Mechanical Systems 363
permanent magnets or electromagnetic windings, surrounds the armature
assembly. Since the magnetic field remains relatively constant, the genera-
tor voltage output depends upon the ratio of the number of motor wind-
ings to the number of generator windings. For instance, if there are twice
as many generator windings as motor windings, the generated DC-volt-
age output will be twice the value of the DC voltage that is input to the
motor section of the dynamotor.Brushless DC Motors
The use of transistors has resulted in the development of brush-
less DC motors, which have neither brushes nor commutator assemblies.
Instead, they make use of solid state switching circuits. The major prob-
lem with most DC motors is the low reliability of the commutator/brush
assembly. The brushes have a limited life and cause the commutator to
wear. This wearing produces brush dust, which can cause other mainte-
nance problems.
Although some brushless DC motors use other methods, the transis-
tor-switched motor is the most common (see Figure 14-10). The motor it-
self is actually a single-phase, AC, permanent-capacitor, induction motor,
with a center-tapped main winding. Transistors, operated by an oscillator
circuit, conduct alternately through the paths of the main winding. The
oscillator circuit requires a feedback winding wound into the stator slots,
in order to generate a control voltage to determine the frequency. A capaci-
tor (C2) is placed across the main winding to reduce voltage peaks and to
keep the frequency of the circuit at a constant value.
The main disadvantage of this motor is its inability to develop a very
high-starting torque. As a result, it is suitable only for driving very low-
torque loads. When used in a low-voltage system, this motor is not very
efficient. Also, since only half of the main winding is in use at any instant,Figure 14-9. Dynamotor construction