ELECTRIC HEATING AND WELDING

ELECTRIC HEATING

When current is passed through a conductor, the conductor becomes hot. When a magnetic material is brought in the vicinity of an alternating magnetic field, heat is produced in the magnetic material.

Similarly it was found that when an electrically insulating material was subjected to electrical stresses, it too underwent a temperature rise (Dielectric heating).

There are various method of heating a material but electric heating is considered to be far superior for the following reasons:

(i)Cleanliness:

Due to complete elemination of dust and ash, the charges to maintain cleanliness are minimum and the material to be heated does not get contaminated.

(ii)Ease of control:

With the help of manual or automatic devices, it is possible to control and regulate the temperature of a furnace with great ease.

(iii)Uniform heating:

Whereas in other forms of heating a temperature gradient is set up from the outer surface to the inner core.

The core being relatively cooler, in case of electric heating, the heat is uniformly distributed and hence the charge is uniformly heated.

(iv)Low attention and maintenance cost:

Electric heating equipments normally do not require much attention and maintenance is also negligible.

Hence labour charges on these items are negligibly small as compared to alternative methods of heating.

Requirement of Heating Material

i) Low Temperature Coefficients of Resistance

Resistance of conducting element varies with the temperature, this variation should be small in case of an element.

Otherwise when switched ON from room temperature to go upto say 1200˚C, the low resistance at initial stage will draw excessively high currents at the same operating voltage.

ii)Resistance coefficient Positive

If temperature is negative the element will draw more current when hot. A higher current means more voltage, a higher temperature or a still lower resistance,

which can instability of operation.

iii)High Melting Point

Its melting point should be sufficiently higher than its operating temperature. Otherwise a small rise in the operating voltage will destroy the element.

iv)High Specific Resistance

The resistivity of the material used for making element should be high. This will require small lengths and shall give convenient size.

v)High Oxidizing Temperature

Its oxidizing temperature should higher than its operating temperature. Otherwise oxidised layers from the surface will flake off changing the resistance of the

filament and giving it a smaller life.

vi)Ductile

To have convenient shapes and sizes, the material used should have high ductility and flexibility.

It should not be brittle and fragile.

vii)Should with stand Vibration

In most industrial process quite strong vibrations are produced. Some furnaces have to open or rock while hot. The element material should withstand the

vibrations while hot and should not break open.

viii)Mechanical Strength

The material used should have sufficient mechanical strength of its own.

CLASSIFICATION OF METHODS OF ELECTRIC HEATING

(i)Power Frequency Method:

Direct resistance heating, indirect resistance heating, direct arc heating, and indirect arc heating.

(ii) High Frequency Heating:

Induction heating and dielectric heating.

Resistance Heating:

This method is based upon the I2R loss. Whenever current is passed through a resistive material heat is produced because of I2 R loss.

There are two methods of resistance heating. They are

i) Direct Resistance Heatingii) Indirect Resistance Heating

Direct Resistance Heating:

In this method of heating the material or change to be heated is taken as a resistance and current is passed through it.

The charge may be in the form of powder pieces or liquid. The two electrodes are immersed in the charge and connected to the supply.

In case of D.C or single phase A.C two electrodes are required but there will be three electrodes in case of three phase supply.

When metal pieces are to be heated a powder of high resistivity material is sprinkled over the surface of the charge to avoid direct short circuit.

The current flows through the charge and heat is produced. This method has high efficiency since heat is produced.

This method has high efficiency since heat is produced is charge itself. Though automatic temperature control is not possible in this method.

But it gives uniform heat and high temperature. One of the major application of the process is salt bath furnaces having an operating temperature between 500˚C to 1400˚C.

An immersed electrode type medium temperature salt bath furnace is shown in figure3.28.

The bath makes use of supply voltage across two electrodes varying between 5 to 20 volts.

For this purpose a special double wound transformer is required which makes use of 3Ф primary and single phase secondary. This speaks of an unbalanced load.

The variation in the secondary voltage is done with the help of an off load tapping switch of the primary side. This is necessary for starting and regulating the bath load.

Advantages :

High efficiency. It gives uniform heat and high temperature.

Application :

It is mainly used in salt bath furnace and water heaters.

Indirect resistance heating

In this method the current is passed through a highly resistance element which is either placed above or below the over depending upon the nature of the job to be performed.

The heat proportional to I2R losses produced in heating element delivered to the charge either by radiation or by convection.

Sometimes in case of industrial heating the resistance is placed in a cylinder which is surrounded by the charge placed in the jackes as shown in figure3.29.

The arrangement provides as uniform temperature. Automatic temperature control can be provided in this case. Both A.C and D.C supplies can be used for this purpose at full mains voltage depending

upon the design of heating element.

Application :

This method is used in room heater, in bimetallic strip used in starters, immersion water heaters and in various types of resistance ovens used in domestic and commercial cooking.

Arc Furnaces

There are two common types of arc furnaces: (1)Three-phase furnace and (2)Single phase furnace.

Three phase furnaces are used in the production of alloy steels. Single phase furnaces are used for the manufacture of gray iron casting also. Three phase furnaces are used for power ratings from 250KVA, 10,000KVA and

capacities upto 25 tonne. Generally graphite electrodes are used. As they are subjected to volatilization, they are to

be replaced. The arc temperature is between 3000 and 3500˚C, so that the process is carried out

between 1500˚C and 2500˚C. The main components of a three phase furnace are:

1)Variable ratio power transformer

2)Reactors

3)Automatic current regulator

4)Control panel

5)Electric motor and tilting motor

6)Circuit breaker and connecting switches.

The chamber in which arc is struck is placed on a metal frame work. The chamber is lined inside with a refractory linning, which is acidic or basic in nature.

The electrodes arc inserted from the top or sides of the chamber, and are placed in such a way as to be replaced easily or adjusted easily.

To have a through mixing, the furnace is made amenable for tilting.

Direct arc furnace

The arc is struck directly with the charge, when a current flows through it and produces intense heat, which results, in high temperature.

Although some furnaces up to 100 tonne are made, generally furnaces up to 25 tonne are in general use.

Stirring action is automatic and gives a uniform product. It is used for alloy steel manufacture and gives a purer product.

Merits:

When compared with cupola method,

It produces purer products It is very simple and easy to control the composistion of the final product during

refining process.

Demerits:

It is very costlier. Eventhough it is used for both melting and refining but wherever electric energy is

expensive it is economical to use cupola for melting and arc furnace for refining.

Application:

The most common application of this type of furnace is to produce steel.

Indirect arc furnace

Electrodes are inserted from the sides and the heat produced is transmitted by radiation to the charge.

As there is no inherent stirring action, the furnace should be rocked.

This furnace is used for only single phase supplies. Also the capacity of the furnace is limited up to 100 tonne.

The furnace is rocked thoroughly to ensure, that the metal will cover the refactory lining and prevent it from reaching high temperatures.

Melting of non-ferrous metals is mostly carried out in this type of furnace. In both the type of furnaces, large quantities of electrodes are used. The energy used is about 500-800kw/tone corresponding to maximum power input, the

power factor is 0.87 and efficiency 70%.

Application:

The main application of this type furnace is melting of non-ferrous metals.

Induction heating:

Induction heating processes make use of currents induced by electromagnetic action in the material to be heated.

Induction heating is based on the principle of transformers. There is a primary winding through which an a.c current is passed.

The coil is magnetically coupled with the metal to be heated which acts as secondary. An electric current is induced in this metal when the a.c current is passed through the

primary coil.

