Special-Purpose Electric Machines

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Special-Purpose Electric Machines

• In addition to the types of machines we have
  studied so far, other types of special-purpose
  machines which operate on the same principle of
  previous machines (DC, synchronous, and induction
  machines) find widespread engineering
  applications in a variety of fields, including
• Robotics
• Vehicle Propulsion
• Aerospace
• Automotive Control

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Two-Pole Brushless DC Motor with Three-Phase
Stator Winding This is in the reality a
permanent-magnet synchronous machine. It is named
DC not because of its structure but due to the
fact that its operating characteristics resemble
those of a shunt DC motor with constant field
current. This characteristic can be obtained by
providing the motor with a power supply whose
electrical frequency is always identical to the
mechanical frequency of rotation of the rotor. To
generate a source of variable frequency, use is
made of DC-to-AC converters (inverters),
consisting of banks of transistors that are
switched on and off at a frequency corresponding
to the rotor speed. Accordingly, these motors
require measurement of the position of the rotor
to determine speed of rotation and to generate a
supply current at the same frequency. The source
of excitation is DC, although the current that
actually flows through the motor windings is AC.
Accordingly, this motor is a synchronous motor.
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Figure 18.1b
A Typical Bbrushless DC Servomotor
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Conventional DC Motors Brushless DC Motors

• Advantages
• Controllability over wide ranges of speed.
• Capability of rapid acceleration
• Convenient control of shaft speed and position.
• Disadvantage
• Commutation (through brushes) causing wear,
  electrical noise, and sparking.

• Advantages
• Controllability over wide ranges of speed.
• Capability of rapid acceleration
• Convenient control of shaft speed and position.
• No mechanical wear or sparking problem.
• Disadvantage
• Need for more complex power electronics.

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Stepping Motor Configurations Motors that convert
digital information to mechanical motion in
printers positioning the heads in magnetic disk
drives in computer disk drives Characteristics
Angle of rotation of the motor is proportional to
the number of input pulses rapid
response-starting-stopping-reversing command
self holding.
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Single-Phase Induction Motor Applications
Refrigerator compressors air conditioners fans
electric tools washer and dryer motors and
others.
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Figure 18.30
Split-Phase Motor Two separate stator windings,
called main and auxiliary windings. The axes of
the two windings are 90o with respect to each
other. The auxiliary winding current is designed
to be out of phase with the main winding current,
as a result of the different reactances of the
two windings. Because of this the motor sees a
two-phase (unbalanced) current that results in a
rotating magnetic field, as in a 3-phase
machines. Once the motor has started, a
centrifugal switch is used to disconnect the
auxiliary winding, since a single winding is
sufficient to sustain the motion of the rotor.
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Capacitor-Start Motor Another method for
obtaining a phase difference between two currents
that will give rise to a rotating magnetic field
is by the addition of a capacitor. Explain the
effect of the capacitor on the starting torque.
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Requirements to be Considered on Motors for a
specific Application

• Starting Characteristics
• Acceleration Characteristics
• Efficiency at Rated Load
• Overload Capability
• Electrical and Thermal Safety
• Cost!