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Title: Notes- All about AC Machines
Description: Notes about AC synchronous and asynchronous Machines
Description: Notes about AC synchronous and asynchronous Machines
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2013
EEE 2015 ELECTRICS
AC Motors‐Synchronous and Asynchronous
AC Motors‐Synchronous and Asynchronous
Electromechanical systems
For energy conversion between electrical and mechanical forms,
electromechanical devices are developed, which can be divided into three
categories:
1
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Examples are microphones, pickups, and speakers
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forces mostly for linear motion drives, such as relays, solenoids (linear actuators),
and electromagnets
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Continuous energy conversion equipment: These devices operate in rotating
mode
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Dr
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12
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Levent ÇETİN
AC Motors‐Synchronous and Asynchronous
Electric Motors
The electric motor is an electromechanical continuous energy conversion
equipment that converts electrical energy into electrical energy mechanical
energy
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Current – In a brush motor when electricity is applied across the motor
termination causes a current flows through a brush and commutator system into
an electromagnetic armature that moves inside a fixed permanent magnet or
an electromagnetic armature that moves inside a fixed permanent magnet or
fixed electromagnetic stator field
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2
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3
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i(t) H(t) B(t) Fmag i(L B) / Vind L(v B) : rotation
Magnetic field is static in DC Motors and rotary in AC Motors
Dr
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12
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AC Motors‐Synchronous and Asynchronous
Alternating Current Motors
The ACM’s are simplier in structure and more economic than DCM’s
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Maintenance of ACM’s is easier
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They can be connected to the AC source
directly
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But, ACM’s
are used more than DCM’s in industry
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AC Motors‐Synchronous and Asynchronous
Motor Components
This AC motor has three main parts, rotor, stator, and enclosure
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Dr
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The stator core is
made up of many thin metal sheets, called laminations
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Stator laminations are stacked
together forming a hollow cylinder
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When the assembled motor
is in operation, the stator windings are connected directly to the power source
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Dr
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12
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There is no electrical power supplied to rotor conductors in induction motors
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There two common type
of constructions for rotor used in an induction motor
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Squirrel cage rotor
2
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In
synchronous rotor construction it is common to use a combination of squiirrel cage and
wound rotor
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AC Motors‐Synchronous and Asynchronous
Motor Components
Enclosure
The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings)
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The rotor fits inside the stator with a slight air gap separating it from the stator
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The enclosure protects the internal parts of the motor from water and other environmental
elements
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Dr
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12
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An induction motor therefore does not require mechanical commutation, separate‐
excitation or self‐excitation for all or part of the energy transferred from stator to rotor
An induction motor's rotor can be either wound type or squirrel‐cage type
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A squirrel cage rotor core is made by stacking thin steel laminations to form a cylinder
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Most squirrel cage rotors are made by die casting
aluminum to form the conductor bars
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Levent ÇETİN
AC Motors‐Synchronous and Asynchronous
AC Induction (asynchronous) Motor
A major difference between the wound rotor motor and the squirrel cage rotor is that the
conductors of the wound rotor consist of wound coils instead of bars
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The
rotating magnetic field induces a voltage in the rotor windings
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Decreasing the resistance causes more current to flow,
increasing rotor speed
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AC Motors‐Synchronous and Asynchronous
Magnetic Field in Stator Coil
The magnetic field of an electromagnet has the same characteristics as a natural magnet,
including a north and south pole
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The polarity of an
electromagnet connected to an AC source changes at the frequency of the AC source
...
p
no magnetic field is
At time 2, current is flowing in a positive
direction, and a magnetic field builds up
around the electromagnet
At time 3, current flow is at its peak
positive value, and the strength of the
electromagnetic field has also peaked
At time 6, current is increasing in the
negative direction
...
The negative half of the cycle
continues through times 7 a nd 8,
returning to zero at time 9
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Dr
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The coils are wound around the soft iron core material of the stator
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When current is applied, each winding becomes an electromagnet, with the two windings
for each phase operating as the opposite ends of one magnet
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The A1 and A2 connected to
phase A of the power supply
...
...
electrical phase
Dr
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12
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Phase B has current flow in the
negative direction and phase C has
current flow in the positive
p
direction
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Magnetic lines of flux leave the B2
north pole and enter the nearest south
pole, C2
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The vector sum of the magnetic fields
is indicated by the arrow
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AC Motors‐Synchronous and Asynchronous
Developing a Rotating Magnetic Field in Stator Coil
t=t1
The following chart shows the progress
of the magnetic field vector as each
phase has advanced 60°
...
Phase A has
current flow in the positive direction
and phase B has current flow in the
negative direction
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The resultant magnetic field
vector has rotated 60° in the clockwise
direction
Dr
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12
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This process repeats 50 times a second for a 60 Hz power source
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Levent ÇETİN
AC Motors‐Synchronous and Asynchronous
Synchronous Speed of an AC Motor
The speed of the rotating magnetic field is referred to as the synchronous speed (NS) of
the motor
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120fAC
ns
p
4 pole motor stator
No
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AC Motors‐Synchronous and Asynchronous
Rotor Rotation
An induction motor induces a current in its rotor so that a magnetic force can exist on
rotor conductors
When current is flowing in a stator winding, the electromagnetic field created cuts across
the nearest rotor bars
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“V” is the relative velocity of the conductor with respect to stator magnetic field
...
So it is evident that the rotor and stator magnetic field have different angular speeds
The induced voltage causes current flow in the conductor
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In a wound rotor current flows through the external circuit, which is
connected to slip rings
Dr
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When power is first applied to the motor with the rotor stopped, this difference in speed is
at its maximum and a large amount of current is induced in the rotor
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This speed difference is called slip
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Slip is also ddependent on load
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Dr
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12
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Poles
2 Pole
4 Pole
6 Pole
6 Pole
8 Pole
Slip Example
Curve
Explanation
Dr
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AC Motors‐Synchronous and Asynchronous
Synchronous Motor
Remember that the rotating magnetic field rotates at a certain speed, that speed is called
synchronous speed
...
Synchronous motors is called so because the speed of the rotor of this motor is same as the
rotating magnetic field
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Poles
2 Pole
4 Pole
6 Pole
8 Pole
Synchronous Speed (no Load)
60 hz
3600
1800
1200
900
50 hz
3000
1500
1000
750
Dr
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That is a squirrel cage, in most of the motors rotor consistsi of both windings and conductor
bars
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When the rotation speed
p
of rotor approaches
pp
the synchronous
y
speed
p
the coil is p
powered
via DC supply to form rotor electromagnet
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AC Motors‐Synchronous and Asynchronous
Mono Phase Induction Machine
Has only one stator coil
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Needs a unit to start to motor: Single phase induction motors require just one power phase
for its operation
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(Fans, washing machines, household
devices… etc
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The single‐phase supply given to the single‐phase winding will produce pulsating field in the
air gap
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AC Motors‐Synchronous and Asynchronous
Mono Phase Induction Machine
Starting methods for single‐phase induction motors:
Single‐phase split‐phase motor
It shows the auxiliary winding also known as starting winding, in space quadrature with the
main winding
...
The starting winding is made of thin wire
so that its impedance is different from that of the main winding
...
The phase difference between the two winding
currents (about 30o) will be sufficient to produce a rotating magnetic field resulting in a
starting torque
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This increase in phase angle will increase the starting torque
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Levent ÇETİN
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Title: Notes- All about AC Machines
Description: Notes about AC synchronous and asynchronous Machines
Description: Notes about AC synchronous and asynchronous Machines