JP2003079168A - Starter for single-phase induction motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a starter for single-phase induction motor which is not burnt even at starting failure, is free of troubles due to dust, and is small-sized and inexpensive. SOLUTION: The starter is provided with an electronic switch which disconnects a capacitor for starting when a detected auxiliary winding voltage, which increases with the increase in rotational speed, reaches a specified voltage; and a time constant circuit which, if the auxiliary winding voltage does not reach the specified voltage even after a certain time has elapsed, forcedly disconnects the capacitor for starting.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a starting device used for a single-phase induction motor. 2. Description of the Related Art A single-phase induction motor comprises a main winding, an auxiliary winding, and a starting capacitor for supplying a leading current to the auxiliary winding at the time of starting. Is generated. When power is supplied to the motor, the rotor gradually increases its rotation speed while generating a starting torque. Next, when the rotor reaches a speed sufficient to continue rotation, it is necessary to disconnect the starting capacitor from the auxiliary winding circuit and operate the motor at a stable current value.
If the same current continues to flow through the auxiliary winding as it is at the time of starting, the current value becomes excessively large, and the motor eventually generates heat and burns out. Conventionally, when a motor reaches a certain rotation speed, a starting circuit including an auxiliary winding and a starting capacitor is disconnected by a centrifugal switch. [0003] A centrifugal switch is a mechanically constructed switch having electrical contacts and thus has a long life, and often breaks down especially when used in a dusty place. Met. In order to solve this problem, an electronic switch having no means for mechanically opening and closing the contact has been devised (see Japanese Patent Application Laid-Open No. Sho 64-43081). The electronic switch utilizes an increase in the voltage of the auxiliary winding as the rotation of the motor increases, and detects that the auxiliary winding voltage has exceeded the reference voltage. Thus, the starting circuit is disconnected. However, in the case of an induction motor, it often happens that the rotation shaft is restricted and cannot be started. [0006] Under such circumstances, when energization of the motor is started, the auxiliary winding voltage does not rise forever, and if energization is further continued, the triac will eventually burn out. In order to avoid burnout, it is necessary to attach a heat sink of sufficient size to the triac, and it is necessary to select a triac that has a rating that can withstand long-term energization. Also took a lot. SUMMARY OF THE INVENTION The present invention detects an auxiliary winding voltage that increases with the rotation speed of an induction motor, and, when the detected voltage exceeds a reference voltage, switches an electronic switch such as a triac. A single-phase induction motor for disconnecting a starting capacitor by using a time constant circuit for forcibly turning off the triac after a lapse of a predetermined time after the start of energization. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventor of the present application has noticed that the starting time of an induction motor is usually about 0.5 seconds, which is extremely short as compared with the operation time of the motor. did. A triac used for intermittently connecting a starting capacitor circuit requires a considerably large heat sink to allow the rated current to flow for a long time. However, when the energization time is very short, the triac is forcibly turned off after a certain starting time, taking advantage of the fact that it can be used sufficiently without specially attaching a heat sink.
It turned out that it should be controlled so as to make it FF. An embodiment of the present invention will be described with reference to FIG. When the power switch 2 is turned on, the AC power supply 1 is connected to the main winding 3. At this time, the comparison voltage shown at point B is
Since the state is lower than the reference voltage shown at the point A, the comparator 1
The output of 9 becomes low level, and the phototriac coupler 11 is turned on via the resistor 15. Triac 6 is triggered by phototriac coupler 11,
Since the ON state is established, the current advanced by the starting capacitor 5 is supplied to the auxiliary winding 4. When the leading current is supplied to the auxiliary winding 4, the rotor of the induction motor gradually increases its rotation speed while generating a starting torque. The voltage across the auxiliary winding 4, which rises as the speed of the rotor increases, is divided by resistors 28 and 29, rectified by a diode 27, and rectified by a capacitor 26.
