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Solving solenoid burnout.

Solving solenoid burnout

Solenoids are elementary-looking electrical devices that make a loud snap when energized and produce mechanical movement on demand. But why do they sometimes fail? Unless you are well versed in the mysteries of magnetism, you may not understand what is causing this problem or what to do to correct the situation.

Here, based on the brochure "What Is A Solenoid?", prepared by Detroit Coil Co, Ferndale, MI, is a primer on the care and feeding of solenoids.

Solenoid basics

Solenoids seem simple: current in a coil creates a magnetic field that flows through an outer C-shaped iron stack into an inner T-shaped plunger. An air gap at the bottom of the plunger creates a resistance that the magnetic field strives to overcome. This magnetic force snaps the plunger downward, quickly eliminating the air gap and holding the solenoid in its activated position. This useful force can be a push or pull, depending on where you hook on to the plunger, top or bottom.

Solenoid designers, however, know that it's not quite this simple. They must add shading coils--two copper loops at the top of the C stack--to generate a lagging current to compensate for the constant reversal of the AC current. This eliminates mechanical buzzing or chatter. Also, they need to insulate between laminations of the C stack to eliminate lateral eddy currents that would otherwise generate wasteful and damaging heat.

Heating up

Heat is the major challenge. Ideally, to get more solenoid force, just increase current. Unfortunately, doubling current increases heating four fold, and even a small increase in current can quickly burn out a coil.

Also, in-rush current is several times holding current. This means that, when the solenoid is blocked open, for whatever reason, the coil will quickly overheat and short out, in a matter of a minute or two.

For this reason, there are two distinctly different solenoid types:

Continuous-duty solenoids are designed to be energized indefinitely without overheating. Holding current is low, and coils readily dissipate the resultant heat.

Intermitten-duty solenoids can pack more punch into the solenoid by allowing much larger in-rush currents on the assumption that ON times will be short and OFF times much longer. They cannot be continuously energized.

Cycle time

A solenoid should close in 8 to 16 milliseconds. Each time it's cycled, it receives a high pulse of in-rush current that generates heat in the coil. The faster a solenoid is cycled, the hotter it gets. If this heat is not given time to dissipate, the coil will overheat, and its resistance will increase, reducing coil current and magnetism. The solenoid will lose power and become too weak to close. Then, in its open position, in-rush current will accelerate, and the coil will burn out.

Troubleshooting tips

Solenoid failures can usually be traced to one or more of these application conditions:

Rating. Be aware of the solenoid's temperature rating. Solenoids are typically rated at 105 C, which means a solenoid too hot to touch may not necessarily be overheating.

Blockage. The most common cause of solenoid failure is when the energized plunger is blocked from closing. High in-rush current quickly burns out the coil. The bobbin melts and nylon runs into the space under the plunger. On a double-solenoid valve, the cause can be energizing both solenoids at the same time.

Voltage change. A drop in supply voltage causes a drop in solenoid force, preventing it from overcoming its load and closing. Check line voltage with an accurate meter, or monitor voltage variation over a 24-hr period. In rare cases, overvoltage can cause excessive holding force and overheating. (Note: because the coil in this case burns out without melting the bobbin, an intact bobbin is confirmation of overvoltage.)

Power-supply mismatch. If a solenoid designed for 60-Hz current is used with 50-Hz current, it will overheat. Conversely, a 50-Hz solenoid on 60-Hz current will not produce rated force.

Ambient temperature. If ambient temperature is too high, the coil will lose its ability to dissipate heat. Resistance increases, current flow and force decrease, the solenoid will not close, and the coil will burn out.

Cycling rate. If cycling rate is too fast, heat will build up faster than can be dissipated. The solenoid becomes too weak to close, receives continuous in-rush current, and burns out.

Shorting. Water-base coolants carrying fine metallic swarf from a grinder or machine tool can splash or soak the coil's leadwires and cause shorting.

Mechanical damage. A solenoid can hammer itself to pieces from excessive force caused by overvoltage or by a reduced load in the return spring it is pulling against. The solenoid's C stack and plunger can distort enough from this mechanical pounding that the air gap disappears and in-rush power is lost.

A final note: coil replacement is simplified in some solenoid designs (a simple matter of removing special clips or a few screws). Take care, however, not to reverse the plunger position on reassembly. The plunger and mating C-stack surfaces have worn a characteristic pattern into each other. Reversing them can cause enough misalignment to create chatter.

For a copy of the brochure, "What is a solenoid?" from Detroit Coil, circle 372.

PHOTO : When a coil burns out, it overheats uniformly, the bobbin melts, and nylon flows into the space under the plunger. Probable cause: plunger was mechanically blocked open.

PHOTO : When a coil shorts out, heating is localized and coil damage indicates this. Probable cause: splashing or soaking with water-based coolants bearing metallic swarf.

PHOTO : Excessive force over time can produce deep grooves in the C stack and plunger laminations that increase as the air gap disappears. Probable cause: overvoltage or reduced force in the return spring.
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Jan 1, 1992
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