JPH0678596A - Trouble diagnostic device for motor driving device - Google Patents

Abstract

PURPOSE:To simplify a circuit configuration, to reduce in size and to improve reliability. CONSTITUTION:The trouble diagnosing device for a motor driver comprises a power source 75, motor coils 73, 74 connected to the power source 75, driving circuits 71, 72 connected to the coils 73, 74 to excite the coils 73, 74, and motor control means 61 for controlling the circuits 71, 72. The device also comprises a divided voltage detector 8 for dividing a driving voltage between the power source 75 and the coils 73, 74 to be detected to output a detection signal of the detected voltage of a predetermined voltage or lower corresponding to the driving voltage. Further, the device also comprises trouble discriminating means 62 receiving the detection signal of the detector 8 to output a normal signal when the detected voltage coincides with a preset normal voltage and an abnormal signal when it does not coincide therewith.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a failure diagnosis device for a motor drive device for driving an electric expansion valve or the like, and more particularly to a detection circuit configuration.

[0002]

2. Description of the Related Art Generally, in an air conditioner, a stepping motor is used to set the valve opening of an electric expansion valve. If the coil of the stepping motor is open (broken) or shorted (short circuit), the stepping motor will stop or the drive circuit will be damaged. Therefore, as disclosed in Japanese Patent Laid-Open No. 59-56819, a current flowing through a motor is conventionally detected by a current transformer, and a motor overload condition and a light load are detected based on a secondary voltage of the current transformer. There is a device that detects a load state and performs a failure diagnosis of a motor.

[0003]

However, in the above-mentioned failure diagnosis device, the secondary voltage of the current transformer is compared with the reference voltage by the comparator to detect the overload state and the light load state of the motor. Therefore, an expensive comparator is required, and it is necessary to provide a highly accurate resistor or the like, so that there is a problem that the cost is high and the circuit configuration is complicated. Further, since the current transformer is provided, there is a problem that the entire apparatus becomes large and the number of parts is large, resulting in lack of reliability.

The present invention has been made in view of the above points, and it is an object of the present invention to simplify the circuit configuration and to reduce the size and improve the reliability.

[0005]

In order to achieve the above-mentioned object, the means taken by the present invention divides the drive voltage to detect it, or operates the transistor with the drive voltage to detect an abnormality. It was done like this.

Concretely, as shown in FIG. 1, the means taken by the invention according to claim 1 is that the power source (75) and the power source (7
5) the motor coil (73, 74) connected to the drive circuit (71, 72) connected to the motor coil (73, 74) to excite the motor coil (73, 74), and the drive circuit (71 It is premised on a motor drive device provided with a motor control means (61) for controlling 71, 72).

The power source (75) and the motor coil (73,
There is provided a voltage division detection circuit (8) which divides and detects the drive voltage between the voltage detector 74 and 74) and outputs a detection signal of a detection voltage equal to or lower than a predetermined voltage corresponding to the drive voltage. Furthermore, when the detection signal of the voltage division detection circuit (8) is received and the detection voltage matches the preset normal voltage when the drive signal of the motor control means (61) is output, the normal signal is not matched. If there is, a failure determination means (62) that outputs an abnormal signal is provided.

Further, the means taken by the invention according to claim 2 is, in place of the failure determining means (62) in the invention according to claim 1, receives the detection signal of the voltage dividing detection circuit (8) and receives the motor control means. (61) variation amount deriving means (63) for deriving the variation amount of the detected voltage at the time of outputting the drive signal, and the detection variation amount derived by the variation amount deriving means (63) becomes a preset normal variation amount. A failure determination means (64) that outputs a normal signal when they match and an abnormal signal when they do not match is provided.

Further, the means taken by the invention according to claim 3 is, instead of the voltage dividing detection circuit (8) and the failure judging means (62) in the invention of claim 1, a power supply (75) and a motor coil (73). , 7
4) has a transistor Tr1 connected between it and the motor coil (7) based on the operating state of the transistor Tr1 when the drive signal of the motor control means (61) is output.
Disconnection detection circuit (9a) that outputs a disconnection signal at the time of disconnection of (3, 74)
And a transistor Tr2 connected between the power source (75) and the motor coil (73, 74), and the motor control means (6
A short circuit detection circuit (9b) that outputs a short circuit signal when the motor coil (73, 74) is short-circuited based on the operating state of the transistor Tr2 when the drive signal is output in 1) is provided.

