Extra low voltage

       Extra low voltage refers to reduced voltages which are used in houses, parks, gardens, swimming pools to eliminate the risk of electric shock. AC voltages below 50 volts and DC voltages below 120 volts are considered to be Extra low Voltage. In many countries, Extra Low Voltage supplies are used to power traffic signals.  This has been facilitated with the advent of LED lighting technology. 

Lighting transformers

    Lighting Transformers are usually used in residences, swimming pools, gardens to specifically feed lighting loads.  These transformers step the low voltage of the distribution system, 230 V to a still lower voltage (Extra Low Voltage) 12V or 24 V.  The use of such low voltage, enables the design of smaller lights.  In applications such as lighting in swimming pool or gardens, lighting transformers producing voltages less than 30 V are used for safety reasons.Lighting transformers are usually hidden in the ceiling or in the ground on in a recess in a wall.  Since the output voltage of the lighting transformer is quite low, they need to be placed very near the lighting load to avoid any drop in the voltage. A single lighting transformers can power more than one lighting load.  Lighting transformers for outdoor applications are designed with weatherproof enclosures. 

The electromagnetic type transformer is designed like a conventional step down transformer.  A toroidal core is usually used to avoid the humming noise from the transformer.  

Vibration motors

     Vibration motors are motors which deliberately generate vibration.  These motors are used in mobile phones to create vibration alerting the user to a call or a message.
      These motors are constructed just like normal motors.  However, they have a mechanically unbalanced weights attached to the output shaft.  These unbalanced weights create vibration.  The frequency and the magnitude of the vibration can be changed by modifying the shape and weight of the counterweights.

Water trees and electrical trees

 Water trees:
            Water trees are tree-like defects, filled with water, which are developed in the insulation of cables. The defects usually originate from defects, voids or contaminants. The trees can cause premature failure of the insulation. Water trees usually propagate in the direction of the electric field. They occur only in the presence of water in the insulation. They are usually invisible to the naked eye in the dry condition.. Water trees reduce the breakdown strength of the cable.
Electrical trees:
      Electrical Trees are formed in the absence of water in dry conditions. They are caused by voids, impurities and defects in the insulation. High electrostatic stress which reverses direction as in AC cables can also accelerate the phenomenon. Occasionally, water trees may evolve into electrical trees. These trees are accompanied with partial discharge which may accelerate insulation failure. Electrical trees are readily visible to the naked eye.

Hipot testing

     Hipot Test is a high voltage test that is used to check the integrity of insulation for high voltage equipments such as busbars, cables, motors etc. The term 'Hipot' is the shortened form of High Potential. The Hipot test is used to ensure that an insulation can withstand a high potential without risk of failure.The Hipot test is alternatively known as Dielectric Withstand test. The test involves the application of a high voltage usually about two times the
operating voltage. Thus a 6.6kV equipment will be tested at a voltage of 13kV.

     The test is conducted for 1 minute to five minutes. If the hipot test is conducted on a transformer winding or an alternator winding, the test is conducted on individual phases. The phases are separated and those phases which are not subjected to the hipot voltage are grounded.

Plugging In Motors

Plugging is a method of braking used in induction motors.  Plugging involves interchanging the supply to two of the stator phase windings.  This method is used in applications which require immediate stop applications.  When the phase supply is reversed, torque is produced in the opposite direction.  This leads to braking of the electric motor.

Motors which are operated this way have a plugging switch.  This switch operates when the stop command is given to the motor circuit.  The operation of this switch applies reverses the supply to two of the windings.  When the motor comes to a complete halt, this reversed supply is disconnected. 

However, this method of braking generates a large amount of heat in the rotor, even greater than that produced by a locked rotor.  This is due to the kinetic energy of the rotor and the coupled load.

Plugging should not be used frequently as the heat produced in this method of braking can cause the rotor bars to overheat and, sometimes, melt.

What are paraistic loads?

  Parasitic loads are electric devices and appliances which draw power even when they are off.  For example, the television which has been switched off using the remote but, is still connected to the power socket continues to draw a small quantity of power.  Parasitic loads are other known as phantom loads or
vampire loads.

Synchronization and paralleling

What is the difference between synchronization and paralleling?
Ans:SYNCHRONIZATION:
It is the process of making phase,voltage,frequency same.

PARALLELING:

It is the process of sharing the load among all the generators.synchronization is done before closing the circuit breaker.after closing it paralleling starts. 

DC supply to AC motor

What will happen if DC supply is given to AC motor?
Ans:AC motor works on the principle of induction.The windings in the AC machine are wound either in star or in delta.(AC machine windings).Only if there is a rotating magnetic field,we have an EMF being  induced and hence current.so, only when these current carrying conductors are placed in the magnetic field, torque will be produced.so, if we give DC supply,no emf,no current, and hence no motion!!moreover,when DC is given,impedance is very low(bcoz,the effect of inductance of coil is getting neglected in DC).So, the winding will be burnt.

Earthing and Grounding

What is the difference between earthing and grounding?
Ans:GROUNDING:
grounding means,keeping that particular terminal as common point.Ex.(take any circuitry,where a number of different voltages are used,0-12v,0-24v,0-110v etc.here all the 0v terminals are grounded and taken as a common terminal.)
EARTHING:
It means making the common point to earth.The potential of this point shall be 0v.Ex.(Equipments are earthed for safety of operator.)

Oil in transformers

What is the purpose of using oil in transformer tanks?which oil is used?
Ans:the function of oil is two-fold.It is used
1)As a coolant
2)As an insulator.
The oil used is mineral oil.Trade names are ASKARELS,PYROCLOR.Good transformer oil should be absolutely free from alkalies,sulphar and particularly from moisture.Hence, the tanks are sealed air-tight.

Efficiency

Three phase induction motor is otherwise called as rotating transformer.but which has higher efficiency?
Ans:As a transformer is a static device,only loss in it are copper loss,core loss.Hence it's efficiency is highest among all electrical machines. It's efficiency is greater than 95%.
 Since 3 phase induction motor is a rotating device, mechanical losses are also included.It's efficiency is around 70%.

Transformer

What will happen if DC supply is given to the transformer?
Ans:Transformers have high inductance,low resistance.When DC is given, only resistance will act.(bcoz inductors,capacitors won't work in DC).since it has low resistance, high current will flow in primary side which will damage windings and insulation.

Electro static precipitator

What is the use of electro static precipitator?
Ans: There are two types of ash produced in thermal power plants.They are bottom ash,fly ash.electro static precipitator is used to collect the fly ash.

Synchronous condenser

When does  synchronous motor act as a synchronous condenser?
Ans;when it is over excited it will act as a synchronous condenser.Sync.condenser is like a capacitor.It can be used for power factor improvement.

Interview questions

Why does humming noise occur in transformer?
Ans:It occurs due to magnetostriction effect.When a ferromagnetic material is subjected to magnetism, change in dimensions will occur in the material.Due to this humming noise occurs in the transformer.

Interview question

Why the earthpin of a 3point pin is bigger in diameter?
Ans: The earth pin has to send the fault current to earth.So it must have smaller resistance. to have smaller resistance, the diameter should be high.It can be understood from R=pl/A.