JPH10243674A - Dc motor protection apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC motor protection apparatus which surely conduct the protecting operation at the starting time without limiting a load in the regular operation. SOLUTION: A reference voltage generating circuit 5 is formed of a resistance voltage dividing circuit 5A for outputting a reference voltage signal Vr by dividing a DC voltage Vc and a reference voltage switching circuit 5B composing a transistor TR1 for switching the level of the reference voltage signal Vr to a lower value by short-circuiting a part R3 of the resistor forming a resistance voltage dividing circuit 5A which turns on when a drive voltage higher than the drive voltage for the regular operation is applied to the motor. When the drive current detecting signal Vi which is proportional to the motor drive current exceeds the reference voltage signal Vr. A protection command signal is generated form the comparing circuit 6 to stop the drive of the motor 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC motor protection device for protecting a DC motor such as a brushless DC motor from overcurrent.

[0002]

2. Description of the Related Art When an excessive drive current flows through an armature of an electric motor, the coil of the armature may be burned out. Therefore, it is desirable to provide a protection device for the electric motor to protect the armature coil by cutting off the drive current when the drive current exceeds the limit value.

In order to quickly start the DC motor, the motor is started by applying a starting voltage higher than the driving voltage at the time of steady operation to the motor, and after the motor is started, the driving voltage is changed to the value at the time of steady operation. May be reduced to As described above, when the motor is driven with the drive voltage at the time of startup being higher than the drive voltage at the time of steady operation, the drive current (armature current) flowing at the time of startup even when the load on the motor is minimum is small. Since the value becomes an excessive value, it is necessary to perform a protection operation for interrupting the driving current even when a driving current determined by the minimum load of the motor and the starting voltage flows at the time of starting. On the other hand, during steady-state operation, the protection operation must not be performed until the motor load exceeds the maximum allowable value, and the protection operation is performed only when an abnormal current that exceeds the drive current at the maximum load flows. It is necessary to make it.

FIG. 4 shows a conventional brushless DC motor and its protection device. In the figure, 1 is 3
Brushless DC motor comprising a stator having phase armature coils 1u, 1v and 1w and a rotor (not shown) having a field constituted by permanent magnets; An inverter circuit having a switching element for switching an exciting current flowing through the armature coils 1u to 1w; 3, a motor control device for controlling the inverter circuit 2; and 4, a drive current flowing through the armature coils 1u to 1w through the inverter circuit 2 A reference voltage generating circuit for generating a reference voltage signal corresponding to the limit value of the drive current;
A comparison circuit 6 compares the drive current detection signal Vi with the reference voltage signal Vr and generates a protection command signal when the drive current detection signal Vi exceeds the reference voltage signal Vr.

To explain each part in more detail, the inverter circuit 2 includes PNP transistors TRu, TRv and TRw whose emitters are connected in common, collectors of these transistors TRu to TRw whose collectors are connected respectively, and whose emitters are connected in common. NPN transistors TRx, TRy and TRz, and transistors TRu-T
A feedback diode Du-Dw connected in parallel with the cathode directed to the emitter side of the transistor TRu-TRw between the emitter and collector of each of the transistors TRx-TRz, and a cathode connected between the collector-emitters of the transistors TRx-TRz. It is composed of feedback diodes Dx to Dz connected in parallel to the collector side of TRz. In the illustrated example, the transistors TRu to T
Rw and TRx to TRz form a switch element on the upper side and a switch element on the lower side of the bridge, respectively, and these switch elements are connected in a three-phase bridge to form a three-phase inverter circuit. Transistor TR
Feedback diodes Du to Dw and Dx to Dz connected in parallel to u to TRw and TRx to TRz, respectively, are connected in a three-phase bridge, and these diodes rectify the voltage induced in the armature coils Iu to Iw to supply power. A three-phase full-wave rectifier circuit that feeds back to the circuit is configured. The common connection point of the emitters of the transistors TRx to TRz is grounded through a current detection resistor Ri, and the current detection resistor Ri detects a drive current flowing through the armature of the motor to generate a voltage signal proportional to the drive current. A current detector 4 that outputs a drive current detection signal Vi is configured.

In the inverter circuit shown in FIG.
The common connection point of the emitters of the transistors TRu to TRw and the ground terminal of the resistor Ri are a positive power terminal 2a and a negative power terminal 2b, respectively. The drive voltage Vd is applied between 2b.

