JPH099672A - Overload protector for motor - Google Patents

Abstract

PURPOSE: To provide an overload protector for a motor which prevents an overload protection from being conducted by a transient change in armature current. CONSTITUTION: A switch circuit 2 is provided between a motor 1 and a terminal (B) of a power supply, and the switch circuit 2 is turned on and off under control of a control circuit 3. An armature current is controlled in a PWM method to equalize a rotational speed of the motor 1 to an indicated speed. A current detecting resistor 4 is provided between the motor 1 and the ground, and a voltage across both ends of the resistor 4 is smoothed by a smoothing circuit 7 to generate a current detection signal Vi. When the current detection signal Vi is over a reference signal Vr, an overload detection signal Vo is generated to limit the armature current to zero or to a given level or below.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overload protection device for protecting a brushless DC motor or a synchronous motor from overload.

[0002]

2. Description of the Related Art A bridge type switch circuit (inverter circuit) is provided between a motor and a power source as a device for controlling the rotation speed of a brushless DC motor or a synchronous motor using a permanent magnet as a field of a rotor. , The ON / OFF control of the switch element of the switch circuit is used to perform PWM control of the armature current of the electric motor, and the average value of the armature current is controlled so that the rotation speed of the electric motor matches the instructed speed. is there.

As an example, FIG. 3 schematically shows the structure of a speed control device for controlling the rotation speed of a brushless DC motor. In FIG. 3, 1 is an armature (stator) M and a rotor (not shown). 2 is a switch circuit provided between a DC power source (not shown) and the motor 1 to turn on / off an armature current supplied to the motor 1, and 3 is a control circuit for controlling switch elements of the switch circuit 2. . In the case of a brushless DC electric motor, a position sensor that detects the position of the magnetic poles of the rotor of the electric motor and a rotary encoder that outputs a speed detection signal composed of a pulse having a frequency proportional to the rotation speed of the rotor are provided. The position detection signal Vp obtained from the position sensor and the speed detection signal Vn obtained from the rotary encoder are input to the control circuit 3.
In some cases, a pulse signal obtained by differentiating the rising and falling edges of the rectangular wave signal obtained from the position sensor is used as the speed detection signal without providing the rotary encoder.

The switch circuit 2 has n (n is an integer of 2 or more) upper side switching elements whose one end is commonly connected to the positive electrode terminal of the DC power source, and one end at each of the other ends of the n upper side switching elements. And a bridge circuit of switch elements having n lower side switch elements whose other ends are commonly connected, and a feedback diode inversely connected to each switch element forming the bridge circuit. The common connection point of the switch elements on the lower side is connected to the negative terminal of the DC power supply through the current detection resistor.

FIG. 4 shows a switch circuit 2 conventionally used when the armature M of the electric motor 1 is a three-phase brushless DC motor having a star-connected three-phase armature coil.
In this switch circuit, three switch elements Su, Sv, S of the three upper sides whose one ends are commonly connected are shown.
A switch element having w and three switch elements Sx, Sy, Sz on the lower side, one ends of which are respectively connected to the other ends of the three switch elements Su, Sv, Sw on the upper side and the other ends are commonly connected. Feedback circuit Du that is connected in anti-parallel to both ends of each of the bridge circuit and the switch elements Su to Sw on the upper side and the switch elements Sx to Sz on the lower side.
.About.Dw and Dx to Dz.

The switch element is a transistor or
IGBT (insulated gate bipolar transistor), M
OSFET etc. can be used. When using a transistor as a switch element, it is necessary to connect a feedback diode from the outside, but a MOSFET having a parasitic diode between the drain and source as a switch element.
When the above is used, since the parasitic diode can be used as a feedback diode, it is not necessary to connect the feedback diode from the outside.

The switch element Su on the upper side of the switch circuit 2
The common connection point of Sz to Sz is the positive pole side DC input terminal t1 of the switch circuit, and the input terminal t1 is connected to the positive pole side power source terminal B connected to the positive pole terminal of the DC power source (not shown). Further, the switch element Sx on the lower side of the switch circuit 2
The common connection point of ~ Sz is the negative side DC input terminal t2 of the switch circuit, and the negative side DC input terminal is connected to the ground terminal E connected to the negative terminal of the DC power source through the current detecting resistor 4. .

The switch element Su on the upper side of the switch circuit
The connection points between Sw and the switch elements Sx to Sz on the lower side are three-phase output terminals 2u to 2w, and the three-phase input terminals of the armature of the three-phase brushless DC motor 1 are connected to these output terminals. ing.