[0010]

With the above construction, in the invention according to claim 1,
The motor control means (61) outputs a drive signal to the drive circuit (71, 72) to excite or demagnetize the motor coils (73, 74) to drive the motor, for example, to open the electric expansion valve in the air conditioner. Control the degree. On the other hand, voltage division detection circuit (8)
Detects the voltage by dividing the drive voltage between the power supply (75) and the motor coils (73, 74), and outputs a detection signal of a detection voltage equal to or lower than a predetermined voltage corresponding to the drive voltage. For example, 5
The detection voltage of V or less is output. Then, in the initial stage of driving the motor or the like, the failure determination means (62) sets the detection voltage of the voltage division detection circuit (8) in advance when the drive signal of the motor control means (61) is output. A normal signal is output when the voltage matches the normal voltage, and an abnormal signal is output when the voltage does not match. This abnormal signal causes an abnormal display, for example.

In the invention according to claim 2, the change amount deriving means (63) receives the detection signal of the voltage dividing detection circuit (8),
The amount of change in the detected voltage when the drive signal of the motor control means (61) is output is derived. And failure determination means
(64) is based on the detected change amount derived by the change amount deriving means (63), a normal signal when the detected change amount matches a preset normal change amount, and an abnormal signal when the detected change amount does not match. If a signal is output and it is abnormal, for example, an abnormal display will be performed.

Further, in the invention according to claim 3, the power source (75)
The transistor Tr1 of the disconnection detection circuit (9a) and the transistor Tr2 of the short circuit detection circuit (9b) are turned on and off based on the change in the drive voltage between the motor coil (73 and 74). Then, the transistor Tr1 of the disconnection detection circuit (9a) is turned off when the motor coils (73, 74) are disconnected at the time of outputting the drive signal of the motor control means (61), and the disconnection detection circuit is turned off. (9a) will output a disconnection signal. Further, the transistor Tr2 of the short-circuit detection circuit (9b) is turned on when the motor coil (73, 74) is short-circuited when the drive signal of the motor control means (61) is output, and the transistor Tr2 is turned on. (9b) will output a short circuit signal.

[0013]

Therefore, according to the invention of claim 1,
The voltage division detection circuit (8) detects the drive voltage between the power supply (75) and the motor coil (73, 74), and detects abnormalities such as disconnection based on the detection voltage of the voltage division detection circuit (8). Since it is possible to detect disconnection etc. based on the analog signal of the voltage division detection circuit (8), it is not necessary to provide a comparator etc. as in the past and it is also necessary to provide a highly accurate resistor etc. There is no. As a result, the circuit configuration can be simplified and the cost can be reduced. Further, unlike the conventional case, since it is not necessary to provide a current transformer in the drive circuit, the entire device can be downsized, and the reliability can be improved by reducing the number of parts.

Further, according to the second aspect of the invention, since the disconnection or the like is detected based on the amount of change in the detection voltage of the voltage dividing detection circuit (8), the power source (75) for driving the motor is used. Even when the power supply (81) for supplying power to the voltage division detection circuit (8) does not fluctuate relatively, it is possible to reliably detect the disconnection and the like. Therefore, in addition to the effect of the invention according to claim 1, Accurate detection can be performed.

According to the invention of claim 3, a power supply is provided.
Since the transistors Tr1 and Tr2 are turned on and off by the drive voltage between the (75) and the motor coils (73, 74) to detect disconnection and short circuit, it is not necessary to provide a comparator or the like unlike the conventional case. At the same time, disconnection and the like can be detected with a small number of parts, so that abnormal detection can be reduced, highly reliable detection can be performed, and reliability can be improved.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings. As shown in FIG. 2, (1) is an air conditioner
It is a plurality of indoor control units for controlling the indoor unit in (2), and is an air conditioning control unit connected to the outdoor control unit (3) for controlling the outdoor unit as shown in FIG. Then, the compressor of the outdoor unit, the four-way switching valve, the outdoor heat exchanger and the outdoor electric expansion valve, and the indoor electric expansion valve and the indoor heat exchanger of the indoor unit are sequentially connected to form a closed circuit refrigerant circuit. The indoor control unit (1) controls the indoor fan, the indoor electric expansion valve, etc., while the outdoor control unit (3) controls the compressor,
It controls the outdoor fan and the outdoor electric expansion valve. The indoor control unit (1) is connected to the remote controller (4), while the indoor control unit (1) is connected to the central controller (5) via the transmission line (21). While transmitting and receiving control signals such as operation signals and set temperature signals between 1) and the remote control (4), the centralized controller (5) above
And the indoor control unit (1) are configured to transmit and receive control signals such as operation signals and set temperature signals.

