Description

ELECTRONIC RELAY FOR SINGLE PHASE INDUCTION

MOTOR

Technical Field

[1] The present invention relates to an electronic relay for a single -phase induction motor, and more particularly, to an electronic relay for a single -phase induction motor, which can integrate and perform a start function and a protection function through program control. Background Art

[2] Generally, a single -phase induction motor is a small motor that is widely used because it can use a single-phase commercial power source, and once the single -phase induction motor is started, a rotor rotates at a synchronous speed according to a pulsating magnetic field. By changing the equilibrium state of the magnetic field to a disequilibrium state at an initial stage, a start method is required for obtaining a start torque.

[3] The single-phase induction motor is classified into a split-phase motor, a capacitor motor and a shading-pole motor based on the start method for obtaining the start torque. The capacitor motor is a motor that obtains the start torque by inserting a capacitor into start winding in series. The phase of a supply current is shifted by the capacitor that is inserted into the start winding and the supply current having the shifted phase flows through the start winding, and thus the equilibrium of an electromagnetic force is disrupted, thereby obtaining the start torque. Subsequently, when a rotor starts to rotate and an angular velocity increases to the certain number of rotation times, the capacitor is separated by a centrifugal switch and thereby the capacitor motor runs normally.

[4] The main start method of a related art single-phase induction motor connects the run winding of a stator to start winding in parallel and makes the start winding thinner than the run winding or connects a capacitor, and thus supplies a current having a different phase to the start winding, making a rotating magnetic field. However, heat, occurring due to the start winding relatively thinner than the run winding, causes damage to a motor. Moreover, a start capacitor that is used for a capacitor start motor cannot continuously endure a start current for several seconds, resulting in damage. Accordingly, as the angular velocity of the start capacitor is closer to a rated speed by use of a mechanical centrifugal switch, the start capacitor is separated from the start winding by breaking a current that flows through the start winding. However, the mechanical centrifugal switch is vulnerable to vibration, and its characteristic is degraded due to me-

chanical/electrical abrasion which is caused by an arc that occurs when switching is frequently performed. The damage of the centrifugal switch causes the breakage of winding due to restriction/overload and the repair/change of a motor.

[5] In the protection relay of the single-phase induction motor, a bimetal scheme uses various structures by capacitance, consumes a relatively high power, represents slow- running characteristic, and is affected by the change of a peripheral temperature. A current detection scheme using a current transformer, which has been improved for solving the demerits of the bimetal scheme, has difficulty in accurately detecting a current when a current higher than a saturation current is supplied to the primary side of the current transformer. Disclosure of Invention Technical Problem

[6] The present invention has been made in an effort to solve the above-described problems. An object of the present invention provides an electronic relay for a single- phase induction motor, which can replace the function of a mechanical centrifugal switch by efficiently controlling a snubberless triac that is a semiconductor device having a relatively high durability, and thus, enables to configure a start circuit that is not affected by vibration, restricts the number of switching times and has a relatively high durability, and enables to be installed even in the external case of a motor or a distributing board, thereby designing the motor relatively easily.

[7] Another object of the present invention provides an electronic relay for a single- phase induction motor, which senses the velocity of a motor with the induced voltage of start winding to check an overload state, and thus can relatively simply add an overload sensing circuit, which enables to easily select capacitance, consumes a low power and is not affected by saturation characteristic, to the inside of a start relay. Accordingly, the electronic relay for a single-phase induction motor can save costs in manufacturing the distributing board of the motor.

[8] Another object of the present invention provides an electronic relay for a single- phase induction motor, which can immediately shift the current phase of start winding without the decrease of speed by controlling a triac (which is used to control the start winding of a motor) through an internal central processing unit (CPU) at the zero point of an alternating current (AC) voltage. Moreover, the electronic relay for a single- phase induction motor which breaks a current by always maintaining the triac in an off state while moving the contact of an internal relay for changing the wiring of start winding, protecting a relay contact from an arc or an inrush current. Technical Solution

[9] To achieve the objects, the present invention provides a relay, which replaces a cen-

trifugal switch being a mechanical contact with a snubberless triac being a semiconductor switching device, detects an induced voltage (which occurs while a motor is being started) by using the internal CPU of the relay and a simple peripheral circuit and checks a point when the start torque of the motor is maximum, and thus efficiently controls the triac, resulting in accurate and stable start and the improvement of the start characteristic of the motor.

