1. Field of the Invention
The present invention relates to a self-diagnosing apparatus for a brushless motor which may be used as a fan motor combined with a temperature sensor for sensing the temperature in a vehicle interior.
2. Description of the Related Art
An automatic air conditioner installed in the interior of a vehicle includes a temperature sensor for sensing the temperature in the vehicle interior. In order to increase the accuracy of interior temperature sensing by the temperature sensor, a fan is provided so that air in the interior of the vehicle can be blown toward the temperature sensor. Direct-current motors have hitherto been used as the motors for such fans. However, a brushless motor has come into use because it generates a low noise level and consumes a low level of electric power.
In actual use of a brushless motor, however, a fault may occur. For example, a short circuit or an open circuit may occur in the coil of the brushless motor or in the drive line for the brushless motor (the coil and the drive line will be generically referred to as "a motor line"). When such a fault occurs, the accuracy of sensing by the temperature sensor decreases, making it impossible for the automatic air conditioner to operate properly. Since a brushless motor generates only a low noise level during its operation, even if a fault, such as above, occurs resulting in an unwanted stoppage of the fan, the driver or passengers of the vehicle cannot be readily informed of the fault, particularly when driving the vehicle. For this reason, a self-diagnosing apparatus for the brushless motor is provided so that a fault, such as above, can be automatically detected and indicated.
FIG. 9 shows, in a block diagram, an example of a conventional self-diagnosing apparatus for a brushless motor. Reference numeral 1 denotes a brushless motor, and reference numeral 2 denotes a drive circuit. The apparatus includes a current detection section 3 and a voltage detection circuit 4.
Referring to FIG. 9, the brushless motor 1 is driven by drive current supplied from the drive circuit 2. The current detection section 3 detects drive current being supplied to the motor 1. On the basis of a detection of the current detection section 3, the voltage detection circuit 4 detects a condition of the motor line, and outputs a detection voltage signal A indicative of the detection. The detection voltage signal A has a high (H) level when the motor line is in a short circuit condition, and has a low (L) level when the motor line is in an open circuit condition. When the brushless motor 1 is in a normally operating condition, the signal A has a voltage x (V) at a level intermediate between "H" and "L" levels.
When the voltage detection circuit 4 has detected a short circuit condition of the motor line, the detection circuit 4 controls the drive circuit 2 in such a manner that no current will flow in the motor line. This operation is referred to as "short circuit protective control".
FIG. 10 is a circuit diagram showing a specific configuration of circuitry corresponding to such a conventional apparatus and a drive circuit. The circuitry includes resistors R1 to R10, transistors TR1 and TR2, diodes D1 to D4, and a comparator CP. Denoted by reference character ML is a motor line connected to a brushless motor, not shown. Reference numerals 2 to 4 denote circuits corresponding to those indicated by the same reference numerals in FIG. 9.
Referring to FIG. 10, when the brushless motor is normally operating, the transistor TR2 is turned on so that a certain bias voltage determined by the resistors R8 and R9 is applied to the base of the transistor TR1, and current IM is supplied to the brushless motor through the transistor TR1 and the resistor R1, the resistor R1 constituting the current detection section 3.
At this time, a voltage VRS, expressed by the following formula (1), is generated across the resistor R1: EQU VRS=R1.times.IM (1)
The voltage VRS is supplied to the voltage detection circuit 4, in which the resistors R2 to R5 cause the comparator CP to output a voltage VSD' expressed as follows: EQU VSD'=k.times.VRS/(1-k) (2)
(where k=R4/(R2+R4), R2 to R5 satisfying the relationship of R2=R3 and R4=R5)
If the resistors R6 and R7 satisfy the relationship of R6 &lt;&lt;R7 and there is simultaneously a diode (diode 2) voltage VD, the voltage detection circuit 4 outputs a voltage VSD expressed as follows: EQU VSD=VSD'-VD (3)
Such a voltage VSD serves as the detection voltage signal A shown in FIG. 9. At the time being described, the diode D1 is forwardly biased and turned on so that the diode D3 is reversely biased and turned off. The diode D4 is also reversely biased and turned off.
