Synchronous Machines - Construction - Principle of Operation
A synchronous machine is a most important type of electric machine. The generating machines found in all generating stations are synchronous machines and are commonly known as synchronous generator or alternators. Synchronous motors are well known for their constant speed operation and are widely used in industries.
The electromechanical energy conversion occurs whenever a change in flux is associated with mechanical motion. In synchronous machines field winding is the primary source of flux. The field winding and the armature winding are placed on a common magnetic circuit composed of two parts – the stator and the rotor
The stator is a stationary member. It is the annular potion of a cylinder inside which the rotor rotates. A sufficient air gap is provided between the stator and the rotor. The armature winding are housed in the slots cut in the stator
There are two types of rotor constructions employed in the synchronous motors. They are cylindrical type rotor and salient pole or projecting pole type rotor. The cylindrical pole rotor has the dc field winding embedded in them. Cylindrical rotor provides greater mechanical strength and permits more accurate dynamic balancing. It is particularly used in high speed turbo generators.
The second type of synchronous motor i.e. the salient pole rotors have projecting poles in it. These projecting poles lessen its mechanical strength. This type of rotor construction is used for low speed applications such as hydroelectric generators. Large number of poles in the rotor makes the rotor larger in diameter and smaller in length.
The rotor axle is carried on two bearings which a housed on the two end covers bolted on the two sides of the rotor. The stator and rotor are made up of silicon steel, a high permeability magnetic material. The field winding is supplied with a separate DC supply from the exciter, through a pair of carbon brushes. The exciter may be an external DC source or a DC generator coupled to the shaft of the synchronous a machine.
Distinguishing features of Salient pole and cylindrical pole rotors.
Salient pole rotor
Cylindrical pole rotors
The salient pole rotors have projecting poles in it.
The cylindrical pole rotors have projecting poles in it.
Mechanical strength is low
Greater mechanical strength
Larger in diameter and smaller in length
Smaller in diameter but larger in length
Have non uniform air gap
Have uniform air gap
Principle of operation
When a DC excitation provided to the rotor creates a fixed North Pole and South Pole in it. The rotor produces a stationary flux in the air gap which links with the stator winding.
Generator is a device which converts mechanical energy into electrical energy. When the rotor is rotated with the help of a prime mover it produces a sinusoidal flux distribution in the air gap which links the armature windings and induces sinusoidally alternating emf in it. The frequency of the induced emf can be calculated using the following formula.
P = number of stator poles.
Ns = Speed of the rotor in rpm.
Motor is a device which converts electrical energy. Let the 3 phase winding of the stator b connected to a 3 phase supply of fixed voltage and frequency. As a result, a 3- phase currents flowing through the stator winding creates a synchronously rotating magnetic field at synchronous speed Ns.
Consider that the rotor is run by an auxiliary means to a speed close to the synchronous speed in the direction of rotation of stator field. Now the rotor pulls into step with the synchronously rotating stator field and runs exactly at the synchronous speed. The electro mechanical torque is developed on the rotor in the direction of rotation of rotor and balances the load torque. It can be noted that the rotor falls back to an angle δ for a given load torque TL. This angle is called torque angle or power angle.
The torque developed by the synchronous motor is given by the following expression.
Hunting in synchronous machines
When a synchronous machine is operated at steady load, certain limited amplitude disturbances are bound to occur on the electrical and mechanical parts of the machine. These disturbances are sudden change in load, sudden change infield current, presence of harmonic variations in load and also in prime mover torque. This disturbance produces oscillations in the machines. This oscillatory behavior is known as hunting. Hunting can be controlled by providing additional damper winding in the rotor.