[10] The CPU continuously senses the induced voltage of start winding being proportional to the angular velocity of a motor even while the motor is running to predict the load state of the motor, and when the velocity of the motor rapidly decreases due to heavy load, the CPU restarts the motor by controlling the triac.

[11] In the overload state of the motor, an operation delay time is calculated differently from a rated load state which is preset by a user, and when the overload state is maintained for longer than the calculated operation delay time, the breakage of motor winding and peripheral equipment are prevented by breaking a motor driving circuit through an internal relay contact for overload protection.

[12] When the switching of forward/reverse rotation is momentarily required while a motor is running, the relay according to the present invention checks that the contact of the triac is in an off state, inverts the current phase of start winding through an internal relay contact for the switching of forward/reverse rotation, and again turns on the triac at a zero-point voltage after the elapse of a certain delay time. Accordingly, the relay according to the present invention minimizes the damage of a peripheral circuit that is caused by an arc or an inrush current and stably realizes forward/reverse start.

Advantageous Effects

[13] A relay according to the present invention enables to easily design a motor.

[14] The relay according to the present invention can save costs in manufacturing the distributing board of the motor. [15] The relay according to the present invention can immediately shift the current phase of start winding without the decrease of speed and protect a relay contact from an arc or an inrush current.

Brief Description of Drawings [16] FIG. 1 is a circuit diagram illustrating the driving circuit of a single-phase induction motor which applies an electronic relay according to an embodiment of the present invention. [17] FIG. 2 is a flowchart illustrating a method for performing a start operation in a CPU, according to an embodiment of the present invention. [18] FIG. 3 is a flowchart illustrating a method for performing restart in the CPU, according to an embodiment of the present invention.

[19] FIG. 4 is a flowchart illustrating a method for performing a protection operation in the CPU, according to an embodiment of the present invention.

[20] FIG. 5 is a flowchart illustrating a method for performing forward/reverse rotation in the CPU, according to an embodiment of the present invention.

[21] FIG. 6 is a flowchart illustrating a method for completing start and performing restart in a split-phase motor, according to an embodiment of the present invention. Mode for the Invention

[22] The advantages, features and aspects of the present invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. However, the following embodiments are merely exemplified for describing the present invention, and should not be construed as limited to the embodiments set forth herein.

[23] FIG. 1 is a circuit diagram illustrating the driving circuit of a single-phase induction motor which applies an electronic relay according to an embodiment of the present invention.

[24] Referring to FIG. 1, a single-phase induction motor 110 includes a rotor and a stator.

A run winding IW and a start winding W2 are wound around the rotor. A start capacitor Cs is connected to the start winding W2 in series. An electronic relay according to an embodiment of the present invention includes a central processing unit (CPU) 130 and a relay circuit 120. The CPU 130 performs a start function, a protection function and a forward/reverse rotation function according to a programmed control algorithm. The relay circuit 120 is directly connected to the single-phase induction motor and senses a motor driving voltage and the induced voltage of the start winding W2 to supply the sensed voltages to the CPU 130. The relay circuit 120 starts and protects the single-phase induction motor 110 according to the control of the CPU 130.

[25] When a power source is applied to the single-phase induction motor 110 and thereby start is completed, the electronic relay for a single-phase induction motor according to an embodiment of the present invention separates the start winding W2 from a power source circuit. The electronic relay detects a voltage that is induced to the start winding W2 according to the angular velocity of the single-phase induction motor while a motor is running, and compares the trip conditions of the motor due to overload which is preset by a user through an electronic circuit that is installed inside the relay by using the detected induced voltage. When the trip conditions are coincident, the electronic relay breaks the driving circuit of the motor, and enables the momentary forward/reverse rotation of the motor when necessary.

[26] The relay circuit 120 detects a circuit portion that detects the induced voltage of the start winding W2 for the start of the motor and controls a triac Ql, a circuit portion that

sets a trip voltage for overload protection and controls a trip contact Sl, a circuit portion that detects a zero-point voltage and a zero-point current for efficiently controlling the triac Ql, a circuit portion that controls a rotation direction for the forward/ reverse rotation of the motor, a power source circuit portion that supplies the power source of the internal circuit of the relay, and a circuit portion that displays the overload state of the motor. The CPU 130 includes a built-in digital input/output terminal, an analog input/output terminal, and a built-in flash memory.