In order to start the motor promptly, a starting voltage Vds higher than the driving voltage Vdn during normal operation is applied to the motor at the time of starting the motor, and the driving current is increased before the driving current exceeds the limit value at the time of starting. The voltage is switched to the value at the time of steady operation.

The motor control device 3 includes, for example, an exciting phase (a phase in which an exciting current flows) and a non-exciting phase (in which no exciting current flows) in accordance with the rotational angle position of the rotor of the motor detected by a position sensor (not shown). ), A speed setting unit for providing a target rotation speed of the electric motor 1, and a relationship between the actual rotation speed and the target rotation speed of the motor detected by a speed detection circuit (not shown). A speed deviation detecting unit for detecting the deviation, and performing a proportional, integral, and differential operation on the deviation detected by the speed deviation detecting unit using a predetermined gain constant, thereby obtaining an exciting current necessary to make the deviation zero. A PID calculation unit for calculating the size,
PWM for generating an PWM signal intermittently with a duty necessary for PWM-modulating the exciting current flowing through the armature coils 1 u to 1 w to the magnitude calculated by the PID calculation unit
The inverter circuit 2 receives the signal generation unit, the phase pattern signal and the PWM signal, and supplies a drive current pulse-width-modulated by the PWM signal to the armature coil in the excitation phase.
And a protection circuit for stopping the supply of the drive signal to the inverter circuit 2 when a protection command signal is given from the comparison circuit 6.

As shown in the figure, in the case of a brushless DC motor having U to W three-phase armature coils 1u to 1w, the motor control device 4 changes, for example, (U phase, (Y phase) → (U phase, Z phase) → (V
Phase, Z phase) → (V phase, X phase) → (W phase, X phase) → (W
Phase, Y phase) → (U phase, Y phase)... The motor is rotated by sequentially changing the excitation phase.

The reference voltage generating circuit 5 includes resistors R1 and R2.
And divides a constant DC voltage Vc obtained from a DC power supply circuit (not shown) to output a reference voltage signal Vr.

The comparator 6 comprises a comparator CP1 and a comparator C
It comprises a resistor Ro and a capacitor Co connected between the output terminal of P1 and the output terminal of a power supply circuit (not shown) and between the output terminal of comparator CP1 and ground, respectively. The reference voltage signal Vr and the drive current detection signal Vi are input to the inverting input terminals, respectively. When the driving current supplied to the armature of the motor through the inverter circuit 2 is less than the limit value, the magnitude of the driving current detection signal Vi is less than the magnitude of the reference voltage signal Vr, and the potential of the output terminal of the comparison circuit CP1 is Is in a low level state. At this time, since the protection circuit provided in the motor control device 3 does not perform the protection operation, the motor 1 continues to rotate.

When the drive current exceeds the limit value, the magnitude of the drive current detection signal Vi exceeds the magnitude of the reference voltage signal Vr, so that the potential of the output terminal of the comparator CP1 becomes high, and the protection circuit 6 A protection command signal is output. At this time, the protection circuit provided in the motor control device 3 stops supplying the drive signal to the transistor of the inverter circuit 2.
Therefore, when the drive current exceeds the limit value, the transistors TRu to TRw and TRx to TRz of the inverter circuit 2 are kept in the cut-off state, the supply of the drive current to the armature coils 1u to 1w is stopped, and the motor is stopped. Can be

A DC motor protection device is constituted by the reference voltage generation circuit 5, the comparison circuit 6, and a protection circuit (not shown) provided in the motor control device 3.

As described above, when a high starting voltage is applied at the time of starting the motor, the drive current limit value at the start is changed to the drive current limit value at the time of steady operation in order to protect the armature coil. Must be set lower. for that reason,
In this type of motor, the drive current limit value at the time of startup is set lower than the drive current determined by the minimum value of the load of the motor and the startup voltage, and the drive current is controlled before the drive current of the motor exceeds the limit value. The voltage is switched to the value at the time of steady operation.

FIG. 5 shows an example of the relationship between the change of the drive current detection signal Vi obtained from the current detector 4 with respect to the drive voltage Vd and the reference voltage signal Vr. In FIG. Is the driving voltage Vd of the driving current detection signal Vi obtained from the current detector 4 when the load is minimum.
, And a straight line b indicates a change of the drive current detection signal Vi obtained from the current detector 4 with respect to the drive voltage Vd when the load of the motor is maximized.

