EEE1012Introduction to Electrical &

Electronics EngineeringChapter 9: Introduction to Electric Machines

by Muhazam Mustapha, October 2010

Learning Outcome

• Be able to theoretically explain the various types of electric motors

• Be able to theoretically explain the various types of electric generators

• Be able to mathematically solve some parameters of DC motors

By the end of this chapter students are expected to:

Chapter Content

• Electric Machines in General

• DC Machines

• Synchronous Machines

• Induction Machines

Electric Machines

Rotating Machines

• Electromechanical machines are commonly rotational in nature

• The machines require one to be static and the other one to be rotating– Stator: stationary– Rotor: rotating

• Both stator and rotor produce magnetic winding whose field will try to align each other – this produces mechanical motion

Rotor and Stator

×

·

· ×

RotorStator

Rotor winding

Stator winding

Current going in

Current coming out

Stator Field

Rotor Field

Commutator Action

Commutator reverses current in coil every half cycle

There can be more than 1 pair of commutators

Windings

• Two types of magnetic windings:– Armature: the winding connects to load– Field: the winding only to produce field

• Either armature or field winding can be located as rotor or stator

• The location of field and armature determines the type of the machine

Machine Types (Generator & Motor)

Type Winding Type Location Current Type

DC Armature Rotor DC

Field Stator DCSynchronous Armature Stator AC

Field Rotor DCInduction Primary Stator AC

Secondary Rotor AC

DC Machines

DC Machines

• DC Machines are hard to construct, but easiest to discuss and analyze

• Hence all our mathematical discussion on machines will be on DC machines

• Other machine type will be covered as theory

Configurations

Separately Excited

La

Ra

Va

Vf

Rf Lf

Ia

If

• DC Machines can be constructed in a few configurations depending on series or parallel structure or the availability of a second power source

Configurations

Shunt Connected

La

Ra

VaVf

Rf

Lf

Ia

If

Series Connected

La

Ra

Va

Vf

Lf

Configurations

Short-Shunt Compound

La

Ra

Va

Ia

Shunt Winding

Long-Shunt Compound

La

Ra

Va

Series Winding

Ia

Shunt Winding

Series Winding

Steady State Equations

La

Ra

VL or Vs

Ia

Eb, ωm

Lf

Rx

Rf

If

• Referring to the following DC machine model, we can deduce some formulas for motor and generator at constant speed

RS

LS

Is

Steady State Equations

fSa

SSaabL

aam

ab

m

mab

III

RIRIEV

IkIEP

T

kE

• Generator

Steady State Equations

afs

SsaabL

aam

ab

m

mab

III

RIRIEV

IkIEP

T

kE

• Motor

Machine Constant

M

pNka 2

• The armature

constant of ka

p = number of magnetic polesN = number of conductors per coilM = number of parallel paths in armature winding

Conversions

mn 2

60

n = round per minute, r/minωm = radian per second, rad/s

1 horse power = 746 watts

Synchronous Machines

Alternator

• Just another word for AC generator

• Normally a permanent magnet or a DC powered electromagnet will be placed at rotor to generate AC current

• Stator would be wound with solenoid that carries the generated energy – there can be more than one windings hence it can generate more than 1 phase of electricity

Alternator

· ×

××

· ·

N

S

Three Phase

Single Phase

Coils at stator

Synchronous Motor

• Virtually identical to alternator

• Needs a DC voltage exciter at rotor to start

• Called synchronous because it spins at the same rate as the AC frequency used to drive it

Induction Machines

Induction Motor

• The stator part is almost identical to synchronous motor

• AC current (single or multi-phase) will be fed into stator – produces spinning field

• There is no power or permanent magnet placed in the rotor

• Rotor and stator are electrically separated

• Then how mechanical force is applied to the rotor?

Induction Motor

• Mechanical motion is possible by the induction process that is identical to the one in transformer

• The changes in the magnetic flux from stator will induce current into the rotor winding and causes magnetic attraction or repel between stator and rotor poles

Induction Motor

• The changes of the magnetic field need to involve the cutting of the rotor coils (Faraday’s Law)

• This cutting is what called ‘slip’ between the rate of stator’s field rotation and the rate of rotor’s spin

• Without the slip induction machine couldn’t work

Induction Motor

• The ‘slippings’ also means that the rotation of rotor is not in-sync with the stator field rotation rate

• This is the main electrical difference between synchronous machine and induction machine

Induction Generator

• Makes use of the same induction concept in induction motor – slipping process

• It requires a starting power at rotor to produce magnetic field for the induction process to start

• After that, the power generated by the generator itself will be used to produce the needed rotor magnetic field