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 three-
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
On aircraft, these motors are either
induction or synchronous machines.
1.1 Stator-Produced Rotating
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
Figure 12-1 Generation of A Rotating Magnetic Field
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.
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
Figure 12-2 A Typical Induction Motor
Figure 12-3 Principle of Induction Motor
Figure 12-4 Torque Generated in Induction Motor
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
Slip speed is the difference between the
rotor speed and the synchronous (stator)
Slip Speed = Synchronous Speed - Rotor
Synchronous Speed = 60f/P
Where f = frequency of supply(Hz), and
P = number of pole pairs in stator
Reversal of rotation occurs if any two of the
motor phases are crossed over.
Loss of a phase occurs when the machine is:
The motor continues to run at a reduced
● Not running
The machine does not start, and fuses or circuit
breakers blow in the other two phases, causing
possible damage to the motor.
12.3 Two-Phase Induction Motor
A rotating magnetic field is produced in a two-
phase induction motor stator by placing the
windings 90° apart, as shown below.
12.3 Two-Phase Induction Motor
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.
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.
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
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.
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).
Synchronous motors are used in
situations where a constant speed is
essential (e.g. gyroscopes).