EE 2 MACHINES EVAL

93問 • 1年前

問題一覧

Two intermators operating in paralel supply of 2500 MW to a load at a power factor of 80% tagging if one of the machine dalivers 1200kWat a power factor of 95% lagging, what is the power factor other machine?

0.66

The speed peed regulation of a synchronous motor is always

zero

The percentage stip in case of a synchronous motor is

zero

The operating speed of a synchronous motor can be changed to new fixed value by

changing frequency

A synchronous motor working at leading power factor can be used as

phase advancer

A 100 hp 1750 rpm, 2300 volts, three phase motor draws 150A and produce a starting torque of 120% of full load value when started at 2300 volts if the compensator to be used and the 80% of the voltage tap is selected, find the starting current

A 75 hp, 3-phase, 8 pole 60 Hz, wyo-connected, cylindrical rotor synchronous motor has a synchronous reactance of 96 ohms per phase. Its rated terminal voltage is 500 volts per phase. Find the value of that makes maximum torque to be 120% of rated torque

430 Volts

In an alternator the voltage generated per phase is proportional to

all of the above

In case of DC machines mechanical losses are primary function of

speed

Iron losses in a DC machine are incependent of variation in

load

In DC generators, current to the external arcuit from armature is given through

commutator

The polarity of a DC generator can be reversed by

reversing the field current

Two generators A and B have 6-poles each. Generator A has wave wound armature while generator B has lap wound armmarture. The ratio of the induced and A and B will be

3:1

Following DC generator will be in a position to build up without any residual magnetism in the poles

self excited generator

In a DC generator the number of mechanical degrees and electrical degrees will be the same when

number of poles is 2

A shunt generator running at 1000 rpm has generated emt as 200 V. If the speed increases to 1200 rpm, the generated emf will be nearly

240 V

The emf induced in the armature of a shunt generator is 600V The armature resistance is 01 ohm if the armature sument is 2004 the terminal voltage wiil be

580 V

In a DC machine stray loss is the sum of

iron loss and mechanical loss

Two generators are running in parallel. One of the generators may nun as motor for which of the following reasons?

that generator takes large share of loads

Two DC shunt generators, each with armature resistance of 0 02 ohm and field resistance of 50 ohm run in parallel and supply a total current of 1000 amperes to the load circuit. If their emfs are 270V and 265V, their bus bar voltage will be

257.4 V

When the shunt field of a compound generator is connected across both series field and armature. Such a convection is known as

long shunt

Drop in speed of a DC generator due to increase in load can be compensated by

increasing the input to the prime mover

No-load speed of which of the following motor will be highest?

series motor

The direction of rotation of a DC series motor can be changed by

interchanging field terminals

Which DC motor will be preferred for machine tools?

shunt motor

Which DC motor will be preferred for constant speed line shafting?

Shunt motor

Differentially compound DC motors can find applications requiring

low starting toque

Which DC motor will be preferred for elevators?

Cumulative compound motor

These day DC motors are widely used in

electron traction

By looking at which part of the motor, it can be easily confirmed that a particular motor is DC motor?

Commutator

If a DC generator fails to build up the probable cause could not be

field resistance less than the critical resistance

in a level compounded DC generator, full load terminal voltage is

equal to no-load terminal voltage

In which of the following applications DC series, motor is invariably used

Starter of a car

Which DC motor is generally preferred for cranes and hoists

Series motor

A 230 V DC shunt motor takes 32 A at full load. The back emf in full load. if the resistance of motor armature and shunt field windings are 0.20 and 115 0 respectively, will be

224 V

Two compound generators are connected in parallel. If generator A has a dhm and is adjusted to take 30% of the line load whale generator & with a series field resistance of 0.001 ohm takes the remainder of the load Determine the current flow in the equalizer when the load is 230 kW at 230 volts

100 A

Brushes of DC machines are made of

carbon

If B is the flux density. I the length of conductor and the velocity of conductor, then induced emf is given by

Blv

For generating large current on DC generators which winding is generally preferred?

lap winding

The rated line current of a 230 volt shunt motor is 56 amp. The shunt field circuit resistance is 230 ohms and the armature circuit resistance is 0.15 ohm. How much external resistance must be connected in the armature circuit to limit the current at starting to 125% of the full load armature current

3.2 ohm

A shunt motor is running at 1200 rpm for a load which requires an armature current of 50 ampere from 230 volt source. At no load, the armature current is 5 amp. If the effect of armature reaction has reduced the air-gap flux by 2% from no load to full load, determine the no load speed. The armature resistance is 0.15 ohm