The following are different types of induction furnaces

1. Core type and 2. Coreless type

Core type is classified into three types. They are

a) Direct core typeb) Vertical core type and c) Indirect core type

Direct core type:

The direct core type induction furnace is shown ion fig. It consist of an iron core, crucible and primary winding connected to an a.c supply. The charge is kept in the cruicible, which forms a single turn short circuited secondary

circuit. The current in the charge is very high in the order of several thousand amperes. The

charge is magnetically coupled to the primary winding. The change is melted because of high current induced in it. When there is no molten

metal, no current will flow in the secondary. To start the furnace molten metal is poured in the oven from the previous charge.

This type of furnace has the following drawbacks.

The magnetic coupling between the primary and secondary is very weak, therefore the leakage reactance is very high. This causes low power factor.

Low frequency supply is necessary because normal frequency causes turbulence of the charge.

If current density exceeds about 5 amps/mm2 the electromagnetic force produced by this current density causes interruption of secondary current.

Hence the heating of the metal is interrupted. It is called pinch effect. The crucible for the charge id of odd shape and inconvenient from the metallurgical point

of view. The furnace cannot function if the secondary circuit is open. It must be closed. For starting the furnace either molten metal is poured into the crucible

or sufficient molten metal is allowed to remain in the crucible from the previous operation.

Such furnace is not suitable for intermittent services.

AJAX WYATT Vertical core type furnace:

It is modified type of core type induction furnace. It has a vertical channel for the charge, thus the crucible used is also vertical. The

construction of ajax wyatt vertical furnace is shown in fig. The principle of operation is that of a transformer in which the secondary turns are

replaced by a closed loop of molten metal. The primary winding is placed on the central limb of the core.

Hence leakage reactance is comparatively low and power factor is high. Inside of the furnace is lined with refactory depending upon the charge.

The top of the furnace is covered with an insulated cover which can be removed for charging. Necessary arrangements are usually made for titling the furnace to take out the molten metal.

The molten metal in the ‘V’ portion acts as a short circuited secondary. When primary is connected to the a.c supply, high current will be accumulated at the bottom and even a small amount of charge will keep the secondary completed.

Hence chances of discontinuity of the circuit is less.

Advantages:

High efficiency and low operating cost. Since both primary and secondary are on the same central core, its power factor is better. The furnace is operated from the normal supply frequency. Chances of discontinuity of the secondary circuit is less, hence it is useful for intermittent

operations.

Applications:

This furnaces is used for melting non ferrous metals like brass, zinc, tin, bronze, copper etc.

Indirect core type induction furnace

Indirect core type induction furnace is shown in fig. I n this type of furnace induction principle has been used for heating metals.

In such furnace an inductively heated element is made to transfer its heat to the change by radiation.

It consists of an iron core linking with the primary winding and secondary. In this case secondary consists of a metal container forming the walls of the furnace.

When the primary winding is connected to the supply, current is induced in the secondary of the metal container.

So heat is produced due to induced current. This heat is transmitted to the charge by radiation.

The portion AB of the magnetic circuit is made up of a special alloy and is kept inside the chamber of the furnace.

The special alloy will loose its magnetic properties at a particular temperature and the magnetic properties are regained when the alloy will cooled.

As soon as the furnace attains the critical temperature the reluctance of the magnetic circuit increases many times and the inductive effect correspondingly decreases thereby cutting off the heat supply.

The bar AB is removable type and can be replaced by other, having different critical temperature. Thus the temperature of the furnace can be controlled very effectively.

Coreless induction furnace:

Coreless induction furnace also operates on the principle of transformer. In this furnace there is no core and thus the flux density will be low.

Hence for compensating the low flux density, the current supplied to the primary should have sufficiently high frequency.

The flux set up by the primary winding produces eddy currents in the charge. The heating effect of the eddy currents melts the charge.

Stirring of the metals takes place by the action of the electromagnetic forces. Coreless furnace may be having conducting or non conducting containers.

Fig shows a coreless induction furnace in which container is made up of conduting material.

The container acts as secondary winding and the charge can have either conducting or non conducting properties.

Thus the container forms a short circuited single turn secondary. Hence heavy current induced in it and produce heat. This heat produced is transferred to the charge by convection.

To prevent the primary winding from high temperature, refactory linings are provided between primary and secondary windings.

Fig shows a coreless induction furnace in which the container is made of ceramic material and the charge must necessarily have conducting properties.

The flux produced by the primary winding produces eddy currents in the charge. The heating effects of the eddy currents melt the charge.

Stirring action in the metals takes place by the action of the electromagnetic forces.

Advantages:

Time taken to reach the melting temperature is less. Accurate power control is possible. Any shape of crucible can be used. The eddy currents in the charge results in automatic stirring. Absence of dirt, smoke, noise, etc. Erection cost is less.

Dielectric heating:

Dielectric heating is also sometimes called as high frequency capacitance heating. If non metallic materials ie, insulators such as wood, plastics, china clay, glass, ceramics

etc are subjected to high voltage A.C current, their temperature will increase in temperature is due to the conversion of dielectric loss into heat.

The dielectric loss is dependent upon the frequency and high voltage. Therefore for obtaining high heating effect high voltage at high frequency is usually employed.

The metal to be heated is placed between two sheet type electrodes which forms a capacitor as shown in fig. The equivalent circuit and vector diagram is also shown in fig.

When A.C supply is connected across the two electrodes, the current drawn by it is leading the voltage exactly 90˚.

The angle between voltage and current is slightly less than 90˚, with the result that there is a inphase component of the current (IR).

This current produces power loss in the dielectric of the capacitor. At normal supply frequency the power loss may be small.

But at high frequencies, the loss becomes large, which is sufficient to heat the dielectric.

Advantages:

Uniform heating is obtained. Running cost is low. Non conducting materials are heated within a short period. Easy heat control.

Applications:

For food processing. For wood processing. For drying purpose in textile industry. For electronic sewing.

Welding:

Welding is the process of joining two similar metals by heating. The metal parts are heated to melting point. In some cases the pieces of metal to be joined are heated to plastic stage and are fused together.

Electric welding:

In electric welding process, electric current is used to produce large heat, required for joining two metal pieces. There are two methods by which electric welding can be carried out. These are

1. Resistance welding and2. Arc welding.

Types of electric welding

1. Resistance welding

a) Butt weldingb) Spot welding c) Seam welding d) Projection welding e) Flash welding

2. Arc welding a) Carbon arc welding b) Metal arc weldingc) Atomic hydrogen arc welding d) Inert gas metal arc weldinge) Submerged arc welding.

Resistance welding:

In resistance welding heavy current is passed through the metal pieces to be welded. Heat will be developed by the resistance of the work piece to the flow of current.

The heat produced for welding is given by

H=I2Rt

Where,

H= Heat developed at the contact area.

I= Current in amperes.

R= Resistance in ohms.

t= time of flow of current.

The fundamental block diagram for resistance welding is shown in fig. The A.C supply is given to the primary winding of the transformer through a controlled

contactor. The welding transformer is a step down transformer. The secondary voltage is in the

order of 1 to 10 volts. But the current may range from 50 to 1000 amperes.

i) Butt welding: In this process heat is generated by the contact resistance between two components. In this type of welding the metal parts to be joined end to end as shown in fig. Sufficient

pressure is applied along the axial direction.

A heavy current is passed from the welding transformer which creates the necessary heat at the joint due to high resistance of the contact area.

