Lecture 4

• AC Machines:

  AC Machines convert AC electrical energy to mechanical energy (Motor) or mechanical energy to AC electrical energy (Generator)

  
• Principle of an Induction Motor:

  •        The three-phase stator windings  are spatially 120o apart 

  •        They are excited with three-phase voltages which are electrically 120o apart

  •        This sets up a rotating magnetic field that also links with the rotor conductors.

  •        EMF is induced in the rotor conductors as they are placed in a rotating magnetic field.

  •        Induced EMF causes currents to flow in the rotor circuit.

  •        Current carrying rotor conductors placed in a rotating magnetic field experience torque. The motor spins!
• Speed Control of Induction Motor:

  • Three-phase stator voltages (Variable voltage- constant frequency).
  • Frequency of the stator voltages (Variable frequency- constant voltage).
  • Both voltage and frequency keeping  the volts/hertz ratio constant

  
• Induction Motor: Variable Voltage - Constant Frequency

  •        Synchronous speed is constant.

  •        Motor flux is NOT constant.

  •        Suitable for variable torque loads where load torque is less at low motor speeds.

  •        Variable slip method- inefficient due to large current at low speeds.

  •        Speed range is significantly reduced for constant torque loads

  
• Induction Motor: Constant Voltage - Variable Frequency

  •        Synchronous speed varies with varying frequency.

  •        Motor flux and hence developed torque reduces with increasing frequency.

  •        Used as a method of speed control above base speeds

  •        Not suitable below base speeds, as it can cause flux saturation

  
• Induction Motor: Variable Voltage - Variable Frequency

  •        To achieve speed control below base speed, voltage and frequency are reduced proportionately to maintain constant flux.

  •        Not suitable for speed control above base speeds, as overvoltage may damage winding insulation.

  •        The motor has low slip characteristics, hence improving efficiency.

  
• Drive implementation:

  Three sinusoidal modulating signals are compared with a high frequency carrier to generate gating signals for each leg of the inverter.

  • The frequency of the modulating signals is the fundamental frequency of stator voltage, and the amplitude modulation index controls the stator voltage.

  
• Principle of a Synchronous Motor

  • Three-phase electricity creates a rotating magnetic field in the stator.
  • The rotor generates its own magnetic field using permanent magnets or DC field windings.
  • Alignment of rotor and stator magnetic fields is hindered by the rotating stator field.
  • Damper windings on the rotor aid initial motor startup as an induction motor.
  • Once spinning, activation of DC field excitation allows synchronization of the two magnetic fields.
• Speed Control of PM Synchronous Motor

  •        In smaller machines with permanent magnets, damper windings are not used to start the machine.

  •        These machines are controlled through an inverter so that the stator voltages are generated such that the rotor can lock itself to the stator magnetic field.

  •        This is done by continuously monitoring the rotor position and generating the inverter modulating signals at constant V/f aligned with the position of the rotor.
• Wind Energy Conversion Systems

  • Power converters play a vital role in controlling not only the speed and torque of electrical motors but also the output of generators.
  • This is particularly crucial in renewable energy systems like wind energy conversion systems (WECS).
  • Variable wind speeds cause the rotor speed in WECS to vary, leading to variable frequency armature voltages.
  • To connect WECS to the grid, a back-to-back converter unit is employed. It maintains a stable output on the grid side while allowing for variable output on the turbine side to match wind characteristics.