As shown in FIG. 5, in the conventional motor protection device, the level of the reference voltage signal Vr is changed by using a straight line a and a straight line V indicating the drive current detection signal Vi when the motor is at the minimum load.
It was set to a value lower than the intersection p with the straight line of d = Vds. By setting the level of the reference voltage signal Vr in this manner, a protection command signal can be generated when the drive current exceeds the limit value at start-up to perform the protection operation regardless of the load of the motor. it can.

[0017]

If the level of the reference voltage signal is set as described above, a protection command signal is generated when the driving current exceeds the limit value regardless of the load at the time of starting the motor. The protection operation can be performed. However, when the reference voltage signal Vr is set as described above,
Even when the load of the motor increases during the steady operation (for example, when the drive current detection signal is as indicated by the straight line b in FIG. 5), the protection operation is performed, so that the load that can be driven by the motor is limited. Was.

In order to prevent the load during steady operation from being limited, the level of the reference voltage signal is set to Vr in FIG.
Although it is conceivable to set the reference voltage signal to a high level as described above, when the level of the reference voltage signal is set in this way, a range in which protection does not work at the time of startup occurs, so that it is impossible to properly protect the motor. .

It is an object of the present invention to provide a DC motor protection device capable of appropriately performing a protection operation at the time of starting without limiting a load during a steady operation.

[0020]

SUMMARY OF THE INVENTION The present invention relates to a DC motor protection device for protecting a DC motor from overcurrent.
The protection device to which the present invention is directed is to detect a drive current flowing through an armature of a DC motor in which a start voltage is applied and a start voltage higher than a drive voltage at the time of steady operation is detected, and a voltage signal proportional to the drive current is detected. As a drive current detection signal, a reference voltage generation circuit for generating a reference voltage signal corresponding to the drive current limit value, and comparing the drive current detection signal with the reference voltage signal to obtain a drive current detection signal. A comparison circuit that generates a protection command signal when a reference voltage signal is exceeded, and a protection circuit that stops supply of drive current to the armature when the protection command signal is generated.

In the present invention, the reference voltage generating circuit
When the motor is started, the value of the reference voltage signal is set to a first value, and when the motor is in a steady operation, the value of the reference voltage signal is set to a second value higher than the first value. And
The first value of the reference voltage signal (a value corresponding to the drive current limit value at startup) is the value of the drive current detection signal when the drive current determined by the minimum value of the load of the motor and the startup voltage flows. And the second value of the reference voltage signal (a value corresponding to the limit value of the drive current during steady operation) is determined by the drive current determined by the maximum load of the motor and the drive voltage during steady operation. Is set higher than the value of the drive current detection signal when the current flows.

The reference voltage generating circuit includes a resistor voltage dividing circuit that divides a constant DC voltage and outputs a reference voltage signal, and a resistor voltage dividing circuit that conducts when a starting voltage is applied to the motor to form a resistor voltage dividing circuit. And a reference voltage changeover switch for canceling a short circuit of a resistor constituting a resistance voltage dividing circuit when a driving voltage at the time of steady operation is applied to the electric motor so as to be cut off. The value of the reference voltage signal output from the resistance voltage dividing circuit is reduced to a first value when the voltage switch is turned on, and the value of the reference voltage signal is reduced when the reference voltage switch is turned off. It can be configured by a circuit configured to increase to the second value.

The reference voltage generating circuit further includes a voltage dividing circuit for dividing a driving voltage applied to the electric motor, and an inverting amplifier for inverting and amplifying an output voltage of the voltage dividing circuit and outputting a reference voltage signal, The value of the reference voltage signal is set to a first value when the starting voltage is applied to the motor, and the value of the reference voltage signal is higher than the first value when the driving voltage during the steady operation is applied to the motor. It can be constituted by a circuit having a value of 2.

As described above, the value of the reference voltage signal is set to the first value at the time of starting the motor, and the value of the reference voltage signal is set to the second value higher than the first value at the time of steady operation of the motor. The level of the reference voltage signal is switched according to the drive voltage, the first value is set lower than the drive current determined by the minimum value of the load of the motor and the starting voltage, and the second value is set to the motor value. If the drive current is set to be higher than the drive current detection signal when the drive current determined by the maximum value of the load and the drive voltage during steady operation flows, when the motor is started, the drive is performed regardless of the load of the motor. The protection operation can be performed when the current exceeds the limit value. In addition, during steady-state operation of the motor, the protection operation can be performed only when an abnormal current that exceeds the drive current at the maximum load flows. From the motor.