Due to the pressure applied, the molten metal forced to produce a bulged joint. This method is suitable for welding pipes, wires and rods.

ii) Spot welding:

Spot welding is usually employed for joining or fabricating sheet metal structure. This type of joint only provides mechanical strength and is not air or water tight.

Spot welding arrangement is shown in fig. The plates to be welded are placed overlapping each other between two electrodes, sufficient mechanical pressure is applied through the electrodes. The welding current flows through electrodes tips producing a spot weld. The welding current and period of current flow depend on the thickness of the plates.

Arc welding:

An electric arc is the flow of electric current through gases. An electric arc is struck by short circuiting two electrodes and then with drawing them

apart by small distance. The current continue to flow across the small gap and give intense heat. The heat developed by the arc is also used for cutting of metal.

Carbon arc welding:

In this process D.C is usually employed. The electrode is made of carbon or graphite and is to be kept negative with respect of the

work. The work piece is connected to positive wire as shown in fig. Flux and filler are also

used. Filler is made up of similar metal as that of metal to be welded. If the electrode is made positive then the carbon contents may flow into the weld and

cause brittleness. The heat from the arc forms a molten pool and the extra metal required to make the weld

is supplied by the filler rod. This type of welding is used for welding copper and its alloy.

Metal arc welding:

In metal arc welding a metal rod of same material as being welded is used as an electrode.

The electrode also serves the purpose of filler. For metal arc welding A.C or D.C can be used.

Electric supply is connected between electrode and work piece. The work piece is then suddenly touched by the electrode and then separated from it a

little. This results in an arc between the job and the electrode. A little portion of the work and the tip of the electrode melts due to the heat generated by

the arc. When the electrode is removed the metal cools and solidifies giving a strong welded

joint.

--------------------------------------------------*****-----------------------------------------------------------------------------------------------**********************---------------------------------------

ELECTRIC DRIVES AND CONTROL

INTRODUCTION

An electric motor is a better prime move for driving mechanical load than hydraulic, steam or diesel engines as it is possible to control the performance of an electric motor is quite easy.

For obtaining electric drives, both A.C and D.C motors are used. However A.C system is preferred.

The utilization of electric energy is always advantageous as it is cheaper. It is easy to maintain the voltage at consumer premises within the prescribed limits and it

is possible to increase or decrease the voltage without appreciable loss of power.

Inspite of the advantages of A.C system sometimes it becomes essential to use D.C energy as industrial drive.

Electric drive

An electric drive is defined as a form of machine equipment designed to convert electrical energy into mechanical energy and provide electrical control of this process.

It is classified into three types. They are

1. Individual drive2. Group drive and 3. Multimotor drive

Advantages of electric drives

It is simple in construction and has less maintenance cost. Its speed control is easy and smooth. It is neat, clean and free from any smoke or flue gases. It requires less space. It can be installed at any desired convenient place. It has comparatively longer life. It can be started immediately without any loss of time. Transmission of power from one place to other can be done with the help of cables in

stead of long shaft etc. It can be remotely controlled. It has high efficiency.

Individual drive

Individual drive consist of single motor is used to drive one individual machine. Most of the industries use this type of drive. In some cases the motor, along with its control equipment, may form an integral part of

the machine, which results in better appearance, cleanliness and safety.

Advantages:

The machines can be installed at any desired position. If there is a fault in one motor other machines will not be affected since they are working

independently.

Each operator has a complete control of his machine. He can vary its speed, if necessary and stop while not in use. Thus no load losses can be eliminated.

Continuity in the production of the industry is achieved. Efficiency of the system is high.

Disadvantages:

The initial cost is high.

Group drive

A group drive consist of a single large motor, which operates a number of machines. The motor is mechanically connected to a long shaft. It is also called line shaft drive. The line shaft is fitted with multistepped pulleys and belts. The driven machines are connected to these pulleys and belts for their required speed. The fig shows the group drive.

Advantages:

When compared with the individual drive,

Its initial cost is less. Only less space is required. It requires little maintenance. In this drive all the operation can be stopped simultaneously.

Disadvantages:

When the motor fails all the operations will be stopped. If most of the machines are idle the main motor will operate on load with less efficiency. Noise level in this drive is quite high. It has low power factor. It is not possible to install any machine at a distance place. Speed control of individual machine is not possible.

Multimotor drive

In multimotor drives separate motors are used for operating different parts of the same mechanism.

Eg in case of an overhead crane, different motors are used for hoisting, long travel motion and cross travel motion.

Such drive is also essential in complicated metal-cutting machine tools, paper making machines, rolling mills.

Fig shows a multimotor drive.

Selection of motors

An industrial process needs a particular electric drive for its successful and efficient operation which in turn calls for appropriate selection of the driving motor.

While selecting a motor, the following factors must be taken into consideration:

a. Electrical characteristics Running characteristics Starting characteristics Speed control Braking

b. Mechanical characteristics Types of enclosures Bearings Transmission of drive Noise level

c. Size of motor and Continuous rating Intermittent or variable load rating Over load capacity Pull out torque

d. Cost Capital cost Running cost

The first three are the technical factors and the last one is the economic factor. Many a time, there are conflicts between the technical and economic factors, but in any

commercial organization, the economic factor overpowers the technical factors as the correct choice of a motor is one which gives the required service at the minimum overall cost.

Since the load on a motor is an integral part of the drive system we study various types of loads.

It is essential that the motor characteristics match with those of the load for stable operation of the system.

Electrical characteristics

Running characteristics

The running characteristics of a motor include the following speed- torque or speed-current characteristics, losses, magnetizing current, efficiency and power factor at various loads.

The magnetizing current and power factor are to be considered in case of A.C motors only.

Starting characteristics

The starting torque developed by a motor should be sufficient to start and accelerate the motor at its load to the rated speed in a reasonable time.

Some motors may be have to start against full load torque. E.g motors driving grinding mills or oil expellers, traction work etc.

At the time of starting a motor, two torques come into play.

The torque required to overcome the static friction and The torque necessary to accelerate the motor and its load to the desired speed.

Starting characteristics of D.C motors

The starting characteristics of D.C motor is the relation between the torque and the armature current.

The torque of a D.C motor is proportional to the product of field flux (Ф) and armature current (Ia).

i.e., T α ФIa

Where

Ia= armature current

Ф= field flux

D.C shunt motor

In DC shunt motor, the field current is constant from no load to full load. Therefore the field flux Ф also constant.

Hence the starting torque is directly proportional to the armature current i.e. (T α Ia). Fig shows the torque current characteristics of D.C shunt motor.

D.C Series motor

In D.C series motor, the field winding is connected in series with the armature. Hence the field current, armature current and load current is same (ie Ia=Ise=IL). hence

field flux and armature flux also same ie., Фse= Фa. Since the series field flux is proportional to the armature current upto saturation point,

the torque produced is proportional to the square of the armature current up to saturation point.

Hence up to OA, the torque current characteristics is in parabolic shape.

ie T α ФIa

upto saturation point, field flux Ф α Ia

Hence T α Ia2

After saturation, the series field flux remains constant. Hence the torque is directly proportional to Ia (T α Ia). hence after A, the torque current

characteristics is a straight line. Since the starting torque is directly proportional to square of armature current, and the

starting torque of D.C series motor is very high. So it can be used where large starting torque is required such as in electric trains, cranes,

lifts and hoists.

