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Classification of Materials

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BE8251 Notes r2017 notes

Single phase induction Motor – Construction, Principle of Operation and Starting methods

Post author By Mr. Padeepz
Post date January 30, 2018
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Single phase induction Motor – Construction, Principle of Operation and Starting methods

The single phase induction motor machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and so forth.

SINGLE PHASE INDUCTION MOTOR

The single phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and so forth.

The single phase motor stator has a laminated iron core with two windings arranged perpendicularly.
One is the main and
The other is the auxiliary winding or starting winding
This “single phase” motors are truly two phase machines.
The motor uses a squirrel cage rotor, which has a laminated iron core with slots.
Aluminum bars are molded on the slots and short-circuited at both ends with a ring.

The single phase induction motor operation can be described by two methods:

Double revolving field theory and
Cross-field theory.

Double revolving theory is perhaps the easier of the two explanations to understand

Double revolving field theory

A single phase ac current supplies the main winding that produces a pulsating magnetic field.
Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions.
The interaction between the fields and the current induced in the rotor bars generates opposing torque.

STARTING METHODS

The single phase IM has no starting torque, but has resultant torque, when it rotates at any other speed, except synchronous speed.

It is also known that, in a balanced two-phase IM having two windings, each having equal number of turns and placed at a space angle of 90⁰(electrical), and are fed from a balanced two-phase supply, with two voltages equal in magnitude, at an angle of 90⁰, the rotating magnetic fields are produced, as in a three-phase IM.

The torque-speed characteristic is same as that of a three-phase one, having both starting and also running torque as shown earlier.

So, in a single phase IM, if an auxiliary winding is introduced in the stator, in addition to the main winding, but placed at a space angle of 90⁰ (electrical), starting torque is produced.

The currents in the two (main and auxiliary) stator windings also must be at an angle of 90⁰ , to produce maximum starting torque, as shown in a balanced two-phase stator.

Thus, rotating magnetic field is produced in such motor, giving rise to starting torque.

The various starting methods used in a single phase IM are described here.

RESISTANCE SPLIT PHASE MOTOR

The schematic (circuit) diagram of this motor is given.

As detailed earlier, another (auxiliary) winding with a high resistance in series is to be added along with the main winding in the stator.

This winding has higher resistance to reactance () ratio as compared to that in the main winding, and is placed at a space angle of from the main winding as given earlier.

The phasor diagram of the currents in two windings and the input voltage is shown.

The current () in the auxiliary winding lags the voltage (V) by an angle, aaXR/°90aIaφ, which is small, whereas the current () in the main winding lags the voltage (V) by an angle, mImφ, which is nearly .

The phase angle between the two currents is (°90aφ−°90), which should be at least .

This results in a small amount of starting torque.

The switch, S (centrifugal switch) is in series with the auxiliary winding.

It automatically cuts out the auxiliary or starting winding, when the motor attains a speed close to full load speed.

The motor has a starting torque of 100−200% of full load torque, with the starting current as 5-7 times the full load current.

The torque-speed characteristics of the motor with/without auxiliary winding are shown.

The change over occurs, when the auxiliary winding is switched off as given earlier.

The direction of rotation is reversed by reversing the terminals of any one of two windings, but not both, before connecting the motor to the supply terminals.

This motor is used in applications, such as fan, saw, small lathe, centrifugal pump, blower, office equipment, washing machine, etc.

CAPACITOR START MOTOR

The schematic (circuit) diagram of this motor is given.

It may be observed that a capacitor along with a centrifugal switch is connected in series with the auxiliary winding, which is being used here as a starting winding.

The capacitor may be rated only for intermittent duty, the cost of which decreases, as it is used only at the time of starting.

The function of the centrifugal switch has been described earlier.

The phasor diagram of two currents as described earlier, and the torque-speed characteristics of the motor with/without auxiliary winding, are shown.

This motor is used in applications, such as compressor, conveyor, machine tool drive, refrigeration and air-conditioning equipment, etc.

Capacitor Start and Capacitor Run Motor

In this motor two capacitors − C_sfor starting, and C_r for running, are used.

The first capacitor is rated for intermittent duty, as described earlier, being used only for starting.

A centrifugal switch is also needed here.

The second one is to be rated for continuous duty, as it is used for running.

The phasor diagram of two currents in both cases, and the torque-speed characteristics with two windings having different values of capacitors, are shown in respectively.

The phase difference between the two currents is (φ_m+φ_a>90⁰) in the first case (starting), while it is90⁰ for second case (running). In the second case, the motor is a balanced two phase one, the two windings having same number of turns and other conditions as given earlier, are also satisfied.

