AC Theory Level 3 Elaborate Exam Study
Questions with Verified Answers | Expert
Graded A+
1. As the phase angle is decreased, the power factor is ? - ANSWER Increased
2. True or False: the apparent power in an AC circuit equals E x I - ANSWER
True
3. What dissipates true power - ANSWER Resistance
4. An industrial plant has a 440-volt, 3-phase, 600-ampere-rated bus feeding
primarily inductive load. Note: the measured voltage, 400 V, and current,
610 A, values for the 200-kilowatt inductive load in the circuit. Solve for PF,
Angle θ, and Preact - ANSWER PF= 47.38%
Step 1: KVA= (I x E x 1.73)/1000
= (610x400x1.73)/1000
= 422.12
Step 2: PF= (kW/KVA) x 100
= (200/422.12) x 100
= 47.38
Angle θ= 61.7
Step 1: Angle θ= acos(kW/KVA)
= acos(200/422.12)
= 61.72
Preact= 371.73 kVARs
,Step 1: Preact= (kW x tan(Angle θ))x1000
= (200 x tan(61.72)) x 1000
= 371732.83 kVARs
5. The average value of an AC sine wave is the mathematical mean of all
instantaneous voltage values in half the sine wave and is equal to ? of the
peak value - ANSWER 0.637
6. A(n) of a voltage indicator is its inability to indicate the voltage amount. -
ANSWER disadvantage
7. Which of the following is an advantage of voltage indicators?
a. Voltage indicators do not have to come in contact with any conductors
to indicate voltage.
b. Voltage indicators may not provide an indication that voltage is
present even when it is.
c. Voltage indicators only indicate the presence of voltage and do not
identify the actual amount of voltage. - ANSWER a. Voltage
indicators do not have to come in contact with any conductors to
indicate voltage.
8. A is an electrical test instrument that indicates the approximate voltage
amount and type of voltage (AC or DC) in a circuit by the movement of a
pointer, and in some models, vibration - ANSWER Voltage tester
, 9. A can test for proper operation of GFCIs and arc fault breakers (AFCIs) -
ANSWER Circuit analyzer
10.Power factor is defined as the ? of theta, and it can be calculated by dividing
the ? in the power triangle by the ? - ANSWER cosine / kilowatts / kilovolt-
amperes
11.Power factor is normally corrected by adding a(n) ? in parallel with the load
- ANSWER capacitor
12.Determine the current flow in a 480-volt, 3-phase, 10-horsepower motor
circuit after the circuit has been corrected to 100% power factor. -
ANSWER 8.98 A
Step 1- VA= W= 7,460
Step 2- I= W/Ex1.73
=7,460/480x1.73
=8.98 A
13.If a 208-volt, 3-phase circuit that has line current of 15 amperes has a power
factor of 80%, how much true power is dissipated by the circuit? -
ANSWER 4,318 W
Step 1- W=VAxPF
=5,398x0.8
=4,318 W
14.A 100-horsepower, 3-phase, 2,400-volt motor operates at 75% power factor.
Calculate the phase angles at 75% PF and 93% PF, and the capacitive VARs
Questions with Verified Answers | Expert
Graded A+
1. As the phase angle is decreased, the power factor is ? - ANSWER Increased
2. True or False: the apparent power in an AC circuit equals E x I - ANSWER
True
3. What dissipates true power - ANSWER Resistance
4. An industrial plant has a 440-volt, 3-phase, 600-ampere-rated bus feeding
primarily inductive load. Note: the measured voltage, 400 V, and current,
610 A, values for the 200-kilowatt inductive load in the circuit. Solve for PF,
Angle θ, and Preact - ANSWER PF= 47.38%
Step 1: KVA= (I x E x 1.73)/1000
= (610x400x1.73)/1000
= 422.12
Step 2: PF= (kW/KVA) x 100
= (200/422.12) x 100
= 47.38
Angle θ= 61.7
Step 1: Angle θ= acos(kW/KVA)
= acos(200/422.12)
= 61.72
Preact= 371.73 kVARs
,Step 1: Preact= (kW x tan(Angle θ))x1000
= (200 x tan(61.72)) x 1000
= 371732.83 kVARs
5. The average value of an AC sine wave is the mathematical mean of all
instantaneous voltage values in half the sine wave and is equal to ? of the
peak value - ANSWER 0.637
6. A(n) of a voltage indicator is its inability to indicate the voltage amount. -
ANSWER disadvantage
7. Which of the following is an advantage of voltage indicators?
a. Voltage indicators do not have to come in contact with any conductors
to indicate voltage.
b. Voltage indicators may not provide an indication that voltage is
present even when it is.
c. Voltage indicators only indicate the presence of voltage and do not
identify the actual amount of voltage. - ANSWER a. Voltage
indicators do not have to come in contact with any conductors to
indicate voltage.
8. A is an electrical test instrument that indicates the approximate voltage
amount and type of voltage (AC or DC) in a circuit by the movement of a
pointer, and in some models, vibration - ANSWER Voltage tester
, 9. A can test for proper operation of GFCIs and arc fault breakers (AFCIs) -
ANSWER Circuit analyzer
10.Power factor is defined as the ? of theta, and it can be calculated by dividing
the ? in the power triangle by the ? - ANSWER cosine / kilowatts / kilovolt-
amperes
11.Power factor is normally corrected by adding a(n) ? in parallel with the load
- ANSWER capacitor
12.Determine the current flow in a 480-volt, 3-phase, 10-horsepower motor
circuit after the circuit has been corrected to 100% power factor. -
ANSWER 8.98 A
Step 1- VA= W= 7,460
Step 2- I= W/Ex1.73
=7,460/480x1.73
=8.98 A
13.If a 208-volt, 3-phase circuit that has line current of 15 amperes has a power
factor of 80%, how much true power is dissipated by the circuit? -
ANSWER 4,318 W
Step 1- W=VAxPF
=5,398x0.8
=4,318 W
14.A 100-horsepower, 3-phase, 2,400-volt motor operates at 75% power factor.
Calculate the phase angles at 75% PF and 93% PF, and the capacitive VARs
