In the case of AC electric coils, the resistance or impedance of the 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).
The high current draw of an open AC solenoid is known as inrush current. And the current draw when the solenoid is closed is called holding current. AC solenoids can only dissipate the heat generated by their holding current. This means it’s very important for the plunger to close completely when an AC solenoid is energized.
In other words, the high inrush current generates more heat than can be continuously dissipated by the solenoid. So if the plunger is not able to be completely pulled into its coil – due to a mechanical problem with the valve for example, then the insulation around the coil windings will burn and the coil will short out.
So what could go wrong with a hydraulic valve that would stop the solenoid plunger from being pulled in completely? Well, contamination is a common cause. When hard or soft particles invade the clearance between the spool and its bore, the solenoid may not have enough power to fully shift the valve’s spool. This is often referred to as “silt-lock”.
If silt-lock is the problem, then replacing the solenoid is a waste of time. Replacing the entire valve will BUY some time – until it too becomes silt-locked. The solution of course, is to get the contamination problem under control.
Another problem presented by the inrush current characteristics of AC solenoids, is the possibility of overheating due to rapid cycling. Each time the solenoid is closed it is subject to the heating affect of the high inrush current. If the solenoid is switched on and off too rapidly, the successive inrush currents can generate more heat than can be dissipated, leading to failure of the solenoid.
Still, an AC solenoid can be cycled quite rapidly. To give you some idea, a class H solenoid, which has insulation rated to 180C, can be safely switched twice per second. But a DC solenoid with class F insulation rated to 150C can be cycled four times per second without any fear of overheating. And the nice thing about DC solenoids is they don’t burn out if the plunger doesn’t completely close – due to silt lock or any other reason. I have more to say about this and similar hydraulic problems in ‘The Hydraulic Troubleshooting Handbook’, available here.
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