Varying the current applied makes it possible to use a smaller solenoid. To prevent the winding from overheating the current is limited by a resistor after the plunger reaches the end of the stroke. This circuit cannot be used with high operating frequencies. The size of the dropping resistor depends on the resistance of the winding.
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There are three ways of damping solenoids:
12.1. Damping by Ohmic resistance
A parallel resistor can be used to limit the voltage surge that occurs when the power to the solenoid is cut off. As a result, however, the drop-out time increases as does the power requirement. Both are reduced as the parallel resistance is reduced.
Suitable circuitry will influence the operating times and service life of the solenoid. In an AC circuit the over-voltage at cut-off is fully damped however this severely delays the drop-out time. In
Protection can be divided into three classes:
If solenoids are to operate reliably, they must be provided with suitable power; the following are a few terms to aid in understanding:
9.1 Nominal voltage
The nominal voltage is that with which a solenoid is normally operated, the tolerance is +5% to -9%.
When selecting a suitable solenoid, temperature must be considered.
8.1. Ambient temperature
The ambient temperature is the temperature (°C) surrounding the solenoid when it is operating. If the range is outside +40°C to -50°C design changes may be required.
The use of solenoids necessitates a certain chronological sequence best clarified with the following terms:
7.1. Power-off pause
The power-off pause (in seconds) is the time between switching off the current and switching it on again.
The stroke is the usable distance travelled by the plunger from its initial position to the end of travel. As the stroke is increased the force is reduced and vice versa.
6.1. Start of Stroke
This is the position of the plunger before it starts its travel. It is also the position when it returns upon conclusion of the complete cycle.
Usually, the correct solenoid for a given application is the smallest one that has adequate magnetic force.
Unlike DC solenoids, AC solenoids provide relatively high operating/cycling frequencies and low cycling times; this results in hard operation which influences service life. Power consumption depends on the position of the plunger.
High cycling frequencies can cause dangerously high thermal loading in AC solenoids and the maximum permissible temperature is therefore the limiting factor for cycling frequency.
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