# Electromagnetism and the Lavet Motor

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Electromagnetism and the Lavet Motor
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Electromagnetism Electromagnetism refers to the physics of an electromagnetic field. This is a field which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles.

Electromagnetism, one step simplified
In other words, a magnetic field is produced by the motion of electric charge (ie: current). This is the same kind of force associated with magnets. The reverse of this statement is also true: An electric field is produced by the motion of a magnetic field.

Electromagnetism, on more step simplified
If you coil a wire and pass a current through it, you create a magnet. If you pass a magnet through a coil of wire, you create an electric current.

Thumbs up! – Right hand rule #1
Everyone make a “thumbs up” with your right hand and point the thumb towards your face. As you pass electric current in the direction of your thumb, it causes a magnetic field in the direction of your fingers. This is called the right hand rule #1.

Thumbs up! – Right hand rule #2
Everyone make a “thumbs up” with your right hand and point the thumb towards your face. See your fingers as a coil of wires. As you pass electric current from your pinky finger to your pointer, it causes a magnetic field in the direction of your thumb. This is called the right hand rule #2.

Thumbs up! – Right hand rule #2
Now everyone make a “thumbs up” with your left hand and butt your hands together. The direction of the magnetic field is dependant upon the direction current flows through the wires of the coil. This is the principal upon which the Lavet motor is driven.

Lavet Motor Construction
The Lavet motor is made up of 3 components: 1. Coil 2. Stator 3. Rotor

Coil The coil is made of a single copper, silver and/or gold wire ~ mm in diameter wrapped around a soft iron core (FeNi). The coil has 2 purposes: 1. To produce a magnetic field to drive the rotor when the motor control supplies a current. 2. To create a voltage induced by the changing magnetic field caused by the rotation of the rotor. Core Wire

Stator The stator is constructed of a soft iron core (FeNi)
The stator has 2 purposes: 1. To complete the magnetic circuit generated by the coil. The majority of the magnetic field generated by the coil is directed through the stator, positioning it to work against the rotor in a specific manner. 2. To bring the rotor into a specific alignment after each rotation. Stator Air Gaps

Rotor The rotor is constructed of a strong permanent magnet glued to a steel pinion, arbor and pivots. The rotor has 2 purposes: 1. To convert the magnetic force generated in the coil into a rotational torque and apply it to the gear train via its pinion. 2. To hold the gear train in place between operations of the motor. Pinion Pinion Pivots Arbor Arbor Magnet

How the Lavet motor works
In a quartz watch, the hands and gear train stays still the vast majority of the time. Therefore we can see that the motor is in operation for only this short portion of time each second. Within this short period of time, all the operations of the Lavet motor occur. Lets take a step by step look at the Lavet motor’s operation, starting at rest.

Step 1 - Rest At rest, the rotor is aligned with the stator according to the stator’s maximum magnetic attraction. A motor’s greatest point of attraction is called a “cogging point”. The shape of the stator, including the air gaps, are responsible for determining this positioning of the rotor.

Step 2 - Impulse When it is time for the motor to operate, the Motor Control sends a brief electrical pulse to the coil. The current flows through the coil, creating a magnetic field. The field flows through the stator and reacts with the field of the rotor. This causes the rotor to rotate, the direction of which is determined by the geometry of the stator. The impulse from the coil is only as big as necessary to drive the rotor past the air gaps. S N S N + -

Step 3 – Aligning with the cogging point.
Once impulse is over, the magnetic field from the coil collapses. The magnetic field from the rotor is now dominant and attempts to align itself with the point of greatest attraction. This force of attraction and the inertia of the rotor keep it rotating towards the next cogging point. The force of attraction increases the closer it aligns with the cogging point, causing the rotor to accelerate.

Step 4 – Coming to rest at the cogging point.
When the rotor reaches the cogging point, it is rotating with enough momentum to overshoot it. As the rotor overshoots, the attraction slows down the rotor and reverses it’s direction back to the cogging point. This repeats for several decaying oscillations until the rotor comes to a rest.

Step 5 - Rest At this point, the rotor has rotated 180˚. In order to operate the motor again, it will need another impulse. Earlier we sent out current left to right with our (+) voltage on the left and (-) on the right. In order to reverse the direction of our magnetic field we will have to reverse the direction of our current flow by reversing the polarity of our voltage.

Step 6 - Impulse For the next impulse, the Motor Control sends a brief electrical pulse to the coil. This time, the voltage and current direction is reversed (right to left). The exact same order of operations happens again. Note: All polarities are the opposite of what they were the last time, yet the direction of rotation is the same (counterclockwise). N S - +

Step 7 – Aligning with the cogging point.
Again, the magnetic field from the coil collapses. The magnetic field from the rotor is now dominant and attempts to align itself with the point of greatest attraction. This force of attraction and the inertia of the rotor keep it rotating towards the next cogging point. The force of attraction increases the closer it aligns with the cogging point, causing the rotor to accelerate.

Step 8 – Coming to rest at the cogging point.
When the rotor reaches the cogging point, it is rotating with enough momentum to overshoot it. As the rotor overshoots, the attraction slows down the rotor and reverses it’s direction back to the cogging point. This repeats for several decaying oscillations until the rotor comes to a rest.

Completed The rotor has now completed one revolution and the set of 8 steps are ready to repeat. Lets take a look at how the complete revolution happens in rapid succession.

Complete Rotor Rotation
Step 1 - Rest Step 5 - Rest Step 2 - Impulse Step 6 - Impulse Step 3 - Aligning with the cogging point Step 7 - Aligning with the cogging point Step 4 - Coming to rest at the cogging point Step 8 - Coming to rest at the cogging point N S S N + - + -

Lavet Motor in Conclusion
Each 180˚ rotation of the rotor occurs during each “tick” of the seconds hand. The rotor rotates at a rate of 30 rpm. The Lavet motor has gained favor within wrist watches due to its relatively simple design and high efficiency.