principles of electric machines and power electronicsli/slides/ch04-2sli.pdf · dc machines third...

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

Chapter 4DC Machines

Third Edition

P. C. Sen

Principles of Electric Machines

and

Power Electronics

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.Fig_1-1

Chapter 4 DC machine

•Main contentsDC machine classifications

DC generator

o Separately excited type

o Self-excited shunt type

o Self-excited compound type


, , or 2a a m a

NpE K K a p a

a

a aT K I

DC machine structure and equations

• DC generator:

• DC motor:


DC machine representation- two circuits

• Field winding circuit

• Armature circuit


• Basic field winding connection: Separately excited type: fielding winding excited from a separate

source—voltage source or permanent magnets

Self-excited type: series, shunt and compound

• Self-excited connection: Shunt winding: large number of turns; small current (less than 5%); in

parallel with armature (connected or disconnected)

Series winding: fewer turns; larger current; in series with armature

Compound winding: both shunt and series

• Rheostat: control field current and vary the field mmf

Interconnection of field and armature circuits


Classifications(a) Separately excited (or permanent magnets)

(b) Self-excited (series-type)

(c) Self-excited (shunt-type)

(d) Self-excited (compound-type—short shunt and long shunt)


• Ra – armature winding resistance

• Rfw - field winding resistance

• Rfc - field winding resistance

• RL - load resistance

• Neglect winding inductance

Equations of separately excited type DC generator

f fw fcR R R f f f

a t a a

a a m

t t L

a t

V I R

E V I R

E K

V I R

I I

Steady-state model of a separately excited dc generator


Armature reaction effects

• Without armature current, flux is

determined by field current

• With armature currentArmature current generate local flux

Flux density increase in one half of the

pole, decreases in another half

Increased flux density cause magnetic

saturation

Net effect: reduction of flux per pole


Impacts of armature reaction:

Flux reduction

Generated voltage decreases


Terminal characteristic: variation of terminal voltage with

load current

A separately excited dc generator

• Due to armature reaction, terminal voltage is slightly lower than predicted

value by equivalent model

•Armature reaction is more pronounced at higher armature currents

𝑉𝑡 = 𝐸𝑎 − 𝐼𝑎𝑅𝑎

𝐼𝑓(𝑒𝑓𝑓) = 𝐼𝑓 − 𝐼𝑓(𝐴𝑅)


Compensating winding

On stator

In series with armature winding

Produced mmf opposes armature

mmf/ enhance field mmf

Produced mmf is proportional to

armature current

Expensive design

Used in larger machines

Used in machine subject to abrupt

changes of armature current


Example

In this book, rated data of DC machine is

considered to apply to armature, regardless of

generator or motor

A 12 kW, 100V, 1000rpm DC shunt generator has an armature resistance of Ra = 0.1Ω, field (shunt) winding resistance of Rfw = 80 Ω and Nf = 1200 turns

per pole. The rated field current is 1 A. The magnetization characteristic at

1000 rpm is given. The machine is operated as a separately excited DC

generator at 1000 rpm with rated field current. Determine:

(a.) terminal voltage at full load neglecting the armature reaction.

(b.) terminal voltage at full load accounting for the armature reaction. Assume

the armature reaction at full load is equivalent to 0.06 field Amperes.

(c)the field current necessary to make the terminal voltage equal to 100 V at

full load.


Shunt (self-excited) DC generator

Armature voltage supplies field current

Close switch SW, voltage builds up

Residual magnetism—initial voltage at certain speed

Circuit schematic without load B-H hysterisis curve


Voltage Buildup Process Without Load

Initially, Ear exists because of residual magnetism

Close switch SW, current If1 flows in fielding winding and

generates voltage Ea1, which is larger than terminal voltage

Ea1 increases current to If2, which generates Ea2

Process continues until point P where no excess voltage is

available to further increase field current

Magnetization curve


Equations of Self-excited dc generator

( )

a t a a

a a m

f f f f fw fc

t L L

a f t

E V I R

E K

V I R I R R

V I R

I I I

f fw fcR R R

𝑉𝑡 = 𝐼𝑡𝑅𝐿


Terminal characteristic of shunt dc generator

(Without armature reaction)

Magnetization curve

1. For each terminal

voltage Vt1, compute

Ia1 from voltage drop

(vertical distance

between Vt1 and Ea1)

2. Perform step 1 on

various terminal

voltages

3. It=0, Ia=If, Vt0…

Terminal characteristics

V-I curve


Compound DC machine

When DC machines deliver current Voltage drop due to Ra

Pole fluxes decreases due to armature reaction

Need A winding Mount on field poles

Connects in series with armature winding

Carries armature current

Provides additional ampere-turns to increase or decrease pole

fluxes

A compound machine has both shunt and series winding Shunt field winding: main field winding, provides major portion of

mmf, carries low current, has many turns

Series winding: fewer turns, carries armature current, provides

mmf for compensation


Equivalent circuit of compound dc machines.

Short shunt Long shunt

t a a a t sr

t a f

V E I R I R

I I I

( )t a a a sr

t a f

tf

fw fc

V E I R R

I I I

VI

R R

sh sr

(eff ) sr sr (AR)

( )a a m

f f f f f f

E K

N I N I N I N I

Flux by shunt

winding

Flux by series

winding

+ Cumulative compound

- Differential compound

- -

-

-

-

Φ Φ


Terminal V-I characteristics of compound dc generators

Depend on : degree of compounding- turn number of

series field winding

Terminal voltage may rise, decrease, remain flat.


Series Generator

sr( )a t a a

t a

E V I R R

I I

Series field winding provides the flux Voltage drop due to Ra

Field circuit is not completed unless a load is connected to machine


Magnetization curve

• If armature reaction is considered, terminal voltage is less.

• ab represents armature reaction in equivalent armature

current


Terminal characteristics

If RL is too large, terminal voltage will be very small

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