D.C generator working and construction

 DC Generator working and construction details:

A shunt-wound DC generator is constructed so that the field winding is in parallel with the armature winding. The voltage of a shunt-wound DC generator decreases with an increase in load current. A series-wound DC generator is constructed so that the field winding is in series with the armature winding. The voltage of a series-wound DC generator increases sharply with an increase in load. In a cumulatively-compounded DC generator, the series and shunt fields aid one another. In a differentially-compounded DC generator, the series and shunt fields oppose one another. The voltage of a flat-compounded DC generator changes less than 5 percent from no-load to full-load. The voltage of an over-compounded DC generator gradually rises with an increasing load.

Here is the construction diagram of dc generator

An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy.

The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. A dynamo machine consists of a stationary structure which generates a strong magnetic field, and a set of rotating windings which turn within that field. On small machines the magnetic field may be provided by a permanent magnet; larger machines have the magnetic field created by electromagnets.

The energy conversion in generator is based on the principle of the production of dynamically induced e.m.f. Whenever a conductor cuts magneticic flux , dynamically induced e.m.f is produced in it according to Faraday's Laws of Electromagnetic induction.This e.m.f causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux.

Generator Construction:

Simple loop generator is having a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnet or electro magnets.In case of without commutator the two ends of the coil are joined to slip rings which are insulated from each other and from the central shaft.Two collecting brushes ( of carbon or copper) press against the slip rings.Their function is to collect the current induced in the coil. In this case the current waveform we obtain is alternating current ( you can see in fig). In case of with commutator the slip rings are replaced by split rings.In this case the current is unidirectional.

Components of a generator:

Rotor: In its simplest form, the rotor consists of a single loop of wire made to rotate within a magnetic field. In practice, the rotor usually consists of several coils of wire wound on an armature.

Armature: The armature is a cylinder of laminated iron mounted on an axle. The axle is carried in bearings mounted in the external structure of the generator. Torque is applied to the axle to make the rotor spin.

Coil: Each coil usually consists of many turns of copper wire wound on the armature. The two ends of each coil are connected either to two slip rings (AC) or two opposite bars of a split-ring commutator (DC).

Stator: The stator is the fixed part of the generator that supplies the magnetic field in which the coils rotate. It may consist of two permanent magnets with opposite poles facing and shaped to fit around the rotor. Alternatively, the magnetic field may be provided by two electromagnets.

Field electromagnets: Each electromagnet consists of a coil of many turns of copper wire wound on a soft iron core. The electromagnets are wound, mounted and shaped in such a way that opposite poles face each other and wrap around the rotor.

Brushes:The brushes are carbon blocks that maintain contact with the ends of the coils via the slip rings (AC) or the split-ring commutator (DC), and conduct electric current from the coils to the external circuit.

How DC generator works?

The commutator rotates with the loop of wire just as the slip rings do with the rotor of an AC generator. Each half of the commutator ring is called a commutator segment and is insulated from the other half. Each end of the rotating loop of wire is connected to a commutator segment. Two carbon brushes connected to the outside circuit rest against the rotating commutator. One brush conducts the current out of the generator, and the other brush feeds it in. The commutator is designed so that, no matter how the current in the loop alternates, the commutator segment containing the outward-going current is always against the "out" brush at the proper time. The armature in a large DC generator has many coils of wire and commutator segments. Because of the commutator, engineers have found it necessary to have the armature serve as the rotor(the rotating part of an apparatus) and the field structure as the stator (a stationary portion enclosing rotating parts).

Motor inner parts

Motor inner parts 

Wave energy system

wave Energy System:

The wave Energy System developed in Australia by BioPower Systems, harnesses the power of ocean waves and converts it into smart grid-connected electricity.

Which method can bring the locomotive to dead stop

# which method can bring the locomotive to dead stop.

(a) Plugging braking
(b) Rheostatic braking
(c) Regenerative braking
(d) None of the above

 

Answer : plugging for dc motor except for dc series motor

            for d.c series motor dynamic breaking is used

Difference between ground and neutral

What is the main difference between Ground and Neutral?

