When hard or soft particles invade the fine clearance between a hydraulic valve spool and its bore, the force required to move the spool increases. In the worst case scenario, the spool can become stuck. This phenomenon is known as silt lock.
Furthermore, if the spool valve’s actuator is an AC solenoid, often the first indication the valve is silt-locked is when the solenoid burns out. To understand why, it’s necessary to learn some electrical theory.
In DC solenoids, the electrical current is constant for a given voltage. The wire diameter and number of turns in the coil are the only variables that affect resistance (all other things equal).
When voltage is applied to a DC solenoid, the current draw rises from zero to the maximum value that can pass through the coil, regardless of the position of the solenoid plunger in relation to the coil.
However, AC coils behave very differently than DC coils. The resistance or impedance of an AC coil is lowest when the solenoid is open, i.e., when the plunger is out. Impedance increases as the plunger is pulled into the closed position. As a result, the current draw of an AC solenoid is highest when the solenoid is open (plunger out) and lowest when the solenoid is closed (plunger in).
