Introduction to Solenoids

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Introduction to Solenoids / Basics of a Solenoid

Two basic laws govern solenoids:

Faraday's Law Ampere's Law

Faraday's Law

The voltage induced in a coil is proportional to the number of turns and rate ofchange of flux. The induced current flows in the direction that opposes the changingflux. Flux has no source or sink (What goes in comes out)

Ampere's Law

The magnetomotive force (mmf) around a closed loop is equal to net currentenclosed by the loop. The objective of solenoid design is to transfer the maximum

amount of NI (energy) from the coil to the working air gap.

Types of Solenoids

There are two main categories of solenoids:

Rotary

Linear

Linear solenoids have applications in appliances, vending machines, door locks, coinchangers, circuit breakers, pumps, medical apparatus, automotive transmissions and

postal machines to name just a few.

Rotary solenoids have applications in machine tools, lasers, photo processing, mediastorage, medical apparatus, sorters, fire door closures, and postal machines, also just to

name a few.

Solenoids are used in almost every conceivable industry in the world and are wellknown as an efficient, affordable and reliable actuation alternative.

Eight Essential Application Considerations when designing a solenoid into your

assembly

Stroke

Force or Torque

Voltage

Current / Power Duty Cycle

Temperature

Operating Time / Speed Environmental

AC / DC

LifeStroke when applying solenoids, keep the stroke as short as possible to keep the size,

weight and power consumption to a minimum.

Force applies to linear products. Starting force is typically more important thanending force. A safety factor of 1.5 is suggested. For example, an application requiring

3 pounds of force should employ a solenoid that provides at least 4.5 pounds of

force. Force is inversely proportional to the square of the air gap with flat face plunger

designs. The air gap is the space in the magnetic circuit allowing the armature to movewithout interference, and the magnetic flux to circulate with minimum resistance

(reluctance).

To determine your requirements for force or torque, you need to consider the


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following:

The actual load you are moving

Return spring force or torque Frictional loads

Temperature rise

Duty cycle Orientation of the solenoid vs. gravity (the weight of the plunger is added orsubtracted depending on how the solenoid is mounted.

In linear solenoids, force can be modified by the shape of the plunger used. A conical

face plunger is used for medium to long stroke applications. The effective air gapchanges to become a fraction of actual stroke. Flat face plungers are used for short

stroke applications. Stepped conical face plungers can provide various

stroke (medium to long) dependent on the angle of the step. These are advantageousfor high holding force requirements.

Torque applies to rotary products. Starting torque is typically more important than

ending torque. A safety factor of 1.5 is suggested. For example, an application

requiring 3 pound of torque should employ a solenoid that provides at least 4.5 poundsof torque. Torque produced by Ledex Rotary Solenoids is inversely proportional to

the total length of the stroke. The longer the stroke, the lower the torque output. The

shorter the stroke, the higher the torque output.Voltage the voltage source determines the coil winding to be used in the appropriate

solenoid. Common DC power supply ratings are 6,12,24,36, and 48 VDC. AC vs. DC

solenoids AC solenoids are most commonly used in household appliances. GenerallyAC solenoids have been specified when there was a high cost to rectify to DC. AC

solenoids typically require twice the in rush power of an equivalent DC solenoid. As a

result, many more DC solenoids are chosen for today's applications.Current / Power Force produced by a DC solenoid is proportional to the square of the

number of turns (N) in the coil winding and current flow (I). This determines theampere turns or NI. Solenoid coil requirements must match the power source.

Duty Cycle The duty cycle of your application is the ratio of the "on-time" divided bythe total time for one complete cycle (on + off). Duty cycle is usually expressed as a

percentage or a fraction (50%, 100%). A more simplistic representation of duty cycle

is to call < 100% duty solenoids "Intermittent" and 100% duty cycle solenoids"Continuous". All intermittent duty solenoids (< 100% duty cycle) also must have a

maximum "on-time" allowed to avoid overheating that can eventually lead to a burned

out coil. The "on-time" must not exceed the power dissipation limits of thecoil. Proper heat sinking and/or additional cooling improves heat dissipation which

allows a broader duty cycle range. Very close attention must be paid to the maximum

"on-time" data provided in conjunction with the duty cycle calculation to avoiddamaging your solenoids. For example, although an application with a one hour cycle

time and a 3 hour off-time might calculate to a 25% duty cycle, this is not realistic in

practice. A more realistic solenoid application might be an on-time of one second and

an off-time of 3 seconds for the same 25% duty cycle.Temperature Both the ambient temperature of the solenoid environment and the self

-heating of the solenoid at work must be considered. The resistance of the coil varies

with temperature which affects force output. The self-heating temperature is dictated


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by the duty cycle. Each 1 increase above 20 C equates to an increase of 0.39% of

rated resistance; thereby reducing force or torque output. There are various ways to

compensate for temperature restrictions: Specify a Class C Coil

Specify an overmolded coil

Use a E Model Rotary solenoid vs. the S Model Actuate at one power level and cut back to a reduced power level for holding(pick and hold)

