The most common use of solenoids is to provide linear actuation for valves and relays in control systems. While a solenoid may seem like a relatively simple device, there are many operations…
The most common use of solenoids is to provide linear actuation for valves and relays in control systems. While solenoids may appear to be relatively simple devices, there are many operating characteristics that make them fairly easy to apply and maintain if understood.
The current flowing through the wire creates a toroidal magnetic force or flux around the wire. The direction and strength of this magnetic flux is related to the direction and strength of the current in the wire, and its direction of rotation is governed by the “right-hand rule”. Wrap your right hand around the wire with your thumb pointing in the direction of current flow (make sure the wire is well insulated before doing this). Your finger will then point in the direction the force field is flowing. When this wire is wound into a coil, the magnetic flux is concentrated in the center of the coil with a strength relative to the number of coils in the coil.
While this magnetic field flows around the coil in a relatively closed atmosphere, the conductivity of ferrous metals was found to pass more easily. It is the overwhelming trend of magnetic flux that finds this “easy way”, creating our magnetic attraction. are all familiar.
The coils are surrounded by a steel shell to collect and provide an easy path for peripheral forces and reduce the electrical energy that supports flux in this unwanted region. The iron plunger floats in the center of the coil and is usually spring loaded, so in the de-energized state an air gap is created between the plunger and the end of the housing or pole piece. When the coil is energized, the plunger retracts the pole piece to close the air gap. This plunger action is used to push or pull the control element of a valve or relay to change its operating state.
The AC solenoid is powered by alternating current from positive peak to zero to negative peak and back again at a rate of 60 full cycles per second. The magnetic field is strongest at negative and positive peaks, but when the current crosses zero, the tension on the plunger decreases and spring pressure begins to retract the plunger. This in and out movement of the plunger creates an unacceptable buzzing or chattering sound.
To correct this, place a small loop of copper wire into the groove of the solenoid pole piece or end block so that the plunger rests against it when fully retracted. Copper rings are highly conductive and can induce or generate relatively high electrical currents through magnetic fields. This induced electric field produces its own magnetic field that lags the main magnetic field by 90 electrical degrees or 1/4 of the AC cycle. When the AC current passes through zero, the shadow ring flux bridges the zero gap, and the plunger contacts the shadow ring, keeping the plunger in place, eliminating hum.
Alternating current also creates different levels of current in the solenoid, depending on the position of the plunger. When the solenoid coil is energized for the first time and the air gap is at its widest point, the magnetic circuit is incomplete and the lower the AC resistance or impedance, the higher the current required by the coil. This high current level is called “surge current” and only occurs in AC circuits.
As the plunger begins to move toward the pole piece, the air gap decreases, the AC resistance or impedance begins to climb, and the resulting coil current begins to decrease until the plunger is fully retracted. The current in the coil now stabilizes to the design level of the coil, called the “holding current”. The inrush current can be 3 to 10 times higher than the hold current, and can cause extreme overheating conditions that can burn the coil if left too long.
A hum of the coil indicates that the plunger is not fully seated. This condition creates an inrush current condition that, if allowed to persist, could cause the coil to overheat and eventually burn out. The humming AC solenoid valve “squeaks” to let you immediately check for dirt or debris in the path used to damage the visor or plunger. Continuous duty coils can withstand the heat from constant holding current, but not from constant inrush current. Replace coils frequently without correcting the cause of humming and burnout.
The operating frequency of the AC solenoid also affects the heat buildup in the coil. Each time the coil is exposed to the heating level of the inrush current, its temperature rises a little. If the cycle frequency exceeds the coil’s ability to dissipate this extra heat, the coil will burn out.
The DC solenoid has a simpler structure, is not hindered by inrush current, and does not require