[27] The CPU 130 is connected to the relay circuit 120 through input/output terminals and controls all operations, as shown in the following Table 1. [28] Table 1 [Table 1] [Table ]

[29] In the relay circuit 120, circuit portions for performing respective functions multiply

use circuit elements, or perform their own function in connection with the CPU 130. For convenience, therefore, the circuit portions are divided for each function and their operations will be described with reference to flowcharts in FIGS. 2 to 6.

[30] FIG. 2 is a flowchart illustrating a method for performing a start operation in the

CPU, according to an embodiment of the present invention. FIG. 3 is a flowchart illustrating a method for performing restart in the CPU, according to an embodiment of the present invention. FIG. 4 is a flowchart illustrating a method for performing a protection operation in the CPU, according to an embodiment of the present invention. FIG. 5 is a flowchart illustrating a method for performing forward/reverse rotation in the CPU, according to an embodiment of the present invention. FIG. 6 is a flowchart illustrating a method for completing start and performing restart in a split-phase motor, according to an embodiment of the present invention.

[31] 1. Zero-point voltage-zero/point current detection circuit

[32] When a power source for driving the motor 110 is turned on at an initial stage, the

CPU 130 makes the reference voltage of a comparator UI a first reference voltage through an output port OUT4. When the triac Ql is turned off, the first reference voltage is a reference voltage for detecting the zero-point voltage of a motor driving voltage in operations S201 and S202.

[33] The both-end voltage of the triac Ql drops across resistors R3 and R4 and is the input voltage of the comparator Ul. The reference voltage of the comparator Ul is the first reference voltage which is the division voltage of a power voltage Vcc that is divided through resistors R5 and R6 according to the output OUT4 of the CPU 130. The output of the comparator Ul is inputted to the input port INl of the CPU 130. At this point, the comparator Ul uses the first reference voltage for detecting a zero-point voltage and a second reference voltage for detecting a zero-point current according to the control of the CPU 130, and thus it configures a window comparison circuit together with the both-end voltage input circuit of the triac Ql.

[34] When the start of the motor 110 is begun, the CPU 130 senses the output signal of the comparator Ul though the input port INl to check the zero-point voltages of the both ends of the triac Ql, and outputs a high-level signal to an output port OUT3 for control of the gate M3 of the triac Ql in operations S203 and S204. When a current is applied to the gate M3 of the triac Ql according to the output OUT3 of the CPU 130 through a transistor Q2 and a resistor R7 that configures a control circuit for control of the gate M3 of the triac Ql, the triac Ql is turned on at a zero-point voltage, thereby protecting the triac Ql and peripheral circuits from an impulse current in operations S204 and S205.

[35] A phase difference between a circuit current and a circuit voltage occurs in an inductive circuit such as a motor, and the triac Ql is again turned off by its operation

characteristic at the zero-point current of a current that is produced for each half cycle. Therefore, the triac Ql is affected by a triac switching noise. Consequently, the comparison circuit varies the signal of the CPU 130 for each half cycle in which a start winding current is closer to a zero-point current while the motor 110 is starting, and the CPU 130 applies a current to the gate M3 of the triac Ql through an output port OUT3 and the transistor Q2, thereby maintaining the turn-on state of the triac Ql in operations S206 to S209. In control of a snubberless triac, particularly, since the amount of a triac gate current is an important component for improving the inherent commutation (dV/dt) characteristic of a triac, predicting the zero-point current of a circuit current and applying an enough gate current to the triac Ql only during a certain delay time is a method that stably controls the triac Ql while minimizing power consumption in a relay.

[36] In a turn-on state, the triac Ql is variable according to element characteristics and peripheral temperatures between the both ends Ml and M2 (which are main contacts) of the triac Ql for each half cycle in which a circuit current is beyond a zero-point current, but the voltage variation of about 1.0 to 1.5 V occurs therein. At this point, the voltages of the both ends of the triac Ql drop across the resistors R3 and R4 to less than the operation voltage of the comparator Ul when the triac Ql is turned on, in order to predict the zero-point current of the circuit current through the voltage variation of the both ends of the triac Ql and apply an enough amount of current to the gate M3 of the triac Ql during the minimum delay time. The dropped voltage is the positive (+) input voltage of the comparator Ul, and the negative (-) reference voltage of the comparator Ul is set across the resistors R5 and R6. Subsequently, by comparing the positive input voltage of the comparator Ul with the negative reference voltage of the comparator Ul, the zero-point current of the start winding current is varied while the motor 110 is starting, and the output signal of the comparator Ul is varied at a time when the zero-point voltage of a motor driving voltage is passed before the start of the motor. Accordingly, the window comparison circuit for sensing the phase inversion of the triac Ql is configured. The reason that the resistor R6 is a variable resistor is for that in which the CPU 130 controls the high limit value and low limit value of the negative reference voltage of the comparator Ul according to the phase inversion of the start winding current that occurs for each half cycle of an AC power source because most quickly and accurately predicting the pass time of the zero- point current of the current and the change section of the pass time is effective for control of the triac Ql. For this, the CPU 130 supplies the second reference voltage having the same phase as a current phase, in order for the comparator Ul to detect the zero-point current of the start winding current that occurs for each half cycle of the AC power source. When the zero-point current is detected, by supplying the second

