Cavitation in hydraulic turbine: causes and effects

Cavitation in hydraulic turbine: causes and effects

Cavitation is formation of vapor bubbles in the liquid flowing through any Hydraulic Turbine. Cavitation occurs when the static pressure of the liquid falls below its vapor pressure. Cavitation is most likely to occur near the fast moving blades of the turbines and in the exit region of the turbines

Causes of Cavitation

The liquid enters hydraulic turbines at high pressure; this pressure is a combination of static and dynamic components. Dynamic pressure of the liquid is by the virtue of flow velocity and the other component, static pressure, is the actual fluid pressure which the fluid applies and which is acted upon it. Static pressure governs the process of vapor bubble formation or boiling. Thus, Cavitation can occur near the fast moving blades of the turbine where local dynamic head increases due to action of blades which causes static pressure to fall. Cavitation also occurs at the exit of the turbine as the liquid has lost major part of its pressure heads and any increase in dynamic head will lead to fall in static pressure causing Cavitation.

Detrimental Effects of Cavitation

The formation of vapor bubbles in cavitation is not a major problem in itself but the collapse of these bubbles generates pressure waves, which can be of very high frequencies, causing damage to the machinery. The bubbles collapsing near the machine surface are more damaging and cause erosion on the surfaces called as cavitation erosion. The collapses of smaller bubbles create higher frequency waves than larger bubbles. So, smaller bubbles are more detrimental to the hydraulic machines.

Smaller bubbles may be more detrimental to the hydraulic machine body but they do not cause any significant reduction in the efficiency of the machine. With further decrease in static pressure more number of bubbles is formed and their size also increases. These bubbles coalesce with each other to form larger bubbles and eventually pockets of vapor. This disturbs the liquid flow and causes flow separation which reduces the machine performance sharply. Cavitation is an important factor to be considered while designing Hydraulic Turbines.

Avoiding Cavitation

To avoid cavitation while operating Hydraulic Turbines parameters should be set such that at any point of flow static pressure may not fall below the vapor pressure of the liquid. These parameters to control cavitation are pressure head, flow rate and exit pressure of the liquid. The control parameters for cavitation free operation of hydraulic turbines can be obtained by conducting tests on model of the turbine under consideration. The parameters beyond which cavitation starts and turbine efficiency falls significantly should be avoided while operation of hydraulic turbines.

Flow separation at the exit of the turbine in the draft tube causes vibrations which can damage the draft tube. To dampen the vibration and stabilize the flow air is injected in the draft tube. To totally avoid the flow separation and cavitation in the draft tube it is submerged below the level of the water in tailrace.

Pelton Turbine

Pelton Turbine

In a Pelton Turbine or Pelton Wheel water jets impact on the blades of the turbine making the wheel rotate, producing torque and power. Learn more about design, analysis, working principle and applications of Pelton Wheel Turbine.

Hydraulic Turbines are being used from very ancient times to harness the energy stored in flowing streams, rivers, and lakes. The oldest and the simplest form of a Hydraulic Turbine was the Waterwheel used for grinding grains. Different types of Hydraulic Turbines were developed with the increasing need for power. Three major types are Pelton Wheel, Francis, and Kaplan Turbine.

Design of Pelton Wheel Turbine

The Pelton Turbine has a circular disk mounted on the rotating shaft or rotor. This circular disk has cup shaped blades, called as buckets, placed at equal spacing around its circumference. Nozzles are arranged around the wheel such that the water jet emerging from a nozzle is tangential to the circumference of the wheel of Pelton Turbine. According to the available water head (pressure of water) and the operating requirements the shape and number of nozzles placed around the Pelton Wheel can vary.

Working Principle of Pelton Turbine

The high speed water jets emerging form the nozzles strike the buckets at splitters, placed at the middle of a bucket, from where jets are divided into two equal streams. These stream flow along the inner curve of the bucket and leave it in the direction opposite to that of incoming jet. The high speed water jets running the Pelton Wheel Turbine are obtained by expanding the high pressure water through nozzles to the atmospheric pressure. The high pressure water can be obtained from any water body situated at some height or streams of water flowing down the hills.