So, only the forward rotating field is present, and the no backward rotating field exists.

The efficiency of the motor under this condition is higher.

Hence, using two capacitors, the performance of the motor improves both at the time of starting and then running.

This motor is used in applications, such as compressor, refrigerator, etc.

Beside the above two types of motors, a Permanent Capacitor Motor with the same capacitor being utilised for both starting and running, is also used.

The power factor of this motor, when it is operating (running), is high.

The operation is also quiet and smooth.

This motor is used in applications, such as ceiling fans, air circulator, blower, etc.

Shaded pole Motor

A typical shaded-pole motor with a cage rotor is shown.

This is a single phase induction motor, with main winding in the stator.

A small portion of each pole is covered with a short-circuited, single-turn copper coil called the shading coil.

The sinusoidally varying flux created by ac (single phase) excitation of the main winding induces emf in the shading coil.

As a result, induced currents flow in the shading coil producing their own flux in the shaded portion of the pole.

Let the main winding flux be φ_m=φ_maxsin_wt

The reversal of the direction of rotation, where desired, can be achieved by providing two shading coils, one on each end of every pole, and by open-circuiting one set of shading coils and by short-circuiting the other set.

The fact that the shaded-pole motor is single-winding (no auxiliary winding) self-starting one, makes it less costly and results in rugged construction.

The motor has low efficiency and is usually available in a range of 1/300 to 1/20 kW.

It is used for domestic fans, record players and tape recorders, humidifiers, slide projectors, small business machines, etc.

The shaded-pole principle is used in starting electric clocks and other single phase synchronous timing motors.

No starting torque is produced in the single phase induction motor with only one (main) stator winding, as the flux produced is a pulsating one, with the winding being fed from single phase supply.

Using double revolving field theory, the torque-speed characteristics of this type of motor are described, and it is also shown that, if the motor is initially given some torque in either direction, the motor accelerates in that direction, and also the torque is produced in that direction.

Then, the various types of single phase induction motors, along with the starting methods used in each one are presented.

Two stator windings − main and auxiliary, are needed to produce the starting torque.

The merits and demerits of each type, along with their application area, are presented.

The process of production of starting torque in shade-pole motor is also described in brief.

In the next module consisting of seven lessons, the construction and also operation of dc machines, both as generator and motor, will be discussed.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

BE8251 Notes r2017 notes

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

Post author By Mr. Padeepz
Post date January 30, 2018
No Comments on Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

Let the applied voltage V1 applied to the primary of a transformer, with secondary open-circuited, be sinusoidal (or sine wave).

Then the current I1, due to applied voltage V1, will also be a sine wave.

The mmf N1 I1 and core flux Ø will follow the variations of I1 closely.

That is the flux is in time phase with the current I1 and varies sinusoidally.

EMF Equation of Single Phase Transformer:

Let the applied voltage V1 applied to the primary of a transformer, with secondary open-circuited, be sinusoidal (or sine wave).

Then the current I1, due to applied voltage V1, will also be a sine wave.

The mmf N1 I1 and core flux Ø will follow the variations of I1 closely.

That is the flux is in time phase with the current I1 and varies sinusoidally.

Let,

N_A = Number of turns in primary

N_B = Number of turns in secondary

Ø_max = Maximum flux in the core in webers = B_max X A f = Frequency of alternating current input in hertz (H_Z)

As shown in figure above, the core flux increases from its zero value to maximum value Ø_max in one quarter of the cycle , that is in ¼ frequency second.

Therefore, average rate of change of flux = Ø_max/ ¼ f = 4f Ø_maxWb/s

Now, rate of change of flux per turn means induced electro motive force in volts.

Therefore,

average electro-motive force induced/turn = 4f Ø_maxvolt

If flux Ø varies sinusoidally, then r.m.s value of induced e.m.f is obtained by multiplying the average value with form factor.

Form Factor = r.m.s. value/average value = 1.11.

Therefore, r.m.s value of e.m.f/turn = 1.11 X 4f Ø_max = 4.44f Ø_max Now, r.m.s value of induced e.m.f in the whole of primary winding

= (induced e.m.f./turn) X Number of primary turns

Therefore,

EA = 4.44f NAØmax = 4.44fNABmA

Similarly, r.m.s value of induced e.m.f in secondary is

EB = 4.44f NB Ømax = 4.44fNBBmA

In an ideal transformer on no load, VA = EA and VB = EB , where VB is the terminal voltage

Voltage Transformation Ratio:

The ratio of secondary voltage to primary voltage is known as the voltage transformation ratio and is designated by letter K.

i.e.

