Dc machines electrical machines – i
-
Upload
soumyadeep-nag -
Category
Engineering
-
view
333 -
download
28
Transcript of Dc machines electrical machines – i
Electrical Machines – I
DC machines
General Idea
Electrical Machine
Electrical Energy Mechanical Energy
Losses – I2R, friction, etc
Types of electrical machines
Types of electrical machines
Rotating type Stationary type
Transformers
Motors and generators
AC motors and generators
DC motors and Generators
Magnets and field
Permanent Magnet
Electromagnet
The magnetic field can be controlled by regulating the amount of current flowing
Faraday’s Coil and Magnet Experiment - Deflection
The amount of deflection
is proportion
al to the amount of
voltage induced
Faraday’s Experiment – Effect of speed of movement of the magnet
The amount of deflection
is proportional
to the amount of
voltage induced
(electromotive force)
Faraday’s Experiment – effect of number of turns of the coil
The amount of deflection
is proportion
al to the amount of
voltage induced
Faraday’s Law• Faraday’s law of electromagnetic induction: - – The induced electromotive force (EMF or voltage) in any closed
circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit. For a coil of wire of N turns EMF is given as: -
– The negative sign indicates that the direction of EMF is opposite to the source that produces it
– is the number of magnetic field lines that a conductor intersects per unit time
dtdNe
dtd
dtde
DC Generators
Lorentz’s Single conductor Experiment in a magnetic field – a generator point of view
I
Conductor
N
W
S
E
F
B
F
B
I
Lorentz force law - generators
•When a conductor is moved in a magnetic field, the electrons experience a force which causes current to flow in the conductor and this current is mutually perpendicular to the direction of the field and the force applied. [For Generators]
•Where θ is the angle between the wire and the magnetic field.• q = charge• v = velocity of electrons •B = magnetic field strength
F = qvBsinθ
Lorentz’s Single conductor Experiment in a magnetic field – a generator point of view
I
Rotational direction
Conductor
N
W
S
E
F
B
F
B
I
Flux distribution at different positions
NS
B
C
D
A
dφ/dt = 0
dφ/dt = 0
dφ/dt = max dφ/dt = max
•dφ/dt = number of lines that the conductor intersects (passes through) per unit time•dφ/dt = 0 because the direction of conductor movement is parallel to the direction of flux
Single loop in a magnetic field – position A
N S
V
a
b
c
d
N
W
S
E
R1
R2
Single loop in a magnetic field – position B
N S
V
-ve
+ve
F
I
F
I
a
bc
d
N
W
S
E
R1
R2
Single loop in a magnetic field – position C
N S
V
a
b
c
dN
W
S
E
R1
R2
Single loop in a magnetic field – position D
N S
V-ve
+ve
a
cb
d
N
W
S
E
R1
R2
I
F
I
F
Single loop in a magnetic field – position A
N S
V
a
b
c
d
N
W
S
E
R1
R2
DC from AC
AC waveform
DC waveform
The Idea of commutation
POSITION B
POSITION D
R1+veR2-ve
R1-veR2+ve
Slip rings and Commutator segments
Slip Rings
Commutator Segments
Left Brush Right Brush
Single loop in a magnetic field – position A
N S
V
N
W
S
Ea
b
c
d
CA
CB
1 2
Single loop in a magnetic field – position B
N S
V
+ve-ve
N
W
S
E
d
bc
aCB
CA
1 2
I
I
F
F
Single loop in a magnetic field – position C
N S
V
N
W
S
Ed
c
b
a1 2
CB
CA
Single loop in a magnetic field – position D
N S
V
+ve -ve
N
W
S
E
a
cb
d
1 2
CA
CB
I
I
F
F
Single loop in a magnetic field – position A
N S
V
N
W
S
Ea
b
c
d
CA
CB
1 2
Converting Impure DC to Pure DC
Effect of number of poles
N S
S
S
N
N
polesofnumberppE
__
Eavg1
Eavg2Eavg2 > Eavg1
Effect of number of conductorsN S
N S
Eavg2
Eavg1Eavg2>Eavg1
zE z = total number of conductors
Effect of field strength and Electromagnets
• Another factor effecting the EMF is the field strength. The strength of Permanent magnets depend upon the material with which it is made. However, the strength of an electromagnet depends upon the amount of current passing through it.
• Ampere’s Law– The magnetic field in space around an electric
current is proportional to the electric current.
