READING MATERIAL FOR B.E STUDENTS

OF RGPV AFFILIATED

ENGINEERING COLLEGES PERSUING IN

ELECTRICAL AND ELECTRONICS BRANCH

Professor MD Dutt Addl General Manager (Retd)

BHARAT HEAVY ELECTRICALS LIMITED

Professor(ex) in EX Department

Bansal Institute of Science and Technology

KOKTA ANANAD NAGAR BHOPAL

Presently Head of The Department ( EX)

Shri Ram College Of Technology

Thuakheda BHOPAL

Sub Code Ex503 Subject Electrical Machine II

UNIT I DC Machines I

RGPV Syllabus

EX 503 ELECTRICAL MACHINES II

UNIT I

DC MACHINES I

Basic construction of DC machines, Types of DC machines and method of excitation, lap and wave winding, EMF equation; armature reaction and methods of limiting armature reaction; Commutation process and methods for improving commutation; Basic performance of DC generators and their performance characteristics; Metadyne and amplidyne; permanent magnet DC motor; Brushless DC motor

INDEX

S No Topic Page1 Basic construction of DC machines 2,3,42 Types of DC machines and method of excitation 5,6,73 Lap and wave winding, EMF equation 7,84 Armature reaction and methods of limiting armature reaction 9,10,11,125 Commutation process and methods for improving commutation 12,13,14,156 Basic performance of DC generators and their performance

characteristics15,16,17,18,

7 Metadyne and Amplidyne; 19,20,218 Permanent magnet DC motor 21,229 Brushless DC motor 23,24,25,2610 27

Q1 Write down the magnetic field system for a D.C. machine?

Ans:- The magnetic field system is the stationary (FIXED ) part of the machine. It produces the main magnetic flux. The outer frame or yoke is a hollow cylinder of cast steel or rolled steel. An even number of pole cores are bolted to the yoke, he yoke serves following purpose:-

a).it supports the pole cores and acts as a protecting cover to the machine.

b) It forms a part of the magnetic circuit.

Each pole cores have pole shoe having curved surface it serves:

i) It supports field coils

ii) It increases the cross sectional area of magnetic circuit and reduces reluctance.

The pole cores are made of silicon sheet steel laminations that are insulated from each other by thin layer of varnish and are riveted together.

Each pole core have one or more field coils placed over it to produce

magnetic field

Q2 Explain with neat diagram armature of a DC machine?

Ans The rotating part of the D.C machine is called the armature. The armature consists of a shaft on which a laminated cylinder, called armature core, is mounted with help of a key for small machines and for large machines they are mounted on a spider. The armature laminations are having slots on the outer surface. The laminations are insulated from each other and tightly clamped together. The purpose of using laminated core is to reduce the eddy current loss.

The insulated conductors are put in the slots of the armature core. The conductors are wedged and bands of steel wire is used to prevent them flying under centrifugal forces. The conductors are connected together. The connection arrangement is called armature wining ( LAP AND

WAVE)

Q3 Construction of Commutator

Ans:- Alternating voltage is induced in the coil rotating magnetic field. To obtain direct current in the external circuit a commutator is needed. The commutator rotates with the armature. The commutator is made from wedge shaped hard drawn silver bearing copper segments. The each segments are insulated from each other by mica and the inner surface is also insulated from shaft. The riser of the commutator is connected to the ends of the armature conductors

The current is collected from armature winding by means of two or more car4bon brushes mounted on the commutator. Each brush is supported in the brush holders. Brush holders are mounted on the brush gear. It connects the stationary circuit to rotating circuit of armature.

Q4 Separately excitated DC machine.

Ans:- There are two type of excitation , namely separately excited and self excited machine.

he seethe field coils are energized by a separate DC source. The connections showing the separately excited d.c. machine is shown here below.

Separately-excited generators are those whose field magnets are energised from an independent external source of d.c. current.

Q5 Series wound DC motor or Generator?

Ans Self-excited generators are those whose field magnets are energised by the current produced

by the generators themselves. Due to residual magnetism, there is always present some flux in the poles. When the armature is rotated, some e.m.f. and hence some induced current is produced which is partly or fully passed through the field coils thereby strengthening the residual pole flux.