reference voltage having a next phase and applying a current to the gate M3 of the triac Ql only during a time when phase inversion is completed, the CPU 130 performs control for the triac Ql to effectively maintain a turn-on state.

[37] Moreover, a varistor Z is connected to the both ends Ml and M2 of the triac Ql in parallel, wherein the operation voltage of the varistor Z is controlled according to the non-repetitive peak off-state voltage of the triac Ql while the motor 110 is running. Accordingly, the varistor Z decreases possibility that the triac Ql and peripheral circuits are damaged by noise that is applied to the voltages of the both ends of the triac Ql.

[38] 2. Induced voltage detection and triac control circuit of start winding

[39] The induced voltage of the start winding W2 that increases in proportion to the angular velocity of the motor 110 is inputted to the analog input port ADCl of the CPU 130 through resistors R 8 and R9 which are a voltage drop circuit, and is converted into a digital value by the analog-to-digital converter of the CPU 130. The converted digital value is stored in a flash memory which is included in the CPU 130. The CPU 130 calculates a speed- voltage variation rate (which is proportional to the acceleration of the motor 110) to check a time when a start torque is in the maximum. When the rapid decrease of the start torque of the motor 110 is begun, by limiting the gate current of the triac Ql through the resistor R7 and the transistor Q2, the CPU 130 changes the current state of the triac Ql into a turn-off state, thereby breaking the start winding W2 in operations S210 to S213.

[40] In a motor, generally, the maximum torque occurs at a time when an angular velocity is about 70 to 80 % of a synchronous speed. When the start of the motor is completed and the induced voltage of the start winding (which is proportional to the angular velocity of the motor) decreases, the CPU 130 continuously checks the level of the decreased induced voltage through the induced voltage detection circuit. When the velocity of the motor is closer to a locked rotor speed, the CPU 130 restarts by the triac Ql.

[41] Relationships between a motor torque and a load torque, an acceleration torque and a speed- voltage variation rate (dVs/dt) are defined. A maximum torque detection method under start that is required for the start control of the single-phase induction motor is proposed as follows. That is, a current that is applied to the run winding Wl and start winding W2 of the stator under start makes a rotating magnetic field. When the rotating magnetic field passes through the rotor, a voltage is applied to the rotor, and a current is produced at the rotor due to the rotor voltage. At this point, a magnetic field is also produced at the periphery of the rotor, and a voltage is again induced to the start winding W2. The level of the induced voltage Vs increases in proportion to the angular velocity (ω) of the rotor. Generally, a motor torque is proportional to the sum of a load

torque and an acceleration torque as follows.

[42] motor torque = load torque + acceleration torque

[43] acceleration torque = J x (dω/dt) = (dVs/dt) (1)

[44] where ω is an angular velocity, J is an inertia moment, Vs is a speed voltage, and dVs/dt is a speed- voltage variation rate.

[45] Accordingly, when the load torque and the inertia moment are constant, the start torque of the motor is increased in proportion to the acceleration or the speed- voltage variation rate of the motor. Therefore, when the CPU of the relay calculates the speed- voltage variation rate through detection of the induced voltage, it is easy to check a time when the maximum torque occurs while the motor is starting, and a time when the start torque of the motor decreases can also be detected effectively.

[46] However, in the case of the split-phase motor which does not use a start capacitor, it is difficult to detect the induced voltage of the start winding under start. Accordingly, it is difficult to check a time when the start torque is the maximum and predict a time when start is completed through the above-described method. Accordingly, as illustrated in FIG. 6, by using the fact that a current rapidly decreases when a locked rotor current is approximately five to seven times higher than a run current and start is completed, the split-phase motor controls the triac Ql at a time when the phase difference between a motor driving voltage "L1-L2" and a start winding current rapidly increases due to the decrease of the current. The phase inversion of the start winding current is performed though the zero-point current/zero-point voltage detection circuit, and a zero-point voltage corresponding to the phase inversion of the motor driving voltage is directly inputted to the input port IN2 of the CPU 130 through a resistor R12 for current limit.