[0025]

FIG. 1 shows an example of the configuration of a brushless DC motor having a protection device according to the present invention.
In the figure, parts that are the same as the parts in FIG. 4 are given the same reference numerals.

In FIG. 1, reference numeral 1 denotes a three-phase armature coil 1.
The brushless DC motor includes a stator having u, 1v, and 1w, and a rotor (not shown) having a field formed by permanent magnets. Reference numeral 2 denotes a three-phase armature coil 1u to 1w of the brushless DC motor 1. Circuit, which includes a switching element for switching an exciting current flowing through the motor, 3 is a motor control device that controls the inverter circuit 2, and 4 is a drive current detection by detecting a drive current flowing through the armature coils 1u to 1w through the inverter circuit 2. A current detector 5 for outputting the signal Vi, 5 is a reference voltage generating circuit, and 6 is a comparing circuit. Of these, the portions other than the reference voltage generating circuit 5 are configured in exactly the same way as the conventional example shown in FIG. .

The reference voltage generating circuit 5 shown in FIG. 1 divides a constant DC voltage to output a reference voltage signal, and a resistance voltage dividing circuit 5A which conducts when a starting voltage is applied to the motor 1 and conducts the resistance. A part of the resistor constituting the voltage dividing circuit 5A is short-circuited,
A reference voltage switching switch 5B is provided, which is turned off when a drive voltage during a steady operation is applied to the electric motor to release a short circuit of the resistance constituting the resistance voltage dividing circuit 5A.

The illustrated resistor voltage dividing circuit 5A is a circuit in which resistors R1 to R3 are arranged in numerical order and connected in series. The terminal on the resistor R3 side of the resistor voltage dividing circuit 5A is grounded, and the terminal on the resistor R1 side of the voltage dividing circuit is connected to the positive output terminal of a constant voltage DC power supply circuit (not shown) in which the negative output terminal is grounded. I have. In this resistance voltage dividing circuit,
The connection point between the resistors R1 and R2 is an output terminal, which is connected to the inverting input terminal of the comparator CP1. The resistance voltage dividing circuit 5A supplies an output voltage obtained by dividing the output voltage Vc of the constant voltage DC power supply circuit to the inverting input terminal of the comparator CP1 as a reference voltage signal Vr.

The reference voltage switch 5B has an NPN transistor TR1 having an emitter grounded and a collector connected to a connection point between the resistors R2 and R3, and a transistor T1.
A resistor R4 having one end connected to the base of R1 and a resistor R5 connected between the base and the emitter of the transistor TR1.
And a zener diode ZD1 connected between the other end of the resistor R4 and the positive output terminal of the power supply circuit for applying the drive voltage Vd to the motor, with the anode facing the resistor R4. The Zener voltage of the Zener diode ZD1 is set to a value between the steady-state value Vdn of the motor drive voltage Vd and the value (start-up voltage) Vds at the time of startup.

In the example shown in FIG. 1, when the starting voltage Vds is applied to the motor, the Zener diode ZD1 conducts, and a base current is supplied to the transistor TR1 through the Zener diode ZD1 and the resistor R4. As a result, the transistor TR1 becomes conductive, and the resistor R3 of the resistor voltage dividing circuit 5A is substantially short-circuited. When the resistor R3 is short-circuited by the transistor TR1 as described above, the reference voltage Vr input to the comparator CP1 from the resistor voltage dividing circuit 5A becomes the first value Vr1 (the applied voltage Vr1 of the resistor voltage dividing circuit).
c is divided by resistors R1 and R2).

When the drive voltage Vd is switched to the steady state value Vdn after the motor is started, the Zener diode Z
Since D1 is turned off, supply of the base current to the transistor TR1 is stopped, and the transistor TR1 is turned off. As described above, when the transistor TR1 is turned off and the short circuit of the resistor R3 is released, the reference voltage signal Vr input to the comparator CP1 from the resistor voltage dividing circuit 5A is changed to the second value Vr2 (of the resistor voltage dividing circuit). The applied voltage Vc is divided by the resistor R1, the resistors R2 and R3).

If the resistances of the resistors R1 to R3 constituting the resistance voltage dividing circuit 5A are represented by the same reference numerals as those representing the resistances, the resistance voltage dividing circuit 5A is in a state where the resistance R3 is short-circuited.
Value of reference voltage obtained from (reference voltage at start-up)
Vr1 is given by the following equation.