Three types of self-excited DC Motors or Generators are there

Shunt wound

The field windings are connected across or in parallel with the armature conductors and have the full voltage of the generator applied across them

Series Wound

the field windings are joined in series with the armature conductors

Q6 Compound wound DC Machines

Compound Wound

It is a combination of a few series and a few shunt windings and can be either short-shunt or long-shunt

Q 7 What do you mean by WAVE and LAP Winding

Ans Armature coils can be connected to the riser of commutator to form either LAP or WAVE winding

WAVE WINDING

The ends of each armature coils are connected to commutator Segment some distance apart, so that only two parallel paths are provided between the positive and negative brushes. Thus wave wound machines have A=2, They are used for High voltage low current machines

LAP WINDING The ends of each armature coil is connected to adjacent segments on the commutator so that total number of parallel path is equal to the total number pf poles, Thus for LAP A=P The Lap winding is used for low voltage high current machines.

Q8 Derive E.M.F equation of a Dc Machine

Ans Generated E.M.F. or E.M.F. Equation of a Generator

Eg = PNZF/60a

Where:

Φ = flux/pole in weber

Z = total number of armature conductors

P = No. of generator poles

a = No. of parallel paths in armature

N = armature rotation in revolutions per minute (r.p.m.)

E = e.m.f. induced in any parallel path in armature Generated e.m.f. Eg = e.m.f. generated in any one of the parallel paths i.e. E.

Q9 What do you mean by armature reaction:

Ans All current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a DC generator affects the flux pattern and distorts the main field. This distortion causes a shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called ARMATURE REACTION

Armature reaction can be better understood with this figure:-

UNDESIRABLE EFFECTS OF ARMATURE REACTION

1. Armature reaction causes a net reduction in the field flux per

pole. Due to this net flux decrease, induced armature e.m.f. decreases and also the torque decreases.

2. Distortion of the main field flux along the air gap i.e. MNA axis shifted. Due to this there is a problem of commutation which results in copper losses, iron losses, sparking etc.

Q 10 Methods How to minimize armature reaction

Ans : Methods To Reduce Armature Reaction

By high Reluctance at POLE TIPS:

1. At the time of construction we use chamfered poles. These poles have larger air gap on the tips and smaller air gap at the centre. These poles provide non-uniform air gap. The effect of armature reaction is more near to edge of poles and negligible near the centre of pole.If air gap is kept non uniform i.e., larger air gap at the edges(Pole Tip) and smaller near the centre of the pole and then armature flux near the pole tip decreases and armature reaction decreases.

2. By Laminated Pole Shoe : We insert Laminated objects in the pole. By having Laminatedpole shoe the reluctance in the armature flux path increases. Hence the armature flux gap gets reduced

3. By Reduction in Armature flux: The effect of Armature Reaction is reduced by creating more reluctance in the path of Armature flux. This is achieved by using field Pole Laminations having several Rectangular holes punched in them. It gives high Reluctance in the path of armature flux. Due

to this armature cross flux reduces whereas main field remains almost unaffected.

4. By having Strong main magnetic field:

During the design of DC machine it should be ensured that the main field m.m.f. is sufficiently strong in comparison with full load armature flux.Greater the main field, lesser will be the distortion.5) By using Inter Poles and Compensating WindingsThe effect of Armature reaction is reduced by interpoles placed in between the main poles. The magnetic axis of interpoles is in the Direction of q-axis. Interpoles windings are connected in series with the armature winding. So that, interpoles flux is able to neutralize the effect of armature flux. The interpoles are narrow and tapered with large air gap.

Interpoles are added [NN & SS]

The Armature Reaction can also be limited by compensating windings. It is the best and most expensive methods. The compensating windings are connected in series with armature winding.Q11 What is commutation How it can be improve

In the armature conductors of a d c generator are alternating in nature .

The commutations process involves the change from a generated alternating current to an externally applied direct current . These induced current flow in one direction when the armature conductor are under north pole . They are in opposite direction when they are under south pole . As conductor pass out of the influence of north pole and enter the south pole , the current in them is reversed . The reversible of current takes place along the MNA or brush axis . When ever a brush spans two commutater segments , the winding element connect to those segments is short circuited . By commutation we mean the change that takes in a winding element during the period of short circuit by a brush. These changes are shown in figure.

In position (a)The current I flowing towards the brush from L.H.S passes round the coil in a clockwise direction

In position (b), this coil carries the same current in the same direction, but the coil is to short circuited by brush

In the position (c) the brush makes contact with bars a and b, thereby short circuiting coil 1.The current is still I from L.H.S and I from R.H.S.

It is seen that these two currents can reach the brush without passing through coil 1

In (d) shows that bar b has just left the brush and the short circuit of coil1 has ended. It is now necessary for the current I reaching the brush from the R.H.S in the anticlockwise direction.

It is seen from above that during the period of short circuit of an armature coil by a brush the current in that coil must be reversed and also brought up to its full value in the reversed direction. The time of short circuit is known as period of commutation.

METHODS OF IMPROVING COMMUTATION

There are three methods for getting sparkles commutation.

