PART 1
SYNCHRONOUS MACHINE
1.1 INTRODUCTION A synchronous machine is an a.c machine in which the rotor moves at a speed which bears a constant relationship to the frequency of currents, in the armature winding. A synchronous machine is one of the important type of electric machines. Large a.c networks operating at constant frequency of 50Hz rely almost exclusively on synchronous generators, also called the alternators, for the supply of electrical energy.
Private, stand-by and peak load plants with diesel or gas-turbine prime movers also have synchronous generators. Synchronous motors provide constant speed industrial drives with the possibility of power factor correction. Synchronous machine are generally constructed in larger sizes. Small size alternators are not economical. The modern trend is to build alternators of very large size capable of generating 500MVA or even more. The synchronous motor is rarely built in small sizes owing to superior performance characteristics and economical construction of induction motors.
1.2 OPERATING PRINCIPLE The operating principle of a synchronous machine is fundamentally the same as that of a d.c machine, but, unlike the latter, in the synchronous machine there is no need to rectify the time varying e.m.f which is induced in the armature winding. Consequently a synchronous machine does not require a commutator. It is in fact quite possible to use a d.c generator as an alternator by replacing a set of collector rings on the shaft and connecting these rings to the proper points on the armature winding; brushes riding on the rings can then be connected to the load. But unlike dc generator, they are to be driven at a very definite constant speed as the frequency of generated e.m.f is determined by that speed. The latter is usually referred to as the synchronous speed, for which reason these machines are frequently called the synchronous generators.
Synchronous generators, because of absence of commutator, are comparatively simple and possess several important advantages over the dc generators.
1.3 CLASSIFICATION OF SYNCHRONOUS MACHINES Practically all medium and large machines are always constructed with revolving field.
Therefore, a synchronous machines, according to the applications may be synchronous generators, synchronous motors or synchronous compensators. A synchronous generator is a synchronous machine which receives mechanical energy from a prime mover (steam turbine, hydraulic turbine or diesel engine) to which it is mechanically coupled and delivers electrical energy. A synchronous motor receives electrical energy from ac supply main and drives mechanical load. Synchronous compensator (or phase modifier) is a synchronous machine designed to operate on no load with its shaft connected neither to a prime mover nor to a mechanical load and is used to control reactive power in power supply networks.
1.4 TYPES The synchronous machines may be single, two or 3-phase types. 1) For the single phase type, the armature has all the coils connected for addition of individual voltages; this provides a pair of output terminals and a single a.c output. 2) For the two phase type, the armature has two sets of windings placed so that, the output from three terminals (one common) are 90o out of phase 3) For the three phase type, the armature has three sets of windings placed so that three outputs with a mutual phase difference of 120o are available. Three winding may be connected either in start or delta. Large ac machines are invariably 3-phase type.
1.5 CONSTRUCTION Based on the construction of the machines, the synchronous machines may be classified as (a) Rotating armature type and (b) Rotating field type.
Since it is immaterial for generation of an induced e.m.f whether a conductor moves across a magnetic field or vice-versa, synchronous generators may be constructed with either the armature or the field structure as the revolving members.
A rotating armature type alternator looks very much like a dc generators except that they are 3-slip rings in place of commutator (or 4 slip-rings if it desired to provide a connection to the generator neutral). In such generators, the required magnetic field is produced by dc electromagnets placed on the stationary member called the stator, and the current generated is collected by means of brushes and slip-rings on the revolving member, called rotor. Such an arrangement is economical for the small low voltage generators.
Rotating armature type alternators are built only in small rating up to about 200 or 250KVA because the voltage generated is small, no difficulty being experienced in collecting such a current. Such machines are suitable for small power plants isolated lighting plants, where medium and large machines are always constructed with revolving field.
