JP2504026B2 - Electronic thermal relay - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to electronic thermal relays that respond to overload in equipment such as induction motors.

(Prior art and its problems) For example, an induction motor has an increased current when it is overloaded. Focusing on this, there is a conventional example described in Japanese Patent Laid-Open No. 59-89517 as an electronic thermal relay that detects an overload state by detecting an overload current in an induction motor.

In this conventional example, as shown in FIG. 1 of the publication, the current of the induction motor is detected by a current transformer, converted to direct current by a rectifying and smoothing circuit, and impedance converted, and further, at the time of CR including a capacitor and a resistor. After passing through a constant circuit, it is compared with a reference voltage in a comparison circuit, and when it exceeds the reference voltage, it is detected that it is an overload.

By the way, in this conventional example, the operating characteristic is the thermal characteristic of the motor (W = I ² t, where W is the heat generation amount of the motor,
(I is the motor current, t is the operating time), and the operating time t is given by the equation of K / I especially in the large current region where the voltage across the capacitor C4 exceeds twice the reference voltage. (Where K is a proportional constant), and in an even larger current region, for example, in a large current region where the voltage across both ends is 6 times the reference voltage or more, the operating time t saturates at a nearly constant value, Had become difficult to protect effectively.

(Object of the Invention) When the output voltage converted from an AC signal to a DC becomes a specified value or more, the present invention reduces the reference voltage according to the excess amount to improve the thermal characteristics of the motor even in a large current range. It is an object of the present invention to enable overload detection with matching operating characteristics so that a motor can be appropriately protected from overload.

(Structure and Effect of the Invention) The present invention relates to a current transformer for current detection, and an AC / DC that converts an AC signal from the current transformer and outputs a DC output voltage.
A conversion circuit, a time constant circuit that smoothes the output voltage from the AC / DC conversion circuit with a predetermined time constant, a reference voltage generation circuit that generates and outputs a reference voltage, and an output voltage from the AC / DC conversion circuit. A reference voltage correction circuit for reducing and correcting the reference voltage from the reference voltage generation circuit according to the above, and outputting the corrected reference voltage as a corrected reference voltage; an output voltage of the time constant circuit and a corrected reference voltage of the reference voltage correction circuit. And a comparison circuit for comparing

According to the present invention, when the output voltage converted from the AC signal to the DC becomes the specified value or more, the reference voltage is reduced according to the excess amount from the specified value, and the reduced reference voltage is In the comparison circuit as the corrected reference voltage,
Since the output voltage is compared with the output voltage of the time constant circuit, the reference voltage is reduced in the large current range, and as a result, overload detection can be performed with operating characteristics that match the thermal characteristics of the motor. Can now properly protect from overload. In particular, the current transformer used to give an AC signal to the AC / DC conversion circuit needs to be one that matches the overload characteristics of the motor, and therefore it is usually 6 times the set current, or A current transformer that can obtain an output proportional to the above current needs to be used, and such a current transformer has a very large form. In the present invention, since the reference voltage is reduced and corrected according to the output voltage, a current transformer that can obtain an output that is proportional to a large current, such as a current that is six times the set current or more. Is avoided, resulting in a smaller device,
Cost reduction is possible.

(Description of Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block circuit diagram of an electronic thermal relay according to an embodiment of the present invention, FIG. 2 is a detailed circuit diagram thereof, and FIG. 3 is an operation waveform diagram of each part.

In FIG. 1, the input terminals T1, T3 and T5 are not shown in the drawing.
It is individually connected to each circuit of the phase power supply. Input terminals T1, T3, T
Normally closed contacts Xb1, Xb2, Xb3 are connected between 5 and the output terminals T2, T4, T6, and current transformers that detect the respective currents.
CT1, CT2, CT3 are connected in series, respectively.

The output of each current transformer is alternating current (AC) to direct current (DC)
It is input to the AC / DC conversion circuit 1 for converting into. The AC / DC conversion circuit 1 converts each given current transformer output into a DC voltage having a level corresponding to its output level, and outputs the DC voltage. Among the current transformer outputs converted by the AC / DC conversion circuit 1, the maximum conversion output is output to the time constant circuit 2, the smoothing circuit 3, and the reference voltage correction circuit 4, respectively. The motor current waveform is shown in FIG. In FIG. 3 (a), the motor is started at time t0. A starting current flows through the motor from time t0 to t1 and a steady current flows from time t1 to t2. Then, after the time t2, the overloaded state of the motor is shown. The output of the AC / DC conversion circuit 1 is shown in FIG. 3 (b). That is, as shown in FIGS. 3 (a) and 3 (b), the output of the AC / DC conversion circuit 1 becomes large at the time of starting the motor, but the output of the AC / DC conversion circuit 1 becomes steady at the time of steady state of the motor. To do. And if the motor is overloaded
The output of the AC / DC conversion circuit 1 becomes larger than that in the steady state.

