Demonstrative Video
Constructional Features of Synchronous Machines
The important parts of a synchronous machine are : \[\bullet \text{Stator} \quad \bullet \text{Rotor} \quad \bullet \text{Miscellaneous}\]
Stator: The outer stationary part of the machine is called stator it has the following important parts:-
Stator frame: outer body of the machine made of cast iron and it protects the inner parts of the machine. It can be also made of any other strong material since it is not to carry the magnetic field. Cast iron is used only because of its high mechanical strength.
Stator Core: made of silicon steel in number of stamping which are insulated from each other. Its function is to provide an easy path for the magnetic lines of force and accommodate the stator winding.
Stator Winding: Slots are cut on the inner periphery of the stator core in which three-phase or one-phase winding is placed. Enameled copper is used as winding material.
Rotor: The rotating part of the machine is called rotor. From construction point of view, there are two types of rotors named as-
- Salient pole type
- Non-Salient pole type
Salient-Pole type:
cost of construction is low
but cannot bear high mechanical stresses at high speeds.
Therefore, suited for medium and low speeds and are usually employed at hydro-electric and diesel power plants
Important Parts:
Spider
Pole core and pole shoe
Field winding or Exciting winding
Damper winding
Spider: made of cast iron to provide an easy path for \(\Phi\). It is keyed to the shaft and at the outer surface, pole core and pole-shoe are keyed to it.
Pole core and pole shoe: Pole core provides least reluctance path for the magnetic field and pole shoe distributes the field over the whole periphery uniformly to produce sinusoidal emf wave
Field winding or Exciting winding: is wound on the former and then placed around the pole core. DC supply is given to it through slip rings. When DC flows through the field winding, it produces the required magnetic field.
Damper winding: At the outermost periphery, holes are provided in which copper bars are inserted and short-circuited at both the sides by rings forming damper winding.
Generally, the segments on individual poles are joined together to form common rings resulting in a short-circuited squirrel cage winding.
Salient pole machines are frequently provided with a damper winding on the rotor to damp rotor oscillations during transient-conditions and to facilitate smooth operation under unbalanced load conditions.
At hydro-electric power plants, usually, salient pole type alternators are placed with their shafts in vertical position.
Non-Salient pole type rotor:
suited for the high speeds.
The steam turbines rotate at a high speed (3000 rpm).
When these turbines are used as prime-mover for generator, a small number of poles are required for given frequency.
Hence, have smaller diameter and larger length.
Non salient pole type rotors have the following parts:
Rotor core: made of silicon steel stampings. It is keyed to the shaft. It provides an easy path to the magnetic flux.
Rotor winding or Exciting winding: It is placed in rotor slots and current is passed through the winding in such a way that poles are formed according to the requirement.
uniform air-gap, means self-inductance of the stator coils does not depend on position of rotor
varying air-gap, results stator self-inductances are time-varying
Thus, salient-pole has d-axis and q-axis reactance
Special Features : Salient & Non-salient Structures
salient pole structure:
These are of larger diameter and shorter length.
Usually, 2/3rd of the pole pitch is covered by the pole shoes.
To reduce eddy current losses, the poles are laminated.
Employed with hydraulic turbines or with diesel engines which are usually operated at low speeds (100 to 375 rpm)
non-salient structure:
They are of smaller diameter and of very long axial length.
Robust construction and noiseless operation.
Less windage (air-resistance) loss.
Better in dynamic balancing.
High operating speed (3000 rpm).
Nearly sinusoidal flux distribution around the periphery, and therefore, gives a better emf waveform than that of salient poles field structure.
No need of providing damper windings because the solid field poles themselves act as efficient dampers.
Miscellaneous Parts:
Brushes: are made of carbon and these just slip over the slip rings. DC supply is given to the brushes. From brushes current flows to the slip rings and then to the exciting winding.
Bearings: are provided between the shaft and outer stationary body to reduce the friction.
Shaft: made of mild steel. Mechanical power is taken or given to the machine through shaft.
Air-gap and its Significance
A very small air-gap increases the stray-load loss and synchronous reactance \(X_d\).
A large air-gap needs larger excitation current. Therefore, a compromise has to be made.
Generally the ratio of air-gap to the pole pitch is between 0.008 and 0.02.