Download electrical engineering technology emt 113/4

FUNDAMENTAL OF
ELECTRICAL ENGINEERING
EMT 113/4
CHAPTER 2:
DC MACHINES
SUBTOPICS
Introduction to DC Machines
DC motors : Principles of Operation,
equivalent circuit & Characteristics
DC generators : Principles of operation,
equivalent circuit & Characteristics
INTRODUCTION TO
DC MACHINES
WHAT ARE DC MACHINES?
 DC generators that convert mechanical energy to DC
electric energy.
 DC motors that convert DC electric energy to mechanical
energy.
• often used as a motor.
• found in many special industrial environments.
Motors drive many types of loads from fans and
pumps to presses and conveyors
• Advantages: easy speed and torque regulation.
• However, their application is limited to mills, mines
and trains. As examples, trolleys and underground
subway cars may use dc motors.
• In the past, automobiles were equipped with dc
dynamos to charge their batteries.
Important parts:
- STATOR : provides mechanical support for the
machine, consists poles and yoke
- ROTOR / ARMATURE : the rotating part,
shrouded by fixed poles on the stator
-COMMUTATOR : mechanical rectifier, which
changes the AC voltage of the rotating
conductors to DC voltage
- BRUSHES : conduct the current from the
commutator to the external circuit
- WINDINGS
•uniform magnetic flux is established by fixed
poles mounted on the inside of the stationary
number called STATOR
•May use permanent magnet as poles or wind
the field windings (excitation coils) around the
poles
• Advantage of wound machine: easy to control
the flux in the machine by regulating the direct
current in the field winding
DC machines, like other
electromechanical energy
conversion devices have
two sets of electrical
windings:
1)
2)
DC motor stator with poles visible
field windings - on
stator
amarture windings on the rotor.
Rotor of a dc motor
DC Machines Construction
• The stator of the dc machine has
poles, which are excited by dc
current to produce magnetic
fields.
• In the neutral zone, in the middle
between the poles, commutating
poles are placed to reduce
sparking of the commutator. The
commutating poles are supplied
by dc current.
• Compensating windings are
mounted on the main poles.
These short-circuited windings
damp rotor oscillations.
• The poles are mounted on an
iron core that provides a closed
magnetic circuit.
• The motor housing supports the
iron core, the brushes and the
bearings.
• The rotor has a ring-shaped
laminated iron core with slots.
• Coils with several turns are
placed in the slots. The
distance between the two legs
of the coil is about 180 electric
degrees.
• The coils are connected in series
through the commutator segments.
• Ends of each coil are connected to a
commutator segment.
• Commutator consists : insulated
copper segments mounted on an
insulated tube.
• Two brushes are pressed to the
commutator to permit current flow.
• Brushes are placed in the neutral
zone, where the magnetic field is
close to zero, to reduce arcing.
• The commutator switches the
current from one rotor coil to the
adjacent coil
• The switching requires the
interruption of the coil current.
• The sudden interruption of an
inductive current generates high
voltages .
• The high voltage produces
flashover and arcing between the
commutator segment and the
brush.
DC MOTORS :
Principles of Operation,
equivalent circuit &
Characteristics
Introduction
TYPES OF DC MOTOR
 Five major types of DC motors:
•
•
•
•
•
Separately excited DC motor
Shunt DC motor
Permanent Magnet DC motor
Series DC motor
Compounded DC motor
 classified according to electrical connections of armature
windings and field windings.
• ARMATURE winding : the winding
which a voltage is induced.
• FIELD windings : the windings that
produce the main flux in the
machines.
• The magnetic field of the field
winding is approximately sinusoidal,
thus AC voltage is induced in the
armature winding as the rotor turns
under the magnetic field of stator.
• The COMMUTATOR and BRUSH
combination converts the AC
generated voltages to DC.
DC Motor Operation
To understand the operation of a DC motor, we
need to know the basic mechanism of the DC
Motor – The Electromagnetism.
LETS REVIEW..!!
Review of Magnetism
Lines of flux define the
magnetic field and are in the
form of concentric circles
around the wire.
The magnetic lines around a
current carrying conductor
leave from the N-pole and reenter at the S-pole.
"Left Hand Rule" states that if you
point the thumb of your left hand in
the direction of the current, your
fingers will point in the direction of
the magnetic field.
The poles of an electro-magnetic coil change when the direction
of current flow changes.
• The motor has a definite relationship between the direction of the magnetic flux,
the direction of motion of the conductor or force, and the direction of the applied
voltage or current.
•Fleming's left hand rule can be used:
- thumb will indicate the direction of motion
- forefinger will indicate the direction of the magnetic field
- middle finger will indicate the direction of current.
•
In either the motor or generator, if the
directions of any two factors are known, the
third can be easily determined.
DC Motor Operation
DC MOTOR OPERATION FLOWS:
1. Uniform magnetic field is created by poles
2. The armature conductors are forces to carry current by
connecting them to DC power source
3. The current direction in the conductors under each pole is
kept the same by commutator
4. According to Lorentz force equation, a current carrying
conductor when placed in a magnetic field experiences a
force that tends to move it (F=ilB)
5. All conductors placed on the periphery of a DC motor are
subjected to the forces
6. These forces cause armature to rotate in the direction of the
torque developed by the motor.
DC Motor Operation
DC Motor Operation : Current
DC Motor Operation : Force
DC Motor Operation : Magnetic Field
Basic principle of operation
• The generated voltage of a DC machines having (p) poles and (Z)
conductors on the armature with (a) parallel path between brushes as
below :
pZ
EA 
 K
2a
where K = pZ /(2πa) = machine constant
• The mechanical torque which also equal to electromagnetic torque,
is found as follows:
e m 
EAI A