1212 rpm

In a motor the armature resistance is 0.1 ohm. When connected across 110 volts mains, the armature takes 20 amp and it speed is 1200 rpm. Determine its speed when the armature takes 50 A from the same mains, with field increased 10%

1061 rpm

In a transformer the energy is conveyed from primary to secondary

by flux

in the transformer the function of a conservator is to

protect the transformer from damage when oil expends due to heating

No-load current of a transformer has

has small magnitude and low power factor

Two transformers operating in parallel will share the load depending upon their

per unit impedance

If R₂ is the resistance of secondary winding of the transformer and K is the transformation ration then the equival secondary resistance referred to primary will be

R2/K^2

What will happen if the transformers working in parallel are not connected with regard to polarity?

incorrect polarity will result in dead short circuit

A synchronous motor can be used as a synchronous capacitor when it is

over-excited

Mostly synchronous motors are of

salient pole type machine

An over-excited synchronous motor takes

leading current

if load (or torque) angle of a 4-pole synchronous motor is 6 electrical, its value in mechanical degrees is

The angle between the rotating stator flux and rotor poles is called... angle.

Torque

Which of the following methods is used to start a synchronous motor?

Damper winding in conjuction with star-delta starter

The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as

synchronizing torque

In a 3-phase, 4-pole, 50Hz synchronous motor, the frequency, pole number and load torque all are haived. The motor speed will be

1500 r.p.m

The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as

pull-in torque

If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will

Stop

Salient pole type alternators are generally used on

Low and meduim speed prime movers

Which of the following onme movers is least efficient?

Steam engine

In which coil the harmonic component of the generated E.M.F Will be more

Full pitch coil

Unbalanced three-phase stater cuments cause

all of the above

The dark and bright lamp method is used for

Synchronizing

In synchronous machine all of the following losses are fixed except

Copper loss

Two alternators 1 and 2 are sharing an inductive load equally. If the excitation of alternator 1 is increased

alternator 2 will deliver less current and alternator 1' will deliver more current

A 250 MW 60 Hz turbine generator set has a speed regulation of 5% based on its own rating. The generator frequency decreases from 60 Hz to a steady value of 59.7 Hz. Determine the increase in the turbine power output

25 MW

A synchronous motor can develop synchronous torque

only at synchronous speed

Under which of the following conditions hunting of synchronous motor is likely to occur?

Periodic variation of load

When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected

the motor stops

The power developed by a synchronous motor will be maximum when the load angle is

90°

An alternater coupled to which prime mover will usually have the highest rotating speed

Steam Turbine

Hydrogen is used tin large alternators mainly to

Cool the machine

A common method of coeling a power transformer is

oil cooling

In a transformer followinding has got more cross-sectional are

low voltage winding

While conducting short-circuit test on a transformer the foliowing side is short-arcuated

low voltage side

The efficiency of a transformer will be maximum when

copper losses = iron losses

A transformer core is laminated to

reduce eddy current losses

An exciter is a nothing but a

D.C Shunt generator

The speed of a 4-pole 60Hz synchronize machine will be

1800 r.p.m

A 25-Hz 120-V/30-V, 500 VA transformer is to be used on a 50-Hz source. If the core flux density is to remain unchanged, determine the maximum permissible primary voltage

240 V

The primary of a transformer has 200 tums and is excited by a 50-Hz 220-V source. What is the maximum value of the core flux?

4.95 mWb

The power mentioned on the name plate of an electic motor indicates

the output power available at the shaft

The current drawn by the armature of DC motor is directly proportional

the torque required

Which of the following load normally needs starting torque more than the rated torque

conveyors

61 A 4400 volts 50 Hz transformer has a core loss of 840 watts, of which one third of which is eddy current loss. Determine the core loss when the transformer is connected to 4400 volt 50 Hz supply

904 W

The chemical used in breather for transformer should have the quality of

Absorbing moisture

The chemical used in breather is

silica gel

Buchholz's relay gives warning and protection against

electrical fault inside the transformer itself

The full-load copper loss of a transformer is 1600 W at half-load, the copper loss be

400 W

In a large generator, dampers

Increase stability

Which of the following is a prime mover?

Steam Turbine

Which of the following is not integral part of synchronous generator system?

Distribution System

The function breather in a transformer is

to arrest flow of moisture when outside air enters the transformer

DC CIRCUITS POWERLINE

premopremo remo · 75問 · 2年前

DC CIRCUITS POWERLINE