Method and apparatus for controlling a brushless DC motor

What is claimed is:

1. A control apparatus for a brushless DC motor which has a permanent magnet rotor and a 3-phase stator winding for applying a rotating magnetic field to the permanent magnet rotor, comprising:

a plurality of first comparison circuits, one provided for each phase, for comparing a specified reference voltage with the stator winding terminal voltages;

a circuit detecting rotor position in response to the first comparison circuits, based on induced voltages generated in the stator windings which are non-conducting when the motor is rotated, the circuit switching conduction to each stator winding in response to detection of the rotor position;

a conduction circuit for supplying a specific conduction pattern to each stator winding when the rotor is stopped;

a second comparison circuit for comparing a reference output determined for the conduction pattern with the outputs of the first comparison circuits when the conduction circuit is conducting; and

an abnormality detection circuit detecting abnormalities based on the comparison results of the second comparison circuit.

2. A control apparatus for a brushless DC motor which has a permanent magnet rotor and a 3-phase stator winding for applying a rotating magnetic field to the permanent magnet rotor, comprising:

a plurality of first comparison circuits, one provided for each phase, for comparing a specified reference voltage with the stator winding terminal voltages;

a circuit detecting rotor position in response to the first comparison circuits, based on induced voltages generated in the stator windings which are non-conducting when the motor is rotated, the circuit switching conduction to each stator winding in response to detection of the rotor position;

a conduction circuit for selectively supplying a first conduction pattern in which all the stator windings take higher voltages than the specified reference voltage and a second conduction pattern in which all the windings take a lower voltage than the specified reference voltage while the rotor is stopped;

a second comparison circuit for comparing a reference output determined for each conduction pattern and the output of the first comparison circuits when the conduction circuit is supplying the first and second conduction patterns; and

an abnormality detection circuit for detecting abnormalities based on the comparison results of the second comparison circuit.

3. A control apparatus for a brushless DC motor which has a permanent magnet rotor and a 3-phase stator winding for applying a rotating magnetic field to the permanent magnet rotor, comprising:

a plurality of first comparison circuits, one provided for each phase, for comparing a specified reference voltage with the stator winding terminal voltages;

a second comparison circuit comparing comparison outputs of the first comparison circuits with a reference signal;

a circuit for detecting rotor position in response to the second comparison circuit, based on induced voltages generated in the stator windings which are non-conducting when the motor is rotated, the circuit switching conduction to each stator winding in response to detection of the rotor position;

a conduction circuit for supplying multiple conduction patterns in which windings of one phase or two phases are at lower voltages than a specified reference voltage and a remaining stator winding is at a higher voltage than the specified reference voltage while the motor is stopped;

a third comparison circuit for comparing a reference output determined for the conduction patterns with the output of the second comparison circuit when the conduction circuit is conducting; and

an abnormality detection circuit for detecting abnormalities based on the comparison results of the third comparison circuit.

4. A control apparatus for a brushless DC motor which has a permanent magnet rotor and a 3-phase stator winding for applying a rotating magnetic field to the permanent magnet rotor, comprising:

a plurality of first comparison circuits, one provided for each phase, which compare a specified reference voltage with the stator winding terminal voltages;

a circuit which detects rotor position in response to the first comparison circuits, based on induced voltages generated in the stator windings which are non-conducting when the motor is rotated, the circuit switching conduction to each stator winding in response to detection of the rotor position;

a conduction circuit which supplies multiple conduction patterns in which windings of one phase or two phases are at lower voltages than a specified reference voltage and a remaining phase stator winding is a higher voltage than the reference voltage while the brushless motor is stopped;

a second comparison circuit which compares a reference output determined for the conduction pattern and the outputs of the first comparison circuits during conduction by the conduction circuit; and

an abnormality detection circuit which detects abnormalities in response to the comparison results of the second comparison circuit.

5. A control apparatus according to one of claims 1 to 3, wherein the conduction circuit conducts current to each winding prior to activation of the brushless DC motor before a starting of motor operation.

6. A control apparatus according to claim 1, 2 or 3, further including an indication circuit for indicating the abnormality, when the abnormality detection circuit has detected an abnormality.

7. A control apparatus according to claim 1, 2 or 3, further including:

a current detection circuit which detects the current flowing in the motor; and

a conduction prohibition circuit which prohibits conduction of current to the stator windings after the current detection circuit has detected an over-current during conduction to the windings by the conduction circuit.

