Transcript chap02_dc machines_230107

EMT 113/4
ELECTRICAL
ENGINEERING
TECHNOLOGY
Chapter 2 :
DC Machines
Chap 2: DC Machines
1
Contents
 Introduction
 DC Machines Construction
 DC motors : Principles of Operation,
Equivalent circuit & Characteristics
 DC generators : Principles of Operation,
Equivalent circuit & Characteristics
 Review
Chap 2: DC Machines
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Introduction:
What is DC Machines?
 Are DC generators that convert mechanical energy to DC electric energy.
 Are DC motors that convert DC electric energy to mechanical energy.
Chapman S.J., “Electric Machinery Fundamentals”
Chap 2: DC Machines
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Introduction
 DC machine can be used as a
motor or as a generator.
 DC Machine is most often used
for a motor.
Cutaway view of a dc motor
 DC motors are found in many special industrial environments
Motors drive many types of loads from fans and pumps to
presses and conveyors
 The major advantages of dc machines are the 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.
Chap 2: DC Machines
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Types of DC motors
 DC motors are classified according to electrical
connections of armature windings and field windings.
 Five major types of DC motors:•
•
•
•
•
Separately excited DC motor
Shunt DC motor
Permanent Magnet DC motor
Series DC motor
Compounded DC motor
Chap 2: DC Machines
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DC Machines Construction
DC motor stator with poles visible
Rotor of a dc motor
Chap 2: DC Machines
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DC Machines Construction
.
DC machines, like other
electromechanical energy
conversion devices have
two sets of electrical
windings
– field windings - on
stator
– amarture windings on the rotor.
Chap 2: DC Machines
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DC Machines Construction
• The stator of the dc motor
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.
Chap 2: DC Machines
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DC Machines Construction
• 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.
Chap 2: DC Machines
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DC Machines Construction
• The coils are connected in
series through the
commutator segments.
• The ends of each coil are
connected to a commutator
segment.
• The commutator consists of
insulated copper segments
mounted on an insulated
tube.
• Two brushes are pressed to
the commutator to permit
current flow.
• The brushes are placed in
the neutral zone, where the
magnetic field is close to
zero, to reduce arcing.
Chap 2: DC Machines
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DC Machines Construction
• 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.
Chap 2: DC Machines
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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 re-enter 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 flow of electrical current in a conductor sets up concentric lines of magnetic flux around the conductor.
Chap 2: DC Machines
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Review of magnetism
The poles of an electro-magnetic coil change when the
direction of current flow changes.
Chap 2: DC Machines
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Review of magnetism
• 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.
– The thumb will indicate the direction of motion
– The forefinger will indicate the direction of the magnetic field
– The 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.
Chap 2: DC Machines
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DC motor Operation
Chap 2: DC Machines
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Current in DC motor
Chap 2: DC Machines
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Magnetic field in DC motor
Chap 2: DC Machines
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Force in DC motor
Chap 2: DC Machines
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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, which is converted
to electrical power. For the motor, e is developed electromagnetic torque,
which used to drive the mechanical load.
Chap 2: DC Machines
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Basic Principles of Operation
ARMATURE winding are defined as the
winding which a voltage is induced.
FIELD windings are defined as 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.
Chap 2: DC Machines
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Basic Principles of Operation
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 min
If the losses of the DC machine are neglected, the electrical power is equal to the
mechanical power
E A I A   m
Chap 2: DC Machines
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Generation of Unidirectional Voltage
As the rotor is rotated at an angular velocity
(), the armature flux linkage () change
and a voltage eaa’ is induced between
terminal a and a’. The expression for the
voltage induced is given by Faraday’s Law
eaa '
d

dt
a) Flux linkage of coil aa’; b) induced voltage;
c) rectified voltage
Chap 2: DC Machines
Two pole DC generator
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Generation of Unidirectional Voltage
The internal generated voltage in the DC machines defined as:
EA  K
Where EA = armature voltage
K = motor constant


= fluks
= rotation per min
Chap 2: DC Machines
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DC Motor Equivalent Circuit
The brush
voltage
drop
RA
External variable resistor
used to control the
amount of current in the
field circuit
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.
Chap 2: DC Machines
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Simplified Equivalent Circuit
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 - Combining Radj with field resistance (RF).
Chap 2: DC Machines
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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
The magnetization curve of a ferromagnetic material ( vs F)
Chap 2: DC Machines
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The Magnetization Curve of a DC
machine
Since the field current (IF) is directly
proportional to magnetomotive force
(mmf) and…….
EA  
EA is directly proportional to the flux,
the magnetization curve is presented as
a plot EA versus field current for a a
given speed.
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).
Chap 2: DC Machines
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The Magnetization Curve
The induced torque developed by
the motor is given as
EA  
 ind  KI A
The magnetization curve of a dc
machine expresses as a plot of EA
versus IF, for a fixed speed ω0
Chap 2: DC Machines
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The equivalent circuit of separately excited dc
motor
Separately excited motor is a motor whose field current is supplied from a
separate constant-voltage power supply.
IF 
VF
RF
IL  IA
VT  E A  I A RA
Chap 2: DC Machines
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The equivalent circuit of a shunt dc motor
VT
RF
VT  E A  I A RA
IF 
A shunt dc motor is a motor whose
field circuit get its power directly
across the armature terminals of the
motor.
IL  I A  IF
Chap 2: DC Machines
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Speed-Torque Characteristics
Consider the DC shunt motor. From the Kirchoff’s Law
VT  E A  I A RA
Induced Voltage
VT  K  I A RA
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)
Chap 2: DC Machines
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Torque-Speed Characteristic
This equation is a straight line with a negative slope. The graph shows the
torque-speed characteristics of a shunt dc motor.
VT
RA



