Linear Solenoids

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Linear Solenoids


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I. Introduction

ASolenoid is an electromagnetic device thatconverts electrical energy into a mechanical force ormotion. They basically consist of an electrical coil

wound around a cylindrical tube with a ferro-magneticactuator or "Plunger" that is free to move or slide "IN"and "OUT" of the coils body.


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I. Introduction

Solenoids are available in a variety of formats with

the more common being:o Linear(Fig. 1.a)

o Rotary ( Fig 1.b)

o Holding

o Latching


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I. Introduction

Fig. 1.a Fig. 1.b


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I. IntroductionThis type of solenoid is generally called a "LinearSolenoid" due to the linear directional movement ofthe plunger. Linear solenoids are available in two basicconfigurations called a "Pull-type" as it pulls theconnected load towards itself when energized, and the"Push-type" that act in the opposite direction pushingit away from itself when energized.


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II. Charateristics

When electrical current flows through a conductor it

generates a magnetic field, and the direction of thismagnetic field with regards to its North and SouthPoles is determined by the direction of the currentflow within the wire. This coil of wire becomes an

"Electromagnet" with its own north and south polesexactly the same as that for a permanent type magnet.


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II. CharateristicsThe strength of this magnetic field can be increased

or decreased by either controlling the amount ofcurrent flowing through the coil or by changing thenumber of turns or loops that the coil has. An exampleof an "Electromagnet" is given below.


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II. CharateristicsWhen an electrical current is passed through the coils

windings, it behaves like an electromagnet and theplunger, which is located inside the coil, is attractedtowards the centre of the coil by the magnetic fluxsetup within the coils body, which inturn compresses asmall spring attached to one end of the plunger. Theforce and speed of the plungers movement isdetermined by the strength of the magnetic fluxgenerated within the coil.


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II. CharateristicsWhen the supply current is turned "OFF" (de-energized) the electromagnetic field generatedpreviously by the coil collapses and the energy storedin the compressed spring forces the plunger back outto its original rest position. The mechanical actionused when the coil is energized can be either a pushmotion (Fig. 2.a) or a pull motion (Fig. 2.b).


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II. Charateristics

Fig. 2.a) Push-type Fig. 2.b) Pull-type


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II. Charateristicso Both Push and Pull types are generally constructed the

same with the difference being in the location of the returnspring and design of the plunger.

o Linear solenoids are useful in many applications thatrequire an open or closed (in or out) type motion such aselectronically activated door locks, pneumatic or hydraulic

control valves, robotics, automotive engine management,irrigation valves to water the garden and even the "Ding-Dong" door bell has one. They are available as open frame,closed frame or sealed tubular types.


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III.Reducing Energy Consumption

One of the main disadvantages of solenoids andespecially Linear Solenoids is that they are "Inductivedevices" which convert some of the electrical currentinto "HEAT", in other words they get hot, and thelonger the time that the power is applied to a solenoidcoil, the hotter the coil will become.


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III.Reducing Energy ConsumptionAlso as the coil heats up, its electrical resistance alsochanges. With a continuous voltage input applied tothe coil, the solenoids coil does not have theopportunity to cool down because the input power isalways on. In order to reduce this self generatedheating effect it is necessary to reduce either theamount of time the coil is energized or reduce the

amount of current flowing through it.


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III.Reducing Energy ConsumptionOne method of consuming less current is to apply a

suitable high enough voltage to the solenoid coil so asto provide the necessary electromagnetic field tooperate and seat the plunger but then once activatedto reduce the coils supply voltage to a level sufficient tomaintain the plunger in its seated position. One way ofachieving this is to connect a suitable "holding"

resistor in series with the solenoids coil, like shown inthe next slide.


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Here, the switch contacts are closed shorting out the

resistance and passing full current to the coil windings.Once energized the contacts which are mechanicallyconnected to the solenoids plunger action open connectingthe holding resistor in series with the solenoids coil.


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III.Reducing Energy ConsumptionUsing this method, the solenoid can be connected to

its voltage supply indefinitely (Continuous Duty Cycle)as the power consumed by the coil and the heatgenerated is greatly reduced and which can be up to 85to 90% using a suitable power resistor. However, thepower consumed by the resistor will also generate acertain amount of heat, I2R (Ohm's Law) and this also

needs to be taken into account.


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III.Reducing Energy ConsumptionAnother more practical way of reducing the heat

generated by the solenoids coil is to use an"Intermittent Duty Cycle". An intermittent duty cyclemeans that the coil is repeatedly switched "ON" and"OFF" at a suitable frequency so as to activate theplunger mechanism. Intermittent duty cycle switchingis a very effective way to reduce the total power

consumed by the coil.


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III.Reducing Energy ConsumptionThe Duty Cycle of a solenoid is the portion of the

"ON" time that a solenoid is energized and is the ratioof the "ON" time to the total "ON" and "OFF" time forone complete cycle of operation and is expressed as apercentage, for example:


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o Then if a solenoid is switched "ON" or energised for30 seconds and then switched "OFF" for 90 seconds

before being re-energized again, one complete cycle,the total "ON/OFF" cycle time would be 120 seconds,(30+90) so the solenoids duty cycle would becalculated as 30/120 secs or 25%.

oA solenoid with a rated Duty Cycle of 100% means thatit has a continuous voltage rating and can therefore beleft "ON" or continuously energised withoutoverheating.

Linear Solenoids

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