The change in momentum (direction as well as speed) of water stream produces an impulse on the blades of the wheel of Pelton Turbine. This impulse generates the torque and rotation in the shaft of Pelton Turbine. To obtain the optimum output from the Pelton Turbine the impulse received by the blades should be maximum for that, change in momentum of the water stream should be maximum possible. That is obtained when the water stream is deflected in the direction opposite to which it strikes the buckets and with the same speed relative to the buckets.

Pelton Turbine Hydroelectric Setup

A typical setup of a system generating electricity by using Pelton Turbine will have a water reservoir situated at a height from the Pelton Wheel. The water from the reservoir flows through a pressure channel to the penstock head and then through the penstock or the supply pipeline to the nozzles, from where the water comes out as high speed jets striking the blades of the Pelton Turbine. The penstock head is fitted with a surge tank which absorbs and dissipates sudden fluctuations in pressure.

For a constant water flow rate from the nozzles the speed of turbine changes with changing loads on it. For quality hydroelectricity generation the turbine should rotate at a constant speed. To keep the speed constant despite the changing loads on the turbine water flow rate through the nozzles is changed. To control the gradual changes in load servo controlled spear valves are used in the jets to change the flow rate. And for sudden reduction in load the jets are deflected using deflector plates so that some of the water from the jets does not strike the blades. This prevents over speeding of the turbine.

Kaplan Turbine

Kaplan Turbine

Kaplan Turbine is designed for low water head applications. Kaplan Turbine has propeller like blades but works just reverse. Instead of displacing the water axially using shaft power and creating axial thrust, the axial force of water acts on the blades of Kaplan Turbine and generating shaft power.

Most of the turbines developed earlier were suitable for large heads of water. With increasing demand of power need was felt to harness power from sources of low head water, such as, rivers flowing at low heights. For such low head applications Viktor Kaplan designed a turbine similar to the propellers of ships. Its working is just reverse to that of propellers. The Kaplan Turbine is also called as Propeller Turbine.

Design of Kaplan Turbine

To generate substantial amount of power from small heads of water using Kaplan Turbine it is necessary to have large flow rates through the turbine. Kaplan Turbine is designed to accommodate the required large flow rates. Except the alignment of the blades the construction of the Kaplan Turbine is very much similar to that of the Francis Turbine. The overall path of flow of water through the Kaplan Turbine is from radial at the entrance to axial at the exit. Similar to the Francis Turbine, Kaplan Turbine also has a ring of fixed guide vanes at the inlet to the turbine.

Unlike the Francis Turbine which has guide vanes at the periphery of the turbine rotor (called as runner in the case of Francis Turbine), there is a passage between the guide vanes and the rotor of the Kaplan Turbine. The shape of the passage is such that the flow which enters the passage in the radial direction is forced to flow in axial direction. The rotor of the Kaplan Turbine is similar to the propeller of a ship. The rotor blades are attached to the central shaft of the turbine. The blades are connected to the shaft with moveable joints such that the blades can be swiveled according to the flow rate and water head available.

The blades of the Kaplan Turbine are not planer as any other axial flow turbine; instead they are designed with twist along the length so as to allow swirling flow at entry and axial flow at exit.

Working of the Kaplan Turbine

The working head of water is low so large flow rates are allowed in the Kaplan Turbine. The water enters the turbine through the guide vanes which are aligned such as to give the flow a suitable degree of swirl determined according to the rotor of the turbine. The flow from guide vanes pass through the curved passage which forces the radial flow to axial direction with the initial swirl imparted by the inlet guide vanes which is now in the form of free vortex.

The axial flow of water with a component of swirl applies force on the blades of the rotor and loses its momentum, both linear and angular, producing torque, and rotation (their product is power) in the shaft. The scheme for production of hydroelectricity by Kaplan Turbine is same as that for Francis Turbine.