Answer:
The difference between Ground and Neutral? NEUTRAL is the origin of all current flow. In a poly-phase system, as its phase relationship with all the three phases is the same, (i.e.) as it is not biased towards any one phase, thus remaining neutral, that’s why it is called neutral. Whereas, GROUND is the EARTH on which we stand. It was perceived to utilize this vast, omnipresent conductor of electricity, in case of fault, so that the fault current returns to the source neutral through this conductor given by nature which is available free of cost. If earth is not used for this purpose, then one has to lay a long. long metallic conductor for the purpose, thus increasing the cost. Ground should never be used as neutral. The protection devices (eg ELCB, RCD etc) work basically on principle that the phase currents are balanced with neutral current. In case you use ground wire as the neutral, these are bound to trip if they are there – and they must be there. at least at substations. And these are kept very sensitive i.e. even minute currents are supposed to trip these. One aspect is safety – when someone touches a neutral, you don’t want him to be electrocuted – do you? Usually if you see the switches at home are on the phase and not neutral (except at the MCB stage). Any one assumes the once the switch is off, it is safe (the safety is taken care of in 3 wire system, but again most of the fixtures are on 2 wire) – he will be shocked at the accidental touching of wire in case the floating neutral is floating too much.

Transformer rating in kva

Why transformer rating is in KVA?

A transformer is basically rated in Kilovolt-amperes (kVA). An example is a 10 kVA Single phase Transformer, by which, the rating is 10 kVA. Have you wondered why transformers are rated in kVA and not in Kilowatts (kW)?
If we dig deeper into the study of transformers, we will see that the copper loss of a transformer depends on the current, while its iron loss depends on its voltage. Therefore, the total transformer loss, which is the copper loss and the iron loss, depends on the volt-ampere and not on the phase angle between current and voltage. Thus, the transformer losses are independent on the load power factor. It is the main reason why transformers are rated in kVA and not in kW.

Single phase transformer

CLEAR EXPLANATION OF SINGLE PHASE TRANSFORMER

Basic Parts of the Distribution transformer

In Transformers Part 1, a transformer was defined as a stationary equipment which transforms power from one voltage level to another through electromagnetic induction. After learning the theory of operation of transformers, it is necessary to know the basic parts of an electrical transformer. Take note that the transformer used in my example is a single phase distribution transformers. These are transformers mostly mounted on pole to deliver power to residential and commercial establishments.
As shown in the figure, the basic parts of a distribution transformer are as follows:
A.  Hand Hole –The hand hole, as the name implies, serves as an access point of a technician to tap changers/mechanisms located  inside the tank without the need of opening the cover. Most modern transformers have tap changers located outside the tank for convenience.
B. Lifting Lugs – This is used, where the hook/rope is connected, for lifting, either using a hoist or a crane.
C. Terminal Markings (Secondary) – The markings provide identification about the terminals of a transformer.
D. LV Bushing – The bushing is made of porcelain. It serves as the output and is the low voltage side of the transformer, which usually supplies power on residential/commercial establishments.
E. Ground Tap – As the name implies, this is where the grounding of the transformer is connected.

F. Transformer Markings – Are markings which indicate the capacity (in Kilovolt-amperes) of a transformer and its voltage output.
G. Radiator Fins –This is the cooling mechanism of a transformer. The cooling mechanism of a transformer depends on the size or rating of the transformer. The bigger the rating, the more cooling mechanisms are used. Take note that in order to maintain operation of the transformer for a long period of time, it is necessary to keep the temperature to be stable.
H. Casing (Tank) – It is generally made of steel. It encloses the core-coil and is the container for the liquid coolant/insulant.
I. HV Bushing – This is the bushing for the high voltage side of the transformer. It is usually made of solid porcelain. This is the input of the transformer.
J. Pressure Relief Device – This is a spring loaded device which releases excess pressure.
K. Mounting Lugs – This is usually used if the transformer is mounted on poles. It is connected on a transformer cluster which is connected on a pole.
L. Tap Changer – This sets the ratio of the HV and LV windings. Most distribution transformers have taps ± 5%, ± 2.5% and 0.
M. Nameplate – Contains all data about the distribution transformer it is connected to.
N. Core – Part of a transformer that serves as a path for the flow of magnetic flux. There are two types of transformer core, the Shell type and the Core type. The core is inside the tank.
• Core Type – The core is in the form of a rectangular frame with coils placed on two vertical sides. They are divided, part of each primary and secondary on each of the two vertical legs.
• Shell Type – The core surrounds the coils, instead of the coils surrounding the core.
O. Windings – Arrangement of conductors wound on an insulating form with each turn insulated from all the other turns. This usually determines the rating of the transformer as the winding is designed by the amount of current it can carry.