D.C compound motor

There are two types of compound motor namely,

1. Cumulative compound motor2. Differential compound motor In cumulative compound motor the series field flux add with the shunt field flux. Hence

the total flux is higher than that of the shunt motor. So the torque developed in this motor is more than that of shunt motor for the same

armature current. In differential compound motor, the series field flux opposes the shunt field flux. Hence

the total flux is lesser than that of the shunt motor as shown in fig.

Starting characteristics of three phase induction motor

Squirrel cage motor

During the starting period, the squirrel cage induction motor has low starting torque and take high starting current.

The condition for maximum starting torque is R2=X2. During the starting period X2 is higher in compare to rotor resistance R2.

Therefore if the rotor resistance R2 increases the starting torque also increases(since T α R2).

It is not possible to increase the rotor resistance on squirrel cage induction motor.

Double cage rotor

The starting torque of a cage motor is increased by providing double cages. The outer cage is made of high resistance metal bars whereas inner cage is made of low

resistance copper bar. The inductance of the inner bar is higher than that of outer. Fig shows the double cage

rotor. At the time of starting, the motor induced current is at the line frequency and hence inner

cage has a high reactance(X2=2 f’L).

Therefore, the rotor current will flow through the outer cage, with the result that the starting torque is high (since T α R2).

During normal running the reactance of the inner cage decreases (since rotor current frequency f’ is decreased) and hence the rotor current flows through the low resistance inner cage.

This gives a high efficiency of the motor.

Slip ring Induction motor

In slip ring induction motor, extra resistance can be added in the rotor circuit during the starting period.

Hence a high starting torque is produced. In addition, it also limits the starting current.

Starting characteristics of synchronous motor

It has no self starting torque. It runs at synchronous speed. The following methods are used to provide the starting arrangement.

i. DC motor coupled to synchronous motor.ii. Pony motor(small I.M.) coupled to synchronous motor.

iii. Provide damper winding on rotor.

Starting characteristics of single phase induction motor

Single phase induction motor is not self starting. It requires some provision for starting. An extra winding known as starting winding is

provided on the stator. The main winding is of high reactance and low resistance. The starting winding is of high resistance and low reactance. They are connected across

the supply. This type of motor is called split phase motor. When the motor picks up the speed at 75%

of synchronous speed, a centrifugal switch is open and disconnects the starting winding. This motor has a very low starting torque. If a capacitor is used for spilt the phase at starting then it is called capacitor start motor .

the main winding is connected directly across the line. The starting winding is connected in series with the capacitor through centrifugal switch

and connected across the single phase supply. Such an arrangement gives a high starting torque. In permanent capacitor motor the capacitor remains in the circuit during starting and

running.

Running characteristics of motors

The running characteristics of a motor include the speed-torque or the speed-current characteristics, losses, magnetizing currents, efficiency and power factor at various loads.

The magnetizing current and power factor are to be considered in case of A.C motors only.

Running characteristics of D.C motors

D.C shunt motor

a) speed current characteristics

In any D.C motor N α (Eb / Ф). When the supply voltage is constant, in DC shunt motor Ф is flux is constant.

N α Eb

N α V-Ia Ra

This indicates that speed of D.C shunt motor decreases with increase in armature current due to loading.

The variation of speed with armature current characteristics is drooping slightly as shown in fig.

The percentage of speed change will be about 5% at full load due to armature resistance drop. But due to armature reaction, the flux is weakened.

Hence the speed will increase. (N α (Eb / Ф)). This increase in speed compensates the drop in speed due to Ia Ra drop.

Therefore the shunt motor is considered as constant speed motor.

b) Speed-Torque characteristics

We know T α ФIa and

N α (Eb / Ф).

In shunt motor field flux Ф= constant

T α Ia ----------------------------- (1)

Ia= KT

N α Eb

N α (V-IaRa) -------------------------- (2)

Put Ia value in equation (2)

N α V-(KT)Ra ---------------------------- (3)

From equation (3) we know that, when the torque increases, speed decreases as shown in fig.

Performance curve

Fig shows the performance curves of D.C shunt motor. These curves are namely torque, speed, current and efficiency., each plotted against output power.

D.C series motor

a) Speed- current characteristics

Consider the speed equation

N α Eb/Ф

N α (V- IaRa)/ Ф

When supply voltage V is kept constant, the speed of the motor will be inversely proportional to flux N α (1 / Ф).

On the light loads the flux produced will be weak and therefore the speed will be dangerously high.

For small value of flux Ф, the speed will be very high. Hence the shape of the curve will be hyperbolic.

When the load current increases, the flux also increases, after saturation the flux remains constant.

Therefore the speed will be constant and low at heavy loads as shown in fig.

b) Speed- Torque characteristics

In any D.C motor N α (V-IaRa)/ Ф

If IaRa drop is negligible N α V/ Ф -------------------------- (1)

We know that, T α Ф Ia

T α Ф. Ф (since Ia α Ф)

T α Ф2

Ф2 =T

Ф2 = √T ------------------------- (2)

Substitute the equation (2) in (1)

N α V/√T

From the equation, speed is inversely proportional to torque. Hence the characteristics curve is hyperbolic in shape.

This is shown in fig. In D.C series motor, as torque increases with decrease of speed. Hence series motor is

suitable for operating cranes, lifts, trains etc.

Performance curve

Fig shows the perfoemance curve of a D.C series motor. These curves are namely torque, speed, current and efficiency each plotted against

output power.

D.C compound motor

Speed – current characteristics

A compound motor has both series field and shunt field.

Compound motors are of two types. If the series field flux and shunt field flux add each other, it is called cumulative

compound motor. If the series field flux opposes the shunt field flux, it is called differential compound

motor. In the cumulative compound motor, the series field emf increases with increase in

armature current. Hence cumulative compound motor has more flux than that of shunt motor.

In any D.C motor, T α Ф Ia

Hence torque of cumulative compound motor is greater than the shunt motor. Since the speed is inversely proportional to flux N α (1 / Ф) cumulative compound motor

has lower speed than the shunt motor. In the case of differential compounded motor the field flux decreases when the armature

current increases, which reduces the torque. (since T α Ф Ia).

But the speed increases with reduction flux ( since N α (1 / Ф)). Hence the speed is greater when compared to shunt motor. The speed Vs armature current and speed torque characteristics of D.C compound motors

are shown in fig. in comparison with the shunt motor.

Running characteristics of three phase induction motor

Running characteristics of squirrel cage induction motor or speed torque characteristics In cage induction motor

Torque (T) = KSE22 R2/ R2

2 +X2

2

Where

k = constant

S = slip

E2 = e.m.f induced in the rotor

R2 = rotor resistance

X2 = rotor reactance

Under normal running condition the rotor frequency (f’=Sf) is small. Hence the rotor reactance (X2= 2πf’L) is also very small. Hence the rotor reactance (X2) is neglected.

T α K1SE22 R2/ R2

2

i.e., T α KSE22/ R2

Since the supply voltage Vis constant, E2 is also constant. Hence the running torque of the motor depends upon the rotor resistance. From the above equation the running torque is inversely proportional to the rotor

resistance R2. Hence at lower value of slip, increasing the running torque the rotor resistance R2 should

be very low. Since the cage motor rotor is short circuited, the rotor resistance is very low. Hence cage

induction motor has good running torque. For various values of R the family of speed torque characteristics shown in fig. when the

load on the motor increases the rotor speed falls down. Then the slip value increases. The torque increases with increase in slip upto rated load. The torque will reach a maximum value at slip S=R2/X2. After the rated load, the

increased load on the motor will increase the slip and on the decrease the torque. Any further more increase in load on the motor results, the motor slowing down and it

finally stops. The stable operating region of the motor lies for the slip values S=0 and that corresponds

to maximum torque. The operating region is hatched in fig.

