What is a DC Generator?

An electromechanical machine which converts input mechanical energy to respective output electric energy is called a generator.

The electrical energy as output from this machine is specified in terms of voltage and current whose waveform does not change with respect to time and has only line resistance as an impedance term. Such an electric power (product of voltage and current) is called DC electric power. Hence, it is called a DC generator.

What is the construction of a DC Generator?

DC motor and generator are same in construction. The only difference is the field applied to the DC machine. It means one can operate a DC machine as DC generator as well as DC motor. The construction of a four-pole DC generator is shown below.

Stator

The stator is an integral part of the DC generator, and its main function is to provide a magnetic field where the coils rotate. This consists of strong magnets, in which there are also the rear poles passing through.

Rotor

The rotor or armature center is the second most important part of the DC generator, and it incorporates fitted metal lines with spaces that can be packed to form a cylinder armature core center. Generally, laminations are introduced to reduce losses due to the current eddy.

Armature windings

Armature center spaces are specially used to hold armature windings. These are in the form of a closed circuit and are connected in series and compliance to improve the current output volume.

Yoke

The outer structure of the DC generator is the yoke and is made of solid metal like steel or iron. It provides the necessary equipment to carry the magnetic flux provided by the poles.

Poles

These are mostly used to hold windings in the field. Usually, those windings are damaged on poles and are connected in series by way of armature windings. In addition, the poles will attach the yoke in a welding manner otherwise using screws.

Pole shoe

The pole shoe is widely used to disperse the magnetic flux and to prevent the winding coil from falling off-grid.

Commutator

Commutator operation is just like a converter that converts AC voltage into DC power, i.e. a rectifier, inside the armature winding into brushes. The commutator is made of a piece of copper, and every piece of copper is secured to every other with the help of mica sheets. It is far located on the machine shaft.

Brushes

A brush alternator uses brushes (or carbon brushes) to help conduct electricity with an alternator or generator. Brushes connect commutator (which moves continuously) to the exterior load (stationary part) and ensures electrical connection between them.

What is the operation of the DC generator?

Under Faraday's law of electromagnetic induction, it is known that when the current conductor is placed in a potentially varying magnetic field, the emf is applied to the conductor. According to Fleming's right-hand rule, the direction of the current induced is changed whenever the direction of the motion of the current carrying conductor changes.

Consider a rotating armature sin a clockwise direction and the right conductor moving downward. When the armature finishes rotating in half, the conductor movement will be reversed i.e upward. Hence, the direction of the current in the armature will also change. On using a split ring commutator, the current reverses when the the armature conductor's connections gets reversed. This gives current in only one direction at the terminals.

There are three types of DC generator on the basis of exitation method, permanent magnet, self excited and separately excited.

• In permanent magnet DC generator, permanent magnets are used to energize the field windings.
• In separately excited DC generator an external source, such as a battery energizes the field windings.
• In self excited DC generator the current within the generator energizes the field windings. Self excited generators are further classified as shunt-wound, series-wounds, and compound wound on the basis of the position of field windings

What are the applications of a DC generator?

Applications for different types of DC generators include the following:

• They are used as portable electric supply system.
• They are used in DC welding as DC supply.
• They are used in labs for testing purpose because they can give variable output voltage.
• They are used for battery charging in various places like industries & power stations.
• They are used to give excitation power supply to the alternators.
• Separately Excited DC generators are the ones which are used for testing purpose in the labs. Secondly, it is used as a supply source to the DC motors.
• Shunt wound DC generator is used for lightening, also to charge a battery box as it gives constant voltage. It can also be used as an excitation source for AC generators or alternators, and finally as a portable generator.
• Series wound DC generators are also known as a constant current source. In regards to application, these generators can be used for supplying an excitation current to the DC locomotives for regenerative braking. These are also used for series arc lightening.
• Compound wound DC generators have both series and Shunt wound DC generators characteristics. This means it can act as  both constant voltage and constant current source when it is made cumulative compounded or differential compounded, respectively. Therefore, it possesses both series wound as well as a shunt wound DC generator application.
• Permanent magnet DC generator generates less DC power comparatively. Due to this reason, it is less used in industry. Talking about application, it is used as dynamos in motorcycles.

Common Mistakes

Remember that, the efficiency of this generator is low due to many losses such as copper, mechanical energy, eddy, etc.

Context and Applications

In each of the expert exams for undergraduate and graduate publications, this topic is huge and is mainly used for:

• Bachelor of Technology in the electrical and electronics department
• Bachelor of Science in physics
• Master of Science in physics

Related Concepts

• AC generator
• Portable generator
• Induction generator

Practice Problems

Q1- The armature of the DC generator is laminated to_________.

(a) reduce the bulk

(b) provide the bulk

(c) insulate the bulk

(d) reduce the eddy current loss

Correct option: (d)

Explanation: The armature of DC generator coils reduces eddy current losses. Eddy's current loss is a circular loss that occurs as a result of rotating weapons. If the coils are not made, there will be a loss of energy and production of a large amount of heat due to exhaustion.

Q2- DC field generator coils are usually made of _________.

(a) copper

(b) mica

(c) cast iron

(d) carbon

Correct option: (a)

Explanation: The DC generator field coil is usually constructed of copper because it has good electrical conductivity, good thermal flexibility, and no corrosion.

Q3- The DC generator may lose the residual magnetism as a result of _________.

(a) vibrations

(b) heating

(c) both

(d) none of these

Correct option: (b)

Explanation: The DC generator may lose the residual magnetism as a result of heating. Residual magnetism is very important to start the generator. If its generator is not available, it will not start.

Q4- In DC generators, the residual magnetism is a system of_________.

(a) 1%

(b) 2.5%

(c) 3.5%

(d) none of these

Correct option: (b)

Explanation: - In DC generators, the residual magnetism is a system of 2.5%.

Q5- In DC generators, the pole shoes are fastened to the pole core by_________.

(a) rivets

(b) welding

(c) countersunk screw

(d) none of these

Correct option: (c)

Explanation: - In DC generators, the pole shoes are fastened to the pole core by a countersunk screw.