WO2007125608A1

WO2007125608A1 - Motor driver and electric apparatus having the same

Info

Publication number: WO2007125608A1
Application number: PCT/JP2006/309129
Authority: WO
Inventors: Masahiro Yasohara
Original assignee: Panasonic Corporation
Priority date: 2006-04-25
Filing date: 2006-04-25
Publication date: 2007-11-08
Also published as: CN101366167A; KR20090003155A; CN101366167B; KR101135777B1

Abstract

A motor driver includes a positive switch element which connects drive windings of a motor to a positive power line and a negative switch element which connects the drive windings to a negative power line. Both of the switch elements are turned on/off based on a control signal supplied from a controller. The controller includes a waveform generator for outputting a driving waveform signal having sine-wave like current waveform and a polarity determiner for determining a polarity of back electromotive force of the drive windings. When the polarity is determined to be positive, the positive switch element is turned on/off, and when the polarity is negative, the negative switch element is turned on/off, so that the current running on the drive windings can shape like sine-wave.

Description

DESCRIPTION

MOTOR DRIVERAND ELECTRIC APPARATUS HAVING THE SAME

TECHNICAL FIELD

The present invention relates to motor drivers good for driving brushless DC motors to be mounted in various electric apparatuses such as air-conditioners, water heaters which use combustion fan motors, air cleaners, and information apparatuses including copying machines and printers. More particularly, it relates to a motor driver that can substantially reduce torque ripple, vibrations and noises during the operation with a simple structure. It also relates to an electric apparatus equipped with the same motor driver.

BACKGROUNDART Electric apparatuses, e.g. air conditioners, water heaters, air cleaners, copying machines and printers, often employ brushless DC motors as driving motors because of their advantages such as longer service life, higher reliability and simpler speed control over other types of motors. (Hereinafter the brushless DC motor is referred to simply as "motor".) A rectangular wave driving method has been widely used as a method of driving motors. This method drives the drive windings of the motor by a rectangular driving waveform. In recent years, however, the market demands that the motor should be driven with less torque ripple, fewer vibrations and at a lower noise level. A sine -wave driving method, which drives the drive windings of the motor by sine-wave driving waveform, now becomes a main stream of the motor driving method in order to meet this market demand.

Japanese Patent Examined Publication No. 3232467 discloses one of prior art about the sine-wave driving method. This prior art sequentially reads sine-wave waveform data stored in a memory in response to a rotational position of the motor. Then this data undergoes a pulse width modulation, and controls respective switching elements of an inverter circuit which powers the drive windings of the motor, so that the motor is driven by sine-wave driving waveform. i

Fig. 8 shows a circuit diagram of the motor driver in accordance with this prior art, and Fig. 9 illustrates an operation of the motor driver shown in Fig. 8. In Fig. 8, motor 501 includes phase-U drive winding 511, phase-V drive winding 513, and phase-W drive winding 515. DC power supply 505 powers these drive windings 511, 513 and 515 via inverter 520.

Inverter 520 includes positive switches 521, 523 and 525 that connect drive windings 511, 513 and 515 to positive power line 501, and also includes negative switches 522, 524 and 526 that connect windings 511, 513 and 515 to negative power line 502. Controller 530 includes waveform generator 531 and pulse width modulator (hereinafter referred to simply as "PWM") 532. Motor driver 500 comprises inverter 520 and controller 530.

Driving waveform signal WF shaping like sine-wave generated by waveform generator 531 in response to a rotational position of motor 501 is fed into PWM 532, which then, based on signal WF, outputs control signals UH, VH,

WH, UL, VL and WL having undergone the pulse width modulation to respective switch elements 521 through 526 of inverter 520. Respective switch elements 521 through 526 are turned on or turned off by these control signals.

Control signals UH, VH and WH have a phase difference of 120 degrees in electrical angles from each other, and supplied from PWM 532. Control signals UL, VL and WL also have a phase difference of 120 degrees in electrical angles from each other, and supplied from PWM 532. An operation of phase-U drive winding 511 connected to output U supplied from inverter 520 is described hereinafter with, reference to Fig. 9 from among the drive windings of motor 501. In Fig. 9, signal CY in triangular waveform is a pulse-width-modulation carrier signal existing in PWM 532. Waveform generator 531 generates driving waveform signal WF shaping like sine-wave in response to a rotational position of motor 501, and signal WF is compared with carrier signal CY by PWM 532. Switch elements 521 and

522 are turned on or turned off complementarity to each other in response to this comparison. Then driving voltage U shown in Fig. 9 is supplied from inverter 520 and applied to phase-U drive winding 511, on which phase-U driving current Iu resultantly runs.

Driving voltage U changes instantaneously and alternately between a positive voltage and a negative voltage of DC power supply 505; however, it becomes shaping like the sine-wave corresponding to driving waveform signal WF on average because of the principle of the pulse width modulation. Thus phase-U drive winding 511 receives a voltage shaping like sine-wave similar to signal WF of phase-U.

In a similar way to what is discussed above, phase-V driving winding 513 and phase- W driving winding 515 also are applied with the voltages shaping like sine-wave by driving voltages V and W supplied from inverter 520.

