The Solenoid

Themes > Science > Physics > Electromagnetism > Magnetostatics > Electromagnetic Application > The Solenoid

A solenoid is simply a special- ly designed electromagnet. Here's how it works.	When current flows through a wire, a magnetic field is set up around the wire.	If we make a coil of many turns of wire this magnetic field becomes many times stronger, flowing around the coil and through its center in a doughnut shape.

Although this magnetic field will flow in air, it flows much more easily through iron or steel--so we add an iron path, or "C" stack around the coil which concentrates the magnetism where we want it.	If we also add an iron path, known as a "T" or plunger, in the center of the coil, the magnetism is concentrated still more.	Because iron is an excellent magnetic conductor and air is a poor one, the movable iron "T" or plunger is drawn by the magnetic field into a position where the magnetism can travel 100% through the metal conductor.

With the addition of this movable plunger we have the basic solenoid as it is today. Now, let's look at some refinements.	Remember, a solenoid op- erates because the magnetism tries to reduce the high resis- tance air gap at the bottom of the plunger. When the plunger is completely closed, the magnetic field flows 100% through a low resistance iron path.	As the plunger is pulled into the coil, the air gap under the plunger is reduced, making the magnetic field stronger and increasing solenoid force. So,...as the solenoid closes, it becomes more powerful.

We have shown that a coil's magnetic field provides motion in only one direction--into the center of the coil. How, then, can we get a push and pull action? To pull, we simply hook on to the top of the plunger. We push from the bottom of the plunger.	An A.C. solenoid operates on current which looks like this. It alternates from positive through "zero" to negative sixty times a second.	The magnetic field is strongest when the alternating current is at its positive and negative peaks. As the current goes through zero, the magnetism and solenoid force decrease, and the load forces the plunger out. When magnetism and force build up again, the plun- ger is pulled back in. This motion of the plunger, in and out, makes the solenoid buzz or chatter.


To eliminate this buzz, and to increase the solenoid hold- ing power, Decco adds two copper loops, called shad- ing coils, to the top of the "C" stack. Current is gen- erated in each of these shad- ing coils, and, most important, this generated current lags be- hind the applied current.	When the applied current is passing through zero, the shading coil current is at its maximum. This low shading coil current provides just enough magnetism to hold the plunger closed when ap- plied current magnetism is at zero, thus eliminating the buzz.	A.C. magnetic fluctuations also cause small stray currents, known as "eddy currents", to move in tiny circular paths within the "C" stack and plun- ger. Eddy currents consume power and cause a heat build-up which reduces sole- noid force. We must minimize them.

Decco makes the "C" stack and plunger of many thin sheets, or laminations, and coats each lamination with insulation. This contains the eddy currents within each lamination.	Magnetism can easily flow in its usual path around the coil, but the eddy currents cannot flow from one lamination to another. By containing the eddy currents within each lam- ination, (limiting their paths), we reduce heating and in- crease solenoid force.	Here's another important fact about solenoid operation. In- creased current in a solenoid coil produces increased mag- netism which increases sole- noid force. So...why don't we rig up a deal to ram all the cur- rent we can into a solenoid?

Because current generates heat, and generates it fast. If you double the current you increase the heat four fold. A small increase in current causes a great rise in tem- perature, which can burn out a coil.	As a solenoid closes, the flow of current decreases. The peak INRUSH CUR- RENT in the coil when the solenoid is open is several times greater than the "sole- noid closed" HOLDING CURRENT due to A.C. resistance (or IMPEDANCE) which increases as the sole- noid closes.	Here is an analogy to help you fix this situation in your mind. Visualize current flow- ing to the coil through a rub- ber tube. This tube runs under the solenoid plunger. As the solenoid closes, it pinches the tube, reducing the flow of cur- rent to the coil.

Remember--when a solenoid is open, it draws a high inrush current, which decreases as the solenoid closes. Now... suppose we energize a solenoid, but mechanically block it open. The high in- rush current will continue to flow in the coil.	This high current will gen- erate more heat than the solenoid can dissipate. The coil wire insulation burns, the bobbin melts, and the coil shorts out--all in a min- ute or two. Applying too heavy a load to a solenoid will hold the plunger open in the same way.	A CONTINUOUS DUTY SOLENOID is one that can be held energized indefinitely without overheating. The heat dissipating ability of this sole- noid is great enough to get rid of all the heat generated by the coil's lower holding current.
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