This invention relates to a motor start-up circuit and more particularly to the characteristics and specifications of a thermistor with positive temperature characteristic used in such a circuit.
FIG. 9 shows a prior art motor-driving circuit for a motor 1 such as a single-phase induction motor used in the compressor of a refrigerator, comprising an auxiliary coil 2 which functions at the time of the start-up of the motor 1 and a main coil 3 for its steady-state operation. The motor start-up circuit for incorporating into such a motor-driving circuit usually includes a thermistor with a positive temperature characteristic (PTC) 4 connected in series with the auxiliary coil 2 for the motor start-up.
A power source 6 is connected to the motor 1 through a switch 5. During an early stage of the motor start-up after the switch 5 is closed and power from the source 6 begins to be supplied to the motor 1, a relatively large current flows through the PTC thermistor 4 to the auxiliary coil 2 to start up the motor 1. After a certain period of time, the resistance of the PTC thermistor 4 increases due to the heat it produces, and the current flowing through the auxiliary coil 2 is thereby reduced.
The resistance of the thermistor, however, does not become infinitely large. As a result, some unwanted current continues to flow through the PTC thermistor 4 to the auxiliary coil 2 even after the start-up of the motor 1, wasting several watts of electric power.
Japanese Patent Publication Tokkai 6-339291disclosed a method of solving this problem to a certain extent. According to this method, as shown in FIG. 1 which uses the same numerals as in FIG. 9 to indicate the same or equivalent components for convenience, the auxiliary coil 2 is connected in series not only with a PTC thermistor for start-up ("the start-up PTC thermistor") 4 but also with a Triac switch (herein referred to simply as a "triac") 7. Another thermistor with positive temperature characteristic ("triac-controlling PTC thermistor") 8 is connected in parallel with the start-up PTC thermistor 4, one of the terminals of this triac-controlling PTC thermistor 8 being connected to the gate G of the triac 7.
When power from the source 6 is supplied to the motor 1 at the time of its start-up, a trigger signal is applied to the gate G of the triac 7 through the triac-controlling PTC thermistor 8, putting the triac 7 in the current-passing condition and allowing a motor start-up current to flow to the auxiliary coil 2 through the start-up PTC thermistor 4. A certain period of time after the start-up of the motor 1, the resistance of the start-up PTC thermistor 4 increases due to the heat generated by itself and the current through the auxiliary coil 2 is thereby reduced. At the same time, the resistance of the triac-controlling PTC thermistor 8 also increases due to its own heat emission, thereby reducing the current to the gate G of the triac 7 and switching off the triac 7.
A very small current will thereafter continue to flow through the triac-controlling PTC thermistor. Since the thermal capacity of the triac-controlling PTC thermistor 8 can be made much smaller than that of the start-up PTC thermistor 4, however, the power required to keep it at a high-temperature, high-resistance condition is much less than that in the case of the circuit shown in FIG. 9.
If a motor start-up circuit as shown in FIG. 1 is actually used for the start-up of a motor in the compressor of a refrigerator with the ambient temperature allowed to change in the range of -10 to +100.degree. C., however, there are often situations where it fails to dependably shut off the current within a specified short period of time (say, about 1-10 seconds). When it is used outdoors in winter, for example, the heat-up time required for the triac-controlling PTC thermistor 8 to raise its resistance by the heat generated by itself may be considerably long, and the wasted power due to the motor noise and start-up may become quite high. The current may even fail to be shut off.
If it is used under a high-temperature condition such as in summer or if it is attached to the compressor or used near the compressor, on the other hand, the triac-controlling PTC thermistor 8 may be already in a heated-up condition or reach a heated-up condition before the motor is to be started up, failing to properly start up the motor.
Japanese Patent Publication Tokkai 7-123759disclosed another technology whereby the time required to shut off the current to the auxiliary coil ("the shut-off time") can be kept constant, independent of variations in the ambient temperature. FIG. 10 shows a motor-driving circuit incorporating a motor start-up circuit according to this technology. In FIG. 10, the components which are the same as or equivalent to those shown in FIG. 9 are indicated by the same numerals and are not repetitively explained.
According to this technology, as shown in FIG. 10, a triac 7 is connected in series with the auxiliary coil 2 and a triac-controlling PTC thermistor 8 is connected to the gate G of this triac 7. This triac-controlling PTC thermistor 8 is connected to a parallel connection of a correction thermistor with positive temperature characteristic 9 and a correction-adjusting resistor 10, and this parallel connection is further connected to a current-limiting resistor 11. This series connection consisting of the triac-controlling PTC thermistor 8, the parallel connection and the current-limiting resistor 11 is itself connected in parallel with the auxiliary coil 2 and the triac 7.
With the motor-start circuit thus structured, the parallel connection of the correction thermistor 9 and the correction-adjusting resistor 10 serves to increase or decrease the current flowing through the triac-controlling PTC thermistor 8 according to the changes in the ambient temperature such that its heat emission is controlled and the heat-up time of this triac-controlling PTC thermistor 8 will remain constant. By this method, therefore, the length of time during which the triac remains in the ON condition and hence a current continues to flow through the auxiliary coil can be kept approximately constant, independent of changes in the ambient temperature.
The motor start-up circuit as shown in FIG. 10 is disadvantageous in that it requires a relatively large number of components. Thus, it is costly and difficult to make it compact. It now goes without saying that a large number of components means there are additional problems to be considered regarding the reliability of their operations.
If a triac is used as in the examples shown in FIGS. 1 and 10, furthermore, the difference in the gate sensitivity of the triac depending on the trigger mode gives rise to a so-called half-wave period as illustrated in FIGS. 11A and 11B before the current is completely shut off. If this half-wave period is too long (such as in excess of 3 seconds), the motor may generate a beat noise or begin to pulsate in its rotary motion.
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