Performance curve

Fig shows the performance curve of three phase squirrel cage induction motor namely slip, current, power factor, efficiency and speed each plotted against power output.

Running characteristics of slip ring induction motor

The running characteristics of slip ring induction motor are same as squirrel cage induction motor.

By introducing resistance in the rotor circuit at running, the torque can be increased.

Running characteristics of double squirrel cage induction motor

The motor is designed to provide improved starting characteristics (i.e. high starting torque with low starting current).

Inner cage has high inductance and low resistance whereas outer cage has high resistance and low inductance.

At the time of starting inner cage offers high reactance. Because the frequency of rotor current is very high. (since at starting slip=1, hence frequency of rotor current f’ increases, since f’ = s f).

Hence most of the current flows through outer cage where resistance is high. Thus more starting torque is developed.

After the motor has picked up its full speed, the frequency of rotor current becomes very low.

Therefore most of the current flows through the inner cage. Hence at running, copper losses are reduced and the efficiency of motor is increased.

The speed- torque characteristics of double cage induction motor are shown in fig.

Running characteristics of single phase induction motor

The speed torque characteristics is similar to three phase induction motor. Fig shows the speed torque characteristics of single phase induction motor. It has no self

starting torque. Separate arrangement is provided to make it self starting. The repulsion start and capacitor start motors are the most common types of single phase

induction motors. Single phase induction motors are used in domestic appliances like fans, refrigerators,

vacuum cleaners etc.

Running characteristics of universal motor.

Universal motor operates on either A.C or D.C supply. Its speed torque characteristics are same as series motor speed-torque characteristics. Fig shows the speed torque characteristics. Universal motors are used in vacuum cleaners, sewing machines, portable drills and other

small power drives.

Speed control

In D.C motor the speed can be controlled by following methods

Armature control method Field control method

In A.C motors, the speed can be controlled by following methods

By changing the supply voltage By changing the supply frequency By changing the no of poles of motor By injecting emf in the rotor circuit By cascading of motors

By injecting resistance in the rotor circuit

Braking

When the load is removed from an electric motor and supplied to it be disconnected it will continue to run for sometime due to inertia.

To avoid danger to the worker or damage to the products manufactured quick stopping of motor is required. It is done by braking.

The braking system should be reliable and quick in action. The braking torque must be controllable.

There are two types of braking.

i) Mechanical brakingii) Electrical braking

Mechanical characteristics of electric motor

While selecting a motor for a particular drive, the mechanical characteristics are also taken into account.

The following features determine the suitability of the motor.

1. Types of enclosures2. Bearings 3. Noise4. Transmission of drive

Types of enclosures

All the major parts of the motors such as windings, bearings, insulation etc are to be protected from the surroundings contaminated air.

In an industry the air surrounding the motors may contain metal, dust, oil, mist, water, dust inflammable fumes etc. also accidents may occur to persons coming in contact with the moving parts.

Therefore it is necessary to provide proper enclosures. The different types of enclosures are as follows.

a) Open type

This type can only be used where the atmosphere and surroundings are free from all contaminations and surrounding air completely dry.

The advantage of this type of motor is that the cost of cooling is very low. But this type is rarely used since there is no protection to the motor parts.

b) Screen protected type In this type of machines openings provided for ventilation are covered with

wire mesh screen. This type of enclosures does not protect the motor against dirt and dust. But larger bodies and big insects cannot enter into the machine.

c) Drip proof type This motor has ventilating opening provided in such a way that drops of liquid

or solid falling on it vertically are prevented to enter inside. This type of motor cannot be used where inflammable dust particles are

present in the surrounding air. Such motors are used in damp atmosphere. E.g Pumpsets.

d) Totally enclosed type This type of motors has solid frames and end shields but no opening for

ventilation. They are cooled by surface radiation only. In this type machines no dirt or

foreign matter can enter and block the air passage. These machines are used for very dusty atmosphere. E.g saw mills, coal handling plants and stone crushing quarries.

e) Splash proof type In this type, the ventilation ducts are provided in such a way that drops of

liquid or solid particles reaching the machine at any angle between vertical and 100˚ from it cannot enter the machine.

f) Flame proof type These enclosures do not communicate an internal fire to the external

environment. Hence these motors are used in coal mines, gas plants, oil refineries etc., where the risk of fire is more.

g) Pipe ventilated type Large sizes of totally enclosed motor employ pipe ventilation. Air is drawn through pipe from outside the building, where clean air is

available and forced to cool the motor.

Bearings

Bearings are the parts of machines which house and support the main shaft.

It provides free rotation of the moving parts with minimum friction. There are two types of bearings usually employed in motors.

1. Ball or Roller bearing2. Sleeve or brush bearing

Ball or Roller bearing

Ball or roller bearing consist of an inner and outer race and cage containing steel roller or balls.

The outer race is attached to the housing(end cover) and the inner race is attached to the shaft.

When the shaft rotates, the steel ball also rotates. Hence the friction of the shaft is minimized.

It has a longer life and maintenance costs are low. It occupy less space. But the initial cost of ball and roller bearings is high. It is used in three phase induction motor where smaller air gap is possible. It is used for chain, belt and gear drives.

Sleeve or brush bearing

Sleeve or brush bearings are normally made of bronze. The rotating shaft is supported by bearing component and is rigidly fixed to the frame of

the machine. It has self lubricating properties due to capillary action. It is lubricated by a metal ring freely rotating on the shaft carrying oil to the bearings. It is mainly used in direct coupled drive such as fan and universal motor. It gives noiseless operation and their life is long. Because of larger wear of bearings, this type of bearing is used in larger air gap induction

motor.

Transmission of drives

Various methods employed for transmission of mechanical power are described below.

1. Direct drive2. Belt drive

3. Rope drive4. Chain drive and 5. Gear drive

Direct drive

In direct drive, motor is coupled directly to the driven machine with the help of solid or flexible coupling.

Flexible coupling protects the motor from jerks. It is more efficient and requires minimum space and it is the simplest method. It can be used where driven and driving machine speed are same.

Belt drive

In belt drive, belt is used to transmit the power from motor to driven machine through pulley system.

The mechanical power wasted due to slip is about 3 to 4 percentage. Maximum power of 300 H.P can be transmitted through this drive.

Advantages

Greater flexibility in the original design of a plant is possible. It gives convenient speed ratio thereby high speed motors can be utilized. The tendency of slipping especially under heavy loads is reduced because it will absorb a

portion of the shock of suddenly applied loads.

Rope drive

This method for transmission for power is used, when it is not possible to employ belt drive.

A number of ropes run in V-grooves over pulleys. The advantages of rope drive are negligible slip and ability of taking sudden loads. It is mainly used in lift and cranes.

Chain drive

Chain drive is very costly in comparison to belt and rope drive. It can be used for high speed ratio (upto 6:1). It is more efficient and transmits large amount of power. It is noiseless, sliplesss and smooth in operation.

Gear drive

Gear drive is used when high speed motor is to drive a low speed machine. The coupling between the two is through a suitable ratio gear box.

Noise

Noise is the another important features to be considered while making the selection of a motor.

It should be kept as low as possible in the workshops, hospitals and other domestic purposes.

The noise may be due to bearing, vibrations, magnetic pulsations and faulty foundations. To reduce noise, journal bearing may be used in place of ball bearings. The motor should be mounted on a heavy concrete or cast iron block. The electrical connections should be made through flexible conduits.