8. A control apparatus according to claim 7, further including an indication circuit which indicates an over-current abnormality when the current detection circuit has detected an over-current during conduction to the windings by the conduction circuit.

9. A control apparatus according to one of claim 1, 2 or 3, wherein the reference voltage is a voltage approximately intermediate between the DC voltage inputs to the brushless DC motor.

10. A control apparatus according to claim 1, 2, 3 or 4, further including a fan which is rotated by the brushless DC motor.

11. A control apparatus according to claim 1, 3 or 4 wherein the specified reference voltage is a neutral point potential of the 3-phase stator winding.

12. An air conditioner having a sealed compressor and a compressor motor, which is built into the sealed compressor, having a permanent magnet rotor and a 3-phase stator winding for applying a rotating magnetic field to the permanent magnet rotor, comprising:

a plurality of first comparison circuits, one provided for each phase, for comparing a specified reference voltage with the stator winding terminal voltages;

a circuit for detecting rotor position in response to the first comparison circuits based on induced voltages generated in the stator windings which are non-conducting when the motor is rotated, the circuit switching conduction to each stator winding in response to detection of the rotor position;

a conduction circuit for supplying a specific conduction pattern to each stator winding when the rotor is stopped;

a second comparison circuit which compares a reference output determined for the conduction pattern with the outputs of the first comparison circuits when the conduction circuit is conducting; and

an abnormality detection circuit which detects abnormalities based on the comparison results of the second comparison circuit.

13. An air conditioner having a sealed compressor and a compressor motor, which is built into the sealed compressor, having a permanent magnet rotor and a 3-phase stator winding for applying a rotating magnetic field to the permanent magnet rotor, comprising:

a plurality of first comparison circuits, one provided for each phase, for comparing a specified reference voltage with the stator winding terminal voltages;

a second comparison circuit comparing comparison outputs of the first comparison circuits with reference signal;

a circuit for detecting rotor position in response to the second comparison circuit, based on induced voltages generated in the stator windings which are non-conducting when the motor is rotated, the circuit switching conduction to each stator winding in response to detection of the rotor position;

a conduction circuit for supplying multiple conduction patterns in which windings of one phase or two phases are at lower voltages than a specified reference voltage and a remaining stator winding is at a higher voltage than the specified reference voltage while the motor is stopped;

a third comparison circuit for comparing a reference output determined for the conduction patterns with the output of the second comparison circuit when the conduction circuit is conducting; and

an abnormality detection circuit for detecting abnormalities based on the comparison results of the third comparison circuit.

14. A method for controlling a brushless DC motor which has a permanent magnet rotor, a 3-phase stator winding for applying a rotating magnetic field to the permanent magnet rotor, comparison circuits, one for each phase, which compare a specified reference voltage with the stator winding terminal voltages, a circuit which detects rotor position based on the outputs of the comparison circuits and an inverter circuit which switches conduction to each stator winding in accordance with the detected rotor position, comprising the steps of:

detecting abnormalities based on the outputs of the comparison circuits;

outputting a specified frequency and voltage from the inverter in a state in which connection between the stator windings and the inverter is broken when an abnormality is detected in the detecting step;

measuring output inter-phase voltages of the inverter when the connection is broken; and

determining an abnormality of the comparison circuits when the inter-phase voltages obtained by the measuring step are approximately the same value, and determining an abnormality of the inverter otherwise.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control apparatus and method for a brushless DC motor.

2. Description of Related Art

In a conventional DC motor, commutations, essentially a mechanical switching operation, control the current through the stator windings. This operation is accomplished in conventional DC motors with brushes and segmented commutators. In such a construction, the brushes wear and require frequent replacement. Sparking and its attendant generation of RF noise cannot be avoided. However, DC motors typically have higher torque and efficiency than AC induction motors. Thus, recently, a brushless DC motor has been developed. Some brushless motors have one or more sensors, for example hall effect devices, to detect the position of a permanent magnet rotor of the DC motor without touching the rotor. Furthermore, sensorless brushless DC motors have been developed which are simpler and less expensive than brushless DC motors with sensors.

A typical sensorless brushless DC motor has three phase stator windings and a permanent magnet rotor. The stator windings are selectively energized or caused to conduct current to apply a rotating magnetic field to the permanent magnet rotor. In the energization pattern, at any particular time only two of the phases are conducting current, leaving one phase not conducting current. The stator windings are energized and deenergized during the rotation of the brushless DC motor, based on a comparison of the induced voltage generated in the non-conducting stator winding with a specified reference voltage. In this motor, three comparator circuits are provided to compare the induced voltage of each winding and the specified reference voltage.

