EET 267 Mid-Term 10-15-09
True/False
Indicate whether the statement is true or false.
____
1. A triangle wave is waveform which “zigzags” up and down in straight lines.
____
2. In an isolation transformer, all windings will have the same number of volts per turn.
____
3. When single-phase loads are tied to a three-phase transformer, the phase currents of the transformer will be
automatically balanced.
Multiple Choice
Identify the choice that best completes the statement or answers the question.
____
____
____
____
____
____
4. Alternating current waveforms repeat the same shape over and over. From the beginning of one shape to the
end of that shape is called a
a. cycle
b. hertz
c. frequency
d. rotation
5. The number of cycles completed in one second is called the
a. cycle
b. hertz
c. frequency
d. rotation
6. Another term used to indicate the same as the RMS value is the _____ value.
a. average
b. peak
c. peak-to-peak
d. effective
7. What is the RMS value of an AC voltage that has a 480 V peak?
a. 678.72 V
b. 305.76 V
c. 339.46 V
d. 753.53 V
8. If an AC waveform is rectified and thereby converted into DC, the voltage value that results is the _____
value.
a. average
b. peak
c. peak-to-peak
d. effective
9. When AC voltage is rectified and converted into DC, the resulting waveform is normally not pure, flat DC.
The bumps that occur in the voltage are called
a. harmonics
b. cosines
c. ripples
d. peaks
____ 10. In a DC circuit, the product of voltage and current is some amount of watts. All of that value is delivered to
the load. This amount of power is called _____ power.
a. apparent
b. reactive
c. true
d. inductive
____ 11. The unit of measure of the power delivered to the load in a DC circuit is
a. hertz
b. volt-amperes
c. VARs
d. watts
____ 12. A load that will convert all of the delivered power into another form of energy is a(n) _____ load.
a. capacitive
b. inductive
c. reactive
d. resistive
____ 13. In an AC circuit, the voltage and current are frequently both sine waves. If they both pass through zero
heading in the same direction at the same time, they are said to be
a. in phase
b. out of phase
c. in leading phase
d. in lagging phase
____ 14. The three-phase connection in which one end of each of the windings are connected together is called the
_____ connection.
a. delta
b. wye
c. open delta
d. open wye
____ 15. In a three-phase connection, the voltage measured from one external voltage connector to another is the _____
voltage.
a. line
b. phase
____ 16. Often in calculations of values in three-phase systems, the square root of three occurs. The approximate value
of this constant is
a. 0.707
b. 1.414
c. 1.732
d. 2.236
____ 17. In a three-phase connection, the current that flows in an externally connected voltage conductor is the _____
current.
a. line
b. phase
____ 18. Line voltage and phase voltage are the same in the
a. delta
b. wye
____ 19. In a three-phase circuit, the true power (watts) is found by
a. multiplying line voltage by line current
b. multiplying apparent power by the power factor
c. dividing power factor by the apparent power
____ 20.
____ 21.
____ 22.
____ 23.
____ 24.
____ 25.
____ 26.
____ 27.
____ 28.
d. multiplying phase voltage by phase current
If a three-phase load is purely reactive, then the power calculated has units of
a. VA
b. W
c. VARs
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?
a. 13.33 A
b. 23.09 A
c. 69.28 A
d. 40 A
A wye-connected alternator that has phase voltages of 277 V is feeding two loads. The first load is wyeconnected 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?
a. 6.93 A
b. 14.93 A
c. 4.62 A
d. 8 A
The transformer winding which is the power input winding is called the _____ winding.
a. primary
b. secondary
c. tertiary
d. tapped
A transformer in which the primary and secondary are physically separated and not connected by a conductor
is called a(n) _____ transformer.
a. isolation
b. step-up
c. step-down
d. turns
A transformer in which the secondary voltage is less than the primary voltage is called a _____ transformer.
a. step-up
b. isolation
c. step-down
d. auto
A transformer in which the secondary voltage is more than the primary voltage is called a _____ transformer.
a. step-down
b. step-up
c. isolation
d. auto
A transformer has a 2:1 turns ratio. 120 V is applied to the primary. What is the secondary voltage?
a. 24 V
b. 60 V
c. 120 V
d. 600 V
A transformer has a 240 V primary and a 60 V secondary. With a 5 ohm load connected, what is the primary
current?
