Synchronous Machine on Infinite Bus Bars: Synchronizing to Infinite Bus Bars – A definite procedure has to be followed in connecting a synchronous machine to bus-bars which for the present purpose will be assumed to be infinite. Infinite bus-bars means a 3phase supply of constant voltage and frequency independent of the load exchanged (fed into the bus-bars or drawn from the bus-bars). Figure 8.27 shows a synchronous machine with terminals a, b, c which is required to be connected to bus-bars with terminals A, B, C by means of a switch S. The machine is run as a generator with its terminals so arranged that its phase sequence is the same as that of the bus-bars. The machine speed and field current are adjusted so as to satisfy the following conditions: The machine terminal voltage must be nearly equal to the bus-bars voltage. The machine frequency is nearly equal to the bus-bars frequency, i.e. the machine speed is close to synchronous. After the above conditions are satisfied the instant of switching on (synchronizing) must be determined such that the two voltages are almost co-phase (the acceptable phase difference is of the order of 5°). This instant is determined with the help of the method described below. Figure 8.28 shows the phasor diagram for phase voltages (line-to-neutral) for the machine and bus-bars. As the two frequencies are not exactly equal, the machine phasors are rotating slowly with respect to the bus-bar phasors at 2 jr 4f rad/s, where 4f is the difference in the two frequencies. At the instant when the two sets of phasors are coincident (cophasal), the voltage The condition can be easily determined by connecting three lamps—one across aA, the other across bC and the third across cB (in order to use standard-voltage lamps it may be necessary to employ potential transformers). The rms values of voltages V aA, Vbc and Vail oscillate at the difference frequency df so that each lamp is alternately dark and bright. At the instant of synchronization, as per the condition (8.40) stated above, the lamp across aA is dark while the other two lamps are equally bright. It is at this instant that switch S is closed. Instead of using lamps, generating stations use an instrument called synchroscope. Once switch S is closed the stator and rotor fields of the machine lock into each other (synchronize) and the machine then onwards runs at synchronous speed. The real power exchange with the mains will now be governed by the loading conditions on the shaft while the reactive power exchange will be determined by the field excitation As a generator is coupled to a prime mover it is easy to follow the above procedure to connect it to the busbars. The same procedure has to be followed for a synchronous motor which must be run initially by an auxiliary device (may be a small dc/induction motor) and then synchronized to the bus-bars. It may be pointed out here that the synchronous motor is non-self-starting. If, for example, switch S is closed in Fig. 8.27 with the rotor stationary, the stator and rotor fields will be moving relative to each other at synchronous speed so as to develop alternating torque with zero average value and as a result the motor would not start. Synchronous motors are made self-starting by providing short-circuited bars on the rotor which produce induction torque for starting PRINCIPLE OF WORKING When the Starting winding in the Synchronous motor is excited by the AC line connection, it immediately sets up a rotating magnetic field. The rotating flux of this field cuts across the damper winding of the rotor and induces secondary currents in the bars of this winding. The reaction between the flux of these secondary current and that of the rotating stator field produced the torque necessary to start the rotor and to bring it to speed. When the motor has been brought up to nearly synchronous speed (as an induction motor because of the damper winding) the DC field poles are excited and the strong flux of these poles causes them to be drawn into step of the stator. During normal operation, the rotor continues to revolve at synchronous speed as if the DC poles were locked to the poles of the rotating magnetic field of the stator. Because a synchronous motor has no slip after the rotor is brought up to full speed, no secondary currents are induced in the bays of the damper windings during normal operation. Application of Synchronous Motor 1. Synchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any electrical power factor, it is used in power system in situations where static capacitors are expensive. 2. Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500 KW, the size, weight and cost of the corresponding three phase induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc. Characteristic Features Of A Synchronous Motor Synchronous motor will run either at synchronous speed or will not run at all. The only way to change its speed is to change its supply frequency. (As Ns = 120f / P) Synchronous motors are not self starting. They need some external force to bring them near to the synchronous speed. They can operate under any power factor, lagging as well as leading. Hence, synchronous motors can be used for power factor improvement. Effect of Excitation and power factor control, V and Inverted V Curves Synchronous motor excitation refers to the DC supply given to rotor which is used to produce the required magnetic flux. One of the major and unique characteristics of this motor is that it can be operated at any electrical power factor leading, lagging or unity and this feature is based on the excitation of the synchronous motor. When the synchronous motor is working at constant applied voltage V, the resultant air gap flux as demanded by V remains substantially constant. This resultant air gap flux is established by the co operation of both AC supply of armature winding and DC supply of rotor winding. W ith constant mechanical load on the synchronous motor,the variation of f i e l d c u r r e n t c h a n g e s t h e a r m a t u r e c u r r e n t d r a w n b y t h e m o t o r a n d a l s o itsoperating power factor. as such,the behavior of the synchronous moto r i s described below under three different modes of field excitation. 1. Normal excitation: The armature current is minimum at a particular value of field current , which is c a l l e d t h e n o r m a l f i e l d e x c i t a t i o n . T h e o p e r a t i n g p o w e r f a c t o r o f t h e motor is unity at this excitation and thus the motor is equivalent to a resistive type of load. 2. under excitation: When the field current is decreased gradually below the normal excitation, the armature current increases and the operating power f a c t o r o f t h e m o t o r decreases. The power factor under this condition is lagging. Thus, the synchronous motor draws a lagging current, when it is under excited and is eq uivalent to an inductive load. 3. Over excitation: When the field current is increased gradually beyond the normal excitation, the armature current again increases and the operating power factor decreases. However, the power factor is leading under this condition. Hence, the synchronous m o t o r d r a w s a l e a d i n g c u r r e n t , w h e n i t i s o v e r e x c i t e d a n d i s e q u i v a l e n t t o a capacitive load The power factor of the synchronous motor can be controlled by varying the field current I f. As we know that the armature current Ia changes with the change in the field current If. Let us assume that the motor is running at NO load. If the field current is increased from this small value, the armature current Ia decreases until the armature current becomes minimum. At this minimum point, the motor is operating at unity power factor. The motor operates at lagging power factor until it reaches up to this point of operation. If now, the field current is increased further, the armature current increases and the motor start operating as a leading power factor. The graph drawn between armature current and field current is known as V curve. If this procedure is repeated for various increased loads, a family of curves is obtained. The Graph plotted between the armature current Ia and field current If at no load the curve is obtained known as V Curve. Since the shape of these curves is similar to the letter “V”, thus they are called V curve of synchronous motor The V curves of a synchronous motor are shown below. The V curves are useful in adjusting the field current. Increasing the field current If beyond the level for minimum armature current results in leading power factor. Similarly decreasing the field current below the minimum armature current result results in lagging power factor. It is seen that the field current for unity power factor at full load is more than the field current for unity power factor at no load. T h e c h a r a c t e r i s t i c c u r v e p l o t t e d between input power factor and the field current for a constant mechanical load on the motor are of the shape of inverted 'V' and are known as inverted 'V' curves. The Inverted V Curve is a graph showing the relation of power factor as a function of field current. The inverted V curve shows the variation of the armature current by varying the field excitation. Obviously armature current caused armature reaction which will oppose the field fluxes or the field they are inversely proportional to each other. Hunting in synchronous motor The phenomenon of oscillation of the rotor about its final equilibrium position is called Hunting. On the sudden application of load, the rotor search for its new equilibrium position and this process is known as Hunting. The Hunting process occurs in a synchronous motor as well as in synchronous generators if an abrupt change in load occurs. Causes of Hunting Sudden changes of load. Faults were occurring in the system which the generator supplies. Sudden change in the field current. Cyclic variations of the load torque. Effects of Hunting It can lead to loss of