AC Motor
AC Motor
After the
introduction of the DC electrical distribution system by Edison in the United
States,
a gradual
transition to the more economical AC system commenced. Lighting worked as well
on AC as on DC.
Transmission of electrical energy covered longer distances at lower loss with
alternating
current. However, motors were a problem with alternating current. Initially, AC
motors were
constructed like DC motors. Numerous problems were encountered due to changing
magnetic fields, as
compared to the static fields in DC motor motor field coils.
Charles P.
Steinmetz contributed to solving these problems with his investigation of
hysteresis
losses in iron
armatures. Nikola Tesla envisioned an entirely new type of motor when
he visualized a
spinning turbine, not spun by water or steam, but by a rotating magnetic field.
His new type of
motor, the AC induction motor, is the workhorse of industry to this day. Its
ruggedness and
simplicity (Figure13.1)
make for long life, high reliability, and low maintenance.
Yet small brushed
AC motors, similar to the DC variety, persist in small appliances
along with small
Tesla induction motors. Above one horsepower (750 W), the Tesla motor
reigns supreme.Modern solid state
electronic circuits drive brushless DC motors with AC waveforms
generatedfrom a DC source.
The brushless DC motor, actually an AC motor, is replacing theconventional
brushed DC motor in many applications. And, the stepper motor, a digital
versionof motor, is driven
by alternating current square waves, again, generated by solid statecircuitry Figure13.2 shows
the family tree of the AC motors described in this chapter.
Cruise ships and
other large vessels replace reduction geared drive shafts with large
multimegawattgenerators and
motors. Such has been the case with diesel-electric locomotives on a
smaller scale for many years.At the system level,
a motor takes in electrical energy in terms of a potentialdifference and a
current flow, converting it to mechanical work. Alas, electric motors are not100% efficient. Some
of the electric energy is lost to heat, another form of energy, due to IRlosses in the
motor windings. The heat is an undesired byproduct of the conversion. It mustbe removed from the
motor and may adversely affect longevity. Thus, one goal is to maximize
motor efficiency,
reducing the heat loss. AC motors also have some losses not encountered by
DC motors:
hysteresis and eddy currents.
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