Electrical Machines and Power

gilamonsterbirdsElectronics - Devices

Nov 24, 2013 (3 years and 11 months ago)

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Introduction.


Direct Current (DC) Machines construction.


DC Machines operation and control.


DC Drives using power electronics.


Alternating Current (AC) Machines construction.


AC Machine operation and control.


AC Drives using power electronics.


Transformer construction.


Transformer operation.


Hard wiring control.


Programmable logic controller (PLC).

Electrical Machines and Power
Electronics

4

Torque
:

Torque

is

a

twisting

or

turning

force

that

tends

to

cause

an

object

to

rotate
.


A

force

applied

to

the

end

of

a

lever,

for

example,

causes

a

turning

effect

or

torque

at

the

pivot

point
.

Torque

(τ)

=

Force

x

Radius

of

rotation

If

10

lbs

of

force

were

applied

to

a

lever

1

foot

long,

for

example,

there

would

be

10

lb
-
ft

of

torque
.

(lbs=
0
.
453
kg),(
1
foot=
30
.
48

centimeters,)

Introduction

5

Introduction

Speed

If an object in motion travels a given distance in a given time.

Speed is the ratio of the distance traveled to the time it takes

to travel the distance.

6

Introduction

Linear Speed

The

linear

speed

of

an

object

is

a

measure

of

how

long

it

takes

the

object

to

get

from

point

A

to

point

B
.


Linear

speed

is

usually

given

in

a

form

such

as

feet

per

second

(f/s)
.


For

example,

if

the

distance

between

point

A

and

point

B

were

10

feet,

and

it

took

2

seconds

to

travel

the

distance,

the

speed

would

be

5

f/s
.

(f=
0
.
3048

meters)

7

Introduction

Angular

(Rotational)

Speed


The

angular

speed

of

a

rotating

object

is

a

measurement

of

how

long

it

takes

a

given

point

on

the

object

to

make

one

complete

revolution

from

its

starting

point
.



Angular

speed

is

generally

given

in

revolutions

per

minute

(RPM)
.


An

object

that

makes

ten

complete

revolutions

in

one

minute,

for

example,

has

a

speed

of

10

RPM
.

8

Introduction

Acceleration


When

an

object

changes

speed,

an

increase

in

speed

is

called

acceleration
.


Acceleration

occurs

when

there

is

a

change

in

the

force

acting

upon

the

object
.


An

object

can

also

change

from

a

higher

to

a

lower

speed
.

This

is

known

as

deceleration(negative

acceleration)
.


A

rotating

object,

for

example,

can

accelerate

from

10

RPM

to

20

RPM,

or

decelerate

from

20

RPM

to

10

RPM
.

9

Introduction

Work


Whenever

a

force

of

any

kind

causes

motion,

work

is

accomplished
.


For

example,

work

is

accomplished

when

an

object

on

a

conveyor

is

moved

from

one

point

to

another
.






Work is defined by the product of the net force (F) applied and
the distance (d) moved.

W = F x d

10

Introduction

Power


Power

is

the

rate

of

doing

work
,

or

work

divided

by

time
.

In

other

words
,

power

is

the

amount

of

work

it

takes

to

move

the

package

from

one

point

to

another

point,

divided

by

the

time
.

11

Introduction

Horsepower


Power

can

be

expressed

in

foot
-
pounds

per

second,

but

is

often

expressed

in

horsepower

(HP)
.

One

horsepower

is

equivalent

to

500

foot
-
pounds

per

second

or

33
,
000

foot
-
pounds

per

minute
.

when torque (lb
-
ft) and speed (RPM) are known.

Power in an electrical circuit is measured in watts (W) or
kilowatts (kW). One horsepower = 746 W=0.746 kW.

12

Introduction

Review 1

1. If 5 lb of force were applied to a radius of 3 feet, the torque would
be ____________ lb
-
ft.

2. Speed is determined by ___________ .


a. dividing Time by Distance


b. dividing Distance by Time


c. multiplying Distance x Time


d. subtracting Distance from Time

3. Work is accomplished whenever ____________ causes motion.

4. A twisting or turning force that causes an object to

rotate is known as ____________ .

