Download DC Machines

Department of Electrical and
Computer Engineering
EE20A - Electromechanical Energy
Conversion
DC Machine
Construction of DC Machines
Commutator
Features of DC Machine
Field Winding
DC Machines- Direction of Power Flow and Losses
DC Machines- Direction of Power Flow and Losses
DC Machines Analysis
Symbols that will be used.
 = flux per pole
p = no. of poles
z = total number of active conductors on the armature
a = no. of parallel paths in the armature winding
Aside: Lap Winding -> a = p
Wave Winding -> a = 2
n = speed of rotation of the armature in rpm
wm = speed in radians per second
DC Machines Connections
+
Ra
If
+
VT
E
-
-
a)
+
Field
E
F
F
Armature
-
b) Separately Excited
DC Machines Connections
+
E
Armature
Field
F
-
F
c) Series
A
+
E
Armature
Field
F
F
d) Shunt
A
DC Machines Connections
A
+
e) Cummulative Compound
S
F
Field
S
E
Armature
F
A
A
+
S
S
E
Armature
d) Differential Compound
F
Field
F
A
DC Machines Connections
A
+
S
E
S
Armature
f) Long Shunt
Field
F
F
A
A
+
S
S
E
Armature
g) Short Shunt
F
Field
F
A
EMF Equation
When the rotor rotates in the field a voltage is developed in the
armature.
- the flux cut by one conductor in one rotation = p
- therefore in n rotations, the flux cut by one conductor = np
-
EMF Equation
EMF induced in the
armature windings
TORQUE EQUATION
EaIa=Tem
- In the DC machine losses are
expressed as rotational losses
due to friction and windage
(F&W).
- The torque equation can then be
rewritten as:-
SHAFT OUTPUT TORQUE = (Te -
TF&W)
DC Generator
Ia
Ea  Nz p
60 a
V  Ea  Ia R a
L
V
I  L
f R
f
Ia  I  I
f L
+
E
Rf
-
+
If
Ra
L
O
A
D
VT
-
Note: VT = VL
i.e. Terminal Voltage is the Load Voltage
OPEN CIRCUIT CHARACTERISTICS
The Open Circuit characteristic is a graph relating Open-Circuit
Armature voltage of a D.C. Generator versus its field current
when the machine is driven at it’s rated speed
DC
Source
Field Regulator
Ra
ZZ
FL
Rf
F
L
OPEN
CIRCUIT
AA
Ea
V
Z
A
A
Field Current
Diagram showing motor connections for the open circuit test, separately excited
The D.C. Generator field is excited by a separate D.C. source and
the current is varied using a generator Field Regulator (a potential
divider).
OPEN CIRCUIT CHARACTERISTICS
Rext is set to its maximum value. The D.C. Generator is driven at
rated its speed. Rext is decrease to a lower value so that the
machine self-excites ( i.e.. Develop an e.m.f).
A
Field Current
Ra
Rext
970 ohms
Z
Rf
ZZ
A
V
VT
Ea
AA
Diagram showing the D.C. Generator as a self-excited shunt machine
EXTERNAL CHARACTERISTIC OF SHUNT GENERATOR
This is a graph relating terminal voltage and the load current of a
D.C. Generator when driven at its rated speed with the field current
maintained at its normal no-load value.
A
A
Field Current
Load Current
Ra
Rext
970 ohms
Z
Rf
ZZ
A
V
Ea
Terminal
Voltage
AA
Diagram showing connections for load test.
L
O
A
D
Summary
E a  V - I a R a Note: For motor
Ea    n  z p
60  a


V - I
  60  a
R


a
a



Speed n
z  p
E
na a

Losses  I2 R a -- Windings(Armature)
a
 I2 R -- Windings(Field )
f
f
and Rotational Losses (Windage and Friction)