Standard rating of motor

The rating of motor is the amount of power which it can deliver without becoming unduly hot. The rating of a motor is classified as follows.

1. Continuous rating.2. Intermittent rating or short time rating.

Continuous rating

This is the rating or the output of a motor which can be delivered continuously for long periods without exceeding the permissible temperature.

This rating is applicable to drives like fans, pumps, textile, mills etc. which operate continuously for long periods.

Intermittent rating or short time rating

This is an output that a motor can give for specified short time without exceeding the permissible temperature rise.

Such motor is loaded for short period of time and is then put off for sometime. During that period the motor cools off as in mixies.

Classes of load duty cycles

As per IS 4722 – 1968 various load time variations are encountered the eight standard classes of duty.

1. Continuous duty.2. Short time duty.3. Intermittent periodic duty.4. Intermittent periodic duty in the starting.

5. Intermittent periodic duty with starting and braking.6. Continuous duty with intermittent periodic loading.7. Continuous duty with starting and braking.8. Continuous duty with periodic speed changes.

Selection of motors for different duty cycles

Continuous duty

Continuous duty denotes the motor operation at a constant load torque to reach steady state temperature. The load time and temperature time graph are shown in fig.

Paper mill drives, compressors, conveyers, centrifugal pumps and fans are some examples of continuous duty.

Short time duty

It denotes the operation of motor at constant load for short period followed by rest to cool down to the original starting temperature.

Short time duty timings are generally 10, 30, 60 and 90 minutes. The load time and the temperature time graph are shown in fig. Crane drivers, drives for household appliances, sluice gate drives, valve drives and

machine tool drives are some examples of short-time duty.

Intermittent periodic duty

It denotes the operation of motor a sequence of indential duty cycle each of constant load and rest period.

In this duty, heating of machine during starting and braking operation is negligible. Fig shows the load time and temperature time graph. Pressing, cutting and drilling

machine drives are some examples of intermittent periodic duty.

Intermittent periodic duty with starting

This is intermittent periodic duty where heat losses during starting cannot be neglected. Thus it consists of a period of starting, a period of operation at a constant load and rest

period. The operating and rest periods are too short to attain the steady state temperature in one

duty cycle. Its characteristics are shown in fig. in this duty heating of machine during braking is

considered to be negligible. Some examples are metal cutting, drilling tool drives, mine hoist drives for lift trucks.

Intermittent periodic duty with starting and braking

This is the periodic duty where heat losses during starting and braking cannot be ignored.

Thus it consists of a period of starting, a period of operation with constant load, a braking period, and a rest period.

Thermal equilibrium is not reached in one duty cycle. Braking is done electrically and is quick. Its characteristics is shown in fig Several machine tool drives, drives for electric suburban trains and mine hoist are some

examples of this duty.

Continuous duty with intermittent periodic loading

The operation of motor has a sequence of indentical duty cycle, each consisting of a period of operation and a period of operation on no load.

Thermal equilibrium is not reached in one duty cycle. Its characteristics are shown in fig. This duty is distinguished from the intermittent periodic duty by a period of running at

constant load is followed by a period of running at no load instead of rest. Pressing, cutting, shearing and drilling machine drives are the examples.

Continuous duty with starting and braking

The operation of motor consists of period of starting, a period of operation at constant load a period of electric braking and there is no rest period.

The characteristics are shown in fig. Blooming mill is an example.

Continuous duty with periodic speed changes

Operation of the motor has a sequence of indentical duty cycle, each cycle is having a period of running at one load and speed and followed by another period of running at different speed and load.

There is no rest period. Its characteristics are shown in fig.

Drives for different industrial application

1. Paper mill- Synchronous motor A paper mill requires a drive which must fulfill the following requirements To manufacture different thickness of papers it is required to vary the speed of entire

series of rolls. Relative speed of rolls should be constant otherwise the paper may be tearing. It is required to adjust the speed at any one group of rolls relative to other in order to

draw the paper.2. Rolling mills or steel mills – separately excited DC motor

Separately excited DC motor is mainly used in rolling mills. The motor required for these mills should have high starting torque about 2 to 2.5

times the rated torque.

It should have strong construction. The ward leonard speed control of D.C motors or slip ring induction motors are used.

3. Textile mills – Double cage induction motor In textile mills group drive is employed. The motors employed must have high starting torque with constant speed. The motors used must be totally enclosed and moisture proof to prevent entry of dust

and moisture enter into machine. Hence totally enclosed, fan cooled, high torque double cage induction motors are

used.4. Cement mills

Various types of loads available in a cement factory and the motor used for them are given belowa) Hammer crusher – Three phase slip ring induction motor

The lime stones are broken into smaller sizes in the crushing mill. For this purpose high starting torque motor is required. Hence three phase slip ring induction motor is used because it has high

starting torque.b) Ball mills – Synchronous motor

In ball mills, the raw materials grind in powder form synchronous motor are used for this process.

c) Rotary driers – Slip ring induction motor The cement slurry is dried by blowers and speed of blower is varied

depending upon the amount of air required to blow. Hence slip ring induction motor with pole changing speed control is

employed.d) Slurring pumps and agitators – Three phase Squirrel cage induction motor

These are used in the wet process Three phase Squirrel cage induction motors used for slurring pumps and

agitators.5. Machine tools – D.C shunt motor or 3Ф Squirrel cage induction motor

The starting torque required is less in most of the machine tools since they start up light.

Therefore 3 phase squirrel cage induction motor is used for machine tool application.

Different speed operation is obtained by using two or three speed motor with suitable gear combination.

D.C shunt motors are used for machine tool application like planners where rapid reversal, and wide speed control are required.

In the case of grinders, totally enclosed motors are used to prevent metallic dust getting into it.

6. Lift and hoists – DC compound motor or 3Ф slip ring induction motor

The essential requirements for a lift are high overload capacity, high smooth accelerating torque of 2 to 2.5 times the full load torque at starting and maximum degree of silence.

D.C compound motor and three phase slip ring induction motor are used for lifts and hoists.

7. Belt conveyor – Double squirrel cage induction motor The conveyors are required to transport bulk materials like coal, are sand on either

flat belt or bucket system. It requires a high starting torque so as to accelerate the load for transport. Hence totally enclosed surface cooled motors are used. Double squirrel cage induction motors are used in belt conveyors.

8. Ship – Synchronous motors Three phase induction motors and synchronous motors are used for very big

ships. A three phase alternator gives the supply to the synchronous motor. The prime mover used for the alternator is steam turbine by varying the voltage

and frequency of alternator the speed of motor is controlled.9. Air compressor - 3 Ф Induction motor

Air compressors are used for pneumatic drill, 3Ф induction motors are used to drive compressors.

Repulsion motor is used for various industrial machinery air compressors. Single phase induction motor is used for small air compressors.

10. Punches and shears For punches and shears D.C cumulative compound motors and A.C 3 Ф slip ring induction motors provided with fly wheel are used.

11. Rotary printing The rotary printing machinery requires variable speed motor. D.C compound motors or A.C 3 Ф induction motors with rotor resistance control

are used for printing machineries.

12. PumpsCentrifugal pump

The load torque varies as square of the speed in a centrifugal pump. At starting the torque required is less. Hence 3 Ф squirrel cage induction motor is used for centrifugal pump. The liquid handled by the pump does not enter the motor. Hence totally enclosed motor is preferred.

Reciprocating pump

A reciprocating pump requires two times the full load torque at starting.

