Application of Magnets

Author: E.E. Kimberly

Lifting magnets of large diameters suspended from cranes are widely used in the handling of steel stock and scrap iron. Magnets for this service are usually energized by direct current. The same type of magnet is used in removing tramp iron from grains before milling and from more valuable machine-tool cuttings of copper, brass, etc.

Many mechanical operations, such as opening and closing switches and valves, are performed by use of magnets. Alternating-current magnets have a more violent action than direct-current magnets and are generally less satisfactory. The action of an a-c magnet is more violent than that of a d-c magnet because the flux is almost as great in the open position of the magnet as in the closed position. The flux in a d-c magnet is determined largely by the armature air-gap, and so is relatively small when the armature is at the starting part of its closing stroke. The closing force is therefore small when the air-gap is large and increases as the stroke proceeds and closes the air-gap.

In the a-c magnet almost all of the impedance is reactance, which is low when the air-gap is large as in the beginning of the closing stroke. The low impedance permits a high inrush of current which magnetizes the core almost as strongly with the long air-gap as the reduced current does with a short air-gap at the end of the closing stroke. The average force then in the closure of an a-c magnet is a greater per cent of the maximum force than is that in a d-c magnet. In some applications in which alternating current only is available, but the violence of the a-c magnet is intolerable, a torque motor is used. The torque motor does not rotate far, but merely turns through a portion of one revolution to accomplish its purpose and then remains stationary though energized. In some cases, particularly on motor control panels, the alternating current is rectified (see Chapter 27) and direct-current magnets are then used. This method gives less violent action and much quieter operation.

It is not difficult to make a satisfactory d-c magnet for moving mechanical parts. An a-c magnet must, however, have a laminated magnetic circuit unless the core is made of special high-resistance iron and must also be fitted with a shading coil to reduce magnetic hum to a satisfactory minimum. These items of construction are not easily achieved in make-shift construction. It is therefore better to buy an a-c magnet for a special use on the open market from an experienced builder than to attempt to make one from materials at hand.