1.6 PARTS OF SYNCHRONOUS MACHINES Synchronous machine consists essentially of two parts, namely: a) The armature (or stator) and b) The field magnet system
1.7 THE ARMATURE (OR STATOR) The armature is an iron ring formed of laminations of special magnetic iron or steel alloy (silicon steel) having slots on its periphery to accommodate armature conductors and is known as stator. The whole structure is held in a frame which may be of cast iron or whelded steel plate. Since the field rotates in between the stator, so that flux of the rotating field cuts the cores of the stator continuously and causes eddy current loss in the stator core. To minimize the eddy current loss, the stator core is laminated. The laminations (usually of thickness 0.5mm or less) are stamped out in complete rings (for smaller machines) or in segments (for larger machines) and insulated from each other with paper or varnish. The stampings also have openings which make axial and radial ventilating duets to provide efficient cooling. A general view of stator and frame is showing in the figure above.
1.8 THE FIELD MAGNET SYSTEM OR ROTOR The magnetic field required for the generation of alternating voltage is provided by field magnets similar to those of a dc machines and these need to be supplied with direct current. This may be had from some external source, but so as to make the alternator self-contained and independent of other source of supply, it is more usual to install a small dc generator (usually a flat compound wound type) for this purpose. The generator is called the exciter and is coupled to the engine or turbine driving the alternator. The exciter armature is connected directly to the alternator field winding, usually without any kind of controlling resistance so that except for variations due to temperature changes, the resistance of the exciter load is constant.
1.9 THE DIFFERENCE BETWEEN A SYNCHRONOUS MACHINE AND A SYNCHRONOUS GENERATOR The synchronous machines may be classified as (i) Salient Pole machines and (ii) cylindrical rotor machines depending upon the type of constructors used for the rotor. The salient pole construction is used for generators and motors of all ranges of output and up to all but the higher speeds. Medium and large sized generators for the highest speeds are of the cylindrical rotor type.
The synchronous generators, based on the type of prime movers to which they are mechanically coupled, may be classified as (i) turbo-generators, (ii) hydro-generators and (iii) diesel engine driven generators.
Turbo-generators are driven by steam turbines. The efficiency of steam turbines is high at large speeds, and therefore, synchronous machines driven by steam turbines (i.e. turbine-generators) are high speed machines. The maximum speed of turbo-generators is 3000 r.p.m corresponding to 2 poles and 50Hz. These have small diameters (limited to about 1.2m) and horizontal configurations.
The synchronous generators driven by water turbines are called water wheel generators or hydro-generators, such generators have to match the specific speed requirements of the turbines driving them.
These generators are low speed machines with speeds between 1,500 and 100 rpm (i.e. numbers of poles ranging from 4 to 60) and of rating up to 750MVA.
Hydro-electric power plants usually have vertical shaft.
EXPERIMENTATION
EXPERIMENT 1
DETERMINE BY EXPERIMENTS THE OPEN AND SHORT CIRCUIT CHARACTERISTICS OF SYNCHRONOUS MACHINES
THE OPEN CIRCUIT TEST
The open-circuit test for any generator is a test carried out in which the actual generated e.m.f. is measured for various values of field current or excitation with the machine operating at normal speed on no-load. With the synchronous generator, the field (usually the rotor) is supplied from the exciter or from a separate dc supply and the current increased in step from zero. At each step, the phase voltage (i.e the generated e.m.f) in stator winding is measured. The circuit for the test is below:
The open-circuit characteristics is a graph of phase e.m.f. against excitation current.
As can be seen, this is a typical B-H type curve and sometimes called a magnetization characteristics.
THE SHORT CIRCUIT TEST
With the terminals of the synchronous generator short-circuited, the e.m.f produced with circulate a short-circuit current Isc through the impedance Zs of the machine. Only a small excitation current is required to produce a large short-circuit current. The circuit for this test is given below:
The short-circuit current Isc is read on one of the ammeters (three ammeters are used only to balance the circuit) for varying values of excitation current until the short-circuit current reaches about twice normal full-load value. This excitation will be quite small, no where near its normal working value, but since the magnetic circuit will be far below saturation, the graph can be extended as a straight line to cover the normal excitation range.