Vr1 = Vc {R2 / (R1 + R2)} (1) In addition, with the short circuit of the resistor R3 opened, the resistor voltage dividing circuit 5
A second value (reference voltage during steady operation) Vr2 of the reference voltage signal Vr obtained from A is given by the following equation.

Vr2 = Vc {(R2 + R3) / (R1 + R2 + R3)}> Vr1 (2) An example of the relationship between the drive current detection signal Vi and the first and second values Vr1 and Vr2 of the reference voltage Vr will be described. This is shown in FIG. In FIG. 2, a straight line a indicates a change in the drive current detection signal Vi obtained from the current detector 4 with respect to the drive voltage Vd when the load on the motor is minimum, and a straight line b indicates
It shows a change of the drive current detection signal Vi obtained from the current detector 4 with respect to the drive voltage Vd when the load of the motor is maximized. The straight line c represents the drive voltage V of the drive current detection signal Vi when a lock current (an abnormal current flowing in a state where the rotor of the electric motor is restricted so as not to move) flows.
This shows the change for d.

In the present invention, as shown in FIG. 2, the first value Vr1 of the reference voltage signal is determined by the driving current when the driving current determined by the minimum value of the load of the motor 1 and the starting voltage Vds flows. It is set sufficiently lower than the value Vis of the detection signal Vi. The first value Vr1 of the reference voltage signal is a value corresponding to the limit value of the drive current when a starting voltage higher than the drive voltage in the steady operation is applied.

The second value Vr2 of the reference voltage signal is a drive current detection signal Vi when a drive current determined by the maximum value of the load of the motor 1 and the drive voltage Vdn during steady operation flows.
Of the drive current detection signal Vi when the abnormal current such as the lock current flows (straight line c).

With the above-described structure, when the high starting voltage Vds is applied to the motor, the level of the reference voltage signal is lowered so that when the drive current reaches the limit value regardless of the load on the motor, The protection command signal can be generated from the circuit 6 to perform the protection operation. Further, at the time of steady operation, the level of the reference voltage signal can be increased to prevent the protection operation from being performed at the time of the maximum load of the motor.

FIG. 3 shows another example of the configuration of the protection device according to the present invention, together with the configuration of the electric motor.
The reference voltage generation circuit 5 detects the drive voltage V applied to the motor.
d, and an inverting amplifier 5C that inverts and amplifies the output voltage of the voltage dividing circuit 5A and outputs a reference voltage signal Vr, and inverts when the starting voltage Vds is applied to the motor. Reference voltage signal Vr output from amplifier 5C
Becomes the first value Vr1, and the value of the reference voltage signal becomes the second value Vr2 higher than the first value when the drive voltage Vdn during the steady operation is applied to the motor. A constant has been set.

In the illustrated example, a voltage dividing circuit 5A for dividing the driving voltage Vd is constituted by a series circuit of the resistors R1 and R2, and a well-known inverting amplifier 5C is constituted by the operational amplifier OP1 and the resistors R6 to R8. .

In the above example, a brushless DC motor is taken as an example of a DC motor to be protected. However, the present invention can be applied to other DC motors that start by applying a higher driving voltage than in a normal operation. Can be applied.

[0041]

As described above, according to the present invention, the level of the reference voltage signal to be compared with the drive current detection signal is reduced when the motor is started, and the level of the reference voltage signal is increased during steady operation of the motor. As a result, at the time of startup, a protection command signal can be generated and the protection operation can be performed when the drive current exceeds the limit value regardless of the load of the electric motor. It is possible to prevent the protection operation from being performed due to an increase in the load on the motor. Therefore, according to the present invention, there is an advantage that the protection operation at the time of starting can be accurately performed without limiting the load that can be driven during the steady operation.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration example of a main part of a protection device according to the present invention together with a configuration example of a motor.

FIG. 2 is a diagram showing a characteristic of a change in a drive current detection signal with respect to a drive voltage used in the protection device of FIG. 1 and a reference voltage signal.

FIG. 3 is a circuit diagram showing another configuration example of a main part of the protection device according to the present invention together with a configuration example of a motor.

FIG. 4 is a circuit diagram showing a configuration of a main part of a conventional protection device together with a configuration of an electric motor.

