EE207 - DC Generator

EE207

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Title: DC Generator

Aim:   1. To obtain the open circuit characteristics of a DC generator by separately exciting the DC winding.

            2. To obtain the load characteristics of

a)     Seperately excited DC motor

b)    Shunt excited DC motor

Apparatus: See Your laboratory manual on page 29.

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THEORY:

DC GENERATOR BASIC THEORY

          An electric machine has two parts, stator and rotor, separated by an air gap. The stator of the machine does not move and normally is the outer frame of the machine. The rotor is free to move and normally is the inner part of the machine. Both stator and rotor are made of ferromagnetic materials. Slots are cut on the inner periphery of the stator and the outer periphery of the rotor. Conductors are placed in the slots of the stator or rotor. They are interconnected to form windings. The winding in which voltage is induced is called armature winding. The winding through which a current is passed to produce the main flux is called the field winding. Permanent magnets are used in some machines to provide the main flux of the machines. There are two types of d.c machines, the d.c generator and the d.c motor. The d.c generator converts mechanical energy into electrical energy. The d.c generator converts mechanical energy into electrical energy. The d.c motor converts electrical energy into mechanical energy. The d.c generator is based on the principles that when a conductor is rotated in a d.c magnetic field, a voltage will be generated in the conductor.

LOAD MAGNETIZATION CURVE

          The load magnetization curve shows the variation of terminal voltage with the field current for a particular value of load current. This characteristic is more or less a replica of the no-load magnetization characteristics but with ordinates reduced by an amount corresponding to the voltage drop in the armature for the particular load. Curve N₃ in the figure shows the load magnetization
characteristics and curve N₁ shows the no load magnetization characteristics. Curve N₂ is deduced by adding armature and brush drop, corresponding to the load current at which the curve has been drawn, to each order of curve N₃.the difference between curve N₁ and curve N₂ is the voltage drop due to armature reaction. For a machine with brushes in geometrical neutral plane, this drop is negligibly small in unsaturated region but becomes appreciable as saturation increases. This is due to the increased demagnetizing armature reaction.

          The load magnetization characteristics for the separately and shunt excited generators is almost the same. The slight difference is due to the fact for shunt machine, the armature current I_a = I_L- I_t , whereas for separately excited machine IA = Il. This results in a slightly different armature reaction and voltage drop in the two cases.

EXTERNAL CHARACTERISTICS

          The external characteristics for a shunt generator are determined for a fixed resistance in the field circuit and at a constant speed. The shape of these characteristics for a shunt generator is similar to that for a separately excited machine but is more drooping because the field current also gets influenced as the load increases. Due to armature reaction and armature resistance drop the terminal voltage is reduced on load and is the field current since the field winding is connected across the armature, thereby reducing the terminal voltage still further. The effect of reduction in field current on induced voltage becomes predominant at higher values of load current especially those greater than rated current of the machine. If the load resistance is continuously decreased, the characteristics actually turns back indicating that the total voltage drop is so large that there is a net decrease in the load current even though the load resistance is decreased.

Load Characteristics of a Separately excited DC generator

In a separately excited dc generator the field coils are energized from an independent source.

The disadvantage of a separately excited d.c. generator is same that we require an external d.c. source for excitation.

But since the output voltage may be controlled more easily and over a wide range (from zero to a maximum), this type of excitation finds many applications.

  Open circuit characteristic

          The O.C.C. of a separately excited generator is determined in a manner described in previous section. It shows the variation of generated e.m f. on no load with field current for various fixed speeds. Note that if the value of constant speed is increased, the steepness of the curve also increases. When the field current is zero, the residual magnetism in the poles will give rise to the small initial e.m.f. as shown.

Internal and External Characteristics

          The external characteristic of a separately excited generator is the curve between the terminal voltage (V) and the load current IL (which is the same as armature current in this case). In order to determine the external characteristic, the circuit set up is as shown in Fig (i). As the load current increases, the terminal voltage falls due to two reasons:

•         (a) The armature reaction weakens the main flux so that actual e.m.f. generated E on

load is less than that generated (E₀) on no load.

•         (b) There is voltage drop across armature resistance (= I_LR_a = I_aR_a).

Due to these reasons, the external characteristic is a drooping curve [curve 3 in Fig (ii)]. Note that in the absence of armature reaction and armature drop, the generated e.m.f. would have been E₀ (Curve 1).

          The internal characteristic can be determined from external characteristic by adding I_LR_a drop to the external characteristic. It is because armature reaction drop is included in the external characteristic. Curve 2 is the internal characteristic of the generator and should obviously lie above the external characteristic.

Note:   The separately excited dc generators has a decided advantage over the self excited generators - it operates in a stable condition with any field excitation. Thus a wide range of output voltage may be obtained. The main disadvantage of a separately excited generator lies in the inconvenience and expense of providing the separate excitation source. For this reason, the use of this type of generator is limited to experimental and testing laboratories where such a source is available and a wide variation of output voltage is desirable.

Load Characteristics of A Shunt Excited DC Generator

In shunt wound DC generators the field windings are connected in parallel with armature conductors as shown in figure below. In these type of generators the armature current I_a divides in two parts. One part is the shunt field current I_sh flows through shunt field winding and the other part is the load current I_L goes through the external load.