a. 12 A
b. 3 A
c. 0.75 A
____ 29.
____ 30.
____ 31.
____ 32.
____ 33.
____ 34.
____ 35.
____ 36.
____ 37.
d. 48 A
A transformer has a 240 V primary and a 120 V secondary. With a 30 ohm load connected, what is the
primary volt-amps?
a. 4
b. 8
c. 480
d. 960
The transformer that converts a high (7200 V) voltage from the power company down to 120 / 240 V for
homes and businesses is called a _____ transformer.
a. auto
b. distribution
c. control
d. household
A transformer supplying a house with 120 / 240 V has a secondary which is center tapped. The conductor
connected to the center is called the _____ conductor.
a. equipment ground
b. hot
c. neutral
d. ungrounded
Most transformer cores are not made from a solid piece of metal. Instead, they are assembled from many thin
sheets of metal. This type of construction is called
a. laminated
b. toroid
c. H core
d. tape wound
A transformer tape wound core is also called a _____ core.
a. toroid
b. solid
c. H
d. E
Any transformer flux that does not follow the core and escapes into the surrounding air is called
a. magnetizing flux
b. flux coupling
c. flux leakage
d. flux reactance
The amount of current that flows into a transformer primary when voltage is first applied is called _____
current.
a. magnetizing
b. excitation
c. activating
d. inrush
A wye-delta transformer has its _____ connected as a wye and its _____ connected as a delta.
a. secondary, primary
b. primary, secondary
If a single three-phase transformer of the proper ratings is not available, three single-phase transformers can
be connected to do the job. When this is done, the connection is called a(n)
a. three-phase bank
b. wye-delta bank
c. wye bank
____ 38.
____ 39.
____ 40.
____ 41.
____ 42.
____ 43.
____ 44.
____ 45.
d. delta bank
In three-phase transformer connections, the primary leads are labeled with the letter _____ and the secondary
leads are labeled with the letter _____.
a. X, H
b. H, X
c. L, X
d. X, L
A simplified schematic diagram of three-phase transformer connections is called a _____ diagram.
a. one-line
b. wiring
c. three-line
d. pictorial
When connecting three transformers for three-phase use with a delta secondary, a voltage test should be done
before closing the secondary. The correct voltage reading is
a. the rated secondary voltage value
b. twice the rated secondary voltage value
c. zero volts
d. the sum of the three transformer primaries
A wye-connected secondary has a phase voltage of 277.14 V. What is the line voltage?
a. 160 V
b. 277.14 V
c. 480 V
d. 554.28 V
A delta-wye transformer has a turns ratio of 9:1. The primary line voltage is 3600 V. What is the secondary
line voltage?
a. 32,400 V
b. 692.8 V
c. 230.95 V
d. 400 V
A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is
connected to a wye-connected three-phase resistive load of 50 Ω in each phase. How much current flow is
there in each load resistor?
a. 9.6 A
b. 4.8 A
c. 2.77 A
d. 5.54 A
A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is
connected to a wye-connected three-phase resistive load of 50 Ω in each phase. How much is the secondary
phase current?
a. 9.6 A
b. 5.54 A
c. 3.2 A
d. 4.8 A
A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the
turns ratio?
a. 1:20
b. 20:1
c. 1:11.55
d. 11.55:1
____ 46. A delta-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the
turns ratio?
a. 11.55:1
b. 1:11.55
c. 20:1
d. 1:20
____ 47. A wye-delta transformer has a secondary phase voltage of 480 V. A delta-connected resistive load has a
resistance of 12 Ω in each leg. What is the current in each load leg?
a. 13.33 A
b. 40 A
c. 23.1 A
d. 69.28 A
____ 48. An open delta connection is made using _____ single-phase transformers.
a. one
b. two
c. three
d. four
____ 49. An open delta transformer is assembled from two 50 kVA transformers. The output power of the open delta
is
a. 100 kVA
b. 86.6 kVA
c. 50 kVA
d. 57.7 kVA
____ 50. For installations that will have both three-phase and single phase loads on a three-phase transformer, the
utility company often provides a(n) _____ three-phase transformer.
a. three wire delta
b. three wire wye
c. open delta with one transformer center tapped