chapter 13 – ac motors.ppt
TRANSCRIPT
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Chapter 13 – AC Motors
AC motors convert AC electrical energy to
Mechanical energy.
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Chapter 13 – AC Motors
• AC motors:
1. the armature of rotor is a magnet (different to DC
motors).
2. the stator is formed by electromagnets (like in DC
motors).
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Effects of AC Supply on
Magnetic Poles
• Consider the rotor to be a permanent magnet.
• 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.
As AC voltage changes, the po les alternate.
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Using AC Supply to Make
an Elementary Motor (1)• Consider the AC voltage at 0 degrees, then, no
current will flow, and there is no magnetism.
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Using AC Supply to Make
an Elementary Motor (2)
• As voltage increases, current starts to flow and
electromagnets gain strength and North and
South poles appear.
• (Use left hand rule to find poles).
• The rotor magnet is pushed CW, and the rotor
and motor starts to rotate.
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Using AC Supply to Make
an Elementary Motor (3)• When voltage decreases, the current decreases
also, the electromagnet loses the strength, and
when V=0 there is no magnetism.
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Using AC Supply to Make
an Elementary Motor (4)• 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.
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Using AC Supply to Make
an Elementary Motor (5)• This process is repeated over and over, as AC
voltage goes through its cycles, and we have
continuous rotation.
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AC Motor Rotation
The whole picture
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Limitation of the
Elementary Motor• The initial position of the rotor determines the
direction of the motor rotation.
• Indicate the rotation in the figures below:
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Practical AC Motor
• By adding another pair of electromagnets thelimitation mentioned before is removed.
• Two electromagnets = Vertical & Horizontal
• Two phases with phase difference = 90 deg.
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Effect of Two Pole-Pairs(Observe the pole rotation)
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Operation of the Practical
AC Motor• Fig. of page 124 shows a CCW rotation
Can you see it?
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Magnetic Poles Revolve in
AC Motors• From the previous slide 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).
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Phase Splitting Method (1)
• So, two voltage sources with 90 degreephase connected to electromagnets
make the rotor turn.
• Question is: Can we do the same using
only one voltage source?
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Phase Splitting Method (2)
• The answer is yes!
• Because we can use inductors and
capacitors to produce a voltage out of
phase with the source!
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Reactor Start AC Motor
(One phase + Inductor)• Two parallel branches connected to the power supply.
• First branch: Start winding through a centrifugal switch.
• Second branch: Run winding (through an inductor).
• The current in the second branch lags the current in the
first branch (Remember “ELI”).
• This phase difference makes motor work.
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Reactor Start AC MotorThe Centrifugal Switch
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Capacitor Start AC Motor
(One phase + Capacitor)
• Here the capacitor provides the phase difference.
• The difference is that the current in the star
winding leads the current in the run winding (ICE).
• Similar effect as with the inductor, but it creates a
motor with higher starting power.• Refrigerators, compressors, air conditioners
8
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Three Types of Capacitor
Start Motors1. Capacitor Start (disconnects
capacitor after motor speed picks up)
2. Capacitor Run (Keeps the capacitor
connected during the operation of the
motor, in order to keep the electric
power consumption low)
3. Capacitor Start-Run (uses two
capacitors, one for starting and one for
running. This further improves Power
Consumption)
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Synchronous Speed
• AC motors always rotate with the speed of their
revolving magnetic field.
• The speed of the revolving poles is the maximum
possible speed of rotation of the motor.
• It is called “Synchronous Speed”.
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Motor Construction
The Stator
• The stator forms a hollow cylinder with coils of
insulated wire inserted into slots of the stator core.
• The coils, plus the steel core form the
electromagnets.
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Motor Construction
The Rotor• There are two types of motor rotors:
• The wound rotor
• The squirrel cage
• The wound rotor has coils of wire wound in the
slots of the rotor (Similar to generator coils).
• The “Squirrel cage” consists of bars of copper or
aluminum electrically connected at each end with
conducting rings.
• As the roto r rotates insid e a magnet ic f ie ld, i t
receives electrom agnet ic induc t ion , then cu rrent
f lows and fo rm the rotor electrom agnet .
0
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Types of Motor Enclosures
1. ODP – Open Drip Proof
2. TENV – Totally Enclosed Non-Ventilating
3. TEFC – Totally enclosed Fan Cooled
4. XP – Explosion Proof
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Types of Motor Enclosures
• ODP – Open Drip Proof
– Air flows through motor (fan blades help flow)
– Used in environments free from contaminants
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Types of Motor Enclosures
• TENV – Totally Enclosed Non-Ventilating
– Protect motor from corrosive and harmful
elements
– Frame fins help to dissipate heat
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Types of Motor Enclosures
• TEFC – Totally enclosed Fan Cooled – Similar to TENV except has external fan for
cooling
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Types of Motor Enclosures
• XP – Explosion Proof
– Similar to TEFC but enclosures are cast iron
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Slip
• Slip is associated with synchronous speed.
• If the motor turned at the same RPM as themagnetic field, there would be no relative motion
between the rotor and the field.
• Therefore, no current would be induced into the
rotor, and no magnetic field would exist.
Rotor speed < syn chronous speed
Slip = synchronous speed – rotor speed
% s l ip = ( Ns – Nr / Ns ) 100
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Three Phase AC Motor
• It has three pairs of electromagnets, connected to
one of the three phases of the power supply.
• It provides a lot higher power that what single
phase motor can deliver.
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AC Motor Data Plate• Each motor has a plate mounted on its frame,
with electrical and mechanical information.
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