In this type of brushless DC motor, an activation malfunction or a short-circuit of a switching transistor of an inverter, which functions as a static commutator, is detected based on the current passing through the stator windings or the motor current. For detection of these malfunctions, the DC motor current is detected by a current detector inserted in series with the current providing circuit of the DC motor. When the detected current is above a specified value, it is determined that a switching transistor of the inverter has short-circuited or the motor failed to start rotating. Then, power is disconnected from the DC motor.

As described above, the induced voltages generated in the non-conducting windings during rotation of the brushless DC motor are compared with a reference voltage. The position of the permanent magnet rotor is detected based on that variation. Sequential current conduction phase switching is executed taking this detected rotor position point as a reference. Since the rotor is not rotating right after the motor is energized, induced voltages are not generated in the non-conducting stator windings because there are no rotating magnetic fields. Therefore, on energization, the stator windings are selectively energized without detection of the rotating position of the rotor in a process called "forced commutation". Then, when a specified time has elapsed after which it is possible to detect the induced voltages generated as a result of forced commutation, commutation based on the detected rotor position starts.

However, when the motor rotor is mechanically constrained, or a drive circuit which drives the switching transistors of the inverter does not operate normally, the motor cannot rotate. Thus, the rotor position cannot be detected even after the specified time has elapsed. At this point, commutation is suspended, and an abnormality, in the form of an activation malfunction, is signaled.

In the above brushless DC motor, the following three abnormalities are considered to have the highest probability:

(a) The motor fails to rotate due to a mechanical failure locking the position of the motor rotor.

(b) Open-phase output due to a failure in the inverter or the drive circuit.

(c) Rotor position cannot be detected due to a failure of an element in a rotor position detection circuit which includes the comparator circuits.

These failures were all expressed as activation malfunctions in the prior art. However, when repairing these failures, the motor must be replaced in response to malfunction (a); the inverter element or drive circuit must be replaced or the wiring between these circuits must be checked in response to malfunction (b); and the position detection circuit must be replaced in response to malfunction (c). Thus, after a failure, the service engineer must discriminate which of these three malfunctions is the cause.

Usually, the circuit components for a DC motor, that is, the inverter, the drive circuit and the position detection circuit, are all mounted on a single substrate. In such a DC motor, when the malfunction is caused by circuit parts, the DC motor can be repaired by replacing that substrate, so that discrimination between malfunctions (b) and (c) is not required. Therefore, it is only necessary to discriminate malfunction (a) from malfunctions (b) and (c).

However, in the prior art there was no device which could discriminate between rotation failure due to motor malfunction (a) and malfunctions (b) and (c). Thus, after an activation malfunction was detected, the service engineer replaced one of the parts of the DC motor. If the DC motor then performed normally, the replaced part was considered faulty. If the activation malfunction re-occurred despite the replacement, one of the unreplaced parts might be faulty. Therefore, the service engineer reinstalled the previously replaced part and replaced one of the unreplaced parts with a new component until the DC motor operated normally. Thus, the work of the service engineer was very inefficient.

Furthermore, if the DC motor was built into a sealed compressor to rotate a compression mechanism, pipes connected to the compressor to carry refrigerant had to be disconnected. Then the compressor itself had to be replaced. Therefore, it is desirable to accurately determine the cause of a failure.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and apparatus for controlling a brushless DC motor which can accurately indicate the cause of a failure.

It is another object of the invention to improve the efficiency of replacing parts of a brushless DC motor after the DC motor has failed.

It is a further object of the invention to provide a method and apparatus for controlling a brushless DC motor which can indicate the cause of a failure before the DC motor is energized.

To achieve the above objects, the present invention provides an improved method and apparatus for controlling a brushless DC motor which has a permanent magnet rotor and a 3-phase stator winding for applying a rotating magnetic field to the permanent magnet rotor. A specified reference voltage is compared with induced voltages in the stator winding which is not energized, and as a result of these comparisons the position of the rotor is determined. The energization of the windings is controlled based on the rotor position. Furthermore, specific conduction patterns can be applied to the stator windings when the rotor is stopped. The voltages detected at the windings are compared with optimal voltages produced when the circuitry is operating properly. Abnormalities are detected when the comparison result is not as expected.