13

14

15

Introduction to Electrical Machines


An

electric

machine

is

a

device

which

converts

electrical

power

(voltages

and

currents)

into

mechanical

power

(torque

and

rotational

speed),

and/or

vice

versa
.




A

motor

describes

a

machine

which

converts

electrical

power

to

mechanical

power
;

a

generator

(or

alternator)

converts

mechanical

power

to

electrical

power
.


16

Introduction to Electrical Machine


To

understand

how

an

electrical

machines

works,

the

key

is

to

understand

how

the

electromagnet

works
.




The

principles

of

magnetism

play

an

important

role

in

the

operation

of

an

electrical

machines
.


17

Review of Electromagnetism


The

basic

idea

behind

an

electromagnet

is

extremely

simple
:

a

magnetic

field

around

the

conductor

can

be

produced

when

current

flows

through

a

conductor
.


In

other

word,

the

magnetic

field

only

exists

when

electric

current

is

flowing


By

using

this

simple

principle,

you

can

create

all

sorts

of

things,

including

motors
,

solenoids,

read/write

heads

for

hard

disks

and

tape

drives
,

speakers
,

and

so

on


MZS
FKEE, UMP

18

Magnetic Field


Unlike

electric

fields

(which

start

on

+q

and

end

on


q),

magnetic

field

encircle

their

current

source
.


The

field

weakens

as

you

move

away

from

the

wire



Ampere’s

circuital

law

-

the

integration

path

length

is

longer


i
d
H





.
A

circular

magnetic

field


develops

around

the

wire


follows

right
-
hand

rules

field

is

perpendicular

to

the

wire

and

that

the

field's

direction

depends

on

which

direction

the

current

is

flowing

in

the

wire


MZS
FKEE, UMP

19

Example of Electromagnetic


An

electromagnet

can

be

made

by

winding

the

conductor

into

a

coil

and

applying

a

DC

voltage
.



The

lines

of

flux,

formed

by

current

flow

through

the

conductor,

combine

to

produce

a

larger

and

stronger

magnetic

field
.



The

center

of

the

coil

is

known

as

the

core
.

In

this

simple

electromagnet

the

core

is

air
.


MZS
FKEE, UMP

20

Adding an Iron Core


Iron

is

a

better

conductor

of

flux

than

air
.

The

air

core

of

an

electromagnet

can

be

replaced

by

a

piece

of

soft

iron
.



When

a

piece

of

iron

is

placed

in

the

center

of

the

coil

more

lines

of

flux

can

flow

and

the

magnetic

field

is

strengthened
.


MZS
FKEE, UMP

21

Strength of Magnetic Field (Cont)


Because

the

magnetic

field

around

a

wire

is

circular

and

perpendicular

to

the

wire
,

an

easy

way

to

amplify

the

wire's

magnetic

field

is

to

coil

the

wire



The

strength

of

the

magnetic

field

in

the

DC

electromagnet

can

be

increased

by

increasing

the

number

of

turns

in

the

coil
.

The

greater

the

number

of

turns

the

stronger

the

magnetic

field

will

be
.


Magnets


A magnet is an object that possesses a magnetic field,
characterized by a North and South pole pair.


A
permanent magnet
(such as this bar magnet) stays
magnetized for a long time.







An
electromagnet

is a magnet that is created when
electricity flows through a coil of wire. It requires a
power source (such as a battery) to set up a magnetic
field.


A Simple Electromagnet


A Nail with a Coil of Wire









Q


How do we set up a magnet?


A


The battery feeds current through the coil of wire.
Current in the coil of wire produces a magnetic field (as long
as the battery is connected).


A Simple Electromagnet


A Nail with a Coil of Wire








Q
-

How do we reverse the poles of this
electromagnet?


A


By reversing the polarity of the battery!


+

-

S

N

The Electromagnet in a Stationary
Magnetic Field


If we surround the electromagnet with a stationary magnetic field, the
poles of the electromagnet will attempt to line up with the poles of the
stationary magnet.












The rotating motion is transmitted to the shaft, providing useful mechanical
work. This is how DC motors work!

OPPOSITE

POLES

ATTRACT!

