter capacitor and the coil and mounted on the aluminum substrate.

The magnetic flux density synthesizer 12 subtracts the V-phase waveform from the U-phase waveform of the hall element 4 to cancel the harmonic component of the U-phase current of the motor. The synthesizer 12 also subtracts the W-phase waveform from the V-phase waveform of the hall element 4 to cancel the harmonic component of the V-phase current of the motor. The synthesizer 12 also subtracts the U-phase waveform from the W-phase waveform of the hall element 4 to cancel the harmonic component of the W-phase current of the motor.

The switching elements Q1, Q3, and Q5 in the upper arm and the switching elements Q2, Q4, and Q6 in the lower arm are coupled together in a full-wave bridge, thereby forming the inverter circuit 6.

The controller 5 controls the inverter circuit 6 in accordance with the signal supplied from the hall element 4 so that the stator coil 2 can be full-wave driven in a prescribed direction and in a prescribed sequence.

The current waveform controller 7 adjusts the output bias current so as to shape the waveform of the motor current into a pattern similar to the waveform from which harmonic components have been removed by the synthesizer 12, while providing feedback signals to the switching elements Q1-Q6 so as to maintain the switching elements in an unsaturated state that is still close to the saturated state.

In the brushless DC motor of the present invention, the DC voltage converter 8 converts the high voltage subjected to the full-wave rectification into a predetermined low voltage of not more than 45V and supplies this low DC voltage to the designated sub-coil 2. Therefore, voltage fluctuations in the AC current do not change the characteristics of the motor. The motor is powered by full-wave driving the inverter circuit 6 at a generally flat low DC voltage, but the harmonic content of the motor current is still eliminated so that the motor produces less noise and vibration. As shown in fig. 3, the filter capacitor 13 may have a small capacity just enough to supplement the voltage in a relatively short period, for example, about 2.1 milliseconds, in the case where the voltage becomes insufficient. Therefore, the filter capacitor 13 can employ a smaller capacitor, for example, a solid-state capacitor characterized by a longer life and a smaller characteristic change caused by the ambient temperature. Solid state capacitors include polymer capacitors, ceramic capacitors, and thin film capacitors.

As shown in fig. 4, Pulse Width Modulation (PWM) control is applicable to the brushless DC motor of the present embodiment. The PWM controller 25 controls the switching elements Q2, Q4, and Q6 of the lower arm in a PWM method, and also controls the inverter circuit 6 in accordance with the signal supplied from the hall element 4 so as to supply power to the designated sub-coil 2 in a full-wave driving in a designated direction and order. The current waveform controller 7 adjusts the ON/OFF operation of the switching element in the lower arm to control the waveform. Instead of the switching elements of the lower arm, the switching elements Q1, Q3, and Q5 of the upper arm may be PWM-controlled. As a result, the number of steps required to adjust the specifications of the apparatus can be reduced.

The operation mode command means 26 provides a command regarding the ON/OFF operation mode (duty) of PWM with a voltage obtained by lowering the low DC voltage.

Coupling sensor 16 detects whether switch 24 selects line terminal 15a or 15b of AC power coupler 15. When the weak output terminal 15b is selected, the operation mode instructing means 26 shortens the ON operation mode of the PWM so that the average voltage applied to the designated sub-coil 2 becomes lower than that when the strong output terminal 15a is selected.

Exemplary embodiment 2

Fig. 5 shows a circuit of a brushless DC motor according to a second exemplary embodiment of the present invention. Elements similar to those in the previous embodiment have the same reference numerals, and their description is omitted here.

The current sensor 30 detects the current of the inverter circuit 6. The current controller 31 adjusts the low DC voltage supplied from the DC voltage converter 8 so that the average current of the inverter circuit 6 becomes equal to the set current. Since withstand voltage and kick-back voltage of the switching element should be considered, the upper limit of the voltage is set to a low DC voltage to be supplied to the inverter circuit 6.