Francis Turbine

                                                      Francis Turbine

In Francis Turbine water flow is radial into the turbine and exits the Turbine axially. Water pressure decreases as it passes through the turbine imparting reaction on the turbine blades making the turbine rotate. Read more about design and working principle of Francis Turbine in this article.

Francis Turbine is the first hydraulic turbine with radial inflow. It was designed by American scientist James Francis. Francis Turbine is a reaction turbine. Reaction Turbines have some primary features which differentiate them from Impulse Turbines. The major part of pressure drop occurs in the turbine itself, unlike the impulse turbine where complete pressure drop takes place up to the entry point and the turbine passage is completely filled by the water flow during the operation.

Design of Francis Turbine

Francis Turbine has a circular plate fixed to the rotating shaft perpendicular to its surface and passing through its center. This circular plate has curved channels on it; the plate with channels is collectively called as runner. The runner is encircled by a ring of stationary channels called as guide vanes. Guide vanes are housed in a spiral casing called as volute. The exit of the Francis turbine is at the center of the runner plate. There is a draft tube attached to the central exit of the runner. The design parameters such as, radius of the runner, curvature of channel, angle of vanes and the size of the turbine as whole depend on the available head and type of application altogether.

Working of Francis Turbine

Francis Turbines are generally installed with their axis vertical. Water with high head (pressure) enters the turbine through the spiral casing surrounding the guide vanes. The water loses a part of its pressure in the volute (spiral casing) to maintain its speed. Then water passes through guide vanes where it is directed to strike the blades on the runner at optimum angles. As the water flows through the runner its pressure and angular momentum reduces. This reduction imparts reaction on the runner and power is transferred to the turbine shaft.

If the turbine is operating at the design conditions the water leaves the runner in axial direction. Water exits the turbine through the draft tube, which acts as a diffuser and reduces the exit velocity of the flow to recover maximum energy from the flowing water.

Power Generation using Francis Turbine

For power generation using Francis Turbine the turbine is supplied with high pressure water which enters the turbine with radial inflow and leaves the turbine axially through the draft tube. The energy from water flow is transferred to the shaft of the turbine in form of torque and rotation. The turbine shaft is coupled with dynamos or alternators for power generation. For quality power generation speed of turbine should be maintained constant despite the changing loads. To maintain the runner speed constant even in reduced load condition the water flow rate is reduced by changing the guide vanes angle.

Hydraulic Turbines

                                                                 Hydraulic Turbines

Hydraulic Turbines transfer the energy from a flowing fluid to a rotating shaft. Turbine itself means a thing which rotates or spins. To know more about what are Hydraulic Turbines, what is the working principle of Hydraulic Turbines and how are they classified, read on through this article series?

Leonardo da Vinci once said “The power of water has changed more in this world than emperors or kings”. It was very rightly stated by him as in present time Hydropower, the power generated from water, has a major contribution to the world’s total power production. This all was made possible by the development of Hydraulic Turbines which can transfer the energy from flowing water to the shafts of dynamos producing electrical power.

Hydraulic Turbines

Hydraulic Turbines have a row of blades fitted to the rotating shaft or a rotating plate. Flowing liquid, mostly water, when pass through the Hydraulic Turbine it strikes the blades of the turbine and makes the shaft rotate. While flowing through the Hydraulic Turbine the velocity and pressure of the liquid reduce, these result in the development of torque and rotation of the turbine shaft. There are different forms of Hydraulic Turbines in use depending on the operational requirements. For every specific use a particular type of Hydraulic Turbine provides the optimum output.

Classification of Hydraulic Turbines: Based on flow path

Water can pass through the Hydraulic Turbines in different flow paths. Based on the flow path of the liquid Hydraulic Turbines can be categorized into three types.

Axial Flow Hydraulic Turbines:

 This category of Hydraulic Turbines has the flow path of the liquid mainly parallel to the axis of rotation. Kaplan Turbines has liquid flow mainly in axial direction.

