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 througha wire, a magnetic field is set up around the wire. If we make a coil of manyturns of wire this magneticfield becomes many timesstronger, flowing around thecoil and through its center ina doughnut shape. Although this magnetic fieldwill flow in air, it flows muchmore easily through iron orsteel--so we add an ironpath, or "C" stack aroundthe coil which concentratesthe magnetism where wewant it. If we also add an iron path,known as a "T" or plunger,in the center of the coil, themagnetism is concentratedstill more. Because iron is an excellentmagnetic conductor and airis a poor one, the movableiron "T" or plunger is drawnby the magnetic field into aposition where the magnetismcan travel 100% throughthe metal conductor. With the addition of thismovable plunger we have the basic solenoid as it is today.Now, let's look at somerefinements. Remember, a solenoid op-erates because the magnetismtries to reduce the high resis-tance air gap at the bottomof the plunger. When the plunger is completely closed,the magnetic field flows 100%through a low resistance ironpath. As the plunger is pulled intothe coil, the air gap under theplunger is reduced, makingthe magnetic field strongerand increasing solenoid force.So,...as the solenoid closes,it becomes more powerful. We have shown that a coil'smagnetic field provides motionin only one direction--into thecenter of the coil. How, then, can we get a push and pullaction? To pull, we simplyhook on to the top of theplunger. We push from thebottom of the plunger. An A.C. solenoid operates oncurrent which looks like this.It alternates from positivethrough "zero" to negativesixty times a second. The magnetic field is strongestwhen the alternating current isat its positive and negativepeaks. As the current goesthrough zero, the magnetismand solenoid force decrease,and the load forces the plungerout. When magnetism andforce build up again, the plun-ger is pulled back in. Thismotion of the plunger, in andout, makes the solenoid buzz or chatter.

 To eliminate this buzz, andto increase the solenoid hold-ing power, Decco adds twocopper 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 currentis passing through zero, theshading coil current is at itsmaximum. This low shadingcoil current provides justenough magnetism to holdthe plunger closed when ap-plied current magnetism isat zero, thus eliminating thebuzz. A.C. magnetic fluctuationsalso cause small stray currents,known as "eddy currents", tomove in tiny circular pathswithin the "C" stack and plun-ger. Eddy currents consumepower and cause a heatbuild-up which reduces sole-noid force. We must minimizethem. Decco makes the "C" stackand plunger of many thinsheets, or laminations, andcoats each lamination withinsulation. This contains theeddy currents within eachlamination. Magnetism can easily flow inits usual path around the coil,but the eddy currents cannotflow from one lamination toanother. By containing theeddy currents within each lam-ination, (limiting their paths),we reduce heating and in-crease solenoid force. Here's another important factabout solenoid operation. In-creased current in a solenoidcoil produces increased mag-netism which increases sole-noid force. So...why don't werig up a deal to ram all the cur-rent we can into a solenoid? Because current generatesheat, and generates it fast.If you double the current youincrease the heat four fold.A small increase in currentcauses a great rise in tem-perature, which can burn outa coil. As a solenoid closes, theflow of current decreases.The peak INRUSH CUR-RENT in the coil when thesolenoid is open is severaltimes greater than the "sole-noid closed" HOLDINGCURRENT due to A.C.resistance (or IMPEDANCE)which increases as the sole-noid closes. Here is an analogy to helpyou fix this situation in yourmind. Visualize current flow-ing to the coil through a rub-ber tube. This tube runs underthe solenoid plunger. As thesolenoid closes, it pinches thetube, reducing the flow of cur-rent to the coil. Remember--when a solenoidis open, it draws a high inrushcurrent, which decreases asthe solenoid closes. Now...suppose we energize asolenoid, but mechanicallyblock it open. The high in-rush current will continueto flow in the coil. This high current will gen-erate more heat than thesolenoid can dissipate. Thecoil wire insulation burns,the bobbin melts, and thecoil shorts out--all in a min-ute or two. Applying too heavy a load to a solenoidwill hold the plunger open inthe same way. A CONTINUOUS DUTYSOLENOID is one that can be held energized indefinitelywithout overheating. The heatdissipating ability of this sole-noid is great enough to get ridof all the heat generated bythe coil's lower holding current. Information provided by: http://www.detroitcoil.com