EXPERIMENT 2
The open-circuit characteristic of a dc shunt generator at 300rpm is
If (A) 0 0.2 0.3 0.4 0.5 0.6 0.7
Eo (V) 7.5 93.0 135.0 165.0 186.0 202.0 215.0
The resistance is adjusted to 354.5( and speed is 300rpm. Determine the following (i) graphically the no-load voltage (ii) critical field resistance (iii) critical speed for the given field resistance (iv) additional resistance inserted in the field circuit to reduce the no-load voltage to 175V.
Plot the 0CC from the given data, draw field resistance line OA representing resistance of 354.5(. The ordinate of the point of intersection of field resistance line and OCC gives the no-load voltage which is 195V.
EXPERIMENT 3
TO DETERMINE THE VOLTAGE REGULATION AND EXTERNAL CIRCUIT CHARACTERISTICS (E.C.C.) CURVE OF A THREE PHASE ALTERNATE (THAT IS LOAD CURVE)
PROCEDURE 1. Make the connection as shown in the circuit above. 2. Keeps the field exciting switch and main switch at “Off” position rotate the rotary field type rotor in marked direction at rated speed with the help of a prime-mover, (if the prime-mover is an engine, the speed can be controlled by governor). 3. Operate the field exciting switch to “ON” position and adjust the field regulator till the alternator build up its rated voltage (Eo) and note down the readings of load ammeter and voltmeter (Eo). 4. Now operate the main switch to “ON” position 5. Keeping the speed and field current constant, increase the load in steps unto rated load and note down the readings of load ammeter and voltmeter at each position in observation table. 6. Plot the graph between terminal voltage and load current.
OBSERVATION TABLE FOR A PARTICULAR VOLTAGE AND LOAD CURRENT
|S/No. |Load Current IL (In Amps.) |Terminal Voltage V (In Volts) |
|1 |0.0 |440 (E0) |
|2 |2.0 |425 |
|3 |3.5 |400 |
|4 |5.0 |370 |
|5 |6.0 |335 |
|6 |6.5 |340 |
N/B: Voltage Regulation,
% Voltage regulation = [pic]
When E8 = No – load voltage
PROCEDURE 1. Make the connection as shown in the diagram above. 2. Keep the field exciting switch at OFF position and rheostat at “ON” position (i.e. at maximum) 3. Rotate the rotary field type rotor in marked direction at rated speed with the help of prime mover (IF the prime mover is an engine, the speed can be controlled by governor) 4. Now keeping the speed constant, increase the field current by reducing the resistance of the rheostat at different positions and note down the readings of ammeter and voltmeter at each position in observation table below. 5. Plot the graph between induced e.m.f. and field current as from the table below.
ASSIGNMENT 1 1) What are the reasons, for the charge in terminal voltage of an alternator with the change in load supplied. 2) What are the special features for non-salient field structure in an alternator and salient pole field structure.
PART II
ALTERNATOR
2.1 INTRODUCTION:
An alternator is a machines which converts mechanical energy into a.c electrical energy. It has three main parts such as stator, rotor and exciter. The working principle of an alternator is that whenever a conductor rotate in the magnetic field or magnetic field rotate and conduct remains stationary, an e.m.f. is induced in the conductor. In all modern machines, the armature is kept stationery and the field rotating. The d.c supply required for exciting the field is generated by a separate dc shunt generator. however, in the modern machines, the alternating current produced in rectified by a bridge rectifier circuit and then supplied to the field system.
Alternators are mainly of two types such as high speed alternators and low speed alternator.
In high speed alternators, cylindrical pole rotor of less numbers of poles are used, these types of alternators are called turbo alternators and are used in thermal power houses whereas, in low speed alternators salient pole rotor of more numbers of poles are used, these types of alternators are called hydro-alternators and are used in hydro power houses. However, alternator can be further subdivided as: 1. With respect to rotation: (a) Rotating Armature types (b) Rotating field type 2. With respect to number of phases (a) Single phase alternator (b) poly phase alternator 3. With respect to excitation (a) Separately excited alternator (b) self excited alternator
2.2 INTRODUCTION AND PRINCIPLE OF SYNCHRONOUS MOTOR The action of motor may be followed by referring to the figure below, which illustrates a 3-phase, 2-pole synchronous machine. The rotor has two poles and the stator has two poles per phase.