Radial Flow Hydraulic Turbines:

 Such Hydraulic Turbines has the liquid flowing mainly in a plane perpendicular to the axis of rotation.

Mixed Flow Hydraulic Turbines:

 For most of the Hydraulic Turbines used there is a significant component of both axial and radial flows. Such types of Hydraulic Turbines are called as Mixed Flow Turbines. Francis Turbine is an example of mixed flow type, in Francis Turbine water enters in radial direction and exits in axial direction.

None of the Hydraulic Turbines are purely axial flow or purely radial flow. There is always a component of radial flow in axial flow turbines and of axial flow in radial flow turbines.

Classification of Hydraulic Turbines: Based on pressure change

One more important criterion for classification of Hydraulic Turbines is whether the pressure of liquid changes or not while it flows through the rotor of the Hydraulic Turbines. Based on the pressure change Hydraulic Turbines can be classified as of two types.

Impulse Turbine:

The pressure of liquid does not change while flowing through the rotor of the machine. In Impulse Turbines pressure change occur only in the nozzles of the machine. One such example of impulse turbine is Pelton Wheel.

Reaction Turbine:

The pressure of liquid changes while it flows through the rotor of the machine. The change in fluid velocity and reduction in its pressure causes a reaction on the turbine blades; this is where from the name Reaction Turbine may have been derived. Francis and Kaplan Turbines fall in the category of Reaction Turbines. 

Hydraulic Turbine

Hydraulic Turbine

Hydraulic turbine is a mechanical component which converts hydraulic energy (i.e. kinetic energy and potential energy) of water in mechanical energy. The mechanical energy so produced is utilized in running an electrical generator which is connected to shaft of the turbine. Thus mechanical energy is converted into electrical energy.

This principle is utilized in the production of hydroelectric power.

Turbine is broadly classified in two types:

Ø  Impulse turbine

Ø  Reaction turbine

Impulse Turbine:

A turbine si said to be as impulse turbine if the runner of the turbine revolves by impact or impulse action of water. To increase the impact of water the entire available energy of water is converted in kinetic energy by passing it through a nozzle. The jet of water smoothly impinges on the bucket through the nozzle due to which the turbine rotates. There is no pressure difference between the inlet and outlet of runner.

Example: Peton turbine

              Cross flow Turbine

              Turbo Turbine

This turbine is also known as free jet turbine. In such type of turbine, it can be written that

Pressure of inlet (p1) = Pressure of outlet (p2)

Velocity of inlet (v1) is very greater than velocity of outlet (v2). (i.e. v1>>v2)

Weight of inlet water (w1) = weight of outlet water (w2)               (neglecting losses in bucket)

Reaction turbine

A turbine is said to be reaction turbine if at the inlet of the turbine water possesses kinetic energy as well as pressure energy. In a reaction turbine runner rotates due to the reaction of pressure difference between the inlet and outlet of the runner. In this case entering water has pressure as well as kinetic energy and when it moves over the runner/blade/bucket both kinetic & pressure energy provides the turning movement of the wheel.

Example: Francis turbine

              Propeller Turbine

              Kaplan Turbine

For this type of turbine, it can be mentioned that;

P1>>P2                                V1>V2                                              & W1<W2

Difference between Impulse and Reaction turbine

Impulse Turbine	Reaction Turbine
The wheel rotates due to the impact of water.	The wheel rotates due to the pressure difference between inlet & outlet.
The turbine is installed over a tail race.	The turbine is entirely submerged on fluid below tail race.
Cashing has no hydraulic function to perform.	Cashing has hydraulic function to perform. It is one of the main which create pressure difference at inlet and outlet.
The flow of fluid can be without loss.	The flow of fluid can’t be regulated without loss.
The wheel does not run full.	The wheel must run full.

Pelton Turbine:

Pelton Turbine is a tangential flow impulse turbine. It is the mostly widely used turbine. The impact of water on the bucket causes the runner to rotate thus develops mechanical energy & is called impulse turbine.

Characteristics of Pelton turbine:

It requires high head above 200m but small quantity of water.