The reference voltage generation circuit 5 includes a current setting volume VR that changes the detected current according to the rated current of the motor. From the reference voltage generation circuit 5, the reference voltage set by the current setting volume VR is output to the reference voltage correction circuit 4.

The reference voltage correction circuit 4 performs a correction operation to reduce the reference voltage from the reference voltage generation circuit 5 according to the output voltage of the AC / DC conversion circuit 1 as shown in FIG. That is, in FIG. 3D, the broken line is the reference voltage from the reference voltage generation circuit 4 (this may be referred to as the pre-correction reference voltage for convenience of explanation), and the solid line is the reference voltage correction circuit 4.
Is a reference voltage corrected by (for convenience of explanation, this may also be referred to as a corrected reference voltage). As is apparent from FIG. 3D, the reference voltage before correction indicated by the broken line is the third reference voltage.
As shown in FIG. 6B, the voltage is reduced in proportion to the output of the AC / DC conversion circuit 1 as indicated by the corrected reference voltage indicated by the solid line. The corrected reference voltage is applied to one input portion of the first comparison circuit 6 and one input portion of the second comparison circuit 7, respectively.

The smoothing circuit 3 is provided for reducing the ripple component and averaging the ripple component when the output of the AC / DC conversion circuit 1 contains the ripple component. Then, the output voltage of the smoothing circuit 3 shown in FIG. 3C is given to the other input portion of the first comparing circuit 6. In this way, the output voltage of the smoothing circuit 3 and the corrected reference voltage from the reference voltage correction circuit 4 applied to both input parts of the first comparison circuit 6 are the first comparison circuit 6
Then the voltage level is compared.

As a result of the comparison in the first comparison circuit 6, when the output voltage of the smoothing circuit 3 exceeds the corrected reference voltage, that is, from the time t3 during the start of the motor to the time t4 in the steady state and the time t5.
In the subsequent overload state, the light emitting diode LED provided in the first comparison circuit 6 lights up to indicate that an overcurrent is flowing, that is, the motor is overloaded. At the same time, the output of the first comparison circuit 6 gives a first comparison circuit output for making it operable to the time constant circuit 2 as shown in FIG. 3 (e).

The time constant circuit 2 supplied with the output of the first comparison circuit 6
The circuit output as shown in FIG. That is,
The output of the time constant circuit 2 gradually increases from time t3 to t4 and after time t5. The output voltage from the time constant circuit 2 is applied to the other input section of the second comparison circuit 7. Second
In the comparator circuit 7, the corrected reference voltage [voltage shown by the solid line in FIG. 3 (d)] applied to both inputs of the comparator circuit 7 and the output voltage from the time constant circuit 2 [shown in FIG. 3 (f)]. Voltage) is compared. When the output voltage of the time constant circuit 2 exceeds the corrected reference voltage, the second comparison circuit 7 outputs a second comparison output corresponding to the comparison result as shown at times t6 to t7 in FIG. 3 (g). It is given to the trip circuit 8. The trip circuit 8 has a trip signal (external trip signal) or a second trip signal input from the outside through its terminal B1.
The output from the comparison circuit 7 is OR-outputted. The output of the trip circuit 8 is given to an OFF (OFF) delay circuit 9. In response to the trip output from the trip circuit 8, the off-delay circuit 9 outputs the off-delay output at times t6 to t8 in FIG. 3 (h) to the drive circuit 10 and the latch circuit 11. The drive circuit 10 given the off-delay output,
Drive the breaking coil. On the other hand, the latch circuit 11 to which the off-delay output is applied has the third circuit until the reset signal is applied by turning on the reset switch SW.
As shown in FIG. 1I, the applied off-delay output is latched, and the latched output is applied to the output circuit 13 via the photocoupler 12 to turn off the output circuit 13. The third from the output terminals 13a and 13b of the output circuit 13
The output as shown in FIG. In this case, the output circuit 13 outputs the on output before the time t6 and outputs the off output after the time t6. Reference numeral 14 is a constant voltage power source for each of the above circuits.

