Principle of work of synchronous moto

Synchronous motor works on the principle of the magnetic locking. When two unlike poles are brought near each other, if the magnets are strong, there exists a tremendous force of attraction between those two poles. In such condition the two magnets are said to be magnetically locked.

       If now one of the two magnets is rotated, the other also rotates in the same direction, with the same speed due to the force of attraction i.e. due to magnetic locking condition. The principle is shown schematically in the Fig.1.

Fig.  1  Principle of magnetic locking

       So to have the magnetic locking condition, there must exist two unlike poles and magnetic axes of two must be brought very close to each other. Let us see the application of this principle in case of synchronous motor.

       Consider a three phase synchronous motor, whose stator is wound for 2 poles. The two magnetic fields are produced in the synchronous motor by exciting both the winding, stator and rotor with three phase a.c. supply and d.c. supply respectively. When three phase winding is excited by a three phase a.c. supply the the flux produced by the three phase winding is always of rotating type, which is already discussed in the previous post. Such a magnetic flux rotates in space at a speed called synchronous speed. This magnetic field is called rotating magnetic field. The rotating magnetic field creates the effect similar to the physical rotation of magnets in space with a synchronous speed. So stator of the synchronous motor produces one magnet which is as good as rotating in space with the synchronous speed. The synchronous speed of a stator rotating magnetic field depends on the supply frequency and the number of poles for which stator winding is wound. if the frequency of the a.c. supply is f Hz and stator is wound for P number of poles, then the speed of the rotating magnetic field is synchronous given by,

                            N_s  = 120f/P r.p.m.

       In this case, as stator is wound for say 2 poles, with 50 Hz supply, the speed of the rotating magnetic field will be 3000 r.p.m. This effect is similar to the physical rotation of two poles with a speed of N_s  r.p.m. For simplicity of understanding let us assume that the stator poles are N₁ and S₁ which are rotating at a speed of N_s. The direction of rotation of rotating magnetic field is say clockwise.

       When the field winding on rotor is excited by a d.c. supply, it also produces two poles, assuming rotor construction to be two pole, salient type. Let these poles be N₂and S₂. 

       Now one magnet is rotating at N_s having poles N₁ and S₁ while at start rotor is stationary i.e. second magnet is stationary having poles N₂ and S₂. If somehow the unlike poles N₁ and S₂ or S₁ and N₂ are brought near each other, the magnetic locking may get established between stator and rotor poles. As stator poles are rotating due to magnetic locking rotor will also rotate in the same direction as that of stator poles i.e. in the direction of rotating magnetic field, with the same speed i.e N_s. Hence synchronous motor rotates at one and only one speed i.e. synchronous speed. But this all depends on existence of magnetic locking between stator and rotor poles. Practically it is not possible for stator poles to pull the rotor poles from their stationary position into magnetic locking condition. hence synchronous motors are not self starting.

Power system protection

This portion of our website covers almost everything related to protection system in power system including standard lead and device numbers, mode of connections at terminal strips, color codes in multi-core cables, Dos and Don’ts in execution. It also covers principles of various power system protection relays and schemes including special power system protection schemes like differential relays, restricted earth fault protection, directional relays and distance relays etc. The details of transformer protection, generator protection, transmission line protection & protection of capacitor banks are also given. It covers almost everything about power system protection.

The switch gear testing, instrument transformers like current transformer testing voltage or potential transformer testing and associated protection relay are explained in detail. The close and trip, indication and alarm circuits different of Circuit breakers are also included and explain.

Objective of power system protection

The objective of power system protection is to isolate a faulty section of electrical power system from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.

Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit breakers automatically open during fault condition due to its trip signal comes from protection relay. The main philosophy about protection is that no protection of power system can prevent the flow of fault current through the system, it only can prevent the continuation of flowing of fault current by quickly disconnect the short circuit path from the system. For satisfying this quick disconnection the protection relays should have following functional requirements.

ABCD Parameters of Transmission

A major section of power system engineering deals in the transmission of electrical power from one particular place (e g. Generating station) to another like substations or distribution units with maximum efficiency. So its of substantial importance for power system engineers to be thorough with its mathematical modeling. Thus the entire transmission system can be simplified to a two port network for the sake of easier calculations.

The circuit of a 2 port network is shown in the diagram below. As the name suggests, a 2 port network consists of an input port PQ and an output port RS. Each port has 2 terminals to connect itself to the external circuit. Thus it is essentially a 2 port or a 4 terminal circuit, having

Supply end voltage = VS

and Supply end current = IS

Given to the input port P Q.

And there is the Receiving end Voltage = VR

and Receiving end current = IR

Given to the output port R S.

As shown in the diagram below.

Now the ABCD parameters or the transmission line parameters provide the link between the supply and receiving end voltages and currents, considering the circuit elements to be linear in nature.
Thus the relation between the sending and receiving end specifications are given using ABCD parameters by the equations below.


VS = A VR + B IR ———————-(1)

IS = C VR + D IR ———————-(2)