Driving voltages U, V and W applied to drive windings 511, 513 and 515 of the respective phases have phase difference of 120 degrees in electric angles. To be more specific, with respect to phase-V drive winding 513, switch elements

523 and 524 of inverter 520 are turned on or turned off complementarily to each other in response to the comparison between carrier signal CY and a phase-V driving waveform signal which has a phase difference of 120 degrees in electric angles from phase-U driving waveform signal WF. With respect to phase- W drive winding 515, switch elements 525 and 526 of inverter 520 are turned on or turned off complementarily to each other in response to the comparison between carrier signal CY and a phase-W driving waveform signal which has a phase difference of 120 degrees in electric angles from phase-U driving waveform signal and phase-V driving waveform signal respectively.

As discussed above, voltages shaping like sine-wave are applied to respective drive windings 511, 513 and 515, so that motor 501 is driven by sine-wave. However, motor driver 500 in accordance with the foregoing prior art has the following fear: During the speed reduction, when motor 501 is put into a driving mode whose driving voltages U, V and W supplied from inverter 520 become lower than the back electromotive force generated on drive windings 511, 513 and 515 of motor 501, the rotating energy of motor 501 returns to DC power supply 505, i.e. regeneration phenomenon occurs. This phenomenon increases the output voltage of DC power supply 505, and sometimes damages motor driver 500 per se or the electric apparatus equipped with motor driver 500.

What is the regeneration phenomenon is described hereinafter. In motor driver 500 in accordance with the foregoing prior art, switch elements 521 and 522 of inverter 520 are turned on and turned off complementary to each other. Switch elements 523 and 524 as well as switch elements 525 and 526 are also turned on and turned off complementary to each other. In this description, the operation of switch elements 521 and 522 is focused. The "turned on/off complementarily to each other" means that while one switch element is turned on, the other one is turned off, and while the other one is turned on, the one switch element is turned off.

Fig. 9 shows control signal. UH that turns on/off switch element 521, and control signal UL that turns on/off switch elements 522. In the timing charts of these control signals, the respective switch elements are turned on during levelΗ, and the respective switch elements are turned off during level-L. Control signals UH and UL shown in Fig. 9 turn on and turn off switch elements 521 and 522 complementarity to each other; however, to be exact, time "td" (referred to as "dead time" or "on delay") is provided at a moment in time when "on" and "off' are switched over. At this instance time "td", both of switch elements 521 and 522 are turned off. This is a known technique for preventing both of switch elements 521 and 522 from being turned on simultaneously and DC power supply 505 from being shorted.

The foregoing regeneration phenomenon discussed as a problem occurs due to the complementary turn-on and turn-off of switch elements 521 and 522. This phenomenon is detailed hereinafter with reference to Fig. 10. In Fig. 10, phase-U drive winding 511 generates back electromotive force Uemf. Fig. 10 shows the operation when an average value (corresponding to driving waveform signal WF) of driving voltage U supplied from inverter 520 becomes smaller than back electromotive force Uemf.

This state, i.e. the average value (driving waveform signal WF) of driving voltage U becomes smaller than back electromotive force Uemf, occurs in reducing the speed of motor 501, e.g. when the wave height of signal WF is lowered, or when motor 501 is accelerated by external force and thus back electromotive force Uemf becomes greater.

First, period "a" shown in Fig. 10 is described. During period "a", switch element 521 is turned on, and switch element 522 is turned off. As a result, drive winding 511 is electrically connected to positive power line 501 of DC power source 505, and driving voltage U instantaneously assumes a voltage of positive line 501. Since driving voltage U is greater than back electromotive force Uemf of drive winding 511 during period "a", current Iu of winding 511 increasingly runs. The increasing amount of current Iu depends on the difference of subtracting back electromotive force Uemf from driving voltage U (hatched sections of period "a" in Fig. 10.) However, when the average of driving voltage U (driving waveform signal WF) is smaller than back electromotive force Uemf, the difference becomes smaller, so that the current increases in only a little amount.

Next, period "b" shown in Fig. 10 is described. During period "b", switch element 521 is turned off and switch element 522 is turned on, so that drive winding 511 is electrically connected to negative power line 502 of DC power supply 505. Driving voltage U thus instantaneously assumes a voltage of negative line 502. Since driving voltage U is smaller than back electromotive force Uemf of drive winding 511 during period "b", current Iu of winding 511 decreasingly runs. The decreasing amount of current Iu depends on the difference of subtracting the driving voltage U from back electromotive force Uemf (hatched sections of period "b" in Fig. 10). When the average of driving voltage U (driving waveform signal WF) is smaller than back electromotive force Uemf, the difference becomes greater, and the current decreases in a greater amount. During period "bl" out of period "b", current Iu runs through switch element 522 or a diode connected inversely and parallely to switch element 522, and runs decreasingly to drive winding 511. Current Iu decreases down to 0 (zero) before it reaches to period "b2". During period "b2", current Iu reverses its direction and runs from drive winding 511 to negative power line 502 via switch element 522. Current Iu is directed such that it is supplied from back electromotive force Uemf, thus current Iu flows along the direction reverse to the normal direction of the motor driving current. During period "c", switch element 521 is turned on and switch element 522 is turned off, which is the same status as during period "a". Thus current Iu increases similar to during period "a">^* however, it increases only a little, and not great enough to reverse its direction again along the original direction. During period "c", current Iu runs from drive winding 511 to positive power line 501 via switch element 521 or a diode connected inversely and parallely to switch element 521. Current Iu is directed such that it is supplied from back electromotive force Uemf, thus current Iu flows along the direction reverse to the normal direction of the motor driving current. This is the same manner as during period "b2" discussed previously.

