Synchronous Machine Explanation and Demonstration

The stator is similar in construction that of a induction motor.The rotor can be Salient or Non-Salient. Field excitation is provided on the rotor by either permanent or electromagnets with number of poles equal to the poles of the RMF caused by stator.

Non-excited rotors are also possible as in case of reluctance motors

The rotor gets locked to the RMF and rotates unlike induction motor at synchronous speed under all load condition.All conventional power plants use synchronous generators for converting power to electrical form.They operate at a better power factor and higher efficiency than equivalent induction machines.

CLICK TO ENLARGE BELOW IMAGES

Synchronous Machine Construction

Synchronous Machine Equivalent Circuit

Synchronous Machine Phasor Diagram

Excitation and Stator induced voltage

 

Effect of Load Change (Field constant)

Effect of Field Change (Load constant)

V curves

Torque versus Load Angle 

Example 1
A six-pole 60 Hz synchronous motor is operating with a developed power of 5 hp and a torque angle of 5o. Find the speed and developed torque. Suppose that the load increases such that the developed torque doubles. Find the new torque angle. Find the pull-out torque and maximum developed power for this machine.









Torque versus Speed

Example 2
An eight-pole, 240 V-rms, 60 Hz, delta connected synchronous motor operates with a constant developed power of 50 hp and a torque angle of 15o at unity power factor. Suppose  the  field current is increased by  20%. Find the new torque angle and power factor. Is the new power factor lagging or  leading? Assume linear magnetic characteristics.

SWITCHED RELUCTANCE MOTORS

The structure of a switched reluctance motor is shown below. This is a 4-phase machine with 4 stator-pole pairs and 3 rotor-pole pairs (8/6 motor). The rotor has neither windings nor permanent magnets.

The stator poles have concentrated winding rather than sinusoidal winding. Each stator-pole pair winding is excited by a converter phase, until the corresponding rotor pole-pair is aligned and is then de-energized. The stator-pole pairs are sequentially excited using a rotor position encoder for timing.

CONSTRUCTION

• STATOR AND ROTOR ARE SALIENT IN STRUCTURE

• STATOR WINDINGS ARE INDEPENDENT CONCENTRATED WINDINGS WHICH ARE EXCITED WITH SWITCHES FROM SOURCE

• NO FIELD WINDINGS HENCE SINGLY EXCITED

• DIAMETRICALLY OPPOSITE ARMATURE WINDINGS ARE CONNECTED TO FORM A PHASE

• FOR BIDERCTIONAL CONTROL AND SELF STARTING,NUM OF ROTOR POLES ARE LESS THAN NUM OF STATOR POLES

• M15,M17,M19 FOR HIGH SPEED AND HYPERCO AND VANADIUM PERMENDUR FOR HIGH PERFORMANCE

• SINGLE STACK AND MULTI STACK CONSTRUCTION POSSIBLE

WORKING

• FIG SHOWS AN 8/6 SINGLE STACK  WHICH HAS 4 PHASES A,B,C,D

• WHEN PHASE A IS EXCITED RELUCTANCE TORQUE CAUSES ROTOR TO TURN UNTIL IT ALIGNS WITH AXIS OF PHASE A.

• EXCITATION IS CHANGED TO B AND A IS DEEXCITED BEFORE ALIGNMENT

• ROTATION IS IN DIRECTION OF ENERGISATION

• DIRECTION OF ROTATION REVERSED BY REVERSING SEQUENCE OF EXCITATION

• SPEED DEPENDS ON MAGNITUDE OF INPUT MICROSTEPPING CAN BE DONE FOR SINGLE STACK ONLY 1 ROTOR AND STATOR

• FOR MULTI STACK OPERATION,NUM OF ROTOR AND STATOR DEPENDS ON NUM OF PHASES

• POSITION OF MIN RELUCTANCE CHANGED WITH HELP OF POSITION SENSORS

• WHEN PHASE IS EXCITED AFTER THE ROTOR PASSES POINT OF MIN RELUCTANCE, REVERSE TORQUE ACTS[REGENERATIVE BRAKING]

The inductance of a stator-pole pair and corresponding phase currents as a function of angular position is shown below.

Applying the stator pulse when the inductance profile has positive slope induces forward motoring torque.

Applying the stator pulse during the time that the inductance profile has negative slope induces regenerative braking torque.

A single phase is excited every 60degree with four consecutive phases excited at 15degree intervals.


The torque is given by:

where  m = inductance slope and 
           i = instantaneous current.