The control circuit 3 externally activates a start command signal Son.
And a command signal input terminal 3a to which a stop command signal Soff is given, which is started when a start command Son is given to the command signal input terminal 3a and is determined by a position detection signal Vp given from a position sensor. Each of the switch elements Su to Sw on the upper side and the switch elements Sx to Sz on the lower side are turned on one by one in a predetermined order so that the armature current flows through the phase coils. The child coil is commutated in a predetermined order to rotate the electric motor.

The control circuit 3 also receives the speed detection signal Vn as an input and turns on / off the switch elements Su to Sw on the upper side of the switch circuit 2 so that the rotation speed of the electric motor detected by the speed detection signal matches the instructed speed. PWM control of the armature current.

The armature current of the electric motor flows from a power source (not shown) through any one of the switch elements on the upper side of the switch circuit 2, the electric motor 1, any of the switch elements on the lower side, and the current detecting resistor 4. Therefore, a current detection signal Vi 'having a voltage value having a magnitude proportional to the armature current is obtained at both ends of the current detection resistor 4. This current detection signal Vi 'is a reference signal Vr which gives a limit value of the armature current.
It is also input to the comparator 5.

In FIG. 3, a battery is used as the reference signal generating means 6 for generating the reference signal Vr, but the reference signal Vr is a DC voltage having a certain magnitude which gives a limit value of the armature current. The reference signal generating means 6 does not have to be a battery. For example, the reference signal generating means 6 can be configured by a circuit or the like that divides the output of the constant voltage DC power supply circuit to generate a constant voltage signal.

When the current detection signal Vi 'exceeds the reference signal Vr, the comparator 5 changes the potential of its output terminal from the high level to the low level or from the low level to the high level to detect the overload detection signal Vo. To occur. Which of the change in the potential of the output terminal of the comparator from the high level to the low level or the change from the low level to the high level is used as the overload detection signal is determined by the configuration of the control circuit 3.

The torque τ generated by the electric motor is given by the following equation.

Τ = KΦIa [Nm] (1) where K is a constant determined by the electric motor, Φ is the magnetic flux given by the field of the rotor, and Ia flows in the armature coil. This is the armature current. In the case of a brushless DC motor, Φ is constant because the rotor field is a permanent magnet.

As is clear from the equation (1), in the electric motor in which the permanent magnets form the field of the rotor, the generated torque is proportional to the armature current. Therefore, when the armature current is detected by the current detecting resistor 4, , The load condition of the motor is known. Further, since the expression (1) is established regardless of the rotation speed of the electric motor, the load of the electric motor can be known from the armature current regardless of the rotation speed of the electric motor.

The control circuit 3 controls the switch elements of the switch circuit 2 so as to limit the armature current to a limit value or below or to shut off the armature current when the overload detection signal Vo is generated. When the overload detection signal Vo is generated, the supply of the armature current to the electric motor is stopped, or the on-duty ratio (the sum of the on-time and the off-time) of the switch element of the switch circuit 2 is The drive current is reduced below the limit value by controlling so that the ON time ratio) becomes smaller. This prevents the drive current from exceeding the limit value and the armature coil from being burned or the switch element of the switch circuit 2 being damaged when overloaded.

In this example, the armature current is limited to a limit value or below according to the current detection resistor 4, the circuit for generating the reference signal Vr, the comparator 5, and the overload detection signal. As described above, the overload switch control means (which is realized by the control circuit 3) for controlling the switch elements of the switch circuit 2 constitutes an overload protection device for the electric motor.

[0019]

In the above-mentioned conventional motor, the voltage across the current detecting resistor 4, which reflects the momentary change of the armature current as it is, is used as it is as the current detecting signal Vi '. If the load protection device reacts excessively and starts the motor, or if the load on the motor changes momentarily and greatly, if the armature current momentarily exceeds the limit value, the overload protection operation is performed. There was a problem that the electric motor stopped and the rotation speed decreased.

Conventionally, the feedback diodes Dx to Sx are directly connected to both ends of the switch elements Sx to Sz on the lower side of the switch circuit.
Since Dz is connected, the armature current flowing by the back electromotive force of the armature coil when the switch elements Su to Sw on the upper side are turned off by the PWM control is fed back without passing through the current detection resistor 4. It circulates to the armature through any of the diodes Dx to Dz. Therefore, even if the voltage across the current detection resistor 4 is detected, it cannot be said that the armature current is always detected accurately, and the current detected by the current detection signal extracted from both ends of the current detection resistor 4 is detected. It cannot be said that the load torque calculated by the equation (1) using the value accurately reflects the load state of the electric motor.

