398 Electrical Power Systems Technology
tors do not operate at undervoltage or overvoltage conditions.
Considerations for Mechanical (Motor) Loads
There are three basic types of mechanical (motor) loads connected
to electrical power systems. These are DC, single-phase AC, and three-
phase AC systems. DC motors are ordinarily used for special applications,
since they are more expensive than other types and require a DC power
source. Typically, they are used for small, portable applications and pow-
ered by batteries, or for industrial/commercial applications with AC con-
verted to DC by rectification systems. A major advantage of DC motors
is their ease of speed control. The shunt-wound DC motor can be used
for accurate speed control and good speed regulation. A disadvantage is
the increased maintenance caused by the brushes and commutator of the
machines. DC shunt-wound motors are used for variable speed drives on
printing presses, rolling mills, elevators, hoists, and automated industrial
machine tools.
Series-wound DC motors have a very high starting torque. Their
speed regulation is not as good as that of shunt-wound DC motors. The
series-wound motor also requires periodic maintenance because of the
brush/commutator assembly. Typical applications of series-wound DC
motors are automobile starters, traction motors for trains and electric bus-
es, and mobile equipment operated by batteries. Compound-wound DC
motors have very few applications today.
Single-phase AC motors are relatively inexpensive. Most types have
good starting torque and are easily provided 120-volt and 240-volt electri-
cal power. Disadvantages include maintenance problems due to centrifu-
gal switches, pulsating torque, and rather noisy operation. They are used
in fractional horsepower sizes (less than one horsepower) for residential,
commercial, and industrial applications. Some integral horsepower sizes
are available in capacitor-start types. Uses include machine cooling sys-
tem blowers, and clothes dryer motors.
Specialized applications for single-phase motors include:
- Shaded-pole motors used for portable fans, record players, dish-
washer pumps, and electric typewriters. They are low-cost and
small, but inefficient. - Single-phase synchronous motors used for clocks, appliance timers,
and recording instruments (compact disk players). They operate at a
constant speed.