5 march 2009

Q1. Give an example where a transformer is used in a dc application?

A1. Inverters produce AC power by switching or chopping polarity of DC input source to produce pulsating AC output. Transformers used in inverter circuits are called inverter transformers. Inverters used in applications where DC power is readily available.

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Q2.What is the problem with a fan that does not rotate unless rotated manually to start?

A2.Couldn't find an answer...still searching.

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Q3.Which is more dangerous - shock due to ac or dc?

A3.still searching an answer..

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done for the day....

4 march 2009

Q1.What is the cause of humming noise made by transformers?

A1. When ferromagnetic transformer core is magnetised, the core length along the alternating flux increases and decreases alternately,with the corresponding increase and decrease of its cross section.This phenomenon is called magnetostriction.As the steel laminations change there dimensions alternately, the ferromagnetic core vibrates and a hummng sound is produced.Degree of humming sound depends on the flux density of the core.Greater the core flux density,greater is the tendency of hmmming in transformers.

Joints in the core are also responsible for humming noise though to a lesser degree.

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Q2. Which is the oil used in transformers?What is the function of transformer oil?

A2.Transformer oil, is usually a highly refined mineral oil that is stable at high temperatures and  has excellent electrical insulating properties.

Functions

*For cooling of transformer.

*provides part of the electrical insulation between internal live parts.

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Q3. What is a CT?What is a PT?

A3. The instrument potential transformer (PT) steps down voltage of a circuit to a low value that can be effectively and safely used for operation of instruments such as ammeters, voltmeters, wattmeters, and relays used for various protective purposes.

The instrument current transformer (CT) steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer.  

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Done for the day.

Bye for now

3rd march 2009

3 transformer related questions today.....

Q1. Does transformer draw any current when secondary is open? why?

A1. Yes ,primary will draw the current from the main supply in order to magnetize the core and to supply for iron and copper losses on no load.There will not be any current in the secondary since secondary circuit is open.

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Q2. What is the difference between a power transformer and a distribution transformer?

A1.

Power transformers are used at the sending and receiving ends of a long ,high voltage power transmission line for stepping up and stepping down the voltage.They are manipulated to operate at or near their rated capacity(KVA rating).Therefore they are disconnected during light load periods.A power transformer does not come in direct contact with the consumer terminals.

The choice of a power transformer out of a large number of competing transformers should be based on full load efficiency.

Distribution transformers are those which change the voltage  suitable for utilisation purposes at the consumer's  premises. A distribution transformer must have its secondary directly connected with the consumer's terminals. The primary of distribution transformer are always energised.

The choice of a distribution transformer out of a number of competing transformers should be done using energy efficiency...and not full load eficiency.

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Q3. Why is transformer rated in KVA rather than in KW ?

A3. This is because rated transformer output is limited by heat and hence by lossesin the transformer.The two type of losses being core losses and ohmic (I^2 R)losses,

The core loss depend on transformer voltage and the ohmic losses depend on transformer current. As these losses depend on transformer's voltage and current and are almost independent of load power factor, the transformer rating is expressed in VA (VxI) or in KVA and not in KW.

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done for today .

bye for now.

2nd march 2009

Q1. Why can Birds touching  power lines do not get a shock?

A1, It’s because the electricity is always looking for a way to get to the ground, but the birds are not touching the ground or anything in contact with the ground. A bird's body is a poor conductor as compared to copper wire, so there's no reason why electrons will take a detour through the bird. 

But if the bird touches another wire too, or somehow comes in contact with ground .... circuit gets completed.and bird gets a shock.

A human if touches a single wire does get a shock unlike a bird because he is in contact with ground...which completes the path of electrons and results in a shock.(see fig.)

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Q2. What will happen if you supply a fan with a dc?

A2.In case of 220v A.C. supply, fan (induction motor) will run since it works 

only on indution principle and to produce EMF, "rate of change of flux" should 

be occuring which is only possible if A.C. supply (A.C. voltage & current)is  

given to the fan..

In case of DC supply, rate of change of flux will be zero,so EMF produce is

zero and there will be no torque & no motion. on the other hand, 

fan will heat up & winding may burn.

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Q3. What is skin effect?

A3. The tendency of alternating current(AC) to distribute itself within the conductor

so thatthe current density near the surface of conductor is greater than that at the

core,in other words, current flows through the 'skin' of the conductor.

Skin effect causes the effective resistance of conductor to increase with frequency

of current.

Skin effect is due to the eddy currents set up by AC current.

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Done for today..

Bye for now