BRIEF DESCRIPTIONS OF THE DRAWINGS

In the accompanying of the drawings:

FIG. 1 is an overall block diagram of a control circuit of an air conditioner using an indoor fan DC motor, an outdoor fan DC motor and a compressor DC motor according to the present invention;

FIG. 2 is a block diagram showing a control circuit for the compressor motor;

FIG. 3 is a schematic transverse cross-sectional view of the compressor motor;

FIG. 4 is a schematic diagram showing the three phase stator winding of the compressor motor;

FIGS. 5(a) to 5(f) are time charts of the conduction of an inverter which drives the compressor motor, and induced voltages in the three phase stator windings;

FIGS. 6(a) to 6(c) are timing charts showing the state of PWM control of the inverter;

FIG. 7 is a table showing the relationships between specific conduction patterns and outputs of a compressor rotor position detection circuit before the compressor motor activation;

FIG. 8 is a table showing forced commutation conduction patterns for the compressor motor;

FIG. 9 is a block diagram showing a control circuit for the indoor fan motor;

FIG. 10 is a table showing the relationships between specific conduction patterns and outputs of a indoor fan motor rotor position detection circuit before activation of the indoor fan motor;

FIG. 11 is a table showing modified specific patterns for the specific conduction patterns shown in FIG. 10;

FIG. 12 a block diagram showing a control circuit for an outdoor fan motor; and

FIG. 13 a table showing the relationships between the specific conduction patterns of the outdoor fan motor and outputs of an outdoor fan motor rotor position detection circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following are descriptions of embodiments of this invention, one embodiment applied as a compressor motor, one embodiment applied as a room indoor fan motor and one embodiment applied as an outdoor fan motor of an air conditioner.

(1) Overall Composition of the Air Conditioner

The basic composition of an air conditioner including all three embodiments and an outline of each embodiment will be explained with reference to FIG. 1.

The refrigerating circuit of the air conditioner includes compressor 2; four-way valve 3, which switches between a heating position and a cooling position; outdoor heat exchanger 4; expansion valve 5; and indoor heat exchanger 6. Compressor 2 has a sealed case 2a including a compression mechanism 2c and compressor DC motor 2M. DC motor 2M drives compression mechanism 2c. The refrigerant sealed inside circulates through the refrigerating circuit as a result of compression mechanism 2c when compressor 2 is energized. Outdoor fan 7 is provided at outdoor heat exchanger 4. Indoor fan 8 is provided at indoor heat exchanger 6. Indoor fan 8 is driven by indoor fan DC motor 8M. Outdoor fan 7 is driven by outdoor fan DC motor 7M. The rotational speed of compressor 2 is controlled by inverter 10 (hereafter called compressor inverter 10) via compressor DC motor 2M. In the same way, the rotational speed of indoor fan 8 is controlled by inverter 11 (hereafter called indoor fan inverter 11) via indoor fan DC motor 8M, and the rotational speed of outdoor fan 7 is controlled by inverter 12 (hereafter called outdoor fan inverter 12) via outdoor fan DC motor 7M. All of inverters 10 to 12 receive DC power via rectifying circuits, described later, from common AC power source 13. Each of inverters 10 to 12 outputs variable frequency AC power.

Compressor inverter 10 inputs DC power obtained from commercial AC power source 13 via voltage doubler rectifying circuit 14 and capacitor 15. The output of compressor inverter 10, that is variable frequency AC power, is supplied to compressor motor 2M. Thus, compressor inverter 10 can drive compressor 2 via compressor DC motor 2M at variable speed.

Indoor fan inverter 11 inputs DC power obtained from AC power source 13 via rectifying circuit 16, capacitor 17, DC/DC converter 18 and voltage-drop type DC/DC converter 19. Indoor fan inverter 11 converts the DC power to variable frequency AC power. Indoor fan 8 is driven at Variable speed by indoor fan Dc motor 8M.

The circuit to drive the outdoor fan 7 is similar to the circuit to drive compressor DC motor 2M. Outdoor fan inverter 12 inputs DC voltage obtained from AC power source 13 via rectifying circuit 20 and capacitor 21, and converts the DC voltage to a variable frequency AC power. Outdoor fan 7 is driven by outdoor fan DC motor 7M at a variable speed.

Inverters 10 to 12 are respectively controlled by individually provided compressor control circuit 22, indoor fan control circuit 23 and outdoor fan control circuit 24. Main control circuit 25 is provided in common for these control circuits 22 to 24. These control circuits 22 to 24 and main control circuit 25 are respectively composed of microprocessors. These control circuits function in accordance with each microprocessor's individual software which is pre-installed in a memory of each microprocessor.

