Basic Electrical Engineering: May 2016

Monday 23 May 2016

Application of synchronous motors

1. They are used in power houses and substations in parallel to the bus-bars to improve the power factor.

2. Synchronous motors are also used to regulate the voltage at the end of transmission lines.

3. In factories having a large number of induction motors, or other power apparatus operating at lagging power, they are employed to improve the power factor.

4. Typical applications of high speed synchronous motors are such drives as fans, blowers, dc generators, lines shafts, centrifugal pumps and compressors, reciprocating pumps and constant speed frequency changers, rubber and paper mills etc.

What is synchronous motor

1. The synchronous motor is one type of 3 phase A.C motor which operate at a constant speed from no load to full load.

2. It is similar in construction to 3-phase A.C generator in that it has a revolving field which must be separately excited from a D.C source.

3. By changing the D.C field excitation,the power factor of this type of motor can be varied over a wide range of lagging and leading values.

4. This motor is used in many application because of its fixed speed from no load to full load, its high efficiency and low initial cost.

5. It is also used to improve the power factor of 3 phase AC industrial circuits.

Wednesday 11 May 2016

Why alternators are rated in kva and not in kw?

Apart from constant loss, the variable loss which occurs in alternators is the copper loss, occurring in the 3-phase winding which depends of R and the square of the current delivered by the generator, As the current is directly related to the apparent power delivered by the generator, the alternators have only their apparent power in VA/KVA/as their power rating.

What is synchroscope?

The best method of synchronizing alternator is by means of a single phase device known as synchroscope, which provides a more accurate indication of synchronization.

Why alternators are synchronized in power plant?

In power stations, alternators are operated in parallel to supply the total load. The need for parallel operation arises due to the following reasons.

1. The total load requirement cannot be met by a singe alternator.

2. Parallel operation increases reliability of electric supply. An outage of one alternator will not cause total power loss to the load

3. If alternators are operating in parallel, one or more of them can be shut down for preventive

maintenance in turn.

4. Parallel operation of alternators leads to economy in operating costs. The less efficient machine can be shut down when the load requirement is less.

What are the conditions for parallel operation of an alternator

What is an excitor?

An excitor is a small generator (usually flat compound wound type ) to supply dc power to the field magnet system or rotor of the alternator.

Monday 9 May 2016

What are the losses in the alternators ?

1. Copper losses

2. Iron losses

3. Friction and windage losses .

What is an armature reaction ?

When the alternator is loaded, current flow through the armature winding which establishes its own flux. there are two fluxes in the air gap. First flux is main flux due to the current through the field winding. Second flux is armature flux, due to the current through the armature. The effect of armature flux on main flux is armature reaction.

What are the causes of changes in voltage of alternators when loaded

1. voltage drop due to resistance of winding

2. Voltage drop due to leakage reactance .

3. Voltage drop due to armature reaction .

How will you adjust the frequency of an alternator

Frequency of an alternator can be adjusted by varying the speed of the prime mover ( as frequency (F) is proportional to speed (N) )

What is the principle of an alternator

The alternator works on the principle of faraday,s law of electromagnetic induction. Whenever a conductor links with a magnetic field either the conductor is moving or the field is moving an emf is induced in the conductor.

1. Faraday's law of electromagnetic induction.

What is an alternator

An alternator or AC generator is a synchronous machine which converts mechanical energy into electrical energy and produces alternating emf.

Alternator

Saturday 7 May 2016

Why buchholz relay is used in transformer

Oil filled transformers with a conservator may have a gas detector relay (Buchholz relay. )These safety devices detect the buildup of gas inside the transformer due to corona discharge, overheating, or an internal electric arc. On a slow accumulation of gas, or rapid pressure rise, these devices can trip a protective circuit breaker to remove power from the transformer.

1. Explain relay

why oil is used in transformer

Transformer oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures for an extended period. To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Very large or high-power transformers may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.

why transformer oil testing is required

Transformer oils are subject to electrical and mechanical stresses while a transformer is in operation. In addition there is contamination caused by chemical interactions with windings and other solid insulation, catalyzed by high operating temperature. The original chemical properties of transformer oil change gradually, rendering it ineffective for its intended purpose after many years. Oil in large transformers and electrical apparatus is periodically tested for its electrical and chemical properties, to make sure it is suitable for further use. Sometimes oil condition can be improved by filtration and treatment

why we are using KVA rating for transformer,not KW rating?

why we are using KVA rating for transformer,not KW rating?

comment your answer .

Thursday 5 May 2016

What is the Full form of bdv test?

Full form of bdv test?

BDV = Break Down Volt test

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