Coupling sensor 16 detects whether switch 24 selects line terminal 15a or 15b of AC power coupler 15. When the weak output terminal 15b is selected, the coupling sensor 16 instructs the set current changer 32 to lower the set current of the inverter circuit 6 than when the strong output terminal 15a is selected.

In the brushless DC motor of the present embodiment, the average current of the inverter circuit 6 is kept constant so that the output torque is normally kept constant as shown in fig. 6. Since the difference in rpm with respect to the variation in load is large, the ventilator to which the brushless DC motor of the present embodiment is applied has an air volume-static pressure characteristic similar to that of the ventilator to which the induction motor is applied. Therefore, the aforementioned structure can prevent an extremely low static pressure or an extremely large air volume at 0 (zero) static pressure, and also achieve a low noise and low power consumption ventilation apparatus.

Fig. 7 tells us that PWM control can be applied to the brushless DC motor of the present embodiment as described in the previous embodiment. The current controller 31 controls the operation mode instructing means 26 that adjusts the ON/OFF operation mode of the PWM so that the average current of the inverter circuit 6 becomes equal to the set current. In other words, the low DC voltage provided by the DC voltage converter 8 is regulated.

Exemplary embodiment 3

Fig. 8 shows a circuit of a brushless DC motor according to a third exemplary embodiment of the present invention. Elements similar to those in the previous embodiment have the same reference numerals, and their description is omitted here.

The rotation signal output device 34 outputs pulses indicating the revolutions per minute of the motor from the waveform detected by the magnetic flux sensor 4. The current command device 33 converts this pulse from frequency to voltage and takes the voltage, i.e. the rpm of the motor. In response to the rpm of the motor, the current command device 33 commands the inverter circuit 6 to increase the current with reference to the set value. In other words, current command device 33 commands circuit 6 to increase the current at a greater number of revolutions per minute.

The current controller 31 adjusts the low DC voltage supplied from the DC voltage converter 8 so that the average current of the inverter circuit 6 becomes equal to the current specified by the current command device 33. Since the withstand voltage and the kickback voltage of the switching element should be considered, the upper limit of the voltage is set to the low DC voltage to be supplied to the inverter circuit 6. As a result, the motor can be operated at a constant voltage without controlling the current even if the rpm is increased in a certain period.

Coupling sensor 16 detects whether switch 24 selects line terminal 15a or 15b of AC power coupler 15. When the weak output terminal 15b is selected, the coupling sensor 16 instructs the set current changer 32 to lower the set current of the inverter circuit 6 than when the strong output terminal 15a is selected.

In the brushless DC motor of the present embodiment, as shown in fig. 9, an increase in rmp increases the current of the inverter circuit 6, and as shown in fig. 10, the torque also increases. In other words, the current command device 33 increases the current of the inverter circuit 6 in response to an increase in the rpm of the motor so that the torque curve can reach a desired gradient.

Therefore, the ventilator to which the brushless DC motor of the present embodiment is applied can obtain the air volume-static pressure characteristic as shown in fig. 11. This characteristic causes the air volume to change only a little even if the pressure loss is changed by the difference of the external wind pressure or the length of the duct.

Fig. 12 tells us that PWM control can be applied to the brushless DC motor of the present embodiment as described in the previous embodiment. The current controller 31 controls the operation mode instructing means 26 to adjust the ON/OFF operation mode of PWM so that the average current of the inverter circuit 6 becomes equal to the current instructed by the current instructing means 33 to change in response to the rpm of the motor.

The upper limit of the current of the inverter circuit 6 is set in consideration of the allowable power consumption of the inverter circuit 6. The upper limit of the ON operation mode of the PWM is set to 100% so that the rise in the number of revolutions per minute is not accompanied by the increase in the current, and the motor is operated in the ON operation mode of 100% for a certain period.

Exemplary embodiment 4

Fig. 13 shows a circuit of a brushless DC motor according to a fourth exemplary embodiment of the present invention. Elements similar to those in the previous embodiment have the same reference numerals, and their description is omitted here.

