DC and AC Machinery

duplex lap winding

A winding in which the number of the parallel paths between the brushes is twice the number of poles is called duplex lap winding.

Flux (WB or max)

It is the number of magnetic lines of forces in a magnetic field

ARMATURE WINDING

It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils

FARADAY'S SECOND LAW OF ELECTROMAGNETIC INDUCTION

The magnitude of the induced emf is proportional to the rate of change of flux linkages

Thermodynamics

The study of how energy gets converted from one form to another

Torque

Twisting force that produce or tends to produce rotation or torsion

SEPARATELYEXCITED DCGENERATORS

A DC generator whose field magnet winding is supplied from an independent external DC source (e.g., a battery etc.) is called a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg = PΦZN/60 A). The greater the speed and field current, greater is the generated e.m.f. It may be noted that separately excited DC generators are rarely used in practice. The DC generators are normally of self-excited type

FORCE ON A CURRENT CARRYING CONDUCTOR LYING IN A MAGNETIC FIELD

A current consists of many small charged particles running through a wire. If immersed in a magnetic field, the particles will be experience a force; they can transmit this force to the wire through which they travel.

FARADAY'S LAW

A law that states an electrical field is induced in any system in which a magnetic field is changing with time

FORCE ON A CURRENT CARRYING CONDUCTOR LYING IN A MAGNETIC FIELD

A magnetic force cannot change the speed of a charged particle, only its direction

INDUCTANCE

A property of an electric circuit by which emf is induced in it as the result of changing magnetic flux. it is also a circuit element, typically a conducting coil, in which emf is generated by electromagnetic induction.

Simplex lap winding

A winding in which the number of parallel path between the brushes is equal to the number of poles is called

TORQUE ON A FLAT COIL IN A UNIFORM MAGNETIC FIELD

Before considering the mathematical nature of the forces on currents in magnetic fields it is worth just looking at the simple magnetic field diagrams that give rise to these effects. These are shown in Figure 1. (a) is the field between two magnets, (b) the field due to a current in a straight wire and (c) the resulting field if they are put together. This last field is known as the "catapult" field because it tends to catapult the wire out of the field in the direction shown by the arrow.

Retrogressive Wave Winding

If after one round of the armature the coil falls in a slot left to its starting slot the winging is called

Progressive Wave Winding

If after one round of the armature the coil falls in a slot right to its starting slot the winging is called

Magnetic Field

It is created When current flows in a wire, a magnetic field is created around the wire. From this it has been inferred that magnetic fields are produced by the motion of electrical charges. A magnetic field of a bar magnet thus results from the motion of negatively charged electrons in the magnet.

Magnetomotive Force (mmf)

It tends to drive magnetic flux through a magnetic circuits and corresponds emf in electric circuits. By an electric current through a number of turns of a wire produces it. AT - mks unit of mmf equal to 1.257 gilbert

SHUNT motoR

Lathe Machines, Centrifugal Pumps, Fans, Blowers,Conveyors, Lifts, Weaving Machine, Spinning machines, etc.

- They support the field or exciting coils - They spread out the magnetic flux over the armature periphery more uniformly - Since pole shoes have larger crosssection, the reluctance of magnetic path is reduced.

POLE CORE AND POLE SHOES Purpose

COMMUTATOR AND BRUSHES

Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

COMPOUND MOTOR

Presses,Shears, Conveyors, Elevators, Rolling Mills, Heavy Planners, etc

Reluctance (Rm)

Property of the magnetic material which provides opposition to flow of magnetic flux in a magnetic circuit. AT/WB - MKS unit of Rm GB/max - CGS unit of Rm

COMPUND moTOR

Provide high starting torque and nearly constant speed thus are used where we require highstarting torque and constant speed

Force on a charge

The amount of attraction or repulsion between charged objects can be put in quantitative terms by the introduction of the electric force. The simplest case to consider is the force between two points charges (charges with a negligible size)

SERIES motor

The armature coils and the series field coils are connected in series. The current passing through the series winding is same as the armature current.

SHUNT motoR

The armature coils and the shunt field coils are connected in parallel. The parallel combination of two windings is connected across a common dc power supply

Energy Transformation

The process of the changing of the energy from one type to another.

electromagnetic induction

The production of an induced emf in a coil in a closed circuit by the relative motion of a magnet and the coil is called the

Long Shunt

The series field are connected in series with the armature coils while the shunt field are connected across the series combination.

Short Shunt

The series field are connected in series with the supply voltage while the shunt field coils and the armature coils are connected in parallel.

Energy Conservation

The total amount of energy remains the same

SELF-EXCITED DC motor

Thefield winding is energized by the residual magnetism and then by the emf induced in the motor. Because of the residual flux, the motor starts to rotate. (i) Series motor; (ii) Shunt motor; (iii) Compound motor;

SEPARATELY EXCITED DC MOTOR

These type of motors are much suitable forapplicationswhich need speed variationfrom low speed tohigher speed.

