Page 660 - Mechatronics with Experiments

646 MECHATRONICS

N S

B
N S
i

Motor action
Speed
v

N S

N B S
i v
V
Voltage
measurement
device
Generator action
FIGURE 8.26: DC motor operating principles: a current carrying conductor in a magnetic
field.

electrical power is converted to mechanical power. This is called the motor action.The
current in the coil is controlled by controlling the terminal voltage and is affected by the
resistive, inductive and back EMF voltages. The electrical circuit relationship is
d (t)
V (t) = R ⋅ i(t) + (8.175)
t
dt
di(t)
̇
= R ⋅ i(t) + L + k ⋅ (t) (8.176)
e
dt
where the d (t)∕dt is the induced voltage as a result of Faraday’s law of induction. It has
two components: the first is due to the self-inductance of coils, and the second is due to the
generator action of the motor.
Notice that when the rotor turns 90 degrees, the moment arm between the forces is
zero and no torque is generated even though each leg of the conductor has the same force.
In order to provide a constant torque independent of the rotor position, for a given magnetic
field strength and current, multiple rotor conductors are evenly distributed over the rotor
armature. In order to switch the current direction for a continuous torque direction, a pair of
brushes and commutators are used. Without the current switching commutation, the motor
would only oscillate as the torque direction would oscillate between clockwise and counter
clockwise direction for every 180 degree rotation. Consider the line connecting the two
brushes and the coils above and below that. At any given position, the current in one half
of the coil is in the opposite direction to the current in the other half of the coils.
Also shown in Figure 8.26 is the generator action of the same device. This is the
result of the Faraday’s induction principle which states that when a conductor is moved in
a magnetic field, a voltage is induced across it in proportion to the speed of motion and