Chapter 12 AC Motors

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Chapter 12 AC Motors
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Chapter 12 AC Motors
• Since AC motors rely on a constant
frequency supply, they are mainly used on
larger aircraft. Motors are generally classified
as follows:
• Large motors have an output of 3 KW
(kilowatt) or more and are normally threephase
machines.
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• Medium to small motors range from 3 KW
down to 50 W and are mostly single-phase
machines. Motors rated at less than 750 W
are referred to as fractional horsepower
(FHP) machines.
• Miniature motors are rated at less than 50
W and are used in instruments and
servomechanisms
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• On aircraft, these motors are either
induction or synchronous machines.
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1.1 Stator-Produced Rotating
Magnetic Field
• When a magnet is rotated within a
three-phase stator, a three-phase voltage
is produced. If this process is reversed (i.e.
by connecting the three-phase supply to a
three-phase stator), a rotating field is
produced, as shown in the following
diagram.
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Figure 12-1 Generation of A Rotating Magnetic Field
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• If a rotor is then placed in the centre of
the rotating magnetic field, a magnetic
field is induced in it, which locks onto the
rotating outer field and turns with it.
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12.2 Induction (Squirrel Cage) Motor
• The induction motor is one of the
most widely used types of AC motor,
which is used on aircraft to drive fuel
pumps, actuators, and air conditioning.
The following diagram shows a typical
machine.
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Figure 12-2 A Typical Induction Motor
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Figure 12-3 Principle of Induction Motor
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Figure 12-4 Torque Generated in Induction Motor
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• When the applied torque equals the load
torque, the motor runs at a speed slightly
less than the stator field. The induction
motor is an asynchronous machine and
possesses following characteristics :
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•  Slip speed is the difference between the
rotor speed and the synchronous (stator)
speed.
• Slip Speed = Synchronous Speed - Rotor
Speed
• Synchronous Speed = 60f/P
• Where f = frequency of supply(Hz), and P =
number of pole pairs in stator
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•  Reversal of rotation occurs if any two of the
motor phases are crossed over.
•  Loss of a phase occurs when the machine is:
• ● Running
• The motor continues to run at a reduced torque.
• ● Not running
• The machine does not start, and fuses or circuit
breakers blow in the other two phases, causing
possible damage to the motor.
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12.3 Two-Phase Induction Motor
• A rotating magnetic field is produced in a twophase
induction motor stator by placing the
windings 90° apart, as shown below.
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12.3 Two-Phase Induction Motor (continue)
• One phase is the reference phase, and the
other is the control phase. By varying the
phasing and the amplitude of the control
phase currents, the direction and speed of
rotation can be controlled. This type of
motor is, however, not as smooth nor as
powerful as a three-phase machine and is
used mainly for autopilot servomotors or
fuel trim motors.
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12.4 Split-Phase Motor
• This is a split-phase induction motor. Two windings,
one capacitive and the other resistive, are both
connected in parallel across a single-phase AC
supply, as shown below.
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12.4 Split-Phase Motor (continue)
• The current in the capacitive winding leads
the current in the resistive winding by
approximately 90° and is known as phase
splitting. This type of motor operates like a
two-phase AC motor and is used to drive
actuators.
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12.5 The Synchronous Motor
• The stator in this type of motor is identical to
that used in an induction motor, except the
rotor in this machine alternatively carries its
own magnetic field windings, which are
supplied from a DC source.
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• The stator is fed with three-phase AC and
produces a rotating magnetic field, which
the rotor follows. This type of motor is a
single speed machine, where the actual
speed is determined by the speed of the
rotating field (i.e. the frequency of the
three-phase input).
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• Synchronous motors are used in
situations where a constant speed is
essential (e.g. gyroscopes).
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END OF CHAPTER 12