Voltage transformation ratio, K = V2/V1 = E2/E1 = N2/N1

Current Ratio:

The ratio of secondary current to primary current is known as current ratio and is reciprocal of voltage transformation ratio in an ideal transformer.

Transformer on No Load:

When the primary of a transformer is connected to the source of an ac supply and the secondary is open circuited, the transformer is said to be on no load.

The Transformer on No Load alternating applied voltage will cause flow of an alternating current I0 in the primary winding, which will create alternating flux Ø.

No-load current I0, also known as excitation or exciting current, has two components the magnetizing component Im and the energy component Ie.

Im is used to create the flux in the core and Ie is used to overcome the hysteresis and eddy current losses occurring in the core in addition to small amount of copper losses occurring in the primary only (no copper loss occurs in the secondary, because it carries no current, being open circuited.)

From vector diagram shown in above it is obvious that

Induced emfs in primary and secondary windings, E1 and E2 lag the main flux Ø by and are in phase with each other.
Applied voltage to primary V1 and leads the main flux Ø by and is in phase opposition to E1.
Secondary voltage V2 is in phase and equal to E2 since there is no voltage drop in secondary.
Im is in phase with Ø and so lags V1 by
Ie is in phase with the applied voltage V1.
Input power on no load = V1Ie = V1I0 cos Ø0 where Ø0 = tan^-1

Transformer on Load:

The transformer is said to be loaded, when its secondary circuit is completed through an impedance or load.

The magnitude and phase of secondary current (i.e. current flowing through secondary) I2 with respect to secondary terminals depends upon the characteristic of the load

i.e. current I2 will be in phase, lag behind and lead the terminal voltage V+2+ respectively when the load is non-inductive, inductive and capacitive.

The net flux passing through the core remains almost constant from no-load to full load irrespective of load conditions and so core losses remain almost constant from no-load to full load.

Vector diagram for an ideal transformer supplying inductive load is shown

Resistance and Leakage Reactance In actual practice, both of the primary and secondary windings have got some ohmic resistance causing voltage drops and copper losses in the windings.

In actual practice, the total flux created does not link both of the primary and secondary windings but is divided into three components namely the main or mutual flux Ø linking both of the primary and secondary windings, primary leakage flux ØL₁ linking with primary winding only and secondary leakage flux ØL₂ linking with secondary winding only.

The primary leakage flux ØL₁ is produced by primary ampere-turns and is proportional to primary current, number of primary turns being fixed.

The primary leakage flux ØL₁is in phase with I₁ and produces self induced emf ØL₁ is in phase with I₁ and produces self induced emf EL₁ given as 2f L₁ I₁ in the primary winding.

The self induced emf divided by the primary current gives the reactance of primary and is denoted by X1.

i.e. X1 = EL1/I1 = 2πfL1I1/I1 = 2FL1,

Similarly leakage reactance of secondary X2 = EL2/E2 = 2fπL2I2/I2 = 2πfL2

Equivalent Resistance and Reactance.

The equivalent resistances and reactance’s of transformer windings referred to primary and secondary sides are given as below Referred to primary side Equivalent resistance,

Equivalent resistance, = X’1 = Referred to secondary side Equivalent resistance,

The equivalent resistance, = X2 + K²X1 Where K is the transformation ratio.

EQUIVALENT CIRCUIT OF Single Phase Transformer

The equivalent impedance of transformer is essential to be calculated because the electrical power transformer is an electrical power system equipment for estimating different parameters of electrical power system which may be required to calculate total internal impedance of an electrical power transformer, viewing from primary side or secondary side as per requirement.

This calculation requires equivalent circuit of transformer referred to primary or equivalent circuit of transformer referred to secondary sides respectively.

Percentage impedance is also very essential parameter of transformer.

Special attention is to be given to this parameter during installing a transformer in an existing electrical power system.

Percentage impedance of different power transformers should be properly matched during parallel operation of power transformers.

The percentage impedance can be derived from equivalent impedance of transformer so, it can be said that equivalent circuit of transformer is also required during calculation of % impedance.

Equivalent Circuit of Transformer Referred to Primary

For drawing equivalent circuit of transformer referred to primary, first we have to establish general equivalent circuit of transformer then, we will modify it for referring from primary side.

For doing this, first we need to recall the complete vector diagram of a transformer which is shown in the figure below.