FIG. 2 is a detailed circuit diagram of FIG. In FIG. 2, the secondary side of each current transformer is individually connected to the resistors R1, R2, R3 of the AC / DC conversion circuit 1. Each resistor R1, R
An AC voltage, which is detected on the secondary side of the current transformer CT, is generated between both ends of R2 and R3, which is proportional to each current when the motor is started, when it is stationary, or when it is overloaded. One end of each resistor R1, R2, R3 is grounded, and the other end is operational amplifier.
Connected individually to the non-inverting input section (+) of IC1, IC2, and IC3.

The anodes of the diodes D1, D2, D3 are individually connected to the output parts of the operational amplifiers IC1, IC2, IC3, respectively. The cathodes of the diodes D1, D2, D3 are individually connected to one ends of resistors R4, R5, R6, respectively. The non-inverting input part (+) of the buffer amplifier IC4, the peak hold capacitor C1 and one end of each of the discharge resistors R7 are commonly connected to the common connection part on the other end side of the resistors R4, R5, R6.

A feedback resistor R8 having the same resistance value as the discharging resistor R7 is connected between the output portion of the buffer amplifier IC4 and the inverting input portion (−). Further, the output section of the buffer amplifier IC4 and the inverting input sections of the operational amplifiers IC1, IC2, IC3 are commonly connected.

In the AC / DC conversion circuit 1 configured as above,
When a motor current (current at start-up, steady state, overload) flows through at least one primary side of the current transformers CT1, CT2, CT3, the windings on the primary side and secondary side of the current transformer An AC voltage is generated across at least one of the resistors R1, R2, R3 due to the secondary current that is approximately proportional to the turns ratio of. The operational amplifiers IC1, IC2, and IC3, whose non-inverting input section is connected to any one of the resistors R1, R2, and R3, receive 100% voltage feedback from the buffer amplifier IC4 to their inverting input section. Therefore, half-wave rectification with a gain of 0 dB (zero decibel) is performed. The waveform of this half-wave rectification is the resistance value of the resistors R4, R5, and R6 and the peak hold capacitor C1 when the waveform rises.
The capacitor C1 is rapidly charged with a time constant of several m (milliseconds) or less depending on the capacitance value of. At the falling edge of the waveform, the rectifying diodes D1, D2, and D3 are reverse-biased and are in the cutoff state, so the charging voltage of the peak hold capacitor C1 is the resistance value of the discharging resistor R7 and its capacitor C1.
The time constant, which is determined by the capacity value of and, decreases in several tens of milliseconds to several hundreds of milliseconds. Further, as a result of the above-mentioned operation being repeated at the fall of this waveform, the DC voltage having an average value close to the peak value of the voltage across the resistors R1, R2, R3 and including a slight ripple [Fig. (B)]. here,
Output from buffer amplifier IC4 to operational amplifier I
Since voltage feedback is applied to C1, IC2, and IC3, by significantly reducing the offset voltage, the error due to the offset voltage can be greatly reduced.

The reference voltage generation circuit 5 divides the output voltage (+ V) of the constant voltage power supply circuit 14 by the resistors R12, R13, R14 and the variable resistor VR to generate a reference voltage (reference voltage before correction). The detection current of the motor overload state is set by changing the reference voltage with the variable resistor VR.

The reference voltage correction circuit 4 is provided with an operational amplifier IC5, and the non-inverted input portion (+) of the operational amplifier IC5 is supplied with a reference voltage before correction from the reference voltage generation circuit 5. The inverting input section (-) of the operational amplifier IC5 is connected to the output section of the operational amplifier IC4 of the AC / DC conversion circuit 1 via the resistor R15. The output of the operational amplifier IC5 is connected to the cathode of diode D4, and the anode of diode D4 is resistor R1.
To the inverting input section of the operational amplifier IC5 via 6,
It is also connected to the non-inverting input section via the resistor R17.

In the reference voltage correction circuit 4 having such a configuration, the non-inverted input portion of the operational amplifier IC5 is compared with the uncorrected reference voltage from the reference voltage generation circuit 5, and the inverted input portion is subjected to AC / DC conversion. When the output voltage from circuit 1 is lower, diode D4 is reverse biased. As a result, the voltage substantially equivalent to the pre-correction reference voltage from the reference voltage generating circuit 5 is the non-inverting input section of the operational amplifier IC6 of the first comparing circuit 6 and the second comparing circuit 7
And the inverting input section of the operational amplifier IC7. On the contrary, when the output voltage of the AC / DC conversion circuit 1 is higher than the reference voltage, the reference voltage correction circuit 4 functions as a subtraction circuit to reduce the reference voltage, and as a result, The reduced reference voltage is supplied to the first comparison circuit 6 and the second comparison circuit 7 as a corrected reference voltage, respectively. The reference voltage reduction amount is determined by the reference voltage generation circuit 5
Resistors R12, R13, R14 and the reference voltage correction circuit 4 resistor R1
5, R16, R17 resistance value and variable resistor VR resistance value influence. Therefore, the reference voltage correction amount can be changed by the detection current set by the variable resistance value VR.

