ENERGY CONVERSION ONE (Course 25741)

1
ENERGY CONVERSION ONE (Course 25741)

• CHAPTER NINE .continued
• DC MOTORS AND GENERATORS

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DC MOTORS AND GENERATORS

• Summary
• 1. The Equivalent Circuit of a DC Motor
• 2. The Magnetization Curve of a DC Machine
• 3. Separately Excited and Shunt DC Motors
• - The Terminal Characteristics of a
  Shunt DC Motor
• - Nonlinear Analysis of a Shunt DC
  Motor
• - Speed Control of Shunt DC Motors
• - The Effect of an Open Field Circuit
• 4. The Permanent-Magnet DC Motor
• 5. The Series DC Motor
• - Induced Torque in a Series DC Motor
• - The Terminal Characteristic of a
  Series DC Motor
• - Speed Control of Series DC Motors.
• 6. DC Motor Starters
• - DC Motor Problems on Starting
• - DC Motor Starting Circuits
• 7. Introduction to DC generators
• 8. Separately Excited Generator
• - Terminal Characteristic of a
  separately Excited DC Generator

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DC MOTORS AND GENERATORSINTRODUCTION

• The same physical dc machine can operate as
  either motor or a generator it depends on
  direction of power flow
• Introduction to DC motors
• dc motors have a significant fraction of
  machinery purchased each year through 1960s
• Reasons existence of dc power system in cars,
  trucks and aircraft
• Another application when wide variations in
  speed are needed
• Before widespread use of power electronic
  rectifier-inverters, dc motors were dominant
  means of speed control
• Even without a dc power source, solid-state
  rectifier chopper circuits used to create
  necessary dc power dc motors used to provide
  speed control
• Today induction motors with solid-state drive
  packages are preferred choice over dc motors for
  most speed control applications, while still in
  some applications dc motors preferred

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DC MOTORS AND GENERATORSINTRODUCTION

• DC motors are compared by their speed regulation
  
• SR ?nl-?fl/?fl x 100
• It is a rough measure of shape of motors
  torque-speed characteristic
• A positive regulation means speed drops with
  increasing load a negative speed regulation
  means speed increases with increasing load
• Magnitude of S.R. approximately show how steep is
  the slope of torque-speed
• Dc motors driven from a dc power supply (unless
  specified) and input voltage assumed constant)
• Five major types of dc motor
• 1- separately excited dc motor 2-shunt dc
  motor
• 3-permnent-magnet dc motor 4- series dc
  motor 5-compounded dc
  motor

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DC MOTOREQUIVALENT CIRCUIT

• Figure below shows a dc motor equivalent cct.
• Armature cct. represented by an ideal voltage
  source EA a resistor RA
• This is thevenin equivalent of entire rotor,
  including coils, interpoles compensating
  windings
• Brush voltage drop represented by a small battery
  Vbrush opposing direction of current flow

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DC MOTOREQUIVALENT CIRCUIT

• A simplified equivalent circuit eliminating the
  brush voltage drop and combining Radj with the
  field resistance shown in (b)
• Some of the few variations and simplifications
• 1- brush drop voltage is often only a very
  tiny fraction of generated voltage in the
  machine. where it is not too critical, brush
  drop voltage may be left out or included in the
  RA.
• 2- internal resistance of field coils is
  sometimes lumped together with variable resistor
  and total is called RF
• 3- Some generators have more than one
  field coil, all of which appear on the equivalent
  circuit

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DC MOTOREQUIVALENT CIRCUIT

• The internal generated voltage is given by
• EA K f?
• and the torque induced is
• ?ind K f IA
• The Magnetization Curve of a DC Machine
• - EA is directly proportional to flux and the
  speed of rotation of the machine
• EA is therefore related to the field current
• field current in a dc machine produces a field
  mmf given by mmfNFIF

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DC MOTOREQUIVALENT CIRCUIT

• mmf produces a flux in the machine in accordance
  with its magnetization curve

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DC MOTOREQUIVALENT CCT

• Since If is proportional to mmf since EA is
  proportional to flux, magnetization curve can
  represented as a plot of EA vs field current for
  a given speed ?0

10
SEPARATELY EXCITED AND SHUNT DC MOTORS

• Equivalent cct. of separately excited dc motor
  shown below

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SEPARATELY EXCITED AND SHUNT DC MOTORS

• separately excited dc motor is a motor whose
  field cct. is supplied by another
  constant-voltage supply
• shunt dc motor is a motor whose field circuit
  gets its power directly from armature terminals
  of motor
• When supply voltage to a motor assumed constant,
  there is no practical difference in behavior
  between these two machines
• Kirchhoffs voltage law KVL equation for armature
  cct. of these motors is VTEAIARA

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TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR

• Terminal characteristic of a motor is a plot of
  output torque versus speed
• If load on shaft of a shunt motor is increased,
  then load torque Tload exceed induced torque Tind
  motor will start to slow down
• Its internal generated voltage EAKf? decrease
• Then IA (VT-EA)/ RA increases
• consequently TindKfIA increases finally Tind
  will equal Tload at a lower mechanical speed

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TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR

• O/P characteristic of shunt dc motor can be
  derived using Tind, EA equations KVl
• Combing these three equations
• VTEAIARA ? VTKf?IARA
• IA Tind /(Kf) ? VTKf? Tind /(Kf) RA
• ? ? VT / (Kf) - Tind/(Kf)2 RA
• This equation is a straight line with a negative
  slope

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TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR

• Torque speed characteristic of a shunt or
  separately excited dc motor

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TERMINAL CHARACTERISTIC of a SHUNT DC MOTOR

• Armature reaction affect the torque speed
  characteristic
• As shown in last slide, as load increase, flux
  weakening effect reduce the flux in shunt motor
• And according to speed equation, reduction in
  flux will increase speed
• If a motor has compensating winding, then there
  would be no flux weakening flux remain constant