As shown in FIG. 3, the indoor control unit (1) is provided with a CPU (6) for controlling the indoor unit, and the CPU (6) is operated by the CPU (6) via an interface (11). Connected to the outdoor control unit (3), the remote control
(4) is connected via the communication microcomputer (12),
A centralized controller (5) is connected to exchange control signals with the remote controller (4) and the like. Further, the CPU (6) of the indoor control unit (1) is connected to the indoor fan motor (13) through the fan control circuit (14) to control the air volume, while the stepping motor (7) for the electric expansion valve
Are connected via a drive circuit (71, 72) and a partial pressure detection circuit (8) to control the opening degree of the indoor electric expansion valve. Further, the CPU (6) includes a room temperature sensor (1
5), the heat exchanger inlet sensor (16), the heat exchanger outlet sensor (17) and the radiation sensor (18) from the outside air temperature, the refrigerant inlet temperature and the refrigerant outlet temperature of the outdoor heat exchanger, each temperature signal of the radiation temperature It is configured to be input.

The stepping motor (7) is provided with two motor coils (73, 74) as shown in FIG. 4, and one end of the motor coils (73, 74) has a driving power source (75), for example, ,
It is connected to a 24V DC power supply (75) through a voltage dividing resistor R1 and the other end is connected to a drive circuit (71, 72), and the drive circuit (71, 72) outputs the CPU (6). Port P0,
It is connected to p1. The CPU (6) is provided with a motor control means (61), and the motor control means (61) outputs "Hi" or "Lo" based on the opening signal of the indoor electric expansion valve.
A w "drive signal is output from the output ports P0, p1 to excite or demagnetize the motor coils (73, 74).

On the other hand, as a feature of the invention according to claim 1,
A voltage dividing detection circuit (8) is connected between the voltage dividing resistor R1 and the motor coil (73, 74). The partial pressure detection circuit (8)
Is configured to divide and detect the drive voltage between the voltage dividing resistor R1 and the motor coil (73, 74) and output a detection voltage signal equal to or lower than a predetermined voltage corresponding to the drive voltage. . Specifically, the voltage dividing detection circuit (8) is
Between the motor coil (73, 74) and the resistor R2 and diode
D1 is connected in series, and the diode D1 is connected to the CPU (6)
Connected to the analog port P2 of the above diode while
A detection power supply (81), for example, a 5V DC power supply (75) is connected between D1 and the analog port P2 via a resistor R3, and the resistor R2 and the diode D1 are connected via a resistor R4. It is configured to be grounded.

For example, when the driving power source (75) is set to 24 V and one motor coil (73, 74) is excited, the drive voltage between the voltage dividing resistor R1 and the motor coil (73, 74) is If the motor coil (73, 74) is normal, 1
It becomes 2V, and when it is an open abnormality, it becomes 24V, and when it is a short abnormality, it becomes 0V. The input voltage Vin (detection voltage) of the analog port P2 is the driving power source.
The current flowing from both resistors R2 and R4 from the (75) side is I1 and the power supply for detection.
Letting I2 be the current flowing through both resistors R3, R4 and diode D1 from the (81) side, the following equation is obtained. I1 ＝ − [{V-5-5 (R2 / R4)} / {(R2 / R4) (R3 + R4) + R3}] …… I2 ＝ {5-I1 (R3 + R4)} / R4 …… Vin = (I1 + I2) R4 + VF ... VF: Forward voltage of diode D1 Therefore, resistance R2 is 200KΩ, resistance R3 is 64KΩ, resistance
If R4 is 33 KΩ, the above voltage division detection circuit (8)
Convert each drive voltage to 5V or less detection voltage Vin
To 3.3V, 24V to 4.5V, 0V to 2.13V
It is configured to be converted into and input to the CPU (6).

