Why hydraulic valve solenoid coils burn-out

Hydraulic Supermarket logo

I got this question from one of our members:

"Could you explain heat generation in hydraulic valve solenoid coils? If the temperature of the coil increases what will happen to the directional control valve's function?"

The short answer to this is: the solenoid coil will fail and therefore the hydraulic valve itself will cease to function.

In 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.

But 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.

Related articles:

An important fact about hydraulic spool valves
How to configure mobile hydraulic valves using power beyond
Would you pass this simple hydraulics test?

If you enjoyed this article, you'll love Brendan Casey's Inside Hydraulics newsletter. It gives you real-life, how-to-do-it, nuts-and-bolts, hydraulics know-how -- information you can use today. Here's what a few members have said about it:

Can't Put It Down
“I get e-mails like this all the time. I never find time to read them. I decided to read Issue #30 and I couldn't put it down. I'll make time from now on.”

Richard A. Shade, CFPS, Project Engineer (Hydraulic Design), JLG Industries Inc.

So Valuable It Earned Me A Raise
“The knowledge I've gained from this newsletter has been so valuable it has earned me a raise!”

Jack Bergstrom, Heavy Equipment Mechanic, Sharpe Equipment Inc.

Love It - Keep Them Coming
“I just love this newsletter. As a Hydraulics Instructor for Eaton, I make copies and distribute them to my students as I address various topics. Please keep 'em coming.”

Michael S Lawrence, Hydraulics Instructor, Eaton Hydraulics Inc.

To get your FREE subscription ($149 value), simply type your first name and primary email address into the form below and hit 'SUBSCRIBE NOW!'

First Name *
Email *

This is a private mailing list that will NEVER be shared for any reason.
You can also unsubscribe at anytime.

Home Page



Copyright © 2000 - 2013 Brendan Casey; HydraulicSupermarket.com