An object of the present invention is to prevent the overload protection operation from being carried out at the time of starting the motor or at the momentary load change, and to carry out the overload protection operation without trouble when the overload state continues. Another object of the present invention is to provide an overload protection device for an electric motor.

[0022]

According to the present invention, there are provided n (n is an integer of 2 or more) upper side switching elements having one end commonly connected to a positive electrode terminal of a DC power source and n upper side switching elements. A bridge circuit of switch elements having n lower side switch elements, one end of which is connected to the other end and the other end of which is commonly connected, and antiparallel connection to each switch element that constitutes the bridge circuit It has a feedback diode and a switch circuit in which the common connection point of the switch element on the lower side is connected to the negative terminal of the DC power supply through a resistor for current detection, and the switch element on the upper side of the switch circuit is turned on / off to control the armature current. And a control circuit for PWM control of the motor, and is an overload protection device for protecting an electric motor that is rotated by being supplied with an armature current from a DC power supply through a switch circuit from an overload.

In the present invention, a smoothing circuit for smoothing the voltage obtained across the current detecting resistor and outputting a current detecting signal corresponding to the average value of the armature current, and a current detecting signal obtained from the smoothing circuit are provided. An overload detection circuit that outputs an overload detection signal when the current detection signal exceeds the reference signal compared to the reference signal, and an armature current of the motor that is less than the limit value when the overload detection signal is output. Overload switch control means is provided for controlling the switch elements of the switch circuit to limit or shut off.

The feedback diodes, which are anti-parallel connected to the switch elements on the upper side of the switch circuit, are connected to both ends of the switch elements on the upper side, and are connected in anti-parallel to the switch elements on the lower side of the switch circuit. The feedback diode is connected between one end of each switch element on the lower side and a circuit connected to the negative terminal of the DC power supply.

As each switch element on the upper side, a transistor, an IGBT or the like may be used, or a MOSFET may be used. When a switch element having no parasitic diode such as a transistor is used as a switch element on the upper side, it is necessary to connect a feedback diode in parallel to each switch element on the upper side. When a MOSFET is used as each switch element on the upper side, a parasitic diode existing between its drain and source can be used as a feedback diode.

A switch element such as a transistor having no parasitic diode is used as the switch element on the lower side, and a feedback diode must be connected between one end of each switch element on the lower side and a circuit connected to the negative terminal of the power supply. There is.

In the present invention, the feedback diode is
The feedback diode is connected in anti-parallel to each switch element. Here, when the feedback diode is connected in anti-parallel to the switch element, the forward direction of the feedback diode is opposite to the conduction direction of the switch element. Is connected in parallel with the switch element. For example, when the switch element is an NPN transistor, the feedback diode is connected in parallel to the collector-emitter circuit of the transistor with its anode facing the emitter side of the transistor.

[0028]

As described above, when the smoothing circuit for smoothing the voltage across the current detecting resistor is provided and the current detecting signal is obtained from the smoothing circuit, the current detecting signal has a waveform without pulsation. Since it hardly changes depending on the momentary change of the child current, it prevents the overload protection operation when the armature current momentarily exceeds the limit value at the time of start-up or momentary load change. Thus, it is possible to prevent the electric motor from stopping and the speed from decreasing.

Further, with the above configuration, since the armature current flows through the current detecting resistor in any case, it is possible to obtain a current detection signal accurately proportional to the average value of the armature current from the smoothing circuit. Using the armature current Ia detected by this current detection signal, the load torque τ can be accurately calculated from the equation (1).

[0030]

1 shows an embodiment in which the present invention is applied to a three-phase brushless DC motor. In FIG. 1, 1 is a brushless DC motor, 2 is a DC power source (not shown) and an armature M of the motor 1. A switch circuit 3 provided between the two is a position detection signal Vp obtained from a position sensor for detecting the position of the magnetic pole of the rotor of the electric motor 1 and a rotational speed detection signal V obtained from a rotary encoder for detecting the rotational speed of the electric motor.
A control circuit for controlling the switch circuit 2 with n and n as inputs, 4 is a current detection resistor inserted in the path of the armature current flowing through the switch circuit 2, 5 is a comparator, 6 is reference signal generating means, and 7 is It is a smoothing circuit.