On the other hand, the CPU (6) has a failure determining means.
(62) is provided, and the failure determination means (62) receives the detection voltage signal of the voltage division detection circuit (8) and is preset when the drive signal of the motor control means (61) is output. The normal voltage is compared with the detected voltage Vin, and when the detected voltage Vin matches the normal voltage, for example, the normal signal is 3.3V, and when the detected voltage Vin does not match, for example, 4.5V or 2 ．
An abnormal signal is output when the voltage is 13V.

Next, the failure diagnosis operation of the stepping motor (7) will be described. This stepper motor
The failure diagnosis operation of (7) is performed at the start of the air conditioning operation. First, one of the two output ports P0 and p1 of the CPU (6) is set to "H".
i "and the other to" Low ".
(8) detects the drive voltage between the voltage dividing resistor R1 and the motor coil (73, 74), converts this drive voltage into the detection voltage Vin of 5V or less, and then the analog port P2 of the CPU (6). Are typing in.

When the motor coil (73) that outputs the "Low" drive signal is normal, the detection voltage Vin of the analog port P2 becomes 3.3V, and the failure determining means
(62) compares the detected voltage Vin with the normal voltage and outputs a normal signal to start the air conditioning operation. If the detection voltage Vin of the analog port P2 is 4.5V, the failure determining means (62) compares the detection voltage Vin with the normal voltage,
An abnormal signal is output by displaying that the open abnormality has occurred, and the abnormality is displayed. Further, when the detected voltage Vin of the analog port P2 is 2.13V, the failure determination means (62) compares the detected voltage Vin with a normal voltage, outputs an abnormal signal as a short circuit abnormality, and displays an abnormality. And so on.

After that, if one of the motor coils (73) is normal, the drive signals of the output ports P0, p1 of the CPU (6) are switched and the above-mentioned operation is performed to carry out the other motor coil (74).
Will be diagnosed.

As described above, according to this embodiment, the voltage dividing detection circuit (8) detects the drive voltage between the voltage dividing resistor R1 and the motor coils (73, 74), and the voltage dividing detection is performed. Since abnormalities such as disconnection are detected based on the detection voltage of the circuit (8), disconnection etc. can be detected based on the analog signal of the voltage division detection circuit (8). It is not necessary to provide a high-precision resistor or the like. As a result, the circuit configuration can be simplified and the cost can be reduced. Further, unlike the conventional case, since it is not necessary to provide a current transformer in the drive circuit, the entire device can be downsized, and the reliability can be improved by reducing the number of parts.

The chain line in FIG. 4 indicates the second aspect of the invention according to claim 2.
In this embodiment, a change amount deriving means (63) and a failure determining means (64) are provided instead of the failure determining means (62) in the previous embodiment. The change amount deriving means (63) receives the detection voltage signal of the voltage division detection circuit (8) and receives the motor control means (61).
Is configured to derive the amount of change in the detected voltage Vin at the time of outputting the drive signal. Further, the failure determination means (64) compares the detected change amount derived by the change amount deriving means (63) with a preset normal change amount, and the detected change amount matches the normal change amount. It is configured to output a normal signal when there is an error and an abnormal signal when there is a mismatch. That is, the detection voltage V when one of the motor coils (73, 74) is excited from the demagnetized state of both the motor coils (73, 74).
The failure diagnosis is performed based on the change amount of in.

Specifically, the voltage of the driving power source (75) is set to 1
6V, power supply for detection (81) 5V, resistance R2 100KΩ, resistance R3 64KΩ, resistance R4 33KΩ, CPU
When the stepping motor (7) in which both output ports P0 and p1 of (6) are set to "Hi" is stopped, the detection voltage Vin of the analog port P2 becomes 4.85V. If one of the output ports P0 and p1 is set to “Low”, the analog port
The detection voltage Vin of P2 is 3.6V when the motor coil (73, 74) is normal, and 4.85V when there is an open abnormality.
If there is a short circuit abnormality, the voltage will be 2.5V. Then, the change amount when one of the motor coils (73, 74) is excited is 1.25V in the normal state and 0V in the open abnormality.
Moreover, it becomes 2.5V in case of short circuit abnormality.

