Solenoids

Electromagnet or solenoid are general terms for all types of electromagnetic actuators.

Basically, electromagnets or solenoids are devices that generate a magnetic field by means of a current-carrying coil, guiding it through suitable iron parts with air gap. Here, magnetic poles are created inbetween which a magnetic force of attraction, the magnetic force, prevails.

If no current is applied to the coil, no electromagnetic force is generated; if the coil current is regulated, the magnetic force can be regulated.

Depending on the construction of the iron parts, the magnetic force is used to carry out linear or rotary movements or to exert holding forces on components, decelerating or fixing them.

Basics Solenoids

Generation of the magnetic field

If the coil is energized, an electrical current flows in dependance of voltage level and electrical resistance.

This current creates a magnetic field in the coil, which is demonstrated by the red field lines in the picture.

The coil is usually made of enamelled copper wire  since copper has a very good electrical conductivity. For extremely large coils, aluminium is also used as conductor material in special cases in order to reduce weight or costs.

Amplification and conduction of the magnetic field

The expansion of the magnetic field considerably depends on the material and its shape through which the magnetic field flows.

Air has a comparatively high resistance against the magnetic field, whereas iron materials such as steel have a high magnetic conductivity and conduct the magnetic field very well.

For this reason, the magnetic field of the coil is guided in steel components. In simplified terms, it can be stated that the magnetic field is amplified by steel components. However, it is correct that there is no amplification, in fact, the magnetic field is weakened only slightly due to the outstanding magnetic conductivity of ferrous materials.

The most ordinary form of an electromagnet is the holding magnet, also called pot magnet. The magnetic circuit consists of a pot-shaped magnet body with internal pole surrounding the coil on 3 sides. The force that can be applied to a magnetic component with this device decreases very rapidly the larger the distance of the component from the so-called pole surface gets.

 

As a rule, if the electromagnet has to function as a linear actuator or linear solenoid,  highest possible forces are required with largest possible strokes.

 

For this purpose, a movable iron part, the armature, is arranged inside the coil at a distance (air gap) from the fixed core.

If the coil is fed with current, the armature moves towards the core and executes the stroke with the dimension of the air gap. The armature's movement  can, for example, be led outside the electromagnet via an armature rod through a hole inside the core.

Magnetic force as a function of stroke and air gap

Inside electromagnets, magnetic force and stroke are decisive variables for their selection and they are mutually dependent.
 

The picture on the left shows a sectional view of a solenoid and the picture above shows the force curve over the stroke or air gap.

The shape of the characteristic curve is determined by the design of the core and armature and can be very accurately determined by using modern calculation tools (finite element method). Fine-tuning is still carried out by adjusting the  sample devices' geometries.

Mechanical characteristics linear solenoids

As an example for other forms of electromagnets, the important mechanical parameters for linear solenoids are explained here in simplified manner. Please download detailed information in our Technical Explanation​​​​​​​.

 

Magnetic force

The magnetic force FM is the exploitable part, i.e. the part of the mechanical force generated inside the linear solenoid, with stroke direction in a horizontal armature position which is reduced by the friction.

    Φ= Useful flux (Wb)

    FR = Friction force (N)

    FM = Magnetic force (N)

    FF = Force applied to the armature by the magnetic field (N)

Stroke force

The stroke force FH is the magnetic force which occurs outside, taking into account the associated component of the armature weight FA.

                  FA = weight force of the armature (N)

                  mA= armature mass (kg)

                  g = 9,81 m/s²

Holding force

The holding force is the magnetic force in the stroke end position, i.e. stroke 0. The moving component, the armature, is in contact with the core, the air gap is almost 0. As a rule, the holding force represents the maximum of the magnetic force-stroke characteristics.

Magnetic force vs. stroke characteristic

Due to the geometric design of the armature and core, three fundamentally different types of characteristic curves can be achieved.

 

    I. decreasing characteristic curve

    II. horizontal characteristic curve

    III. increasing characteristic curve

 

The most common curves are the increasing characteristic curve, particularly suitable for spring counterforces, and the horizontal characteristic curve, particularly suitable for constant counterforces.

