K.Rajkumar, Department of Electrical and Electronics Engineering Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.

1. Explain the constructional details of Alternator in detail with neat sketches. (16 Marks) Construction of Alternators: An alternator has 3,-phase winding on the stator and a d.c. field winding on the rotor. a. Stator It is the stationary part of the machine and is built up of silicon steel laminations having slots on its inner periphery. A 3-phase winding is placed in these slots and serves as the armature winding of the alternator. The armature winding is always connected in star and the neutral is connected to ground. b. Rotor Fig. 1. Salient Pole Rotor The rotor carries a field winding which is supplied with direct current through two slip rings by a separate d.c. source. This Low and medium-speed alternators (120-400 r.p.m.) such as d.c. source (called exciter) is generally a small d.c. shunt or those driven by diesel engines or water turbines have salient pole compound generator mounted on the shaft of the alternator. type rotors due to the following reasons: (a) The salient field poles Rotor construction is of two types, namely; would cause an excessive windage loss if driven at high speed and 1. Salient (or projecting) pole type would tend to produce noise. (b) Salient-pole construction cannot 2. Non-salient (or cylindrical) pole type be made strong enough to withstand the mechanical stresses to Salient pole type: which they may be subjected at higher speeds. In this type, salient or projecting poles are mounted on a large Since a frequency of 50 Hz is required, we must use a large circular steel frame which is fixed to the shaft of the alternator as number of poles on the rotor of slow-speed alternators (Used in shown in Fig. (1). The individual field pole windings are connected hydro turbines and Diesel Engines). Low- speed rotors always in series in such a way that when the field winding is energized by possess a large diameter to provide the necessary spate for the the d.c. exciter, adjacent poles have opposite polarities. poles. Consequently, salient-pole type rotors have large diameters and short axial lengths. Non-salient pole type: In this type, the rotor is made of smooth solid forged-steel radial cylinder having a number of slots along the outer periphery. The field windings are embedded in these slots and are connected in series to the slip rings through which they are energized by the d.c. exciter. The regions forming the poles are usually left unslotted as shown in Fig. (2). It is clear that the poles formed are non-salient i.e., they do not project out from the rotor surface.

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K.Rajkumar, Department of Electrical and Electronics Engineering Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.

2. Discuss the powerfactor improvement using Synchronous Condenser. (Or) Write notes on Synchronous Condensor.(8Marks) SYNCHRONOUS CONDENSER A synchronous motor takes a leading current when over-excited and, therefore, behaves as a capacitor. An over-excited synchronous motor running on no-load is known as synchronous condenser. When such a machine is connected in parallel with induction motors or other devices that operate at low lagging power factor, the leading kVAR supplied by the synchronous condenser partly neutralizes the lagging reactive kVAR of the loads. Consequently, the power factor of the system is improved. The following figure shows the power factor improvement by synchronous condenser method. The 3 phase load takes current IL at low lagging power factor cosϕL. The synchronous condenser takes a Fig. 2. Cylinderical Rotor current Im which leads the voltage by an angle ϕm. The resultant High-speed alternators (1500 or 3000 r.p.m.) are driven by steam current I is the vector sum of Im and IL and lags behind the voltage turbines and use non-salient type rotors due to the following by an angle f. It is clear that ‘ϕ’ is less than ‘ϕL’ so that cosϕ is reasons: greater than cosϕL. Thus the power factor is increased from cosϕL to a. This type of construction has mechanical robustness and gives cosϕ. noiseless operation at high speeds. Synchronous condensers are generally used at major bulk supply substations for power factor improvement. b. The flux distribution around the periphery is nearly a sine wave Advantages and hence a better e.m.f. waveform is obtained than in the case i. By varying the field excitation, the magnitude of current of salient-pole type. drawn by the motor can be changed by any amount. This Since steam turbines run at high speed and a frequency of 50 Hz helps in achieving stepless control of power factor. is required, we need a small number of poles on the rotor of high- ii. The motor windings have high thermal stability to short speed alternators (also called turboalternators) (Used with steam circuit currents. tubines). We can use not less than 2 poles and this fixes the highest iii. The faults can be removed easily. possible speed. For a frequency of 50 Hz, it is 3000 r.p.m. The next Disadvantages lower speed is 1500 r.p.m. for a 4-pole machine. Consequently, i. There are considerable losses in the motor. turboalternators possess 2 or 4 poles and have small diameters and ii. The maintenance cost is high. very long axial lengths. iii. It produces noise. iv. Except in sizes above 500 RVA, the cost is greater than that of static capacitors of the same rating. 2

K.Rajkumar, Department of Electrical and Electronics Engineering Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.