In practice, it is used to have salient poles on the rotor, but the armature winding is housed in slots in the concave periphery of stator. In the figure salient type stator poles have been shown for convenience only.
As the rotor is excited from dc supply, so the poles of the rotor retain the same polarity throughout but the polarity of the stator poles changes as it is connected to an ac supply mains.
Let us consider the rotor as stationary and in the position shown in figure (a).
At this instant the rotor s-pole is attracted to the stator N-pole and therefore the rotor tends to rotate in clockwise direction. After half a period
[i.e after [pic] second whereas T = [pic]] the polarity of stator poles is reversed but the polarity of the rotor poles remains the same. See figure (b)
At this instant, the rotor s-pole is repelled by the stator s-pole, being similar in nature and therefore, rotor tends to rotate in counter clockwise direction. Thus the figure acting upon the rotor of a synchronous motor is not unidirectional but pulsating one and due to t inertia of the rotor, it will not move in any direction.
So the synchronous motor has got no self-starting torque.
Now let consider the rotor rotating in clockwise direction, which is possible by external means and in the position shown in figure (a). AT this instant, as mentioned earlier, the rotor s-pole is attracted to stator N-pole and so the torque act on the rotor in clockwise direction. After half a period the stator polarity is reversed, i.e. stator s-pole becomes N-pole and N-pole become S-pole but if the rotor is rotated as such a speed by some external means at the starting moment that rotor s-pole advances by a pole pitch so that it is again under the influence of stator N-pole, as in the figure below.
The torque acting on the rotor will be again clockwise. Hence a continuous (unidirectional) torque will be obtained. Now if the external means is removed the rotor will continue to rotate in clockwise direction under the influence of clockwise continuous torque acting on the rotor.
Hence to obtain a continuous torque, it is necessary that the rotor rotates at such a speed that it moves through the distance equal to pole pitch in half the period.
2.3 V- AND INVERTED V-CURVES
It has been already discussed how the variation in field current affects the power factor at which synchronous motor operates. Hence when the excitation of a synchronous motor is increased above 100%, first power factor improves until it becomes unity; at this instant the current drawn from the supply mains is minimum and is in phase with the supply mains is minimum and is in phase with supply voltage.
With further increase in field current the power factor becomes leading one and decreases and current drawn from supply main increases. The variation of current and power factor of a synchronous motor with a variation of field current (excitation) and for a constants load will be shown in the figure below.
Because of their shape, excitation – armature current curves are called the v-curves of a synchronous motor.
In case of a synchronous motor driving a constant mechanical load variation in field or dc excitation current will not only affect the power factor but also the current drawn by the motor.
Except for the change in copper losses due to variation in armature current and a slight charge in core loss due to variation in flux power input to the motor is almost constant for a constant load.
The power drawn by a 3-phase synchronous motor is given by;
P = [pic] where VL is the line voltage, IL, line current or armature current and Cos( is the power factor.
Since input power p and supply voltage v is constant, decrease in power factor causes increase in armature current and vice-versa. Hence variation in excitation or in field current causes the variation in armature current curves drawn between loads are known as “V-curves drawn between armature current and field current for different constant loads are known as “v-curves” due to their shape similar to English letter V.
The V-curves of a synchronous motor give relation between armature current and field current for different power inputs.
Similarly the variation of power factor with a variation in field current (dc excitation) for a constant load gives inverted V-curves in the same diagram, above.
EXPERIMENT 2.1
TO STUDY THE PARTS OF AN ALTERNATOR
PROCEDURES 1. Separate the coupling of alternator and prime-mover. Remove the coupling form the shaft of the alternators 2. Remove the carbon brush of rotary field winding 3. Put one dots on the top of the frame and front end cover, and two dots on the same position on the frame and rear and cover with centre punch. 4. Unscrew the front end grease plate of the alternator 5. Now unscrew the front end cover and remove it by mallet. Similarly, unscrew the rear end cover and remove the rotor after carefully slipping. 6. Place the rotor on rotor stand and clean each part with air blower and study them namely as frame, stator, stator winding, type of rotor winding, types of exciter, slipping, front end cover, rear end cover, carbon brush, shaft, cooling fan, eye bolt, bed plate, ball bearings, terminal box. See figure above. 7. reassemble the parts of alternator in the same manner in which they are dismantled and check for free rotating of rotor.
