DIRECT CURRENT MOTOR

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CHAPTER 2

• DIRECT CURRENT MOTOR

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CHAPTER OUTLINE
2.1 OVERVIEW OF DIRECT CURRENT MOTOR 2.2
CONSTRUCTION 2.3 PRINCIPLE OF OPERATION 2.4 TYPES
OF DC MOTOR POWER FLOW DIAGRAM 2.5 SPEED
CONTROL
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LEARNING OBJECTIVES

• Upon completion of the chapter, the student
  should be able to
• State the principle by which machines convert
  mechanical energy to electrical energy.
• Discuss the operating differences between
  different types of generators
• Understand the principle of DC generator as it
  represents a logical behavior of dc motors.

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2.1 Overview of Direct Current Machines

• Direct-current (DC) machines are divided into dc
  generators and dc motors.
• DC generators convert mechanical energy to
  electrical energy by using the principle of
  magnetic induction
• DC motor convert electrical energy to mechanical
  energy by supplying dc power using the principle
  of magnetic induction

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2.1.1 DC Motor

• DC motors are everywhere! In a house, almost
  every mechanical movement that you see around you
  is caused by an DC (direct current) motor.

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2.2 Construction

• Major parts are rotor (armature) and stator
  (field).

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2.2 Construction

• More loops of wire higher rectified voltage
• In practical, loops are generally placed in slots
  of an iron core
• The iron acts as a magnetic conductor by
  providing a low-reluctance path for magnetic
  lines of flux to increase the inductance of the
  loops and provide a higher induced voltage.
• The commutator is connected to the slotted iron
  core. The entire assembly of iron core,
  commutator, and windings is called the armature.
  The windings of armatures are connected in
  different ways depending on the requirements of
  the machine.

Loops of wire are wound around slot in a metal
core
DC machine armature
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2.2.1 Armature Windings

• Lap Wound Armatures
• are used in machines designed for low voltage and
  high current
• armatures are constructed with large wire because
  of high current
• Eg - used in starter motor of almost all
  automobiles
• The windings of a lap wound armature are
  connected in parallel. This permits the current
  capacity of each winding to be added and provides
  a higher operating current
• No of current path, C2p pno of poles

Lap wound armatures
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2.2.1 Armature Windings

• Frogleg Wound Armatures
• mostly used in practical nowadays
• designed for use with moderate current and
  moderate armatures voltage
• the windings are connected in series parallel.
• Most large DC machines use frogleg wound
  armatures.

Frogleg wound armatures
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2.2.1 Armature Windings
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2.2.2 Field Windings

• Most DC machines use wound electromagnets to
  provide the magnetic field.
• Two types of field windings are used
• series field
• shunt field

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2.2.2 Field Windings

• Series field windings
• connected in series with the armature
• are made with relatively few windings turns of
  very large wire and have a very low resistance
• usually found in large horsepower machines wound
  with square or rectangular wire

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2.2.2 Field Windings

• Series field windings
• The use of square wire permits the windings to be
  laid closer together, which increases the number
  of turns that can be wound in a particular space
• Square and rectangular wire can also be made
  physically smaller than round wire and still
  contain the same surface area

Square wire contains more surface than round wire
Square wire permits more turns than round wire in
the same area
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2.2.2 Field Windings

• Shunt field windings
• constructed with relatively many turns of small
  wire, thus, it has a much higher resistance than
  the series field
• intended to be connected in parallel with, or
  shunt, the armature
• high resistance is used to limit current flow
  through the field

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2.2.2 Field Windings

• When a DC machine uses both series and shunt
  fields, each pole piece will contain both
  windings.
• The windings are wound on the pole pieces in such
  a manner that when current flows through the
  winding it will produce alternate magnetic
  polarities.

Both series and shunt field windings are
contained in each pole piece
S series field F shunt field
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2.2.3 Machine Windings Overview
Winding
Separately Excited
Self excited
Lap C2p
Wave C2
Frogleg
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2.3 Principle of Operation

• DC motor consist of rotor-mounted windings
  (armature) and stationary windings (field poles).
  
• In all DC motors, except permanent magnet motors,
  current must be conducted to the armature
  windings by passing current through carbon
  brushes that slide over a commutator, which is
  mounted on the rotor.

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2.3 Principle of Operation

• The brushcommutator combination makes a sliding
  switch that energizes particular portions of the
  armature, based on the position of the rotor.
• This process creates north and south magnetic
  poles on the rotor that are attracted to or
  repelled by north and south poles on the stator,
  which are formed by passing direct current
  through the field windings. It's this magnetic
  attraction and repulsion that causes the rotor to
  rotate.