 KI A
In the case of a generator:
m is the input mechanical torque, (converted to electrical power)
For the motor:
e is developed electromagnetic torque, (drive the mechanical load)
• The induced or generated DC voltage (EA) appearing between
the brushes is a function of the field current (IF) and the speed of
rotation () of the machine. This generated voltage is :
EA  K I F
'
Where
K’ = voltage constant
 = rotation per mina
• If the losses of the DC machine are neglected, the electrical
power is equal to the mechanical power
E A I A   m
Important Notice
Equation of Induced voltage
when speed, w (in radian per
second /
Angular speed)
EA  K
K  pz / 2a
Where
p : no of poles
z : no of conductors
a : no of current path
Equation of Induced
Voltage when speed, n
(revolution per minute/
Run per minute/
rotation per minute
(rpm)
EA  K `n
K ` pz / 60a
Where
p : no of poles
z : no of conductors
a : no of current path
The Magnetization Curve of a DC machine
• The internal generated voltage in the
motor
EA  K
• From the equation,
EA is directly proportional to the flux () in
the motor and speed of the motor ().
• The field current (IF) in DC machines
produces a field magnetomotive force
(mmf)
• This magnetomotive force (mmf)
produces a flux () in the motor in
accordance with its magnetization curve.
IF  mmf  flux
• Since the field current (IF) is directly
proportional to magnetomotive force
(mmf) and
• EA is directly proportional to the flux,
the magnetization curve is presented as
a plot EA versus field current for a given
speed.
• The induced torque developed by the
motor is given as
 ind  KI A
The magnetization curve of a dc
machine expresses as a plot of EA
versus IF, for a fixed speed ω0
NOTE : To get the maximum possible power, the motors and
generators are designed to operate near the saturation point on the
magnetization curve (at the knee of the curve).
Equivalent Circuit
RA
External variable resistor
used to control the
amount of current in the
field circuit
Equivalent circuit of dc motor
The brush
voltage
drop
Armature circuit (entire
rotor structure)
Field Coils
NOTE: Because a dc motor is the same physical machine as a dc generator, its
equivalent circuit is exactly the same as generator except for the direction of
current flow.
Simplified Equivalent Circuit
Simplified quivalent circuit of dc motor
• The brush drop voltage (Vbrush ) is often only a very tiny fraction of the generated
voltage in the machine – Neglected or included in RA.
• Internal resistance of the field coils is sometimes lumped together with the
variable resistor and called RF
Separately excited DC motor
VF
IF 
RF
IL  I A
Separately excited motor is a
motor whose field current is supplied
from a separate constant-voltage
power supply.
VT  E A  I A RA
Shunt DC motor
VT
IF 
RF
IL  I A  IF
A shunt dc motor is a motor
whose field circuit get its power
directly across the armature
terminals of the motor.
VT  E A  I A RA
Shunt DC Motor :
Terminal Characteristics
• Consider the DC shunt motor. From the Kirchoff’s Law
VT  E A  I A RA
• Induced Voltage
EA  K
• Substituting the expression for induced
voltage between VT and EA.
VT  K  I A RA
• Since, then current IA can be expressed
as

