Lecture 2

• Dc Machine:

  DC Machines are generators that convert mechanical energy into DC electrical energy and motors that convert DC electrical energy into mechanical energy.

  
• DC Generator Principles:

  1. Basic Principle: A DC generator converts mechanical energy into electrical energy through electromagnetic induction.
  2. Construction: Consists of a stationary part called the stator and a rotating part called the rotor.
  3. Field Magnets: Produce a magnetic field within the generator.
  4. Armature: A coil of wire that rotates within the magnetic field.
  5. Commutator: Device that reverses the direction of current flow in the armature coil every half rotation, ensuring a unidirectional output.
• Voltage Induced in a Rotating Loop:

  1. Faraday's Law: States that a changing magnetic field induces an electromotive force (EMF) in a loop of wire.
  2. Principle of Operation: When the armature coil rotates within the magnetic field, the magnetic flux through the coil changes, inducing a voltage.
  3. Maximum Voltage: Occurs when the coil is perpendicular to the magnetic field lines.
  4. Minimum Voltage: Occurs when the coil is parallel to the magnetic field lines.
  5. Expression for Induced Voltage (EMF): EMF=−𝑁𝑑𝜙/dt  
• What process is required to obtain DC voltage at the output of a DC generator?

  Rectification of the induced alternating current (AC).
• What component in the DC generator performs the rectification function?

  The commutator.
• What are the stationary components that make contact with the commutator?

  Carbon brushes.
• Besides the commutator, what other component may be used to smooth out voltage fluctuations?

  A smoothing capacitor or additional circuitry.
• How can the output voltage of a DC generator be regulated?


  By adjusting the strength of the magnetic field (field current) or by varying the speed of rotation of the armature coil.
• Torque induced in a rotating loop:

  
• Methods of speed control:

  1.        Adjusting the field resistance RF (and thus the field flux)

  2.         Adjusting the terminal voltage applied to the armature.

  3.         Inserting a resistor in series with the armature circuit.
• Armature voltage control:

  • Increasing armature voltage, increases armature current
  • This in turn increases the induced torque in the motor
  • As induced torque exceeds the load torque, motor speeds up
  • Which in turn increases the motor back EMF
  • This decreases the current and hence the induced torque till,
  • The motor now settles at an increased speed!
• Power Converters for DC Motor Drives – Full converter fed dc drive

  • Semi-Controlled Thyristor Converter
  • Firing angle based turn ON control
  • Converts fixed AC input to controllable DC output

  
• Power Converters for DC Motor Drives – DC-DC Chopper fed DC driver

  •        Fully-Controlled IGBT/MOSFET Converter

  •        Duty ratio based turn ON and turn OFF control

  •        Converts fixed DC input to controllable DC output
• Average output voltage:

  
• What does continuos conduction mode is favoured by:

  • High armature inductance.
  •  Smaller values of back emf i.e. motor operation at low speeds.
  •  Small values of firing angle
  •  High load current
• Control Arrangement for DC Motor Drives - Speed Loop

  •        Input to the system is a reference speed signal.

  •        Output of the system is the speed measured by a tacho generator.

  •        Tacho-Generator produces a feedback voltage proportional to the speed of the motor.

  •        The speed control loop generates a  current command from the speed error
• Control Arrangement for DC Motor Drives – Current Loop

  •        The current loop makes sure the motor armature current follows the current command.

  •        The measured motor current is compared to the reference current and the error is used to control the firing angle.