Synchronous Machine

 Synchronous Generator Construction

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.

A dc current must be supplied to the field circuit on the rotor. Since the rotor is rotating, a special

arrangement is required to get the dc power to its field windings. The common ways are:

Salient Rotor of a Synchronous Machine

Non- Salient Rotor Synchronous Machine

•  supply the dc power from an external dc source to the rotor by means of slip rings and brushes.
•  Supply the dc power from a special dc power source mounted directly on the shaft of the synchronous generator.

A dc current must be supplied to the field circuit on the rotor. Since the rotor is rotating, a special

arrangement is required to get the dc power to its field windings. The common ways are:

1.  supply the dc power from an external dc source to the rotor by means of slip rings and brushes.
2.  Supply the dc power from a special dc power source mounted directly on the shaft of the synchronous generator.

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.

Small synchronous machines – use slip rings and brushes.

Larger machines – brushless exciters are used to supply the dc field current.

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.

The Equivalent Circuit of a Synchronous Generator

The voltage EA is the internal generated voltage produced in one phase of a synchronous generator. If the machine is not connected to a load (no armature current flowing), the terminal voltage will be equivalent to the voltage induced at the stator coils. This is due to the fact that there are no current flow in the stator coils hence no losses. When there is a load connected to the generator, there will be differences between EA and Vφ. These differences are due to:

1. Distortion of the air gap magnetic field by the current flowing in the stator called armature reaction. 
2. Self inductance of the armature coil
3.  Resistance of the armature coils
4. The effect of salient pole rotor shapes.

We will explore factors a, b, and c and derive a machine model from them. The effect of salient pole rotor shape will be ignored, and all machines in this chapter are assumed to have nonsalient or cylindrical rotors.

Armature Reaction

When the rotor is spun, a voltage EA is induced in the stator windings. If a load is attached to the

terminals of the generator, a current flows. But a 3-phase stator current flow will produce a magnetic field of its own. This stator magnetic field will distorts the original rotor magnetic field, changing the resulting phase voltage. This effect is called armature reaction because the armature (stator) current affects the magnetic field, which produced it in the first place.

Refer to the diagrams below, showing a two-pole rotor spinning inside a 3-phase stator.

Equivalent Circuit of a Synchronous Machine

.....to be continued.....