Electromagnets

Definition

An electromagnet is the general term for all types of electromagnetic actuators.

The most important information in brief

• The electromagnetic principle is particularly suitable for realising small strokes, fast, with great force and little effort in terms of actuation and mechanical integration.
   
• When a current flows through a coil, electromagnets generate a magnetic field that is guided by iron parts to an air gap, where magnetic poles with attractive force, the so-called magnetic force, are formed.
   
• The magnetic force is variable by controlling the coil current and is used for linear or rotary movements as well as for holding or braking components, but in particular often as a drive for solenoid valves.
   
• Important mechanical parameters are magnetic force, lifting force, holding force and lifting work. The force-stroke characteristic can be falling, horizontal or rising.
   
• Electrical parameters of electromagnets are nominal voltage or limit current, duty cycle, rated current, test current and nominal power.

How do electromagnets work?
 

In principle, electromagnets are devices that generate a magnetic field by means of a current-carrying coil and guide it through a suitable iron part to an air gap. Magnetic poles form there, between which a magnetic attraction, the magnetic force, prevails. If there is no current at the coil, no electromagnetic force is generated; if the coil current is regulated, the magnetic force is adjustable. Depending on the design of the iron parts, the magnetic force is used to perform linear and rotational movements or to generate holding forces on components, thereby braking or fixing them.

If the coil is energized, an electrical current flows in dependence 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 cost.

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.

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.

The magnetic force F_M 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_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 occurs outside, taking into account 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. 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.

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.

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. 

The Rated work W_N 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 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.

For the rated voltage U_N, 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) .

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 which the magnetic force values specified in the data sheets refer to.

It is calculated as follows:

    I_Pr = 0,9 U_N / R_W

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 called 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 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 are 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
t_ein turn-on instant

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 ϑ_23.

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

    t_S  =  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“.

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

• DC Solenoids

and

• 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 following 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 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 current control instead of 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 influence of the temperature-dependent resistance change 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 counterforce's extent

Applications which call for special positioning accuracy are equipped with position sensors, making it possible for a control loop to be set up with which reaching and holding certain positions can be controlled precisely.

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 in order to return the solenoid to its initial position, e.g. spring force, weight force, etc.

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

Reverse solenoids (without position zero) 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 of them.

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

As a basic rule and analogically to linear solenoids, a reversible rotary solenoid version with 2 rotary solenoids working against each other can be put into practice.

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

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

Holding magnets create a magnetic field throughout a power supply connection.

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 coil additionally installed.

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