Switched reluctance motors are growing in popularity because of their simple design and robustness of construction. They also offer the advantages of only having to provide positive currents, simplifying the inverter design. Also, shoot-through faults are not an issue because each of the main switching devices is connected in series with a motor winding. However, the drawbacks of this type of motor are the pulsating nature of their  torque and they can be acoustically noisy (although improved mechanical design has mitigated this problem.)



ADVANTAGES

• ROTOR HAS NO WINDINGS COMMUTATOR OR BRUSH

• TORQUE –INERTIA RATIO IS HIGH

• HIGH RELIABILITY,WIDE SPEED RANGE,LOW COST

• FAST RESPONSE,RUGGEDNESS,FAULT TOLERANCE

• NO SHOOT THROUGH AND CROSS OVERS IN CONVERTER

• NO PERMANENT MAGNET

• OC VOLTAGE AND SHORT CIRCUIT CURRENT AT FAULTS IS ZERO

DISADVANTAGES

• ROTOR POSITION SENSORS REQUIRED

• TORQUE RIPPLES ARE HIGH

• ACOUSTIC NOISE IS PRESENT

 
APPLICATIONS

• FLUID PUMPS,VACUUM BLOWERS

• PROCESS CONTROL INDUSTRIES

• HYBRID/ELECTRIC VEHICLES

• ELECTROMECHANICAL BRAKE SYSTEM

• ELECTRIC POWER STEERING

• STARTER GENERATOR SYSTEM

• FUEL PUMP OPERATION

Variable Reluctance Motors

A variable reluctance motor has double saliency, i.e. both the rotor and stator have saliency. There are two groups of variable reluctance motors: stepper motors and switched reluctance motors. Stepper motors are not suitable for variable speed drives.

Ref: A. Hughes, “Electric Motors and Drives”, 2nd. Edn. Newnes

SYNCHRONOUS RELUCTANCE MOTOR

A synchronous reluctance motor has the same structure as that of a salient pole synchronous motor except that it does not have a field winding on the rotor.

The stator has a 3-phase, symmetrical winding which creates a sinusoidal rotating field in the air gap. This causes a reluctance torque to be created on the rotor because the magnetic field induced in the rotor causes it to align with the stator field in a minimum reluctance position. The torque developed in this type of motor can be expressed as:

The reluctance torque stability limit can be seen to occur at (see figure below).

Iron laminations separated by non-magnetic materials increases reluctance flux in the qe-axis. With proper design, the reluctance motor performance can approach that of an induction motor, although it is slightly heavier and has a lower power factor. Their low cost and robustness has seen them increasingly used for low power applications, such as in fiber-spinning mills.

Brief About Water Connection Components

Components generally include a usage meter; a service feed pipe, water heater and a distribution system.
In a public water system, the public utility provides water supply through a water main and service line to the edge of the public right-of-way, where the water usage meter is located. The service feed pipe from the structure then connects to the usage meter. For structures connected to public water sup- ply, only the service feed pipe and the on-site distribution system are covered in this manual.

In an on-site water supply system, the source of water is a private on-site well, and the owner is responsible for the operation and maintenance of the well. The water service feed pipe is the main pipeline that connects the on- site water well to the distribution system within a structure. The distribution system includes the pressure tank and all the pipes and water delivery accessories (faucets, showers, etc.) in a structure.

CLICK IMAGES TO ENLARGE

Components of a public potable water supply system in a non-velocity flow area

Components of a public potable water supply system in a velocity flow area

The components of an on-site potable water system

The two main dangers that floodwaters present to water supply systems are:

1. Damage to pipes and to the on-site well head from the effects of velocity flow, wave action, and debris impact.

2. Water supply contamination in the well, service feed pipe, and distribution system.


In general, the figures in this chapter attempt to illustrate some general prac- tices that meet the requirements of the National Flood Insurance Program (NFIP). Local codes and health/sanitary regulations permit many variations that also meet NFIP regulations. Please refer to your local code officials for specific practices that may meet both the NFIP regulations and local code.

Characteristic Curves of Hydraulic Turbines

Characteristic curves  of a  hydraulic  turbine  are the  curves,  with the  help of which the exact behavior and performance of the turbine under different working conditions, can be known.  These curves  are plotted from the results  of the tests performed on the turbine under different working conditions.

The important parameters which are varied during a test on a turbine are :

1. Speed (W)

2.Head (H)

3. Discharge (Q)

4. Power (P)

5. Overall efficiency (o)

6.Gate opening.

