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Motion System Design
MSD 101: Ac solenoids

MSD 101: Ac solenoids

Ac solenoids are a type of electromagnetic actuator, consisting of a magnetizing coil and a plunger. Applying current to the coil produces a magnetic field that draws in the plunger, reducing the air gap as well as the reluctance of the flux path. As the gap closes, the magnetic field grows stronger, increasing solenoid force. Removing the current lets the plunger return to its original (open) position, usually with the help of a spring.

Solenoid force is a function of the field strength, which varies with plunger position and coil current. When the plunger is open, it produces the least amount of force; when it’s closed, the maximum amount. Rated force should match the load, or the solenoid may hammer itself to pieces (in the case of a small load) or burn up (in the case of a large load).

Questions & Answers

Q: How does heat affect performance?
Heat is the number one enemy of solenoids. Not only does it reduce pulling force, it can melt coil insulation and cause a short. Anything that reduces current, both in amplitude and frequency, will reduce heat. Steel laminations, like those used in transformer cores, also help.

Q: What are shading coils?
Ac solenoids are prone to chatter as load forces battle with fluctuating plunger forces. To stem the noise and increase holding power, some solenoids have additional windings (shading coils) atop the “C” stack that magnetically latch onto the plunger.

Q: Why is there a limit on how fast a solenoid can be cycled?
Solenoids produce most heat when turning on, and thus require a given amount of time between “on” cycles to cool. When cycled too quickly, heat (from inrush current) builds up faster than the solenoid can dissipate it. Eventually the solenoid will become too weak to close and will burn out.

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