IA 
ind
K
VT  K 
 ind
K
RA
• Finally, solving for the motor's speed yield
VT
RA



2 ind
K ( K)
This equation is a straight line with a negative slope.
Torque-speed characteristic of a shunt or separately excited dc motor
Shunt DC Motor : Terminal Characteristic
• Affect of Armature Reaction (AR) will reduce flux as the load
increase (ind also increase), so it will increase motor speed ().
If the motor has compensating winding, the flux () will be
constant.
VT
RA



2 ind
K ( K)
Torque-speed characteristic of a motor with armature reaction
present.
Shunt DC Motor : Speed Control
1 : Changing The Field Resistance (flux affected)
 V 
1. Increasing RF causes IF   T  to
 RF   decrease.
2. Deceasing IF decreases . (graph flux vs current)
3. Decreasing  lowers EA
 K   
 VT  E A  


RA


4. Decreasing EA increases IA 
5. Increasing IA increases  ind ( K  I A )
with the change in IA dominant over the change in flux ().
6. Increasing τind makes
 ind  load
and the speed ω increases.
Shunt DC Motor : Speed Control
7. Increasing speed to increases EA = K again.
8. Increasing EA decreases IA.
9. Decreasing IA decreases until
 ind   load
at a higher speed ω
Decreasing RF would reverse the whole process, and the speed of the motor
would drop.
The effect of field resistance speed
control on a shunt motor’s torque
speed characteristic: over the motor’s
normal operating range
Shunt DC Motor : Speed Control
2: Changing The Armature Voltage
Armature voltage control of
a shunt (or separately
excited) dc motor.
1. An increase in VA increases IA [= (VA  – EA)/RA]
2. Increasing IA increases
3. Increasing τind makes
 ind ( KI A )
 ind  load
4. Increasing ω increases EA (=Kω  )
increasing ω.
Shunt DC Motor : Speed Control
5. Increasing EA decreases IA [ = (VA – EA)/RA]
6. Decreasing IA decreases τind until
 ind   load
at a higher ω.
The effect of armature voltage speed
control on a shunt motor’s torque
speed characteristic
The speed control is shiftted by this
method, but the slope of the curve
remains constant
Shunt DC Motor : Speed Control
3 : Inserting Resistor in Series with Armature Circuit
Add resistor in
series with RA
Equivalent circuit of DC shunt
motor
The effect of armature resistance
speed control on a shunt motor’s
torque – speed characteristic
Additional resistor in series will drastically increase the slope of the
motor’s characteristic, making it operate more slowly if loaded
Shunt DC Motor : Speed Control
VT
RA



2 ind
K ( K)
The above equation shows if RA increase, speed will
decrease
This method is very wasteful method of speed control, since
the losses in the inserted resistor is very large. For this it is
rarely used.
Series DC Motor
Series DC Motor: DC motor whose field windings consists of relatively few turns
connected in series with armature circuit
Equivalent circuit of a series
DC motor.
The Kirchhoff’s voltage law equation for this motor
VT  E A  I A ( RA  RS )
Series DC Motor : Induced Torque
• The induced or developed torque is given by
 ind  KI A
• The flux in this motor is directly proportional to its armature current.
Therefore, the flux in the motor can be given by
  cI A
where c is a constant of proportionality. The induced torque in this machine is
thus given by
 ind  KI A  KcI A
2
This equation shows, torque in the motor is proportional to the square
of armature current. So, series motor give more torque per ampere
than any other dc motor, therefore it is used in applications requiring
very high torque, e.g. starter motors in cars, elevator motors, and
tractor motors in locomotives.
Series DC Motor : Terminal Characteristic
• To determine the terminal characteristic of a series dc motor,
analysis will be based on the assumption of a linear magnetization curve,
and the effects of saturation will be considered in a graphical analysis
• The assumption of a linear magnetization curve implies that the flux in
the motor given by :
  cI A
• The derivation of a series motor’s torque-speed characteristic starts
with Kirchhoff’s voltage law:
VT  E A  I A ( RA  RS )
From the equation;
be expressed as:
 ind  KI A  KcI A2
IA 
 ind
Kc
the armature current can
• Also, EA = K, substituting these expression yields:
VT  K 
We know
IA 

c
 ind
Kc
( RA  RS )
;
• Substituting the equations so the induced torque
equation can written as
 ind  KcIA
2
K 2
 
c
Therefore, the flux in the series motor can be written as :
c

 ind
K
• Substituting the previous equation for VT yields:
 ind
c
VT  K
 ind  
( RA  RS )
K
Kc
RA  RS
VT
1


