ADVANTAGES AC MOTOR
THEN DC MOTOR.
•design to eliminates the need of brushes &
commutator.
•less maintainance allow ac motor operate longer
then dc motor.
•rotor of ac motor made from laminated steel
and
than copper coil press on core for dc motor
which
reduce maintenance.
•using variable-frequency drive, the speed of ac
motor can be adjusted easily than DC motor.
TYPE OF AC MOTOR
• SERIES AC MOTOR
-call as universal motor.
•A universal motor works on the same principle as a DC
motor, force is created on the armature conductors due
to the interaction between the main field flux and the
flux created by the current-carrying armature
conductors.
• A universal motor develops unidirectional torque
regardless of
whether it operated on AC or DC supply which
starting torque
very high.
•Low speed if large load & high speed for small load.
•do not operate in polyphase ac power.
•Lower efficiency then ac and dc motor.
•the instantaneous magnetic polarities of the armature and field
oppose each other, and motor action results. Now, reverse the
current by reversing the polarity of the input
•the field magnetic polarity still opposes the armature magnetic
polarity. This is because the reversal affects both the armature
and the field.
•The AC input causes these reversals to take place continuously.
Synchronous motor
. Synchronous motors have the characteristic of
constant speed between no load and full load.
capable of correcting the low power factor of
an inductive load when they are operated
under certain conditions.
often used to drive DC generators.
designed in sizes up to thousands of
horsepower either single-phase or multiphase
machines.
SYNCHRONOUS MACHINE
Salient Rotor Machine
•The stator has a
laminated iron-core
with slots and three
phase windings
placed in the slots.
•The rotor has salient
poles excited by dc
current.
•DC current is
supplied to the
rotor through slip-
rings and brushes.
Synchronous Machine
Round Rotor
Machine
•The stator is a ring
shaped laminated
iron-core with
slots.
•Three phase
windings are
placed in the slots.
•Round solid iron
rotor with slots.
•A single winding is
placed in the slots.
Dc current is
supplied through
PRINCIPLE OPERATION
Application of three-phase AC power to the stator causes a
rotating magnetic field to be set up around the rotor and
energized with dc (it acts like a bar magnet).
The strong rotating magnetic field attracts the strong rotor
field activated by the DC. make a strong turning force on the
rotor shaft. able to turn a load as it rotates in step with the
rotating magnetic field.
When ac is applied to the stator, a high-speed rotating
magnetic field appears immediately. This rotating field
rushes past the rotor poles so quickly that the rotor does not
have a chance to get started. In effect, the rotor is repelled
first in one direction and then the other. A synchronous
motor in its purest form has no starting torque. It has torque
only when it is running at synchronous speed.
•A squirrel-cage type of winding is added to the rotor
of a synchronous motor to cause it to start.
Simply, the windings are heavy copper bars shorted
together by copper rings.
•A low voltage is induced in these shorted windings by
the rotating three-phase stator field causes a magnetic
field that interacts with the rotating field of the stator.
Because of the interaction, the rotor begins to turn,
following the stator field; the motor starts.
INDUCTION MOTOR
• Simple, rugged construction cost relatively little to
manufacture.
Stator of an induction motor
•There is no difference between squirrel cage
and slip – ring motor stators.
The stator or the stationary part consists of
three-phase winding held in place in the slots of
a laminated steel core which is enclosed and
supported by a cast iron or a steel frame as
shown in Fig 1a.
•The phase windings are placed 120 electrical
degrees apart, and may connected in either star
or delta externally, for which six leads are
brought out to a terminal box mounted on the
frame of the motor.
•When the stator is energized from a three –
phase voltage it will produce a rotating
magnetic field in the stator core.
ROTOR OF A SQUIRREL CAGE & SLIP-RING INDUCTION
MOTOR
SQUIRREL CAGE SLIP RING
1. Conductors are not insulated from 1. The rotor is wound with an
the core insulated windings similar to the
stator windings
2. The conductors are placed in 2. The rotor phase windings are wye
parallel to the shaft and embedded connected with the open end of cash
in the surface of the core. At each phase brought out to a slip ring
ends, the rotor conductors are all mounted on the rotor shaft.
short-circuited by continuous end
rings.
3. No external connection to the 3. The rotor windings are not
rotor connected to the supply. The slip
rings and brushes merely providing
a mean of connecting an external
variable control resistance in the
rotor circuit.
4. Widely used; rugged, cheaper 4. Less extensively used; higher cost
and less maintenance and greater maintenance costs.
DIFFERENCES BETWEEN A SQUIRREL CAGE AND
SLIP-RING ROTORS
The squirrel Cage rotor possesses the following advantages:
1.cheaper and more robust
2.slightly higher efficiency and power factor
3.explosion proof, since the absence of slip-rings and brushes
eliminates risk of sparking.
The advantages of the slip-ring rotor are:
4.the starting torque is much higher and the starting current
much
lower
2.the speed can be varied by means of external rotor
resistors.
•COMPARISON BETWEEN SYNCHRONOUS MOTOR AND
INDUCTION MOTOR
No Aspects Synchronous motor Induction motor
Synchronous speed Less than synchronous
1 Speed constant. Independent of speed. Decreases with
load condition. increasing load.
Operates at all power Operates at only lagging
2 Power factor factors whether lagging or power factor.
leading.
3 Efficiency Very good. Good.
4 Cost Costlier. Cheaper.
5 Starting Not self-starting. Self-starting.
6 Speed No question. Can be controlled to small
control units.
Used for mechanical load Limited to supply of
7 Application and also to improve mechanical load.
power factor as
synchronous condenser.
Two-phase Rotating Magnetic Fields
The waveforms are numbered to match their associated phase. At
position 1, the current flow and magnetic field in winding 1-1A is at
maximum (because the phase voltage is maximum). The current flow
and magnetic field in winding 2-2A is zero (because the phase voltage is
zero). The resultant magnetic field is therefore in the direction of the 1-
1A axis. At the 45-degree point (position 2), the resultant magnetic field
lies midway between windings 1-1A and 2-2A. The coil currents and
magnetic fields are equal in strength. At 90° (position 3), the magnetic
field in winding 1-1A is zero. The magnetic field in winding 2-2A is at
maximum.
Now the resultant magnetic field lies along the axis of the 2-2A
winding as shown. The resultant magnetic field has rotated clockwise
through 90° to get from position 1 to position 3. When the two-phase
voltages have completed one full cycle (position 9), the resultant
magnetic field has rotated through 360°. Thus, by placing two windings
at right angles to each other and exciting these windings with voltages
90° out of phase, a rotating magnetic field results.
At point 1, the magnetic field in coils 1-1A is maximum with
polarities as shown. At the same time, negative voltages are being
felt in the 2-2A and 3-3A windings. These create weaker magnetic
fields, which tend to aid the 1-1A field.
At point 2, maximum negative voltage is being felt in the 3-3A
windings. This creates a strong magnetic field which, in turn, is
aided by the weaker fields in 1-1A and 2-2A.
As each point on the voltage graph is analyzed, it can be seen
that the resultant magnetic field is rotating in a clockwise direction.
When the three-phase voltage completes one full cycle (point 7),
the magnetic field has rotated through 360°.