During period "d", switch element 521 is turned off, and switch element 522 is turned on, which is the same status as during period "b". Thus current Iu keeps decreasing greatly similar to during period "b". Current Iu runs from drive winding 511 to negative power line 502 via switch element 522. Thus current Iu is supplied from back electromotive force Uemf in a greater amount than that of during period "b2".

Current Iu should be supplied toward back electromotive force Uemf of drive winding 511 in order to drive the motor, ' however, contrary to this normal status, if current Iu is kept being supplied from back electromotive force Uemf, current Iu flows from drive winding 511 toward positive power line 501 via switch element 521 or the diode connected inversely and parallely to switch element 521 every time when switch element 521 is turned on and switch element 522 is turned off. This phenomenon occurs in due course in other drive windings 513 and 515, so that the currents flow from respective drive windings 511, 513 and 515 toward a positive electrode of DC power supply 505 via positive power line 501.

In other words, motor 501 works as an electric generator, so that motor 501 conversely powers DC power supply 505 which is expected to power motor 501, i.e. regeneration occurs.

In general, DC power supply 505 is designed such that it has a capacity of supplying an electric current and it is not expected to absorb an electric current. The regeneration phenomenon allows the electric current to flow from drive windings 511, 513 and 515 into DC power supply 505, so that an output voltage from DC power supply 505 increases, which sometimes damages power supply 505, motor driver 500 or an electric apparatus having motor driver 500.

DISCLOSUKE OF INVENTION

The motor driver of the present invention includes an inverter and a controller. They comprises the following elements : the inverter including positive switch elements which connect the drive windings of plural phases of the motor to a positive power line, and negative switch elements which connect those drive windings to a negative power line; and the controller including a waveform generator which outputs a signal showing a ratio of on-period vs. off-period of the positive or negative switch elements as a driving waveform signal of the drive windings, and a polarity determiner which determines a polarity of back electromotive force generated on the drive windings.

When the polarity determiner determines the back electromotive force of the drive winding to be positive, the controller outputs a control signal, which controls a ratio of on-period vs. off-period of the positive switch element, to the inverter. When the polarity determiner determines the back electromotive force of the drive winding to be negative, the controller outputs a control signal, which controls a ratio of on -period vs. off-period of the negative switch element, to the inverter. This mechanism allows the controller to turn on or turn off the positive or negative switch element of the inverter with its control signal, and allows the inverter to drive the drive windings of the respective phases with an alternating current shaping like sine -wave. The foregoing structure allows the motor to be driven by the sine -wave current free from the regeneration phenomenon even if the motor is put in a driving mode in which an average of the driving voltage supplied from the inverter becomes lower than the back electromotive force generated on the drive windings of the motor. (The driving voltage is supplied from the inverter and has undergone PWM control. The regeneration phenomenon returns the rotating energy of the motor to the DC power supply.) Therefore, even if the motor suddenly reduces its speed or the motor is forcibly accelerated by external force, this status does not cause to increase an output voltage of the DC power supply, so that the apparatus is not damaged. As a result, the present invention can provide a motor driver, excellent in safety and reliability, working with little torque ripple, less vibrations, and less noises.

The electric apparatus of the present invention includes an apparatus unit, a motor mounted in the apparatus unit, and the motor driver comprising the inverter and the controller. This motor driver employs the motor driver described above, so that the present invention can provide the electric apparatus, excellent in safety and reliability, working with less vibrations and less noises.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a circuit diagram of a motor driver in accordance with a first embodiment of the present invention.

Fig. 2 illustrates an operation of the motor driver shown in Fig. 1. Fig. 3 illustrates an operation of the motor driver shown in Fig. 1 with low back electromotive force of drive windings of the motor.

Fig. 4 shows an operating waveform of a motor driver in accordance with a second embodiment of the present invention. Fig. 5 shows an operating waveform of a motor driver in accordance with a third embodiment of the present invention.

Fig. 6 shows a construction of an electric apparatus (outdoor unit of airxonditioner) in accordance with a fourth embodiment.

Fig. 7 shows a construction of an electric apparatus (inkjet printer) in accordance with a fifth embodiment.

Fig. 8 shows a circuit diagram of a motor driver of prior art.

Fig. 9 illustrates an operation of the motor driver shown in Fig. 8.

Fig. 10 illustrates an operation of the motor driver shown in Fig. 8 with low back electromotive force of drive windings of the motor.