The decreasing characteristic curve is rarely used for linear solenoids, it is applied where high friction forces need counterforces.

Work

The Work W is defined as the integral of the magnetic force over the magnetic stroke.

 

In simple words, this work demonstrates the area below the magnetic force-stroke curve. 

Rated work

The Rated work WN specified in the data sheets is calculated as the product of the magnetic force FM in the initial stroke position s1 and the magnetic stroke s. 

If a solenoid needs to be selected for a new application on the basis of existing specifications regarding stroke and magnetic force, the calculated nominal stroke energy can be compared with the values in data sheets in order to determine an adequate solenoid size, depending on the operating mode. If necessary, please contact your local Technical office.

 

Electrical characteristics Solenoids

Important electrical parameters for DC solenoids are explained below. Unless otherwise stated, voltage and current specifications are arithmetic average values for direct currents.

Rated voltage

For the Rated voltage UN, a voltage device is designed and marked. Unless otherwise stated, the values of the data sheets are based on a rated voltage of 24 V. Regarding other nominal voltages, deviations from the specified magnetic forces may occur, due to the different insulation components in the exciter windings, upwards (usually at > 24 V) as well as downwards (usually at < 24 V) .

Voltage tolerance

The permanently permissible voltage change on DC solenoids is ±10 % of the rated voltage.

 

Rated current

The Rated current IB is the current which is generated at nominal voltage and an exciter winding temperature of +20° C. The current is determined by the voltage of the exciter winding.  It can be determined by dividing the rated power specified in the data sheets by the rated voltage.

Test current

The Test current IPR is the current which the magnetic force values specified in the data sheets refer to.

 

It is calculated as follows:

    IPr = 0,9 UN / RW

RW stands for the operating temperature resistance of the exciter winding.

The factor 0.9 considers the smallest permissible supply voltage due to the permissible voltage tolerance.

Rated power

The Rated power PN specified in the data sheets refers to the rated voltage and rated current. Since the rated current is based on a temperature of +20°C, the rated power for MSM is called P20. Unless otherwise stated, the rated voltage is 24V.

 

Thermal behaviour and operating mode

Self-heating of solenoids

The electrical resistance of the solenoid coil means that an electromagnet heats up as soon as it is energized. The maximum temperature (steady-state temperature) in the coil is reached when the heat supplied by electrical energy corresponds to the amount of heat dissipated by heat radiation; a heat conduction and a thermal equilibrium is achieved.

If a higher electrical power is supplied, the magnetic force increases and a higher winding temperature (steady-state temperature) is reached.

During designing the windings of electromagnets at MSM, care is taken in order to ensure that the temperature of the winding is below the permissible maximum temperature for the insulating materials, taking into account the ambient conditions and the operating mode.

ϑ13 ambient temperature at the end of the measurement
ϑ11 reference temperature (consideration of temperature, influences of media)
ϑ14 upper ambient temperature
ϑ16 initial temperature at the beginning of the measurement
ϑ21 upper limit temperature
ϑ23 steady-state temperature
Δϑ31 excessive temperature
Δϑ32 steady-state excessive temperature
Δϑ33 temperature rise limit
Δϑ34 hot spot difference
tein turn-on instant

Operating modes of solenoids

Solenoids are made for different operating modes, depending on requirements.

 

In continuous operation (S1 or 100% ED), the duty cycle takes as much time as needed to reach the steady-state temperature ϑ23.

During Intermittent operation (S3), switch-on time and currentless breaks alternate in regular or irregular intervals; here, the breaks are so short that the device does not cool down to its reference temperature or output temperature.

 

The permissible duty cycle is in % and is calculated as follows:

    %ED = duty cycle / cycle time * 100

 

 

 

    t5  =  duty cycle

    t6  =  de- energized period

    tS  =  cycle time 

   

The force, power and time values indicated on the electromagnet data sheets refer to a cycle time of 5 minutes (300 sec.). For this cycle time, the following permissible maximum values for the duty cycle are indicated in the table on the left-hand side.