The smoothing circuit 3 is composed of a resistor R11 having one end connected to the output part of the operational amplifier IC4, and a smoothing capacitor C2 connected between the other end of the resistor R11 and the ground part. The smoothing circuit 3 is for removing the ripple component contained in the output of the AC / DC conversion circuit 1.

The first comparison circuit 6 includes an operational amplifier IC6,
Output part of the operational amplifier IC6 and constant voltage power supply 1
A series circuit of a resistor R18 and a light emitting diode LED connected between the + V terminal of 4, a diode D5 and a capacitor C3
It is composed of a parallel circuit and a resistor R19.

The time constant circuit 2 includes resistors R10 and R20 and a capacitor C4.
And a switch circuit using a transistor FET.

Operational amplifier IC6 in the first comparison circuit 6
The output voltage of the motor is lower when the output voltage from the smoothing circuit 3 applied to its inverting input is lower than the uncorrected reference voltage from the reference voltage generating circuit 5 applied to its non-inverting input, that is, the motor. If is not overloaded, it goes high. As a result, the light emitting diode LED is turned off, and the switch circuit formed by the transistor FET connected to the output part of the first comparison circuit 6 is turned on, that is, the source and drain of the transistor FET are short-circuited. On the contrary, when the motor is overloaded, the output voltage of the operational amplifier IC6 becomes low level (low value), the light emitting diode LED lights up and the switch circuit turns off, that is, between the source and drain of the transistor FET. Opens.

The second comparison circuit 7 is composed of an operational amplifier IC7, the non-inverting input section of which is connected to the output section of the time constant circuit 2 and the inverting input section of which is connected to the output section of the reference voltage correction circuit 4. ing.

When the output voltage of the AC / DC conversion circuit 1 is less than or equal to the reference voltage from the reference voltage generation circuit 5, the output voltage of the operational amplifier IC6 of the first comparison circuit 6 is high level, and therefore the transistor FET is on. .

When the transistor FET is on, the time constant circuit 2
Since the voltage across the capacitor C4 is charged only to a value obtained by dividing the output voltage of the AC / DC conversion circuit 1 by the resistors R10 and R20, the second comparison circuit 7 does not operate.

On the other hand, when the output voltage of the AC / DC conversion circuit 1 exceeds the reference voltage, the first comparison circuit 6 operates and the light emitting diode
Since the LED is turned on and the transistor FET is turned off, the voltage across the capacitor C4 starts rising to the output voltage of the AC / DC conversion circuit 1. When the voltage across the capacitor C4 reaches the reference voltage, the second comparison circuit 7 operates and the output of the operational amplifier IC7 becomes approximately the power supply voltage + V.

In this way, the operation of the second comparison circuit 7 is controlled by the first comparison circuit 6. Now, even when the resistance value of the resistor R20 is made extremely small (for example, zero), the first comparison circuit 6
Control function is satisfied, but hot start characteristics are no longer satisfied.

Here, considering the thermal characteristics of the motor, when the motor is unused for a long time, the coil of the motor is at room temperature (cool). Therefore, even if the motor is started in the overloaded state in such a state, it takes a relatively long time until the coil of the motor reaches the heat resistant temperature. In contrast to this, the temperature of the motor coil rises when the motor is operating for a long time. Therefore, if the motor is overloaded in this state, the heat resistant temperature of the motor is reached in a short time. The former is called a cold start, and the latter is called a hot start. And, generally, the operation time of the hot start is set to about half of that of the cold start. When the resistance value of the resistor R20 is set to zero, the operation time of hot start becomes the same value as that of cold start. The hot start characteristic is obtained by the voltage division ratio of resistors R20 and R10.

In the above description, in order for the first comparison circuit 6 to control the second comparison circuit 7, the second comparison circuit 7 needs to operate at a lower voltage than the first comparison circuit 6. Therefore, in this embodiment, attention is paid to the input bias current of the operational amplifier, and when the resistance value of the resistor R10 is r10 and the resistance value of the resistor R11 is r11, r10> r11. That is, when the input bias currents of the operational amplifiers IC6 and IC7 are the same, the offset voltage due to the input bias current is proportional to the resistance value, and because of the relationship of r10> r11, the second comparison circuit 7 is lower by this offset voltage. It will work with voltage.