The rotation signal output device 34 outputs pulses indicating the revolutions per minute of the motor from the waveform detected by the magnetic flux sensor 4. The rmp range sensor 39 converts this pulse from frequency to voltage, and acquires an rmp range including the rpm of the motor at that time.

Current command device 40 commands inverter circuit 6 to vary the current in response to a range of revolutions per minute. More specifically, the current command device 40 issues a command to the inverter circuit 6 so that a larger number of revolutions per minute is accompanied by a stepwise increase in current corresponding to each of the ranges of revolutions per minute.

The current sensor 30 detects the current of the inverter circuit 6. The current controller 31 adjusts the low DC voltage supplied from the DC voltage converter 8 so that the average current of the inverter circuit 6 becomes equal to the current specified by the current command device 40. Since the withstand voltage and the kickback voltage of the switching elements Q1-Q6 should be considered, the upper limit of the voltage is set to a low DC voltage to be supplied to the inverter circuit 6. As a result, the motor can be operated at a constant voltage without controlling the current even if the rpm is increased in a certain period.

Coupling sensor 16 detects whether switch 24 selects line terminal 15a or 15b of AC power coupler 15. When the weak output terminal 15b is selected, the coupling sensor 16 instructs the set current changer 32 to lower the set current of the inverter circuit 6 than when the strong output terminal 15a is selected.

As shown in fig. 14, in the brushless DC motor of the present embodiment, the torque is increased stepwise at a large number of revolutions per minute of the motor. In other words, the current command device 40 increases the current of the inverter circuit 6 stepwise in response to an increase in the motor rpm so that the torque can reach a desired gradient. On the broken line of the torque characteristic as shown in fig. 14, a lag between the decrease and the increase of the rpm is set, and the width of the lag is appropriately set so as not to cause troubles such as parasitic oscillation. The stepwise change of the current makes a frequency-to-voltage converter available with a smaller dynamic range.

The ventilation apparatus to which the brushless DC motor of the present embodiment is applied has the air volume-static pressure characteristic as shown in fig. 15. This characteristic causes the air volume to change only a little even if the pressure loss is changed by the difference of the external wind pressure or the length of the duct.

Fig. 16 tells us that PWM control can be applied to the brushless DC motor of the present embodiment as described in the previous embodiment. The current controller 31 controls the operation mode instructing means 26 to adjust the ON/OFF operation mode of PWM so that the average current of the inverter circuit 6 becomes equal to the current instructed by the current instructing means 33 to change in response to the rpm of the motor. For the same reason as described in the previous embodiment, it is necessary to add an upper limit to the current of the inverter circuit 6.

Exemplary embodiment 5

Fig. 17 shows a circuit of a brushless DC motor according to a fifth exemplary embodiment of the present invention. Elements similar to those in the previous embodiment have the same reference numerals, and their description is omitted here.

The voltage reduction means 46 is external to the motor via terminals 44 and 45. The low DC voltage supplied by the DC voltage converter 8 is applied to the inverter circuit 6 through the voltage reduction device 46.

The foregoing structure makes it possible to obtain a brushless DC motor directly coupled with an AC power supply, and to smoothly adjust the speed of the motor, that is, not stepwise, so that the air volume of the ventilation apparatus can be arbitrarily set. Fig. 18 tells us that PWM control can be applied to the brushless DC motor of the present embodiment as discussed in the previous embodiment.

Exemplary embodiment 6

Fig. 19 shows a circuit of a brushless DC motor according to a sixth exemplary embodiment of the present invention. Elements similar to those in the previous embodiment have the same reference numerals, and their description is omitted here.

The voltage reduction device 46 is external to the motor via terminals 53 and 54. The PWM controller 25 controls switching elements Q2, Q4, and Q6 in the lower arm. The current waveform controller 51 adjusts the ON/OFF operation of the switching element of the lower arm so that the waveform of the motor current becomes a pattern similar to the waveform from which the harmonic component has been removed by the magnetic flux density synthesizer 12. The reference voltage generating circuit 52 generates a constant output reference voltage by lowering the low DC voltage supplied from the DC voltage converter 50. Then, the reference voltage is applied to the PWM controller 25 through the voltage reduction device 46 outside the motor as an operation mode command voltage that instructs the ON/OFF operation mode of PWM.