PREFERRED EMBODIMENTS OF THE INVENTION

Exemplary embodiments of the present invention are demonstrated hereinafter with reference to the accompanying drawings. Embodiment 1 Fig. 1 shows a circuit diagram of the motor driver in accordance with the first embodiment of the present invention, and Fig. 2 illustrates an operation of the motor driver shown in Fig. 1. Fig. 3 illustrates an operation of the motor driver shown in Fig. 1 with lower back electromotive force of drive windings of the motor. In Fig. 1, motor driver 100 in accordance with the first embodiment includes inverter 20 and controller 30.

Inverter 20 has positive switch elements 21, 23 and 25 which electrically connect drive windings 11, 13 and 15 of a plurality of phases (three phases) of motor 1 to positive power line 101, and it also has negative switch elements 22, 24 and 26 which electrically connect drive windings 11, 13 and 15 to negative power line 102.

Controller 30 includes waveform generator 31 and polarity determiner 33. Generator 31 outputs a signal showing a ratio of on-period vs. off-period of the positive or negative switch element 21 through 26 as a driving waveform signal of the drive windings. Polarity determiner 33 determines respective polarities of back electromotive force generated on drive windings 11, 13 and 15.

When polarity determiner 33 determines the back electromotive force of drive windings 11, 13 and 15 to be positive, controller 30 outputs a control signal, which controls a ratio of on-period vs. off-period of positive switch elements 21, 23 and 25, to inverter 20. When polarity determiner 33 determines the back electromotive force of drive windings 11, 13 and 15 to be negative, controller 30 outputs a control signal, which controls a ratio of on-period vs. off-period of negative switch elements 22, 24 and 26, to the inverter. This mechanism allows controller 30 to turn on or turn off the positive or negative switch elements 21 through 26 of the inverter with its control signal, and allows inverter 20 to drive respective drive windings 11, 13 and 15 of the plural phases with an alternating current shaping like sine -wave. The structure of the motor driver in accordance with this first embodiment is further detailed with reference to Fig. 1. In Fig. 1, motor 1 is coupled to DC power supply 5 via inverter 20. To be more specific, positive power line 101 is connected to a first terminal of positive switch element 21, and a second terminal of positive switch element 21 is connected to a first terminal of negative switch element 22. A second terminal of negative switch element 22 is connected to negative power line 102 of DC power supply 5. A junction point common to both positive switch element 21 and negative switch element 22, i.e. the junction point between the second terminal of positive switch element 21 and the first terminal of negative switch element 22, is connected to a first end of phase-U drive winding 11 of motor 1.

In a similar way, positive power line 101 is connected with a first terminal of positive switch element 23, and a second terminal of positive switch element 23 is connected to a first terminal of negative switch element 24. A second terminal of negative switch element 24 is connected to negative power line 102. A junction point common to both positive switch element 23 and negative switch element 24, i.e. the junction point between the second terminal of positive switch element 23 and the first terminal of negative switch element 24, is connected to a first end of drive winding 13 of phase-V of motor 1.

Likewise, positive power line 101 is connected with a first terminal of positive switch element 25, and a second terminal of positive switch element 25 is connected to a first terminal of negative switch element 26. A second terminal of negative switch element 26 is connected to negative power line 102. A junction point common to both positive switch element 25 and negative switch element 26, i.e. the junction point between the second terminal of positive switch element 25 and the first terminal of negative switch element 26, is connected to a first end of drive winding 15 of phase- W of motor 1. A second end of phase-U drive winding 11, a second end of phase-V drive winding 13 and a second end of phase- W drive winding 15 are connected to each other, thereby forming a neutral point.

Controller 30 outputs control signals UH, VH and WH, which respectively turn on or turn off positive switch elements 21, 23 and 25, to respective third terminals of positive switch elements 21, 23 and 25. Controller 30 also outputs control signals UL, VL and WL, which respectively turn on or turn off negative switch elements 22, 24 and 26, to respective third terminals of negative switch elements 22, 24 and 26.

Controller 30 includes PWM (pulse width modulator) 32 in addition to waveform generator 31 and polarity determiner 33. Waveform generator 31 outputs driving waveform signal WF to PWM 32 so that the driving current waveforms of drive windings 11, 13 and 15 can shape like sine-wave.

Polarity determiner 33 determines polarities of back electromotive force generated on drive windings 11, 13 and 15, then outputs the determined signals to PWM 32. PWM 32 compares driving waveform signal WF with carrier signal CY in terms of voltage, so that signal WF undergoes the pulse width modulation. PWM 32 also outputs the result of pulse width modulation of signal WF as control signals UH, VH, WH, UL, VL and WL of controller 30 to inverter 20 in response to the determined signal supplied from polarity determiner 33.

An operation of motor driver 100 discussed above is demonstrated hereinafter. Fig. 2 illustrates the operation of motor driver 100 shown in Fig. 1. In Fig. 2, signal CY shaping like triangular wave is a PWM carrier signal existing in PWM 32. In general, the frequency of carrier signal CY is set at a frequency substantially higher than an electric angle cycle generated by the rotation of motor 1; however, Fig. 2 shows a relatively low frequency in order to make the description simpler.