The flow is tangential

It has specific speed varying between 10-25 for single jet and 50 for double jet.

Main components of Pelton turbine

Nozzle

Nozzle is provided at the end of penstock. The pressure energy of water passing through the penstock is converted into kinetic energy and is further increased by nozzle at atmospheric pressure in the bucket. When the water trikes the bucket fitted on the periphery of runner mechanical energy is produced.

Bucket

The bucket of Pelton wheel has a shape of a double hemispherical cup. Bucket is divided by a common dividing straight edge. The two hemispherical cups gives the advantage that the axial force in both the cups neutralizes each other and avoids axial thrust.

These buckets are made of cast iron (for low head) cast steel or stainless steel is used for high head

Cashing

Cashing in Pelton turbine has no hydraulic function but it is provided to prevent splashing of water. It is made strong enough to resist reaction of jet.

Hydraulic brake

The turbine goes on revolving for a long time, even if the jet is closed. Thus to stop the rotation small brake nozzle is provided which opposes the rotation of bucket and stops.

Francis Turbine

Francis turbine is a radial flow reaction turbine. The flow in Francis Turbine is radial at both entry and exist of head which is less than 200m, Francis turbine is more suitable than Pelton as Pelton becomes slow and unwidely.

 Characteristics of Francis turbine

It operates in medium head (between 30-200m) and needs medium quantity of water.

Water flows in radial direction.

Francis turbine has specific speed varying from 50-300 rpm.

Main components of Francis Turbine:

Scroll Cashing:

The water from penstock is supply to the turbine casing which is spiral in shape. The casing plays an important role in hydraulic function. It surrounds the runner and guide blade. As water enters the casing it forces the water to flow into them at high pressure causing runner to rotate.

Guide Mechanism:

It performs two functions:

Ø  To direct the water in correct angle to the runner vane

Ø  To regulate the quantity of water according to load variation

Guide mechanism is held stationary in the casing and surrounds the runner. These vanes are generally made of cast iron.

Runner

It is fitted to the shaft which is coupled to electric generator. It is made of cast iron for small turbine or cast steel for large turbine. It can also make up of stainless steel and sometimes of non-ferrous metal like bronze.

Draft Tube:

It is a tube in passage having gradually increasing cross-sectional area. Its upper end is connected to exist of runner and lower end sufficiently submerged to tail race. It helps in increasing to woe done of turbine by reducing pressure.

Propeller Turbine

It is an axial flow propeller turbine. It has 4-8 vanes fixed to Boss. The shape of the runner is similar to the propeller of a ship hence these turbines are known as propeller turbine.

Characteristics of propeller turbine:

·         It can operate at low head (below 30m) but it requires large discharge

·         The flow in propeller turbine is in axial direction

·         Its specific speed varies from 300-1000 rms.

·         It is vertical turbine shaft.

·         Components are same as Francis turbine.

Kaplan Turbine

It is an axial flow reaction turbine. It is the modified propeller turbine as the blade on Kaplan turbine is adjustable (i.e. blades angle can be changed by means of oil pressure servo motor). This type of turbine can give good efficiency even at different load condition and water flow over the blade without shock.

Characteristics of Kaplan Turbine:

·         It is suitable for low head (3-30m) & it requires large discharge

·         It consists of axial flow

·         Its specific speed varies from 300-1000 rpm.

·         It can give overall efficiency up to 92%

·         It is vertical shaft turbine.

Basic Types of Drilling Machines

Basic Types of Drilling Machines

Drilling machines or drill presses are one of the most common machines found in the machine shop. A drill press is a machine that turns and advances a rotary tool into a work piece. The drill press is used primarily for drilling holes, but when used with the proper tooling, it can be used for a number of machining operations. The most common machining operations performed on a drill press are drilling, reaming, tapping, counter boring, countersinking, and spot facing.

There are many different types or configurations of drilling machines, but most drilling machines will fall into four broad categories: upright sensitive, upright, radial, and special purpose.