Out of the above six parameters, three parameters namely speed (N), Head (H) and discharge (Q) are independent parameters.

Out of the three independent parameters (N, H, Q), one of the parameter is kept constant
(say  H)  and  the  variation  of the other  four  parameters with   respect  to  any one  of the remaining two  independent  variables (say N  and  Q) are plotted and  various  curves  are obtained. These curves are called characteristic curves.

The followings are the important characteristic curves of a turbine :

•    Main Characteristic Curves or Constant Head Curve.

•    Operating Characteristic Curves or Constant Speed Curve.

•    Muschel Curves or Constant Efficiency Curve.

•    Main characteristic Curves or Constant Head Curves.


Main characteristic curves are obtained by maintaining a constant head and a constant gate opening (G.O.) on the turbine. The speed of the turbine is varied by changing load on the turbine. For each value of the speed, the corresponding values of the power (P) and discharge (Q) are obtained. Then the overall efficiency (o) for each value of the speed is calculated. From these readings the values of unit  speed (Nu)  unit  power  (Pu)  and  unit discharge (Ou) are determined. Taking Nu  as abscissa, the values of Qu, Pu, P and o are plotted. By changing the gate opening, the values of Qu Pu and o and Nu are determined and taking Ns as abscissa, the values of Qu, Pu and o are plotted. Fig. below shows the main   characteristic   curves   for   a   Pelton   wheel   and   Fig.   below   shows   the   main characteristic curves for reaction (Francis and Kaplan) turbines.

VELOCITY TRIANGLES, WORKDONE, EFFICIENCY OF PELTON WHEEL INLET AND OUTLET VECTOR DIAGRAMS

Let V = Velocity of the jet
u = Velocity of the vane (cups) at the impact point
u =    DN/ 60

where  D = Diameter of the wheel corresponding to the impact point
= Pitch circle diameter.
At inlet the shape of the vane is such that the direction of motion of the jet and the
vane is the same. i.e., Ȑ = 0, ș = 0
Relative velocity at inlet    Vr  = V  —u

Hydraulic efficiency

This is the ratio of the work done per second per
head at inlet to the turbine. Energy head at inlet = V2/2g

SPECIFIC SPEED

The  speed  of  any  water  turbine  is  represented  by  N  rpm.  A  turbine  has  speed, known as specific speed and is represented by N
‘Specific speed of a water turbine in the speed at which a geometrically similar turbine would run if producing unit power (1 kW) and working under a net head of
1 m. Such a turbine would be an imaginary one and is called specific turbine.

KAPLAN TURBINE

It  is  an  axial  flow  reaction  turbine.  This  operates in  an  entirely  closed  conduit  to tailrace. Kaplan turbine is employed, where a large quantity of water  is available. It  consists  of  spiral  casing,  guide  mechanism  and  draft  tube  of  Kaplan  turbine runner are similar to those of Francis turbine.





WORKING PRINCIPLE OF KAPLAN TURBINE


Kaplan turbine consist of the following parts

a) Spiral or scroll air tight casing

b) Guide mechanism

c ) Runner and main shaft

d) Draft tube   

e) Governor

In the Kaplan turbine the runner has 4 to 6 blades attached to the hub or boss. The water enters the turbine in the axial direction. Since only a few blades are used the contact surface with water and hence the frictional resistance is reduced. The blades are  made  of stainless  steel.  The  runner blades  are so  arranged  that  their angle of inclination  can  be  adjusted  while  running  Hence  the  kaplan  turbine  is  also  called variable pitch propeller turbine.

The  whole  mechanism  is  enclosed  in  a  central  boss,  which  is  operated  by  the governor  through  the  action  of  servomotor.  By  this  means,  the  blade  angles  are automatically  adjusted  while  running  according  to  the  power  developed  by  the turbine. In the Kaplan turbine both guide vane angle and runner blade angle may be varied. This results in higher efficiency.

Working

The  water  from  the  scroll  casing  flows  over  the  guide  vanes.  It  is  then  deflected through  90°   and  enters  the  adjustable  runner  vanes.  The   water  enters  with maximum  potential  energy  and  with  little  kinetic  energy.  It  flows  through  the :‘ lades  in  the  axial  direction.  The  force  exerted  on  the  vanes  causes  the  shaft  to rotate. In this turbine only 3 to 6 blades are used and they are fixed at equidistance. The  runner is  in  the  form  of  boss  having  a  bigger  diameter.  As  the  blades  of  the runner as well as guide blades can be adjusted during operation,  the governing of the turbine is easy. The water after doing work passes on to the tail race through a draft tube. The specific speed of this turbine is between 300 to 1000 rpm.