Waveform generator 31 generates ^' driving waveform signal WF shaping like sine-wave in response to a rotational position of motor 1. Signal WF is compared with carrier signal CY in terms of voltage by PWM 32, so that a PWM (pulse width modulation) signal of which pulse width varies in response to signal WF is generated. Either one of positive switch element 21 or negative switch element 22 of inverter 20 is turned on or turned off in response to the PWM signal. Positive switch element 21 and negative switch element 22 shown in Fig. 1 are not turned on and turned off complementary to each other as the conventional counterparts shown in Fig. 8 are done, but either one of the switch elements is turned on and off while the other one stays in off-status. The polarity of back electromotive force Uemf, detected by polarity determiner 33, of phase-U drive winding 11 determines which switch element, namely, which one of positive switch element 21 or negative switch element 22, is turned on or off and which one stays in off-status.

To be more specific, as shown in Fig. 2, when the polarity of back electromotive force is positive, a PWM signal resulting from the comparison between carrier signal CY and driving waveform signal WF is reflected on control signal UH, which is supposed to turn on or turn off positive switch element 21, thereby turning. on or off switch element 21 based on the PWM signal. At this time, control signal UL is fixed at level L in order to maintain negative switch element 22 in off status.

On the other hand, when the polarity of back electromotive force is negative, a PWM signal resulting from the comparison between carrier signal CY and driving waveform signal WF is reflected on control signal UL, thereby turning on or off negative switch element 22 based on the PWM signal. At this time, control signal UH is fixed at level L in order to maintain positive switch element 21 in off status.

The motion of control signals UH and UL are shown in Fig. 2. Level H indicates that the respective switch elements are turned on, and level L indicates that they are turned off. As discussed above, in response to the polarity of back electromotive force

Uemf, either one of positive switch element 21 or negative switch element 22 is turned on or turned off by a PWM signal generated in response to sine-wave like signal WF, and the other one stays in off status. This mechanism allows phase U drive winding 11 to have driving waveform shaping like sine-wave.

The foregoing mechanism is further detailed hereinafter. Current Iu should run through phase-U drive winding 11 against back electromotive force Uemf so that motor 1 can generate torque. A positive polarity of Uemf directs current Iu toward phase-U drive winding 11, and current Iu being directed toward this direction can be controlled in sine-wave shape by turning on or off positive switch element 21 and maintaining negative switch element 22 in off status. In other words, current Iu can be controlled in increasing direction by turning on positive switch element 21 for applying a voltage of positive power line 101 to phase-U drive winding 11. Current Iu can be controlled in decreasing direction by just turning off switch element 21. The turn-off of switch element 21 allows applying a voltage of negative power line 102 to phase-U drive winding 11, so that current Iu is controlled in decreasing direction. Because current Iu is directed toward winding 11, and the turn-off of switch element 21 brings the diode coupled inversely and parallely to negative switch element 22 into conduction.

As discussed above, when back electromotive force Uemf is positive, current Iu can be controlled to be like sine -wave by turning on or off positive switch element 21.

On the other hand, in the case of back electromotive force Uemf having a negative polarity, motor 1 can generate torque when current Iu flows in the direction of flowing from phase-U drive winding 11. Current Iu flowing in this direction can be controlled in sine-wave shape by turning on or off negative switch element 22 and maintaining positive switch element 21 in off status. This condition is quite opposite to the case when force Uemf has a positive polarity.

In other words, current Iu can be increased in the direction of flowing from phase-U drive winding 11 by turning on negative switch element 22 for applying a voltage of negative power line 102 to phase-U drive winding 11. Current Iu can be decreased by just turning off switch element 22. The turn-off of negative switch element 22 allows applying a voltage of positive power line 101 to phase-U drive winding 11, so that current Iu flowing from winding 11 is controlled in decreasing direction. Because the turn-off of switch element 22 brings the diode coupled inversely and parallely to positive switch element 21 into conduction. When back electromotive force Uemf assumes a negative polarity, current Iu can be thus controlled to be like sine-wave by turning on or off negative switch element 22.

As discussed above, in the respective cases of the polarities of back electromotive force Uemf, positive switch element 21 or negative switch element 22 is turned on or off, and driving voltage U shown in Fig. 2 is resultantly supplied from inverter 20 and is applied to phase-U drive winding 11.

Driving voltage U alternately and instantaneously changes between the positive voltage and the negative voltage of DC power supply 5; however, it becomes a sine-wave like voltage in average in response to driving waveform signal WF based on the pulse width modulation principle. As a result, a sine-wave like voltage similar to signal WF is applied to phase-U drive winding 11.

The foregoing description refers to phase-U drive winding 11. In a similar way, a sine -wave like voltage, formed of driving voltages V or W supplied from inverter 20, can be applied to drive windings 13 and 15 of phases V and W. •

Respective driving voltages U, V and W applied to drive windings 11, 13 and 15 have a phase difference of 120 degrees in electrical angles from each other. This is realized by turning on and off switch elements 23 and 24 of inverter 20 in response to a result of the comparison between carrier signal CY and a phase -V sine -wave like driving waveform signal which has the phase difference of 120 degrees in electrical angles from phase -U driving waveform signal WF. With respect to phase-W drive winding 15, this is realized by turning on and off switch elements 25 and 26 of inverter 20 in response to a result of the comparison between carrier signal CY and a phase-W sine-wave like driving waveform signal which has the phase difference of 120 degrees in electrical angles from phase-U and phase-V driving waveform signals.