Use a latching solenoid

Use a multiple winding solenoid Operate intermittently, not at continuous duty

Use a larger solenoid

Use a heat sink Add a cooling fan

The limiting factor of operating temperature of a solenoid is the insulation material of

the magnet wire used. Insulation classes:

Class B- 130 C Class F- 155 C

Class H- 180 C

Class C- 220 CA typical solenoid requires 10% of the normal current to remain energized. To

accomplish this, use one of the following:

Mechanical hold in resistor Capacitor discharge and hold in resistor

Transistorized hold in circuit

Pulse-width modulation Pick and Hold

Dual voltage Multiple coils

Operating Time / Speed Factors affecting time and speed include the mass of theload, available power / watts and stroke. De-energizing also plays an important role

and is affected by the air gap, coil suppression, the plunger or armature return

mechanism, and residual magnetism. The air gap is the space in the magnetic circuit allowing the armature to move

without interference, and the magnetic flux to flow with minimum resistance

(reluctance). The smaller the air gap, the longer it takes for the magnetic field resultingfrom the excited coil to diminish. This causes a longer de-energizing time.

The application of electronic protection devices to reduce spikes caused by

interrupting the current in the coil is necessary to ensure protection of your switchingdevice. Coil suppression tends to increase the de-energizing time of the solenoid.

Since solenoids have force in one direction only, there must be some restoring

force (such as gravity or a spring) to take the solenoid back to the starting or de-

energized position. This positions the solenoid for the next operation. Air gap surfaces of a solenoid become the north pole and south pole of a

magnet when energized. When the solenoid is off, a small but measurable magnetic

attraction between the poles still exists called residual magnetism. Residual magnetism


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can be reduced by hyperannealing the solenoid parts of by increasing the size of the air

gap.

Environmental Many environmental factors must be noted when choosing asolenoid. These include temperature, sand/ dust, humidity, shock, vibration, altitude,

vacuum, chemicals and paper dust.

Solenoid Life Life is determined by / optimized by the: Bearing system and shaft surface finish Side loading and load alignment

Preventing the pole pieces from slamming together

Reducing impact shock upon energizingSolenoid life expectations range from 50 thousand cycles to over 100 million cycles.

Custom Solenoids - 80% of solenoids used are custom designs. Typical modifications

include termination, lead wires, plunger configurations, shaft extensions, mountingchanges and linkages.

Application Hints -

To achieve extended life, try the following options:

Drive the load from the armature end of a rotary solenoid rather than thebase end

Use vespel or oilite bearings in a low profile solenoid design

Use dual ring bearings or a groove in the shaft to act as a lube reservoir Use glass-filled or carbon-filled nylon couplings

To achieve increased holding torque / force performance try the following

options: Use indented ball races in a rotary solenoid

Use flat pole pieces

Use latching solenoids To determine the temperature at which a coil has stabilized follow this

sequence of steps: Measure the coil resistance at room temperature

Measure the current at the stabilized temperature and determine the coilresistance using Ohm's Law

Divide this resistance by the resistance at room temperature to obtain

the resistance factor Using the resistance factor chart, read the temperature at which the

solenoid coil has stabilized.

To compensate for temperature rise: Mount the solenoid on a metal surface (heat sink)

Use a cooling fan

Use a larger solenoid Operate at < 100% duty cycle

Consider a higher insulation class

Use a solenoid with multiple windings

Use a pick and hold circuit such as PWM


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Comparison of AC vs. DC Solenoids

As a general rule, DC solenoids are typically preferred over AC solenoids for severalreasons. For example, when space constraints are tight, a DC solenoid will usually give

better performance in a smaller package than will a comparable AC solenoid. In addition,DC solenoids have the capability of being modified in such a fashion as to prevent thepole faces from making contact at the end of the stroke. In AC solenoid applications, this

type of noise dampening would very likely cause premature overheating and failure. AC

solenoids require great care to insure precise alignment of the plunger's pole face to thestator pole face. This facilitates the pole faces making contact with as much surface area

as possible in the energized state which will reduce the amount of hum or chattering in

the AC unit. However, in a similar DC application, a slight gap between the pole piecesat the end of the stroke can have a drastic effect on improving the overall life of the

solenoid.

When looking inside the AC solenoid stator cavity at the stator pole piece, there is asmall ring inserted into the face of the stator pole. This ring is known as a shading coil

and is designed to obtain minimal pulsing in force. What this means is that the coil splits

the pole into two separate parts which causes the flux of these parts to be out of phase. Ifnot for this shading coil, the chattering sound that is commonly associated with AC

solenoids would be more noticeable.

Bottoming out of the pole pieces during each stroke is a requirement on AC solenoids.

Naturally, with this metal-to-metal contact, some deformation of the pole pieces is

possible. As the pole pieces deform and less surface area makes contact between the

pieces, the AC solenoid hums louder and louder. In contrast, a DC solenoid does not

have to have the pole pieces come in contact during each cycle. Rather, if the pole piecesdo not make contact, life of the solenoid is extended. Life of an AC solenoid is typically

lower than that of a similar DC counterpart.

Introduction to Solenoids

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