26

Faraday’s Law


The effect of magnetic field:


Induced Voltage from a Time Changing
Magnetic Field


Production of Induced Force on a Wire


Induced Voltage on a Conductor Moving in a
Magnetic Field

MZS
FKEE, UMP

27

Voltage Induced from a time changing
magnetic field

28

Voltage Induced in a conductor moving in a
magnetic field


Faraday’s Law for moving conductors :
For coils in which wire
(conductor) is moving thru the magnetic flux, an alternate approach is
to separate the voltage induced by time
-
varying flux from the voltage
induced in a moving conductor.



This situation is indicates the presence of an electromagnetic field in a
wire (conductor). This voltage described by Faraday’s Law is called as
the flux cutting or
Electromotive force
, or
emf
.



The value of the induced voltage is given by





E =

Blv


where


E

= induced voltage (V)


B

= flux density (T)


l

= active length of the conductor in the magnetic field (m)


v

= relative
speed

of the conductor (m/s)



The polarity of induced
voltage is given by the
right
-
hand rule.

29

Induced Force (Cont)


The motion of the bar
produces an

electromagnetic
force.
The polarity of the
emf

is

+
ve

where the current
enters the moving bars. The
moving bar

generates a
‘back’
emf

that opposes the current.



The instantaneous electrical power into the bar = mechanical
output power


TRANSFORMERS

Badariah Bais

KKKF163 Introduction to EE Sem II
2006/07

31

Transformer


Made up of inductors.


Not electrically connected.


An ac voltage applied to the primary induces an ac voltage in the secondary.

32

Types of Transformer

Step
-
up transformer


-


provides a secondary voltage that is
greater than

the primary voltage.

Step
-
down transformer


-


provides a secondary voltage that is
less than

the primary voltage.

Isolation transformer


-
provides a secondary voltage that is
equal
to

the primary voltage.

-
to isolate the power supply electrically from
the power line, which serves as a
protection.

33

Transformer


secondary voltage

The turns ratio of a transformer is equal to the voltage ratio of the component:

)
(
)
(
1
2
1
2
t
v
t
v
N
N

or

)
(
)
(
1
1
2
2
t
v
N
N
t
v

For example:

ac
ac
V
V
t
v
N
N
t
v
30
)
120
(
4
1
)
(
)
(
1
1
2
2



34

Transformer


secondary current

Assuming the transformer is 100% efficient, then

or

1
2
P
P

)
(
)
(
)
(
)
(
1
1
2
2
t
i
t
v
t
i
t
v

)
(
)
(
)
(
)
(
1
2
1
2
t
i
t
v
t
v
t
i

)
(
)
(
1
2
1
2
t
i
N
N
t
i

35

Example

Consider the source, transformer, and load shown in the circuit below. Determine
the rms values of the currents and voltages (a) with the switch open and (b) with
the switch closed.

36

Example

Consider the source, transformer, and load shown in the circuit below. Determine
the rms values of the currents and voltages (a) with the switch open and (b) with
the switch closed.

V
rms
V
110
)
(
1

Solution

Voltage applied to the primary,

V
rms
V
N
N
rms
V
22
)
110
(
5
1
)
(
)
(
1
1
2
2



(a) With the switch open, the secondary current is zero. Hence, the primary
current is also zero.

(b) With the switch closed,

A
R
rms
V
rms
I
L
2
.
2
10
22
)
(
)
(
2
2



A
rms
I
N
N
rms
I
44
.
0
)
2
.
2
(
5
1
)
(
)
(
2
1
2
1



Badariah Bais

KKKF163 Introduction to EE Sem II
2006/07

37

Transformer Rating


The rating of a transformer is stated as Volt
Ampere (VA) that it can transform without
overheating.


The transformer rating can be calculated as
either V
1
I
1

or V
2
I
2

where I
2

is the full load
secondary current.

DC Motor Construction

DC

motors

provide

very

precise

control

for

industrial

applications
.


DC

motors

can

be

used

With
:

conveyors,


elevators,

robots

marine

applications,


material

handling,


paper,

plastics,

rubber,

steel,

and

textile

applications
.

38

DC Motor Construction

Construction

DC motors are made up of the
following components :

• Frame

• Shaft

• Bearings

• Main Field Windings


(Stator)

• Armature (Rotor)

• Commutator

• Brush Assembly

39

40

DC Machine Construction

Figure 8.3 Details of the commutator of a dc motor
.

