The on-off solenoid
Designs
The principle of the on-off solenoid is to move the armature from a starting position into a final position by means of magnetic force. The final position is defined by the design; the armature strikes the pole.
Such single-acting solenoids are reset by means of external forces, for example by a weight or a spring.
The double-acting solenoid comprises two single-acting solenoids. The armature is moved in one or the other direction, depending on which exciter winding is activated. When the coil is de-energized, it is fixed in the central position by the resetting forces.
The design of the return stroke solenoid is similar to the previous design, but here the armature can move from one stroke end position to the opposite one, depending on the activated exciter winding.
Forces and characteristic curves
The available mechanical force is referred to as the magnetic force.
The holding force is the magnetic force obtained in the stroke end position. This can be influenced by inserting a non-magnetic anti-sticking disk between armature and pole. The holding force is comparable to the adhesive force of a holding solenoid.
The reset force returns the armature to its initial position; to do so, it must be greater than the sum of adhesive force, friction and possibly armature weight.
The possible stroke is calculated through integration of the magnetic force across the stroke, from the stroke start to the stroke end position.
The rated current relates to the rated voltage and a coil temperature of 20 degrees Celsius. This definition is necessary, since the ohmic resistance is temperature-dependent. The product of current and voltage value specifies the electric power currently absorbed by the solenoid.
A power-saving circuit uses the effect that the holding force is several times the magnetic force. When the rated voltage is applied to the solenoid, the armature moves into the stroke end position, where the power can be considerably reduced.
The solenoid is generally cooled by means of the connection with the higher-level system or alternatively – although not as satisfactory – by means of the ambient air.
The solenoid coils heat up during operation, and temperature limits must therefore be observed. The heat can be dissipated from the coil by means of design measures or coolant.
Generally, the devices comprise materials in insulating material class F, so that a limit temperature of 155 degrees Celsius is permissible.
A working cycle denotes a complete switching on and off process, the number of switching operations specifies the number of working cycles and the switching frequency the number of switching operations per hour.
The on-period is first of all defined as the time between switching the exciter current on and off, followed by the idle interval and ending with switching on again.
The sum of both times is called the cycle duration.
The relative on-period is the ratio of on-period to cycle duration and is specified in percent.
In short-time operation the on-period is not sufficient to reach the steady-state temperature. The device constantly cools down to the reference temperature. The manufacturer can select the smallest possible solenoid for the user, on the basis of the relative on-period.
Application example 1
On-off solenoid for active steering
This is a lifting/holding solenoid, which overexcites when the car engine starts ...
Application example 2
On-off solenoid for brake assistant and ESP
Many drivers do not use the maximum braking effect in dangerous situations...
Application example 3
On-off solenoid for transfer case
This on-off solenoid is a constituent of a transfer case drive, in addition to the DC motor ...
Application example 4
Twin solenoid for mining
The times when coal seams were mined with a pickaxe are long gone. Likewise ...