The phasor diagram shown in Fig.3 neglects the armature resistance. i.e, Ra=0. X tan  s   Hence =90o. Ra Input power per phase =VIacosϕ 2 For the case Ra=0, stator copper loss, Ia Ra=0. Hence input power is equal to the mechanical power developed by the motor (Pm). i.e, Pm= VaIacosϕ --- (1) Referring to the phasor diagram in Fig 3,

AB  Er cos  I a X s cos

Also, AB  Eb sin  I a X s cos

Eb sin  or I a cos  --- (2) X s Substituting (2) in (1),

Fig 2. Powerfactor improvement using Synchronous Condenser VE b sin  Pm  X s v. As a synchronous motor has no self-starting torque, then- It is clear from the above relation that mechanical power increases fore, an auxiliary equipment has to be provided for this with torque angle δ and its maximum value reached when δ=90o. purpose. VE P  b per phase 3. Derive the power developed by the synchronous motor. m max  X s (8Marks) Under this condition, the poles of the rotor will be mid-way between N and S poles of the stator. 4. Compare the Synchronous Motor with Induction Motor. (6 Marks) S.No Remarks Synchronous Motor Induction Motor 1 Speed Remains Constant Decreases with load irrespective of load 2 Powerfactor Can be operated at Operates at lagging any powerfactor p.f only 3 Excitation Requires D.C No excitation is Excitation at the required. Fig.3. Phasor Diagram of Under excited Synchronous Motor rotor 3

K.Rajkumar, Department of Electrical and Electronics Engineering Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy.

S.No Remarks Synchronous Motor Induction Motor excitation, i.e the motor is over-excited, operates at leading power 4 Economy Economical for the Economical for factor. speed below 300 speed above 600 Fig(1) shows the variation of armature current and power factor r.p.m r.p.m 5 Self-starting No self starting. It Self-starting with field current at no load, half load and full load conditions. requires additional arrangement. 6 Construction Complicated Simple 7 Starting More Starting Less Starting torque Torque torque 5. Explain how the V and inverted V curves can be obtained in synchronous motor. (8Marks) A synchronous motor is a double-excited machine, its armature winding is energised from an a.c source and its field winding from d.c source. When synchronous motor is working at constant applied voltage, the resultant air gap flux demanded by applied voltage remains constant. This resultant air gap flux is established by both a.c in armature winding and d.c in the field winding. If the field current is sufficient enough to set up the air-gap flux, as demanded by constant applied voltage then magnetizing current or lagging reactive VA requied from the a.c source is zero and therefore motor operates at unity power factor. This field current, which causes unity power factor operation of the synchronous motor, is called normal excitation or normal field current. If the current less than the normal excitation, i.e, the motor is under excited, then the deficiency in flux must be made up by the 6. Discuss the procedure to obtain Xd (Direct axis reactance) and armature winding m.m.f. In order to do the needful, the armature Xq (Quadrature Axis Reactance) of a synchronous generator. winding draws a magnetizing current or lagging reactive VA from (8Marks) the a.c source and as a result of it, the motor operates at a lagging The unsaturated values of Xd and Xq of a 3-Phase synchronous power factor. In case the field current is made more than its normal machine can be easily determined experimentally by conducting the 4

K.Rajkumar, Department of Electrical and Electronics Engineering Dhanalakshmi Srinivasan Institute of Technology, Samayapuram, Trichy. following test known as slip test. The rotor of the synchronous machine is driven by means of a prime mover (usually a DC motor in the laboratory) at a speed close to the synchronous speed in the proper direction but not equal to it. The armature is supplied with a low voltage 3-Phase balanced supply through a variac, while the field circuit is kept open. The armature current varies between two limits since it moves through, since the synchronously rotating armature MMF acts through the varying magnetic reluctance paths as it goes from inter-polar axis to pole axis region. The values of Xsd and Xsq are determined based on the applied voltage and the armature current values. The ratio of applied voltage to the minimum value of the armature current gives the direct axis synchronous reactance

Xsd. The ratio of applied voltage to the maximum value of the armature current gives the the quadrature-axis reactance Xsq. For more accurate determination of these values the oscillogram of the armature current and voltage can be recorded.

Vt X d  imin 2

Vt X q  imax 2

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