Three most important characteristic of shunt wound dc generators are discussed below:

Magnetic or Open Circuit Characteristic of Shunt Wound DC Generator

This curve is drawn between shunt field current(I_sh) and the no load voltage (E₀). For a given excitation current or field current, the emf generated at no load E₀ varies in proportionally with the rotational speed of the armature. Here in the diagram the magnetic characteristiccurve for various speeds are drawn. Due to residual magnetism the curves start from a point A slightly up from the origin O. The upper portions of the curves are bend due to saturation. The external load resistance of the machine needs to be maintained greater than its critical value otherwise the machine will not excite or will stop running if it is already in motion. AB, AC and AD are the slops which give critical resistances at speeds N₁, N₂ and N₃. Here, N₁ > N₂ > N₃.

Critical Load Resistance of Shunt Wound DC Generator

This is the minimum external load resistance which is required to excite the shunt wound generator.

Internal Characteristic of Shunt Wound DC Generator

          The internal characteristic curve represents the relation between the generated voltage Eg and the load current IL. When the generator is loaded then the generated voltage is decreased due to armature reaction. So, generated voltage will be lower than the emf generated at no load. Here in the figure below AD curve is showing the no load voltage curve and AB is the internal characteristic curve.

External Characteristic of Shunt Wound DC Generator

          AC curve is showing the external characteristic of the shunt wound DC generator. It is showing the variation of terminal voltage with the load current. Ohmic drop due to armature resistance gives lesser terminal voltage the generated voltage. That is why the curve lies below the internal characteristic curve.

The terminal voltage can always be maintained constant by adjusting the of the load terminal.

When the load resistance of a shunt wound DC generator is decreased, then load current of the generator increased as shown in above figure. But the load current can be increased to a certain limit with (upto point C) the decrease of load resistance. Beyond this point, it shows a reversal in the characteristic. Any decrease of load resistance, results in current reduction and consequently, the external characteristic curve turns back as shown in the dotted line and ultimately the terminal voltage becomes zero. Though there is some voltage due to residual magnetism.

We know, Terminal voltage

Now, when IL increased, then terminal voltage decreased. After a certain limit, due to heavy load current and increased ohmic drop, the terminal voltage is reduced drastically. This drastic reduction of terminal voltage across the load, results the drop in the load current although at that time load is high or load resistance is low.

That is why the load resistance of the machine must be maintained properly. The point in which the machine gives maximum current output is called breakdown point (point C in the picture).

For Procedure and Circuit Diagrams, See your Electrical Laboratory Manual from pages 29-32. 

 Fill Up Tables in Your Manual and Continue To….

PRECAUTIONS:

1.     Ensure that all connections are checked by the experiment supervisor before power is switched on.

For More Precautions, See General Electrical Lab Precautions

Answers to Questions:

       

       There are three conditions necessary to induce a voltage into a conductor.

1.     A magnetic field

2.     A conductor

3.     A Relative motion between the two (The Magnetic field and the Conductor)

Self excitation

Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils. The rotor iron retains a magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field creates a weak voltage in the stator coils, which in turn increases the field current, until the machine "builds up" to full voltage.

Starting

Self-excited generators must be started without any external load attached. An external load will continuously drain off the buildup voltage and prevent the generator from reaching its proper operating voltage.

Field flashing

If the machine does not have enough residual magnetism to build up to full voltage, usually provision is made to inject current into the rotor from another source. This may be a battery, a house unit providing direct current, or rectified current from a source of alternating current power. Since this initial current is required for a very short time, it is called "field flashing". Even small portable generator sets may occasionally need field flashing to restart.

The critical field resistance is the maximum field circuit resistance for a given speed with which the shunt generator would excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open circuit characteristics of the generator at a given speed.

2…. COMPARISON AND DIFFERENCE BETWEEN SEFLF EXCITED AND SEPARATELY EXCITED DC GENERATORS

The magnetic fields in DC generators are most commonly provided by electromagnets.

A current must flow through the electromagnet conductors to produce a magnetic field. In order for a DC generator to operate properly, the magnetic field must always be in the same direction. Therefore, the current through the field winding must be direct current. This current is known as the field excitation current and can be supplied to the field winding in one of two ways. It can come from a separate DC source external to the generator (e.g., a separately excited generator) or it can come directly from the output of the generator, in which case it is called a self-excited generator. In a self-excited generator, the field winding is connected directly to the generator output.

The field may be connected in series with the output, in parallel with the output, or a combination of the two. Separate excitation requires an external source, such as a battery or another DC source. It is generally more expensive than a self-excited generator. Separately excited generators are, therefore, used only where self-excitation is not satisfactory. They would be used in cases where the generator must respond quickly to an external control source or where the generated voltage must be varied over a wide range during normal operations. Varying Generator Terminal Voltage DC generator output voltage is dependent on three factors (1) the number of conductor loops in series in the armature, (2) armature speed, and (3) magnetic field strength. In order to change the generator output, one of these three factors must be varied. The number of conductors in the armature cannot be changed in a normally operating generator, and it is usually impractical to change the speed at which the armature rotates. The strength of the magnetic field, however, can be changed quite easily by varying the current through the field winding.

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About Uniben Engineering

Stephen Djes is a passionate Graduate of Engineering from the University of Benin, and he is geared towards helping fellow engineering students in the great institution of UNIBEN to do better at academics.