Basic Stepper Motor Layout
One of the main components of the quartz watch movement, the stepper motor consists of a permanent magnet rotor, a magnetic stator, and a coil. In the above diagram, there is a dotted line to show the magnetic field on the stator. The stator will carry the magnetic field in a loop inside itself, like a wire carries electricity. Instead of carrying an electrical current, the stator acts as a magnetic circuit.
Since there is an air gap in the circuit, the two ends of the stator act as magnetic poles. The ends of the stator is called the pole faces. On a quartz watch movement, the stator curves around to meet the rotor. The rotor has a permanent magnet built into it.
The Operation of a Stepper Motor in a Quartz Watch
In this picture, the coil has current flowing in one direction, which is producing a magnetic field in the stator. The pole faces of the stator have a north pole on the left face and a south pole on the right face. I showed the poles in red to highlight that they are being changed by the current flowing in the coil which is also shown in red.
As stated before, the rotor is a permanent magnet with its own north and south pole. The north pole of the rotor is attracted to the south pole of the stator and the south pole of the rotor is attracted to the north pole of the stator. Therefore, the rotor takes up the position shown.
Now let us reverse the current in the coil. This will reverse the magnetic fields in the stator, so that the north and south poles of the pole faces will be reversed. The magnetic poles of the rotor now face like poles on the stator (i.e., the north pole of the rotor is facing the north pole of the stator. As you know, like poles repel one another, so the rotor will spin around due to this fact. As the rotor starts to spin, its poles become attracted to the opposite poles of the stator.
Current pulses of Stepper Motor
Here is a key point. The poles of the magnet in the rotor are attracted to the pole faces on the stator even when no current is flowing through the coil. This means that the rotor is stable when at rest and resists any tendency to turn. Therefore, it is not necessary to keep the current flowing to the coil. All that stepper motor requires is a brief impulse of current, just long enough to drive the rotor in the opposite position. The current can now be switched off and the rotor remains in its new position.
In most watches the current pulse is very brief, just a few milliseconds. Maintaining the pulse any longer that this draws current unnecessarily and reduces the battery life.
The Problem with Turning 180˚
At this point it should be clear that each time the current is reversed in the coil, the rotor spins through 180 degrees. By adding a pinion to the rotor, we can now connect to other to a train of wheels. Unfortunately, the problem that we are left with is this. How do we control the direction the rotor turns?
Controlling the Rotor Direction
As shown previously, the rotor could just as well turn clockwise or counterclockwise. For the purposes of running a watch this does not work. If we could somehow make the rotor turn in a circle, our problem would be solved. This is done by arranging the rotor so that it sits slightly rotated one way. This is the secret to making sure the rotor only turns in one direction.
Offset Rest Position of Rotor
In this picture you can see that the stator is made to curve in towards the rotor. You will also recall that the stator is made from magnetic material so that the rotor is attracted to it. By shaping the pole face, like in the picture, the smallest gap between the rotor and the pole face is several degrees clockwise from horizontal. The rotor will rest with its pole faces several degrees clockwise from horizontal. This gives us the offset we need to ensure the rotor always turns clockwise. Like before, I have shown the induced magnetic poles in brackets.
How a stepper motor works in a quartz watch
Now let us look at the entire sequence of how the stepper motor works in a quartz watch.
With the motor at rest the rotor poles are at the closest points of the stator pole faces.
The “Forward” current in the coil causes the rotor to turn clockwise.
As the rotor reaches its new position, it is held in place by the magnetic field in the coil.
As the current pulse finishes, the rotor rotates a little further to its “Rest” position. The rotor poles are adjacent to the closest points of the stator pole faces.
Now the “Reverse current in the coil causes the rotor to turn clockwise again.
The rotor reaches a new position and is held in place by the magnetic field in the coil.
The current pulse in the coil finishes, the rotor rotates a little further to its rest position. The rotor poles are adjacent to the closest points of the stator pole faces.
In conclusion, the two current pulses, one in each direction, turns the motor clockwise 360˚.
In my next article, I will describe the function of the quartz oscillator.