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What is the significance of vector grouping in Power Transformers?

Answer:Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phace difference betweent the current and voltage. EG. DYN11 means Delta primary, Wye Secondry and the current is at 11 o clock reffered to the voltage. 

Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, padestal fan, bracket fan etc) which are find in the houses ? 

Answer:Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run. 

Give two basic speed control scheme of DC shunt motor?

Answer:1. By using flux control method:in this method a rheostat is connected across the field winding to control the field current.so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled 2.armature control method:in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop(IaRa)can be varied,and since speed is directly proportional to Eb-IaRa the speed can be controlled. 

what is the principle of motor?

Answer:Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque. 

what is meant by armature reaction?

Answer:The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux. 

Give two basic speed control scheme of DC shunt motor?

Answer:1. By using flux control method:in this method a rheostat is connected across the field winding to control the field current.so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled 2.armature control method:in this method a rheostat is connected across armature wdg.by varying the resistance the value of resistive drop(IaRa)can be varied,and since speed is directly proportional to Eb-IaRa the speed can be controlled. 

what is the difference between synchronous generator & asynchronous generator? 

Answer:In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing.This type of generators are used in windmills. 

What is the Polarization index value ? (pi value)and simple definition of polarization index ?

Answer:Its ratio between insulation resistance(IR)i.e meggar value

for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5 . 

Why syn. generators r used for the production of electricity?

Answer:synchronous machines have capability to work on different power factor(or say

different imaginary power varying the field emf. Hence syn. generators r used for the production of electricity. 

What is the difference between synchronous generator & asynchronous generator?

Answer:In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing.This type of generators are used in windmills. 

1 ton is equal to how many watts?

Answer:1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower,

12,000 * 0.0003929 = 4.715 hp therefore 1 ton = 4.715*.746 = 3.5 KW. 

why syn. generators r used for the production of electricity?

Answer:synchronous machines have capability to work on differentpower factor(or say different imaginary pow varying the field emf. Hence syn. generators r used for the production of electricity. 

Enlist types of dc generator?

Answer:D.C.Generators are classified into two types 1)separatly exicted d.c.generator 2)self exicted d.c.generator, which is further classified into;1)series 2)shunt and 3)compound(which is further classified into cumulative and differential). 

What is Automatic Voltageregulator(AVR)?

Answer:AVR is an abbreviation for Automatic Voltage Regulator.It is important part in Synchronous Generators, it controls theoutput voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator. 

What is an exciter and how does it work? 

Answer:There are two types of exciters, static exciter and rotory exciter.purpose of excitor is to supply the excitation dc voltage to the fixed poles of generator.Rotory excitor is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotory poles through slip ring and brushes( conventional alternator). if it is an ac excitor, out put of ac excitor is rectified by rotating diodes and supply dc to main fixed poles.ac excitor is the ac generator whose field winding are stationary and armature rotates. initialvoltage is built up by residual magnetism.It gives the starting torque to the generator. 

Difference between a four point starter and three point starter?

Answer:The shunt connection in four point stater is provided separately form the line where as in three point stater it is connected with line which is the drawback in three point stater 

Why use the VCB at High Transmission System ? Why can't use ACB?

Answer:Actually the thing is vacuum has high arc queching property compare to air because in VCB ,the die electric strengths equal to 8 times of air . That y always vaccum used as inHT breaker and air used as in LT . 

What is the difference between surge arrestor and lightning arrestor?

Answer:LA is installed outside and the effect of lightning is grounded,where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge. 

What happens if i connect a capacitor to a generator load?

Answer:Connecting a capacitor across a generator always improves powerfactor,but it will help depends up on the engine capacity of the alternator,other wise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an alternator while it is picking up or without any other load. 

Why the capacitors works on ac only?

Answer:Generally capacitor gives infinite resistance to dc components(i.e., block the dc components). it allows the ac components to pass through. 

Explain the working principal of the circuit breaker?

Answer:Circuit Breaker is one which makes or breaks the circuit.It has two contacts namely fixed contact & moving contact.under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions(when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuitArc is extinguished by the Arc Quenching media like air, oil, vaccum etc. 

How many types of colling system it transformers?

Answer:1. ONAN (oil natural,air natural)

2. ONAF (oil natural,air forced)

3. OFAF (oil forced,air forced)

4. ODWF (oil direct,water forced)

[V1] 5. OFAN (oil forced,air forced)

What is the function of anti-pumping in circuit breaker?