Let us consider the transformation ratio be,

In the figure right, the applied voltage to the primary is V₁ and voltage across the primary winding is E₁.

Total current supplied to primary is I₁.

So the voltage V₁ applied to the primary is partly dropped by I₁Z₁ or I₁R₁ + j.I₁X₁ before it appears across primary winding.

The voltage appeared across winding is countered by primary induced emf E₁.

The equivalent circuit for that equation can be drawn as below,

From the vector diagram above, it is found that the total primary current I₁ has two components, one is no – load component I_o and the other is load component I₂′.

As this primary current has two a component or branches, so there must be a parallel path with primary winding of transformer.

This parallel path of current is known as excitation branch of equivalent circuit of transformer.

The resistive and reactive branches of the excitation circuit can be represented as

The load component I₂′ flows through the primary winding of transformer and induced voltage across the winding is E₁ as shown in the figure right.

This induced voltage E₁transforms to secondary and it is E₂ and load component of primary current I₂′ is transformed to secondary as secondary current I₂. Current of secondary is I ₂.

So the voltage E₂ across secondary winding is partly dropped by I₂Z₂ or I₂R₂ + j.I₂X₂ before it appears across load. The load voltage is V₂.

From above equation, secondary impedance of transformer referred to primary is,

So, the complete equivalent circuit of transformer referred to primary is shown in the figure below,

Approximate Equivalent Circuit of Transformer

Since Io is very small compared to I₁, it is less than 5% of full load primary current, Iochanges the voltage drop insignificantly.

Hence, it is good approximation to ignore the excitation circuit in approximate equivalent circuit of transformer.

The winding resistanceand reactance being in series can now be combined into equivalent resistance and reactance of transformer, referred to any particular side. In this case it is side 1 or primary side.

Equivalent Circuit of Transformer Referred to Secondary

In similar way, approximate equivalent circuit of transformer referred to secondary can be drawn.

Where equivalent impedance of transformer referred to secondary, can be derived as

VOLTAGE REGULATION

The voltage regulation is the percentage of voltage difference between no load and full load voltages of a transformer with respect to its full load voltage.

Explanation of Voltage Regulation of Transformer

Say an electrical power transformer is open circuited, means load is not connected with secondary terminals.

In this situation, the secondary terminal voltage of the transformer will be its secondary induced emf E₂.

Whenever full load is connected to the secondary terminals of the transformer, rated current I₂ flows through the secondary circuit and voltage drop comes into picture.

At this situation, primary winding will also draw equivalent full load current from source.

The voltage drop in the secondary is I₂Z₂ where Z₂ is the secondary impedance of transformer.

Now if at this loading condition, any one measures the voltage between secondary terminals, he or she will get voltage V₂across load terminals which is obviously less than no load secondary voltage E₂ and this is because of I₂Z₂ voltage drop in the transformer.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Single phase induction Motor – Construction, Principle of Operation

BE8251 Notes r2017 notes

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Post author By Mr. Padeepz
Post date January 30, 2018
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Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer.

A TRANSFORMER is a device that transfers electrical energy from one circuit to another by electromagnetic induction (transformer action).

The electrical energy is always transferred without a change in frequency, but may involve changes in magnitudes of voltage and current.

Single Phase Transformer

A TRANSFORMER is a device that transfers electrical energy from one circuit to another by electromagnetic induction (transformer action).

The electrical energy is always transferred without a change in frequency, but may involve changes in magnitudes of voltage and current.

Because a transformer works on the principle of electromagnetic induction, it must be used with an input source voltage that varies in amplitude.

There are many types of power that fit this description; for ease of explanation and understanding, transformer action will be explained using an ac voltage as the input source.

BASIC OPERATION OF A Single Phase Transformer

In its most basic form a transformer consists of:

A primary coil or winding. A secondary coil or winding. Core that supports the coils or windings.

Refer to the transformer circuit in figure as you read the following explanation:

The primary winding is connected to a 60 hertz ac voltage source.

The magnetic field (flux) builds up (expands) and collapses (contracts) about the primary winding.

The expanding and contracting magnetic field around the primary winding cuts the secondary winding and induces an alternating voltage into the winding.

This voltage causes alternating current to flow through the load.

The voltage may be stepped up or down depending on the design of the primary and secondary windings.

AN IDEAL Single Phase Transformer

An ideal transformer is shown in the adjacent figure.

Current passing through the primary coil creates a magnetic field.

The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils.