When the motor line ML is in an open circuit condition, substantially no current IM flows through the resistor R1 constituting the current detection section 3 so that the voltage VRS across the resistor R1 (expressed by formula (1)) substantially equals 0 (VRS=0). Consequently, the voltage output from the voltage detecting circuit 4 (calculated by the formulae (2) and (3)) approximately equals 0 (VSD.apprxeq.0). Thus, the signal A at a "L" level is obtained.
When the motor line ML is in a short circuit condition, the diode D4 is turned on, whereby the potential at point P becomes equal to the voltage VD of the diode D4. As a result, the transistor TR2 is turned off, and the transistor TR1, in which the bias voltage at its base increases, is also turned off. Consequently, the magnitude of current IM flowing in the motor line is brought to 0. This is the short circuit protective control.
The turning off of the transistor TR2 causes the diode D1 to be reversely biased and turned off. As a result, the output resistor 7, to which a line voltage E3 is supplied through the resistor R10 and the diode D3, causes the voltage detection circuit 4 to output a voltage VSD expressed as follows provided that the relationship of R10 &lt;&lt;R7 stands: EQU VSD=E3-VD (4)
Thus, the signal A at a "H" level is obtained. In formula (4), VD represents the voltage of the diode D3. At this time, the diode 2 is reversely biased and turned off.
If the circuitry operable as above is arranged such that the output voltage VSD expressed by the formula (3) is lower than the output voltage VSD expressed by the formula (4), it is possible to distinguish, on the basis of the magnitude of voltage VSD, each of a normally operating condition of the brushless motor, an open circuit condition of the motor line ML and a short circuit condition of the motor line ML from the others. An example of arrangement for this purpose will be described below.
In this example, it is assumed that E1=E3=+5 (V), E2=+12 (V), R8=470 (.OMEGA.), R9=1.8 (k.OMEGA.), R1=15 (.OMEGA.), and IM=30 (mA). In the voltage detection circuit 4, R2=R3=10 (k.OMEGA.), R4=R5=R7=47 (k.OMEGA.), R6=1 (k.OMEGA.), and R10=820 (.OMEGA.). It is also assumed that the diode voltage VD=0.6 (V).
Under the above assumption, voltage VSD during a normally operating condition of the brushless motor can be calculated from the formulae (1), (2) and (3) as follows: From formula (1), VRS=30 (mA).times.15 (.OMEGA.)=450 (mA). Therefore, from formulae (2) and (3), EQU VSD=450 (mA).times.(47/10)-0.6 (V)=1.5 (V).
On the other hand, when the motor line ML is in a short circuit condition, from formula (4), EQU VSD=5 (V)-0.6 (V)=4.4 (V)
Thus, voltage VSD during a short circuit condition is apparently greater than voltage VSD during a normal operation of the brushless motor.
The circuitry shown in FIG. 10 is combined with a determination circuit in which various voltage ranges, such as those shown in FIG. 11, are set. The determination circuit is arranged to determine within which of these ranges a voltage VSD output from the voltage detection circuit 4 falls, so as to determine a normally operating condition of the brushless motor, a short circuit condition of the motor line ML, or an open circuit condition of the motor line ML.
With the conventional-self-diagnosing apparatus, however, it has been impossible to determine a condition referred to as "a motor lock condition", in which the brushless motor becomes unable to rotate due, for instance, to the entrance of foreign matter. This is for the following reason: when the motor is in a locked condition, the magnitude of current IM flowing in the motor line ML is not so great as that of current which can flow in the motor line ML in a short circuit condition; as a result, a voltage VSD output from the voltage detection circuit 4 at this time falls within the same voltage range as output voltages VSD obtainable in a normally operating condition of the brushless motor.