The foregoing structure makes it possible to obtain a brushless DC motor directly coupled with an AC power supply, and to smoothly adjust the speed of the motor, that is, not stepwise, so that the air volume of the ventilation apparatus can be arbitrarily set.

7 th exemplary embodiment

Fig. 20 shows a circuit of a brushless DC motor according to a seventh exemplary embodiment of the present invention. Elements similar to those in the previous embodiment have the same reference numerals, and their description is omitted here.

The voltage reduction device 46 is external to the motor via terminals 53 and 54. The reference voltage generation circuit 52 lowers the reference voltage supplied from the DC voltage converter 50, thereby generating a constant-output reference voltage. Then, the reference voltage is applied to the set current changer 32 through the voltage lowering device 46 outside the motor.

The current changer 32 is provided to change the reference value depending on the applied voltage. The current command means 33 commands the inverter circuit 6 to maintain a constant current, or to change the current with reference to a reference value, or to change the current stepwise with reference to a reference value, in response to an output signal supplied from the rotation signal output means 34. At this time, since the withstand voltage and the kickback voltage of the switching element should be considered, the upper limit of the voltage is set to the low DC voltage to be supplied to the inverter circuit 6. As a result, the motor can be operated at a constant voltage without controlling the current even if the rpm is increased in a certain period.

In the present embodiment, the current of the inverter circuit 6 is increased at a larger rpm and, conversely, is decreased at a smaller rpm. As a result, the shaft torque of the motor increases at a larger rpm. The ventilation apparatus to which the brushless DC motor of the present embodiment is applied obtains a characteristic that the volume of air is changed only a little even if the pressure loss is changed by the difference of the external wind pressure or the length of the duct. Fig. 21 tells us that PWM control can be applied to the brushless DC motor of the present embodiment as discussed in the previous embodiment.

Next, an electric apparatus to which the brushless DC motor of the present invention is applied will be described. The brushless DC motor is suitable for electric apparatuses such as a ventilator and a blower. Fig. 22 shows a front view, a side view, and a top view of a ventilator to which the brushless DC motor of the present invention is applied.

In fig. 22, the ventilator 59 includes a brushless DC motor 58 and a centrifugal blower 60, wherein the motor 58 rotates a sirocco fan of the blower 60 to blow air. The motor discussed in the previous embodiment may be used as the motor 58 so that the ventilation apparatus of the present invention enjoys the advantages of the motor.

The brushless DC motor of the present invention directly coupled with an AC power supply can reduce torque ripple and reduce a rate of change of instantaneous torque, thereby suppressing noise and vibration. Such a motor has the following advantages: the current is small; the available range of the load torque is wide; low power consumption over a high power output range; the circuit size is reduced; the quality is high; the service life is long; uneven rotation is suppressed; the characteristic does not change with the change of the power supply voltage; and rpm-torque characteristics comparable to those of induction motors. Also, such a motor can reduce the number of steps required to adjust its specifications, and thus, such a motor is often used to be installed in electric equipment including the centrifugal blower 60, such as ventilation equipment, water heaters, air conditioners, air filters, dehumidifiers, dryers, and fan filter units. Further, since such a motor has an rpm-torque characteristic that a shaft torque increases at a large rpm, it is often used to be mounted to an electric apparatus including a centrifugal blower and an air volume-static pressure characteristic that requires an air volume hardly to change with a change in static pressure, as follows: ventilation equipment, water heaters, air filters, air conditioners, and blower units for cleaning rooms.

Industrial applicability

The brushless DC motor of the present invention is suitable for installation in ventilation equipment and blowers.