Kc
Kc  ind
• Disadvantage of series motor can be seen immediately
from this equation. When the torque on this motor goes to
zero, its speed goes to infinity.
In practice, the torque can never go entirely to zero,
because of the mechanical, core and stray losses that
must be overcome.
• However, if no other load is connected to the motor, it can
turn fast enough to seriously damage itself.
NEVER completely unload a series motor, and NEVER
connect one to a load by a belt or other mechanism that
could break.
Figure : The ideal torque- speed characteristic of a series dc motor
Series DC Motor : Speed Control
Method of controlling the speed in series motor :
1. Change the terminal voltage of the motor. If the terminal voltage is increased,
the speed also increased, resulting in a higher speed for any given torque.

VT
1
Kc  ind

R A  RS
Kc
2. By the insertion of a series resistor into the motor circuit, but this technique
is very wasteful of power and is used only for intermittent period during the
start-up of some motor.
Compounded DC Motor
A compound DC motor is a motor with both a shunt and a series field
Two field windings : - One is connected in series with armature
(series field)
- Other is connected in parallel with the armature
(shunt field).
series
shunt
shunt
series
The equivalent compound DC motor
a) Long-shunt connection (cumulative compounding), (b) Short-shunt
connection (differential compounding)
Compounded DC Motor
• In long shunt compound dc motor, the series field is
connected in series with armature and the combination is
in parallel with the shunt field.
•In the short shunt field compound dc motor, the shunt
field is in parallel with armature and the combination is
connected in series with the series field.
• If the magnetic fluxes produced by both series field and
shunt field windings are in same direction, that is, additive,
the dc motor is cumulative compound. If the magnetic
fluxes are in opposite, the dc motor is differential
compound.
Compounded DC Motor
• The Kirchhoff’s voltage law equation for a compound dc motor is:
VT  E A  I A ( R A  RS )
• The currents in the compounded motor are related by :
IA  IL  IF
VT
IF 
RF
• The net magnetomotive force given by
F net = F F ± FSE - FAR
FF = magnetmotive force (shunt field)
FSE = magnetomotive force (series field)
FAR = magnetomotive force (armature reaction)
The effective shunt field current in the compounded DC motor
given by:
N SE
FAR
I  IF 
IA 
NF
NF
*
F
NSE = winding turn per pole on series
winding
NF = winding turn per pole on shunt
winding
The positive (+) sign is for cumulatively compound motor
The negative (-) sign is for differentially compound motor
Cumulatively Compounded DC Motor:
Torque Speed Characteristic
• Has a higher starting torque than a shunt motor (whose flux is constant) but a
lower starting torque than a series motor (whose entire flux is proportional to
armature current).
• It combines the best features of both the shunt and the series motors. Like a
series motor, it has extra torque for starting; like a shunt motor, it does not over
speed at no load.
• At light loads, the series field has a very small effect, so the motor behaves
approximately as a shunt dc motor.
• As the load gets very large, the series flux becomes quite important and the
torque speed curve begins to look like a series motor’s characteristic.
• A comparison of these torque speed characteristics of each types is shown in
next slide.
Fig (a) The torque-speed characteristic of a cumulatively
compounded dc motor compared to series and shunt motors with
the same full-load rating.
Fig. (b) The torque-speed characteristic of a cumulatively
compounded dc motor compared to a shunt motor with the same
no-load speed.
Cumulatively Compounded DC Motor :
Speed Control
The techniques available for control of speed in a
cumulatively compounded
dc motor are the same as those available for a shunt
motor:
1. Change the field resistance, RF
2. Change the armature voltage, VA
3. Change the armature resistance, RA
The arguments describing the effects of changing RF or VA
are very similar to
the arguments given earlier for the shunt motor.
Differentially Compounded DC Motor:
Torque Speed Characteristic
• The shunt magnetomotive force and series magnetomotive force subtract from
each other.
• This means that as the load on the motor increase,
IA increase and the flux in the motor decreased,
(IA)
As the flux decrease, the speed of the motor increase, ()
This speed increase causes an-other increase in load, which further increase IA,
Further decreasing the flux, and increasing the speed again.
• All the phenomena resulting the differentially compounded motor is unstable and
tends to run away.
• This instability is much worse than that of a shunt motor with armature reaction,
and make it unsuitable for any application.
DC Motor Starter
In order for a dc motor to function properly on the job, it must have some
special control and protection equipment associated with it. The purposes
of this equipment are:
1. To protect the motor against damage due to short circuits in the
equipment
2. To protect the motor against damage from long term overloads
3. To protect the motor against damage from excessive starting currents
4. To provide a convenient manner in which to control the operating speed
of the motor
DC Motor Problem on Starting
• DC motor must be protected from physical damage during the starting period.
• At starting conditions, the motor is not turning, and so EA = 0 V.
• Since the internal resistance of a normal dc motor is very low, a very high
current flows, hence the starting current will be dangerously high, could severely
damage the motor, even if they last for only a moment.
• Consider the dc shunt motor:
VT  E A VT
IA 

