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7/28/2019 AC Motors Lesson
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AC MotorsPresented by Tracy Baptiste
At the end of Session 1 you will be able to:
1. State the two types of AC motors
2. State the advantages and disadvantages of induction motors
3. Stipulate where induction are used
4. identify the construction features of the stator
and the rotors used in the induction motor5. Correlate the 3-phase current to the flux
position and hence the rotor position in a 3-
phase induction motor 1
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Introduction
1. Synchronous motor – the magnetic fieldcurrent is supplied by a separate DC power
source;
2. Induction motor – the magnetic field currentis supplied by magnetic induction
(transformer action) into their field windings.
2
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The Induction Motor
• Induction motors are used worldwide inmany residential, commercial, industrial, andutility applications.
• It can be part of a pump or fan, or connectedto some other form of mechanicalequipment such as a winder, conveyor, ormixer.
• Main Advantages : cheap and low maintenance
• Main Disadvantage: speed control is not easy
3
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Overview of Three-Phase Induction Motor
4
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Induction Motor Stator Construction
5
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Induction Motor Rotor construction
• Two types:
1. Squirrel cage (SCIM): Rotor winding is composed of copper bars embedded in the rotor slots and
shorted at both end by end rings
This is the most common type Simple, low cost, robust, low maintenance
6
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/rotor winding
Short circuits all
rotor bars.
Squirrel cage Rotor
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2. Wound rotor (WRIM): Rotor winding is woundby wires. The winding terminals can be
connected to external circuits through slip ringsand brushes.Easy to control speed, more expensive.
8
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Arrangement of Rotor & Stator
• 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
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The Induction principle
• Coils placement in motor createsrotating, magnetic field in stator
• Rotating magnetic field cuts rotor
bar and induces current in rotor
• Rotor current creates magnetic field
on rotor• Attraction of rotor to stator creates
torque and, hence, horsepower
10
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3-phase stator-winding illustration
11
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Principle of Operation
• Torque producing mechanism
A
B
C
0
0
12
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Principle of Operation
• Torque producing mechanism
A
B
C
13
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Principle of Operation
• Torque producing mechanism
A
B
C0
0
14
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Principle of Operation
• Torque producing mechanism
A
B
C
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Principle of Operation
• Torque producing mechanism
A
B
C
0
0
16
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Principle of Operation
• Torque producing mechanism
A
B
C
17
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Principle of Operation
• Torque producing mechanism
A
B
C
0
0
18
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Single-Phase Induction Motors (SPIM)
At the end of Session 2 you will be able to:1. Describe a Universal Motor
2. Describe the Principle of Operation of SPIM
3. State the types of SPIM
4. Build a SPIM
19
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Types of Single Phase Induction Motors
Recall• In a series dc motor, when both the stator and
rotor fields are reversed, the motor runs in the
same direction1. This principle is used in the universal motor
20
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Types of Single Phase Induction Motors
2. Shaded pole motor
21
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Types of Single Phase Induction Motors
3. Repulsion Induction motors
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Types of Single Phase Induction Motors
3. (a) Split-Phase Induction motors
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24
SPIMThe Centrifugal Switch
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Types of Single Phase Induction Motors
3. (b) Split-Phase Resistor-Start Induction
motors
25
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Types of Single Phase Induction Motors
4. Capacitor Induction motors
a) Capacitor Start AC Induction Motor
26
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Types of Single Phase Induction Motors
4. Capacitor Induction motors
(b)Permanent Split Capacitor (PSC) Induction Motor
OR Capacitor Run AC Induction Motor
27
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Types of Single Phase Induction Motors
4. Capacitor Induction motors
(c) Capacitor Start – Capacitor Run Induction Motor
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Now to activity 2
L
N
N
N
L2
L1
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30
Session 3: Synchronous Speed
Generally,
• Ns=120f/p, where p→# poles
• or Ns = 60f/p, where p→ # pole pairs (or pp)
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Slip
• Slip is associated with synchronous speed.
• If the motor turned at the same RPM as the magnetic 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 < synchronous speed
Slip(S) = synchronous speed(Ns) – rotor speed(Nr) in rpm
% slip = ( Ns – Nr / Ns ) 100
31
Session 4: Synchronous motors
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Session 4: Synchronous motors
– just as a DC generator can be used as a DC motor, so AC
generators (or alternators) can be used as synchronousAC motors
– three phase motors use three sets of stator coils
• the rotating magnetic field drags the rotor around with it
– single phase motors require some starting mechanism
– torque is only produced when the rotor is in sync with
the rotating magnetic field
• not self-starting – may be configured as an induction motor
until its gets up to speed, then becomes a synchronous motor
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• Synchronous machines are AC machines that have afield circuit supplied by an external DC source.
• In a synchronous motor, a 3-phase set of statorcurrents produces a rotating magnetic field causing therotor magnetic field to align with it. The rotor magneticfield is produced by a DC current applied to the rotor
winding.• Field windings are the windings producing the main
magnetic field (rotor windings for synchronousmachines);
• armature windings are the windings where the mainvoltage is induced (stator windings for synchronousmachines).
33
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Synchronous rotor
34
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Synchronous Motors (continued)
•
Construction
– Stator identical to that of a three-phase inductionmotor – now called the “armature”
– Energize from a three-phase supply and developthe rotating magnetic field
– Rotor has a DC voltage applied (excitation)
– Rotor could be a permanent-magnet type
36
C t ti f h
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Construction of synchronous
machines
Two common approaches are used to supply a DC current to the field circuits on the
rotating rotor:
1. Supply the DC power from an external DC
source to the rotor by means of slip rings
and brushes;
2. Supply the DC power from a special DC
power source mounted directly on the
shaft of the machine.
Slip rings are metal rings completely encircling the shaft of a machine but insulated from it.
One end of a DC rotor winding is connected to each of the two slip rings on the machine’s
shaft. Graphite-like carbon brushes connected to DC terminals ride on each slip ring
supplying DC voltage to field windings regardless the position or speed of the rotor.
C t ti f h
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Construction of synchronous
machines
A rotor of large
synchronous machine
with a brushless exciter
mounted on the sameshaft.
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Synchronous Motors (continued)
• Constant speed fixed by system frequency
• DC for excitation
•
low starting torque: suited for low loadapplications
– clocks, turn tables
• Can improve power factor: suited for highelectricity use systems
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Synchronous Motors (continued)
• Operation
– Magnetic field of the rotor “locks” with the
rotating magnetic field – rotor turns at
synchronous speed
40
ess on :
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ess on :Direct-On-Line (D.O.L)/Across the Line(A.T.L.)
Motor Starter
STOP
START
M.
M
O/L
M
MOTOR
L1
L2
L3M
M
120V a.c.
Pilot lamp
C.B.
C.B.
95
96
13 14
A1 A2
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D.O.L/A.T.L.
Motor Starter Components
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Reduced voltage starting
motor control
• Star delta motor control circuit and power
circuit
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Star Delta47
U1,W2 U2,V1
V2,W1
Reduced voltage starting
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Reduced voltage startingAuto-Transformer closed transition starter
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Induction motor control
reduced voltage starting
primary resistance starting
• With one resistor