BASIC WORKING PRINCIPLE OF Single Phase Transformer

A transformer can be defined as a static device which helps in the transformation of electric power in one circuit to electric power of the same frequency in another circuit.

The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings.

The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux.

A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance.

PRINCIPLE OF Single Phase Transformer

The working principle of the transformer can be understood from the figure below.

As shown above the transformer has primary and secondary windings.

The core laminations are joined in the form of strips in between the strips you can see that there are some narrow gaps right through the cross-section of the core.

These staggered joints are said to be ‘imbricated’. Both the coils have high mutual inductance.

A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source of alternating voltage.

Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electro-motive force.

The so produced electro-motive force can be explained with the help of Faraday’s laws of Electromagnetic Induction as

e=M*dI/dt

If the second coil circuit is closed, a current flows in it and thus electrical energy is transferred magnetically from the first to the second coil.

The alternating current supply is given to the first coil and hence it can be called as the primary winding.

The energy is drawn out from the second coil and thus can be called as the secondary winding.

In short, a transformer carries the operations shown below:

Transfer of electric power from one circuit to another.

Transfer of electric power without any change in frequency.

The transfer with the principle of electromagnetic induction.

The two electrical circuits are linked by mutual induction

Construction of Single Phase Transformer

Two coils of wire (called windings) are wound on some type of core material.

In some cases the coils of wire are wound on a cylindrical or rectangular cardboard form.

In effect, the core material is air and the transformer is called an AIR-CORE TRANSFORMER.

Transformers used at low frequencies, such as 60 hertz and 400 hertz, require a core of low-reluctance magnetic material, usually iron.

This type of transformer is called an IRON-CORE TRANSFORMER. Most power transformers are of the iron-core type.

The principle parts of a transformer and their functions are:

The CORE, which provides a path for the magnetic lines of flux.

The PRIMARY WINDING, which receives energy from the ac source.

The SECONDARY WINDING, which receives energy from the primary winding and delivers it to the load.

The ENCLOSURE, which protects the above components from dirt, moisture, and mechanical damage.

(i) CORE

There are two main shapes of cores used in laminated-steel-core transformers.

One is the HOLLOWCORE, so named because the core is shaped with a hollow square through the center.

This shape of core.

Notice that the core is made up of many laminations of steel it shows how the transformer windings are wrapped around both sides of the core.

(ii) WINDINGS

As stated above, the transformer consists of two coils called WINDINGS which are wrapped around a core.

The transformer operates when a source of ac voltage is connected to one of the windings and a load device is connected to the other.

The winding that is connected to the source is called the PRIMARY WINDING.

The winding that is connected to the load is called the SECONDARY WINDING.

The primary is wound in layers directly on a rectangular cardboard form.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Basic Equations of DC Motor and Applications of DC Motor

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

Single phase induction Motor – Construction, Principle of Operation

BE8251 Notes r2017 notes

Basic Equations of DC Motor and Applications of DC Motor

Post author By Mr. Padeepz
Post date January 30, 2018
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Basic Equations of DC Motor and Applications of DC Motor

Basic Equations of DC Motor and Applications of DC Motor D.C Shunt Motors:

It is a constant speed motor.

Where the speed is required to remain almost constant from no load to full load.

Where the load has to be driven at a number of speeds and any one of which is nearly constant.

VOLTAGE EQUATION OF MOTORS

Let in a d.c. motor

V = applied voltage

Eb = back e.m.f.

Ra = armature resistance

Ia = armature current

Since back e.m.f. Eb acts in opposition to the applied voltage V, the net voltage across the armature circuit is V-Eb.

The armature current Ia is given by

APPLICATIONS OF DC MOTORS:

D.C Shunt Motors:

It is a constant speed motor.Where the speed is required to remain almost constant from no load to full load.

Where the load has to be driven at a number of speeds and any one of which is nearly constant.

Industrial use:

Lathes

Drills

Boring mills

Shapers

Spinning and Weaving machines.

D.C Series motor:

It is a variable speed motor.

The speed is low at high torque.

At light or no load ,the motor speed attains dangerously high speed.

The motor has a high starting torque. (elevators,electric traction)

Industrial Uses:

Electric traction

Cranes

Elevators

Air compressor

D.C Compound motor:

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

Industrial uses:

Presses Shears

Reciprocating machine.

CLASSIFICATION OF DC MOTOR

DC motors are more common than we may think.

A car may have as many as 20 DC motors to drive fans, seats, and windows.

They come in three different types, classified according to the electrical circuit used.

In the shunt motor, the armature and field windings are connected in parallel, and so the currents through each are relatively independent.