Main controller 25 transmits `Operate`/`Stop` instructions (a) to each control circuit 22 to 24, based on `Operate`/`Stop` instructions transmitted from a remote control unit. At the same time, it transmits compressor rotational speed instructions (b) to compressor control circuit 22 so that the difference between room temperature T.sub.a detected by room temperature sensor 26, for example, a thermistor, and set room temperature T.sub.s, set by room temperature setting device 27, that is, temperature deviation .DELTA.T, will approach zero. Main controller 25 further transmits indoor fan rotational speed instructions (c) to control circuit 23 and transmits outdoor fan rotational speed instructions (d) to control circuit 24.

Each DC motor 2M, 7M and 8M, is a synchronous motor having a permanent magnet rotor. Each DC motor 2M, 7M and 8M, together with each position detection device and each inverter 10, 11, 12 (electronic rectifier) forms a brushless DC motor. In this invention, position detection is performed based on the induced voltages in stator windings of DC motors 2M, 7M and 8M, without providing an independent position sensor (in other words "sensorless"). That is, the current conduction combination of the stator windings is set so that one phase is always disconnected from a current supply. Therefore, it is possible to detect the rotor position based on the induced voltage in the non-conducting stator winding at the time of rotor rotation.

The non-conducting phase must be disconnected from the current supply for a sufficient portion of the rotational cycle to permit position detection based on induced voltage. Normally, in a three phase motor, each stator winding, is energized in a series of cycles of positive conduction for an electrical angle of 120.degree., non-conduction for 60.degree., negative conduction for 120.degree. and thereafter non-conduction for 60.degree..

Current detection resistor 29 which has a low resistance, for detecting the input current, and voltage divider resistors 28 are provided on the input side of compressor inverter 10. Voltage divider resistors 28 extract a voltage intermediate of the input DC voltage. The respective detection signals are applied to compressor control circuit 22. The output voltage of compressor inverter 10, e.g., the voltage at the input terminal of compressor DC motor 2M built into compressor 2, is also applied to compressor control circuit 22. Compressor control circuit 22 calculates frequency instructions for compressor inverter 10 based on these input signals, together with `Operate`/`Stop` instructions (a) and compressor rotational speed instructions (b) to control compressor inverter 10.

The input current for indoor fan inverter 11 is applied to indoor fan control circuit 23 after it is detected via current detector 30, which is composed of a low value resistor. In addition, an input voltage to and an output voltage from indoor fan inverter 11 are input to indoor fan control circuit 23. In indoor fan control circuit 23, the neutral point potential of indoor fan DC motor 8M is generated from the output voltage of indoor fan inverter 11, i.e., the input voltage of indoor fan DC motor 8M. Indoor fan control circuit 23 calculates and outputs control instructions for DC/DC converter 18, voltage-drop type DC/DC converter 19 and indoor fan inverter 11 based on these input voltage values, together with `Operate`/`Stop` instructions (a) and indoor fan rotational speed instructions (c) to control the indoor fan inverter 11.

For outdoor fan inverter 12, in a manner similar to compressor inverter 10, the input current and the input voltage are respectively detected via low resistance current detection resistor 32 and voltage divider resistors 31, and are applied to outdoor fan control circuit 24. The output voltage of outdoor fan inverter 12 is also applied to outdoor fan control circuit 24. Outdoor fan control circuit 24 controls outdoor fan inverter 12 by calculating and outputting control instructions for outdoor fan inverter 12 based on these input voltage values, together with `Operate`/`Stop` instructions (a) and outdoor fan rotational speed instructions (d).

Further details of the compositions of the controllers for compressor 2 (compressor DC motor 2M), indoor fan 8 (indoor fan DC motor 8M) and outdoor fan 7 (outdoor fan DC motor 7M), will be explained in that order.

(2) Compressor Control

<Composition of Main Control Circuit and Compressor Control Circuit>

FIG. 2 shows the details of compressor inverter 12, the main system for compressor 2 and compressor control circuit 22.