If the relationship of r10> r11 cannot be satisfied functionally, for example, the operational amplifier configuration is determined so that the input bias current of the operational amplifier IC6 becomes smaller than the input bias current of the operational amplifier IC7, or the operational amplifier IC6 The purpose can be achieved by a method in which a voltage obtained by dividing the reference voltage of is used as the reference voltage of the operational amplifier IC7.

The trip circuit 8 is an external trip terminal B1, B2 for checking the operation of the system using this device and forcibly tripping due to an abnormal heating signal of a motor with a thermistor.
And transistors TR1, TR4 and diode D6.

The off delay circuit 9 includes a resistor R21 having a relatively short time constant.
And a capacitor C5, a resistor R23 having a relatively high resistance value connected in parallel to the capacitor C5, and an operational amplifier IC8. The operational amplifier IC8 of the off-delay circuit 9 turns on when the charging potential of its capacitor C5 reaches the ground potential level. The time it takes for the capacitor C5 to reach the ground potential level is
Although the time constant between R21 and capacitor C5 is short, it is comparatively short, but on the contrary, capacitor C5 discharges through discharge resistor R23 having a high resistance value, and therefore its discharge time constant is comparatively short. long. Therefore, the operational amplifier IC8
Will take longer to turn off.

The output of the off-delay circuit 9 is output to two systems of a drive circuit 10 and a latch circuit 11. The drive circuit 10 is a transistor
The rat circuit 11 including TR2 is configured to include a thyristor SCR and a reset switch SW that is turned on when the breaker is reset and turned off when the reset is completed. The output given from the off delay circuit 9 to the drive circuit 10 is
The transistor TR2 is operated to excite the coil X1 which operates the built-in circuit breaker. On the other hand, the output of the off delay circuit 9 output to the latch circuit 11 is input to the gate of the thyristor. As a result, the thyristor SCR anode
Conduction between the cathodes and the anode of the thyristor SCR
The current flowing through the light emitting diode 12a of the photocoupler 12 connected in parallel to the cathode becomes zero. As a result, the phototransistor 12b is turned off and the transistor TR3 whose base is connected to the phototransistor 12b is turned off. 1
3 is an output circuit including the transistor TR3.

A Zener diode ZD4 and a capacitor C10 are provided between the other end of the resistor R28, one end of which is connected to the power input terminal A1, and the power input terminal A2 to absorb and absorb external surges and noise.
And C11 are connected, and the constant voltage circuit IC3 is connected.

A Zener diode ZD3 that creates a ground potential level and a resistor R29 that limits the Zener current are connected between the output + V of the constant voltage circuit IC3 and the power supply input terminal A2, and the connection between the Zener diode ZD3 and the resistor R29 is grounded. . The capacitor C9 connected in parallel with the Zener diode ZD3 is for absorbing the noise generated by the Zener diode ZD3.

The resistor R9 is
The operational amplifier IC4 is a resistor for discharging the electric charge stored in the capacitors C2 and C4 because it does not operate.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of an embodiment of the present invention, FIG. 2 is a detailed circuit diagram of FIG. 1, and FIG. 3 is an operation waveform diagram of the embodiment circuit. 1 ... AC / DC conversion circuit, 2 ... time constant circuit, 3 ... smoothing circuit,
4 ... Reference voltage correction circuit, 5 ... Reference voltage generation circuit, 6 ... First comparison circuit, 7 ... Second comparison circuit, 8 ... Trip circuit, 9
… Off-delay circuit, 10… Drive circuit, 11… Latch circuit,
13 ... Output circuit, 14 ... Constant voltage power supply.

Claims (1)

(57) [Claims] 1. A current transformer for current detection, an AC / DC conversion circuit for converting an AC signal from the current transformer to output a DC output voltage, and an output from the AC / DC conversion circuit. A time constant circuit that smoothes the voltage with a predetermined time constant, a reference voltage generation circuit that generates and outputs a reference voltage, and a reference voltage from the reference voltage generation circuit according to the output voltage from the AC / DC conversion circuit. It is provided with a reference voltage correction circuit that performs reduction correction and outputs the corrected reference voltage as a corrected reference voltage, and a comparison circuit that compares the output voltage of the time constant circuit and the corrected reference voltage of the reference voltage correction circuit. An electronic thermal relay.