1. Resistance commutation2. Voltage commutation3. Compensating winding

Resistance commutation

The method of improving commutation consists of using carbon brushes . This makes the contact resistance between the commutator segment and brushes high. This high contact resistance has tendency to force the current in the short circuited coil to change according to the commutation requirements.Voltage commutationIn this method arrangement is made to induce a voltage in the coil under going commutation , which will neutralize the reactance voltage. The injected voltage is in opposition to the reactance voltage and if the value of injected voltage is made equal to reactance voltage ,quick reversal of current in the short circuited coil will take place and there will be sparkles commutation.a)Brush ShiftThe effect of armature reaction is to shift the MNA in the direction of rotation in case of generator and against the MNA in case of Motor. b) Commutating or Inter polesInterpoles are narrow poles attached to the stator yoke, and placed exactly midway between the main poles. The compole windings are connected in series with the armature, because the interpoles must produce fluxes that are directly proportional to the armature currentThe armature and interpole mmf’s are affected simultaneously by the same armature current. Consequently the armature flux which tends to shift the MNA is neutralized. The interpoles must induce a voltage in the conductors undergoing commutation that is opposite to the voltage caused by the neutral plane shift and reactance voltage.i)For a generator, the polarity of the interpole must be the same as that of the next main pole further ahead in the direction of rotation. ii) For a motor, the polarity of the interpole must be the opposite as that of the next main pole in the direction of rotation.

COMPENSATING WINDINGSCompensating windings are the most effective way for eliminating the problem of armature reaction and flash over by balancing armature mmf. The compensating windings are placed in the pole faces parallel to the rotor armature conductors. These windings are connected in series with the armature windings. The compensating winding produces an mmf that is equal and opposite to the armature mmf.The major draw back with compensating winding is that they are very costly. There use can be justified for the following special casesa) In large machines subject to heavy overloads or plugging

b) In small motors subject to sudden reversal and high acceleration.

Q12 Draw the characteristics of a separately and shunt excited DC generator

In general, three characteristics specify the steady-state performance of a DC generators:

1. Open-circuit characteristics: generated voltage versus field current at constant speed. This is also called magnetizing characteristics

2. Internal characteristic: It is plot between generated voltage versus load current

3. External characteristic or Load characteristic: terminal voltage load current at

constant speed.

The terminal voltage of a dc generator is given by

Open-circuit and load characteristics separately excited DC generator

It can be seen from the external characteristics that the terminal voltage falls slightly as the load current increases. Voltage regulation is defined as the percentage change in terminal voltage when full load is removed, so that from the external characteristics,

V t=Ea−I a Ra

=[ f ( I f , ωm )−Armature reaction drop ]−I a Ra

Voltage regulation=Ea−V t

V t

×100

EXTERNAL CHRACTERISTICS

Self-Excited DC Shunt Generator

Maximum permissible value of the field resistance if the terminal voltage has to build up. OPEN CIRCUIT CHARACTERISTICS

Q13 Basic performance of DC Generator what are main characteristics

Since the field current If in a shunt generator is very small the voltage drop IfRa can be neglected and

V=Ea V=If Rf is a straight line and is called resistance line the figure gives below the voltage build up in DC shunt generator for various field circuit resistance. A decrease in resistance of the field circuit reduces the slope of the field resistance line resulting in higher voltage. If the speed is constant An increase in the resistance of the field circuit increases the slope of the field resistance line, resulting lower voltage If the field resistance is increased to Rc which is termed as the critical resistance of the field, the field resistance line becomes tangent to the initial part of the magnetizing curve, when the field resistance is higher than this value, the generator fails to excite .

The figure above shows the variation of no load voltage with fixed Rf and variable speed of the armature. The magnetizing curve varies with the speed and its ordinate for any field current is proportional to the speed of the generator. The following conditions must be satisfied for voltage build up

1. There must be sufficient residual flux in the field poles.2. The field terminals should be such connected that the field current increases the

flux in direction of residual flux3. The field circuit resistance should be less than the critical field resistance.

Characteristics of COPOUND DC GENERATORDepending upon the number of series field turns, the cumulatively compound generators may be over compounded, flat compounded and under compounded. For over compounded generator the full load terminal voltage is higher than No load voltage, for flat compounded generator the full load terminal voltage is equal to the no load voltage. In an under compounded generator the full load terminal voltage is less than the no load voltage.

