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..A clearly definable goal of electronic evolution has been the replacement of mechanical devices with solid-state devices. The basic idea, of course, is to improve re liability. As often as not, enhanced performance also attends the changeover from mechanical to electronic operation. This is because more precise timing can generally be attained, and operation occurs in more favorable surroundings. With regard to ac motors, it has often appeared somewhat incongruous to use a centrifugal switch in the capacitor-start induction motor; part of the rationale for selecting the induction motor is to eliminate switching contacts. When you use mechanical switching, you will generally be subjected to more frequent maintenance.
The scheme illustrated in FIG. 3 dispenses with the centrifugal switch, substituting a triac instead. The gate circuit of the triac is coupled to the ac line by means of a current transformer. The primary of this transformer consists of one or more turns of heavy conductor so that motor operation is not appreciably affected by inserting the transformer primary in series with the line. The secondary winding has a selected number of turns so that the triac is triggered by the inrush of current through the running winding of the motor. When the triac is turned on, the starting winding is energized. As the motor approaches running speed, the current through the primary of the current transformer is no longer sufficient to cause the triac to be triggered through the secondary winding. As a result, the triac turns off and the power is no longer supplied to the starting winding. Capacitor C1 and resistor R1 form a snubbing network to prevent inadvertent triggering of the triac from the inductive kickback of the starting winding.
This simple electronic switch can often be placed inside the motor itself. Not only does it effectively substitute for the centrifugal switch used on most capacitor start induction motors, but it similarly replaces the current relay used for the same purpose on many of these motors. Aside from the obvious advantages of eliminating mechanical switches, the freedom from sparking can be a worthwhile feature when the motor is operated in a volatile atmosphere.
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