What are actuating solenoids?
Neutral and polarised magnets
Neutral linear solenoids are characterised by the fact that they create their magnetic field exclusively via the current flow in the coil. In contrast to this, polarised linear solenoids feature one or more continuous magnets (permanent solenoids), which also create a magnetic field without electrical power. With the help of the coil current, the magnetic field of the permanent magnet is modified so that the desired effect is able to be achieved, e.g.
Linear solenoids and rotary solenoids
While linear solenoids are used to create linear actuating movements, rotary solenoids are used for rotary drive of a shaft. All explanations of linear solenoids apply to rotary solenoids accordingly.
Function of the actuating solenoid
What kind of activation processes of actuating solenoids exists?
Operation at the voltage source (normal operation)
Solenoids are normally operated with supply voltage so that the voltage tolerances or internal resistance of the voltage source and lines have an influence on the operating behaviour. In general, these are combined for tolerance-related nominal voltage. Note that the specified upper temperature limits of the coil must be maintained at maximum voltage (→ maximum power consumption) for functionality, but the projected power-force characteristic must be reached in case of minimal voltage and maximum coil temperature.
Heating and voltage tolerance lower the magnetic force of a solenoid significantly below the force at nominal conditions. The magnetic force of the solenoid now amounts to approx. 50% of the force at nominal conditions.
The magnetic force of the solenoid now amounts to approx. 50% of the force at nominal conditions.
The consequences are clear differences in terms of force, switching time, and activation noise, depending on the level of heat and operating voltage.
The forces and switching times indicated in the data sheets are achieved by magnets at operating temperature
DC voltage controller
The tolerance of operating voltage is clearly limited. For the residual tolerance of ± 2%, for example, and a medium coil temperature of 155°C, the force drops to approx. 60% under warm conditions, compared to nominal conditions.
DC current controller
This voltage is variable for the DC current controller. The maximum permitted current results from the thermal class and the quality of heat dissipation to the environment. The magnetic circuit is optimised to the calculated current.
This current can be applied in any operating condition. The supply-side always features a voltage reserve to compensate the increased resistance resulting from heating.
Advantages of current regulation:
In any case, the maximum coil resistance already included during specification of the nominal current, whereby the gained mechanical work is comparable with that of the DC voltage controller.
Shortened activation duration
If actuating solenoids are used in continuous operation, the option of operating with shortened activation duration is available. Due to a changed coil configuration, the nominal power is increased compared to the permanent operation. The advantage related to increased stroke work related to the design size.
High-speed excitation to shorten the attraction time t1
|During high-speed excitation the overall resistance is increased via upstream activation of hmic resistance to the magnet, and the time constant is also reduced. The operating voltage UB is clearly higher than the nominal voltage of the magnet UN. During activation (I=0) via high voltage (UB >>UN) with a simultaneously reduced electric time constant |
the magnetic coil is excited faster than without resistance. The activation delay time and attraction times are reduced accordingly. In stationary operation with I = IN = const., the operating behaviour and the mechanical work of the magnets correspond with the normal operation.
Note that the preliminary resistance constantly consumes a power loss of during stationary operation.
In case of over-excitation, the solenoid receives increased voltage during the attraction process to
Another form of over-excitation consists of initially applying the entire operating voltage to a partial coil (attraction coil (LM1)). After the end of the armature movement, this is connected in series with the so-called holding winding (LM2), so that the nominal current is adjusted
he difference of "over-excitation" to the operating mode “shortened activation duration” consists of the fact that the power is reduced to a thermally safe value after the attraction process. The solenoid is also able to be operated like a 100% activation duration device. The voltage is reduced either time-controlled or via limit position detection. Because the holding force is present at nominal power by way of the magnet configuration, it is sensible to utilise the over-excitation as far as possible so that the attraction force corresponds with nominal operation.
The increased attraction power causes additional heating during each attraction process, which is why a maximum number of activations Z (switching cycles per h) must be specified for this mode. Over-excitation enables the stroke work to be increased similarly well as by using shortened activation duration, however without the disadvantage of long shut-off phases. The over-excitation power is normally specified so that permanent operation is possible. The theoretical maximum switching frequency therefore results in
In case of increased over-excitation power, currentless pauses may be required to maintain the temperature thresholds, depending on the switching frequency. These are determined by application-specific tests. The statements regarding the influence of coil warming on solenoid current, force, and activation time apply analogously to over-excitation. Over-excitation can also be combined with current and voltage regulation to produce the advantages indicated above.