ELECTRIC MACHINERY

PART 3
SYNCHRONOUS GENERATOR

  A DC current is applied to the rotor winding, which then produces a rotor magnetic field. The rotor is then turned by a prime mover (eg. Steam, water etc.) producing a rotating magnetic field. This rotating magnetic field induces a 3-phase set of voltages within the stator windings of the generator.
  "Field windings" applies to the windings that produce the main magnetic field in a machine, and "armature windings" applies to the windings where the main voltage is induced. For synchronous machines, the field windings are on the rotor, so the terms "rotor windings" and "field windings" are used interchangeably.
  Generally a synchronous generator must have at least 2 components:

    Rotor Windings or Field Windings
      Salient Pole
      Non Salient Pole
    Stator Windings or Armature Windings
  The rotor of a synchronous generator is a large electromagnet and the magnetic poles on the rotor can either be salient or non salient construction. Non-salient pole rotors are normally used for rotors with 2 or 4 poles rotor, while salient pole rotors are used for 4 or more poles rotor.


Non-salient rotor
Non-salient rotor for a synchronous machine

synchronous machine


  Slip rings are metal rings completely encircling the shaft of a machine but insulated from it. One end of the dc rotor winding is tied to each of the 2 slip rings on the shaft of the synchronous machine, and a stationary brush rides on each slip ring.
  A "brush" is a block of graphitelike carbon compound that conducts electricity freely but has very low friction, hence it doesn't wear down the slip ring. If the positive end of a dc voltage source is connected to one brush and the negative end is connected to the other, then the same dc voltage will be applied to the field winding at all times regardless of the angular position or speed of the rotor.
  Some problems with slip rings and brushes:
      They increase the amount of maintenance required on the machine, since the brushes must be checked for wear regularly.
      Brush voltage drop can be the cause of significant power losses on machines with larger field currents.
  A brushless exciter is a small ac generator with its field circuit mounted on the stator and its armature circuit mounted on the rotor shaft. The 3-phase output of the exciter generator is rectified to direct current by a 3-phase rectifier circuit also mounted on the shaft of the generator, and is then fed to the main dc field circuit. By controlling the small dc field current of the exciter generator (located on the stator), we can adjust the field current on the main machine without slip rings and brushes. Since no mechanical contacts occur between the rotor and stator, a brushless exciter requires less maintenance.

controlling the small dc field current of the exciter generator

A brushless exciter circuit : A small 3-phase current is rectified and used to supply the field circuit of the exciter, which is located on the stator. The output of the armature circuit of the exciter (on the rotor) is then rectified and used to supply the field current of the main machine.


  To make the excitation of a generator completely independent of any external power sources, a small pilot exciter can be used.
  A pilot exciter is a small ac generator with permanent magnets mounted on the rotor shaft and a 3-phase winding on the stator. It produces the power for the field circuit of the exciter, which in turn controls the field circuit of the main machine. If a pilot exciter is included on the generator shaft, then no external electric power is required.

pilot exciter | generator shaft

A brushless excitation scheme that includes a pilot exciter. The permanent magnets of the pilot exciter produce the field current of the exciter, which in turn produces the field current of the main machine.


  Synchronous generators are by definition synchronous, meaning that the electrical frequency produced is locked in or synchronized with the mechanical rate of rotation of the generator. A synchronous generator's rotor consists of an electromagnet to which direct current is supplied. The rotor's magnetic field points in the direction the rotor is turned. Hence, the rate of rotation of the magnetic field in the machine is related to the stator electrical frequency by:

stator electrical frequency


  Voltage induced is dependent upon flux and speed of rotation, hence from what we have learnt so far, the induced voltage can be found as follows:

induced voltage


For simplicity, it may be simplified to as follows:

induced voltage





Synchronous Motor V-Curves Experiment

  The rotor of the dynamometer is a permanent magnet cup inside the coil assembly of the "stator" that is free to move. The current in the coil is controlled by the power supply. Because of the phase lag between the rotor field (permanent magnet) and the stator assembly, a drag torque is produced which is proportional to applied current. This torque is measured by the swing of the stator assembly. In the digital models, the torque and the speed are read directly on the readout unit.
  The "no-load" position on the dynamometer is when the power supply knob is completely CCW. To load the motor, increase the stiffness or drag between the rotor and stator assembly, increase the current in the coil by turning the potentiometer knob of he power supply in the CW direction. The torque readings of this unit are in lb-in. The power in watts can be calculated from the following expression:
The power in watts can be calculated ...

where; n is the speed in RPM, T is the torque in lb-in, and P is the power in watts.

Procedure:
1. Connect the circuit shown in the figure with the dynamometer decoupled. Connect the motor to a suitable voltage source through a three-phase wattmeter. Start the motor as induction motor by opening the switch of the field circuit. When the speed of the motor gets to about the synchronous speed, move the field circuit switch to the synchronous-run position.
2. Vary the motor field current from 0.1A to 1A and record the input power, line voltage, armature current, and field current.
3. Couple the dynamometer with the motor with the knob of the power supply fully CCW.
4. Repeat step 2 for 75%, 100%, and 125% of the rated motor current at unity power factor.
5. Adjust the field current for 0.8 power factor leading at about 75% of the armature current and hold it constant at this value for a complete load run. Record all meter readings at about five load intervals.

Report:
1. Plot the V-curves from data of steps 2 and 4 on one graph and the pf vs. field current on another graph. Draw curves through the points of unity power factor, 0.8 power factor leading, and 0.8 power factor lagging.
2. Plot power factor versus armature current from data of step 5. (Constant Excitation)
3. Explain the results of step 4 for a 50% load by a phasor diagram.
4. Explain the results of step 5 by a phasor diagram.
5. Can you predict the results of step 5 from the V-curves?
6. Explain how a synchronous motor assumes load at constant speed.

synchronous motor