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Sep 10, 2023

Mechanisms: Solenoids

Since humans first starting playing with electricity, we’ve proven ourselves pretty clever at finding ways to harness that power and turn it into motion. Electric motors of every type move the world,

Since humans first starting playing with electricity, we’ve proven ourselves pretty clever at finding ways to harness that power and turn it into motion. Electric motors of every type move the world, but they are far from the only way to put electricity into motion. When you want continuous rotation, a motor is the way to go. But for simpler on and off applications, where fine control of position is not critical, a solenoid is more like what you need. These electromagnetic devices are found everywhere and they’re next in our series on useful mechanisms.

A physicist will tell you a solenoid simply a coil of wire through which current can be passed. That’s it. Other than in the physics lab, though, such a simple device is not of much mechanical use, so what we tend to think of as a solenoid is slightly more complicated. A practical solenoid has a coil, but it’s also going to have several mechanical parts to make it work as an actuator.

A plunger solenoid is a good example of the basics. The air core of the solenoid’s coil is partially occupied by an iron or mild steel plunger, held in place by a return spring. When current is applied to the coil, a magnetic field forms, and the plunger is pulled forcefully into the solenoid’s core. When current stops flowing, the magnetic field collapses, and the return spring forces the plunger back to the resting state. This is characteristic of most solenoids — they’re either actuated or they’re not. This makes them great for jobs that require something to be positioned in either one position or another over a short distance, like valves that stop the flow of liquid through a pipe or tubing.

Plunger solenoids range in size from the very tiny to the ludicrously large. On the small side, plunger solenoids see service as actuators for microfluidics valves in scientific and medical applications, and in the drive head for the impact style of dot-matrix printers (yes, each one of those dots is the plunger of a solenoid).

You likely interact with medium-sized solenoids on a daily basis. The click at the beginning and end of your refrigerator’s ice maker is what switches the water on and off to refill the tray. You’ll hear a similar click in fountain soda machines. And those pinball wizards among us will attest that the forces throwing that silver ball around the playfield are generated by solenoids.

Stepping up the scale, there’s a fairly large solenoid inside the starter motor of almost every car and truck on the road, at least those with internal combustion engines. The solenoid sits atop the starter motor and is responsible for connecting and disconnecting the starter from the system. The solenoid’s plunger is attached to the motor drive shaft via a lever. When the ignition key is turned, the solenoid coil is energized, pulling the plunger in and moving the lever out along the now-spinning motor shaft. This drives a pinion gear out to engage with the engine flywheel to crank the engine until it starts.

Other styles of solenoid are available, including rotary solenoids. These are exactly what they sound like: actuators that can rotate between two positions. Designs vary, but the most common types have a permanent magnet rotor on a shaft inside the solenoid’s core. When the coil is energized, the rotor experiences a torque due to the magnetic field, much like the rotor of a permanent magnet motor. The rotor only moves to a physical stop, though, and is returned to the resting position by a spring when the coil is de-energized. If the polarity of the coil is reversed, then the rotor and shaft can swing the other way, making this style of rotary solenoid bistable. Other rotary solenoids use a metal disc with ramped grooves and ball bearings; when the plunger is sucked into the core, the ball bearings force the disc and shaft to rotate along the grooves.

As electrically simple devices, solenoids can run on either AC or DC. A DC solenoid tends to be quieter because the magnetic field is constant while the coil is energized. An AC solenoid tends to chatter as the magnetic field varies and the force of the return spring overcomes it at the instant the current changes direction in the coil. This tendency can be mitigated by the use of a shading ring to alter the magnetic circuit of AC solenoids. A shading ring is just a small copper ring that sits inside the core of the solenoid so it contacts the plunger when it’s fully retracted. The magnetic field of the energized coil induces a current inside the ring, which in turn creates its own magnetic field that lags the phase of the solenoid’s field by 90°. When the solenoid’s field falls to zero as the AC waveform passes the zero point, the magnetic flux from the shading ring keeps the solenoid retracted, eliminating the bothersome chatter.

While any solenoid will run on AC or DC, care needs to be taken to observe the coil’s specs. Solenoids represent an inductive load, and so their impedance is much higher in AC applications. So if a solenoid rated for 24 VAC were powered by 24 VDC, it would probably burn out quickly as the current through it would exceed the design specs. This could be avoided with a current limiting resistor or by lowering the DC supply voltage.

Like their cousin the relay, solenoids have the potential to damage whatever circuit is controlling them. When the current flowing through a solenoid or relay coil is removed, the voltage spikes as the magnetic field collapses. If that spike gets into sensitive components, like a transistor driving the coil, the device could be destroyed. The classic remedy for this with DC coils is the snubber diode, connected in parallel across the coil with the anode on the negative side. The snubber gives the induced current somewhere to go when the power is removed from the coil to prevent it from inducing the high voltage spike. Snubber diodes won’t work on AC coils, so an RC snubber, with a small resistance and capacitance in series with each other placed in parallel across the coil, serves the same purpose.

This is only a brief look at what solenoids are and do, and how to incorporate these mechanisms into your designs.