41

DC Machine Construction

Figure 8.4 DC motor stator with poles visible.

42

DC Machine Construction

Figure 8.5 Rotor of a dc motor.

43

DC Machine Construction

Figure 8.6 Cutaway view of a dc motor.

DC Motor Construction

Basic Construction

Field windings are mounted on pole pieces to form electromagnets.

In smaller DC motors, the field may be a permanent magnet.

The armature is inserted between the field windings.

The armature is supported by bearings .

Carbon brushes are held against the commutator.

44

DC Motor Construction

Armature


The armature rotates between the poles of the field windings.

The armature is made up of a shaft, core, armature windings, and
a commutator.

The armature windings are wound and then placed in slots in the
core.

45

DC Motor Construction

Brushes


Brushes

ride

on

the

side

of

the

commutator

to

provide

supply

voltage

to

the

motor
.


Dirt

on

the

commutator

can

inhibit

supply

voltage

from

reaching

the

armature
.


The

action

of

the

carbon

brush

against

the

commutator

causes

sparks

which

may

be

problematic

in

hazardous

environments
.

46

DC Motor Operation

Magnetic Fields

In small DC motors, permanent magnets can be used for the stator.
However, in large motors used in industrial applications the stator is
an electromagnet.

When voltage is applied to stator windings an electromagnet with
north and south poles is established.

The resultant magnetic field is static (nonrotational).

47

DC Motor Operation

Magnetic Fields

A

DC

motor

rotates

as

a

result

of

two

magnetic

fields

interacting

with

each

other
.


The

first

field

is

the

main

field

that

exists

in

the

stator

windings
.


The

second

field

exists

in

the

armature
.

Whenever

current

flows

through

a

conductor

a

magnetic

field

is

generated


around

the

conductor
.

48

DC Motor Operation

Right
-
Hand

Rule

for

Motors

A

relationship,

known

as

the

right
-
hand

rule

for

motors,

exists

between

the

main

field,

the

field

around

a

conductor,

and

the

direction

the

conductor

tends

to

move
.

If

the

thumb,

index

finger,

and

third

finger

are

held

at

right

angles

to

each

other

and

placed

as

shown

in

the

following

illustration

so

that

the

index

finger

points

in

the

direction

of

the

main

field

flux

and

the

third

finger

points

in

the

direction

of

electron

flow

in

the

conductor,


the

thumb

will

indicate


direction

of

conductor

motion
.


49

DC Motor Operation

Right
-
Hand

Rule

for

Motors


As

can

be

seen

from

the

following

illustration,

conductors

on

the

left

side

tend

to

be

pushed

up
.

Conductors

on

the

right

side

tend

to

be

pushed

down
.

This

results

in

a

motor

that

is

rotating

in

a

clockwise

direction
.

We

will

see

later

that

the

amount

of

force

acting

on

the

conductor

to

produce

rotation

is

directly

proportional

to

the

field

strength

and

the

amount

of

current

flowing

in

the

conductor
.

50

DC Motor Operation

CEMF

Whenever

a

conductor

cuts

through

lines

of

flux

a

voltage

is

induced

in

the

conductor
.

In

a

DC

motor

the

armature

conductors

cut

through

the

lines

of

flux

of

the

main

field
.

The

voltage

induced

into

the

armature

conductors

is

always

in

opposition

to

the

applied

DC

voltage
.

So

it

is

known

as

CEMF

(counter

electromotive

force)
.


The

amount

of

induced

CEMF

depends

on

the

number

of

turns

in

the

coils,

flux

density,

and

the

speed

of

the

motor
.

51

DC Motor Operation

Commutation

In

the

following

illustration

of

a

DC

motor

only

one

armature

conductor

is

shown
.

Half

of

the

conductor

has

been

shaded

black,

the

other

half

white
.


The

conductor

is

connected

to

two

segments

of

the

commutator
.

In

position

1

the

black

half

of

the

conductor

is

in

contact

with

the

negative

side

of

the

DC

applied

voltage
.

Current

flows

away

from

the

commutator

on

the

black

half

of

the


conductor

and

returns

to

the


positive

side,

flowing

towards

the


commutator

on

the

white

half
.

