Notebook

The three terminals of the armature are short circuited .

The machine is driven at approximately synchronous rated speed and measurements of armature short circuit currents are made for various values of field currents usually up to and above rated armature current.

The machine is driven at approximately synchronous rated speed and measurements of armature short circuit currents are made for various values of field currents usually up to and above rated armature current.

In conventional synchronous machines the short circuit characteristics is practically linear because the iron is unsaturated up to rated armature current

To obtain the open circuit characteristics the machine is driven at rated speed without the load. Readings of the line-to-line voltage are taken for various values of field current. The voltage, except in very low voltage machines, is stepped down by the means of a potential transformer.

Experimental set up-

If not for the magnetic saturation of the iron, the open circuit characteristics would be linear as represented by the air gap line.

The three-phase alternator has three single-phase windings spaced so that the voltage induced in any one is phase-displaced by 120 degrees from the other two.

The voltage waveforms generated across each phase are drawn on a graph phase-displaced 120 degrees from each other.

•The three phases are independent of each other.

•One point from each winding can be connected to form a  neutral and thus make a wye connection.

•The voltage from this point to any one of the line leads will be the phase voltage. The line voltage across any two line leads is the vector sum of the individual phase voltages. The line voltage is 1.73, (Ö3 ), times the phase voltage.

Since the windings form only one path for current flow between phases, the line and phase currents are equal.

A three-phase stator can also be connected so that the phases form a “delta” connection.

•In the delta connection the line voltages are equal to the phase voltages, but the line currents will be equal to the vector sum of the phase currents.

•

•Since the phases are 120 degrees out of phase, the line current will be 1.73, (Ö3 ), times the phase current. Both "wye" and the "delta" connections are used in alternators.

The two poles of the stator winding are connected to each other so that the AC voltages are in phase, so they add.As the rotor (field) turns, its poles will induce AC voltages in the stator (armature) windings. Since one rotor pole is in the same position relative to a stator pole as any other rotor pole, both the stator poles are cut by equal amounts of magnetic lines of force at any time. As a result, the voltages induced in the two poles of the stator winding have the same amplitude or value at any given instant. 

Angle in Electrical and Mechanical Units

Consider a synchronous machine with two magnetic poles. The idealized radial distribution of the air gap flux density is sinusoidal along the air gap. When the rotor rotates for one revolution, the induced emf, which is also sinusoidal, varies for one cycle as illustrated by the waveforms in the diagram below. If we measure the rotor position by physical or mechanical degrees or radians and the phase angles of the flux density and emf

by electrical degrees or radians, in this case, it is ready to see that the angle measured in mechanical degrees or radians is equal to that measured in electrical degrees or radians, 

i.e.

where θ   is the angle in electrical degrees or radians and θ_m   the mechanical angle.

Stator and Rotor

The armature winding of a conventional synchronous machine is almost invariably on the stator and is usually a three phase winding. The field winding is usually on the rotor and excited by dc current, or permanent magnets. The dc power supply required for excitation usually is supplied through a dc generator known as exciter, which is often mounted on the same shaft as the synchronous machine. Various excitation systems using ac exciter and solid state rectifiers are used with large turbine generators.

    There are two types of rotor structures: round or cylindrical rotor and salient pole rotor as illustrated schematically in the diagram below. 

Generally, round rotor structure is used for high-speed synchronous machines, such as steam turbine generators, while salient pole structure is used for low-speed applications, such as hydroelectric generators. The pictures below show the stator and rotor of a hydroelectric generator and the rotor of a turbine generator.

1 Introduction

With the development of the technology and the way in which human labour is get-ting minimized and the comforts increasing tremendously the use of electrical energy is ever increasing. Basically electric power is the main source of energy for carrying out many func-tions, as it is a clean and efficient energy source, which can be easily transmitted over long distances. With the availability of Transformer for changing the voltage levels to a very high value (of say 132kV to 400kV) the use of AC power has increased rapidly and the DC power is used only at remote places where AC power cannot be supplied through power lines or cables or for a few social purposes. A synchronous generator is an electrical machine producing alternating emf (Elec- tromotive force or voltage) of constant frequency. In our country the standard commercial frequency of AC supply is 50 Hz. In U.S.A. and a few other countries the frequency is 60 Hz. The AC voltages generated may be single phase or 3-phase depending on the power supplied. For low power applications single phase generators are preferable. The basic prin- ciples involved in the production of emf and the constructional details of the generators are discussed below.

1.1 Generation of emf

In 1831 Faraday discovered that an emf can be induced (or generated) due to relative motion between a magnetic field and a conductor of electricity. This voltage was termed as the induced emf since the emf is produced only due to motion between the conductor and the magnetic field without actual physical contact between them. The principle of electromagnetic induction is best understood by referring to Fig. 1. The magnetic field is produced by the two fixed poles one being the north pole from which the magnetic flux lines emerge and enter into the other pole known as the south pole. It was found that the magnitude of the voltage induced in the conductor is proportional to the rate of change of flux lines linking the conductor.

MCQ 4.1 The slip of an induction motor normally does not depend on

(A) rotor speed (B) synchronous speed

(C) shaft torque (D) core-loss component

MCQ 4.2 A 220 V, 15 kW, 100 rpm shunt motor with armature resistance of 0.25 Ω,

has a rated line current of 68 A and a rated field current of 2.2 A. The

change in field flux required to obtain a speed of 1600 rpm while drawing a

line current of 52.8 A and a field current of 1.8 A is

(A) 18.18% increase (B) 18.18% decrease

(C) 36.36% increase (D) 36.36% decrease

MCQ 4.3 The locked rotor current in a 3-phase, star connected 15 kW, 4 pole, 230 V, 50 Hz induction motor at rated conditions is 50 A. Neglecting losses and magnetizing current, the approximate locked rotor line current drawn when the motor is connected to a 236 V, 57 Hz supply is

(A) 58.5 A (B) 45.0 A

(C) 42.7 A (D) 55.6 A

MCQ 4.4 A single phase 10 kVA, 50 Hz transformer with 1 kV primary winding draws 0.5 A and 55W, at rated voltage and frequency, on no load. A second transformer has a core with all its linear dimensions 2 times the corresponding dimensions of the first transformer. The core material and lamination thickness are the same in both transformer. The primary winding of both the transformers have the save number of turns. If a rate voltage of 2 kV at 50 Hz is applied to the primary of the second transformer, then the no load current and power, respectively, are

(A) 0.7 A, 77.8 A (B) 0.7 A, 155.6W

(C) 1A, 110W (D) 1A, 220W

MCQ 4.5 A 4 point starter is used to start and control the speed of a

(A) dc shunt motor with armature resistance control

(B) dc shunt motor with field weakening control

(C) dc series motor

(D) dc compound motor

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