Anti-residual Springs

With low force applications plungers may hold in on residual flux. To prevent this anti-residual springs are available. The force / stroke characteristics will be modified when the springs are fitted.

AC, DC Solenoid Force / Stroke Curve

When a solenoid is fully open it has a large air gap. The reluctance of this air gap keeps the magnetic field small and the force correspondingly low. As the plunger closes, the reluctance falls and the magnetic field increases. For this reason, the force obtainable from a solenoid increases progressively as the plunger closes. The curves show the force exerted with the coil at full working temperature. The force exerted by a cold solenoid is always higher. Force variations with temperature are greatest on DC solenoids. Note: AC solenoids must be allowed to close completely otherwise excessive noise from buzzing caused by the AC frequency can be heard. DC solenoids have no such restriction.

Matching Solenoid to Load

The force / stroke curves give the nominal force that will be available from the plunger at any particular plunger position. There will also be a matching duty cycle which will be the force required by the solenoids load throughout the stroke. In some cases this may be for practical purposes constant, as, for example, when a solenoid lifts a dead weight.

Operate Time

At any point in the operating stroke the difference between the force available from the solenoid and the force required to drive the load will be the force available to accelerate the load and plunger.
This means, of course, that the more excess power there is available from the solenoid the faster the solenoid will operate. The closing time of the solenoid is approximately doubled as its mechanical load is increased from 70% of what it will pull to the maximum. For reasonably fast operation 25% excess power is advisable. As a general principle, the use of excessively large solenoids for the duty is not, however, good practice, as unabsorbed energy must be taken up on impact.
Capacitor discharge circuits can be used to provide very fast closing while keeping the power in the hold position to a reasonable value.