Switch circuit 2 used in this embodiment
As shown in FIG. 2, the three upper switch elements Su to Sw whose one end is commonly connected to the power supply terminal B connected to the positive electrode side output terminal of the DC power supply and the switch elements Su to Sw on the upper side are Three lower side switch elements Sx, one end of which is connected to the other end and the other end of which are commonly connected
A switching element bridge circuit having Sz, and feedback diodes Du to Dw and Dx to Dz connected in antiparallel to the respective switching elements forming the bridge circuit. The common connection point of Sz is connected to the ground terminal E connected to the negative terminal of the DC power source through the current detection resistor 4.

Transistors, IGBTs or the like may be used as the switch elements Su to Sw on the upper side.
May be used. MOS as switch elements Su to Sw
When the FET is used, a parasitic diode exists between the respective drain sources, so that the parasitic diode can be used as the feedback diodes Du to Dw.

In the present invention, the switch element S on the lower side is
The feedback diodes Dx to Dz, which are respectively connected in anti-parallel to x to Sz, are not directly connected to both ends of the switch elements Sx to Sz but to the switch elements Sx to Sz.
Is connected between one end of the circuit and a circuit (ground terminal E in the illustrated example) connected to the negative terminal of the DC power supply. Therefore, as the switch elements Sx to Sz on the lower side, it is necessary to use switch elements having no parasitic diode at both ends, for example, transistors.

In the switch circuit 2 described above, the common connection point at one end of the switch elements Su to Sw and the common connection point at the other end of the switch elements Sx to Sz are the DC input terminals t1 and t2 of the bridge circuit, respectively. The connection points between the other ends of the switch elements Su to Sw and the one ends of the switch elements Sx to Sz are three-phase output terminals 2u to 2w, respectively. The three-phase input terminals of the armature M of the electric motor 1 are connected to the output terminals 2u to 2w, respectively.

The smoothing circuit 7 can be composed of, for example, a capacitor connected in parallel to both ends of the current detecting resistor 4. The output of the smoothing circuit 7 is input to the comparator 5 as the current detection signal Vi. The comparator 5 outputs the overload detection signal Vo when the current detection signal Vi exceeds the reference signal Vr. The other points are the same as those of the conventional example shown in FIG. 3, and the control circuit 3 receives the position detection signal Vp as an input and commutates the armature current to the three-phase armature coil. One switch element and one switch element on the lower side are electrically connected in a predetermined order. The control circuit 3 also turns on / off the switch elements Su to Sw on the upper side of the switch circuit 2 so that the rotation speed of the electric motor detected by the speed detection signal Vn matches the instructed rotation speed, and performs PWM control of the armature current. .

The control circuit 3 also controls the switch element of the switch circuit 2 so as to limit or cut off the armature current to a value below the limit value when the overload detection signal Vo is generated. When the overload detection signal Vo is generated, the supply of the armature current to the electric motor is stopped, or the on-duty ratio (the sum of the on-time and the off-time) of the switch element of the switch circuit 2 is provided. The ratio of the ON time with respect to (1) is controlled to be smaller, and the drive current is reduced below the limit value.

Current detecting resistor 4 and reference signal generating means 6
A smoothing circuit 7, a comparator 5, and an overload switch control for controlling the switch element of the switch circuit 2 so as to limit or cut off the armature current to a limit value or less according to the overload detection signal. The means (implemented by the control circuit 3) constitutes an overload protection device for the electric motor.

In the above embodiment, the armature current is PW.
Since the M control is performed, the armature current has a finely pulsating triangular waveform, and the voltage appearing at both ends of the current detection resistor 4 also has a finely pulsating triangular waveform. If the voltage across the resistor 4 for current detection is directly input to the comparator 5 as a current detection signal, the comparator 5 reacts sensitively to an instantaneous change in the armature current. Even when the armature current momentarily exceeds the limit value, the comparator 5 outputs the overload detection signal Vo to operate the overload switch control means realized by the control circuit 3 to stop the motor. , Reduce the rotation speed of the motor.

On the other hand, when the smoothing circuit 7 for smoothing the voltage across the current detecting resistor 4 is provided as in the above embodiment and the output voltage of the smoothing circuit is used as the current detecting signal Vi, The current detection signal Vi has a waveform with no pulsating current and hardly changes due to an instantaneous change in the armature current. Therefore, the armature current momentarily exceeds the limit value at the time of start-up or momentary load change. In this case, it is possible to prevent the overload protection operation from being performed. Therefore, it is possible to prevent the electric motor from stopping or the speed from decreasing at the time of startup or when a momentary load change occurs.

If the motor overload condition continues,
The current detection signal Vi obtained from the smoothing circuit 7 is the reference signal V
Since r is exceeded, the overload protection operation can be performed without any trouble.