Answer:when breaker is close at one time by close push button,the anti pumping contactor prevent re close the breaker by close push button after if it already close. 

what is stepper motor.what is its uses?

Answer:Stepper motor is the electrical machine which act upon input pulse applied to it. it is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle.so, in automation parts it is used. 

how to calculate capacitor bank value to maintain unity power factor with some suitable example?

Answer:KVAR= KW(TAN(COS(-1)#e)- TAN(COS(-1)#d) )

#e= EXISTING P.F.

#d= DESIRED P.F.

Tell me in detail about c.t. and p.t. ?(Company:reliance)

Answer:The term C.T means current transformer,and the term P.T means potential transformer.In circuit where measurements of high voltage and high current is involved they are used there.Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device.so, CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on linkage of electromagneticflux produced by primary with secondary.They work on the ratio to they are designed.E.g if CTis of ratio 5000\5A and it has to measure secondary current of 8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter .and after measuring 8A we can calculate the primary current.same is the operation of PT but measuring voltage. 

There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why? 

Answer:The motor has max load current compare to that of transformer because the motor consumes real power.. and the transformer is only producing the working flux and its not consuming.. hence the load current in the transformer is because of core loss so it is minimum. 

what is power factor? whether it should be high or low? why?

Answer:Power factor should be high in order to get smooth operation of the system.Low power factor means losses will be more.it is the ratio of true power to apperent power. it has to be ideally 1. if it is too low then cable over heating & equipment overloading will occur. if it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping.(if pf is poor ex: 0.17 to meet actual power load has to draw more current(V constant),result in more lossesif pf is good ex: 0.95 to meet actual power load has to draw less current(V constant),result in less losses). 

What is the difference between Isolator and Circuit Breaker?

Answer:Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. it is manually operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. first the load on it must be made zero and then it can safely operated. its specification only rated current is given.But circuit breaker is onload automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current. 

what is boucholz relay and the significance of it in to the transformer?

Answer:Boucholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself other wise it trips the circuit by its own tripping mechanism. 

What is SF6 Circuit Breaker?

Answer:SF6 is Sulpher hexa Flouride gas.. if this gas is used as arc quenching medium in a Circuitbreaker means SF6 CB. 

what is ferrantic effect?

Answer:Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage. The Ferranti effect is a rise in voltage occurring at the receiving end of a long transmission line, relative to the voltage at the sending end, which occurs when the line is energized but there is a very light load or the load is disconnected. This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance are responsible for producing this phenomenon.[1] The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied.[2] The relative voltage rise is proportional to the square of the line length.[3] Due to high capacitance, the Ferranti effect is much more pronounced in underground cables, even in short lengths. 

what is meant by insulation voltage in cables? explain it?

Answer:It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable. 

Why we do 2 types of earthing on transformer (ie:)body earthing & neutral earthing , what is function. i am going to install a 5oo kva transformer & 380 kva DG set what should the earthing value? 

Answer:The two types of earthing are Familiar as Equipment earthing and system earthing. In Equipment earthing: body ( non conducting part)of the equipment shouldd be earthed to safegaurd the human beings.system Earthing : In this neutral of the supply source ( Transformer or Generator) should be grounded. With this,in case of unbalanced loading neutral will not be shifted.so that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment( transformer or alternator)and selection of relying system earthing will be further classified into directly earthed,Impedance earthing, resistive (NGRs) earthing. 

What is the difference between MCB & MCCB, Where it can be used?

Answer:MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition.under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A. 

Where should the lighting arrestor be placed in distribution lines?

Answer:Near distribution transformers and out going feeders of 11kv and incomming feeder of 33kv and near power transformers in sub-stations. 

Define IDMT relay?

Answer:It is an inverse definite minimum time relay.In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates.It is inverse in the sense ,the tripping time will decrease as the magnitude of fault current increase. 

What are the transformer losses?

Answer:TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R). Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer. 

What is meant by regenerative braking?

Answer:When the supply is cutt off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load(resitor) across the armature winding and the motor should have maintained continuous field supply. so that back e.m.f voltage is made to applyacross the resistor and due to load the motor stops quickly.This type of breaking is called as "Regenerative Breaking". 

Why is the starting current high in a DC motor?

Answer:In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage,Eb = Back emf in Motor,Ia = Armature current,Ra = Aramture resistance).At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased. 

What are the advantages of star-delta starter with induction motor?

Answer:(1). The main advantage of using the star delta starter is reduction of current during the starting of the motor.Starting current is reduced to 3-4 times Of current of Direct online starting.(2). Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.

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