UPRIGHT SENSITIVE DRILL PRESS

The upright sensitive drill press is a light-duty type of drilling machine that normally incorporates a belt drive spindle head. This machine is generally used for moderate-to-light duty work. The upright sensitive drill press gets its name due to the fact that the machine can only be hand fed. Hand feeding the tool into the work piece allows the operator to "feel" the cutting action of the tool. The sensitive drill press is manufactured in a floor style or a bench style.

UPRIGHT DRILL PRESS

The upright drill press is a heavy duty type of drilling machine normally incorporating a geared drive spindle head. This type of drilling machine is used on large hole-producing operations that typically involve larger or heavier parts. The upright drill press allows the operator to hand feed or power feed the tool into the work piece. The power feed mechanism automatically advances the tool into the work piece. Some types of upright drill presses are also manufactured with automatic table-raising mechanisms.

RADIAL ARM DRILL PRESS

The radial arm drill press is the hole producing work horse of the machine shop. The press is commonly referred to as a radial drill press. The radial arm drill press allows the operator to position the spindle directly over the work piece rather than move the work piece to the tool. The design of the radial drill press gives it a great deal of versatility, especially on parts too large to position easily. Radial drills offer power feed on the spindle, as well as an automatic mechanism to raise or lower the radial arm. The wheel head, which is located on the radial arm, can also be traversed along the arm, giving the machine added ease of use as well as versatility. Radial arm drill presses can be equipped with a turn-on table or tilting table. This gives the operator the ability to drill intersecting or angular holes in one setup.

SPECIAL PURPOSE DRILL MACHINES

There are a number of types of special purpose drilling machines. The purposes of these types of drilling machines vary. Special purpose drilling machines include machines capable of drilling 20 holes at once or drilling holes as small as 0.01 of an inch.

Gang Drilling Machines

The gang style drilling machine or gang drill press has several work heads positioned over a single table. This type of drill press is used when successive operations are to be done. For instance, the first head may be used to spot drill. The second head may be used to tap drill. The third head may be used, along with a tapping head, to tap the hole. The fourth head may be used to chamfer.

Multiple Spindles Drilling Machine

The multiple spindle drilling machines is commonly referred to as a multi spindle drill press. This special purpose drill press has many spindles connected to one main work head.

All of the spindles are fed into the work piece at the same time. This type of drilling machine is especially useful when you have a large number of parts with many holes located close together.

Micro-Drill Press

The micro drill press is an extremely accurate, high spindle speed drill press. The micro drill press is typically very small and is only capable of handling very small parts. Many micro drill presses are manufactured as bench top models. They are equipped with chucks capable of holding very small drilling tools.          

Turret Type Drilling Machine

Turret drilling machines are equipped with several drilling heads mounted on a turret. Each turret head can be equipped with a different type of cutting tool. The turret allows the needed tool to be quickly indexed into position. Modern turret type drilling machines are computer-controlled so that the table can be quickly and accurately positioned.

DRILLING MACHINE

DRILLING MACHINE

Introduction: The drilling machine or drill press is one of the most common and useful machine employed in industry for producing forming and finishing holes in a work piece. The unit essentially consists of:

1. A spindle which turns the tool (called drill) which can be advanced in the work piece either automatically or by hand.

2. A work table which holds the work piece rigidly in position.

Working principle: The rotating edge of the drill exerts a large force on the work piece and the hole is generated. The removal of metal in a drilling operation is by shearing and extrusion.

Types of Drilling Machines: A wide variety of drilling machines are available ranging from the simple portable to highly complex automatic and numerically controlled machines are as follows:

1. Portable drilling machine: It is a small light weight, compact and self- contained unit that can drill holes up to 12.5 cm diameter. The machine is driven by a small electric motor operating at high speed. The machine is capable of drilling holes in the work pieces in any position.

2. Sensitive drill machine/press: This is a light weight, high speed machine designed for drilling small holes in light jobs. Generally the machine has the capacity to rotate drills of 1.5 to 15.5 cm at high speed of 20,000 rev. /min.

