Chap.4 DC Motors

 

Chapter 4

DC Machines

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Outline

 

1. Introduction 2. Cl Claass ssif ific icat atio ion n of of DC DC Mot Motor orss 3. Motor Cha Chara raccteristics 4. Efficiency 5. Speed Control 6. Starting of of DC DC Motor  7. Clas Classi sifi fica cati tion on of DC Ge Gene nera rato tors rs 8. Vol olta tage ge Bu Buil ild d Up Pr Proc oces esss 9. Gen eneera rato torr Char Charaacte teri rist stiics 10. Load Flow and and Efficie Efficiency ncy

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Introduction

 

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extensively used in industry.

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In these machines, conversion of energy from electrical to mechanical form or vice versa results from the following two electromagnetic phenomena: When n a con conduc ductor tor mo moves ves in a mag magnet netic ic fie field ld,, volt voltage age 1. Whe is induced in the conductor. 2. Whe When n a cur curren rent-c t-carr arryi ying ng co condu nducto ctorr is is pla placed ced in a magnetic field, the conductor experiences a mechanical force.

•  A wide variety of volt–ampere or or torque-speed characteristics can be obtained from various connections of the field windings. • f ield winding is placed on the stator and the armature winding on the rotor.  •  A simple DC Machine shown in the the following Fig. 3

 

Simple DC Machine

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 Animated Simple DC Machine

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Construction Of DC Machine

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Magnetizatio Magnetization n Curve

 

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If we plot the generated voltage on the Y the  Y axis and axis and field current on the X axis then axis  then the magnetization curve will be as shown in figure below.

•  Magnetization curve of a DC  Machine has a great importance because it represents the saturation of the magnetic circuit. • For this reason this curve is also called saturation curve.

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Magnetization curve of a DC machine 

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The internal generated voltage E  A is directly proportional to the flux in the machine and the speed of its rotation. The field current in a DC machine produces a field mmf F  =  = N F I F , which produces a flux in the machine according to the magnetization curve.

or in terms of internal voltage vs. field current for a given speed.

To get the maximum possible power per weight out of the machine, most motors and generators are operating near the saturation point on the magnetization curve. Therefore, when operating at full load, often a large increase in current I   may be needed for small increases F  in the generated voltage E  A.

 

Classsification of DC Machines • Each DC machine can act as a generator or a motor. • Hence, this classification is valid for both: DC generators and DC motors. • DC machines are usually classified on the basis of their field excitation method. • This makes two broad categories of dc machines; 1. Sepa Separa rate tely ly exc excit ited ed and 2. Self Self-e -exc xcit ite ed. 9

Classsification of DC Machines

 

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Separately excited: In separately excited dc machines, the field winding is source.. That means the field winding is supplied from a separate power source electrically separated from the armature circuit. Separately excited DC generators are not commonly used because they are relatively expensive due to the requirement of an additional power source or circuitry. circuitry. They are used in laboratories for research work, for accurate speed control of DC

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motors with Ward-Leonar Ward-Leonard d unsatisfactory. system and in. few other applications selfexcited DC generators are unsatisfactory In this type, t ype, the stator where field flux may also be provided with the help of permanent magnets (such as in the case of a permanent magnet DC motors). A PMDC motor may be used in a small toy car. Self-excited: In Self-excited:  In this type, field winding and armature winding are interconnected in various ways to achieve a wide range of performance characteristics (for example, field winding w inding in series or series or parallel with the armature winding).In self-excited type of DC D C generator, generator, the field winding is energized by the current produced by themselves. A small amount of flux is magnetism.. So, initially, always present in the poles due to the residual magnetism current induces in the armature conductors of a dc generator only due to the residual magnetism. magnetism. The field flu flux x gradually increases as the induced 10 current starts flowing through the field winding.

Classsification of DC Machines

 

Self-excited machines can be further classified as – 

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Series wound  – In this type, field winding is connected in series with the armature winding. Therefore, the field winding carries whole load current (armature current). That is why series winding is designed with few turns of thick wire and the resistance is kept very low (about 0.5 Ohm). Shunt wound –  Here, –  field is connected in parallel with field the armature winding. Hence, thewinding full voltage is applied across the winding. Shunt winding is made with a large number of turns and the resistance is kept very high (about 100 Ohm). It takes only small current which is less than 5% of the rated armature current. Compound wound – In – In this type, there are two sets of field winding. One is connected in series and the other is connected in parallel with the armature winding. Compound wound machines are further divided as • Short shunt – field winding is connected in parallel with only the armature winding • Long shunt –  –  field winding is connected in parallel with the combination of series field winding and armature winding 11

 

Classsification of DC Machines

Fig: Different connections of dc machines. (a) Separately excited dc machine. (b) Series dc machine. (c) Shunt dc machine. (d) Compound dc machine.

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Separately Excited DC Motors

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Series DC Motors

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Cmpound DC Motors

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Example

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Power Flow and Efficiency

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Example

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Control ol Techn Techniques iques Speed Contr

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Control ol Techn Techniques iques Speed Contr

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Control ol Techn Techniques iques Speed Contr

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Control ol Techn Techniques iques Speed Contr

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Control ol Techn Techniques iques Speed Contr

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Control ol Techn Techniques iques Speed Contr

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Control ol Techn Techniques iques Speed Contr

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Starting of DC Motor 

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DC Generators

 

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The dc machine operating as a generator is driven by a prime mover at a constant speed and the armature terminals are connected to a load. In many applications of dc generators, knowledge of the variation of the terminal voltage with load current, known as the external or (terminal) characteristic, is essential. 27

 

1. Separately excited DC Generator  • The defining equations are the following:

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1. Separately excited DC Generator 

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 Armature Reaction

 

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In a DC machine, the main field is produced by field coils. In both the generating and motoring modes, the armature carries current and a magnetic field is established, which is called the armature flux. The effect of armature flux on the main field is called the armature reaction.

• Much of the armature mmf can be neutralized by using a compensating  winding, which is fitted in slots cut on main pole faces. • the  MMF produced by compensating winding opposes the armature mmf. • The compensating winding is connected in series with the armature winding so that its mmf is  proportional to armature armature mmf. 30

 

2. Shunt (Self-Excited) Generator 

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Voltage buildup in a self-excited dc generator.

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Effect of field Resistance

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External Characteristic Curve

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3. Series Generator 

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4. Compound Machine

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External Characteristic Curve

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Power Flow and Efficiency