SYNCHRONOUS MACHINES & CHARACTERISTICS
Synchronous machines are principally used as alternating current generators. They supply the electric power
used by all sectors of modern society. Synchronous machine is an important electromechanical energy
converter. Synchronous generators usually operate in parallel forming a large power system supplying electrical
power to consumers or loads. For these applications the synchronous generators are built in large units, their
rating ranging from tens to hundreds of Megawatts. These synchronous machines can also be run as
Synchronous machines are AC machines that have a field circuit supplied by an external DC source.
Synchronous machines are having two major parts namely stationary part stator and a rotating field system
In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The
rotor is then driven by external means producing a rotating magnetic field, which induces a 3-phase voltage
within the stator winding.
Field windings are the windings producing the main magnetic field (rotor windings for synchronous machines);
armature windings are the windings where the main voltage is induced (stator windings for synchronous
Types of synchronous machines
According to the arrangement of armature and field winding, the synchronous machines are classified as
rotating armature type or rotating field type.
In rotating armature type the armature winding is on the rotor and the field winding is on the stator. The
generated emf or current is brought to the load via the slip rings. These type of generators are built only in small
In case of rotating field type generators field windings are on the rotor and the armature windings are on the
stator. Here the field current is supplied through a pair of slip rings and the induced emf or current is supplied to
the load via the stationary terminals.
Based on the type of the prime movers employed the synchronous generators are classified as
Hydrogenerators : The generators which are driven by hydraulic turbines are called
hydrogenerators. These are run at lower speeds less than 1000 rpm.
Turbogenerators: These are the generators driven by steam turbines. These generators are run at very
high speed of 1500rpm or above.
Engine driven Generators: These are driven by IC engines. These are run at aspeed less than 1500 rpm.
Construction of synchronous machines
Salient pole Machines: These type of machines have salient pole or projecting poles with concentrated
field windings. This type of construction is for the machines which are driven by hydraulic turbines or
Nonsalient pole or Cylindrical rotor or Round rotor Machines: These machines are having cylindrical
smooth rotor construction with distributed field winding in slots. This type of rotor construction is
employed for the machine driven by steam turbines.
The stator is the outer stationary part of the machine, which consists of
The outer cylindrical frame called yoke, which is made either of welded sheet steel, cast iron.
The magnetic path, which comprises a set of slotted steel laminations called stator core pressed into the
cylindrical space inside the outer frame. The magnetic path is laminated to reduce eddy currents,
reducing losses and heating. CRGO laminations of 0.5 mm thickness are used to reduce the iron losses.
A set of insulated electrical windings are placed inside the slots of the laminated stator. The cross-sectional area
of these windings must be large enough for the power rating of the machine. For a 3-phase generator, 3 sets of