[47] Referring to FIG. 6, in the case of the split-phase motor, the phase difference of about 20 to 30 degrees occurs between a motor driving voltage and a start winding current according to winding types when the triac Ql is turned on and the start of a motor is begun. When the start winding current decreases due to a voltage that is induced to the start winding W2 at a state in which the velocity of the motor is higher than about 70 to 80 % under start, the CPU 130 checks a time when the phase difference between a current and a voltage increases in consideration of that in which the phase difference between the voltages of the both ends of the triac Ql (which has the same phase as those of the motor driving voltage and the start winding current) rapidly increases. When the shift of the phase becomes slow, the CPU 130 turns off the triac Ql within a certain delay time. When the phase difference does not occur for longer than a certain time while the motor is starting, the CPU 130 determines the state of the motor as a locked rotor state under start and immediately turns off the triac Ql, thereby protecting the start winding in operations S601 to S605.

[48] Moreover, although the induced voltage detection circuit for restart under the normal run of the motor may sense the velocity of the motor through detection of the induced voltage like a capacitor start motor, a low-cost circuit may be configured in the following phase-difference comparison scheme without a separate analog-to-digital converter. By using that in which the phase difference between the motor driving voltage and the voltages of the both ends of the triac Ql decreases when the velocity of the motor decreases due to a locked rotor load even though the vector sum of the motor driving voltage and the induced voltage is applied across the both ends of the triac Ql under the run of the motor and thereby the phase difference between power voltages occurs, the induced voltage detection circuit continuously senses the phase difference between the voltages. When the phase difference between the voltages decreases by a value greater than a certain value, the gate of the triac Ql is triggered and restart is performed according to the control of the CPU 130 in operations S606 to S609. That is, the induced voltage detection circuit senses the phase difference between a circuit voltage and a start winding current under start and senses the phase difference between the circuit voltage and the voltages of the both ends of the triac Ql under run. As a result, when necessary, restart is performed.

[49] 3. Trip voltage setting and trip contact control circuit for overload protection

[50] The following description will be made on the operation principle of the trip voltage setting and trip contact control circuit for overload protection using that in which the induced voltage of the start winding W2 is proportional to the angular velocity of the motor.

[51] When a voltage proportional to the angular velocity of the motor is induced to the start winding W2, the induced voltage becomes a low-voltage AC signal by dropping through the resistors R8 and R9, and the vector sum of the motor driving voltage and the induced voltage becomes a low-voltage AC signal by dropping through resistors RlO and Rl 1. The low- voltage AC signals are inputted to the analog input ports ADCl and ADC2 of the CPU 130, they is respectively converted into digital signals through the analog-to-digital converter of the CPU 130 and the digital signals are stored in the flash memory.

[52] In the operation of a user setting unit for trip condition and reset, when a user sets the limit value of the induced voltage that trips the motor driving circuit through a knob switch which is disposed in the external case of the relay for performing the function of a variable resistor for setting, the limit value of the induced voltage is inputted to the CPU 130 through the variable resistor R2 connected to the knob switch and the analog input port ADC3. Subsequently, the limit value of the induced voltage is converted into a digital value through the analog-to-digital converter of the CPU 130 and is stored in the flash memory. Accordingly, when an induced voltage measurement value

is less than the trip setting value by using a start winding induced voltage measurement value and the trip setting value which are stored in the flash memory, the CPU 130 outputs a high-level signal through the output port OUTl after the elapse of an operation delay time based on an excess rate. The high-level signal applies a current to a coil Sl and a transistor Q3, which are the control circuit of an overload trip contact Sl, to open the contact Sl, and opens a magnet switch contact MCl for motor control to break the driving circuit of an overload motor. At this point, the CPU 130 checks a system power source state that is detected through the motor driving voltage detection circuit, and when necessary, it applies a compensation value for a low voltage or an overvoltage to an induced voltage measurement value. Moreover, a light emitting diode D3 for overload display maintains a turn-on state or a flicker state during an overload operation time, and when a trip is completed, the light emitting diode D3 maintains the turn-on state until before reset. In a rated speed while the motor is normally running, a user finds a point in which a detected induced voltage is closer to an induced voltage value for overload setting by turning the knob switch for overload setting to the left and right. Herein, the found point is a point in which the light emitting diode D3 for overload display maintains the turn-on state or the flicker state. According to an embodiment of the present invention, that is, a simple overload operation voltage setting method is to find a point, in which the flicker of the light emitting diode D3 stops, by continuously turning the knob switch. Additionally, an equation, in which the internal circuit of the relay senses a motor voltage for compensating a start winding induced voltage and a low voltage to calculate the angular velocity of the motor while the motor is running, is as follows. The following Table 2 represents signs that are used in the following equations. [53] Table 2

