
The Lavet-type stepping motor has widespread use as a drive in
electro-mechanical
In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two system ...
clock
A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and t ...
s and is a special kind of single-phase
stepping motor
A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can be commanded to move and hold at one of these steps without any po ...
. Both analog and stepped-movement quartz clocks use the Lavet-type stepping motor (see
Quartz clock
Quartz clocks and quartz watches are timepieces that use an electronic oscillator regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks and watches are at least a ...
). Through miniaturization, it can be used in
wristwatch
A watch is a portable timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached ...
es and requires very little power, making a battery last for many years. The French engineer
Marius Lavet
Marius Lavet (February 8, 1894 in Clermont-Ferrand – February 14, 1980 in Paris) was a French engineer who invented in 1936 the principle of stepper motor. He studied at Supélec, the School of Electric Power, French private engineering school. ...
invented this kind of drive and described it in 1936 in his patent application FR823395.
Like other single-phase motors, the Lavet motor is only able to turn in one direction, which depends on the geometry of its
stator
The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric ...
. The
rotor
Rotor may refer to:
Science and technology
Engineering
*Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator
*Helicopter rotor, the rotary wing(s) of a rotorcraft ...
is a permanent
magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nic ...
. In a clock, a circuit generates the bipolar pulse train, which alternately delivers a positive and a negative voltage to the coil for short periods (providing a correct mechanical output to move a second hand). The motor can be built with a strong magnet and large stator to deliver high torque, but it is mostly built small, to drive the load through a low gear ratio.

The stator core looks a lot like one in a
shaded-pole motor and defines the rotational direction according to the position of holes, grooves, or shade windings through the stator. However, unlike a shaded-pole motor, the grooves are at backward positions, and the positions where the rotor settles after each cycle are well determined, which is not the case for induction motors in general, where slip and load affect the angle that the rotor turns each cycle.
Essential for the movement of the Lavet motor are the cogging points of the rotor, which differ depending on whether the stator coil is energized or unenergized. The cogging points with no current are caused by
reluctant force against a direct magnetic field, rather than retarding the propagation of an alternating magnetic flux, and in practice are the angles where the air volume between the poles of the magnetic rotor and the bulk of the stator is minimised.
Movement of the common two step Lavet motor:
: (a) currentless stator, north pole of rotor points to the upper left,
: (b) energized stator, rotor moves clockwise, and north pole points to the right afterwards,
: (c) after energization of the stator has declined, rotor moves further until north pole points downright,
: (d) stator energized in opposite direction, rotor moves clockwise, and north pole points to the left,
: (a') after energization of the stator has declined, rotor moves to its initial position (a).
To make a Lavet motor turn, the current through its stator coil must change direction each step (bipolar) followed by an interval without current while the rotor moves to its reluctant position.
Aside from clock drives, there are many variations of Lavet's concept. One example are types of
dashboard For business applications, see Dashboard (business).
A dashboard (also called dash, instrument panel (IP), or fascia) is a control panel set within the central console of a vehicle or small aircraft. Usually located directly ahead of the driv ...
instruments in cars.
[Hermann Winner, Stephan Hakuli, Gabriele Wolf: ]
Handbuch Fahrerassistenzsysteme: Grundlagen, Komponenten und Systeme für aktive Sicherheit und Komfort
'. Vieweg+Teubner, 2009, S. 337.
Animation

Animation of a Lavet-type stepping motor used in an analog quartz clock movement.
Literature
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References
{{Commonscat, Lavet Type Stepping Motors
Electric motors
Timekeeping components