2 ind
K ( K)
ind  then , with constant VT,
otherwise it affect the torque-speed curve
Torque-speed characteristic of a shunt or separately excited dc motor
Chap 2: DC Machines
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Torque-Speed 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.
Chap 2: DC Machines
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Torque-Speed Characteristic
In order for the motor speed to vary linearly with torque, the other term in
this expression must be constant as the load changes.
The terminal supplied by the dc power source is assumed to be constant –
if not, then the voltage variations will effect the shape of the torque-speed
curve.
However, in actual machine, as the load increase, the flux is reduced
because of the armature reaction. Since the denominator terms decrease,
there is less reduction in speed and speed regulation is improved (as
shown in previous slide).
If a motor has compensating windings, of course there will be no fluxweakening problem in the machines, and the flux in the machine will be
constant
Chap 2: DC Machines
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Speed Control of Shunt DC Motor
Two common ways in which the speed () of a shunt dc machine can
be controlled.
• Adjusting the field resistance RF (and thus the field flux)
• Adjusting the terminal voltage applied to the armature.
The less common method of speed control is by
• Inserting a resistor in series with armature circuit.
Chap 2: DC Machines
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1 : Changing The Field Resistance
 VT  to decrease.

1. Increasing RF causes IF  
 RF  
2. Deceasing IF decreases .
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.
Chap 2: DC Machines
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1: Changing The Field Resistance
7. Increasing speed to increases EA = K again.
8. Increasing EA decreases IA.
9. Decreasing IA decreases  ind 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
Chap 2: DC Machines
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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
increasing ω.
4. Increasing ω increases EA (=Kω  )
5. Increasing EA decreases IA [ = (VA – EA)/RA]
6. Decreasing IA decreases τind until
 ind   load
Chap 2: DC Machines
at a higher ω.
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2: Changing The Armature Voltage
The speed control is shiftted by this
method, but the slope of the curve
remains constant
The effect of armature voltage speed control on a shunt motor’s
torque speed characteristic
Chap 2: DC Machines
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3 : Inserting Resistor in Series with Armature
Add resistor in series
with R
Circuit
A
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
Chap 2: DC Machines
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3 : Inserting Resistor in Series with Armature
Add resistor in series
with R
Circuit
A
VT
RA



2 ind
K ( K)
The above equation shows if RA
increase, speed will decrease
Equivalent circuit of DC shunt
motor
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.
Chap 2: DC Machines
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The Series DC Motor
Equivalent circuit of a series
DC motor.
The Kirchhoff’s voltage law equation for this motor
VT  E A  I A ( RA  RS )
Chap 2: DC Machines
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Induced Torque in a Series DC Motor
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 that a series motor give more torque per ampere than any
other dc motor, therefore it is used in applications requiring very high torque,
example starter motors in cars, elevator motors, and tractor motors in locomotives.
Chap 2: DC Machines
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The Terminal Characteristic of a Series DC
Motor.
To determine the terminal characteristic of a series dc motor, an 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;
expressed as:
 ind  KI A  KcI A2 the armature current can be
IA 
 ind
Kc
Chap 2: DC Machines
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The Terminal Characteristic of a Series DC
Motor.
Also, EA = K, substituting these expression yields:
 ind
VT  K 

We know I A 
c
IA 
Kc
( RA  RS )
;

c
W
Substituting the equations so the induced torque equation can written as
e
K 2
 
c
 ind
k
n
o
Therefore, the fluxw
in the series
motor can be written as :
;
c

 ind
K
Chap 2: DC Machines
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The Terminal Characteristic of a Series DC
Motor.
Substituting the previous equation for VT yields:
 ind
c
VT  K
 ind  
( RA  RS )
K
Kc
The resulting torque – speed relationship is

VT
1
Kc  ind
R A  RS

Kc
One 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.
Chap 2: DC Machines
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The Terminal Characteristic of a Series DC
Motor.
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.
Fig : The ideal torque- speed
characteristic of a series dc
motor
Chap 2: DC Machines
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Speed Control of Series DC Motor.
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. This
is only one efficient way to change the speed of a series motor.

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.
Chap 2: DC Machines
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