[Table 2] [Table ]

E: Induced voltage in a rotorE': Induced voltage in an auxiliary windingN: Rotor speedφ: Stator magnetic fieldφ': Rotor magnetic fieldk: Particular constant based on a motor structure V: Line voltage

[54] The stator magnetic field (φ) increases in proportion to the voltage of the stator or a motor voltage as expressed in Equation 2 below. The rotor induced voltage (E) is proportional to the intensity of the stator magnetic field and the rotor speed (N).

[55] E = Kφn = kVN (2)

[56] As described above, if it is assumed that the induced voltage (E') of the start winding

W2 is proportional to the rotor magnetic field (φ ¹ ) and the rotor speed (N) and the constant (k) is constant, the induced voltage (E) in an auxiliary winding is expressed as

Equation 3 below.

[57] E' = kφ'N = kEN = K ² VN ² (3)

[58] where N ² = (l/k ² )(E'/V).

[59] ordingly, the rotor speed (N) is proportional to a square root (which is "induced voltage (E) in an auxiliary winding/line voltage (V)") as expressed in Equation 4 below.

(4)

[61] 4. Rotation direction control circuit for forward/reverse rotation of a motor

[62] The following description will be made with reference to FIG. 5 on the rotation direction control circuit for forward/reverse rotation of the motor.

[63] When a motor reverse start command is inputted to the CPU 130 through user control, the CPU 130 checks the turn-off state of the triac Ql and outputs a high-level signal through the output port OUT2. Subsequently, the high-level signal excites a coil S2 through a transistor Q4 to switch the sliding contact of a relay switch S2, and thus the phase of a start winding current is inverted in operations S501 to S508.

[64] At this point, the CPU 130 waits during a certain time for protecting relevant circuits from an arc until the sliding contact of the relay switch S2 terminates moving, and checks the zero-point voltage of a start winding voltage to turn on the triac Ql through the resistor R7 and the transistor Q2 that are the triac control circuit. Consequently, a current is applied to the start winding W2, and the moment forward/reverse rotation of the motor therefore is performed.

[65] On the other hand, when a forward start command is inputted to the CPU 130, the

CPU 130 again controls the triac Ql after the sliding contact of the relay switch S2 terminates moving by the above-described order, momentarily switching a rotation direction.

[66] The forward/reverse rotation control circuit may be applied to the braking function of the motor. In this case, the forward/reverse rotation control circuit checks the speed decrease of the motor through the induced voltage detection circuit, and controls the triac Ql and the magnet switch MCl before the velocity of the motor increases in a reverse direction, thereby breaking the motor driving circuit.

[67] 5. Power source circuit for supplying a power source to the internal circuit of the relay

[68] The following description will be made on the power source circuit for supplying a power source to the internal circuit of the relay.

[69] When the power source is supplied to the relay, a voltage drops to a low voltage AC signal through a resistor Rl and a capacitor Cl, and the dropped voltage is half- wave

rectified through a diode Dl. The half- wave rectified voltage is converted into a direct current (DC) voltage through a zenerdiode D2. The DC voltage is smoothed through a capacitor C2 and thereby becomes a power source that is supplied to the CPU 130 and peripheral circuits. For directly inputting the zero-point voltage and zero-point current of the triac Ql to the CPU 130, the reference voltage Vcc of an internal circuit may be connected to have the same phase as that of the gate contact M3 of the triac Ql.

[70] In an embodiment of the present invention, for convenience and understanding, each filter circuit, a forward/reverse user command input circuit, a reset type selecting circuit and clamp diodes for protecting the overvoltage of the CPU 130 has been omitted in the accompanying drawings.

[71] While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims. Industrial Applicability

[72] An electronic relay according to the present invention may be applied to a single- phase induction motor.