Compressor 2 includes rotary type compressor mechanism 2C and compressor motor 2M which drives rotary type compressor mechanism 2C. Compressor DC motor 2M is housed inside a sealed case which is common to compressor mechanism 2C. As shown in FIG. 3, compressor DC motor 2M is composed of stator 200 which has 6 salient-pole teeth arranged spatially at 60.degree. intervals and 4-pole rotor 220 having 4 permanent magnets which is rotatably supported inside the stator 200. Three phase stator windings 211 to 216 for applying a rotating magnetic field to rotor 220 are wound on salient-pole teeth 201 to 206. Rotation axis 221 extends through the center of rotor 220. As shown in FIG. 4, three phase windings 211 to 216 are star-connected. The output voltage of compressor inverter 10 is applied to three phase winding terminals R, S and T.

Rectifying circuit 14 is a known voltage doubler rectifying circuit composed of diodes and capacitors. It produces a DC voltage of approximately 280V by voltage doubling rectification of the 100V AC voltage of commercial AC power source 13, and inputs that DC voltage to compressor inverter 10. Compressor inverter 10 together with compressor DC motor 2M compose a brushless DC motor. Connector 35, which functions as a circuit breaker, is provided between compressor inverter 10 and compressor DC motor 2M. Constant voltage supply circuit 40, connected to the output terminals of rectifying circuit 14, generates constant voltage V.sub.dd. Compressor inverter 10 includes transistors Tr1 to Tr6 and outputs variable frequency AC power from its AC output terminals U, V and W.

As shown in FIGS.5(a) to 5(c), the voltage supplied to compressor DC motor 2M from compressor inverter 10 repeats a pattern for each phase winding of positive conduction for an electrical angle of 120.degree., non-conduction for 60.degree., negative conduction for 120.degree. and non-conduction for 60.degree. during one cycle. It is set to maintain a phase difference of 120.degree. between each phase. In this way, when compressor DC motor 2M is rotating, induced voltages r, s and t, which vary from a first state to a second state or from the second state to the first state, are generated in each phase winding during the non-conducting periods, as shown in FIGS.5(d) to 5(f). Therefore, time period t.sub.0 from the switch-OFF time of two transistors which form one phase of compressor inverter 10, until the induced voltage of the phase crosses a reference voltage during this non-conduction period, is measured. Specified transistors are switched ON or OFF after the time period t.sub.0 has elapsed from the time the induced voltage crosses the reference voltage. For example, as shown in FIGS. 5(c) and 5(f), time period t.sub.0 is measured from the switch-OFF time point of transistor Tr6 (W phase negative side=T phase winding negative voltage) until the T phase induced voltage crosses the reference voltage. After the measurement, of transistor Tr1 is switched OFF (U phase positive side=R phase winding positive voltage) and transistor Tr3 is switched ON (W phase positive side=T phase winding positive voltage) when the time period t.sub.0 has passed from the point in time when the induced voltage crosses the reference voltage. In this way sensorless DC motor control can be performed by rotor position detection.

The above theoretical description corresponds to the principle of commutatorless sensorless DC motor control. In practice, in compressor inverter 10, one of a pair of transistors for one phase is energized to provide a pulse width modulated (PWM) current to DC motor 2m in order to control the compressor rotational speed. Therefore, each phase voltage V.sub.r, V.sub.s and V.sub.t of the motor windings is as shown in FIGS. 6(a)-6(c). In this case, during the non-conduction period (angle), the phase voltage is changed by the PWM switching of the transistor. However, induced voltages can be detected based on non-conduction phase voltage V.sub.r, V.sub.s or V.sub.t. Time period t.sub.0 is measured as the time from the start of a non-conduction period (points A) until the induced voltage of the non-conduction phase becomes half of the input DC voltage of compressor inverter 11, that is the neutral point potential of the input DC voltage (point P), during this non-conduction period. Then, specified transistors are switched 0N or OFF after the time period t.sub.0 has elapsed from the time the induced voltage became half of the input DC voltage.

Compressor control circuit 22 is provided with over-current detection circuit 42, which detects over-currents in the input circuit of compressor inverter 10 based on the voltage drop across current detection resistor 29. Inverter drive circuit 41 drives transistors Tr1 to Tr6 of compressor inverter 10. The detection output signals of over-current detection circuit 42 are transmitted to inverter drive circuit 41 and short circuit transistor discrimination device 60. The phase output terminal potentials of compressor inverter 10, obtained via voltage divider resistors 43, 44 and 45, are compared in comparators 46, 47 and 48 with the neutral point potential of the input DC voltage provided by voltage divider resistors 28. These comparison result signals are input to a comparison input terminal of comparator 50 via voltage divider circuit 49. Voltage divider circuit 49 is composed of resistors R1, R2 and R3, which are connected in series to each output terminal of comparators 46, 47 and 48, and resistor R4 which is connected in common between the other terminals of reSistors R1, R2 and R3 and the constant DC voltage V.sub.dd terminal. The node of resistor R4 and resistors R1, R2 and R3 is connected to the input terminal of comparator 50. A reference voltage for comparison, which is obtained from constant DC voltage V.sub.dd by voltage divider circuit 51, composed of resistors R5 and R6, is input to a reference input terminal of comparator 50. A position detection circuit which detects a rotor position of compressor motor 2M is composed of voltage divider resistors 43 to 45 and comparators 46 to 48 and 50, and voltage divider circuits 49 and 51. Voltage divider circuits 49 and 51 and comparator 50 form a majority decision circuit (described hereafter) in which the output of comparator 50 depends on the state of the outputs of the majority of comparators 46-48.

