Delmars Unit 26 Three-Phase Circuits
The alternator must supply the _____ for each of the loads. capacitance line current inductance voltage
line current
What does IL(Load) represent? phase current of the load phase voltage of the alternator phase voltage of the load line current of the load
line current of the load
In a three-phase connection, what is the voltage measured between the lines? line voltage phase voltage
line voltage
In three-phase power systems, the delivered power _____. falls to zero three times per cycle falls to zero once per cycle never falls to zero falls to zero at least three times per cycle
never falls to zero
When calculating the power of a pure resistive load, the voltage and current are in phase with each other and the power factor is _____. one two three four
one
Load 1 consists of three resistors with a resistance of 6 ohms each, connected in wye and the phase voltage is less than the line voltage by a factor of 1.732. What is the phase current? 56 A 96.992 A 40.416 A 53.888 A
53.888 A
Which of these formulas is used to calculate total current delivered by the alternator?
IL(alt)=
The circuit power factor is the ratio of apparent power and true power. True False
True
If a three-phase load is a pure reactive load, then the power calculated has units of _____. VA W VARs
VARs
Line voltage and phase voltage are the same in the _____ connection. delta wye
wye
Which type of load is the above image depicting? wye delta
wye picture has 240V
What does EL(Load) represent? phase current of the load phase voltage of the alternator phase voltage of the load line voltage of the load
line voltage of the load
What does IP(Load) represent? phase current of the load phase voltage of the alternator phase voltage of the load line voltage of the load
phase current of the load
In a three-phase connection, what is the voltage measured across a single winding? line voltage phase voltage
phase voltage
What does EP(Load) represent? phase current of the load phase voltage of the alternator phase voltage of the load line voltage of the load
phase voltage of the load
In a wye-connected system, the line voltage is higher than the phase voltage by a factor of _____. square root of two square root of three two three
square root of three
Load 2 is formed from three inductors in a delta with an inductive reactance of 10 ohms each. What is the phase current? 56 A 96.992 A 40.416 A 53.888 A
56 A
Which of these formulas is used to calculate three-phase watts of a three-phase power circuit?
P=
Which of these formulas is used to calculate circuit power factor?
PF=
When Load 2 is made up of inductors, the reactive power can be calculated using the line values of voltage and current supplied to the load. True False
True
In calculating apparent power (VA) in a three-phase circuit, the phase values of voltage and current are multiplied by _____. three four five six
three
The horsepower rating of three-phase motors and the kilovolt-ampere rating of three-phase transformers are about _____ greater than for single-phase motors or transformers with a similar frame size. 75% 100% 150% 200%
150%
A delta connected set of windings has a phase voltage of 480 V. What is the line voltage? 277.14 V 480 V 831.36 V 1440 V
480 V
A delta connected set of windings has a phase voltage of 100 V. What is the line voltage? 57.7 V 100 V 173.2 V 300 V
100 V
A delta-connected set of windings has a phase current of 10 A. What is the line current? 5.77 A 10 A 17.32 A 30 A
10A
Three-phase voltages are _____ out of phase with each other. 80° 90° 100° 120°
120°
A delta-connected three-phase alternator is driving a wye-connected resistive load. The alternator output voltage is 480 V. The load resistors are 12 Ω each. What is the phase current in the alternator? 13.33 A 23.09 A 39.28 A 40 A
40 A
Load 3 is formed from three capacitors with a capacitive reactance of 8, connected in a wye and the phase voltage isless than the line voltage by a factor of 1.732. What is the phase current? 56 A 96.992 A 40.416 A 53.888 A
40.416 A
A wye-connected alternator that has phase voltages of 277 V is feeding two loads. The first load is wye-connected inductors that have 40 Ω of XL. The second load is delta-connected capacitors that have XC of 60 Ω. How much is the alternator phase current? 6.93 A 14.93 A 24.62 A 28 A
6.93 A
In a balanced three-phase system, the conductors need to be only about _____ the size of conductors for a single-phase, two-wire system of the same kilovolt-ampere (kVA) rating. 25% 30% 50% 75%
75%
When determining the amount of capacitance needed to correct power factor, what is the first step? Determine the capacitance value of each capacitor. Determine the reactive power of the circuit. Determine the apparent power of the circuit. Determine the power of the circuit.
Determine the apparent power of the circuit
When resistors of Load 1 are connected in a delta, the line current is the same as the phase current. True False
False
Another name for the three-phase wye connection is the _______________ connection.
Star
IL(Load 1) represents the line current to Load 1 (resistive). True False
True
If Load 1 is pure resistive, true power can be calculated using the line and phase current values. True False
True
If Load 3 is pure capacitive, the current and voltage are 90 degrees out of phase with each other and the power factor is zero. True False
True
Line current and phase current are the same in the _____ configuration. delta wye
delta
In a three-phase circuit, the true power (watts) is found by _____. multiplying line voltage by line current multiplying apparent power by the power factor dividing power factor by the apparent power multiplying phase voltage by phase current
multiplying apparent power by the power factor
The three-phase connection in which one end of each of the windings are connected together is called a(n) _____ connection. delta wye open delta open wye
wye
If the phase windings of the alternator are connected in a wye, the phase current is the same as the line current. True False
True
Which of these formulas is used to calculate apparent power of a three-phase power circuit?
VA=
A three-phase connection in which three separate inductive loads have been connected together is a(n) _____ connection. delta wye open delta open wye
delta
Which type of load is the above image depicting? wye delta
delta picture shows 480V
