Synchronous Machines-1 Study notes For Electrical Engineering

Synchronous Machines

Synchronous machines are vital components in electrical power systems, employed for power generation, transmission, and distribution. These machines operate in synchronism with the grid frequency, providing stable and reliable power. With their unique characteristics and capabilities, synchronous machines form an integral part of the infrastructure that powers our modern world.

• Synchronous machines are one of two types: the stationary field or the rotating dc magnetic field. The stationary field synchronous machine has salient poles mounted on the stator—the stationary member. The poles are magnetized either by permanent magnets or by a dc current.
• The armature, normally containing a three-phase winding, is mounted on the shaft. The armature winding is fed through three slip rings (collectors) and a set of brushes sliding on them. This arrangement can be found in machines up to about 5 kVA in rating.
• For larger machines—all those covered in this book—the typical arrangement used is the rotating magnetic field. The rotating magnetic field (also known as revolving-field) synchronous machine has the field-winding wound on the rotating member (the rotor), and the armature wound on the stationary member (the stator).
• A dc current, creating a magnetic field that must be rotated at synchronous speed, energizes the rotating field-winding. The rotating field winding can be energized through a set of slip rings and brushes (external excitation), or from a diode bridge mounted on the rotor (self-excited).
• A synchronous generator is an electrical machine producing alternating emf (Electromotive force or voltage) of constant frequency.
• The synchronous motor operates at a precise synchronous speed and hence is a constant-speed motor. Unlike the induction motor, whose operation always involves a lagging power factor, the synchronous motor possesses a variable-power-factor characteristic and hence is suitable for power-factor correction applications.
• A synchronous motor operating without mechanical load is called a compensator. It behaves as a variable capacitor when the field is overexcited, and as a variable inductor when the field is under-excited. It is often used in critical positions in a power system for reactive power control.