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.

If the coil is energized, an electrical current flows subject to 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 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.

The espansion of the magnetic field very much depends on the material and its shape through which the magnetic field is flowing.

Air is comparatively highly resistant against the magnetic field, while 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, infact, the magnetic field is weakened only by a very small extent due to the good 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 lead outside the electromagnet via an armature rod through a hole inside the core.

In electromagnets, magnetic force and stroke are decisive variables for the selection and are mutually dependent.
 

The picture on the left shows a sectional view of a solenoid and above it 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 estimated using modern calculation tools (finite element method). The fine-tuning is still carried out by adjusting the geometry of sample devices.

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

    Φ_N = Useful flux (Wb)

    F_R = Friction force (N)

    F_M = Magnetic force (N)

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

The Stroke force F_H is the magnetic force which acts on the outside under consideration of the associated component of the armature weight F_A.

                  F_A = Weight force of the armature (N)

                  m_A= Armature mass (kg)

                  g = 9,81 m/s²

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

    II. horizontal characteristic curve

    III. increasing characteristic curve

The most common 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, application areas are where high friction forces are to be worked against.

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

Put simply, the work corresponds to the area below the magnetic force-stroke curve. 

The Rated work W_N, which is specified in the data sheets, is calculated as the product of the magnetic force F_M in the initial stroke position s₁ and the magnetic stroke s. 

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

The Rated voltage U_N is the voltage for which a voltage device is designed and with which it is marked. Unless otherwise stated, the values given in the data sheets are based on a rated voltage of 24 V. The voltage rating U_N is the voltage for which a voltage device is designed and with which it is marked. With other nominal voltages, deviations from the specified magnetic forces can occur both upwards (usually at > 24 V) and downwards (usually at < 24 V) due to the different insulation components in the exciter windings.

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

The Rated current I_B 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.

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

It is calculated as follows:

    I_Pr = 0,9 U_N / R_W

where 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.

The Rated power P_N 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 named P₂₀. Unless otherwise stated, the rated voltage is 24V.

The electrical resistance of the solenoid coil means that an electromagnet heats up as soon as voltage is applied to it. 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 and 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.

When designing the windings of electromagnets at the factory, care is taken 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
t_ein turn-on instant

Solenoids are designed for different operating modes depending on requirements.

In continuous operation (S1 or 100% ED), the duty cycle is so long that the steady-state temperature ϑ₂₃ is practically reached.

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

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

    %ED = duty cycle / cycle time * 100

    t₅  =  duty cycle

    t₆  =  de- energized period

    t_S  =  cycle time 

The force, power and time values given in 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 given in the table on the left.

In short-time operation (S2), the duty cycle is so short that the steady-state temperature is not reached. The currentless pause 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“.

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

• DC Solenoids

und

• AC Solenoids.

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.

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

• Single acting solenoids, they perform linear movements.

• Shotbolt Locking Units as a special form of single acting solenoids, the armature rod is particularly stable, therefore transverse forces can be absorbed.

• Rotary solenoids for rotary movements over a limited angle (max. 110°)

•  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.

Unless otherwise stated, solenoids are ON/OFF actuators. If the force of an solenoid is to be controlled, it is executed as a proportional solenoid. This subdivision is primarily to be found in hydraulics. But also pneumatic valves are partly operated with proportional solenoids.

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

The usually increasing characteristic curve is well suited for working against a spring, as this also has an increasing characteristic.

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. 

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.

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.

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.

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