Worksheets13 Retake
Total questions: 40
Worksheet time: 20mins
A delta-connected alternator has a rated voltage of 240 volts per phase. The alternator supplies a load consisting of three 24 Ω resistors connected in wye. What is the line voltage of the alternator?
120 V
138.6 V
208 V
240 V
If the phase voltage is 120 V, what is the line voltage for a wye connection?
120 V
173 V
208 V
240 V
Power factor correction is used for which of the following purposes?
To reduce line loss and increase system capacity
To increase circuit reactance
To decrease circuit resistance
To increase power consumption
Three-wire delta systems normally are:
Polarized
Grounded
Center-tapped
Un-grounded
The ratio between the peak value and the root mean square (RMS) value of a periodic waveform is known as the:
Form factor
Crest factor
Power factor
Ripple factor
A coil with an impedance of 116 Ω and a power factor of 0.22 is connected in series with a 50 μF capacitor to a 120 V, 60 Hz supply. Calculate the new power factor of the circuit.
0.392
0.707
0.914
1.000
When a circuit containing power factor correction capacitors is disconnected from its source:
The capacitors could discharge through other parts of the circuit
The capacitors immediately discharge to zero volts
The capacitors retain no charge once power is off
The capacitors short-circuit the supply terminals
An unbalanced wye-connected three-phase, four-wire system has a line-to-line voltage of 600 V. What is the line-to-neutral voltage?
208 V
600 V
347 V
416 V
Which of the following voltage characteristics represents an ungrounded three-phase wye connection?
120/208 V, 4-wire
277/480 V, 4-wire
347/600 V, 4-wire
138/240 V, 3-wire
A wye-connected load draws 100 A per phase. What is the line current?
173 A
86.6 A
100 A
57.7 A
Choose the equation that best represents the relationship between line current (Il) and phase current (Ip) for a delta-connected load that draws a line current of 37 A.
Il=Ip×3
Ip=Il/3
Il=Ip/3
Ip=Il×3
The instrument shown is primarily used on three-phase systems to perform which of the following functions?
Frequency measurement
Power quality analysis
Voltage regulation
Power factor correction
Which of the following equations correctly describes phase reactive power in an unbalanced three-phase load?
Q = Iline × Vline × sinθ
Q = Iphase × Vphase × sinθ
Q = Iphase × Vline × sinθ
Q = Iline × Vphase × sinθ
Which of the following equations correctly describes phase apparent power in an unbalanced three-phase load?
VARS = Ip× Vp × cosθ
W = Ip × Vp × cosθ
VA = Ip × Vp × cosθ
S = Ip × Vp
A resistor has a value of 14 Ω. The current through this resistor is 1.5 A. What is the voltage drop across the resistor?
36 V
21 V
18 V
9.3 V
Use the information provided to determine the correct line voltage in a delta connection.
120 V
831 V
480 V
277 V
Use the information provided to determine the correct phase voltage in a wye connection.
208 V
240 V
347 V
120 V
In a three-phase system, the voltages are separated by what angle?
60°
90°
120°
180°
Study the diagram of the motor on the left and determine the phase rotation.
Positive phase sequence, clockwise rotation – ABC
Positive phase sequence, anti-clockwise rotation – CBA
Negative phase sequence, anti-clockwise rotation – ABC
Negative phase sequence, clockwise rotation – CBA
Study the diagram of the motor on the right and determine the phase rotation.
Positive phase sequence, clockwise rotation – ABC
Positive phase sequence, anti-clockwise rotation – CAB
Negative phase sequence, anti-clockwise rotation – ABC
Negative phase sequence, clockwise rotation – CAB
Using the information provided, how many cycles occur in the waveforms shown above for a three-phase system?
1
1.5
2
2.5
Using the information provided, how many phase occur in the waveforms shown above for a three-phase system?
1
2
3
4
How many revolutions does a 60Hz rotating magnetic field complete per second?
1
120
0
60
The following loads are connected in delta to a 240 V three-phase supply:
(1) A–B: 10 A at 0.9 pf lag,
(2) B–C: 15 A at pf 1,
(3) C–A: 20 A at 0.9 pf lag.
Calculate the current in line C. ☠️
33.5 A
25.0 A
20.8 A
17.3 A
Three 35 Ω resistors are connected in delta across a 240 V, 60 Hz three-phase supply. What is the line current?
6.86 A
9.81 A
11.9 A
14.2 A
To determine the total true power of a balanced three-phase system, which of the following equations should be used?
P=Iline×Vline×3
P=Iline×Vline×3×pf
P=Iphase×Vphase×3×pf
P=Iphase×Vline×pf
How many poles are required for a 3600 rpm machine operating at 60 Hz to complete one mechanical rotation per cycle?
1
2
3
4
When adding capacitors to a circuit to correct the overall power factor, the capacitors must be connected in:
Parallel with the load
Series with the supply
Series with the load
Parallel with the supply only
Adding capacitors to a circuit to correct the overall power factor closer to unity will cause the:
Current in the circuit to increase
Real power to increase
Power factor to decrease
VARs supplied to the circuit to be reduced
Choose the instrument that is primarily used to evaluate the condition of three-phase systems.
Power quality analyzer
Wattmeter
Clamp ammeter
Phase sequence indicator
Use the information provided to respond. How many phases are present in a four-wire three-phase system?
1
2
3
4
When three equal loads are connected in wye and then reconnected in delta, the total power of the three loads becomes:
The same
Twice as great
Three times greater
One-third as much
In a wye-connected system, the line voltage (VL) is related to the phase voltage (VP) by which of the following relationships?
VL=VP/1.732
VL=VP×1.732
VP=VL×1.732
VP=VL/3
In a delta-connected system, if the line current is 50 A, what is the phase current?
50 A
86.6 A
100 A
173 A
When converting a wye connection to a delta connection, how does the total power change if the loads are equal?
It remains the same
It reduces by one-third
It doubles
It triples
In a wye-connected system, what happens to the line current when the load becomes unbalanced?
It varies depending on the phase
It remains the same
It decreases in all phases
It increases in all phases
Which of the following is a characteristic of an unbalanced load in a wye connection?
Equal phase currents
Unequal phase voltages
Constant power factor
Zero sequence current
When analyzing a balanced wye-connected load, which of the following equations is used to calculate total power?
P = 3 × V_phase × I_phase × cosθ
P = V_line × I_line
P = V_phase × I_line
P = 3 × V_line × I_phase
In a delta-connected system, if one phase experiences a fault, how does it affect the overall system performance?
It leads to an increase in current in the remaining phases
It reduces the voltage across all phases
It has no effect on the system
It causes the system to trip immediately
When analyzing a delta-connected load, which of the following is true regarding the line current?
It is equal to the phase current
It is the sum of the phase currents
It is less than the phase current
It is greater than the phase current