RA
RA
When EA = 0 and RA is very small, then the current IA will be very high.
Two methods of limiting the starting current :
• Insert a starting resistor in series with armature to limit the current flow (until
EA can build up to do the limiting). The resistor must be not permanently to
avoid excessive losses and cause torque speed to drop excessively with
increase of load.
• Manual DC motor starter, totally human dependant
Inserting a Starting Resistor in Series & Manual
DC Motor
Fig : A shunt motor with a starting
Fig : A Manual DC Motor
resistor in series with an
armature. Contacts 1A, 2A and 3A Human dependant:
• Too quickly, the resulting current flow
short circuit portions of the
would be too large.
starting resistor when they close
• Too slowly, the starting resistor could
burn-up
DC Motor Efficiency Calculations
To calculate the efficiency of a dc motor, the following losses must be
determined :
•
•
•
•
•
Copper losses (I2R losses)
Brush drop losses
Mechanical losses
Core losses
Stray losses
Pconv = Pdev = EAIA=indω
Pout =out m
Pin =VTIL
I2R losses Mechanical
losses
Core loss Stray losses
 Electrical or Copper losses : losses that occur in the Armature and
field windings of the machine. The copper losses for the armature
and field winding are given by :
Armature Loss PA = IA2RA
Field Loss PF = IF2RF
Must consider RS for series
PA = Armature Losses
and compound DC
PF = Field Circuit Losses
Motors
• The resistance used in these calculations is usually the winding
resistance
at normal operating temperature
 Brush Losses : power loss across the contact potential at the
brushes of the machines. It is given by the equation:
PBD = VBDIA
 Magnetic or core loss : Hysteresis and eddy current losses
occuring in the metal of the motor.
 Mechanical loss : Friction and windage losses.
• Friction losses include the losses caused by bearing friction and the
friction
between the brushes andcommutator.
• Windage losses are caused by the friction between rotating parts
and air inside the DC machine’s casing.
 Stray losses (or Miscellaneous losses) : losses that cannot be
placed in one of the previous categories. (Is about 1% of full loadRULE OF THUMB) [[pg 318,Electric Machinery and Transformers,
BHAG S. GURU] and [pg 525, Electric Machinery Fundamentals,
STEPHEN J. CHAPMAN]
 Rotational losses : when the mechanical losses, Core losses and Stray
losses
are lumped together. [pg. 193 Electromechanical Energy Devices and
Power
System, ZIA A. ZAMAYEE & JUAN L. BALA JR.]
It also consider as combination between mechanical and core losses at no
load
and rated speed.[pg 317, Electric Machinery and Transformers, BHAG S.
GURU] and [pg 593, Electric Machinery Fundamentals, STEPHEN J.
CHAPMAN]
Motor efficiency :


Poutput
Pinput
X 100%
Pinput  Plosses
Pinput
X 100%
Speed Regulation
The speed regulation is a measure of the change speed from no-load to full
load. The percent speed regulation is defined
Speed Regulation (SR):
 nl   fl

X 100%
 fl
or
nl   fl

X 100%
 fl
+Ve SR means that the motor speed will decrease when the load on its shaft is
increased.
-Ve SR means that the motor speed increases with increasing load.