B-Basic Theory of Electric Motor Current

Diagnostic Faults in
Electrical Motor

Theory Application to
Electric Motors

The Basic of Electric Motor
Rotate Magnetic Field
Alternating current
The phase
windings A, B and
C are placed 120
degrees apart.

Rotate Magnetic Field
Alternating current
The phase windings and number of poles

Types of Electric Motor
Theory application to electric motors

AC MOTOR
Asynchronous/Induction Motors
• The induction motor is so named for the induced current
flowing in the secondary winding (the rotor) by action of the
primary winding. No direct electrical connection is made; it is
a result of the magnetic field being established by the stator
winding.
• The principle purpose of the stator winding is to establish a
rotating magnetic field in the stator core that will induce a
voltage in the rotor core.
• The rotor “becomes” a magnet, with a North and South pole,
which in turn follows the moving magnetic fields in the stator.

Construction
• The three basic parts of an AC motor are the rotor, stator, and
enclosure.
• The stator and the rotor are electrical circuits that perform as
electromagnets.

AC MOTOR
STATOR
ROTOR
OTHERS  SHAFT, BEARING, FAN, END COVER, TERMINAL BOX
MAIN PARTS 

STATOR
FRAME
To provide
mechanical
protection
and support
for windings
INSULATED
ELECTRICAL
WINDINGS
Consist of copper
wires, insulated with
varnish, which are
fitted into insulated
slotted laminations
Slots are made from
high grade alloy
steel to reduce the
effects of eddy
currents

SHAFT
.. Shaft is placed inside the rotor, so when rotor rotates
then actually shaft rotates ..

BEARINGS
-Rotator is mounted on bearings to reduce friction on both sides
-Usually ball and roller bearings are used to suit heavy duty, trouble
free running and enhanced service life.

FAN
.. Used to for adequate circulation of cooling air ; securely
keyed onto the rotor shaft ..

TERMINAL BOX
.. Used for holding stator windings and rotor windings ..

END COVERS
.. Provide support for the rotor assembly ..

AC MOTOR

Stator
Stator: The stationary electrical part of the
motor. It contains a number of windings
whose polarity is changed all the time
when an alternating current (AC) is
applied. This makes the combined
magnetic field of the stator rotate.
The stator insulation design is classified.
This clas- sifi-cation is defined in IEC
62114, which have different insulation
classes (temperature classes) and
temperature rises (∆T).
The stator can be designed to handle
various voltages, frequencies and
outputs and a varying number of poles.

Construction (Stator construction)
The stator is the stationary electrical part of the motor.
The stator core of a National Electrical Manufacturers Association (NEMA)
motor is made up of several hundred thin laminations.
Stator laminations are stacked together forming a hollow cylinder. Coils
of insulated wire are inserted into slots of the stator core.
Electromagnetism is the principle behind motor operation. Each
grouping of coils, together with the steel core it surrounds, form an
electromagnet. The stator windings are connected directly to the power
source.

STATOR WINDING
CONFIGURATION
3-PHASE

ROTOR
.. rotating part of the motor ..
Two types of Rotors based on its structure :
Squirrel-Cage Rotor and Slip-Ring Rotor

Rotor
When the stator's moving magnetic field
cuts across the rotor conductor bars, a
current is produced. This current
circulates through the bars and creates
magnetic fields around each rotor bar.
As the magnetic field in the stator
keeps changing, so does the field in the
rotor. This interaction is what causes the
rotor to move.

Construction (Rotor construction)
• The rotor is the rotating part of the electromagnetic circuit.
• It can be found in two types:
• Squirrel cage
• Wound rotor
• However, the most common type of rotor is the “squirrel cage” rotor.

Construction (Rotor construction)
Wound Rotor
Squirrel-Cage Rotor
/rotor winding
Short circuits all
rotor bars.

ROTOR - Squirrel-Cage Rotor
ROTOR BAR

Enclosure
The enclosure consists of a frame (or yoke) and two end
brackets (or bearing housings). The stator is mounted inside the
frame. The rotor fits inside the stator with a slight air gap
separating it from the stator. There is NO direct physical
connection between the rotor and the stator.
Stator
Rotor
Air gap
 The enclosure also protects the electrical
and operating parts of the motor from
harmful effects of the environment in which
the motor operates. Bearings, mounted on
the shaft, support the rotor and allow it to
turn. A fan, also mounted on the shaft, is
used on the motor shown below for cooling.

