Introduction :
In an AC system voltage level can be increased or decreased (according to need) very easily with the help of a transformer, therefore, this system is exclusively used for generation, transmission and distribution of electric power. The mechanical energy is converted into electrical energy with the help of an AC machine called alternator or synchronous generator. However, when the same machine can be used to convert electrical energy into mechanical energy, then it is known as a synchronous motor Thus, the same machine can be operated as a generator or as a motor and in general, it is called as a synchronous machine.
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These machine always run at synchronous speed which is given by NS = 120 f/P
Where f = frequency, P = Pole
Working principle :
A synchronous machine is just an electro-mechanical transducer which converts mechanical energy into electrical energy or vice-versa.
Construction :
The important parts of a synchronous machine are given below:
1. Stator 2. Rotor 3. Miscellaneous
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(1) Stator :
The outer stationary part of the machine is called stator;
It has the following important parts:
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(i) Stator frame : It is the 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.
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(ii) Stator Core : The stator core is made of silicon steel material. It is made from 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.
(iii) Stator Winding : Slots are cut on the inner periphery of the stator core in which three-phase or one-phase winding is placed. Enamelled copper is used as winding material.
(2) Rotor : The rotating part of the machine is called rotor. From construction point of view, there are two types of rotors named as
(i) Salient pole type rotor
(ii) Non-salient pole type rotor.
(i) Salient pole type rotor : In this case, projected poles are provided on the rotor. The cost of construction of salient pole type rotors is low, moreover sufficient space is available to accommodate field winding but these cannot bear high mechanical stresses at high speeds. Therefore, salient pole type construction is suited for medium and low speeds and are usually employed at hydro-electric and diesel power plants as synchronous generators. Since the speed of these machines (generators) is quite low, to obtain the required frequency, the machines have large number of poles as shown in below Figure. To accommodate such a large number of poles, these machines have larger diameter and small length.For a speed of 200 rpm (alternators coupled with water turbines) the diameter of the machines is as large as 14 metre and length is only 1 metre.
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The salient pole type rotor has the following important parts:
(a) Spider : Spider is made of cast iron to provide an easy path for the magnetic flux. It is keyed to the shaft and at the outer surface, pole core and pole-shoe are keyed to it.
(b) Pole core and pole shoe : It is made of laminated sheet material. Pole core provides least reluctance path for the magnetic field and pole shoe distributes the field over the whole periphery uniformly to produce sinusoidal wave form of the generated emf.
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(c) Field winding or Exciting winding: Field winding is wound on the former and then placed around the pole core. DC supply is given to it through slip rings. When direct current flows through the field winding, it produces the required magnetic field.
(d) 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 similar to that used in induction machines with squirrel cage rotors. 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.
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(ii) Non-salient pole type rotor : A non-salient pole alternator is shown in Fig. (e). In this case, there are no projected poles but the poles are formed by the current flowing through the rotor (exciting) winding. Non-salient pole type construction is suited for the high speeds. The steam turbines rotate at a high speed (3000 rpm). When these turbines are used as prime-mover for this machine working as a generator, a small number of poles are required for given frequency. Hence, these machines have smaller diameter and larger length.
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Non salient pole type rotors have the following parts:
(a) Rotor core: Rotor core is made of silicon steel stampings. It is keyed to the shaft. At the outer periphery slots are cut in which exciting coils are placed. It provides an easy path to the magnetic flux.
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(b) 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
(3) Miscellaneous Parts : The following are few important miscellaneous parts;
(i) Brushes: 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.
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(ii) Bearings: Bearings are provided between the shaft and outer stationary body to reduce the friction. The material used for their construction is high carbon steel.
(iii) Shaft: Shaft is made of mild steel. Mechanical power is taken or given to the machine through shaft.
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Working :
1) Synchronous Motor
Working of Synchronous Motor depend on the interaction of the magnetic field of the stator with the magnetic field of the rotor. The stator supplied with 3 phase power. Thus , Stator winding produces a 3 phase rotating magnetic field , rotor winding of synchronous motor is excited by DC source. magnetic field produced by stator is rotating in nature, so this field is cut by rotor conductors, due to that emf is generated in rotor conductors by this emf one more magnetic field is produced but in opposite direction. Rotor winding is supplied by DC source so rotor produce a permanent magnet poles. Both magnetic field interact each other(magnetic locking between stator and rotor ) and rotate with same speed (synchronous speed).
which is given by NS = 120f/P
where f= frequency, P= No of poles
Question season (write in comment if you know)
(1) Do you know any application of
Synchronous Machine ?
(2) Power frequency in US ?
(3) Power frequency in india ?
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