The current through the field winding can be controlled with a field rheostat (variable resistor), thus allowing a wide variation in the motor speed over a large range of load conditions.

This type of motor is used for driving machine tools or fans, which require a wide range of speeds.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

Single phase induction Motor – Construction, Principle of Operation

BE8251 Notes r2017 notes

CLASSIFICATION OF DC MOTOR

Post author By Mr. Padeepz
Post date January 28, 2018
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CLASSIFICATION OF DC MOTOR

CLASSIFICATION OF DC MOTOR is more common than we may think.

A car may have as many as 20 DC motors to drive fans, seats, and windows.

They come in three different types, classified according to the electrical circuit used.

In the shunt motor, the armature and field windings are connected in parallel, and so the currents through each are relatively independent.

The current through the field winding can be controlled with a field rheostat (variable resistor), thus allowing a wide variation in the motor speed over a large range of load conditions.

This type of motor is used for driving machine tools or fans, which require a wide range of speeds.

In the series motor, the field winding is connected in series with the armature winding, resulting in a very high starting torque since both the armature current and field strength run at their maximum.

However, once the armature starts to rotate, the counter EMF reduces the current in the circuit, thus reducing the field strength.

The series motor is used where a large starting torque is required, such as in automobile starter motors, cranes, and hoists.

The compound motor is a combination of the series and shunt motors, having parallel and series field windings.

This type of motor has a high starting torque and the ability to vary the speed and is used in situations requiring both these properties such as punch presses, conveyors and elevators.

Types of DC motor

Shunt Wound
Series Wound
Compound wound

Shunt Motor

In shunt wound motor the field winding is connected in parallel with armature.

The current through the shunt field winding is not the same as the armature current.

Shunt field windings are designed to produce the necessary m.m.f. by means of a relatively large number of turns of wire having high resistance.

Therefore, shunt field current is relatively small compared with the armature current

Series Motor

In series wound motor the field winding is connected in series with the armature.

Therefore, series field winding carries the armature current.

Since the current passing through a series field winding is the same as the armature current, series field windings must be designed with much fewer turns than shunt field windings for the same mmf.

Therefore, a series field winding has a relatively small number of turns of thick wire and, therefore, will possess a low resistance.

Compound Wound Motor

Compound wound motor has two field windings; one connected in parallel with the armature and the other in series with it.

There are two types of compound motor connections

1) Short-shunt connection

2) Long shunt connection

When the shunt field winding is directly connected across the armature terminals it is called short-shunt connection.

When the shunt winding is so connected that it shunts the series combination of armature and series field it is called long-shunt connection.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

Single phase induction Motor – Construction, Principle of Operation

BE8251 Notes r2017 notes

Principles of DC motor and Operation of DC motor

Post author By Mr. Padeepz
Post date January 28, 2018
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Principles of DC motor and Operation of DC motor:

Principles of DC motor it is based on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left-hand rule and whose magnitude is given by

Force, F = B I l newton

Where B is the magnetic field in weber/m². I is the current in amperes and

l is the length of the coil in meter.

The force, current and the magnetic field are all in different directions.

If an Electric current flows through two copper wires that are between the poles of a magnet, an upward force will move one wire up and a downward force will move the other wire down.

BACK OR COUNTER EMF

When the armature of a dc motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence an e.m.f. is induced in them.

The induced e.m.f. acts in opposite direction to the applied voltage V (Lenz’s law) and is known as back or counter emf Eb.

SIGNIFICANCE OF BACK EMF

The presence of back emf makes the dc motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load.

Back emf in a dc motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

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BE8251 Notes r2017 notes

Working of DC motors

working of dc motors

Working of DC motors

The Working of DC motors are explained in here.

DC MOTOR

A machine that converts dc power into mechanical energy is known as dc motor.

Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.

The direction of the force is given by Fleming’s left hand rule.

How DC motors work?

There are different kinds of D.C. motors, but they all work on the same principles.

When a permanent magnet is positioned around a loop of wire that is hooked up to a D.C. power source, we have the basics of a D.C. motor.

In order to make the loop of wire spin, we have to connect a battery or DC power supply between its ends, and support it so it can spin about its axis.

To allow the rotor to turn without twisting the wires, the ends of the wire loop are connected to a set of contacts called the commutator, which rubs against a set of conductors called the brushes.

The brushes make electrical contact with the commutator as it spins, and are connected to the positive and negative leads of the power source, allowing electricity to flow through the loop.