In order to provide an accurate indication of when the induced voltages change state, the voltage between voltage divider resistors 28 should be one-half of the difference between the maximum and minimum voltages developed between the resistors of dividers 43,.44 and 45. As practical examples of the values of the voltage divider resistors, resistor 28 connected to the higher voltage is 560 k.OMEGA., and resistor 28 connected to the lower voltage is 20 k.OMEGA.. The resistor of each of dividers 43-45 which is connected to a motor winding is 270 k.OMEGA., and the other resistor of each divider 43.OMEGA.45 is 20 k.OMEGA.. AlSo, in resistor circuit 49 and voltage divider circuit 51, R1=R2=R3=100 k.OMEGA., R4=75 k.OMEGA. and R5=R6=82 k.OMEGA..

In this embodiment, a voltage which is 1/2 of voltage V.sub.dd is input to the reference input terminal of comparator 50. The respective output terminals of comparators 46 to 48 are functionally cut off from resistor R4 at the time of the H output because of an `open collector` (or `open drain`) configuration, and at the time of the L output the output terminals are connected to resistor R4. Therefore, when one of comparators 46 to 48 has a H output and the other two have L outputs, the H output comparator becomes cut off from voltage divider circuit 49. The output terminal resistors of the two L output comparators are parallel. Thus, their total resistance value becomes halved (in the case of the above example, 50 k.OMEGA.). As a result, the comparison input to comparator 50 becomes (50/(50+75))*V.sub.dd (<(1/2)*V.sub.dd). Accordingly, the value is less than the reference voltage (1/2)*V.sub.dd. Thus, the output of comparator 50 becomes L. When two of comparators 46 to 48 have H outputs and the other has a L output, the comparison input to comparator 50 becomes (100/(100+75))*.sub.Vdd (>(1/2)*V.sub.dd). Accordingly, the value is greater than the reference voltage (1/2)*V.sub.dd. As a result, the output of comparator 50, becomes H. In this way, a majority decision circuit is formed by voltage divider circuits 49, 51 and comparator 50, in which the output of comparator 50 depends on the state of the outputs of the majority of comparators 46-48.

The rotor position of compressor DC motor 2M is determined by position determination circuit 52 based on the output of comparator 50, or the output of the majority decision circuit. Position determination circuit 52 is connected to activation control circuit 57, rotational speed detection circuit 53 and pattern comparison circuit 54. `Operate` instruction (a), transmitted from main control circuit 25, is input to specific pattern output circuit 55, and compressor rotational speed. instructions (b) are input to rotational speed comparison circuit 56. When specific pattern output circuit 55 has received `Operate` instruction (a), specific pattern output circuit 55 transmits specific drive patterns, described later, to inverter drive circuit 41, pattern comparison circuit 54 and short-circuit transistor discrimination circuit 60. For example, specific pattern output circuit 55 outputs six signals, one to control each of transistors TR1-TR6 through drive circuit 41. Pattern comparison circuit 54 compares the specific patterns input from specific pattern output circuit 55 with the comparison output of comparator 50, or output of majority decision circuit. As a result to this comparison, pattern comparison circuit 54 transmits an `Activation Permit` signal to activation control circuit 57 or an `Abnormality` signal to abnormality detail indication circuit 58. When activation control circuit 57 receives an `Activation Permit` signal from pattern comparison circuit 54, it transmits activation control signals for voltage determination circuit 59 and conducting phase determination circuit 61.

