11-DC Generators Part2

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11-DC Generators Part 2Text: 5.9 – 5.16

ECEGR 450

Electromechanical Energy Conversion

Overview

• Introduction

• DC Generator Types

• Voltage Regulation

• Losses• Separately Excited Generator

• Self Excited Generators

• Maximum Efficiency Criterion

© H. Louie, 2008 2

DC Generator Types

• DC generators can be classified by excitationmethod

Separate

• Excitation current supplied by external source

Self

• Excitation current self supplied

• Permanent Magnet (PM) generators can beconsidered separately excited generators

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DC Generator Types

• Self-excited generators can also be classifiedbased upon how the excitation winding isconnected:

Series

Shunt (parallel)

Compound (combination of series and shunt)

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Voltage Regulation

• In all dc generators, as current (load) increases,the terminal voltage drops

Ohmic losses in the armature Armature reaction

• The voltage drop is desired to be minimal

• Voltage Regulation is a metric for quantifying thevoltage drop with respect to load

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Voltage Regulation

VR: percent voltage regulation (%)

VnL: terminal voltage under no load (V)

VfL: terminal voltage under full load (V)

• Ideal voltage regulation is 0%

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100nL fL

fL

V V VR

V



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Losses

• No machine is 100 efficient

• General categories of losses:

Mechanical

Magnetic Copper

Stray Load

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Mechanical Losses

• Losses due to:

Friction of bearings

Friction between brushes and commutator

Drag on the armature (caused by the air around it)• Losses tend to increase with rotational speed

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Magnetic Losses

• Losses due to:

Hysteresis

Eddy-currents

• Operating the machine in the linear region and ata low flux density (make the machine physicallylarger) decreases magnetic losses

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Rotational Losses

• Mechanical and magnetic losses are oftengrouped together as “rotational losses”

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Copper Losses

• Copper has a non-zero resistance, so power isdissipated when current flows through it

• Power Loss is equal to i2R• Contributors to copper losses:

Armature-winding loss

Shunt field-winding loss

Series field-winding loss

Interpole field-winding loss

Compensating field-winding loss

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Stray Load Loss

• Stray load loss: a “catch all” term for the lossesthat are unaccounted for in the previous

categories Commutation losses

Distorted flux due to armature reaction

• Approximately equal to 1% in large (>100horsepower)

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Efficiency

• Efficiency of a machine is the ratio of outputpower to input power

• Output power

Po: output power (W)

Pr: rotational losses (W)

Pcu: copper losses (W)

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rP

o s m cu

P T P

Efficiency

• Efficiency in percent is then:

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100 100o o

in s m

P P

P T

Generator Types

• We next consider how thefield windings are powered

• Three types considered:

Separately excited

Shunt

Series

Compound

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cross sectionfield windings

N

S

N

S

Separately Excited Generator

• DC generator in which a external dc source isused to generate the field current

• External source can be

Battery

Another DC generator

Rectified AC

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• Equivalent circuit shown

vt: generator terminal voltage (V)

vf : applied field winding voltage (V)

Rfw: field winding resistance (Ohm)

Rfx: adjustable field winding resistance (Ohm)

Ra: armature resistance (Ohm)

Nf : field winding turns per pole

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+

-

+

-Ea

RL

Rfw

Rfx

Ra

Nf vt

+

-

iL

f ield circuit generator circuit

vf if


• Notes: we will assume that the generator isoperating in steady state

mechanical energy does not change Inductance acts as a short

• Rfx is used to control the field current, and hencethe flux

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• Defining equations are:

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+

-

+

-Ea

RL

Rfw

Rfx

Ra

Nf vt

+

-

iL

f ield circuit generator circuit

vf if

( )f f fw fx f f

a t a a

L a

v i R R i R

E v i R

i i


• If if and m are constant, then Ea is independentof the armature current

• As load increases (iL increases), the terminal

voltage drops due to Ra• Vtnl = Ea (no load terminal voltage = induced

emf)

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load

v t

vtnl

including armature reaction

Shunt Generator

• Instead of using an external dc circuit, connectthe terminals of the generator to the fieldwinding

• This is known as a “shunt generator”

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Shunt Generator

• Equivalent circuit shown

• Defining Equations:

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+

-Ea

RL

Rfw

Rfx

Ra

Nf

vt

+

-

iL

if

( )t f fw fx f f

t a a a

a L f

v i R R i R

v E i R

i i i

Shunt Generator

• It is interesting to examine what happens to ashunt generator under no-load

• Under no load ia = if • Rf is usually large since vt can be large

Large number of turns of small gauge

• Ea will be 0 since there is no flux created by fieldwinding (ia = 0)