A sine-wave like voltage is thus applied to respective drive windings 11, 13 and 15, so that they are driven by an alternate current shaping like sine -wave.

Next an operation in an unfavorable driving mode is described about phase U for representing other phases V and W. The unfavorable driving mode refers to a case such as motor 1 suddenly reduces its speed, or motor 1 is forcibly accelerated by external force, so that driving voltages U, V and W supplied from inverter 20 become lower than the back electromotive force generated on drive windings 11, 13 and 15 of motor 1. Fig. 3 illustrates an operation in the case when an average

(corresponding to driving waveform signal WF) of driving voltage U supplied from inverter 20 is lower than back electromotive force Uemf generated on phase-U drive winding 11. This situation, i.e. the average of driving voltage U, namely, driving waveform signal WF, becomes lower than force Uemf, occurs when the wave height of signal WF is lowered in order to reduce the speed of motor 1, or force Uemf becomes greater because motor 1 is forcibly accelerated by external force. An enlarged portion of driving voltage U shown in the lower section of

Fig. 3 shows the area where the polarity of back electromotive force Uemf is positive. In this area, as previously described, positive switch element 21 is turned on or off, thereby controlling the driving waveform of drive winding 11 in a sine-wave shape while negative switch element 22 stays in off status.

The following description refers to the area where back electromotive force Uemf assumes a positive polarity! however, a similar operation is done in the area where force Uemf assumes a negative polarity.

In the enlarged portion of Fig. 3, period "a" is described first. During period "a", positive switch element 21 is turned on while negative switch element 22 stays in off status. Drive winding 11 is thus electrically connected to positive power line 101 of DC power supply 5, and driving voltage U instantaneously assumes a voltage of line 101. During period "a", driving voltage U is higher than back electromotive force Uemf of drive winding 11, so that current Iu of winding 11 flows increasingly. The increasing amount of current Iu depends on the difference of subtracting back electromotive force

Uemf from driving voltage U (hatched sections during period "a" in Fig. 3).

However, when the average of driving voltage U (driving waveform signal WF) is smaller than back electromotive force Uemf, the difference becomes smaller, so that the current increases in only a little amount.

During period "b", positive switch element 21 is turned off while negative switch element 22 is kept in off status. The turn-off of switch element 21 brings a diode connected inversely and parallely to switch element 22 into conduction, so that drive winding 11 is electrically connected to negative power line 102 of DC power supply 5. Driving voltage U thus instantaneously assumes a voltage of negative line 102. Since driving voltage U is smaller than back electromotive force Uemf of drive winding 11 during period "b", current Iu of winding 11 decreasingly runs. The decreasing amount of current Iu depends on the difference of subtracting the driving voltage U from back electromotive force Uemf (hatched sections during period "b" in Fig. 3). When the average of driving voltage U, i.e. driving waveform signal WF, is smaller than back electromotive force Uemf, the difference becomes great, so that the current decreases in a greater amount.

During period "bl" out of period "b", current Iu runs through a diode connected inversely and parallely to negative switch element 22, and runs decreasingly to drive winding 11. Current Iu decreases down to 0 (zero) before it reaches to period "b2". During period "b2", current Iu stays at 0 (zero) because negative switch element 22 is kept in off status. Thus current Iu never flows like it is supplied from back electromotive force Uemf as described in the prior art shown in Fig. 10. During period "b2", since current Iu stays at 0 (zero), back electromotive force Uemf of winding 11 per se can be observed at an output from inverter 20.

During period "c", positive switch element 21 is turned on as it is turned on during period "a". Negative switch element 22 stays in off status. Thus current Iu increases in a little amount as it does during period "a". During period "c", current Iu flows from positive power line 101 toward drive winding 11 via positive switch element 21.

During period "d", positive switch element 21 is turned off, and negative switch element 22 stays in off status. Thus current Iu decreasingly flows to drive winding 11 via the diode connected inversely and parallely to switch element 22, and it decreases down to 0 (zero) at the end of period "dl". During period "d2" following period "dl", since negative switch element 22 remains in off status, current Iu stays in 0 (zero) level. In other words, during period "d2", current Iu never flows like it is supplied from back electromotive force Uemf as described in the prior art shown in Fig. 10.

A mechanism similar to what is discussed above works also in phase-V drive winding 13 and phase-W drive winding 15. Therefore, it can be concluded that the current in the motor driver in accordance with the first embodiment never flows from the respective back electromotive force of drive windings 11, 13 and 15 toward DC power supply 5, namely, never flows inversely to its fundamental flowing direction. Thus the regeneration never occurs.

