AC Motors Lesson

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



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



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



Overview of Three-Phase Induction Motor

4



Induction Motor Stator Construction

5



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



/rotor winding

Short circuits all

rotor bars.

Squirrel cage Rotor

7



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



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

9



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



3-phase stator-winding illustration

11



Principle of Operation

• Torque producing mechanism

A

B

C

0

0

12





A

B

C

13





A

B

C0

0

14





A

B

C

15





A

B

C

0

0

16





A

B

C

17





A

B

C

0

0

18



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



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




2. Shaded pole motor

21




3. Repulsion Induction motors

22




3. (a) Split-Phase Induction motors

23



24

SPIMThe Centrifugal Switch




3. (b) Split-Phase Resistor-Start Induction

motors

25




4. Capacitor Induction motors

a) Capacitor Start AC Induction Motor

26





(b)Permanent Split Capacitor (PSC) Induction Motor

OR Capacitor Run AC Induction Motor

27





(c) Capacitor Start – Capacitor Run Induction Motor

28



Now to activity 2

L

N

N

N

L2

L1

29



30

Session 3: Synchronous Speed

Generally,

• Ns=120f/p, where p→# poles

• or Ns = 60f/p, where p→ # pole pairs (or pp)



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



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

32



• 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



Synchronous rotor

34



35



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



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



Construction of synchronous

machines

A rotor of large

synchronous machine

with a brushless exciter

mounted on the sameshaft.




• 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

39




• Operation

– Magnetic field of the rotor “locks” with the

rotating magnetic field – rotor turns at

synchronous speed

40

ess on :



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



42

D.O.L/A.T.L.



D.O.L/A.T.L.

Motor Starter Components



44



45



Reduced voltage starting

motor control

• Star delta motor control circuit and power

circuit



Star Delta47

U1,W2 U2,V1

V2,W1




Reduced voltage startingAuto-Transformer closed transition starter



Induction motor control

reduced voltage starting

primary resistance starting

• With one resistor




Part winding starters

• Two step starting:

AC Motors Lesson

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