AC solenoids are electromagnetic devices that operate on alternating current. They have higher current ratings and a shading coil to prevent chatter caused by the zero voltage cross point. When energized, the magnetic field attracts a plunger connected to a secondary mechanism. The strength of the field is in direct relation to the current, which poses a problem for AC solenoids as the current constantly flows from peak positive to peak negative voltage. The shading coil creates a magnetic field that holds the plunger in place while the main field weakens near the zero volt point, preventing vibration. AC solenoids can be used on a DC supply but not vice versa.
An AC solenoid is an electromagnetic switching or actuating device specifically designed to operate on an alternating current (AC) supply. These solenoids are, in many respects, physically similar to their direct current (DC) counterparts except for higher main coil current ratings and the inclusion of a shading coil. The higher current ratings of the AC solenoid coils are necessary due to the fact that, for any voltage, AC solenoids tend to operate at higher currents. The shading coil is to prevent “chatter” caused by the zero voltage cross point in the AC cycle.
Solenoids are simple electromagnetic devices used to provide remote switching or actuating motion for secondary mechanisms and consist of a coil of wire wound around an insulated hollow core. A spring loaded plunger is positioned with its end near the core opening and connected to the secondary mechanism via a linkage on the other end. When the coil is energized with an adequate electric current, a strong magnetic field is generated around it. This magnetic force attracts the plunger, causing it to move elegantly in the core against the spring tension, activating the secondary mechanism in the process. When current is interrupted to the coil, the spring returns the plunger to its neutral position and resets the solenoid.
The magnetic field which provides the force required to provide the actuating movement is in phase with the current applied to the coil. This means that there is in direct relation to that current and, if the current decreases, also the strength of the field and consequently the strength of the solenoid output. In the case of a DC solenoid, this phenomenon poses no problem as the current supplied to the coil is of constant amplitude. The same cannot be said of the AC solenoid, however, as the current is constantly flowing from a peak positive voltage, through a zero voltage to a peak negative voltage. This cycle occurs between 50 and 60 times per second on average AC power.
As the voltage approaches its zero point, the magnetic force becomes so weak that the plunger spring momentarily pulls it out of the core until the voltage again passes the zero point towards the opposite peak value. This causes the plunger to constantly bounce or “chatter” as it is alternately released and reactivated. This can cause excessive wear and overheating of the solenoid and, if allowed to continue, cause eventual destruction of the solenoid. To prevent these vibrations, an AC solenoid uses an extra coil known as a shading coil.
This coil creates a magnetic field that is 90 degrees out of phase and slightly weaker than that of the main coil. This field is strong enough to hold the plunger in place while the main field weakens near the zero volt point, thus preventing vibration. This means that a properly rated AC solenoid can be used on a DC supply, but not the other way around. However, care must be taken when replacing AC and DC solenoids, as the coil on an AC solenoid is typically rated at a higher current than a DC coil of similar voltage to handle the typically higher AC currents.
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