DC solenoids are used in many applications that require movement of a part to be arrested in some way, to be released when an event occurs. An example of such an application would be the garage door. A solenoid keeps the garage door locked down until a signal reaches it to release the door – to allow a vehicle to go in or out – a simple operation as long as the door operates as intended. However, there may be times when the door does not, and one of the reasons could be the solenoid failing to activate.
If the solenoid is easily accessible, the movement of its plunger or parts attached to it can indicate whether it is functioning as intended. However, some solenoids must be located at remote locations that are difficult to reach and therefore, pose difficulties for visual fault diagnosis. However, there is a way to remotely sense whether there is proper plunger movement when the solenoid is switched on.
Many types of valves, relays and contactors use electromechanical solenoids. Typically, these operate from 12-24 V DC and 110-230 V AC systems, consuming power ranging from 8-20 Watts. Electromechanical solenoids consist of a movable iron or steel slug named the plunger or armature, and an electromagnetically inductive coil wound around it.
During actuation, when the plunger has to be pulled into the coil, the solenoid needs high current. Once actuated, the solenoid can hold the armature in the pulled-in position with only about 30% of its nominal current – this is called the hold current. If the solenoid coil consistently operates at the nominal current, high power dissipation raises the temperature of the coil and plunger. Therefore, immediately after the plunger has moved, reducing the current to the hold current helps to reduce power consumption and minimize the temperature rise in the solenoid. This is another reason to detect the plunger movement in a solenoid.
Two popular methods used to detect plunger movement depend on one, Hall sensors and two, on excitation current profile. However, Hall sensors cannot detect faulty or slow movement of the plunger, while the excitation current profile depends on the working temperature of the solenoid. Therefore, these are not particularly suitable for detecting faulty operation of solenoids.
A third and more reliable method of detecting plunger movement in solenoids depends on the current profile from the Back EMF generated by the movement. The solenoid operates when an excitation voltage energizes the solenoid coil. Current passing through the coil causes a distribution of magnetic flux through the plunger. The current increases until the magnetic flux is strong enough to move the plunger.
As soon as the plunger starts to move, its movement produces a magnetic flux in opposition to the main magnetic flux. This induces back EMF in the solenoid coil opposing the excitation voltage – momentarily reducing the current through the coil. Note that this reduction happens only because of the plunger movement and not because of anything else.
Temperature does not affect the dip seen in the current due to plunger movement. Hence, this method is a reliable indication of detecting plunger movement in solenoids.
