The usual shape of an electric motor is that of a cylinder, whether it be fed with alternating or direct current. Its job is to receive electric power input and to convert this, as efficiently as possible, into mechanical output by applying a twisting force to a central shaft so as to produce rotary motion. In perhaps 99 percent of all such machines, the outer part of the motor, a hollow iron cylinder having current-carrying coils, is the stationary part (the stator), usually secured to a bedplate on the floor. The inner cylinder, generally known as the rotor, is mounted on a bearing at each end of the machine.
Our understanding of the working principles of an electric motor (as with electromagnetism in general) is nothing more than the accumulated experience of the best ways of making such machines. There is no fundamental change in the action of a motor if it is built in such a way that its structure looks exactly like a rotary machine that has been sliced open along a radial plane and unrolled. Instead of producing a twisting force (or torque) on a rotary shaft, a linear motor produces force between primary and secondary members (the stator and rotor of ordinary motors) in a straight line.
Linear motors have found many uses in applications where accuracy and speed are required under heavy loads, such as in riveting or punching machines. Unlike conventional electric motors, they have only one moving part and do not need gears, ball bearings, or similar mechanical devices that are prone to wear or breakdown. Linear motors are also found powering conveyor belts, textile loom shuttles, and sliding doors. A more unusual type of linear motor can be found in an electromagnetic pump, which uses a liquid metal conductor instead of a solid one.
Types of motor
There are many different types of rotating electric motor, including induction, synchronous, reluctance, commutator (both AC and DC), and hysteresis motors. Each type has its linear counterpart. The most popular rotating motor, the induction motor, uses AC power directly and requires no brushes or rubbing contacts between stationary and moving parts. The robust nature of the rotor, which usually consists of copper or aluminum bars cast into slots in a steel cylinder and joined together at each end of the cylinder by a thick bar of the same metal as the bars, has resulted in over 95 percent of the world’s electric drive power being of this type. For similar reasons, the linear induction motor has now also proved to be the most popular.
A glance at a linear motor shows at once that either its primary or secondary member must be extended or relative motion will separate the two very quickly. In the induction motor, it is less expensive to elongate the simpler cast secondary member, for it is cheaper to construct and does not require a supply of electric current. Even so, any substantial extension is expensive, and in 1903, the double-sided sandwich motor was developed, in which the secondary member was reduced to a single aluminum sheet.
This motor was rejected by industry on the grounds that because the magnetic field had to be driven through two air gaps plus the substantial thickness of nonmagnetic sheet that forms the secondary, its efficiency, power factor, power-to-weight ratio, and power-to-cost value were all bound to be much too low to be profitable. This belief continued until the late 1950s, when research at Manchester University, Britain, investigated some of the complex phenomena in linear motors, demonstrated how the motor’s shortcomings could be made less severe than previously thought possible, and showed that in a number of applications the shortcomings were greatly outweighed by the advantages of direct linear drive.
A less common type of linear motor is the tubular motor. It resembles a linear particle accelerator, consisting of a long tube surrounded by concentric coils. When the coils are fed with a polyphase supply, the result is a traveling electromagnetic wave. If a conductor is placed in the tube and the supply switched on, the conductor experiences a force and moves at the same velocity as the traveling wave. A powerful, fast-moving wave in a suitable tubular motor can be used as a missile launcher or some other kind of electromagnetic cannon.