Therefore, at the start of the air conditioning operation, the two output ports P0 and p1 are both set to the "Hi" state, and in this state, the detection voltage Vin (4.8) detected by the voltage dividing detection circuit (8).
5V) is stored in the change amount deriving means (63). After that, one of the output ports P0 and p1 is set to "Low". In this state, the change amount deriving means (63) derives the change amount of the detection voltage Vin from the detection voltage Vin detected by the voltage dividing detection circuit (8). Then, the failure determination means (64) determines the failure of the motor coil (73) based on this change amount. That is, the change amount is 1.2
In the case of 5V, a normal signal is output and the air conditioning operation is started. Further, when the change amount is 0V, it is determined that there is an open abnormality, and when the change amount is 2.5V, it is determined that there is a short circuit abnormality and an abnormality signal is output to display an abnormality. After that, the other output ports P0 and p1 are set to "Low" and the above-described operation is repeated to perform the failure diagnosis of the other motor coil (74).

As described above, according to the present embodiment, the stepping motor (7) is driven because the disconnection or the like is detected based on the amount of change in the voltage detected by the voltage dividing detection circuit (8). Even when the driving power supply (75) and the detection power supply (81) that supplies power to the voltage division detection circuit (8) do not relatively change, disconnection or the like can be reliably detected. That is, 1
5V even if the 6V driving power supply (75) fluctuates
There is a case where the detection power supply (81) does not fluctuate, and at that time, a fluctuation such as an open abnormality can be detected regardless of the power fluctuation. As a result, in addition to the effect of the previous embodiment, highly accurate detection can be performed.

FIG. 5 shows a third embodiment of the invention according to claim 3, and instead of the voltage dividing detection circuit (8) of each of the above embodiments, a disconnection detection circuit (9a) for detecting an open abnormality. ) And a short circuit detection circuit (9b) for detecting a short circuit abnormality. The disconnection detection circuit (9a) includes a PNP type transistor Tr1, the emitter of the transistor Tr1 is connected between the driving power source (75) and the voltage dividing resistor R1, and the base is a voltage dividing resistor. A resistor R5 is connected between R1 and the motor coil (73, 74), and a resistor R6 and a Zener diode D2 are connected in series to the collector. Furthermore, the resistance
CP is connected between R6 and Zener diode D2 via resistor R7.
It is connected to the first input port P3 of U (6). And
The disconnection detection circuit (9a) is configured to output a disconnection signal if the transistor Tr1 is in the off state when the drive signal of the motor control means (61) is output. Also,
The short circuit detection circuit (9b) is a PNP transistor Tr2.
And the emitter of the transistor Tr2 has a power source (9
It is connected to 1) and its base is resistor R8 and diode.
It is connected to one end of the motor coil (73, 74) via D3 and D4, the collector is grounded via the resistor R9, and CP
It is connected to the second input port P4 of U (6). And
The short circuit detection circuit (9b) is configured to output a short circuit signal when the transistor Tr2 is turned on when the drive signal of the motor control means (61) is output.

Next, the failure diagnosis operation of the stepping motor (7) will be described. First, at the start of the air conditioning operation, one of the output ports P0 and p1 of the CPU (6) is set to "H".
The drive signal is switched after the other is set to "i" and the other is set to "Low". At the time of outputting the drive signal, if the motor coil (73, 74) on the "Low" side of the drive signal is normal, disconnection occurs. The transistor Tr1 of the detection circuit (9a) is
A voltage between the voltage dividing resistor R1 is applied between the emitter and the base to turn it on, and a normal 4.5V "Hi" signal held at a constant voltage by the Zener diode D2 causes the first input port P3 of the CPU (6). Will be entered. In addition, the short circuit detection circuit (9
In the transistor Tr2 of b), the base voltage is higher than the emitter voltage and the transistor Tr2 is in the off state, and the normal signal of "Low" is the CPU.
It will be input to the second input port P4 of (6).

On the other hand, when the motor coil (73, 74) on the "Low" side of the drive signal is in an open abnormality, the transistor Tr1 of the disconnection detection circuit (9a) has the same potential between the emitter and the base and is turned off. Then, the disconnection signal of "Low" is input to the first input port P3 of the CPU (6). If the motor coil (73, 74) on the “Low” side of the drive signal has a short circuit abnormality, the short circuit detection circuit (9
In the transistor Tr2 of b), the base voltage becomes lower than the emitter voltage and the transistor Tr2 is turned on, and the "Hi" short circuit signal is input to the second input port P4 of the CPU (6).
Then, an abnormality display or the like is performed by the disconnection signal and the short circuit signal of the motor coil (73, 74).