Even if the unfavorable driving mode happens, i.e. an average of the driving voltage supplied from inverter 20 becomes lower than the back electromotive force generated on the drive windings of motor 1, in other words, the driving waveform signal becomes lower than the back electromotive force, the motor can be driven by the sine -wave driving signal free from the regeneration that returns the rotating energy of motor 1 to DC power supply 5. Therefore, if the motor reduces its speed suddenly, or is forcibly accelerated by external force, DC power supply 5 does not increase its output voltage, so that the electric apparatus is not damaged. As a result, the present invention can provide a reliable motor driver with little torque ripple, less vibrations, less noises and excellent in safety.

Embodiment 2

Fig. 4 shows an operating waveform of a motor driver in accordance with the second embodiment of the present invention. In the first embodiment discussed previously, the average (corresponding to driving waveform signal WF) of driving voltage U supplied from inverter 20, namely, the sine-wave like driving voltage waveform, is in phase with back electromotive force Uemf. This is shown in Figs. 2 and 3. However, in practice, the current running on the drive windings is influenced by inductance of the windings, so that the phase of the current delays from that of the driving voltage. The back electromotive force becomes out of phase from the winding current, so that the motor reduces its driving efficiency.

To overcome this disadvantage, the phase of the driving voltage waveform supplied from inverter 20 is advanced ahead of time, so that the out-of-phase between the back electromotive force and the winding current can be cancelled. This is called "phase-advancing control", which is regularly carried out.

Fig. 4 shows the operating waveform of the motor driver in accordance with the second embodiment, and the waveform has undergone this phase -advancing control. As shown in Fig. 4, the phase can be advanced by outputting driving waveform signal WF from waveform generator 31 shown in Fig. 1 ahead of back-electromotive-force Uemf.

When the phase-advancing control is carried out, positive switch element 21 is turned on or off with control signal UH at a positive polarity of Uemf (during period "UP" in Fig. 4), and negative switch element 22 is turned on or off with control signal UL at a negative polarity of Uemf (during period "UN" in Fig. 4). This operation remains unchanged from the first embodiment, and an advantage identical to that of the first embodiment is obtainable.

Embodiment 3

Fig. 5 shows an operating waveform of a motor driver in accordance with the third embodiment of the present invention. The average (corresponding to driving waveform signal WF) of driving voltage U supplied from inverter 20 does not necessarily shape like sine-wave, but motor 1 can be driven resultantly by sine-wave provided the following condition is satisfied: the voltages between the respective first ends of drive windings 11, 13 and 15 shape like sine-wave, or voltages between the neutral points and respective first ends of drive windings 11, 13 and 15 shape like sine-wave. For instance, driving waveform signal WF shown in Fig. 5 can be pulse-width modulated, and inverter 20 outputs this signal as driving voltage U. In this case, the waveforms generated between the neutral point and the respective first ends of drive windings 11, 13 and 15 shape like sine-wave as indicated by waveform F in Fig. 5, so that the motor is driven by sine-wave similar to the first embodiment.

In the case of using driving waveform signal WF shown in Fig. 5, positive switch element 21 is turned on or off with control signal UH when the polarity of Uemf is positive (during period "UP" in Fig. 5), and negative switch element 22 is turned on or off with control signal UL when the polarity of Uemf is negative (during period "UN" in Fig. 5). This operation produces an advantage identical to that of the first embodiment.

Embodiment 4

An electric apparatus equipped with a motor driver is demonstrated hereinafter. Fig. 6 shows a construction of the electric apparatus (outdoor unit of air-conditioner) in accordance with the fourth embodiment. In Fig. 6, the electric apparatus in accordance with the fourth embodiment is outdoor unit

201 of an air-conditioner equipped with motor driver 203. Outdoor unit 201 comprises apparatus unit 211, motor (fan motor) 208 mounted in apparatus unit 211, and motor driver 203 including an inverter and a controller.

Motor driver 203 can employ one of the motor drivers demonstrated in embodiments 1 through 3. Motor driver 203 comprises the following elements-' the inverter having positive switch elements for connecting drive windings of plural phases of the motor to a positive power line, and negative switch elements for connecting the drive windings to a negative power line; and the controller having a waveform generator for outputting a signal indicating a ratio of on-period vs. off-period of the positive or the negative switch elements as a driving waveform signal, and a polarity determiner for determining a polarity of back electromotive force generated on the drive windings.

When the polarity determiner determines the back electromotive force of the drive windings to be positive, the controller outputs a control signal, which controls a ratio of on-period vs. off-period of the positive switch elements, to the inverter. When the polarity determiner determines the back electromotive force of the drive windings to be negative, the controller outputs a control signal, which controls a ratio of on -period vs. off-period of the negative switch elements, to the inverter. This mechanism allows the controller to turn on or turn off the positive or negative switch elements of the inverter with its control signal, and allows the inverter to drive the respective drive windings of the plural phases with an alternating current shaping like sine -wave.

This electric apparatus, namely, the outdoor unit, is further detailed with reference to Fig. 6, which shows a structure of the outdoor unit of the air-conditioner. In Fig. 6, outdoor unit 201 is divided by partition plate 204 standing on bottom plate 202 into compressor room 206 and heat exchanger room 209. Compressor 205 is placed in room 206. Heat exchanger 207 and blower fan motor 208 are placed in room 209. Box 210 carrying electrical components is placed over partition plate 204.