Stator
A
B
C
Current of
Each
Winding
Flux
EMF
Stator
EMF
A+B+C
Rotor
Inducing
Rotor
Current
of Rotor
Force of
Rotor
Nr >> Slip
Fr = S*Fs

Rotating Magnetic Field
When a 3 phase stator winding is connected to a 3 phase voltage
supply, 3 phase current will flow in the windings, which also will induced
3 phase flux in the stator.
These flux will rotate at a speed called a Synchronous Speed, ns. The
flux is called as Rotating magnetic Field
Synchronous speed: speed of rotating flux
Where; p = is the number of poles, and
f = the frequency of supply
p
f
n s
120


a Fc
-93 10 113 216
-1.5
-1
-0.5
0
0.5
1
1.5
a’
c’ b’
b c
a
a’
c’ b’
b c
a
a’
c’ b’
b c
a
a’
c’ b’
b c
Fb
Fa F
Fb
Fc
F
Fa
F
Fb
Fc Fc Fb
F
Space angle () in degrees
F
Fa Fc
Fb
t = t0= t4
t = t1
t = t2 t = t3
t = t0= t4
RMF(Rotating Magnetic Field)

Induction Motor Speed
At what speed will the IM run?
 Can the IM run at the synchronous speed, why?
 If rotor runs at the synchronous speed, which is the same speed of the
rotating magnetic field, then the rotor will appear stationary to the rotating
magnetic field and the rotating magnetic field will not cut the rotor. So, no
induced current will flow in the rotor and no rotor magnetic flux will be
produced so no torque is generated and the rotor speed will fall below the
synchronous speed
 When the speed falls, the rotating magnetic field will cut the rotor windings
and a torque is produced

Synchronous Speed ROTOR
p
f
n s
120

If, for example, the frequency of the
applied power is 50 Hz, the
synchronous speed is 3000 min-1
for a 2-pole motor.

Induction Motor Speed
 IM will always run at a speed lower than the synchronous speed
 The difference between the motor speed and the synchronous
speed is called the Slip
Where, nslip = slip speed
nsync = speed of the magnetic field
nm = mechanical shaft speed of the motor
slip syn c m
n n n
 

So far, so good. But of course we already
know that AC motors are known as
asynchronous motors. This is because
the rotor field does not follow the stator
field in perfectly synchronous motion.
Slip
This difference in speed between rotor and
stator fields, is called slip and is measured
in %. Slip is a key factor and is
necessary to produce torque. The
greater the load - torque - the greater slip.

Slip and Rotor Speed
Slip s
The rotor speed of an Induction machine is different from the speed of
Rotating magnetic field. The % difference of the speed is called slip.
Where; ns = synchronous speed (rpm)
nr = mechanical speed of rotor (rpm)
under normal operating conditions, s= 0.01 ~ 0.05, which is very small
and the actual speed is very close to synchronous speed.
Note that : s is not negligible
)
1
( s
n
n
OR
n
n
n
s s
r
s
r
s





Slip and Rotor Speed
Rotor Speed
 When the rotor move at rotor speed, nr (rps), the stator flux will circulate
the rotor conductor at a speed of (ns-nr) per second. Hence, the
frequency of the rotor is written as:
Where; s = slip
f = supply frequency
sf
p
n
n
f r
s
r


 )
(
f
s
f
i
ii
ii
p
n
n
f
n
n
Rotor
At
i
p
n
f
n
stator
At
Note
r
r
s
r
p
f
r
s
s
p
f
s
.
:
)
(
)
(
)
.....(
120
)
(
:
)
.....(
120
:
:
120
120










When the rotor is blocked (s=1) , the frequency of the induced voltage is equal
to the supply frequency
On the other hand, if the rotor runs at synchronous speed (s = 0), the
frequency will be zero

Where does torque
and speed come
from?

EXERCISE TIME
HOW TO READ MOTOR NAME
PLATE

Frequency of motor excitation
58
Magnetic field speed, RPM (Ns) = 120 x F / # pole
Slip frequency (SF) = Magnetic Field Speed - Actual Speed
Pole pass frequency (Fp) = Slip frequency X # poles
Rotor bar pass frequency (RBPF) = # bars x RPM
Stator Slot Pass Frequency = # stator slot x RPM

Example
• Sebuah motor listrik di name plate tertera 1480 RPM. Jumlah
rotor bar = 40.
Berarti motor listrik ini mempunyai 4 pole, sehingga :
Magnetic Field speed (Ns) = 120 x 50 / 4 = 1500 RPM
Slip frekuensi (SF) = 1500 - 1480 RPM = 20 RPM = 0.33 Hz
Pole pass frekuensi (Fp) = 4 x 20 RPM = 80 RPM
RBPF = 40 x 1480 RPM = 59200 RPM = 986.67 Hz.
59

B-Basic Theory of Electric Motor Current

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