A double cage induction motor is suitable for reciprocating pump. 3 Ф slip ring induction motor is also used for this type of pump. D.C shunt motor is used where D.C supply is available.

13. Draught fan The single phase split phase induction motors are used for draught fan. The single phase split phase induction motor has shunt characteristics and so the

operating speed is almost constant.14. Ceiling fans

Single phase capacitor start and run motors are used for ceiling fan.15. Cranes – D.C series motor

The D.C series motors are used for cranes because they have high starting torque. Because they have high starting torque, which helps the motor to reach the speed

in a short time and also prevents the motor from stalling in case of heavy loads. A.C 3 Ф slip ring inductions are also used for cranes. For starting and special adjustments proper graded rotor resistance is used with

slip ring induction motor.

16. Mines The various loads in a mine are winders, ventilating fans, conveyors, compressors

and pumps. The winder consists of two cages and a rope for transporting material from

bottom of the mine to the surface. Acceleration and braking operations are repeated. A 3 Ф slip ring induction motor

with ward leonard speed control is used for winder. Ventilating fans are used for circulating fresh air. A 3 Ф squirrel cage induction

motor is used for ventilating fan if no speed control is not required. Conveyors require a high starting torque, so a double squirrel cage motor is used. Compressor is used to provide compressed air for pneumatic drills used for

mining operations. It requires shunt characteristics and so 3 Ф squirrel cage induction motor is used.

Centrifugal pumps are used to pump out the water falling through the rock layers. It requires high starting torque therefore a 3 Ф slip ring induction motor is used for pumps.

17. Domestic appliances – Universal motor or Single phase induction motor Small universal motor is used for various domestic appliances such as for

domestic refrigerators, shavers, vacuum cleaner, mixi, cloth washing machines etc.,

Choice of drive

Choice of drive is governed by the following factors:

(i) Speed of driving and driven machines(ii) Convenience(iii) Space available(iv) Clutching arrangement required(v) Cost

The choice of motor speed is the most important factor as it not only affects the performance of motor but also overall cost.

The dimension and, therefore, the first cost of a motor for a given output are approximately inversely proportional to the speed, so for the some output kW the cost of a high speed motor is less than that of a slow speed motor.

In case of induction motor, the efficiency and power factor decreases with decrease in speed.

Thus for a low-speed drive high speed motor using a reduction gear is usually found cheaper than a low-speed direct-coupled motor.

Power requirement calculation

1. Continuous duty and constant load: For most of the applications, the rating can be determined from the equation given

as under:P = TN/975η kW ---------------------------------- (1)

Where,

T = Load torque, kg-m,

N = Speed, r.p.m., and

η = Product of the efficiency of the driven equipment and that of transmitting device.

In case of linear motion, the rating of the motor required is given by,

P = F x v/2 x 102 η kW ------------------------------------ (2)

Where,

F = force caused by the load, Kg, and

v = velocity of motion of the load, m/s.

Equation (2) is directly applicable in case of hoisting mechanisms. It is also suitable for lifts or elevators, it should be modified as follows:

P = F x v/2 x 102 η kW

The velocity of normal passenger lift cabins vary from 0.5 to 1.5 m/s.

In case of pumps, the rating can be determined from the following relation: P = ρQH/102 η kW

Where,

ρ = Density of liquid pumped, kg/m3,

Q = Delivery of pumps, m3/s, and

H = Gross head (static head + friction head), m.

η varies from 0.8 to 0.9 for reciprocating pumps and from 0.4 to 0.8 for centrifugal pumps.

The rating of a fan motor is given by,

P = Qh/102 η kW

Where,

Q = volume of air or any other gas, m3/s, and

h = Pressure in mm of water or kg/m2.

For small power fans, the efficiency η may be taken as 0.6 and for large power ones it may reach a value up to 0.8.

The rating of a motor used in metal shearing lathes can be found from the relation.

P = F x v/ 102 x 60 η kW

Where,

F = shearing force, kg,

v = Velocity of shearing, m/min, and

η = Mechanical efficiency of the lathe.

2. Motor rating for variable load:

The following are the commonly used methods for determination of motor rating for variable load drives

(i) Method of average losses(ii) Equivalent current method(iii) Equivalent torque method(iv) Equivalent power method.

Method of average losses (Qav)

The method consists of finding average losses Qav in the motor when it operates according to the given load diagram.

These losses are then compared with Qnom, the losses corresponding to the continuous duty of the machine when operated at its nominal rating.

This method presupposes that when Qav = Qnom, the motor will operate without temperature rise going above the maximum permissible for the particular class of insulation.

In this case, Qm = Qav / Aλ = Qnom/ Aλ. The loss diagram of the electric motor is shown. The rating of the electric motor

can be found from method of successive approximations. The losses of the motor are calculated for each portion of the load diagram by

referring to the efficiency curve of the motor. The average losses are given by

Qav = Q1t1 + Q2t2 + Q3t3 +………. + Qntn/ t1 + t2 + t3 +…….+ tn ------------ (1)

The average losses as found from eqn (1) are compared with losses of selected motor at rated frequency.

In case the two losses are equal or differ by a small amount the motor is selected. However, in case the losses differ considerably, another motor is selected and the

calculations repeated till the motor having almost the same losses or the average losses is found.

This method is accurate and reliable for determining the average temperature rise of the motor during one work cycle.

The disadvantage of the method of equal losses is that it is tedious to work with and also many a times the efficiency curve is not readily available and the efficiency has to be calculated by means of empirical formulae which may not be accurate to work with.

Equivalent current method

This method is based on the assumption that the actual variable current may be replaced by an equivalent current Ieq which produces the same losses in the motors as the actual current.

Ieq = √(I12t1 + I2

2t2 + I32t3 + ……+ In

2tn) / ( t1 + t2 + t3 + ……+ tn) The heating and cooling conditions in self ventilated machines depend upon its

speed. At low speed the cooling conditions are poorer than at normal speeds. The equivalent current as found from eqn should be compared with the rated

current of the motor selected and the conditions Ieq < Inom should be met. The machine selected should also be checked for its overload capacity. For D.C motors………….. Imax/ Inom < 2 to 2.5 For induction motors ……… Imax/ Inom < 1.65 to 2.75 In case the overload capacity of the motor selected is not sufficient it becomes

necessary to select a motor of high power rating. It may not be easy to calculate the equivalent current especially in cases where the

current load diagram is irregular as shown in fig. The equivalent current in such cases is calculated from the following expression:

Ieq = √ [(1/1∑n ) t∫∑t i2 dt] The value of the integral may be found with the help of integral. The current values obtained by this method are sufficiently accurate for practical

purposes.

Equivalent torque and equivalent power methods

For the selection of suitable capacity of the motor it often becomes necessary to use torque or power load diagrams.

The equivalent torque or power is found in the same manner as the equivalent current.

Assuming constant flux and constant power factor, the torque is directly proportional to current and, therefore, the equivalent torque is:

Teq = √ [T12t1 + T2

2t2 + T32t3 +…. + Tn

2tn/ t1 + t2 + t3 +……..+ tn]

The equation for equivalent power follows directly from above eqn. as power is directly proportional to the torque.

At constant speed or where the changes in speed are small, the equivalent power is given by:

Peq = √ [P12t1 + P2

2t2 + P32t3 +…. + Pn

2tn/ t1 + t2 + t3 +……..+ tn]

The “equivalent current method” is the most accurate out of all the above methods discussed above.