synchronism. It can cause variations of the supply voltage producing undesirable lamp flicker. The possibility of Resonance condition increases. If the frequency of the torque component becomes equal to that of the transient oscillations of the synchronous machine, resonance may take place. Large mechanical stresses may develop in the rotor shaft. The machine losses increases and the temperature of the machine rise. Reduction of Hunting Use of damper windings Uses of flywheels:-The prime mover is provided with a large and heavy flywheel. This increases the inertia of the prime mover and helps in maintaining the rotor speed constant. By designing synchronous machines with suitable synchronizing power coefficients. Construction of three -phase synchronous machines An alternator consists of two main parts namely, the stator and the rotor. the stator is the stationary part of the machine. It carries the armature winding in which the voltage is generated. the output of the machine is taken from the stator. the rotor is the rotating part of machine. the rotor produces the main field flux. Stator: the various parts of the stator include the frame, stator core, stator windings and cooling arrangement. Frame may be cast iron for small-size machines and welded steel type for large size machines. In order to reduce hysteresis and eddy-current losses, the stator core is assembled with high grade silicon content steel lamination. A 3-phase winding is put in the slots cut on the inner periphery of the stator as show in fig the winding of each phase is distributed over a several slots. When current flow in a distributed winding it produces an essentially sinusoidal apace distribution of emf. Rotor: There are two types of rotor construction namely, the salient pole and cylindrical rotor type. Salient Pole Rotor: The term Salient means protruding or projecting thus a salient- pole rotor consists of poles projecting out from the surface of the rotor core. Since the rotor is subjected to changing magnetic fields, it is made of thin steel laminations to reduce eddy current losses. The salient poles rotor has concentrated winding on the poles. Damper bars are usually inserted in the pole faces to damp out the rotor oscillations during the sudden change in load condition. A salient pole synchronous machines has nonuniform air gap. the air gap is minimum under the pole centers and it is maximum in between the poles. the pole faces are so shaped that the radial air gap length increases from pole centre to pole tips so that the flux distribution in the air gap is sinusoidal. this will help the machine generate sinusoidal emf. The individual field pole windings are connected in series to give alternate north south polarities. the end of field winding are connected to a d. c. source through brushes on the slip rings. the slip rings are metal rings mounted on the shaft insulated from it. they are used to carry current to or from the rotating part machine via carbon brushes. and the and the Salient pole generator have a large number of poles, and operate at lower speeds. A salient pole generator has comparatively a large diameter and short axial length. the large diameter accommodates a large number of poles. Salient pole generators driven by water turbines are called hydroalternators orhydrogenerators. Cylindrical Rotor: A cylindrical rotor machine is also called a non-salient pole rotor machines.it has its rotor so constructed that it forms a smooth cylinder. the construction is such that there are no physical poles to be seen as in salient pole construction. Cylinder rotor are made from solid forgings of high grade nickel chrome-molybdenum steel. In about two third of the rotor periphery, slots are cut in regular intervals and parallel to the shaft. The d.c. field wings are accommodated in these slots. the winding of distributed type. the unslotted portion of the rotor forms pole faces. The smooth rotor of the machine makes less winding losses and the operation is less noisy because of uniform air gap. fig Show end views of 2 pole and 4 pole cylindrical rotors. cylindrical-rotor machines are driven by steam or gas turbines. cylindrical-rotor synchronous generators are called turboalternator or turbogenerator.such machine are always horizontal configuration installation.the machine are built in a number of ratings from 10MVA to 1500MVA. Armature reaction In an alternator, a three phase voltage is induced at the stator winding, due to the rotating magnetic field from the rotor. The stator circuit is also known as armature circuit. When there is no load across the stator, the entire voltage induced at the stator winding appears as the terminal voltage .However, if we connect a load across the stator, current flows across it ,which produces its own flux known as stator flux .The produced stator flux distorts the main flux as a result of which the terminal voltage across the machine is not equal to the voltage initially induced . This effect of the stator( armature) is known as armature reaction. The effect of the armature reaction on the terminal voltage of the alternator is not same for all conditions. In case the load is at unity power factor, the affect of armature reaction is cross magnetizing type. When the load is at unity power factor,Stator current is at phase with induced emf…….