AC MACHINES (ALTERNATOR) POWERLINE REVIEWER

premopremo remo · 75問 · 2年前

通報

問題一覧

The voltage applied to field of a rotating cylindrical rotor type alternators

low dc voltage

Alternator are usually designed to generate

Definite Frequencies

The power factor of an alternator is determined by its

load

What kind if rotor is most suitable for turbo-alternator which are designed to run at high speed?

Non-salient pole type

Distribution factor Kd is defined as the ratio of emf's of

distributed to concentrated winding

For a 3- phase winding with 6 slots per pole per phase and with coil span of 15 slot, the value of pitch factor is

0.966

A 4 pole 3 phase synchronous machjne has 48 slots. Calculate the armature winding didtribution factor

0.958

A 4 pole, 1800 rpm alternatir will generate emf at _______ frequency

60 Hz

A 60 cycle alternator has a speed of 120 rpm. How many poles has it?

60 poles

Increasing the speed of alternator, what will happen?

Increase frequency

An alternator on open circuit generates 360 volts at 60 Hz when the field current is 3.6 Amp. Neglecting saturation, determine the generated emf at a frequency of 30 Hz and a field current if 3.0 Amp

150 Volts

A three phase wye connected synchrous machine had a stator with 2000 turns per phase. What rotor flux would be required to produce of voltage of 5 kV line to line

5.4 mWb

Calculate generated emf per phase of a 10 pole, 3 phase, 60 Hz alternator with 5 slot per pole per phase and 4 conductor per slot intwo layers. The winding is short by 3 slots & the flux per pole is 0.12 Weber

2908 Volts

Out of the following conditions, the one which does not have to be met by alternators working parallel is

the machine mush have equal rating

It is never advisable to connect a stationary alternator to live bus bar because

It will get short circuited

Adjusment of field excitation of one of the two alternator operating in parallel will

change its power factor

The power factor of an alternator is 75%> The operator is ordered to increase the power factor to 80%> What shall he do?

decrease the excitation

In order to transfer the load from ine alternator to other alternator, when they are operating in parallel.

prime mover power input is increased

Two alternator opersting in parallel supply of 2500 kW to a load at a power factor of 80% lagging. If one of the machine delivers 1200 kW at a power factor of 95% lagging, what is the power supplied by the other machine?

1300 kW

A generator at no-load operates 61 Hz, slope 1 MW/Hz load 1=1.0 MW and load 2=0.8MW are connected. Find the frequency before and after adding the load 2

60 Hz & 59.2 Hz

Two 60 Hz genersting units rated 250 MW and 400 MW have governor speed regulation of 6% and 6.4% resp. from no load to full load. They are operating in parallel and share a load of 500 MW. Determine the load shared by each unit

190 MW & 310 MW

A 30 kVA,220 volt wye connected 3 phase salient pole synchronous generator supplies rated load at 0.707 lagging power factor. The reactance per phase are Xd= 2 Xq=4 ohms. Determine the percent voltage regulation.

224%

A three phase salient pole 440 volts wye connected synchronous generator operates at a power angle of 20° while developing power of 36 KW. The machine constant are Xd = 2Xq = 5 ohms and Ra is negligible. Calculate the percent voltage regulation

78%

A 150 kVA, 1000 volts three phase wye-connected alternator has an open circuit emf 230 volts has a synchronous reactance of 1.2 ohm per phase and an armature resistance of 0.5 ohm per phase. Calculate the percent regulation at full load with 0.8 lagging power factor

21.8%

A 30 kVA, 220 Volt, wye connected, phase salient pole synchrous generator supplies rated load at 0.707 power factor. The reactance per phase are Xd=2Xq= 4 ohms. Determine the percent voltage regulation.

224%

In large capacity ac generator, the moving part is

Poles

Hydrogen is used tin large alternator mainly to

Cool the machine

Salient pole type alternator are generally used on

Low and medium speed primemovers

It plays an important role in over speed protection of a generator?

Governor

One of the advantages of distributing the winding in alternator is

600

A 60 Hz alternator has a speed of 300 rpm. How many poles has it?

The induced emf in each coil side of a certain alternator winding is calculated as 10 volts. if the measured voltage across the coil is 19.32 volts, the winding is short by how many electrical degrees?