FIELD WINDING

They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles.

hysterisis loss

This loss is due to the reversal of the magnetization of the armature core. Every portion of the rotating core passes under N and S pole alternately, thereby attaining S and N polarity respectively. The core undergoes one complete cycle of magnetic reversal after passing under one pair of poles. If P is the number of poles and N, the armature speed in rpm, then the frequency of magnetic reversals is f = PN/120.

FORCE ON A CURRENT CARRYING CONDUCTOR LYING IN A MAGNETIC FIELD

When a charged particle enters a uniform magnetic field in a direction perpendicular to that field, its motion is continuously changed by the magnetic forc

ENERGY STORED IN A MAGNETIC FIELD

When a conductor carries a current, a magnetic field surrounding the conductor is produced. The resulting magnetic flux is proportional to the current. If the current changes, the change in magnetic flux is proportional to the time-rate of change in current by a factor called inductance (L). Since nature abhors rapid change, a voltage (electromotive force, EMF) produced in the conductor opposes the change in current, which is also proportional to the change in magnetic flux. Thus, inductors oppose change in current by producing a voltage that,in turn, creates a current to oppose the change in magnetic flux; the voltage is proportional to the change in current.

TORQUE ON A FLAT COIL IN A UNIFORM MAGNETIC FIELD

When a current carrying loop is placed across a magnetic field, it has the tendency to be rotated either clockwise or counterclockwise dependent on the direction of the magnetic field and the current. Its direction of rotation is determined using the right hand rule.

dc generator

When conductor slashes magnetic flux, an emf will be generated based on the electromagnetic induction principle of Faraday's Laws. This electromotive force can cause a flow of current when the conductor circuit is closed.

Eddy current loss

When the armature core rotates, it also cuts the magnetic flux. Hence, an emf is induced in the body of the core according to the laws of electromagnetic induction. This emf, though small, sets sup large current in the body of the core due to its small resistance. This current is known as the eddy current.

FARADAY'S FIRST LAW OF ELECTROMAGNETIC INDUCTION.

Whenever the flux linking a coil or current changes, an emf is induced in it.

PARALLEL COIL OPPOSING

this is where connected coils decreasing the total equivalent inductance compared to coils that have zero mutual inductance.

PARALLEL COIL AIDING

this is where connected coils increasing the total equivalent inductance.

DC motors

use magnetic fields that occur from the electrical currents generated, which powers the movement of a rotor fixed within the output shaft. The output torque and speed depends upon both the electrical input and the design of the motor

SERIES motor

used in a vacuum cleaner, traction systems, sewing machines, cranes, air compressors etc.

Eye Bolt

used to lift heavy motors with a hoist or crane to prevent motor damage.

SHUNT motoR

used where constant speed is neededsuch as in fixed speed applications.

SERIES motor

used where high starting torque required. These motors are only used where the variation of speed is possible. series motors are not suitable for constant speed applications

Slot/Teeth

for mechanical support, protection from abrasion and further electrical insulation, non-conduction slot liners are often wedged between the coils and the slot walls. The magnetic material between the slots is called teeth.

Short Shunt

in which only shunt field winding is in parallel with the armature winding;

Long Shunt

in which shunt field winding is in parallel with both series field and armature winding

direct current (DC) motor

is a type of electric machine that converts electrical energy into mechanical energy. DC motors take electrical power through direct current, and convert this energy into mechanical rotation

DC machine

is an electro-mechanical energy conversion device. When it converts mechanical power into DC electrical power, it is known as a DC generator. On the other hand, when it converts DC electrical power into mechanical power it is known as a DC motor. Although battery is an important source of DC electric power, but it can supply limited power. There are some applications where large quantity of DC power is required (such as in chemical and metal extraction plants, for electroplating and electrolysis processes etc.), at such places DC generators are used to deliver power. In short, we can say that DC machines have their own role in the field of engineering. In this chapter, we shall focus our attention on the common topics of DC generators

hysterisis loss

is due to the reversal of magnetization of transformer core whenever it is subjected to alternating nature of magnetizing force .Whenever the core is subjected to an alternating magnetic field, the domain present in the material will change their orientation after every half cycle. The power consumed by the magnetic domains for changing the orientation after every half cycle is called Hysteresis loss

ENERGY STORED IN A MAGNETIC FIELD

is equal to the work needed to produce a current through the inductor.

SHAFT

is made of mild steel with a maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine. The rotating parts like armature core, commutator, cooling fan etc. are keyed to the shaft.