(1) ∅𝑓=∅𝑓𝑚sinwt ( Flux at the armature due to main field flux ) Emf (E) = -w∅𝑚.coswt (Emf at the armature)……. (2) Angle between ∅ and Emf (E) is 90 degrees………… (3) Armature flux : ∅ (a) is directly proportional to stator current. So, ∅ (a) is in phase with E. [From (1)] ……….(4) From (3) and (4) ∅ and ∅ (a) are at quadrature. Hence, the effect is cross magnetizing. So, voltage at no load is only slightly greater than voltage at full load. Hence, the voltage regulation is positively small. In case the load is at lagging power factor, the affect of armature reaction is demagnetizing type. Angle between phi(f) and Emf (E) is 90 degrees. phi(f) leads Emf(E) by 90 degrees………(1) Stator current I lags Emf ( E) at 90 degrees. phi (a) lags Emf at 90 degrees…………..(2) From, (1) and (2) phi(a) and phi(f) are opposite. They cancel each other. So, voltage at no load is higher than the voltage at the full load. Hence, voltage regulation is positively large. In case the load is at lagging power factor, the affect of armature reaction is magnetizing type. Angle between phi(f) and Emf (E) is 90 degrees. phi(f) leads Emf(E) by 90 degrees………(1) Stator current I leads Emf ( E) at 90 degrees. phi (a) leads Emf at 90 degrees…………..(2) From, (1) and (2) phi(a) and phi(f) are supporting eachother. They add up . So, voltage at no load is lower than the voltage at the full load. Hence, voltage regulation is negative. Salient Pole Synchronous Machine Two Reaction Theory In the cylindrical rotor synchronous machine, the air gap is uniform. The pole structure of the rotor of a salient pole machine makes the air gap highly non-uniform. Consider a 2 pole, salient pole rotor rotating in the anticlockwise direction within a 2 pole stator as shown in the figure below. The axis along the axis of the rotor is called the direct or the d axis. The axis perpendicular to d axis is known as the quadrature or q axis. The direct axis flux path involves two small air gaps and is the path of the minimum reluctance. The path shown in the above figure by ϕq has two large air gaps and is the path of the maximum reluctance. The rotor flux BR is shown vertically upwards as shown in the figure below. The rotor flux induces a voltage Ef in the stator. The stator armature current Iawill flow through the synchronous motor when a lagging power factor load is connected it. This stator armature current I a lags behind the generated voltage Ef by an angle Ψ. The armature current produces stator magneto motive force Fs. This MMF lags behind Ia by angle 90 degrees. The MMF FS produces stator magnetic field Belong the direction of Fs. The stator MMF is resolved into two components, namely the direct axis component Fd and the quadrature axis component Fq. If, ϕd is the direct axis flux Φq is the quadrature axis flux Rd is the reluctance of the direct axis flux path Therefore As, Rd < Rq, the direct axis component of MMF Fd produces more flux than the quadrature axis component of the MMF. The fluxes of the direct and quadrature axis produce a voltage in the windings of the stator by armature reaction. Let, Ead be the direct axis component of the armature reaction voltage. Eaq be the quadrature axis component of the armature reaction voltage. Since each armature reaction voltage is directly proportional to its stator current and lags behind by 90 degrees angles. Therefore, armature reaction voltages can be written as shown below. Where, Xad is the armature reaction reactance in the direct axis per phase. Xaq is the armature reaction reactance in the quadrature axis per phase. The value of Xad is always greater than Xaq. As the EMF induced by a given MMF acting on the direct axis is smaller than for the quadrature axis due to its higher reluctance. The total voltage induced in the stator is the sum of EMF induced by the field excitation. The equations are written as follows:- The voltage E’ is equal to the sum of the terminal voltage V and the voltage drops in the resistance and leakage reactance of the armature. The equation is written as The armature current is divided into two components; one is the phase with the excitation voltage E f and the other is in phase quadrature to it. If Iq is the axis component of Ia in phase with Ef. Id is the d axis Ia lagging Ef by 90 degrees. Therefore, Combining the equation (4) and (5) we get Combining the equation (6) and (7) we get Let, The reactance Xd is called the direct axis synchronous reactance, and the reactance Xq is called the quadrature axis synchronous reactance. Combining the equations (9) (10) and (11), we get the equations shown below. The equation (12) shown above is the final voltage equation for a salient pole synchronous generator.