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2.3 Principle of Operation

• The POLARITY of the voltage depends on the
  direction of the magnetic lines of flux and the
  direction of movement of the conductor.
• To determine the direction of current in a given
  situation, the RIGHT-HAND RULE is used.
• In operation, the electrical current supplied to
  the motor is used to generate magnetic fields in
  both the rotor and the stator. These fields push
  against each other with the result that the rotor
  experiences a torque and consequently rotates.

• thumb in the direction the conductor is being
  moved
• forefinger in the direction of magnetic flux
  (from north to south)
• middle finger will then point in the direction
  of current flow in an
• external circuit to which the voltage is applied
  

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2.3 Principle of Operation

• The operation of a DC motor is dependent on the
  workings of the poles of the stator with a part
  of the rotor/armature.
• The stator contains an even number of poles of
  alternating magnetic polarity, each pole
  consisting of an electromagnet formed from a pole
  winding wrapped around a pole core.
• When a DC current flows through the winding, a
  magnetic field is formed.
• The armature also contains a winding, in which
  the current flows in the direction illustrated.
• This armature current interacts with the magnetic
  field in accordance with Ampère's law, producing
  a torque which turns the armature

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2.3 Principle of Operation
The advantages of DC Motor

• The greatest advantage of DC motors may be a-
  speed control. Since speed is directly
  proportional to armature voltage and inversely
  proportional to the magnetic flux produced by the
  poles, adjusting the armature voltage and/or the
  field current will change the rotor speed.
• Today, adjustable frequency drives can provide
  precise speed control for AC motors, but they do
  so at the expense of power quality, as the
  solid-state switching devices in the drives
  produce a rich harmonic spectrum. The DC motor
  has no adverse effects on power quality.

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2.3 Principle of Operation
The drawbacks of DC Motor

• Power supply, initial cost, and maintenance
  requirements are the negatives associated with DC
  motors
• Rectification must be provided for any DC motors
  supplied from the grid. It can also cause power
  quality problems.
• The construction of a DC motor is considerably
  more complicated and expensive than that of an AC
  motor, primarily due to the commutator, brushes,
  and armature windings. An induction motor
  requires no commutator or brushes, and most use
  cast squirrel-cage rotor bars instead of true
  windings two huge simplifications.

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2.4 Types of DC Motor

• Self excited DC motor
• Series DC motor
• Shunt DC motor
• Compound DC motor
• Separately excited DC motor
• Permanent magnet DC motor

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2.4.1.1 Series Motor

• Series motors connect the field windings in
  series with the armature.
• Series motors lack good speed regulation, but are
  well-suited for high-torque loads like power
  tools and automobile starters because of their
  high torque production and compact size.

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2.4.1.1 Series Motor
Series Motor Power Flow Diagram
P? is normally given Pin Pout total
losses Where, Pca armature copper loss Pcf
field copper loss P?stray, mech etc
Pm Ea ia
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Example 2.1

• A dc machine in Figure 1 is consumed a 6.5kW when
  the 12.5 A of armature current is passing thru
  the armature and field resistance of 3.3? and
  2.0? respectively. Assume stray losses of 1.2kW.
  Calculate
• a) terminal voltage, VT
• b) back emf, Ea
• c) net torque if the speed is at 3560rpm
• d) efficiency of the machine
• 520V, 453.75V, 12N-m, 68.8

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Example 2.2

• A 600V 150-hp dc machine in Figure 2 operates at
  its full rated load at 600rpm. The armature and
  field resistance are 0.12? and 0.04?
  respectively. The machine draws 200A at full
  load. Assume stray losses 1700W. Determine
• a) the armature back emf at full load, Ea
• b) developed/mechanical power and
  developed/mechanical torque
• c) assume that a change in load results in the
  line current dropping to 150A. Find the new speed
  in rpm and new developed torque. Hint
  EaK1K2ia?

568V, 113.6kW, 1808Nm, 811.27rpm, 1017Nm
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2.4.1.2 Shunt Motor

• Shunt motors use high-resistance field windings
  connected in parallel with the armature.
• Varying the field resistance changes the motor
  speed.
• Shunt motors are prone to armature reaction, a
  distortion and weakening of the flux generated by
  the poles that results in commutation problems
  evidenced by sparking at the brushes.
• Installing additional poles, called interpoles,
  on the stator between the main poles wired in
  series with the armature reduces armature
  reaction.