8. Check the insulation resistance between windings and between the earth and winding with megger. It should not be less than 2m(. you should understand that in the stator of 3( alternator there are three windings placed 120 electrical degree aparts. Each winding has separate coils insulated from each other. The three similar ends of three windings, A’ B’ C’ are joined together to create star (neutral) point for the system and the remaining three terminals A, B, C are the phase terminals of the alternator as in the figure above. T he neutral point of the alternator is generally earthed at the generating station.
Four wires are brought out as supply terminals.
PRECAUTIONS 1. Avoid hammering as far as possible. Use mallet for this purpose. 2. Mark the position of side covers with centre punch before dismantling them. 3. keep all the parts carefully.
PART III
POTTER METHOD OF DETERMINATION OF VOLTAGE REGULATION OF AN ALTERNATOR
3.1 INTRODUCTION The regulation obtained by mmf and emf methods is based on the total synchronous reactance (the sum of reactance due to armature leakage flux and due to armature reaction effect).
This method is based on the separation of reactances due to leakage flux and that due to armature reaction flux, therefore, it is more accurate.
For determination of voltage regulation by this method, the data required are; i) Effective resistance of armature winding ii) Open-circuit characteristic iii) Field current to circulate full-load current in the stator, and iv) Zero – power factor full-load voltage characteristics – a curve between terminal voltage and excitation, while the machine is being run on synchronous speed and delivering full-load current at zero power factor.
3.2 HOW POTTER COULD BE DETERMINED PROCEDURE:
The machines is run at rated synchronous speed by a prime-mover. A purely inductive load (variable load reactors) is connected across the armature terminal and the excitation or field current is raised so as to cause flow of full load armature current. The value of the reactance is then increase step by step in such a way that the excitation current is adjusted to a value that cause full-load rated armature current to flow.
The armature terminals voltages are varied from 125% to 25% of the rated voltage in steps, maintaining the speed and rated armature current constant throughout the test.
The armature terminal voltages and excitation currents are noted at each step.
The curve drawn between terminal voltage and excitation current gives the zero power factor (lagging) characteristic.
3.3 RESULT
When the zero power factor (lagging) characteristics is to be used only for obtaining the potier reactance, it is sufficient to determine the point representing rated armature current and rated voltage. This is indicated in point B in the graph above.
The alternator can be loaded alternatively by an under-excited synchronous motor.
There is a definite relationship between the zero power factor (lagging) characteristic and an open-circuit characteristic of an alternator. The zero power factor characteristic curve is of exactly the same shape, as the 0CC but it is shifted vertically downward by leakage reactance drop IXL and horizontally by the armature reaction mmf. Zero power factor full-load voltage excitation characteristic can be drawn by knowing two points A and B.
Point A is obtained from a short-circuit test with full load armature current.
Hence 0A represents excitation (field current) required to overcome demagnetizing effect of armature reaction and to balance leakage reactance drop at full load.
Point B is obtained when full load current flows through the armature and wattmeter reads zero, as discussed above.
From B, line BC is drawn equal and parallel to A0. then a line is drawn through C parallel to initial straight parts of 0CC (parallel to OG), intersecting the OCC at D. BD is joined and a perpendicular DF is dropped on BC.
The triangle BFD is imposed at various points 0CC to obtain corresponding points on the zero factor curve.
In (BDF the length BF represents armature reaction excitation and the length DF represents leakage reactance drop (IXL). this is known as potier reactance voltage drop and the triangle is known as potier triangle. The potier reactance is given as
XP = [pic]
3.4 IMPORTANT NOTE
(1) In case of cylindrical rotor machines, potier reactance is nearly equal to armature leakage reactance.