Appears as a DC voltage at both ends of the circuit, and is applied to the point B as a comparison voltage via the resistor 18. On the other hand, the power supply voltage is rectified by the diode 12, divided by the resistors 14 and 21, appears as a DC voltage across the capacitor 22, and is applied to the point A as a reference voltage. The reference voltage at point B, which rises as the speed of the rotor increases, is set to exceed the reference voltage at point A when the speed of the rotor becomes sufficient to continue operation. I have. In this example, the reference voltage is designed to be proportional to the power supply voltage, but may be a constant voltage irrespective of the power supply voltage. When the comparison voltage at point B exceeds the reference voltage at point A, the output of the comparator 19 is immediately inverted to a high level, so that the phototriac coupler 11 connected via the resistor 15 is connected. Is turned off, and the triac 6 is not triggered, so that the starting circuit composed of the starting capacitor 5 and the auxiliary winding 4 is cut off and no current flows. The motor completes the starting operation by the operation described above. At this time, if the motor is overloaded or the rotating shaft is restrained,
Since the rotor cannot reach the normal rotation speed and the auxiliary winding voltage does not increase, the output of the comparator does not reverse. Therefore, the triac 6 continues to be energized, generating excessive heat, and eventually burns out. In order to solve this problem, a time constant circuit including a resistor 16 and a capacitor 23 is provided.
That is, since the voltage across the capacitor 23 is 0 V immediately after the power is turned on, the voltage at the point C is also 0 V. However, as time passes, the capacitor 23 is gradually charged through the resistor 16 and the voltage at the point C is reduced. To rise. When the voltage at the point C exceeds the voltage at the point B, the voltage at the point B is increased via the diode 17, and when the voltage at the point B exceeds the voltage at the point A, which is the reference voltage, the output of the comparator 19 becomes Since the phototriac coupler 11 is turned off and turned off, the triac 6 is not triggered and is turned off. Thus, the resistance 16 and the capacitor 2
By setting the time constant of the circuit consisting of 3 slightly longer than the time required for starting the electric motor, the triac can avoid burning. As shown in FIG. 3, the surface temperature of the triac 6 without the time constant circuit at this time rises with the passage of time, and eventually exceeds the burning temperature of the triac. If the time exceeds a preset time, the surface temperature of the triac 6 gradually decreases. Therefore, a time constant is set so that the maximum temperature does not exceed the triac burnout temperature as shown in FIG. Since the present invention is configured as described above, it has the following effects. Even if the load on the motor is excessive or the rotation speed cannot reach the predetermined speed due to the restriction of the rotating shaft, it is possible to avoid accidents such as excessive heat generation in the triac and burning. Further, since it is not necessary to attach an excessive heat radiating plate which is not required at the time of normal startup, it greatly contributes to downsizing of the apparatus. Since the triac only needs to have the minimum required rating, the cost can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing an embodiment of the present invention. FIG. 2 is a circuit diagram of a general single-phase induction motor using a centrifugal switch. FIG. 3 is a graph showing the surface temperature of a triac when the rotating shaft is constrained. [Description of Signs] 1 AC power supply 2 Power switch 3 Main winding 4 Auxiliary winding 5 Starting capacitor 6 Triac 7 Snubber capacitor 8 Snubber resistor 9, 10 Resistor 11 Phototriac coupler 12 Diodes 13, 14, 15, 16 Resistor 17 Diode 18 Resistor 19 Comparator 20, 21 Resistor 22, 23, 24 Capacitor 25 Zener diode 26 Capacitor 27 Diode 28, 29 Resistance 30 Centrifugal switch

Claims (1)

Claims: 1. A main winding connected to an AC power supply, an auxiliary winding connected to the AC power supply via a starting capacitor, and a means for detecting the auxiliary winding voltage. A single-phase induction motor comprising: a means for comparing a reference voltage with the auxiliary winding voltage; and an electronic switch for disconnecting the starting capacitor when a detection voltage of the auxiliary winding exceeds the reference voltage. A starting device for a single-phase induction motor, comprising a time constant circuit, wherein the electronic switch is set to forcibly disconnect the starting capacitor after a lapse of a predetermined time.