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Shunt Generator

• However, generally there is residual magnetism inthe stator and a small amount of voltage will be

induced This increases ia, which increase Ea and so on

The process does not continue for ever

Saturation of the stator limit the process

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Shunt Generator

• Voltage build-up process

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if

v t

vtnl

Er

field resistance line

magnetization curve

Shunt Generator

• The no-load voltage depends upon the field-circuit resistance

• Smaller resistances increase the rate of build-up

•

If the resistance is too large (greater than the “critical resistance”) then voltage build-up doesnot occur

• See Figure 5.24 for an example

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Shunt Generator

• Under no load: ia = if Vt is nearly equal to Ea since iaRa is small

• As il increases

iaRa increases

Armature reaction demagnetization effect increases

• Hence, Ea decreases

This further lowers if and Ea

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Shunt Generator

• If the load resistance continues to decrease, theload current will also start to decrease

due to the decrease in terminal voltage

• If the terminals are shorted, the field currentbecomes zero, but current still flows due to theresidual magnetism Er

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Shunt Generator

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Load current iL

v t

vtnl

rated load

with Ra drop

Shunt Generators

• Shunt generators must operate in the saturatedregion

• Otherwise, an increase in load would decreasethe field current, which would have a large effecton Ea

• This would further drop if , and so on

• Operation in the saturated region desensitizes thechange in flux due to the change in field current

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Series Generator

• Assume now that the field winding is placed inseries with armature and external circuit

• Known as a “Series Generator”

•

A series field diverter resistance (Rd) is used tocontrol the flux

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Series Generator

• Equivalent circuit

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+

-Ea

Rs

Rd

Ra

Ns

id

iL vt

+

-

is

ia

t a a a s s

a L s d

s s d d

v E i R i R

i i i i

i R i R

Series Generator

• When under no load, the produced flux in thefield is zero

Ea is equal to Er

• As load increases, flux increases

Ea increases

• Terminal voltage drops due to series resistanceand armature reaction

• Ea and vt are functions of the load current

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Series Generator

• Note: il = ia• Terminal voltage increases with load current

• As il increases, it is possible to drive the terminalvoltage to zero due to armature reaction

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Load current iL

v t

With armature and field windingdrops and armature reaction

Magnetization curve

Compound Generator

• Terminal voltage:

Decreases with load in a shunt generator

Rises with load in a series generator

• Combine them into a single generator

• Known as a “Compound Generator”

• Several types, depending on how they are wound

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Compound Generator

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Series winding

Shunt winding

S

Cumulative

Series winding

Shunt winding

S

Differential

if if

is is

(mmfs add) (mmfs subtract)



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Compound Generator

• Short-shunt compound:

series winding is in between the shunt and load

• Long-shunt compound:

Shunt winding connected directly across the load

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Compound Generator

• A long-shunt cumulative generator

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+

-Ea

Rfw

Rd

Ra

ia

if

Rfx

Nf

NsRs

id

vt

-

+

if il

Compound Generator

• A long-shunt differential generator

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+

-Ea

Rfw

Rd

Ra

ia

if

Rfx

Nf

NsRs

id

vt

-

+

if il

Compound Generator

• In any configuration:

Shunt winding provides the majority of the flux

Series winding controls the total flux

• Adjusting the current through the series windingallows for three different degrees of compounding

Under-compound

Normal compound

Over-compound

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Compound Generator

• Under-compound generator

Full-load voltage is slightly higher than in a shunt

generator, but still lower than no-load voltage Voltage regulation is better than in a shunt

generator

• Flat-compound generator

Full-load voltage is equal to the no-load voltage

Voltage regulation is better than in a shuntgenerator

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Compound Generator

• Over-compound generator

Full-load voltage is higher than no-load voltage

Useful when connected to a long transmission line(to compensate for the voltage drop)

Compound generators are usually over-compound

See text for more details and comparison of generator types (Figure 5.32)

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Maximum Efficiency

• Generator efficiency varies with load

• Operating at maximum efficiency is desirable

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o t L

2in t L L a r

t L

2

t L L a r

P v i

P v i i R P

v i

v i i R P

Maximum Efficiency

• iLm: load current under maximum efficiency

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( )Lm

Lm

2 2

t Lm a r

2

a r

rLm

a

0v i i R P

i R PP

iR

Maximum Efficiency

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2 2

2 2

2

Separately Excited:

Shunt:

( )

Series

( )

Lm a r f f

Lm a r f a f

Lm a s r

i R P i R

i R P i R R

i R R P

Maximum Efficiency

• Short-shunt:

• Long shunt:

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( ) ( )2 2Lm a s r f a f i R R P i R R

( ) ( )2 2Lm a s r f a f si R R P i R R R

Reading Assignment

• Text Chapter 6

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11-DC Generators Part2

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