E
The EMF equation
zpaEn
aznpE
b
b
6060
Eb= generated EMF (volts) (back EMF - motors)p = number of polesn = speed (RPM)z = total number of conductorsa = total number of parallel paths
1 parallel path
2 parallel path
Electromagnet
I
I
NS
II
NS
Electromagnet
• Metals offer an easier path for the flow of magnetic flux
Stator frame
Field coils
Physical structure of DC machines
Physical structure of DC machines
DC Motors
Lorentz’s Single conductor Experiment in a magnetic field – a motors point of view
F
B
I
F
B
I
Rotational direction
Conductor
N
W
S
E
Lorentz’s force law - Motors
• Lorentz force: - If a current carrying conductor (with current I A) is placed in a magnetic field (with field B T), it experiences a force (F N) in a direction mutually perpendicular to the magnetic field and the current. [For Motors]
sinBILF
• B = Magnetic field intensity• I = current in the conductor• L = length of the conductor• θ = angle between the current (or conductor) and the field
Single loop in a magnetic field – position A
N S N
W
S
Ea
b
c
d
CA
CB
1 2 • Two brushes are shorted, hence no current flows through the conductors. +
DC motors
+
N S
+N S
N S
N S
A
B
B
A
A
B
A
B
F
FF
F
Electrical circuit of a DC machine
AIPZT
TIETPalso
IEPRIIEVI
RIEVRIEVRIEV
am
maa
mm
aam
aaaaa
aaa
aab
aab
2
_
2
•It is quite evident that the torque of the motor is directly proportional to the flux produced by magnets and the armature current.
V
Eb = Ea = armature voltage
atm IkT
Steady state characteristics – No load
apzk
kVn
nkVnkEaznpE
e
e
e
ea
a
60
60
VERIVERIEV
a
aaa
aaa
volts
250 500
speed
500
1000
RPM
Ke = 2
• current drawn is very small under no-load or Ia is 0 or of the order of 1e-3• no-load current basically supplies the frictional losses of the machine• We can also say that the no-load speed is directly proportional to the applied voltage
Torque under no-load is the torque required to overcome the frictional losses which are extremely small
Steady state characteristics – When loaded
lma
t
ma
atm
am
TTTkTI
IkTAIPZT
2 Torque
speedRPM
load
motor
900
X
• Acceleration and deceleration is governed by the sign of the accelerating torque• Under steady-state conditions the armature current is constant and the motor torque is equal to the load torque as there is no acceleration
ltee
aaa
aaa
TkkR
kVn
RIVERIEV
900time
Transient characteristics
lm TT
Steady state characteristics – When loaded - Droop
lte
a
e
TkkR
kVn
125V 250V 375V 500V
RPM
500 1000
Torque
Slope = Droop
• Droop is the fall in speed as load is applied• Droop is directly proportional to the armature resistance•Machines with least amount of droop are chosen so that expensive controls can be avoidedΔn
ΔT
Full Load
No-load Δn = 5 to 10%
Connections
Stator frame
Field coils
Types of Electric circuit of a DC machine
• Permanent magnet excited• Separately excited• Shunt Excited
– Constant Speed applications
• Series Excited– Heavy torque
Shunt motors used for constant speed applications. Why?
increasesTIkTincreasesI
increasesEVaznpEE
RVI
m
atm
a
a
ab
shsh
60
V
• This increase in torque causes the machine to accelerate and hence prevent the speed from falling.
• armature current control• armature current becomes extremely high
Series motors used for constant high torque applications. Why?
current
Flux lines
Load added
ase II Φ α Ia
• The Series motor is able to overcome high load torque as the flux is proportional to the load current.
• armature current and flux increase• flux and current control• poor speed regulation due to high series resistance and saturation• higher voltage required due to voltage division
Speed control of DC motors
Control System
DC motorPID Controller-
Reference Speed (1750
RPM) +
Load or Disturbances
Δω
ω
Separately Excited machine rated 5Hp
Ki/sΔω
Kp
sKd
+
V
lte
a
e
TkkR
kVn
Proportional control
Integral control
Derivative control
PID control
dttdeku
dtteku
teku
dd
t
ii
pp
)(
)(
)(
0
Proportional control sets the voltage proportional to the
current value of error in speed
integral control sets the voltage proportional to the accumulated error in speed over a certain time period
Derivative controller sets the voltage proportional to the rate at which the error in speed is approaching zero
Over many time steps the error accumulates and sums up. Seeking
action.
di
p skskksu )(
Reduction of oscillations and snappy response but amplifies
noise.
PID Control
Parameter: Rise Time Overshoot Settling Time S.S.Error Kp Decrease Increase Small Change Decrease Ki Decrease Increase Increase Eliminate Kd Small Change Decrease Decrease None
The End