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Shunt Motor (power flow diagram)
2.4.1.2 Shunt Motor
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Example 2.3

• Example
• A voltage of 230V is applied to armature of a
  machines results in a full load armature currents
  of 205A. Assume that armature resistance is 0.2?.
  Find the back emf, net power and torque by
  assuming the rotational losses are 1445W at full
  load speed of 1750rpm.
• 189V, 37.3kW, 203.5Nm

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2.4.1.3 Compound Motor

• the concept of the series and shunt designs are
  combined.

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2.4.1.3 Compound Motor
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2.4.2 Separately Excited Motor

• There is no direct connection between the
  armature and field winding resistance
• DC field current is supplied by an independent
  source
• (such as battery or another generator or prime
  mover called an exciter)

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2.4.2 Separately Excited Motor
Circuit analysis
Where p no of pole pair n speed
(rpm) Zno of conductor
?Flux per pole (Wb) C no of
current/parallel path 2p (lap winding) 2
(wave winding)
KVL
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2.4.3 Permanent Magnet Motor

• PMDC is a dc motor whose poles are made of
  permanent magnets.
• Do not require external field circuit, no copper
  losses
• No field winding, size smaller than other types
  dc motors
• Disadvantage cannot produce high flux density,
  lower induce voltage

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2.5 Speed Control
Shunt motor and separately excited dc motor

• Torque speed characteristic for shunt and
  separately excited dc motor

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2.5 Speed Control
Shunt motor and separately excited dc motor

• By referring to the Torque speed characteristic
  for shunt and separately excited dc motor
• note that, there are three variables that can
  influence the speed of the motor, V, If and Ra
• Thus, there are three methods of controlling the
  speed of the shunt and separately excited dc
  motor,
• Armature terminal voltage speed control
• Field speed control
• Armature resistance speed control

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2.5 Speed Control
Shunt motor and separately excited dc motor

• Armature resistance speed control
• Speed may be controlled by changing Ra
• The total resistance of armature may be varied by
  means of a rheostat in series with the armature
• The armature speed control rheostat also serves
  as a starting resistor.
• From ?-n characteristic,

Will be changed
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2.5 Speed Control
Shunt motor and separately excited dc motor

• Torque speed characteristic (Ra speed control)

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2.5 Speed Control
Shunt motor and separately excited dc motor

• Advantages armature resistance speed control
• Starting and speed control functions may be
  combined in one rheostat
• The speed range begins at zero speed
• The cost is much less than other system that
  permit control down to zero speed
• Simple method
• Disadvantages armature resistance speed control
• Introduce more power loss in rheostat
• Speed regulation is poor (S.R difference nLoaded
  nno loaded)
• Low efficiency due to rheostat

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2.5 Speed Control
Shunt motor and separately excited dc motor

• Field Speed Control
• Rheostat in series with field winding (shunt or
  separately exct.)
• If field current, If is varied, hence flux is
  also varied
• Not suitable for series field
• Refer to ?-n characteristic,
• Slope and nNL will be changed

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2.5 Speed Control
Shunt motor and separately excited dc motor

• Torque speed characteristic (field speed control)

If1 lt If2 lt If3 ?1 lt ?2 lt ?3
Base speed
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2.5 Speed Control
Shunt motor and separately excited dc motor

• Advantages field speed control
• Allows for controlling at or above the base speed
• The cost of the rheostat is cheaper because If is
  small value
• Disadvantages field speed control
• Speed regulation is poor (S.R difference nLoaded
  nno loaded)
• At high speed, flux is small, thus causes the
  speed of the machines becomes unstable
• At high speed also, the machines is unstable
  mechanically, thus there is an upper speed limit

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2.5 Speed Control
Shunt motor and separately excited dc motor

• Armature terminal voltage speed control
• Use power electronics controller
• AC supply ?rectifier
• DC supply ?chopper
• Supply voltage to the armature is controlled
• Constant speed regulation
• From ?-n characteristic,
• C and nNL will be change
• Slope constant

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2.5 Speed Control
Shunt motor and separately excited dc motor

• Torque speed characteristic (armature terminal
  voltage speed control)

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2.5 Speed Control
Shunt motor and separately excited dc motor

• Advantages armature terminal voltage speed
  control
• Does not change the speed regulation
• Speed is easily controlled from zero to maximum
  safe speed
• Disadvantages armature terminal voltage speed
  control
• Cost is higher because of using power electronic
  controller

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THANK YOU FOR YOUR ATTENTION!!