 

In short-time operation (S2), the duty cycle is so short that the steady-state temperature is not reached. The currentless break is so long that the device practically cools down to the reference temperature.

 

Short-time duty is characterized by the indication of the duty cycle

e.g. „S2 20s“.

Types of solenoids

Solenoids are distinguished and classified according to various criteria. In the following, each criteria is described individually, in practice, combinations can be seen.

 

Differentiation according to voltage type

Depending on the type of voltage, a distinction is made between

 

 

• DC Solenoids

 

 

and

 

AC Solenoids.

Differentiation according to type of construction

For all types of electromagnets, a distinction is made between the following designs

 

 

 

closed design,

the magnetic body or another type of coating encloses the excitation winding on all sides, 

and

 

open design,

here the excitation winding is only partially enclosed.

Differentiation according to the type of movement

Depending on the type of movement of a solenoid, this term is used:

 

 

 

 

  • Shotbolt Locking Units as a special form of single acting solenoids, the armature rod is particularly stable, therefore transverse forces can be absorbed.
  •  Holding magnets: they generate a magnetic field which applies attraction forces to a ferromagnetic counterpart.   
  • AC Vibrators for generating small stroke movements that can cause spring-mass systems to vibrate.

Differentiation according to controllability

Unless otherwise stated, solenoids are ON/OFF actuators. If a solenoid's  force needs to be controlled, it is executed as a proportional solenoid. This subdivision can primarily be found in hydraulics. However, pneumatic valves are partly operated with proportional solenoids as well.

ON/OFF solenoids are designed to move the armature to the end position after being energized.

 

The usually increasing characteristic curve is suitable to work against a spring as it has an increasing characteristic as well.

Proportional solenoids are equipped with a horizontal or slightly falling characteristic by suitable design measures. 

In contrast to ON/OFF solenoids, the proportional solenoids are controlled via a current control instead of with nominal voltage.

In order to limit the thermal load, the maximum permissible current is specified as the so-called limit current IG. The current control eliminates the influences of the temperature-dependent change in resistance of the coil winding.

The magnetic force can now be adjusted by changing the current flowing through the coil. If the solenoid works against a force, a stroke point will be set depending on the magnitude of the counterforce.

Applications in which special positioning accuracies are required are equipped with position sensors, which enable a control loop to be set up with which the reaching and holding of a position can be precisely controlled. 

Explanation of the device types

Acting Solenoids

Depending on the type of motion cycle, a distinction is made between single, double and reverse acting solenoids.

 

 

 

Single acting solenoids are electromagnets in which the movement from the initial position to the end position is effected by electromagnetic force. An external force is required to return the solenoid to its initial position, e.g. spring force, weight force, etc.

 

 

Double acting solenoids  (with zero position) are electromagnets in which the movement after excitation of the relevant coil takes place from zero position in one of the two opposite directions. Return to zero position is achieved by external restoring forces after switching off. The zero position is the initial position for both directions.

 

 

Reverse solenoids (without zero position) are electromagnets in which the movement takes place after excitation of the relevant coil from one end position to the other or vice versa.

 

Rotary Solenoids

Solenoids that perform a rotary motion are called rotary solenoids.

 

 

 

The single-acting version is the most common for rotary solenoids.

 

 

 

 

 

 

If a return movement is required, a return spring is adapted via a suitable spring cage.

 

Analogous to the linear solenoids, a version as a reversible rotary solenoid with 2 rotary solenoids working against each other can be realized.

Holding magnets

Retaining magnets are electromagnets that generally have no or only a very small stroke.

 

Depending on the type of function, the counterpart is attracted by a magnetic field in the energised or de-energised state.

Holding magnets build up a magnetic field as long as they are connected to the power supply.

Permanet holding magnets constantly attract magnetic counterparts. The magnetic field is generated by an integrated permanent magnet. If the devices are connected to the power supply, this magnetic field is largely neutralized by the magnetic field of the additionally installed coil.