This method may be used to determine the motor capacity for all uses except where it is necessary to take into account the changes in so ‘constant losses’ i.e. the iron and mechanical losses.

The “equivalent torque method” cannot be used for cases where equivalent current method cannot be applied.

It cannot be used for selection of motor rating for cases in which the field flux does not remain constant like D.C series motors and for squirrel cage induction motors under starting and braking conditions.

The disadvantage of the “equivalent power method” is that it cannot be used for motors whose speed varies considerably under load, especially when dealing with starting and braking conditions.

Power factor improvement

Apparent, Active (True or Real) and Reactive power and Power Factor

Every circuit has two components(i) Active component and(ii) Reactive component.

“Active component” consumes power in the circuit while “reactive component” is responsible for the field which lags or leads the main current from the voltage.

In fig active component is I active = I cos Ф, and reactive component is I reactive = I sin Ф.

I = √ [(I active) 2 + (I reactive) 2](i) Apparent power (S):

It is given by the product of r.m.s. values of applied voltage and circuit resistance.

S = VI = (I x Z) .I = I2Z volt-amperes (VA)(ii) Active or true or real power (P or W):

It is the power which is actually dissipated in the circuit resistance.P = I2R = VI cos Ф watts.

(iii) Reactive power (Q):

A pure inductor and a pure capacitor do not consume any power, since in a half cycle what so ever power is received from the source by these components the same is returned to the source.

This power which flows back and forth(i.e., in both directions in the circuit) or reacts upon itself is called “reactive power”.

It may be noted that the current in phase with the voltage produces active or true or real power while the current 90º out of phase with the voltage contributes to reactive power.

In a R-L circuit, reactive power which is the power developed in the inductive reactance of the circuit, is given as:

Q = I2XL = I2Zsin Ф = I. (IZ) sin Ф

= VI sin Ф volt-amperes-reactive (VAR)

These three powers are shown in fig Relation between VA, W and VR

W = VA cos Ф

VAR = VA sin Ф

VA = W/ cos Ф

VA = VAR/ sin Ф

Power factor (p.f) = W/VA = True power/ Apparent power The larger bigger units of apparent, true and reactive power are kVA (or MVA),

kW(or kW) and kVAR (or MVAR) respectively. The power factor depends on the reactive power component. If it is made equal to

the active power component, the power factor becomes unity.

Causes of low power factor

(i) All A.C motors (except overexcited synchronous motors and certain types of commutators motors) and transformers operate at lagging power factor.

(ii) Due to typical characteristics of the arc, are lamps operate at low power factor.

(iii) When there is increase in supply voltage, which usually occurs during low load periods (such as lunch hours, night hours etc.,) the magnetizing current of inductive reactances increases and power factor of the electrical plant as a whole decreases.

(iv) Arc and induction furnaces etc. operate at a very low lagging power factor.

(v) Due to improper maintenance and repairs of motors the power factor at which motors operate fall.

Advantages of power factor improvement

The installation of power factor improvement device, to raise the power factor results in one or more of the following effects and advantages:1) Reduction in investment in the system facilities per kW of the load supplied.2) Reduction in circuit current.3) Reduction in copper losses in the system due to reduction in current.4) Increase in voltage level at load.5) Improvement in power factor of the generators.6) Reduction in kVA loading of the generators and circuits.7) Reduction in kVA demand charges for large consumers.

Methods of power factor improvement

The various methods employed for power factor correction are:1) Use of static capacitors.2) Use of synchronous condensers.3) Use of phase advancers.4) Use of phase compensated motors.

The above methods of power factor improvement are discussed below:

Use of static capacitors:

It is known that static capacitor/ condenser takes current which leads the voltage by nearly 90º.

Thus if condenser is connected across an inductive load resultant quadrature component of the whole combination will be difference of leading component of condenser current (Ic) and lagging component of lead current (I sin Ф1) as shown in fig.

In view of reduced magnitude of quadrature component of current, p.f of the whole combination is improved from cos Ф1 to cos Ф2.

Power factor of the system can be improved by placing static capacitors in series with the liner as shown in fig.

Capacitors connected in series with the line neutralize the line reactance. The capacitors, when connected in series with the line, are called “series

capacitor”, and when connected in parallel with the equipment, are called “shunt capacitors”.

Shunt capacitors are used in factories, plants and also on transmission lines. Series capacitors are used on long transmission lines as they provide automatic

compensation with the variations in load.

Advantages of capacitors

1) Small losses (less than 0.5 percent) or higher efficiency (say 99.6).2) Low initial cost.3) Easy installation.4) Little maintenance.5) Long life.6) Greater reliability in service.7) Flexible in operation.8) No restriction on the choice of site for capacitor and can be installed in

relatively small banks located near the load.

Besides p.f improvement, capacitors are employed to perform the following functions also:

1) To reduce losses.2) To reduce voltage regulation of the line.3) To meet a demand for reactive power.4) To utilize fully the capacities of generators, transformers and transmission and

distribution network.

Use of synchronous condenser:

An over- excited synchronous motor running on no load is called the synchronous condenser or synchronous phase advancer.

It behaves like a capacitor, the capacitance reactance of which depends upon the motor excitation.

Power factor can be improved by using synchronous condensers like shunt capacitors connected across the supply.

IL = current taken by the industrial load,

ФL = Angle of lag,

IM = current drawn by the synchronous motor,

ФM = Angle of lead,

I = Resultant current and

Ф = angle lag

Synchronous condensers are usually built in large units and are employed where a large quantity of corrective kVAR is required.

From the fig we observe that angle of lag(Ф) is much smaller than ФL ; thus overall factor is improved from cos ФL to cos ФM by the use of synchronous condenser.

Advantages

A finer control can be obtained by varying the field excitation. Inherent characteristics of synchronous condensers of stabilizing variations in the

line voltage and thereby automatically aid in regulation. Possibility of overloading a synchronous condenser for short periods. Improvement in the system stability and reduction of the effect of sudden changes

in load owing to inertia of synchronous condenser.

Disadvantages

The cost is higher than that of static capacitors of the same rating, except in size above about 5000 kVAR.

Higher maintenance and operating costs comparatively. Comparatively lower efficiency, due to losses in rotating parts and heat losses. Increase of short-circuit currents when the fault occurs near the synchronous

condenser. For starting synchronous condensers an auxiliary equipment is required. Possibility of synchronous condensers falling out of synchronism causing

interruption of supply. During operation noise is produced.

Use of phase advancers:

The p.f of an induction motor falls mainly due to its exciting current drawn from the A.C. supply mains, because exciting current lags behind the voltage by 90º.

It may be improved by equipping the set with an “A.C. exciter” or “phase advancer” which supplies this exciting current to the rotor at slip frequency.

Such an excitor may be mounted on the same shaft as the main motor or may be suitably driven from it.

Use of phase advancer is not generally economical in connection with motors below 150 kW output but above this size, phase advancers are frequently employed.

Shunt and series type of phase advancers are available according to whether the exciting winding of the advancer is connected in parallel or series with the rotor winding of the induction motor.

Use of phase compensated motors:

As mentioned earlier, the use of phase advancers may not be economical for induction motors below 150 kW output.

Power factor improvement of the system is achieved by the use of phase compensated motors such as torda, osnos and scharge motors.

These motors are however very costly and require more maintenance than plain induction motors.

As such these motors are chosen when we are sure that they will be loaded to rated output for most of the time and that they will effect more saving in the energy cost due to higher p.f than the additional expenses incurred on them.-----------------------------*********************-------------------------------