For procedures, see your electrical laboratory manual.

Title: DC Generator

Aim

1. To obtain the open circuit characteristics of a DC generator by separately exciting the field winding.
2. To obtain the load characteristics of
— Separately excited DC generator
— Shunt excited DC generator

Apparatus

1. One Power supply panel with meters
2. One 500 Rheostat
3. One Tachometer
4. One 0 - 200 Rheostat, on panel

Theory



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

According to Faraday’s laws of electromagnetic induction, whenever a conductor is placed in a varying magnetic field (OR a conductor is moved in a magnetic field), an emf (electromotive force) gets induced in the conductor. The magnitude of induced emf can be calculated from the emf equation of the dc generator. If the conductor is provided with a closed path, the induced current will circulate within the path. In a DC generator, field coils produce an electromagnetic field and the armature conductors are rotated into the field. Thus, an electromagnetically induced emf is generated in the armature conductors. The direction of the induced current is given by Fleming’s right-hand rule.

A DC generator can be used as a DC motor without any constructional changes and vice versa is also possible. Thus, a DC generator or a DC motor can be broadly termed as a DC machine.

Types Of A DC Generator:

DC generators can be classified into two main categories; Separately excited and Self-excited.
(i) Separately excited: In this type, field coils are energized from an independent external DC source.
(ii) Self-excited: In this type, field coils are energized from the current produced by the generator itself. Initial emf generation is due to residual magnetism in field poles. The generated emf causes a part of the current to flow in the field coils, thus strengthening the field flux and thereby increasing emf generation. Self-excited dc generators can further be divided into three types -
 (a) Series wound - field winding in series with the armature winding
(b) Shunt-wound - field winding in parallel with the armature winding
(c) Compound wound - a combination of series and shunt winding

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 dc generator is the 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.

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 the figure below. In these types of generators, the armature current Ia divides into two parts. One part is the shunt field current Ish flows through shunt field winding and the other part is the load current IL goes through the external load.

Answers to Questions:

1. There are three conditions necessary to induce a voltage into a conductor.
— A magnetic field
— A conductor
— 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 housing 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.
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.