As described above, according to this embodiment, the drive voltage between the voltage dividing resistor R1 and the motor coil (73, 74) causes the transistors Tr1 and Tr2 to be turned on and off to detect disconnection and short circuit. Therefore, unlike the conventional case, it is not necessary to provide a comparator and the like, and since it is possible to detect a disconnection or the like with a small number of parts, detection failures are reduced and highly reliable detection can be performed. ,
It is possible to improve reliability.

In each of the embodiments, the air conditioner is used.
Stepping motor for indoor electric expansion valve in (2) (7)
However, it goes without saying that the failure diagnosis device of the present invention can be applied to various motors.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of the present invention.

FIG. 2 is a system diagram of an air conditioner.

FIG. 3 is a circuit block diagram of an indoor control unit.

FIG. 4 is a control block diagram of an indoor electric expansion valve.

FIG. 5 is a control block diagram of an indoor electric expansion valve according to a third embodiment.

[Explanation of symbols]

 1 Indoor control unit 7 Stepping motor 6 CPU 8 Voltage division detection circuit 9a Disconnection detection circuit 9b Short circuit detection circuit 61 Motor control means 62,64 Failure determination means 63 Change amount derivation means 71,72 Drive circuit 73,74 Motor coil 75 for driving Power supply 81 Power supply for detection Tr1, Tr2 Transistor

Claims (3)

[Claims] 1. A power supply (75), a motor coil (73, 74) connected to the power supply (75), and a motor coil (73, 74) connected to the motor coil (73, 74) to excite the motor coil (73, 74). Drive circuit (7
1, 72) and a motor control device (61) for controlling the drive circuit (71, 72), a drive between the power supply (75) and the motor coil (73, 74) The voltage is divided and detected, and a voltage division detection circuit (8) that outputs a detection signal of a detection voltage equal to or lower than a predetermined voltage corresponding to the drive voltage, and a detection signal of the voltage division detection circuit (8) are received. The motor control means (61) is provided with a failure determination means (62) that outputs a normal signal if the detected voltage matches a preset normal voltage when outputting the drive signal, and outputs an abnormal signal if they do not match. A failure diagnosis device for a motor drive device characterized in that 2. A power source (75), a motor coil (73, 74) connected to the power source (75), and a motor coil (73, 74) connected to the motor coil (73, 74) to excite the motor coil (73, 74). Drive circuit (7
1, 72) and a motor control device (61) for controlling the drive circuit (71, 72), a drive between the power supply (75) and the motor coil (73, 74) The voltage is divided and detected, and a voltage division detection circuit (8) that outputs a detection signal of a detection voltage equal to or lower than a predetermined voltage corresponding to the drive voltage, and a detection signal of the voltage division detection circuit (8) are received. A change amount deriving means (63) for deriving a change amount of the detection voltage when the drive signal of the motor control means (61) is output, and a detected change amount derived by the change amount deriving means (63) are set in advance as normal. A failure diagnosis device for a motor drive device, comprising: a failure determination means (64) that outputs a normal signal if the change amount matches and an abnormal signal if the change amount does not match. 3. A power source (75), a motor coil (73, 74) connected to the power source (75), and a motor coil (73, 74) connected to the motor coil (73, 74) to excite the motor coil (73, 74). Drive circuit (7
1, 72) and a motor control means (61) for controlling the drive circuit (71, 72), in the motor drive device, connected between the power supply (75) and the motor coil (73, 74) Disconnection detection circuit for outputting a disconnection signal when the motor coil (73, 74) is disconnected based on the operating state of the transistor Tr1 when the drive signal of the motor control means (61) is output ( 9a) and a transistor Tr2 connected between the power source (75) and the motor coil (73, 74), and the operating state of the transistor Tr2 when the drive signal of the motor control means (61) is output. A fault diagnosis device for a motor drive device, comprising: a short-circuit detection circuit (9b) that outputs a short-circuit signal when the motor coils (73, 74) are short-circuited.