(2) In case of salient pole machines, the magnetizing circuit is more saturated and the armature leakage reactance is smaller than the potier reactance.
3.5 POTIER REGULATION DIAGRAM
OV is drawn horizontally to represent terminal voltage, V on full load and OI is drawn to represent full load current at a given power factor.
VE is drawn perpendicular to phasor OI and equal to reactance drop (IXL), neglecting resistance drop.
Now phasor OE represents generated emf E. from OCC field excitation I, corresponding to generated emf E is determined, OI is drawn perpendicular to phasor OE to represent excitation required to induced emf OE on open circuit. I1I2 is drawn parallel to load current phasor OI to represent excitation equivalent to full-load armature reaction. OI2 gives total excitation required.
Note that, it the load is thrown off, then terminal voltage will be equal to generated emf corresponding to field excitation OI2. hence emf Eo may be determined from OCC corresponding to field excitation OI2. Phasor OEo will lag behind phasor OI2 by 90o. EEo represent voltage drop due to armature reaction. Now regulation can be obtained from the relation
% Regulation = [pic]
PRACTICAL EXAMPLES 1
Tests on a 15,00KVA, 11,000 volts, 3-Q, 50Hz, star connected alternator gave the following results.
Field AT per pole, thousands, 10 18 25 30 40 45 50
Open circuit line emf, kv 5.0 8.4 10.0 11.2 12.8 13.3 13.65
Full-load current, zero power - 0 - - - 10.0 -
Factor test, line pd kv
Find the armature reaction amper – turns, the leakage reactance and the regulation for full load at 0.8 pf lagging. Neglect resistance.
From the data drawn OCC between line voltage and field current. Full load zero power factor curve should be drawn too.
Take point A to be (18.0) and point B (45, 10.2) that should be known from the (BDF.
Armature reaction ampwer turns = BF = 15,500 AT/Pole
Full-load reactance drop = Df = 1.15V – 1,150 volts
Leakage reactance drop per phase = [pic]
Full-load current = [pic]
Leakage reactance per phase = 1,150 x 11
PART IV
DETERMINATION OF LOAD CHARACTERISTICS OF AN ALTERNATOR
INTRODUCTION
While current and the speed remain constant, the terminal voltage of an alternator changes with the change in the load or armature current. The relationship between the terminal voltage V and load current I of an alternator is known as its load characteristic.
This diagram showing the curve variation of terminal voltage V with load current I for constant excitation (or no-load voltage Eo) for three different power factor loads were given.
The curves are plotted in terms of percentage values because general operating conditions can be illustrated better in this way.
Normally the terminals voltage falls with the increase in load current but when the power factor is leading one, the load characteristic curve may rise at first. Each curve is nearly straight at the beginning but tends to drop because, with the increase in load current, the angle of lag between current and emf, owing to the original field increases.
The highest current is obtained when the alternator terminals are short circuited, the value being given as Ise = [pic] amperes.
Where Eo is no-load terminal voltage and Zs is the synchronous reactance.
All curves meet at the short-circuit points.
In modern alternators, the steady short-circuit current is not much greater than full-load rated current. This is purposely arranged to prevent excessive current in the event of a short-circuit.
THE EFFECT OF VARIATION OF POWER FACTOR ON TERMINAL VOLTAGE (LOAD CURRENT, EXCITATION AND SPEED REMAINING UNCHANGED)
On changing the power factor from leading to lagging one, if the load current, excitation and speed are kept constant, the terminal voltage falls. Since when the power factor is leading (highly capacitive load) the effect of armature flux is to help the main flux, hence to generate more emf but when power factor is lagging (highly inductive load) the effect of armature flux is to oppose the main flux hence to generate less emf. Thus at lagging power factor the terminal voltage falls from that on leading power factor mainly owing to decrease in generated emf.
LABORATORY METHOD OF DETERMINATION OF SYNCHRONOUS REACTANCE
As already defined, the sum of leakage reactance XL and fiction reactance Xa, to account for the voltage drop due to armature known as synchronous reactance Xs and it is given as Xs = [pic] where Zs is the synchronous impedance and Re is the effective resistance.