Fan motor 208 is formed of a brushless DC motor with a blower-fan mounted on motor's rotary shaft, and driven by motor driver 203 housed in box 210. The rotation of fan motor 208 entails the blower fan to spin, which generates wind for cooling heat exchanger room 209.

Since motor drive 203 can employ one of the motor drivers demonstrated in embodiments 1 through 3, the electric apparatus of the present invention, i.e. outdoor unit 201 of the air-conditioner, can enjoy the advantage of the motor driver of the present invention.

Embodiment 5

Fig. 7 shows a construction of an electric apparatus (inkjet printer) in accordance with the fifth embodiment. In Fig. 7, inkjet printer 310 (hereinafter referred to simply as "printer") employs a driving system which comprises the following elements • carriage motor 301 for scanning an object with print head 307 mounted to the carriage; and paper feeding motor 306 for feeding paper 308.

Carriage motor 301 employs a brushless DC motor which is driven by motor driver 300. Paper feeding motor 306 employs a stepping motor.

Rotation of paper feeding motor 306 transfers its rotating force to paper feeding roller 305, so that paper 308 is fed via roller 305 to this side in Fig. 7. Carriage motor 301 has pulley 302 at its rotary shaft, and timing belt 303 is looped over pulley 302. Print head 307 is mounted to belt 303, and discharges liquid ink through nozzles (not shown) onto paper 308. Rotation of carriage motor 301 in positive direction or reverse direction allows print head 307 to scan an object from side to side along guide-shaft 304 in Fig. 7. Scanning by print head 307, discharging of ink from head 307, and feeding of paper 308 allow forming an image on paper 308.

Motor driver 300 can employ one of the motor drivers demonstrated in embodiments 1 through 3, so that the printer including the motor driver of the present invention can enjoy the advantage of the motor driver of the present invention.

Other than the electric apparatuses demonstrated in embodiments 4 and 5, there are various electric apparatuses which can employ the motor driver of the present invention. They are, e.g. copying machines, optical media apparatuses, apparatuses using hard-disks, air-conditioners (indoor units), air cleaners, water heaters, refrigerators, vacuum cleaners, washing machines, FA apparatuses, and apparatuses using inverters.

Industrial Applicability

A motor driver of the present invention can drive a motor with a sine-wave like driving signal free from a regeneration phenomenon, i.e. rotating energy of the motor returns to the DC power supply. In other words, if the motor reduces its speed suddenly, or is forcibly accelerated by external force, the DC power supply does not increase its output voltage, so that the electric apparatus is not damaged. As a result, the present invention can provide a reliable motor driver working with little torque ripple, less vibrations, less noises and excellent in safety. The motor driver of the present invention can be suitable for driving information apparatuses which are ^' supposed to undergo frequent acceleration/deceleration control and need less vibrations as well as less noises, e.g. copying machines, apparatuses using hard-disks, and optical media apparatuses. The motor driver is also suitable for driving the apparatuses that can be subjected to a gale such as typhoon, e.g. fan motors of air-conditioners, combustion fan motors of water heaters, and it is also good for driving blower fan motors of air cleaners.

Claims

1. A motor driver comprising- an inverter including: a positive switch element which connects drive windings of a plurality of phases of a motor to a positive power line! and a negative switch element which connects the drive windings to a negative power line, and a controller including: a waveform generator which outputs a signal showing a ratio of on-period vs. off-period of the positive or the negative switch element as a driving waveform signal of the drive windings; and a polarity determiner which determines a polarity of back electromotive force generated on the drive windings, wherein when the polarity determiner determines the back electromotive force of the drive windings to be positive, the controller outputs a control signal, which controls a ratio of on-period vs. off-period of the positive switch element, to the inverter, and when the polarity determiner determines the back electromotive force of the drive windings to be negative, the controller outputs a control signal, which controls a ratio of on-period vs. off-period of the negative switch element, to the inverter, wherein thus the controller can turn on or turn off the positive or negative switch element of the inverter with its control signal, and the inverter can thus drive the drive windings of the plurality of phases with an alternating current shaping like sine -wave.

2. An electric apparatus including a motor driver, the apparatus comprising: an apparatus unit; a motor mounted to the apparatus unit; and the motor driver comprising-' an inverter including- a positive switch element which connects drive windings of a plurality of phases of the motor to a positive power line; and a negative switch element which connects the drive windings to a negative power line, and a controller including- a waveform generator which outputs a signal showing a ratio of on -period vs. off-period of the positive or the negative switch element as a driving waveform signal of the drive windings; and a polarity determiner which determines a polarity of back electromotive force generated on the drive windings, wherein when the polarity determiner determines the back electromotive force of the drive windings to be positive, the controller outputs a control signal, which controls a ratio of on-period vs. off-period of the positive switch element, to the inverter, and when the polarity determiner determines the back electromotive force of the drive windings to be negative, the controller outputs a control signal, which controls a ratio of on-period vs. off-period of the negative switch element, to the inverter, wherein thus the controller can turn on or turn off the positive or negative switch element of the inverter with its control signal, and the inverter can thus drive the drive windings of the plurality of phases with an alternating current shaping like sine -wave.