FRANCIS TURBINE

The Francis turbine is mixed flow reaction turbine. This turbine is used for medium heads with medium discharge. Water enters the runner and flows towards the centre of wheel in the radial direction and leaves parallel to the axis of the turbine.

Description of main parts

Francis turbine consists mainly of the following parts

a) Spiral or scroll casing  

b) Guide mechanism

c) Runner and turbine main

d) Draft tube

Spiral casing of scroll

The  casing  of  the  francis  turbine  is  designed  in  a  spiral  form  with  a  gradually increasing area. The advantages of this design are

i) Smooth and even distribution of water around the runner.

ii) Loss of head due to the formation of eddies is avoided.

iii) Efficiency of flow of water to the turbine is increased.

In big units stay vanes are provided which direct the water to the guide vanes. The casing is also provided with inspection holes and pressure gauge connection.

The selection of material for the casing depends upon the head of water to be supplied

For a head  —    upto 30 metres    —concrete is used.
For a head  —    from 30 to 60 metres  —    welded rolled steel plates are used.
For a head of above 90 metres .    — cast steel is used.

GUIDE MECHANISM

The guide vanes or wicket gates are fixed between two rings. This arrangement is in the form of a wheel and called guide wheel. Each vane can be rotated about its pivot centre. The opening between the vanes can be increased or decreased by adjusting the  guide  wheel.  The  guide  wheel  is  adjusted  by  the  regulating  shaft  which  is operated  by  a  governor.  The  guide  mechanism  provides  the  required  quantity  of
water  to  the  runner  depending  upon  the  load  conditions.  The  guide  vanes  are  in general made of cast steel.

RUNNER AND TURBINE MAIN SHAFT

The flow in the runner of a modern Francis turbine is partly radial and partly axial. The runners may be classified as

i) Slow  

ii) Medium  

iii) Fast

The  runner  may  be  cast  in  one  piece  or  made  of  separate  steel  plates  welded together.  The  runner  made  of  CI  for  small  output,  cast  steel  or  stainless  steel  or bronze for large output. The runner blades should be carefully finished with high degree of accuracy. The runner may be keyed to the shaft which may be vertical or horizontal. The shaft is made of steel and is forged it is provided with a collar for transmitting the axial thrust.

DRAFT TUBE

The water after doing work on the runner passes on to the tall race through a tube called draft tube. It is made of riveted steel plate or pipe or a concrete tunnel. The cross  —section of the tube increases gradually towards the outlet. The draft tube connects   the   runner   exit   to   the   tail   race.   This   tube   should   be   drowned approximately 1 metre below the tail race water level.

Working principles of Francis turbine

The  water  is  admitted  to  the  runner  through  guide  vanes  or  wicket  gates.  The opening between the vanes can be adjusted to vary the quantity of water admitted to the turbine. This is done to suit the load conditions.

The water enters the runner with a low velocity but with a considerable pressure. As the water flows over the vanes the pressure head is gradually converted into velocity head. This kinetic energy is utilised in rotating the wheel Thus the hydraulic energy is converted into mechanical energy. The out going water enters the tail race after passing through the draft tube. The draft tube enlarges gradually and the enlarged end is submerged deeply in the tail race water.  Due to this arrangement a suction head is created at the exit of the runner.

PELTON TURBINE

Among  different  types  of impulse  turbines,  Pelton  wheel is  the  only  turbine  being used  at  present.  It  was  discovered  in  1880  by  an  American  Engineer  Lester A.   Pelton.   It   operates   under   very   high   heads   (upto   1800   m)   and   requires comparatively lesser quantity of water.

Working principle of  Pelton turbines 

From the head race in the mountains water is conveyed to the turbines installed in the power house through the penstocks. The lower end of the penstock is joined with a nozzle in the turbine casing. Water is delivered by the nozzle at a high velocity on the buckets. These buckets are mounted on the periphery of a circular wheel (also known as runner) which is generally mounted on a horizontal shaft. The quantity of water coming out of the nozzle or nozzles can be controlled by regulators (governing arrangement)  in  case  of  big  installations  and  by  hand  wheels  in  case  of  small installations.

