Actuators - Introduction to Biomechatronics - page 100

FIGURE 3-8
Application of
Faraday's law for a
single-loop coil.
If current continues to flow into the coil, it will rotate about the axis O - O' by 90 ◦
until the coil is orientated vertically and then will stop because the forces in the conductors
will pass through the rotation axis and the turning moment will have reduced to zero.
If the current was reversed and the coil was just past the equilibrium point, then it
would rotate by a further 180 ◦ and then stop again. To allow for continuous motion, some
form of switching or commutation is required to automatically reverse the direction of the
current at the correct angle. In brushed motors, these are performed using a mechanical
method, while in brushless motors the position of the coil is sensed electronically using a
Hall switch (see Chapter 2), and commutation also occurs electronically.
3.2.2.1 Single-Coil DC Motor
Mechanical commutation using brushes is shown in Figure 3-9. It can be seen that a pair
of brushes remain in contact with the coil over angles from 0 ◦ to 90 ◦ . As with the previous
figure, the force on the segment AB is downward, and the force on segment CD is upward.
This produces a torque in the clockwise direction. There is a short period in the region
around 90 ◦ where the brushes are disconnected and the motor must rely on its momentum
to carry it past this region. The sense of the current into the coil is now reversed with the
force on CD being downward and that on AB being upward, which continues to produce
a small torque in the clockwise direction. As the angle increases, the torque reaches a
maximum at 180 ◦ before decreasing to zero at 270 ◦ , where the commutation switches
again.
This crude motor is impractical for a number of reasons, the most important of which
is the large variation in the torque experienced, as shown graphically in Figure 3-10.
3.2.2.2 Multiple-Coil Direct Current Motor
If a second coil with its commutation mechanics is added at 90 ◦ to the single coil, the
amount of ripple will be reduced significantly, as can be seen in Figure 3-11. In real motors,
the number of coils is generally increased well beyond two coil segments with the result
that there is very little torque ripple.
A second problem with these motors is the fact that they rely on a single turn, with
the result that the amount of torque generated is limited. Real motors have multiturn coils
to maximize the torque output.