Transcript Electrical Machine & Applications 1

ELECTRICAL MACHINES
DET 204/3
JIMIRAFIZI BIN JAMIL
CHAPTER 3
Direct Current (DC) Motor
Electrical Machine & Applications 1
DC Motor
There are four major types of DC motor in general use:
• Separately excited DC motor
• Shunt DC Motor
• Series DC Motor
• Compounded DC Motor
Electrical Machine & Applications 1
Speed Regulations
• DC motors are often compared by their speed regulations.
• Speed Regulations (SR) of the a motor is defined by
SR 
nl  fl
x 100%
fl
n nl  n fl
SR 
x 100%
n fl
Electrical Machine & Applications 1
The Equivalent Circuit of a DC Motor
Armature circuit represented by voltage source, EA and a
resistor RA.
The brush voltage drop represented by battery, Vbrush opposing
the direction of current flow in the machine.
The field coils, which produce the magnetic flux are represented
by inductor LF and resistor RF.
The separate resistor, Radj represents an external variable
resistor used to control the amount of current in the field circuit.
Electrical Machine & Applications 1
The Equivalent Circuit of a DC Motor
The brush voltage drop is often only a very tiny fraction of
the generated voltage in a machine so the voltage drop
may be left out or approximately included in the value of
RA.
The internal resistor in the field coils is sometimes lump
together with the variable resistor, and the total is called
RF.
Electrical Machine & Applications 1
The Equivalent Circuit of a DC Motor
The internal generated voltage in DC motor is
E A  K
The induce torque developed by DC motor is
ind  KI A
Electrical Machine & Applications 1
Separately Excited and Shunt DC motors
VF
IF 
RF
VT  E A  I A R A
IL  IA
Separately excited DC motor
IF = VT
RF
VT  E A  I A R A
IL  IA  IF
Shunt DC motor
Electrical Machine & Applications 1
Example
A 50hp, 250V, 1200rpm DC shunt motor with
compensating windings has an armature resistance of
0.06Ω. Its filed circuit has a total resistance of 50Ω, which
produces a no load speed of 1200rpm. There are 1200
turns per pole on the shunt field winding. Find:
1.
2.
3.
4.
the speed of this motor when its input current is 100A
the speed of this motor when its input current is 200A
the speed of this motor when its input current is 300A.
the induced torque of this motor for above conditions.
Electrical Machine & Applications 1
Solution
VT  250V
n  1200rpm
2n 2 x 1200


 125.67 rads 1
60
60
E A  K
The relationship between the speeds and internal
generated voltages of the motor at two different load
conditions is
E A1  K1
(1)
EA2  K2
(2)
The flux is constant and
no armature reaction
Electrical Machine & Applications 1
Solution
E A1 K1

E A 2 K2
At no load, n  1200rpm
IA = 0A
E A1  VT  250V
1)
VT
250
IA  IL  IF  IL 
 100 
 95A
RF
50
Electrical Machine & Applications 1
Solution
EA at this load will be
EA  VT  IA R A  250  95(0.06)  244.3V
The resulting speed of this motor is
E A1 K1

E A 2 K2
E A 2 1 244.3x125.67
2 

 122.8rads 1
E A1
250
60 60 x122.8
n2 

 1173rpm
2
2
Electrical Machine & Applications 1
Answer
2)
IA = 195A,
EA = 238.3V,
n2 = 1144rpm
3)
IA = 295A,
EA = 232.3V,
n2 = 1115rpm
Electrical Machine & Applications 1
Solution
4)
Pconv  E A I A  ind
E A IA
ind 

At IL = 100A
ind
244.3x 95

 189 Nm
122.8
At IL = 200A
ind  388Nm
At IL = 300A
ind  587 Nm
Electrical Machine & Applications 1
Series DC Motor
VT  E A  I A (R A  R s )
I L  I A  IS
Electrical Machine & Applications 1
Compounded DC Motor
A compounded DC motor is a motor with both a shunt
and a series field.
Current flowing into
magnetomotive force.
a
dot
produces
a
positive
If current flows into the dots on both field coils, the
resulting magnetomotive forces add to produces a larger
total magnetomotive force. It is called cumulative
compounding.
Electrical Machine & Applications 1
Equivalent circuit of compounded DC motor
VT  E A  I A (R A  R s )
IA  IL  IF
VF
IF 
RF
Long shunt connection
Fnet  F F  FSE - FAR
N SE
FAR
I  IF 
IA 
NF
NF
*
F
Short shunt connection
Electrical Machine & Applications 1
DC Generator
There are five major types of DC generators:
1. Separately excited generator. In a separately excited generator, the
field flux is derived from a separately power source independent of
the generator itself.
2. Shunt generator. In a shunt generator, the field flux is derived by
connecting the field circuit directly across the terminals of the
generator.
3. Series generator. In a series generator, the field flux is produced by
connecting the field circuit in series with the armature of the
generator.
4. Cumulatively compounded generator. In a cumulatively compounded
generator, both a shunt and a series field are present, and their
effects are additive.
5. Differentially compounded generator. In a differentially compounded
generator, both a shunt and a series field are present, but their effects
are subtractive.
Electrical Machine & Applications 1
DC Generator
DC generators are compared by their voltages, power
ratings, efficiencies, and voltage regulations.
Voltage regulation, VR is defined by
Vnl  Vfl
VR 
x 100%
Vfl
Electrical Machine & Applications 1
Separately Excited Generator
IL  IA
IF 
VF
RF
VT  E A  I A R A
Electrical Machine & Applications 1
Shunt DC Generator
IA  IF  IL
VT  E A  I A R A
Electrical Machine & Applications 1
Series DC Generator
I A  IS  I L
VT  E A  I A (R A  R S )
Electrical Machine & Applications 1
The Cumulatively Compounded DC Generator
Total Magnetomotive force
Fnet  F F  FSE - FAR
N F I*F  N F I F  N SE I A  FAR
I*F  I F 
Cumulatively compounded dc generator with a long
shunt connection
N SE I A FAR

NF
NF
IA  IF  IL
VT  E A  I A (R A  R S )
VT
IF 
RF
Cumulatively compounded dc generator with a short
shunt connection
Electrical Machine & Applications 1
Differentially Compounded DC Generator
With a long shunt connection
Fnet  F F  FSE - FAR
N I  N F I F  N SE I A  FAR
*
F F
Equivalent shunt field
current,
I eq  
N SE I A FAR

NF
NF
N SE I A FAR
I  IF 

NF
NF
*
F
Electrical Machine & Applications 1
Assignment 4
A 12kW, 240V, 1200 rpm, separately excited DC
generator has armature and field winding resistances
of 0.20Ω and 200Ω, respectively. At no load, the
terminal voltage is 240V, the field current is 1.2A, and
the machine runs at 1200rpm. When the generator
delivers rated current to a load at 240V, calculate
a) The generated voltage, EA
b) The field circuit voltage, VT
c) The developed torque, τder