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Mechanical and Electrical Systems
SKAA 2032
Introduction to Electrical
Machines
Dr. Asrul Izam Azmi
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
Introduction
• One of energy can be obtained from the other
form with the help of converters.
• Converters that are used to continuously
translate electrical input to mechanical output
or vice versa are called electric machines.
• The process of translation is known as
electromechanical energy conversion.
Electrical
system
e, i
Electric
Machine
Mechanical
system
T, n
Motor
Energy flow
Generator
• An electrical machine is link between an electrical
system and a mechanical system.
• Conversion from mechanical to electrical: generator
• Conversion from electrical to mechanical: motor
Electrical
Machines
DC
machine
AC
machine
Synchronous
machine
Induction
machine
• Machines are called AC machines (generators
or motors) if the electrical system is AC.
• DC machines (generators or motors) if the
electrical system is DC.
Electrical
system
e, i
Coupling
magnetic
fields
Mechanical
system
T, n
Two electromagnetic phenomena in the electric
machines:
• When a conductor moves in a magnetic field,
voltage is induced in the conductor.
• When a current-carrying conductor is placed
in a magnetic field, the conductor experiences
a mechanical force.
AC Rotating Machines
Electrical
Machines
DC
machine
AC
machine
Synchronous
machine
Induction
machine
AC Motors
Basic Idea
• A motor uses magnets to create motion.
• The fundamental law of all magnets:
– Opposites attract
– Likes repel.
• Inside an electric motor, these attracting and
repelling forces create rotational motion
Basic Idea
Magnetic field of a straight conductor
• The magnetic field lines around a long wire which
carries an electric current form concentric circles
around the wire.
• Right hand rule-1
Basic Idea
Magnetic field of a circular conductor
• Right hand rule-1 gives the direction of the magnetic
field inside and outside a current-carrying loop.
Basic Idea
Magnetic field of a coil of wire
• A solenoid is a long coil of wire
• The field inside a solenoid can be very uniform and very
strong.
• The field is similar to that of a bar magnet.
Basic Idea
• The use of soft metal increases the magnetic field
strength
• Use right hand rule-2 / eye rule to determine direction of
magnetic field in a coil
Basic Idea
• Fleming’s left hand rule for motors
• Don’t be confused with Fleming’s right hand rule for
generator
Working Principle
Elementary AC motor
• Consider a rotor → formed by permanent magnet.
• Consider a stator → formed by coil of conductor to
create AC electromagnetic field
Working Principle
• An AC Current flowing through conductors energize
the magnets and develop N and S poles.
• The strength of electromagnets depends on current.
• First half cycle current flows in one direction.
• Second half cycle it flows in opposite direction.
Working Principle
• Consider the AC voltage at 0 degrees, then, no
current will flow, and there is no magnetism.
Supplied voltage
Initial position of the rotor
Working Principle
• As voltage increases, current starts to flow and
electromagnets gain strength and North and South
poles appear.
• The rotor magnet is pushed CW, and the rotor and
motor starts to rotate.
Working Principle
• When voltage decreases, the current decreases also,
the electromagnet loses the strength, and when V=0
there is no magnetism.
Working Principle
• Now, AC voltage builds up as part of the negative
cycle.
• Then, current flows in opposite direction, and the
magnets reverse polarity.
• Therefore, the CW rotation continues.
AC Motor Rotation
Limitation of the Elementary Motor
• The initial position of the rotor determines the
direction of the motor rotation.
Practical AC Motor
• By adding another pair of electromagnets the
limitation mentioned before is removed.
• Example: Two electromagnets = Vertical & Horizontal
• Three phase system has three electromagnets
Practical AC Motor
Practical AC Motor
Counter clockwise
rotation
Practical AC Motor
• We can see that the poles rotate around the
circumference of the motor.
• The rotor, no matter how it is positioned at rest, will
be locked-in with the magnetic field and will turn in
one direction only.
• (Same rotation as
the poles).
Induction Motor
• Most AC motors are induction motors
• Induction motors are favored due to their
ruggedness (no brush), simplicity and cheap.
• 90% of industrial motors are induction motor.
• Application
– (1-phase): washing machines, refrigerators, blenders, juice
mixers, stereo turntables, etc.
– (2-phase) induction motors are used primarily as
servomotors in a control system.
– (3-phase): pumps, compressors, paper mills, textile mills,
etc.
Induction Motor
• The single-phase induction motor is the most
frequently used motor in the world
• Most appliances,
such as washing
machines and
refrigerators, use
a single-phase
induction
machine
• Highly reliable
and economical
Induction Motor
• For industrial applications, the three-phase
induction motor is used to drive machines
• Large three-phase
induction motor.
(Courtesy
Siemens).