52

DC Motor Operation

Commutation

At

position

3

current

flows

away

from

the

commutator

in

the

white

half

and

toward

the

commutator

in

the

black

half
.

Current

has

reversed

direction

in

the

conductor
.


This

is

known

as

commutation
.

53

DC Motor Operation

Types

of

DC

Motors

The

field

of

DC

motors

can

be

a

permanent

magnet,

or

electromagnets
.

Permanent

Magnet

Motors

The

permanent

magnet

motor

uses

a

magnet

to

supply

field

flux
.

Permanent

magnet

DC

motors

have

excellent

starting

torque

capability

with

good

speed

regulation
.

A

disadvantage

of

permanent

magnet

DC

motors

is

that

these

motors

can

be

found

on

low

horsepower

applications

only
.

Another

disadvantage

is

that

torque

is

usually

limited

to

150
%

of

rated

torque

to

prevent

demagnetization

of

the

permanent

magnets
.

54

DC Motor Operation

Series

Motors


In

a

series

DC

motor

the

field

is

connected

in

series

with

the

armature
.

The

field

must

carry

the

full

armature

current
.

The

series

motor

develops

a

large

amount

of

starting

torque
.


However,

speed

varies

widely

between

no

load

and

full

load
.


Series

motors

cannot

be

used

where

a

constant

speed

is

required

under

varying

loads
.

Additionally,

the

speed

of

a

series

motor

with

no

load

increases

to

the

point

where

the

motor

can

become

damaged
.

Some

load

must

always

be

connected

to

a

series
-
connected

motor
.

Series
-
connected

motors

generally

are

not


suitable

for

use

on

most

variable

speed

drive


applications
.

55

DC Motor Operation

Shunt

Motors

In

a

shunt

motor

the

field

is

connected

in

parallel

(shunt)

with

the

armature

windings
.

The

shunt
-
connected

motor

offers

good

speed

regulation
.

The

field

winding

can

be

separately

excited

or

connected

to

the

same

source

as

the

armature
.


An

advantage

to

a

separately

excited

shunt

field

is

the

ability

to

provide

independent

control

of

the

armature

and

field
.


56

DC Motor Operation

Compound

Motors

Compound

motors

have

a

field

connected

in

series

with

the

armature

and

a

separately

excited

shunt

field
.

The

series

field

provides

better

starting

torque

and

the

shunt

field

provides

better

speed

regulation
.


57

DC Motor Operation

Review

2

1
.

The

field

in

larger

DC

motors

is

typically

an____________

.

2
.

Whenever

____________

flows

through

a

conductor

a

magnetic

field

is

generated

around

the

conductor
.

3
.

Voltage

induced

into

the

conductors

of

an

armature

that

is

in

opposition

to

the

applied

voltage

is


known

as____________

.

4
.

Identify

the

following

motor


types
.

58

DC Motor Operation

DC

Motor

Ratings

The

nameplate

of

a

DC

motor

provides

important

information

necessary

for

correctly

applying

a

DC

motor

with

a

DC

drive
.

The

following

specifications

are

generally

indicated

on

the

nameplate
:



Manufacturer



Horsepower

at

Base

Speed



Maximum

Ambient

Temperature



Insulation

Class



Base

Speed

at

Rated

Load



Rated

Armature

Voltage



Rated

Field

Voltage



Armature

Rated

Load

Current



Winding

Type

(Shunt,

Series,

Compound,

Permanent

Magnet)



Enclosure

59

DC Motor Operation

Typically

armature

voltage

in

the

U
.
S
.

is

either

250

VDC

or

500

VDC
.

The

speed

of

an

unloaded

motor

can

generally

be

predicted

for

any

armature

voltage
.


For

example,

an

unloaded

motor

might

run

at

1200

RPM

at

500

volts
.

The

same

motor

would

run

at

approximately

600

RPM

at

250

volts
.

60

Review

3

1
.

One

way

to

increase

motor

speed

is

to

____________armature

voltage
.



a
.

increase



b
.

decrease

2
.

CEMF

is

zero

when

the

armature

is

____________

.



a
.

turning

at

low

speed



b
.

turning

at

max

speed



c
.

not

turning



d
.

accelerating

3
.

A

____________

-

connected

motor

is

typically

used

with

DC

drives
.

61