Advantages of solenoids

Electromagnetic systems are basically in competition with a large number of other types of actuators, for example:

 

  • Electromotive drives with spindle, gear or crank loop
  • Electromechanical cylinders
  • Pneumatic actuators consisting of cylinder and valve
  • Piezo actuators
  • Voice Coil Actuators
  • Magnetostrictive actuators or translators
  • Actuators with shape memory metals
  • Actuators with thermobimetals

The electromagnetic actuator has the following qualitative advantages:

  • Fast motion with high dynamics
  • High forces
  • Low technical effort compared to an electromotive solution:
    • Simple electrical control
    • No mechanical interface (gearbox, etc.) required
    • Fixed stroke
  • Wide working temperature range
  • Low number of moving parts
  • Low susceptibility to interference
  • High degree of protection and explosion protection according to ATEX/IECEx possible
  • Long service life
  • Maintenance free
  • Large variety of types
  • Standard devices available from stock

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What is the difference between a pulling or a pushing solenoid? 

 

The distinction between pulling and pushing solenoids results from the constructive possibilities of flange-mounting at the moving component, the armature. 

The figure shows the basic design of an solenoid, especially a linear solenoid in the unpowered initial position. When energized, the armature and armature rod move in the direction of the arrow.

 

If the solenoid is to be used in pressure mode, it is flange-mounted to the armature rod.

If the armature is hinged - possibly also via a 2nd armature rod not shown here - then the solenoid is inserted pulling.

Product examples of pulling solenoids: (G TA F, G MC X, Pulling valve solenoid with needle)

 

Product examples pushing solenoids:  (G AA, G MC X, G SC X)

Depending on the installation, different designs can be used both as pulling and pushing solenoids: (G TC A, G FC X)

 

 

How to connect a DC solenoid ?
 

This depends on the available voltage source.

A DC solenoid is directly connected to a DC power source. The voltage must correspond to the nominal voltage of the device.

If AC voltage is available as the voltage source, the electrical connection can be made via a rectifier.

If the supply voltage is 230 V, the DC magnet must be designed for 205 V when silicon rectifiers are used.

The solenoid generates an overvoltage when it is switched off, how can this be limited?

 

The inductance associated with a DC solenoid causes high switch-off overvoltages, which can cause the electrical insulation to break through or damage other electrical components in the circuit.

The most common method of limiting the cut-off overvoltage is a diode protective circuit.

For further information, please refer to our Technical Explanation GXX.

On which parameters does the force of an solenoid depend?

The force of an solenoid is directly related to the current flowing through the coil. If a higher current flows, the force of the solenoid increases.

In addition, the size of the air gap or the position of the armature has a considerable influence.

The solenoid heats up during continuous operation. Is this a malfunction?

 

The self-heating of an energized solenoid is physically conditioned and basically no malfunction. If current flows through the coil of the device, the electrical energy is converted into thermal energy due to the ohmic resistance of the coil. This causes the solenoid to heat up. Depending on the design and ambient conditions, this results in considerable surface temperatures. If a device is operated under incorrect conditions, the temperature can rise above the permissible temperature, which can lead to damage to the device. The maximum permissible temperature (limit temperature) is between 90°C and 250°C and is determined by the choice of insulation materials.

Please refer to our Technical Explanation GXX for further information.

What does the term duty cycle mean?

 

The duty cycle t5 is the time in which the supply voltage is applied to the solenoid.

t5 = duty cycle (s)

t6 = de-energized period (s)

tS = cycle time (s)

P = power (W)

t = time (s)

The relative duty cycle ED given in % is the percentage ratio of duty cycle t5 to cycle time tS.

 

%ED = duty cycle / cycle time * 100

 

In which way is it possible to reduce the self-heating of an solenoid?

 

The heating of a solenoid can be influenced from two directions:

  • Reduction of the supplied energy

or

  • Improvement of heat dissipation through cooling

The use of a holding current reduction Z KD H 211 is recommended to reduce the supplied energy.