30°

Calculate the electrical displacement between adjacent slots of a 16 pole alternator with 144 slots

20°

The winding of a 3 phase 6 pole 60 cycle alternator has five sixth pitch. What is the pitch factor?

0.966

Determine the distribution factor for a 3 phase winding in which there are 5 slots per pole per phase

0.957

Determine the distribution factor for a 3 phase winding in which there are 18 slots per pole

0.956

In an alternator for a coil having a span of 2/3 of pole pitch, the coil span factor will be

0.866

Which fractional pitch will eliminate the seventh harmonic feom the voltage wave form of alternator?

6/7

Determine the distribution factor for a 3 phase winding in which there are 3 slots per pole per phase.

0.960

A 60 cycle alternator has a speed of 720 rpm. How many poles has it?

10 poles

What is the frequency of the generated emf of a 10 pole alternator driven at 600 rpm?

50 Hz

A 6 pole three phase AC generator has 72 slots, the coil span is 12 slots. What is the pitch factor?

1.00

A 3 phase, 60 Hz 10 pole alternatir has 120 slots and 4 turns in every coil. The coil pitch is short by 2 slots. The winding distribution factor of the alternator is

0.957

The effective voltage for a 5 turn coil on a 6 pole generator is 10.6 volts. The flux per pole is 0.00795 weber. Calculate the frequency of the machine

60 hz

A three phase alternator has three coils each rated at 1330 volts and 160 amperes. What is the voltage of this alternator when connected in wye?

2300 volts

A three phase alternator has three armatyre coils each rated 1200 volts and 120 amperes. What is the kVA rating of this unit?

432

Find the synchronous impedance of a 3 phase synchronous generator in which a given field current produces an armature current of 200 amperes on short circuit and a generated emf of 1050 volts on open circuit

3.0 ohms

If in a 3 phase alternator field current of 50 A produces a full load current of 200 A on short circuit and 1730 V on open circuit then its synchronous impedance is

5 ohms

A single phase generator with full load current of 450 amperes at 2400 volts has 0.08 ohms resistance and synchronous reactance is 2.8 ohms. Determine the voltage regulation in percent at 75% power factor lagging

A 25 kVA 3 phase wye connected 400 V synchronous generatir has a synchronous impedance of 0.5+j1.6 ohms per phase. Determine the full load voltage regulation at 0.8 power factor leading

-5.5%

A 3 phase 100 kVA, 2300 volts, 60 hz, 4 pole wye connected alternator having a synchronous impedanxe of 1.2+j20 ohms per phase deliver 34.5 kW at 0.866 power factor, current lagging to a 2300 volts line. Compute the voltage regulation?

A 25 kVA 220 volt three phase alternator deliver rated kVA at 0.84 power factor. The effective ac resistance between the armature winding terminals is 0.18. The field takes 9.3 amperes at 115 volts. If the frictiin and windage loss is 460 qattss and the core loss is 610 watts. What is the efficiency?

86.35%

Synchronous of alternator means

connecting alternator in parallel

Damper of Amortisseur windings are used in alternators to

prevent hunting

Increasing the power output of alternator worling in parallel to a large system, the operator must adjust

Governor

A belt driven generator supplies 875 kW at 90% efficiency. The loss in the driven belt is 2.5%. Calculate the ho of the engine needed to drive the generator. A 100 kVA. 400 volt wye connected salient pole synchronous generator runs at full load and 80% power factor leading. If Xd=2Xq=1.1 ohms per phase and Ra is negligible, calculate the voltage regulation

-26.4%

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 frequency in the turbine power output

25 MW

Damper in large generator

increase stability

A generator at no loaf operates 61.2 Hz, slope 1mHz/W. Load 1=1200W and load 2=900W. Find the frequency before and after adding the load 2

60 Hz & 59.1 Hz

Overheating of generator winding

reduces life of the machine

Two alternatir are to be put in parallel. Which of the following factors should be identical for both?

All of the above

A lower voltage alternatir for the same power rating will be

larger in size

Dirt accumulation in generator can cause all of the following except

low power factor

Which if the following is the common synchronous speed in rpm between 50 Hz and be 60 Hz alternator

600

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