When short-circuit transistor discrimination circuit 60 discriminates a short-circuit transistor in compressor inverter 10, based on the specific pattern signals output from specific pattern output circuit 55 and an abnormality detection signal from over-current detection circuit 42, short circuit transistor discrimination circuit 60 transmits that discrimination signal to abnormality detail indication circuit 58. As will be described below, the specific drive patterns include two conduction patterns for detecting the short-circuit of compressor inverter 10. In one pattern, all of the positive side transistors Tr1 to Tr3 are turned on and all of the negative side transistors Tr4 to Tr6 are turned off (pattern a) shown in FIG. 7). In the other pattern, all of the positive side transistors Tr1 to Tr3 are turned off and all of the negative side transistors Tr4 to Tr6 are turned on (pattern b) shown in FIG. 7). If an abnormality signal, i.e., an over-current detections signal, is transmitted from over-current detection circuit 42 while these two conduction patterns are supplied to compressor inverter 10, short-circuit transistor discrimination circuit 60 detects the short-circuit by receiving the abnormality signal and the two conduction patterns described above transmitted from specific pattern output circuit 55. Furthermore, short-circuit transistor discrimination circuit 60 distinguishes pattern a) from b), and determines which side of the transistors is short-circuited based on the distinction. Short-circuit transistor discrimination circuit 60 is easy to construct using logic circuits, such as exclusive OR and AND logic circuits.

When a forced operation instruction is transmitted from forced operation switch 62, that is transmitted to inverter drive circuit 41 via forced operation circuit 63, compressor 2 is forcibly operated via compressor inverter 10. When a motor abnormality is discriminated by motor abnormality discrimination circuit 64 based on the output of position determination circuit 52 and the output of activation control circuit 57, that discrimination signal is transmitted to abnormality detail indication circuit 58 to indicate the abnormality detail. It is also transmitted to activation control circuit 57 as an `Activation Not Permit` signal.

As indicated above, compressor control circuit 22 can be achieved by a microprocessor programmed with appropriate software or with logic circuits.

The operation of the above described circuit configuration will now be explained.

<Specified Pattern Output Before Activation>

When the air conditioner receives an `Operate` instruction from the user via the remote control unit, main control circuit 25 supplies `Operate` instruction (a) to specific pattern output circuit 55 in compressor control circuit 22. Specific pattern output circuit 55 transmits specified three phase output patterns, which have been preset or stored in the compressor control circuit 22, to inverter drive circuit 41. The specified three phase output patterns are shown in FIG. 7. First, pattern a) is output for a period of 50 ms (milliseconds). Then, after a stopped period of 20 ms has elapsed, output of the next pattern b) is performed for a 50 ms period. Thereafter, outputs are performed in sequence up to pattern f).

<Detection for Compressor Inverter 10 Short-Circuit Abnormality>

Pattern a) and Pattern b) are for detecting short-circuit abnormalities of transistors Tr1 to Tr6 in compressor inverter 10. When over-current detection circuit 42 detects an over-current during the output of these patterns, it is determined that a transistor short-circuit abnormality exists. At the same time, short-circuit transistor discrimination circuit 60 discriminates whether the short-circuit transistor is in the positive arm transistors (Tr1 to Tr3) or in the negative arm transistors (Tr4 to Tr6) based on the specific pattern a) and b). That is say, if over-current is detected during the output pattern a), at least one of positive arm transistors (Tr1 to Tr3) is short-circuited. While if over-current is detected during the output pattern b), at least one of negative arm transistors (Tr4 to Tr6) is short-circuited. The transistor short-circuit abnormality and the positive side/negative side discrimination result are displayed by abnormality detail indication circuit 58 based on the discrimination result by short-circuit transistor discrimination circuit 60.

At the same time, at the moment that over-current detection circuit 42 has detected an over-current, it immediately transmits a conduction prohibition instruction to inverter drive circuit 41 so that the damage will not spread further. By this conduction prohibition instruction, inverter drive circuit 41 does not drive transistors Tr1 to Tr6 of compressor inverter 10 until the conduction prohibition instruction has been reset after completion of repairs. For this reason, in the case that over-current detection circuit 42 detects an over-current during the output patterns a) or b), patterns c) to f) are not thereafter executed.

When transistors Tr1 to Tr6 are assembled in one package, all transistors Tr1 to Tr6 will be replaced, taking the package as a unit, at the time of repair and service. Thus, specification of the broken-down transistor is not required. Therefore, only the fact that there is a transistor short-circuit abnormality need be displayed by abnormality detail indication circuit 58.

<Detection of Position Detection Circuit or Inverter Drive Circuit 41 Abnormality>

When no over-current has been detected during the output of pattern a) or b), there