The  impact  of  water  on  the  buckets  causes  the  runner  to  rotate,  thus  develops mechanical energy. After doing work on the buckets water is discharged in the tail race.  Being  impulse  turbine  it  must  run  at  atmospheric  pressure  and  therefore, these are located above the tail race. The buckets are so shaped that water enters tangentially in the middle and discharges backward and flows again tangentially in both  the  direction  to  avoid  thrust  on  the  wheel  (as  shown  in  the  line  sketch).

Actually the jet is deflected by 1600. To produce electric energy these are coupled with the electric generators.

HYDRAULIC TURBINES —DEFINITION

The hydraulic turbine is a prime mover that uses the energy of flowing water and converts  is  into  the  mechanical  energy  in  the  form  of  rotation  of  the  runner.  (A prime mover is a machine which uses the raw energy of a substance and converts it into the mechanical energy.) Since the fluid medium is water, these turbines are also known   as   the   ‘ water   turbines’ .   Hydraulic   turbines   coupled   with   hydro    — generators form the so  —called ‘ hydrounits’ which are widely used now a days for generating electrical power.

CLASSIFICATION OF TURBINES


Hydraulic turbines may be classified in the following ways:

i) According to the type of energy at inlet.

a) Impulse turbine
b) Reaction turbine.

ii) According to the direction of flow through runners.

a) Tangential flow
b) Radial flow
c) Axial flow
d) Mixed flow turbines.

iii) According to the head and quantity of water

a) High head turbines  —which work under high heads (above 250m) but with less quantity of water.

Example: Pelton wheel

b) Medium head turbines  —work under medium heads (60m to 25m) —they
require relatively large quantity of water. Example: Francis turbines

c) Low head turbines  —work under heads less than 60m  —they require a very la quantity of water.

Example: Kaplan turbine

iv) According to position of shaft

a) Horizontal turbines  —These turbines have horizontal shafts.

Example: Pelton wheel

b) Vertical turbines  —These turbines have vertical shafts.

Example: Francis and Kaplan turbines.

EULER EQUATION OF TURBO MACHINES

In the Euler equation for work done or energy transfer, in case of a series of radial
curved vanes was derived as,

Work done per unit mass per second =  Vwiui  ± Vwo uo
or Energy transfer, E/unit mass/s = Vwiui  ± Vwo uo

•    This  is  the  fundamental  equation  of  hydraulic  machines,  i.e.,  turbines  and pumps and is known as Euler’ s equation. The equation expresses the energy conversion in a runner (wheel of a turbine) or an impeller (wheel of a pump).

•    The equation in its present form indicates the energy transfer to the wheel by the fluid, which gives motion to the wheel. This is the principle of motion of the turbines.

•    Negative value  of E indicates the energy transfer by the wheel to the  fluid, which can be used to raise the pressure energy of the fluid, or the fluid can be raised to higher altitudes. This principle applies to centrifugal pumps.

where
From inlet velocity triangle, -

Vi  = Absolute velocity of the jet at the inlet ui  = Velocity of the vane at the inlet
Vri  = Relative velocity of the jet at the inlet
Ȑt  = Angle of the absolute velocity at the inlet with the direction of motion of the
vane
= Nozzle angle (also known as guide vane angle)
ȕi  = Angle of the relative velocity with the direction of motion of the vane
Vwi  = Vane angle at the inlet.
Vfi  = Component of the absolute velocity in x-direction
Vrwi  = Velocity of whirl at the inlet

V = Component of the absolute velocity in y-direction
V = Component of the relative velocity in x —d i rection Outlet Velocity Triangle, Let    Vo  = Absolute velocity of the jet at the outlet
uo  = Velocity of the vane at the outlet
Vro  = Relative velocity of the jet at the outlet

Ȑo  = Angle of the absolute velocity at the outlet with the direction of the vane
ȕo  = Angle of the relative velocity with the direction of motion of the vane
= Vane angle at the outlet
Vwo  = Component of the absolute velocity in x-direction
Vfo  = Velocity of whirl at the outlet
Vrwo  = Component of the absolute velocity in y-direction

The equation is valid when direction of Vrwo  is opposite to the direction of u.
The equation shows that the energy transfer E in a fluid machine is the sum of,

•    Difference is squares of absolute fluid velocities

•    Difference in squares of peripheral rotor velocities

•    Difference in squares of relative fluid velocities.

the first   term represents the change in kinetic energy of the fluid. the second term represents the  effect  of centrifugal  head  and  represents  the  pressure change  from that.
The third term represents the pressure change due to the change in relative kinetic energy. Second and third terms constitute static pressure effects.
Flow  in  the  fluid  machines  can  be  tangential  (tangent  to  the  wheel,  radial,  axial
(Parallel to the shaft) or mixed (radial and axial).

