Electrical World

In the applications where high starting torque and constant speed are desired then synchronous induction motors can be used. It has the advantages of  both synchronous and induction motors. The synchronous motor gives constant speed whereas induction motors can be started against full load torque.        Consider a normal slip ring induction motor having three phase winding on the rotor as shown in the Fig. 1. Fig. 1        The motor is connected to the exciter which gives d.c. supply to the motor through slip rings. One phase carries full d.c. current while the other two carries half of the full d.c. current as they are in parallel. Due to this d.c. excitation, permanent poles (N and S) are formed on the rotor.        Initially it is run as an slip ring induction motor with the help of starting resistances. When the resistance is cut out the motor runs with a slip. Now ...

The important characteristics of the synchronous motor is its constant speed irrespective of the load conditions, and variable power factor operation. As seen earlier its power factor can be controlled by controlling its excitation. For overexcitation its power factor is leading in nature, which is very important from the power factor correction point of view.        Due to constant speed characteristics, it is used in machine tools, motor generator sets, synchronous clocks, stroboscopic devices, timing devices, belt driven reciprocating compressors, fans and blowers, centrifugal pumps, vacuum pumps, pulp grinders, textile mills, paper mills line shafts, rolling mills, cement mills etc.        The synchronous motors are often used as a power factor correction device, phase advancers and phase modifiers for voltage regulation of the transmission lines. This is possible because the excitation of the synchronou...

When synchronous motor is over excited it takes leading p.f. current. If synchronous motor is on no load, where load angle δ is very small and it is over excited (E b > V) then power factor angle increases almost upto 90 o . And motor runs with almost zero leading power factor condition. This is shown in the phasor diagram Fig. 1. Fig. 1 Synchronous condenser        This characteristics is similar to a normal capacitor which takes leading power factor current. Hence over excited synchronous motor operating on no load condition is called as synchronous condenser or synchronous capacitor. This is the property due to which synchronous motor is used as a phase advancer or as power improvement device. 1.1 Disadvantage of Low Power Factor        In various industries, many machines are of induction motor type. The lighting and heating loads are supplied through transformers. The induction motors and transforme...

There is a specific procedure of connecting synchronous machine to infinite bus bars. Infinite bus bar is one which keeps constant voltage and frequency although load varies. The Fig. 1 shows a synchronous machine which is to be connected to the bus bars with the help of switch K.         If the synchronous machine is running as a generator then its phase sequence should be some as that of bus bars. The machine speed and field current is adjusted in such a way so as to have the machine voltage same as that of bus bar voltage. The machine frequency should be nearly equal to bus bar frequency so that the machine speed is nearer to synchronous speed. Fig. 1         When the above conditions are satisfied, the instant of switching for synchronization should be determined. This can be determined by lamps dark method, Lamps bright and dark method or by using synchroscope.     ...

It is seen that, when synchronous motor is on no load, the stator and rotor pole axes almost coincide with each other. When motor is loaded, the rotor axis falls back with respect to stator. The angle by which rotor retards is called load angle or angle of retardation δ .        If the load connected to the motor is suddenly changed by a large amount, then rotor tries to retard to take its new equilibrium position.        But due to inertia of the rotor, it can not achieve its final position instantaneously. While achieving its new position due to inertia it passes beyond its final position corresponding to new load. This will produce more torque than what is demanded. This will try reduce the load angle and rotor swings in other direction. So there is periodic swinging of the rotor on both sides of the new equilibrium position, corresponding to the load. Such a swing is shown in the Fig. 1. Fig. 1  H...

The analysis of salient pole synchronous motor is based on the Blondel's two reaction. The direct and quadrature axis components of current and reactance are same as defined earlier for the synchronous generators. Thus,                       X d = Direct axis reactance                       X q  = Quadrature axis reactance                       I d  = Direct axis component of I a                        I q = Quadrature axis component of I a        The complete phasor diagram of lagging p.f. is shown in the Fig. 1. ...

The Blondel diagram of a synchronous motor is an extension of a simple phasor diagram of a synchronous motor.        For a synchronous motor, the power input to the motor per phase is given by,                     P in  = V ph I ph cosΦ                                               ............ per phase        The gross mechanical power developed per phase will be equal to the difference between  P in  per phase and the per phase copper losses of the winding.                    Copper loss per phase = (I aph ) 2   R a . . .  ...

The value of δ for which the mechanical power developed is maximum can be obtained as, Note : Thus when R a is negligible, θ = 90 o for maximum power developed. The corresponding torque is called pull out torque. 1.1 The Value of Maximum Power Developed        The value of maximum power developed can be obtained by substituting θ = δ in the equation of P m .        When R a is negligible,     θ = 90 o  and cos (θ) = 0 hence, . . .               R a = Z s cosθ   and X s = Z s sinθ        Substituting   cosθ = R a /Z s in equation (6b) we get,         Solving the above quadratic in E b we get,        As E b is completely dependent on excitation, the equation (8) gives the excitation limits for a...

Consider the phasor diagram of a synchronous motor running on leading power factor cosΦ as shown in the Fig. 1. Fig. 1        The line CD is drawn at an angle θ to AB.        The lines AC and DE are perpendicular to CD and AE.        and angle between AB = E bph and I aph is also ψ.        The mechanical per phase power developed is given by,        In triangle OBD,                   BD = OB cosψ = I a  Zs cosψ                   OD = OB sin ψ = Ia  Zs sin        Now    BD = CD - BC = AE - BC       Substituting in (2),       ...

Net input to the synchronous motor is the three phase input to the stator. . . .                       P in = √3 V L I L cosΦ W         where         V L = Applied Line Voltage                           I L = Line current drawn by the motor                           cosΦ = operating p.f. of synchronous motor        or                P in  = 3 ([er phase power)                      ...

Case i) Under excitation, E bph < V ph .        Z s = R a + j X s = | Z s |   ∟θ Ω        θ = tan -1 (X s /R a )        E Rph  ^ I aph = θ, I a lags always by angle θ.        V ph = Phase voltage applied        E Rph = Back e.m.f. induced per phase        E Rph = I a x Z s V            ... per phase        Let p.f. be cosΦ, lagging as under excited,        V ph  ^ I aph = Φ        Phasor diagram is shown in the Fig. 1. Fig. 1 Phasor diagram for under excited condition        Applying cosine rule to Δ OAB,         (E bph ) 2 = (V ph ) 2 ...