In the switch circuit shown in FIG. 4, when the switch elements Su to Sw on the upper side are turned off by PWM control, for example, if the switch element Sx is in the on state, the reverse induced voltage of the electric motor 1 is generated. By the electric motor 1
→ switch element Sx → terminal t2 → feedback diode Dz connected to both ends of the switch element Sz → closed loop of the motor 1 or the motor 1 → switch element Sx → terminal t2 → feedback diode Dy at both ends of the switch element Sy → motor 1 Armature current flows in the closed loop of. Since these currents do not flow through the current detection resistor 4, the armature current cannot be detected accurately.

On the other hand, when the feedback diodes Dx to Dz are provided as shown in FIG. 2, the switch elements Su to S on the upper side are provided.
When w is turned off by PWM control, the back electromotive force of the motor causes, for example, the motor 1 → switch element Sx →
A current flows through the resistor 4 → ground circuit → diode Dz → motor 1 circuit or motor 1 → switch element Sx → resistor 4 → earth circuit → diode Dy → motor 1 circuit, and the armature current constantly causes the current detection resistor 4 to flow. Since it flows, the armature current can be accurately detected.

Therefore, according to the above embodiment, the average value Ia of the armature current can be accurately detected even when the armature current is PWM-controlled, and the current detection signal Vi obtained from the smoothing circuit 7 can be obtained. The load state of the electric motor can be detected by calculating the load torque τ from the equation (1) using the armature current Ia detected by

In the above embodiment, the brushless DC motor is taken as an example, but the present invention is also applied to the case where the synchronous motor using the permanent magnet for the field of the rotor is PWM-controlled by using the bridge type switch circuit. Can be applied.

[0045]

As described above, according to the present invention, since the smoothing circuit for smoothing the voltage across the current detecting resistor is provided and the current detecting signal is obtained from the smoothing circuit, the current detecting signal is obtained. Can be made into a waveform with no pulsating current, and an overload detection signal can be prevented from being generated by an instantaneous change in the armature current. Therefore, it is possible to prevent the overload protection operation from being performed when the armature current momentarily exceeds the limit value at the time of startup or momentary load change.
It is possible to prevent the motor from stopping or the speed from decreasing at the time of start-up or a momentary load change.

Further, according to the present invention, since the armature current can always flow through the current detection resistor, the smoothing circuit obtains the current detection signal which is accurately proportional to the average value of the armature current, and the load of the motor is obtained. The state can be detected accurately.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a switch circuit used in the embodiment of FIG.

FIG. 3 is a circuit configuration diagram showing a configuration of a conventional electric motor overload protection device.

4 is a circuit diagram showing a configuration of a switch circuit used in the device of FIG.

[Explanation of symbols]

 1 Motor 2 Switch Circuit 3 Control Circuit 4 Current Detection Resistor 5 Comparator 6 Reference Signal Generating Means 7 Smoothing Circuit

Claims (1)

[Claims] 1. An n-side (n is an integer of 2 or more) upper side switching element whose one end is commonly connected to a positive electrode terminal of a DC power source, and one end is connected to the other ends of the n upper side switching elements, respectively. And a switching element bridge circuit having n lower side switching elements whose other ends are commonly connected, and a feedback diode antiparallelly connected to each switching element forming the bridge circuit. A switch circuit in which the common connection point of the switch elements on the lower side is connected to the negative terminal of the DC power supply through a current detection resistor, and a control circuit for performing on / off control of the switch elements on the upper side of the switch circuit to perform PWM control of the armature current. An overload protection device for protecting an electric motor that is rotated by being supplied with an armature current from a DC power supply through the switch circuit, comprising: A smoothing circuit that smoothes the voltage obtained across the current detection resistor and outputs a current detection signal corresponding to the average value of the armature current; and compares the current detection signal obtained from the smoothing circuit with a reference signal. An overload detection circuit that outputs an overload detection signal when the current detection signal exceeds a reference signal, and limits the armature current of the electric motor to a limit value or less when the overload detection signal is output, or An overload switch control means for controlling the switch element of the switch circuit so as to be cut off, and the feedback diode connected in antiparallel to each switch element of the upper side of the switch circuit is a switch diode of the upper side. A feedback diode connected to both ends of the switch circuit and connected in antiparallel to each switch element on the lower side of the switch circuit is connected to one end of each switch element on the lower side and the DC power source. It is connected motor for overload protection device according to claim between the negative electrode terminal lead circuit.