•    In a radial or mixed flow machine, all the three terms are effective as there is change  in  absolute  and  relative  velocities  of  the  fluid  as  well  as  in  the peripheral velocity of the rotor.

•    In  an  axial   — flow  machine,  the  second  term  is  not  involved  as  the  fluid remains at the same radial distance during its travel through the machine so that a = u ; only first and third terms are effective.

In case of tangential flow machines also, the second term is ineffective because the fluid  enters  and  leaves  at  the  same  radial  distance  so  that  ui   =  uo.  But  usually, tangential  flow  machines  are  impulse  type  of  machines  that  work  under  constant pressure  (atmospheric).  Neglecting  the  effect  of  elevation  and  friction,  there  is  no change in the relative velocities by the application of Bernoulli’ s equation. Thus the third  term  also  vanishes  and  the  energy  transfer  is  only  due  to  the  change  in  the kinetic energy of the fluid.

FLUID MACHINES

A fluid machine is a device which convert the energy stored by a fluid into mechanical energy or vice versa. The energy stored by a fluid mass appears in the form of potential, kinetic and intermolecular energy. The mechanical energy, on the other hand, is usually transmitted by rotating shaft. Machines using liquid (mainly water, for almost all practical purpose) are termed as hydraulic machines. In this chapter we shall discuss, in general, the basic fluid mechanical principle governing the energy transfer in a fluid machine and also a brief description of different kinds of hydraulic machines along with their performance.

FLUID                       TYPES OF TURBINE

Water                          Hydraulic Turbine
Steam                          Steam Turbine
Froen                          Vapour Turbine
Gas or air                    Gas Turbine
Wind                           Wind Mills

Similarly, fluid machines which convert shaft power to fluid power by raising the energy content per unit mass of the fluid are classified as follows:

Fluid                                                Types of Machine

Water and Other liquids                     Pumps

Air and Other Gases                          Fans and Propellers
(with slight pressure rise) 

Air and Other Gases                          Blowers and Compressors
(with higher pressure rise)



CLASSIFICATION OF FLUID MACHINES 


The turbines in general are classified in two ways:

• According to the direction of flow of water through the runner

• According to the action of water on the runner blades.

In order to classify machines according to the direction of flow of water through the
runner, three mutually perpendicular directions for flow of water are chosen


RADIAL FLOW MACHINE


The path of water particles is wholly or mainly in the plane of rotation. i.e., the water
enters the runner at the outer periphery, flows inwards in the radial direction and leaves at
a different radius as shown in figure 4.1(b).


AXIAL FLOW MACHINES

The water mainly flows through the runner in a direction parallel to the axis of rotation as shown in figure 4

MIXED FLOW MACHINES


The flow in the runner may not be merely in one direction.
turbines, water enters radially inwards and emerges out axially so that parallel to the axis of the shaft as shown in figure 4.1(d).

In mixed flow the discharge in According to the action of water on moving blades, the turbine way be placed in one of the two general categories: i Impulse ii Reaction.

In a hydroelectric power scheme, water in a very large quantity is stored in a high
level reservoir. In an impulse turbine, the water is brought to the turbine entrance through penstock pipes ending in one or more fixed nozzles. The entire pressure energy of water is converted into the kinetic energy of an unconfined jet. The jet of fluid then strikes the blades of the runner and loses practically all of its kinetic energy, i.e., the velocity of water at the exit of the runner is just sufficient to enable
it to move out the runner. The static pressure of water at the entrance to the runner
is equal to the static pressure at exit and the rotation of the wheel is caused purely due to the tangential force created by the impact of the jet, and hence an impulse turbine. The most common impulse turbine is called Pelton turbine.

Tangential flow machine

The water strikes the blades or buckets of the runner in a direction tangential to the path of rotation. The tangential direction is perpendicular to both axial and radial directions as shown in figure 4.1(c).


EXCHANGE OF ENERGY


•    A machine wherein rotary motion is obtained by centrifugal forces which result
from a change in the direction of high velocity fluid jet that issues from a nozzle.

A  hydraulic  turbine  uses  the  potential  and  kinetic  energy  of water  and  convert  it  into usable  mechanical energy.  The fluid  energy is available  in the  natural or artificial high level water reservoirs which are created by constructing dams at appropriate places in the flow  path  of rivers.  When  water  from  the  reservoir  is  taken to  the  turbine,  transfer  of energy  takes  place  in  the  blade  passages  of  the  unit.  The  mechanical  energy  made available at the turbine shaft is used to run an electric generator which is directly coupled
to the turbine shaft.