5 is a diagram showing a characteristic of a change in a drive current detection signal with respect to a drive voltage used in the protection device of FIG. 4 and a reference voltage signal.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 brushless DC motor 2 inverter circuit 3 motor control device 4 current detector 5 reference voltage generation circuit 5A resistance voltage dividing circuit 5B reference voltage switching switch 5C inverting amplifier 6 comparison circuit

Claims (3)

[Claims] 1. A drive current flowing through an armature of a DC motor to be started by applying a start-up voltage higher than a drive voltage at the time of steady operation, and a voltage signal proportional to the drive current being detected. And a reference voltage generating circuit for generating a reference voltage signal corresponding to the limit value of the drive current, and comparing the drive current detection signal with the reference voltage signal to determine whether the drive current detection signal is a reference voltage. A DC motor protection device comprising: a comparison circuit that generates a protection command signal when a signal is exceeded; and a protection circuit that stops supplying drive current to the armature when the protection command signal is generated. The voltage generation circuit sets the value of the reference voltage signal to a first value when the motor is started, and sets the value of the reference voltage signal higher than the first value during a steady operation of the motor. The first value of the reference voltage signal is set lower than the value of the drive current detection signal when the drive current determined by the minimum value of the load of the motor and the starting voltage flows, The second value of the reference voltage signal is set to be higher than the value of the drive current detection signal when a drive current determined by the maximum value of the load of the motor and the drive voltage during steady operation flows. DC motor protection device. 2. A driving current detection signal, which detects a driving current flowing through an armature of a DC motor to be started by applying a starting voltage higher than a driving voltage at the time of steady operation, and outputs a voltage signal proportional to the driving current. And a reference voltage generating circuit for generating a reference voltage signal corresponding to the limit value of the drive current, and comparing the drive current detection signal with the reference voltage signal to determine whether the drive current detection signal is a reference voltage. A comparison circuit for generating a protection command signal when the signal is exceeded, and a protection circuit for stopping supply of drive current to the armature when the protection command signal is generated. The protection command signal is generated when a drive current that exceeds a limit value set lower than the drive current determined by the minimum value of the load and the start-up voltage is generated. In a DC motor protection device in which the magnitude of the reference voltage signal is set so that the protection command signal is generated only when an abnormal current exceeding the drive current flowing to the armature flows at a large load, the reference voltage generation The circuit divides a fixed DC voltage and outputs a reference voltage signal, and short-circuits a part of a resistor constituting the resistance voltage dividing circuit which becomes conductive when a starting voltage is applied to the motor. A reference voltage switching switch that is turned off when a drive voltage during a steady operation is applied to the electric motor to cancel a short circuit of a resistor that constitutes the resistance voltage dividing circuit; The value of a reference voltage signal output from the resistance voltage dividing circuit is reduced to a first value when the changeover switch is turned on, and the reference voltage is reduced when the reference voltage changeover switch is turned off. The first value of the reference voltage signal is used to detect a drive current when a drive current determined by a minimum value of a load of the electric motor and a starting voltage flows. The second value of the reference voltage signal is set to be lower than the value of the driving current detection signal when the driving current determined by the maximum value of the load of the motor and the driving voltage during the steady operation flows. DC motor protection device characterized in that it is also set high. 3. A drive current detection signal which is detected by detecting a drive current flowing through an armature of a DC motor which is started by applying a start-up voltage higher than a drive voltage at the time of steady operation and which is proportional to the drive current. And a reference voltage generating circuit for generating a reference voltage signal corresponding to the limit value of the drive current, and comparing the drive current detection signal with the reference voltage signal to determine whether the drive current detection signal is a reference voltage. A comparison circuit for generating a protection command signal when the signal is exceeded, and a protection circuit for stopping supply of drive current to the armature when the protection command signal is generated. The protection command signal is generated when a drive current that exceeds a limit value set lower than the drive current determined by the minimum value of the load and the start-up voltage is generated. In the DC motor protection device, wherein the magnitude of the reference voltage signal is set such that the protection command signal is generated only when an abnormal current exceeding the drive current flowing to the armature flows at a large load, The circuit includes a voltage dividing circuit that divides a driving voltage applied to the motor, and an inverting amplifier that inverts and amplifies an output voltage of the voltage dividing circuit and outputs the reference voltage signal. The value of the reference voltage signal is set to a first value when a voltage is applied, and the value of the reference voltage signal is higher than the first value when a drive voltage during steady operation is applied to the motor. The first value of the reference voltage signal is larger than the value of the drive current detection signal when the drive current determined by the minimum value of the load of the motor and the starting voltage flows. Set low The second value of the reference voltage signal is set to be higher than the value of the drive current detection signal when a drive current determined by the maximum value of the load of the motor and the drive voltage during steady operation flows. DC motor protection device characterized by the above-mentioned.