Magnetic Field

is represented using magnetic field lines (lines of force , flux lines) that show the shape, direction and strength of the field.

Magnetism

is the force exerted by magnets when they attract or repel each other. Magnetism is caused by the motion of electric charges

ARMATURE CORE

is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed to the shaft

lap winding

is the winding in which successive coils overlap each other. In this winding the finishing end of one coil is connected to one commutator segment and the starting end of the next coil situated under the same pole and connected with same commutator segment. Lap Winding is forms a loop as it expands around the armature core. The total current output divides equally between them. Lap wound generators produce high current, low voltage output

BEARING

may be ball or roller, these are fitted in the end housings. Their function is to reduce friction between the rotating and stationary parts of the machine. Mostly high carbon steel is used for the construction of bearings as it is very hard material.

Lorentz law,

on the other hand, describes the force that a charged particle experiences when it is in the presence of an electric field and a magnetic field. The force on the charged particle is proportional to the strength of the magnetic field, the charge of the particle, and the velocity of the particle

Lorentz force law

plays a crucial role in the principle of motor operation. An electric motor converts electrical energy into mechanical energy by utilizing the interaction between magnetic fields and electric currents.

Conduit Box

point of connection of electrical power to the motor's stator windings

Relative Permeability

ratio of the permeability of material to the permeability of air or vacuum.

SERIES COIL OPPOSING

sources of electromotive force (emf) which give the ability to the current to flow in opposite direction.

SERIES COIL AIDING

sources of electromotive force (emf) which give the ability to the current to flow in the same direction.

Faraday's law

states that a changing magnetic field can induce an electromotive force (EMF) in a conductor. This means that if a conductor is moved through a magnetic field or if the magnetic field is changed, an EMF will be induced in the conductor. The magnitude of the induced EMF is proportional to the rate of change of the magnetic field.

Stator core

stationary part of the motor's electromagnetic circuit. The stator is electrical circuit that performs as electromagnet. The stator core is made up of many thin metal sheets called laminations. Laminations are used to reduce energy losses that would result if a solid core were used.

Stator winding

stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core. When the assembled motor is in operation, the stator windings are connected directly to the power source. Each grouping of coils, together with the steel core it surrounds, becomes an electromagnet when current is applied. Electromagnetism is the basic principle behind motor operation..

Permeability

the ability of a material to conduct magnetic flux through it

Commutator

the coils on the armature are terminated and interconnected through the --- which is insulated from each other. The --- rotates with the rotor and serves to rectify the induced voltage and the current in the armature both of which are AC

Mutual Inductance

the ratio of emf in a circuit to the corresponding change of current in a neighboring circuit. Measures the mutual induction between two magnetically linked circuits, given as the ratio of the induced emf to the rate of charge of current producing it, measured in Henries (H)

Self Inductance

the ratio of emf produced in a circuit by self induction to the rate of change of current producing it, expressed in Henries (H)

Magnetic Field

the space surrounding a magnet, in which magnetic force is exerted. If a bar magnet is placed in such a field, it will experience magnetic force. However, the field will continue to exist even if the magnet is removed. The direction of magnetic field at a point is the direction of the resultant force acting on a hypothetical North Pole placed at that point

Brushes

these are conducting carbon graphite spring loaded to ride on the commutator and act as interface between the external circuit and the armature winding

SELF-EXCITED DCGENERATORS

A DC generator whose field magnet winding is supplied current from the output of the generator itself is called a self-excited generator. There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely; (i) Series generator; (ii) Shunt generator; (iii) Compound generator

Torque

Driving force of electric motor

ENERGY STORED IN A MAGNETIC FIELD

Due to energy conservation, the energy needed to drive the original current must have an outlet. For an inductor, that outlet is the magnetic field—the energy stored by an inductor is equal to the work needed to produce a current through the inductor.

Magnetic Field Strenght/ Force/ Intensity (H)

Field strength at any point within a magnetic field is numerically equal to the force experienced by a N-pole of one Weber placed at that point. OERSTED - cgs unit of magnetic field strength equal to gilbert per centimeter. 1 oersted = 79.577 AT/m

Magnetic Flux Density (Tesla or Gauss)

Flux per unit area

SEPARATELY EXCITED DC MOTOR

In a ___ dc motor, a separate DC supply is given to both the field winding and armature winding. Both are electrically isolated and have separate voltage ratings

SHUNT GENERATOR

In a ___ generator, the field winding is connected in parallel with the armature winding so that terminal voltage of the generator is applied across it. The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load. Figure below shows the connections of a shunt-wound generator.

