principles of electric machines and power electronicsli/slides/ch04-2sli.pdf · dc machines third...
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.
Chapter 4DC Machines
Third Edition
P. C. Sen
Principles of Electric Machines
and
Power Electronics
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Chapter 4 DC machine
•Main contentsDC machine classifications
DC generator
o Separately excited type
o Self-excited shunt type
o Self-excited compound type
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, , 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:
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DC machine representation- two circuits
• Field winding circuit
• Armature circuit
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• 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
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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)
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• 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
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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
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Impacts of armature reaction:
Flux reduction
Generated voltage decreases
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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)
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.Fig_4-33
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
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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.
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.Fig_4-34
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
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.Fig_4-34
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
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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
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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
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.Fig_4-41
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
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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
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.Fig_4-42
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.
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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
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Magnetization curve
• If armature reaction is considered, terminal voltage is less.
• ab represents armature reaction in equivalent armature
current
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Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.Fig_4-45
Terminal characteristics
If RL is too large, terminal voltage will be very small