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WorksheetsAC Machi-es
Total questions: 40
Worksheet time: 20mins
To adjust the power of the alternator, adjust the _______.
power factor
governor
field current
frequency
In a 6-pole alternator, how many mechanical degrees is the equivalent of 180 electrical degrees?
60
30
120
90
A 60 Hz alternator has a speed of 300 rpm. How many poles has it __________.
12
6
24
16
What kind of load of an alternator has for a negative voltage regulation?
Resistive
Inductive
Inductive and resistive
Capacitive
For two alternators operating in parallel, if the load shared by one of them is to be increased, its field excitation is _________.
to be strengthened keeping input torque the same
to be weakened keeping input torque the same
to be kept constant but input torque should be increased
to be kept constant but input torque should be decreased
Two exactly similar turbo-alternators are rated 5 MW each. They are running in parallel. The speed-load characteristics of the driving turbines are such that the frequency of alternator one drops uniformly from 60 Hz on no-load to 57.5 Hz on full-load, that of alternator two from 60 Hz to 58 Hz. How will the two machines share a load of 8,000 kW?
3,556 kW, 4,444 kW
4,000 kW, 4,000 kW
3,500 kW, 4,500 kW
3,636 kW, 4,364 kW
What is the formula for the synchronous speed of a motor?
Ns = 120P/f
Ns= 120fP
Ns= 120f/P
Ns=60f/P
What affects the speed of a rotating field?
torque and frequency
torque and no. of poles
frequency and number of poles
frequency and horsepower
A 60-Hz induction motor has 2 poles and runs at 3510 rpm. What is the synchronous speed of the induction motor?
3200 rpm
3600 rpm
3800 rpm
2400 rpm
An induction motor with 8 poles, 60 Hz is operated with a slip of 3%. Calculate the rotor speed.
1, 684
1,024
873
927
The nameplate speed of a 60 Hz, 3-phase induction motor is 1,175 rpm. What is its number of poles?
2
4
6
8
The rotor of a 3-phase, 60 Hz, 4-pole inductor motor takes 120 kW at 3 Hz. What is the rotor speed of the induction motor?
1710 rpm
1810 rpm
1800 rpm
1700 rpm
If the speed of an induction motor increases, what happens to the rotor frequency?
Increases
Decreases
does not change
frequency has no relation to the speed
When the frequency of a synchronous motor increases, what happens to its speed?
Decreases
Increases
remains the same
none of these
A three-phase, 60 HZ induction motor has a full-load slip of 5% and full-load rotor current per phase of 10 A. The resistance of its wye-connected rotor is 0.035 ohm per phase. Determine the internal power developed.
200 w
210 w
220 w
160 w
A 6-pole, three-phase, 60 Hz wound rotor induction motor delivers 25 hp at 1,150 rpm. If the rotational losses of the motor are 300 watts, what is the rotor power input in KW?
21.66
18.95
19.77
22.74
Which of the following is NOT a reason why single-phase induction motor run?
two windings are displaced from each other
hysteresis
one of the current is made out of phase between the lines
shaded poles
What is the main reason that alternating current replaced DC for general used?
AC voltages conveniently transformed
AC generators do not need slip rings
AC minimizes magnetic fields
A given AC current does not heat a power line as much as the same as DC current
Which of the following is not a basic element of transformer?
Core
Primary winding
Secondary winding
Mutual flux
Which of the following transformer has only one winding?
Current
distribution
autotransformer
power
What type of transformer bank is used to convert 2-phase to 3-phase power?
open-delta
scott-T
wye-delta
delta-wye
Transformers are used to change the value of the following EXCEPT:
alternating current
alternating voltage
power
resistance
Which is not a characteristic of an ideal transformer?
finite permeability
infinite permeability
no losses
no leakage flux
Ideal transformer assumptions do NOT include __________.
zero resistance of the winding
zero reactance of the winding
no saturation of the core
no leakage flux
Open circuit test on transformers is conducted to determine _____.
hysteresis loss
copper loss
core loss
eddy current loss
Copper loss of a transformer are determined by means of a/an ________ test.
open-circuit
polarity
short-circuit
inductive-circuit
To protect the power transformer against fault, the CT will have _______.
Delta-delta connection
Delta-wye connection
Wye-delta connection
Wye-wye connection
A 200:5 current transformer is used with a 10-amps ammeter. If the meter reads 3.75 amps, determine the line current.
300 A
150 A
180 A
45 A
At a 115 kV substation, the PT ratio is 1,000 and the CT ratio is 1200/5. The potential going into the wattmeter is 115 volts. What is the MW indicated when the wattmeter reads 800 watts?
150 MW
192 MW
15 MW
19.2 MW
A transformer takes 5.17 kW at a power factor of 0.85 when connected to a 2.3 kV distribution circuit. What is the current input in Amp.?
2.51
2.45
2.65
2.81
A transformer has a primary winding of 2000 turns and voltage of 2400 v and current of
8.66 - j5 with an impedance z2 connected across the secondary winding. The secondary winding has 500 turns. What is the value of the secondary impedance?
8.66 + j5 ohm
26 + j15 ohm
15.32 + j10 ohm
13 + j7.5 ohm
The primary of a transformer has 200 turns and is excited by a 60 Hz, 200 V source.
What is the maximum value of the core flux?
6.12 mWb
4.13 mWb
3.41 mWb
5.31 mWb
A transformer is rated 2-kVA, 230/115 v, 60 Hz. Because of an emergency, this transformer has to be used on a 50-Hz system. If the flux density in the transformer core is to be kept the same as at 60 Hz and 230 v, what voltage should be applied at 50 Hz?
195.83 V
200 V
183.33 V
191.67 V
An 8 MVA, 69/13.2 kV transformer has a 6.75% impedance. What is the actual impedance?
0.1470150.147015 ohms
147.015 ohms
14.7015 ohms
1.47015 ohms
A 7.5 MVA, 69/13.8 kV, 3-phase transformer has 8% impedance. What is the per unit impedance at 10 MVA base?
0.24
0.106
0.06
0.096
In a single-phase transformer, the core losses is 60 W at 40 Hz and 95 W at 60 Hz both measured at the same maximum value of flux density. Find the core losses in W at a frequency of 50 Hz.
69
77
89
56
Three single-phase transformers rated 25 kVA each are connected in delta-delta to supply a balanced load of 75 kVA. One transformer is removed for repair, what is the kVA of each remaining transformer?
43.3
50
57.7
34.3
A rice mill will be served by two units of 50 KVA transformers, 7.620KV/240V. The transformers will be installed as open-delta bank to serve the three-phase load of the rice mill. What is the maximum kVA load that this bank can carry without overloading?
75.5
86.6
50
100
An excellent indicator of the condition of the oil in the power transformer is obtained by dividing the IFT (interfacial tension) by the NN (neutralization number). This ratio is known as the ________________.
Neutralization Number
Myers Index Number
Polarization Index Number
Transformer turn ratioa
The starting / in rush current of the transformer is generally _________ times of its full load current.
12
20
10
6