Housing
Motor
Construction of Induction Motor
• An induction motor is composed of a rotor, (armature)
• A stator containing windings connected to a poly-phase
energy source
• The pair of coils correspond to the phases of electrical
energy available.
• Each pair connected in
series creating
opposite poles:
– 1 pole for North and 1
pole for South.
Induction Motor
Stator frame showing slots for
windings.
Stator with (a) 2-phase and (b) 3-phase windings.
Induction Motor
• It has a stator and a rotor like other type of motors.
• 2 different type of rotors:
– Squirrel-cage winding,
– Wound-rotor
• Both three-phase and single-phase motors are widely used.
• Majority of the motors used by industry are squirrel-cage
induction motors
• A typical motor consists of two parts:
– An outside stationary stator having coils supplied with AC current
to produce a rotating magnetic field,
– An inside rotor attached to the output shaft that is given a torque
by the rotating field.
Squirrel-cage Rotor
• Rotor is from laminated iron core
with slots.
• Metal (Aluminum) bars are
molded in the slots instead of a
winding.
• Two rings short circuits the
bars.–Most of single phase
induction motors have SquirrelCage rotor.
• One or 2 fans are attached to the
shaft in the sides of rotor to cool
the circuit.
Wound Rotor
• It is usually for large 3 phase
induction motors.
• Rotor has a winding the same as
stator and the end of each phase
is connected to a slip ring.
• Three brushes contact the three
slip-rings to three connected
resistances (3-phase Y) for
reduction of starting current and
speed control.
• Compared to squirrel cage
rotors, wound rotor motors are
expensive and require
maintenance of the slip rings and
brushes, so it is not so common
in industry applications
• Wound rotor induction motor
was the standard form for
variable speed control before the
advent of motor
Slips
• It is virtually impossible for the rotor of an AC induction
motor to turn at the same speed as that of the rotating
magnetic field.
• If the speed of the rotor were the same as that of the
stator, no relative motion between them would exist, and
there would be no induced EMF in the rotor.
• Without this induced EMF, there would be no interaction of
fields to produce motion. The rotor must, therefore, rotate
at some speed less than that of the stator if relative motion
is to exist between the two.
• The percentage difference between the speed of the rotor
and the speed of the rotating magnetic field is called slip.
• The smaller the percentage, the closer the rotor speed is to
the rotating magnetic field speed.
Slips
SLIP 
NS  N R
100%
NS
where
NS : synchronous speed or the rotating magnetic field (rpm)
NR : rotor speed (rpm)
The synchronous speed (NS) of a motor is given by:
NS 
120 f
NP
where
F : frequency of the rotor current (Hz)
NP : number of poles
Example Problem
A two pole, 60 Hz AC induction motor has a full load
speed of 3554 rpm. What is the percent slip at full
load?
NP
Torque
• Torque is a rotational force.
• The torque of an AC induction motor is dependent
upon the strength of the interacting rotor and stator
fields and the phase relationship between them.
T  K Φ I R cosθ R
where
T : torque
K: constant
Φ: stator magnetic flux (Wb)
IR : rotor current (A)
cos θR : power factor of rotor
Voltage and frequency induced in the rotor
• The voltage and frequency induced in the rotor both
depend on the slip. They are given by the following
equation.
f2 = s f
E2 = s Eoc (approx.)
f2 = frequency of the voltage and current in the rotor [Hz]
f = frequency of the source connected to the stator [Hz]
s = slip
E2 = voltage induced in the rotor at the slip s
Eoc = open-circuit voltage induced in the rotor when at rest [V]
Active Power in a Induction Motor
Efficiency () =
Poutput
Pinput
Example 1
• Calculate the synchronous speed of a 3-phase
induction motor having 20 poles when it is
connected to a 50 Hz source.
Source frequency = 50 Hz,
Synchronous speed ns =
number of poles = 20
120 f
p
120 x 50
=
20
ns = 300 r/min
Example 2
• A 0.5 hp, 6-pole induction motor is excited by
a 3-phase, 60 Hz source. If the full-load is 1140
r/min, calculate the slip.
Source frequency = 60 Hz,
number of poles = 6
Full load/rotor speed = 1140 r/min
Synchronous speed ns =
120 f
p
120 x 60
=
6
ns = 1200 r/min
Induction Motor
Slip speed: ns – nR = 1200 – 1140 = 60 r/min
Slip: s = (ns - nR) / ns
= 60/1200
= 0.05 or 5%
Example 3
A single phase, 4 poles induction motor gives the following
data:
Output 373 W ; 230 V
Frequency : 50 Hz., Input current 2.9 A
Power factor: 0.71 ; Speed: 1410 r.p.m.
a) Calculate the efficiency of the motor
b) Determine the slip of the motor when delivering
the rated output