The electricity flowing through the loop creates a magnetic field that interacts with the magnetic field of the permanent magnet to make the loop spin.

PRINCIPLES OF OPERATION

It is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left-hand rule and whose magnitude is given by

Force, F = B I l newton

Where B is the magnetic field in weber/m2. I is the current in amperes and

l is the length of the coil in meter.

The force, current and the magnetic field are all in different directions.

If an Electric current flows through two copper wires that are between the poles of a magnet, an upward force will move one wire up and a downward force will move the other wire down.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

Single phase induction Motor – Construction, Principle of Operation

BE8251 Notes r2017 notes

APPLICATIONS OF DC GENERATOR

Post author By Mr. Padeepz
Post date January 28, 2018
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APPLICATIONS OF DC GENERATOR

APPLICATIONS OF DC GENERATOR are explained in this page

DC Separately Exited Generator:

As a supply source to DC Motors, whose speed is to be controlled for certain applications.

Where a wide range of voltage is required for the testing purposes.

DC Shunt Generator:

The terminal voltage of DC shunt generator is more or less constant from no load to full load .

Therefore these generators are used where constant voltage is required.

For electro plating

Battery charging

For excitation of Alternators.

DC Series Generator:

The terminal voltage of series generator increases with load current from no load to full load .

Therefore these generators are,

Used as Boosters

Used for supply to arc Lamps

DC Compound Generator:

Differential Compound generators are used to supply dc welding machines.

Level compound generators are used to supply power for offices, hostels and Lodges etc.

Over compound generators are used to compensate the voltage drop in Feeders.

An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction.

The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR

Basic Equations of DC Motor and Applications of DC Motor

Construction of Single Phase Transformer, Principle of Operation of Single Phase Transformer

Basic Equations of Single Phase Transformer and Applications of Single Phase Transformer

Single phase induction Motor – Construction, Principle of Operation

BE8251 Notes r2017 notes

CHARACTERISTICS OF DC GENERATOR

Post author By Mr. Padeepz
Post date January 28, 2018
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CHARACTERISTICS OF DC GENERATOR:

Characteristics of DC Generator – The three most important characteristics of curves of a d.c generator are

Open Circuit Characteristic (O.C.C.)

This curve shows the relation between the generated e.m.f. at no-load (E0) and the field current (If) at constant speed.

It is also known as magnetic characteristic or no-load saturation curve.

Its shape is practically the same for all generators whether separately or self-excited.

The data for O.C.C. curve are obtained experimentally by operating the generator at no load and constant speed and recording the change in terminal voltage as the field current is varied.

Internal or Total characteristic (E/Ia)

This curve shows the relation between the generated e.m.f. on load (E) and the armature current (Ia).

The e.m.f. E is less than E0 due to the demagnetizing effect of armature reaction.

Therefore, this curve will lie below the open circuit characteristic (O.C.C.).

The internal characteristic is of interest chiefly to the designer.

It cannot be obtained directly by experiment.

It is because a voltmeter cannot read the e.m.f. generated on load due to the voltage drop in armature resistance.

The internal characteristic can be obtained from external characteristic if winding resistances are known because armature reaction effect is included in both characteristics

External characteristic (V/IL)

This curve shows the relation between the terminal voltage (V) and load current (IL).

The terminal voltage V will be less than E due to voltage drop in the armature circuit.

Therefore, this curve will lie below the internal characteristic.

This characteristic is very important in determining the suitability of a generator for a given purpose.

It can be obtained by making simultaneous.

CHARACTERISTICS OF DC GENERATOR in Series:

Fig. shows the connections of a series wound generator.

Since there is only one current (that which flows through the whole machine), the load current is the same as the exciting current.

(i) O.C.C.

Curve 1 shows the open circuit characteristic (O.C.C.) of a series generator.

It can be obtained experimentally by disconnecting the field winding from the machine and exciting it from a separate d.c. source.

(ii) Internal characteristic

Curve 2 shows the total or internal characteristic of a series generator.

It gives the relation between the generated e.m.f. E. on load and armature current.

Due to armature reaction, the flux in the machine will be less than the flux at no load.

Hence, e.m.f. E generated under load conditions will be less than the e.m.f. EO generated under no load conditions.

Consequently, internal characteristic curve generated under no load conditions.

Consequently, internal characteristic curve lies below the O.C.C. curve; the difference between them representing the effect of armature reaction.

(iii) External characteristic

Curve 3 shows the external characteristic of a series generator.

It gives the relation between terminal voltage and load current IL.