Construction: The machine has only a hand feed mechanism for feeding the tool into the work piece. This enables the operator to feel how the drill is cutting and accordingly he can control the down feed pressure. Sensitive drill presses are manufactured in bench or floor models, i.e., the base of machine may be mounted on a bench or floor.

The main operating parts of a sensitive machine/drill press are Base, Column, Table, and Drill Head.

1. Base: The base is a heavy casting that supports the machine structure; it provides rigid mounting for the column and stability for the machine. The base is usually provided with holes and slots which help to Bolt the base to a table or bench and allow the work-holding device or the work piece to be fastened to the base.

2. Column: The column is a vertical post that Column holds the worktable and the head containing the driving mechanism. The column may be of round or box section.

3. Table: The table, either rectangular or round. Drill machine/press in shape supports the work piece and is carried by the vertical column. The surface of the table is 90-degree to the column and it can be raised, lowered, and swiveled around it. The table can be clamp/hold the required the work piece. Slots are provided in most tables to allow the jigs, fixtures, or large work pieces to be securely fixed directly to the table.

4. Drilling Head: The drilling head, mounted close to the top of the column, houses the driving arrangement and variable speed pulleys. These units transmit rotary motion at different speeds to the drill spindle. The hand feed lever is used to control the vertical movement of the spindle sleeve and the cutting tool.

The system is called the sensitive drilling machine/press as the operator is able to sense the progress of drill with hand-faced.

D.C. Motor

D.C. Motor

A machine that converts electrical power to mechanical power is known as DC motor. Its operation is based on the principle that when a current carrying conductor is placed in magnetic field the conductor experiences a mechanical force. The direction of this force is given by Fleming’s left hand rule and magnitude is given by F=BIL newton.

Basically there is no constructional different between DC motor and DC generator. The same machine can work as generator and motor. A DC machine work as a DC generator when it is driven by prime mover. The same machine work as DC motor when electrical energy is supplied to it.

Operating principle

The operating principle of the DC motor is based on the principle that when a current carrying conductor is placed in a magnetic field, force is developed on its ends. The direction of force is given by Fleming’s left hand rule and the magnitude of force is given by F=BIL newton.

Application of DC motor

An application of D. C. Motors doesn’t indicate the use only, it means why and where we use a D. C. Motor.

As we all know D. C. Motors are of three types: -

Shunt Motors

Series Motors

Compound Motor

Shunt Motors

There are three kind of character sticks for a motor viz. Speed-Torque, Speed-Current and Torque-Current characteristics. After analyzing all three characteristics for D. C. Shunt Motor it is observed that it is an approximately constant speed motor. It is therefore, used where

The speed is required to remain almost constant from no-load condition to full load-condition.

The load has to be driven at a number of prefer and any one of which is required to remain nearly constant.

Industrial Use: - Lathes, Drills, Boring Mills, Shapers, Spinning and Weaving Machines etc.

Series Motors:

After analyzing all three characteristics for D. C. Series Motor it is observed that it is a variable speed motor. It means speed it low at high torque and vice-versa. However, at light or no-load, the motor tends to attain dangerously high speed. The motor has a high starting torque. It is therefore, used where

Large starting torque is required like in Elevators and Electric Traction.

The load is subjected to heavy fluctuations and the speed is automatically required on sewing machines etc.

Industrial Use : - Electric traction, brands, elevators, air compressors, vacuum cleaners, hair drier, sewing machines etc.

Compound Motors

D. C. Compound Motor is of two types. It is therefore, used where, specification required for particular motor

Differential-compound motors are rarely used because of their poor torque characteristics.

Cumulative-compound motors are used where a fairly constant speed is required with irregular loads or suddenly applied heavy load

Industrial Use: Presses, Shears, Reciprocating Machines etc.