Q14 explain metadyne and amplydyne

An ordinary DC generator Can be modified to a matadyne by providing an additional pair of brushes on D-axis as shown in figure. The quadrature axis(q axis) brushes are short circuited and the output is

obtained from d axis brushes . The stator has control field winding , A current If flows through the control field winding. When the generator is rotating at a constant speed, the control field wing mmf, Ff induces an emf Eaq between the quadrature axis brushes qq’, This emf is given by

Eaq = Kqf If

Where Kqf is constant If is the field current

Since the qq’ are short circuited, a quadrature axis armature current Iq flows and q axis mmf Fq is established. Since the impedance of the short circuited path is low , only a small control field current if will produce a much larger quadrature axis armature current. Due to commutator action this magnetic field is stationary in space. An emf is generated in the armature by its rotation in the quadrature axis flux, This emf appears across the direct axis brushes dd’and is given by

Ead- KdqI q

Kdq is constant and I q is quadrature axis armature current

For a given Ff a load resistance steady values of Id and Iq are reached. Any increase in Id reduces Eaq as seen by equation above this reduces Iq . Therefore Ead and Id are reduced, hence for given value of control field excited current If the output current Id remains substantially constant over a wide range load variation. That is a Metadyne behave as constant current generator

AMPLIDYNE

The most common version of the matadyne is the amplidyne. The amplidyne consists of the basic matadyne in which a compensating winding is connected in series with the power output brush terminals. The d axis mmf which opposes the control field mmf in matadyne is cancelled by the compensating winding . The compensating winding is located in the d axis on the stator. The compensating winding carries the load current Id and produces a flux which opposes the flux produced by the d axis armature current. The negative feedback effect of the load current is therefore minimized .The d axis flux

now mainly depends on the field winding current. This gives a wider control over the net d axis flux.

Q15 what do you mean by PMDC

A permanent magnet DC (PMDC) motor is the DC motor whose pole are made of permanent magnets. Figure shows two pole PMDC motor. The permanent magnets of the PMDC motor are radially magnetized and mounted on the inner periphery of cylindrical steel stator. The stator also serves as the return path for the magnetic flux. The rotor has conventional DC armature with commutator segments and brushes.

Most of the PMDC motors operates on 6V,12V or 24V dc supply obtained from batteries or rectifiers.

The torque is produced by interaction between axial current carrying rotor conductors and the magnetic flux produced by the permanent magnets. In a PMDC motor Flux Φis constant so the back EMF depends on

E=K1N

τ c= K1Ia

where K1 =K Φ is called speed voltage constant or torque constantt

V=E+IaRa

V = K1N +IaRa

N =(V-IaRa)/ K1

Advantages

1. There is no field circuit copper loss, this increases the efficiency2. No space is required for field winding, these motors are smaller than

corresponding wound pole motors.

Disadvantages

1. There is risk of demagnetizing of poles may be caused by large armature current, excessive heating

2. The magnetic field is always present even when motor is not in use, The motor is totally enclosed so that foreign magnetic matter may not enter or get attracted by PM.

3. The PM can produce high flux density as compare to externally supplied DC shunt field . Therefore PMDC motor has lower induced torque compare to shunt motor.

Q16 what is DC brushless motor

Ans Brushless DC Motor(BLDC)

Principles

• A BLDC is simply a normal dc motor turned inside out, that means the coil is on the out side and the magnets are inside

• The stator consists of several coils which current is led through Creating a magnetic field that makes the rotor turns

Construction

• Stator consists of stacked steel laminations with windings placed in

the slots that are axially cut along the inner periphery

• Rotor is made of permanent magnet and can vary from two to eight pole pairs with alternate North (N) and South (S) poles. Ferrite magnets and Rare earth alloy magnets are used in rotor

•

Unlike a brushed DC motor, the commutation of a BLDC motor is controlled electronically.

• It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence.

• Rotor position is sensed by different ways some of them are

• 1)Hall sensors 2)Optical encoders

• :-When a magnetic field applied to a system with electric current a hall voltage Perpendicular to the field and to current is generated. This was discovered by

Edwin Hall in 1879.

• Halls Sensors sense the position of the coils The Decoder Circuit turns appropriate switches on and off The voltage

• through the specific coils turns the motor

• Torque speed characteristics of BLDC

ADVANTAGES

• Increased Reliability & Efficiency

• Longer Life

• Elimination of Sparks from Commutator

• Reduced Friction

• Faster Rate of Voltage & Current

DISADVANTAGES

• Requires Complex Drive Circuitry

• Requires additional Sensors

• Higher Cost

• Some designs require manual labor (Hand wound Stator Coils)

APPLICATION

• Consumer: Hard Drives, CD/DVD Drives, PC Cooling Fans, toys, RC airplanes, air conditioners

• Medical: Artificial heart, Microscopes, centrifuges, Arthroscopic surgical tools, Dental surgical tools and Organ transport pump system.