V= E-Ia(Ra+Rse)

Therefore, external characteristic curve will lie below internal characteristic curve by an amount equal to ohmic drop [i.e., Ia(Ra+Rse)] in the machine.

The internal and external characteristics of a d.c. series generator can be plotted from one another as shown in Fig.

Suppose we are given the internal characteristic of the generator.

Let the line OC represent the resistance of the whole machine i.e. Ra+Rse.

If the load current is OB, drop in the machine is AB i.e.

AB = Ohmic drop in the machine = OB(Ra+Rse)

Now raise a perpendicular from point B and mark a point b on this line such that ab = AB.

Then point b will lie on the external characteristic of the generator.

Following similar procedure, other points of external characteristic can be located.

It is easy to see that we can also plot internal characteristic from the external characteristic.

CHARACTERISTICS OF DC GENERATOR – Shunt DC generator:

Fig shows the connections of a shunt wound generator.

The armature current Ia splits up into two parts; a small fraction Ish flowing through shunt field winding while the major part IL goes to the external load.

(i) O.C.C.

The O.C.C. of a shunt generator is similar in shape to that of a series generator as shown in Fig.

The line OA represents the shunt field circuit resistance.

When the generator is run at normal speed, it will build up a voltage OM.

At no-load, the terminal voltage of the generator will be constant (= OM) represented by the horizontal dotted line MC.

(ii) Internal characteristic

When the generator is loaded, flux per pole is reduced due to armature reaction.

Therefore, e.m.f. E generated on load is less than the e.m.f. generated at no load.

As a result, the internal characteristic (E/Ia) drops down slightly as shown in Fig.

(iii) External characteristic

Curve 2 shows the external characteristic of a shunt generator.

It gives the relation between terminal voltage V and load current IL.

V = E – IaRa = E -(IL +Ish)Ra

Therefore, external characteristic curve will lie below the internal characteristic curve by an amount equal to drop in the armature circuit [i.e., (IL +Ish)Ra ] as shown in Fig

Critical External Resistance for Shunt Generator

If the load resistance across the terminals of a shunt generator is decreased, then load current increase?

However, there is a limit to the increase in load current with the decrease of load resistance.

Any decrease of load resistance beyond this point, instead of increasing the current, ultimately results in reduced current.

Consequently, the external characteristic turns back (dottedcurve) as shown in Fig.

The tangent OA to the curve represents the minimum external resistance required to excite the shunt generator on load and is called critical external resistance.

If the resistance of the external circuit is less than the critical external resistance (represented by tangent OA in Fig, the machine will refuse to excite or will de-excite if already running.

This means that external resistance is so low as virtually to short circuit the machine and so doing away with its excitation.

There are two critical resistances for a shunt generator viz.,

(i) critical field resistance

(ii) critical external resistance.

For the shunt generator to build up voltage, the former should not be exceeded and the latter must not be gone below

Characteristics compound generator:

In a compound generator, both series and shunt excitation are combined as shown in Fig.

The shunt winding can be connected either across the armature only (short-shunt connection S) or across armature plus series field (long-shunt connection G).

The compound generator can be cumulatively compounded or differentially compounded generator.

The latter is rarely used in practice.

Therefore, we shall discuss the characteristics of cumulatively compounded generator. It may be noted that external characteristics of long and short shunt compound generators are almost identical.

External CHARACTERISTICS OF DC GENERATOR:

Fig. shows the external characteristics of a cumulatively compounded generator.

The series excitation aids the shunt excitation.

The degree of compounding depends upon the increase in series excitation with the increase in load current.

(i) If series winding turns are so adjusted that with the increase in load current the terminal voltage increases, it is called over-compounded generator.

In such a case, as the load current increases, the series field m.m.f. increases and tends to increase the flux and hence the generated voltage.

The increase in generated voltage is greater than the IaRa drop so that instead of decreasing, the terminal voltage increases as shown by curve A in Fig.

(ii) If series winding turns are so adjusted that with the increase in load current, the terminal voltage substantially remains constant, it is called flat-compounded generator.

The series winding of such a machine has lesser number of turns than the one in over-compounded machine and, therefore, does not increase the flux as much for a given load current.

Consequently, the full-load voltage is nearly equal to the no-load voltage as indicated by curve B in Fig

(iii) If series field winding has lesser number of turns than for a flat compounded machine, the terminal voltage falls with increase in load current as indicated by curve C m Fig. Such a machine is called under-compounded generator.

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PRINCIPLE OF DC GENERATOR AND OPERATION OF DC GENERATOR