Transcript Capacitor-Start Motor - School of Electrical Engineering and

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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A Typical Bbrushless DC Servomotor
Figure 18.1b
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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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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.
Figure
18.30
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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
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Starting Characteristics
Acceleration Characteristics
Efficiency at Rated Load
Overload Capability
Electrical and Thermal Safety
Cost!
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