The power generated by utilizing the potential  and kinetic  energy of water has the
advantages of

•    High efficiency
•    Operational flexibility
•    Low wear and tear
•    Ease maintenance.

Despite the heavy capital cost involved  in constructing dams and reservoirs,  in running
pipelines  and  in  turbine  installation  (when  compared  to  an  equivalent  thermal  power plant)  different  countries  have tried to tap all their water power resources. Appropriate types of water turbines have been installed for most efficient utilization.

ORIFICE METER

The  Venturi  meter  described  earlier  is  a  reliable  flow-measuring  device. Furthermore, it causes little pressure loss.

For these reasons it is  widely used, particularly for large-volume liquid  and gas flows. However this meter is relatively complex to construct and hence expensive.  Especially  for  small  pipelines,  its  cost  seems  prohibitive,  so simpler devices such as orifice meters are used.

The orifice meter consists of a flat orifice plate with a circular hole drilled in it. There  is  a  pressure  tap  upstream  from  the  orifice  plate  and  another  just downstream.

The principle of the orifice meter is identical with that of the venturi meter.

The reduction of the cross section of the flowing stream in passing through the orifice increases the velocity head at the expense of the pressure head, and manometer measures the reduction in pressure between the taps.

Bernoulli's equation  provides a  basis for correlating the increase  in velocity head with the decrease in pressure head.

VENTURIMETER

In  this  meter  the  fluid  is  accelerated  by  its  passage  through  a  converging cone of angle 15-20o.

The pressure difference between the upstream end if the cone and the throat are measured and provide the signal for the rate of flow.

The fluid  is  then  retarded in  a  cone of  smaller  angle  (5-7o) in  which  large proportion of kinetic energy is converted back to pressure energy.

Because of the gradual reduction in the area there is no vena contracta and the flow area is a minimum at the throat so that the coefficient of contraction
is unity.

The attraction of this meter lies in its high-energy recovery so that it may be used where only a small pressure head is available, though its construction
is expensive.

To  make  the  pressure  recovery  large,  the  angle  of  downstream  cone  is small, so boundary layer separation is prevented and friction minimized.

Since separation does not occur in a contracting cross section, the upstream cone can be made shorter than the downstream cone  with but little friction, and space and  material are thereby conserved.

Although Venturimeter can be applied to the measurement of gas, they are most commonly used for liquids.

The following treatment is limited to incompressible fluids.

The basic equation for the venturimeter is obtained by writing the Bernoulli equation for incompressible fluids between the two sections a and b. Friction is neglected, the
meter is assumed to be horizontal.

Full Wave Rectifier

In the previous Power Diodes tutorial we discussed ways of reducing the ripple or voltage variations on a direct DC voltage by connecting capacitors across the load resistance. While this method may be suitable for low power applications it is unsuitable to applications which need a "steady and smooth" DC supply voltage. One method to improve on this is to use every half-cycle of the input voltage instead of every other half-cycle. The circuit which allows us to do this is called a Full Wave Rectifier.

Like the half wave circuit, a full wave rectifier circuit produces an output voltage or current which is purely DC or has some specified DC component. Full wave rectifiers have some fundamental advantages over their half wave rectifier counterparts. The average (DC) output voltage is higher than for half wave, the output of the full wave rectifier has much less ripple than that of the half wave rectifier producing a smoother output waveform.

In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle. A transformer is used whose secondary winding is split equally into two halves with a common centre tapped connection, (C). This configuration results in each diode conducting in turn when its anode terminal is positive with respect to the transformer centre point C producing an output during both half-cycles, twice that for the half wave rectifier so it is 100% efficient as shown below.

Full Wave Rectifier Circuit

The full wave rectifier circuit consists of two power diodes connected to a single load resistance (R_L) with each diode taking it in turn to supply current to the load. When point A of the transformer is positive with respect to point C, diode D₁ conducts in the forward direction as indicated by the arrows. When point B is positive (in the negative half of the cycle) with respect to point C, diode D₂ conducts in the forward direction and the current flowing through resistor R is in the same direction for both half-cycles. As the output voltage across the resistor R is the phasor sum of the two waveforms combined, this type of full wave rectifier circuit is also known as a "bi-phase" circuit.