Claims (8)

1. A brushless dc motor directly coupled to an ac power source, the motor comprising:

(a) a stator including a stator coil;

(b) a rotor including a rotor magnet;

(c) a magnetic flux sensor that detects a magnetic flux density distribution of the rotor magnet;

(d) an inverter circuit including a plurality of switching elements coupled in a full-wave bridge including an upper arm and a lower arm;

(e) an alternating current power supply coupler;

(f) a rectifier full-wave rectifying a voltage of an alternating current power supply;

(g) a direct-current voltage converter for converting the rectified voltage supplied from the rectifier into a low direct-current voltage and applying the low direct-current voltage as a power source to the inverter circuit; and

(h) a controller for controlling the inverter circuit according to a signal provided from the magnetic flux sensor so as to supply a low direct current voltage to the designated sub-coil in a full-wave driving method;

(i) a current controller for constantly adjusting an average current value applied to the inverter circuit at a set current;

(j) a set current changer for changing the set current regulated by the current controller;

wherein the AC power coupler includes a weak output terminal and a strong output terminal, and

wherein the set current changer reduces the set current adjusted by the current controller to be lower when the weak output terminal is selected than the set current determined when the strong output terminal is selected.

2. A brushless dc motor directly coupled to an ac power source, the motor comprising:

(a) a stator including a stator coil;

(b) a rotor including a rotor magnet;

(c) a magnetic flux sensor that detects a magnetic flux density distribution of the rotor magnet;

(d) an inverter circuit including a plurality of switching elements coupled in a full-wave bridge including an upper arm and a lower arm;

(e) an alternating current power supply coupler;

(f) a rectifier full-wave rectifying a voltage of an alternating current power supply;

(g) a direct-current voltage converter for converting the rectified voltage supplied from the rectifier into a low direct-current voltage and applying the low direct-current voltage as a power source to the inverter circuit;

(h) a controller for controlling the inverter circuit according to a signal provided from the magnetic flux sensor so as to supply a low direct current voltage to the designated sub-coil in a full-wave driving method; and

(i) current command means for commanding an average current value supplied to the inverter circuit;

(j) a current controller for constantly adjusting an average current value supplied to the inverter circuit at a command value;

(k) an output device for outputting a signal of the rpm of the motor based on the signal provided by the magnetic flux sensor,

wherein the current command means provides the average current value to the inverter circuit in response to the motor rpm command.

3. The brushless dc motor of claim 2, wherein the ac power coupler includes a weak output terminal and a strong output terminal, and

wherein the commanded average current value is lower when the weak output is selected than the commanded value determined when the strong output is selected.

4. The brushless dc motor according to claim 2, further comprising:

detecting means for detecting in which motor rpm range the motor rpm is included,

wherein the current command means provides the average current value to the inverter circuit in response to the motor rpm range command.

5. The brushless dc motor according to claim 1, further comprising:

a current instruction device for instructing an average current value supplied to the inverter circuit,

a line terminal for connecting a voltage reduction device located outside the motor,

wherein a signal voltage that instructs an average current value supplied to the inverter circuit is applied to the current instruction device via a voltage reduction device that is located outside the motor, and

the current command device commands the inverter circuit to operate a constant current based on the signal voltage.

6. The brushless dc motor according to claim 2, further comprising:

a line terminal for connecting a voltage reduction device located outside the motor,

wherein a signal voltage that instructs an average current value supplied to the inverter circuit is applied to the current instruction device via a voltage reduction device that is located outside the motor, and

the current command device commands the inverter circuit to change the current in response to the motor rpm based on the signal voltage.

7. The brushless dc motor of claim 4, further comprising:

a line terminal for connecting a voltage reduction device located outside the motor,

wherein a signal voltage that instructs an average current value supplied to the inverter circuit is applied to the current instruction device via a voltage reduction device that is located outside the motor, and

wherein the current command means commands the inverter circuit to change the current in response to the range of revolutions per minute of the motor based on the signal voltage.

8. An electric device in which the brushless dc motor according to claim 1 or 2 is installed.