Reciprocating electric motor
A motor with back-and-forth armature movement, now rare.
A reciprocating electric motor moves its armature back and forth instead of rotating it. Early examples, like those built by Daniel Davis in the 1840s, were among the first electric motors. Although uncommon today, they still appear in specialized uses such as linear compressors for cryogenics and as educational toys.
Daniel Davis was an early manufacturer of these motors. Many early designs copied the layout of contemporary steam engines, swapping the piston and cylinder for an electromagnetic solenoid.
The motor uses an alternating magnetic field to push and pull the armature. A single field coil can be placed at one end of the armature’s travel, or a coil can be used at each end. If the armature is a permanent magnet, the coils can both attract and repel it. With two coils, they are wound so their like poles face each other; for example, when both poles facing the armature are negative, one attracts the armature’s south pole while the other repels its north pole. When the armature reaches its limit, the coil polarity reverses. Alternatively, the armature can be made of ferromagnetic material, like in a solenoid. In that case, the coils simply switch on and off instead of reversing polarity. A single-coil motor with a non-magnetic armature needs a spring or other return mechanism to pull the armature back after attraction—the same principle used in an electromechanical buzzer. A dual-coil motor energizes each coil in turn. If the motor is adapted for rotary motion, the return mechanism includes a crankshaft and flywheel.
This motor is extremely simple, making it easy to build demonstration models for teaching. As a practical motor, it has drawbacks. Magnetic field strength drops quickly with distance, and the gap between armature and coil necessarily grows large, reducing power and starting force. Vibration is also a problem.
Applications include linear compressors (such as a design from the Cryogenic Engineering Group at the University of Oxford), plunger pumps, some electric shavers, and toys. Educational toy versions can be DIY projects, and some have been patented, for instance in 1929 and 1963.
- Early maker
- Daniel Davis
- Decade of early examples
- 1840s
- Type of movement
- back and forth
- Common early layout
- followed the general layout of steam engines, replacing piston-and-cylinder with an electromagnetic solenoid
- Example application
- linear compressors for cryogenics
- Example educational toy patent years
- 1929, 1963
Lore & Background
Daniel Davis was an early maker of reciprocating electric motors. As can be seen in these examples, early motors of this type often followed the general layout of the steam engines of the day, simply replacing the piston-and-cylinder with an electromagnetic solenoid. A reciprocating electric motor uses an alternating magnetic field to move its armature back and forth, rather than circularly as in a conventional electric motor. A single field coil may be placed at one end of the armature's possible movement, or a field coil may be used at each end. The armature may be a permanent magnet, in which case the coil or coils can exert both repulsive and attractive force on the armature. If there are two coils, they will be wound and connected so that their like poles face each other, so that when (for example) the poles facing the armature are both negative, one pole will attract the armature's south pole while the other will repel its north pole. When the armature reaches the extreme of its movement, polarity to the coils is reversed. The armature may instead be made of ferromagnetic material, as in an electromagnetic solenoid. In this case the current in the coils will alternate between on and off, rather than between polarities. A single-coil motor with a non-magnetic armature would require a spring or some other 'return' mechanism to move the armature away from the coil upon completion of the 'attract' cycle. An 'interrupter'-style electromechanical buzzer operates on this same principle. A dual-coil motor would alternately energize the two coils. Where the motor is adapted to produce rotary motion, the return mechanism consists of a crankshaft and flywheel.
Reader's Guide
The reciprocating electric motor is notable primarily as an early form of electric motor, exemplified by the work of Daniel Davis in the 1840s. Its design directly mirrored contemporary steam engines, substituting an electromagnetic solenoid for the piston-and-cylinder. Although now rare, it persists in niche applications such as linear compressors for cryogenics, with a design produced by the Cryogenic Engineering Group at the University of Oxford. It also appears in some electric shavers and as educational toys, some of which were patented in 1929 and 1963. The motor is extremely simple, making it suitable for demonstration models for teaching. However, it has several practical disadvantages: magnetic field strength drops off rapidly with increasing distance, which reduces output power and starting force, and vibration is an issue. Its legacy lies in illustrating an early conceptual bridge between steam power and electromagnetism, and in serving specialized linear-motion roles where rotary conversion is unnecessary.
Did You Know?
- Early reciprocating electric motors often followed the layout of steam engines, replacing the piston-and-cylinder with an electromagnetic solenoid.
- A single-coil reciprocating motor with a non-magnetic armature requires a spring or other return mechanism.
- Some electric shavers use reciprocating motors.
- Educational toy versions of this motor were patented in 1929 and 1963.
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