Brushed (Field Energized) Motors (Motors using wound rotors)

The traditional DC motor needs two current supplies, one through the stator windings to provide the magnetic field and the other through the rotor windings to interact with the magnetic field to generate the motive force. There are three ways of accomplishing this, each one resulting in unique characteristic motor performance. Because they all use wound rotors, they all need a commutator to feed the current into the rotor windings.

Speed is controlled by varying the rotor voltage and hence the rotor current, or by varying the magnetic flux in the air gap by changing the current in the field windings.

With access to both the field and rotor windings, all DC motors offer the facility of simple speed and torque control.

Series Wound

The series wound motor has only one voltage supply to the motor and the field winding is connected in series with the rotor winding.

Characteristics

The series motor has poor speed regulation. It delivers increasing torque with increased motor current but this is at the expense of speed which falls with increasing torque demands.

This motor has a very high starting torque because there is zero back EMF at zero speed however as the speed builds up so does the back EMF causing a reduction in torque.

Increasing the load on the motor tends to slow it down, but this in turn lowers back EMF and increase the torque to accommodate the load.

Speed control is possible by varying the supply voltage.

Under no load conditions the speed will accelerate to dangerous levels possibly causing destruction of the motor. The motor can be reversed by reversing the connections on either the field or the rotor windings but not both.

Regenerative braking is not possible since the field current needs to be maintained but it collapses when the rotor current passes through zero and reverses.

Applications

The series DC motor is an industry workhorse for high and low power, fixed, and variable speed electric drives.

Applications range from cheap toys to automotive applications.

They are inexpensive to manufacture and are used in variable speed household appliances such as sewing machines and power tools.

Its high starting torque makes it particularly suitable for a wide range of traction applications.

Shunt Wound

The shunt wound motor also has only one voltage supply to the motor but in this case the field winding is connected in parallel with the rotor winding.

Field Weakening

The speed of a shunt wound motor can be controlled to a limited extent without affecting the supply voltage, by "field weakening". A rheostat in series with the field winding can be used to reduce the field current. This in turn reduces the flux in the air gap and since the speed is inversely proportional to the flux, the motor will speed up. However the torque is directly proportional to the flux in the air gap so that the speed increase will be accompanied by a reduction in torque.

Characteristics

The shunt wound motor turns at almost constant speed if the voltage is fixed. The motor can deliver increasing torque, without an appreciable reduction in speed, by increasing the motor current.

As with the series wound motor, the shunt wound motor can be reversed by reversing the connections on either the field or the rotor windings.

Regenerative braking is possible. Self-excitation maintains the field when the rotor current reverses.

Applications

Fixed speed applications such as automotive windscreen wipers and fans.

Separately Excited

The separately excited motor has independent voltage supplies to the field and rotor windings allowing more control over the motor performance.

Characteristics

The voltage on either the field or the rotor windings can be used to control the speed and torque of a separately excited motor.

Applications

Train and automotive traction applications

Permanent Magnet Motors

As the name implies, these motors use permanent magnets rather than electromagnets to provide either the rotor or the stator field. They are used extensively in small DC motors and to an increasing extent in traction applications.

Rotor Magnets

These are by far the most common types of permanent magnet motors. They have no rotor windings but use permanent magnets to supply the rotor field and they behave like shunt wound DC motors with a fixed shunt current.

Their major advantage is the elimination of the commutator.

Field Magnets

These motors have no field winding but use permanent magnets to provide the magnetic field. Current is still supplied to the rotor via commutator as in other brushed motors and the speed can be controlled by varying the voltage on the rotor windings. In this way their behavior is similar to a series wound DC motor.

Universal Motors

In a series wound DC motor, reversing either the field winding leads or the rotor winding leads will reverse the direction of the motor. However, simply reversing the leads from the power supply will have no effect on the direction of rotation since it is equivalent to reversing the current through both the individual windings - in effect a double reversal. In other words the motor will turn in the same direction even though the current through the series windings is reversed. This means that the motor can run on alternating current as well as direct current since the direction of rotation is independent of the direction of the current through the series windings.

Universal motors are often used in power tools and household appliances such as vacuum cleaners and food mixers.