The most common use for electric solenoids is to provide lineal actuation for valves and relays in control systems. Although solenoids appear to be relatively simple devices, there are a number of operating characteristics which, when understood, will make their application and maintenance considerably easier.
Electricity travelling through a wire creates a magnetic force, or flux, in a circular fashion around that wire. The direction and strength of that flux is relative to the direction and strength of the electric current in the wire and its rotational direction is identified by the “right hand rule.” Wrap your right hand around the wire with your thumb pointing in the direction of the current flow (make sure the wire is well insulated before doing this). Your fingers will then point in the direction of the force field flow. When this wire is wound into a coil, the flux is focused in the centre of the coil with a strength relative to the number of coils of wire.
Although this magnetic force field flows around the coil in a relatively insular atmosphere, it finds the conductivity of ferrous metals much easier to travel through. It is the overpowering tendency for the magnetic flux to find this “easy path” that creates the magnetic attraction that we are all familiar with.
The wire coil is surrounded by a steel case to collect and provide an easy path for the peripheral forces and reduce the electrical energy supporting the flux in this unwanted area. An iron plunger is floated in the centre of the coil and is generally spring loaded so that in a de-energized state, an air gap is created between the plunger and the end of the housing or pole piece (see figure 1). When the coil is energized, the plunger retracts towards the pole piece in an effort to close the air gap. This plunger action is used to push or pull the control member of a valve or relay to change its operating state.
An AC solenoid is operated by current alternating from a positive peak through zero to a negative peak and back again at a rate of 60 complete cycles per second. The magnetic field is strongest at the negative and positive peaks but as the current passes through zero, the pulling force on the plunger is decreased and the spring pressure starts to retract the plunger. This in-and-out motion of the plunger creates an unacceptable buzzing or chattering sound.
To correct this, a small ring of copper wire is put into a groove in the pole piece or end block of the solenoid so that the plunger seats against it when it is fully retracted. The copper ring, being very conductive, allows a relatively high level of electric current to be induced or generated in it by the magnetic field. This induced electric field produces its own magnetic field, which lags the primary field by 90 electrical degrees or 1/4 of the AC cycle. As the AC current passes through zero, the shading ring flux bridges the zero gap, and providing the plunger is contacting the shading ring, holds the plunger in position, eliminating the buzz.
Alternating current also creates varying levels of current flow in solenoid coils, depending on the position of the plunger. When the solenoid coil is first energized and the air gap is at its widest point, the magnetic circuit is incomplete, the AC resistance or impedance is low and the electrical current demanded by the coil is high. This high current level is called “inrush current” and only occurs in AC circuits.
As the plunger starts to move towards the pole piece, reducing the air gap, the AC resistance or impedance starts to climb and the resulting coil current starts to decrease until the plunger is fully retracted (see figure 3). The electrical current in the coil stabilizes at this point to the design level of the coil, called the “holding current.” Inrush current can be three to 10 times higher than holding current and can cause extreme overheating conditions, resulting in coil burnout if it is prolonged.
A buzzing coil indicates the plunger is not fully seated. This condition causes an inrush current situation which, if allowed to persist, can lead to an overheated coil and eventual burnout. A buzzing AC solenoid coil is “screaming” at you to check the shading ring for damage or the plunger path for dirt or debris without delay. A continuous-duty coil is capable of withstanding the heat generated by a constant holding current but not by a constant inrush current. Too often, the coil is changed without correcting the cause of the buzzing and burnout.
The frequency of operation of an AC solenoid will also affect the heat buildup in the coil. Each time the coil is subjected to the heat-generating levels of inrush current, its temperature rises a little higher. If the cycle frequency goes beyond the capacity of the coil to dissipate this additional heat, the coil will burn out.
DC solenoids are simpler in construction and are not hampered by inrush currents and the need for shading rings. The heat generated by the resistance to current flow of the coil windings is constant and weaker regardless of the plunger position. Burnout, therefore, is seldom encountered with DC coils.
The flux generated around a coil charged with direct current (electricity travelling in one direction only) induces its own electric current in the coil wire in a reverse direction. This induced current, although weaker than the primary current, impedes the buildup of the magnetic flux to maximum strength (see figure 2). The resulting delay in actuation time will, generally speaking, produce a slower action than a similar-size AC solenoid.
Any load that generates a magnetic force, such as a motor or a solenoid, is classified as an inductive load. When the power is shut off, the collapsing magnetic field generates or induces a high voltage pulse of electricity in a reverse direction to that of the main current. This induced pulse is characteristically up to 10 times higher than the line voltage but has a very low flow or current. It happens with AC current but is stronger when using DC. The spark that is seen as a switch opens or the sparks generated around the brushes in your power tools are caused by this induced electric pulse or voltage spike.
Voltage spikes can be very harmful to other components in electrical circuits such as rectifiers and almost all electronic equipment. Spikes can also induce other electric noise or low voltage signals in adjacent lines close to lines carrying voltage spikes. This electrical noise can create false signals in transistor-based electronic control equipment but can usually be controlled by installing inexpensive suppressing devices around the coils or motors causing the problem.
Ted Grove, corporate training manager for Wainbee Limited of Mississauga, Ont., is an experienced fluid power trainer. Visit www.mromagazine.com on the Internet and click on the Past Issues button to view Practical Automation columns from previous issues of Machinery & Equipment MRO.