In your data sheets, different magnetic forces are specified for different operating modes or duty cycles.  What is it all about?

 

With solenoids, it is possible to increase the magnetic force at the expense of the permissible duty cycle by installing coils with different powers.  For this reason, the magnetic force specifications for

 

Continuous operation (operating mode S1 / relative duty cycle 100%)

sowie

Intermittent operation (operating mode S3 / relative duty cycles 40% / 25% / 15% / 5% )

angegeben.

Below is an excerpt from an exemplary data sheet for linear solenoids.

If you look at the magnetic force values for the different strokes column by column, you can see how the magnetic force increases with decreasing duty cycle.

 

The relative duty cycle for solenoids refers to 5 minutes (=300s).

This means in the concrete example for the duty cycles listed in the table:

In the case of several switch-on processes within the 300s, the switch-on times and currentless pauses are added together.

The force of the solenoid used is too low. How do I proceed? 

 

In the first step, we recommend to check to what extent the required force can be reduced by optimizing the overall application.

With solenoids, it should be noted that the magnetic force generally decreases with increasing air gap / stroke. If the stroke can be reduced and possible idle strokes reduced, this will result in the solenoid being operated in the range of the higher magnetic force.

In principle, solenoids can be used to increase performance by reducing the duty cycle. For this purpose, the device is designed at the factory with an adapted coil winding.

Your responsible Technical office will be happy to help you solve your problem.

Which voltage is used to operate a solenoid?

 

As a rule, a solenoid is operated at its rated voltage, which is indicated on the rating plate together with the operating mode or relative duty cycle.

The nominal voltage must be specified when ordering a solenoid. The standard voltage and the maximum possible nominal voltage are specified in the respective data sheets.

For proportional solenoids, the permissible limit current IG is relevant instead of the nominal voltage.

I would like to operate my solenoid with a voltage other than the nominal voltage. Is this possible?

 

As a general rule, solenoids have to be operated exclusively at rated voltage.

If a solenoid is not operated at its rated voltage, the technical data given do not apply. If the voltage is below the nominal voltage, the force values are not reached and the solenoid is not fully used. If the operating voltage is above the nominal voltage, the device delivers greater forces, but the coil is overloaded, the magnet overheats, and there is a danger that the device will be destroyed and may start to burn.

Taking into account a reduced duty cycle, a solenoid can be overexcited using a suitable control (holding current reduction). More information can be found in the data sheet Z KD H.

Does the direction of movement of a solenoid change when the polarity at the connectors is changed?

No.

If the polarity is changed, the direction of the electromagnetic flow in the solenoid changes, but the direction of movement of the armature remains the same, because the direction of the force effect in the air gap does not change.

This changes as soon as a permanent magnet is installed in the magnetic circuit. In this case we speak of a polarized magnet system. In the case of polarized solenoids, care must therefore be taken to ensure that the polarity of the connection is correct.

Which electrical power does the electromagnet need?

 

The required electrical power (W) is specified in the technical data sheet depending on the relative duty cycle. The corresponding current must be calculated from the nominal voltage and the nominal power.

What are the differences between proportional and ON/OFF solenoids?

 

On the outside there are no differences between proportional and ON/OFF  solenoids. The differences can be found in the shape of the characteristic curve, in the precision of the bearing and magnetic circuit as well as in the electrical control. While a proportional solenoid can be used to set up a position control system that allows the stroke points to be approached repeatedly and with repeat accuracy, a ON/OFF solenoid is designed to reach the end positions without intermediate positions.

Why can I not implement position control with a standard linear solenoid?

Linear solenoids of our standard program are usually ON/OFF solenoids with an increasing characteristic. This characteristic is not suitable for position control.

A ON/OFF solenoid is designed for its nominal voltage. The current flowing through the coil depends on the resistance of the coil, which in turn changes with the temperature of the coil. Therefore, the required repeat accuracy for position control is not guaranteed.

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