As the spaces between each half-wave developed by each diode is now being filled in by the other diode the average DC output voltage across the load resistor is now double that of the single half-wave rectifier circuit and is about  0.637V_max  of the peak voltage, assuming no losses.

Where: V_MAX is the maximum peak value in one half of the secondary winding and V_RMS is the rms value.

The peak voltage of the output waveform is the same as before for the half-wave rectifier provided each half of the transformer windings have the same rms voltage value. To obtain a different DC voltage output different transformer ratios can be used. The main disadvantage of this type of full wave rectifier circuit is that a larger transformer for a given power output is required with two separate but identical secondary windings making this type of full wave rectifying circuit costly compared to the "Full Wave Bridge Rectifier" circuit equivalent.

The Full Wave Bridge Rectifier

Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop "bridge" configuration to produce the desired output. The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below.

The Diode Bridge Rectifier

The four diodes labelled D₁ to D₄ are arranged in "series pairs" with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below.

The Positive Half-cycle

During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch "OFF" as they are now reverse biased. The current flowing through the load is the same direction as before.

The Negative Half-cycle

As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637V_max. However in reality, during each half cycle the current flows through two diodes instead of just one so the amplitude of the output voltage is two voltage drops ( 2 x 0.7 = 1.4V ) less than the input V_MAX amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply)

Typical Bridge Rectifier

Although we can use four individual power diodes to make a full wave bridge rectifier, pre-made bridge rectifier components are available "off-the-shelf" in a range of different voltage and current sizes that can be soldered directly into a PCB circuit board or be connected by spade connectors. The image to the right shows a typical single phase bridge rectifier with one corner cut off. This cut-off corner indicates that the terminal nearest to the corner is the positive or +ve output terminal or lead with the opposite (diagonal) lead being the negative or -ve output lead. The other two connecting leads are for the input alternating voltage from a transformer secondary winding.

The Smoothing Capacitor

We saw in the previous section that the single phase half-wave rectifier produces an output wave every half cycle and that it was not practical to use this type of circuit to produce a steady DC supply. The full-wave bridge rectifier however, gives us a greater mean DC value (0.637 Vmax) with less superimposed ripple while the output waveform is twice that of the frequency of the input supply frequency. We can therefore increase its average DC output level even higher by connecting a suitable smoothing capacitor across the output of the bridge circuit as shown below.

Full-wave Rectifier with Smoothing Capacitor

The smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage. Generally for DC power supply circuits the smoothing capacitor is an Aluminium Electrolytic type that has a capacitance value of 100uF or more with repeated DC voltage pulses from the rectifier charging up the capacitor to peak voltage. However, their are two important parameters to consider when choosing a suitable smoothing capacitor and these are its Working Voltage, which must be higher than the no-load output value of the rectifier and its Capacitance Value, which determines the amount of ripple that will appear superimposed on top of the DC voltage. Too low a value and the capacitor has little effect but if the smoothing capacitor is large enough (parallel capacitors can be used) and the load current is not too large, the output voltage will be almost as smooth as pure DC. As a general rule of thumb, we are looking to have a ripple voltage of less than 100mV peak to peak.

The maximum ripple voltage present for a Full Wave Rectifier circuit is not only determined by the value of the smoothing capacitor but by the frequency and load current, and is calculated as:

Bridge Rectifier Ripple Voltage

Where: I is the DC load current in amps, ƒ is the frequency of the ripple or twice the input frequency in Hertz, and C is the capacitance in Farads.

The main advantages of a full-wave bridge rectifier is that it has a smaller AC ripple value for a given load and a smaller reservoir or smoothing capacitor than an equivalent half-wave rectifier. Therefore, the fundamental frequency of the ripple voltage is twice that of the AC supply frequency (100Hz) where for the half-wave rectifier it is exactly equal to the supply frequency (50Hz).

The amount of ripple voltage that is superimposed on top of the DC supply voltage by the diodes can be virtually eliminated by adding a much improved π-filter (pi-filter) to the output terminals of the bridge rectifier. This type of low-pass filter consists of two smoothing capacitors, usually of the same value and a choke or inductance across them to introduce a high impedance path to the alternating ripple component. Another more practical and cheaper alternative is to use a 3-terminal voltage regulator IC, such as a LM78xx for a positive output voltage or the LM79xx for a negative output voltage which can reduce the ripple by more than 70dB (Datasheet) while delivering a constant output current of over 1 amp.

In the next tutorial about diodes, we will look at the Zener Diode which takes advantage of its reverse breakdown voltage characteristic to produce a constant and fixed output voltage across itself.