Pull of Magnets and Solenoids

Author: E.E. Kimberly

In order to make the most effective use of the ampere-turns on an electromagnet, it is necessary that a complete iron path be provided for the flux when the movable part or armature is in the closed position. Fig. 7-13 (a) shows the elements of a d-c magnet used in a d-c motor starter to close the contacts X and Y of a portion of the starter circuit. The force with which the armature is held may be expressed by the equation

(7-10)

in which F is the force, in pounds; B is the flux density at the pole face, in lines per square inch; and A is the area of the pole face, in square inches.

Fig. 7-13. Examples of One Type of Magnet and One Type of Solenoid

When the armature is in its open position, the force available to start its closure is less than the holding force after it is closed, because the reluctance of the air-gap reduces the flux that reaches the armature. The forces at the open and intermediate positions can be calculated only after a rather complicated flux-density plot has been made to ascertain how much flux actually reaches the armature. In practice the air-gap is made relatively small; and, if greater motion is desired, it is obtained by the mechanical advantage of a lever.

A solenoid is actually a coil of wire, but the term is frequently used to specify a type of electromagnet in which the air-gap is placed inside the coil where flux leakage can occur less readily. Fig. 7-13 (b) shows the elements of a solenoid. A solenoid has a longer effective movement than a magnet of the type in (a). When a solenoid is magnetized with alternating current from a constant-voltage source, the flux from the high current with the core at the open position is not wasted so much in leakage as in the type of magnet in (a). If equation (7-10) for the force F is to be used to calculate the force exerted by an a-c magnet or solenoid, B must be the root-mean-square value of the flux density.