SERIES GENERATOR

In a ___ generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load. The figure below shows the connections of a series-wound generator. Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purposes e.g., as boosters

COMPUND GENERATOR

In a ___ generator, there are two sets of field windings on each pole—one is in series and the other in parallel with the armature

wave winding

In a ___ winding, we connect the end of one coil to the start of another coil of the same polarity. The coil side (A - B) progresses forward around the armature to another coil side and goes on successively passing through North and South poles until it returns to a conductor (A1-B1) lying under the starting pole. This winding forms a wave with its coil, that's why we call it a wave winding. Since we connect the coils in series, it is also referred to as a series winding.

SEPARATELY EXCITED DC MOTOR

SEPARATELY EXCITED DC MOTOR

T

T/F All magnets have north and south poles. Opposite poles are attracted to each other, while the same poles repel each other.

T

T/F DC compound motors have both series as well as shunt winding. In a compound motor, if series and shunt windings are connected such that series flux is in direction as that of the shunt flux then the motor is said to be cumulatively compounded. And if the series flux is opposite to the direction of the shunt flux, then the motor is said to be differentially compounded.

T

T/F Separately excited Dc motors got very accuratespeed

T

T/F Some substances can be magnetized by an electric current. When electricity runs through a coil of wire, it produces a magnetic field. The field around the coil will disappear, however, as soon as the electric current is turned off.

T

T/F To become magnetized, another strongly magnetic substance must enter the magnetic field of an existing magnet. The magnetic field is the area around a magnet that has magnetic force

Eddy current loss

The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called EDDYCURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss. Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow

induced emf.

The current produced by the relative motion of the coil or the magnet is called an induced current and is said to be setup by an

Current in the same direction

The field strength in the space between the conductors is decreased due to the two fields there being in the opposition to each other. Hence, the two conductors are attached towards each other.

Current in the opposite direction

The field strength is increased in the space between the two conductors due to the two fields being in the same direction there. Because of the lateral repulsion of the lines of force, the two conductors expensive a mutual force of repulsion.

hysterisis loss

The loss depends upon the volume and grade of iron, maximum value of flux density βmax and frequency of magnetic reversals. For normal flux densities (i.e. up to 1.5 Wb/m2), hysteresis loss is given by the Steinmetz formula. According to this formula

FORCE ON A CURRENT CARRYING CONDUCTOR LYING IN A MAGNETIC FIELD

The magnetic force on a charged particle depends on the relative orientation of the particle's velocity and the magnetic field

Energy Transfer

The movement of energy from one location to another

YOKE

The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding. It provides protection to the rotating and other parts of the machine from moisture, dust etc.

VOltage regulation

amount of output voltage drop as load is added fromno-load to full- load

speed regulation

amount of speed drops as load is added from no-load to full load

dc generator

an electrical machine which converts mechanical energy into direct current electricity. This energy conversion is based on the principle of production of dynamically induced emf.

POLE COLE AND POLE SHOES

are fixed to the magnetic frame or yoke by bolts.

Rotor

carries the armature winding. The armature is the load carrying member. It is cylindrical cylinder

Rotor

carries the armature winding. The armature is the load carrying member. The rotor is cylindrical cylinder.

Stator

carries the field winding and poles. The stator together with the rotor constitutes the magnetic circuit or core of the machine

Enclosure

consists of a frame (or yoke) and two end brackets (on bearing housings). not only holds the motor's components, together it also protects the internal components from moisture and containments. The degree of protection depends on the enclosure type. In addition, the type of enclosure affects the motor's cooling.

DC generator is an energy converter that turns mechanical energy into electrical. This change in the form of energy happens based on the principle of electromagnetic induction which means wherever a change in the magnetic flux happens associated with a conductor, an EMF or an electromagnetic force is induced in it. This induction causes a current to flow in case the conductor circuit is closed. Imagine an armature rotating clockwise and a conductor at the left moving upwards. Now when the armature accomplishes a half rotation, the direction of the movement of the conductor will be reversed downward. So, the direction of the current in every armature will be changing. But with a split ring commutator, connections of the armature conductors get reversed when a current reversal occurs. Therefore, we get unidirectional current at the terminals.

working principle of dc generator

In an electric motor, an electric current is passed through a wire that is placed in a magnetic field. As per the Lorentz force law, the current-carrying wire experiences a force perpendicular to both the magnetic field and the direction of current flow. This force causes the wire to move in a circular motion around a fixed axis, which in turn causes the shaft of the motor to rotate. So, the relationship between Lorentz law and the principle of motor operation can be summarized as follows: Lorentz force law provides the fundamental physical principles that explain the motion of the wire in the magnetic field of the motor, while the principle of motor operation utilizes this motion to create the rotation of the motor shaft, which in turn leads to the conversion of electrical energy into mechanical energy When a current-carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. In other words, when a magnetic field and an electric field interact, a mechanical force is produced. The DC motor or direct current motor works on that principle. This is known as motoring action.

working principle of dc motor