RM&T MID TERM
If Load 2 is formed from three inductors in delta with an inductive reactance of 10 ohms each. What is the phase current?
56 A
In a balanced tthree-phase system, the conductor need to be only about _____ the size of conductors for a single-phase, two-wire system of the same kilovolt-ampere (kVA) rating.
75%
In an open delta with a center-tapped transformer, label the two ends of the center-tapped transformer as L2 and L3 and label the high leg as L1. The current flow in the neutral (center tap) is equal to _________.
AN = A1 + (A2 - A3)
An open delta transformer is assembled from two 50 kVA transformers. The output power of the open delta is _________.
86.6 kVA
In a T connection, how much is it preferred for the teaser transformer to contain?
86.6%
A square wave voltage varies between 0 V and 12 V. The positive pulses are 75 us wide and there is 25 us gap between them. What is the DC average of the waveform?
9 V
What function is performed by dielectric oil?
All of the above is true
Series field windings are connected in series with the armature. They are wound with _____ turns of fairly _____ wire.
few; large
Permanent magnet motors have _____ efficiency than conventional field-wound motors
higher
What should the ohmmeter indicate when testing checks for shorted windings between the primary and secondary?
infinity
A transformer in which the primary and secondary are physically separated and not connected by a conductor is called a(n) ________ transformer.
isolation
The alternator must supply the _____ for each of the loads.
line current
What does I L(load) represent?
line current of the load
In a three-phase connection, what is the voltage measured between the lines?
line voltage
A device that converts electrical energy into mechanical energy is called a(n) __________.
motor
In three-phase power systems, the delivered power?
never falls to zero
What king of magnets does the armature or rotor of a brushless DC motor contain?
permanent
What does EP (Load) represent?
phase voltage of the load
In calculating apparent power (VA) in a three-phase circuit, the phase values of voltage and current are multiplied by ________.
three
An open delta connection made using _________ single-phase transformers.
two
When a transformer is shown on a schematic diagram, what is the common practice to indicate polarity of the transformer windings?
using dots
What connection is used primarily for grounding purposes?
zig-zag connection
Which connection is mainly used to establish a neutral point for the grounding of fault currents?
zig-zag connection
What are permanent magnets in a PM motor?
pole pieces
When making the T connection, the main transformer is connected directly across the _____________.
power line
The speed of the disc servomotor can be varied by changing the amount of voltage supplied to the armature. The voltage is generally varied using ______.
pulse-width
Which type of generator must be self-excited?
series
What type of generator can either self-excited or separately excited?
shunt
What is a DC generator that uses permanent magnets as its field?
magneto
A delta connected set of windings has a phase voltage of 480 V. What is the line voltage?
480 V
A wye-connected secondary has a phase voltage of 277.14 V. What is the line voltage?
480 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 ohms in each phase. What is the current flow of each load resistor?
5.54 A
In a T connection, the main transformer must contain a center or what percentage tap for both primary and secondary windings?
50%
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 factor of 1.732. What is the phase current?
53.888 A
A wye-delta transformer has a turns ratio of 2:1. It has secondary line voltage of 480 V. The secondary is connected to a wye-connected three-phase resistive load of 50 ohms in each phase. What is the primary line current?
1.6 A
A delta-connected set of windings has a phase current of 10 A. What is the line current?
17.32 A
When increasing the amount of output voltage when loop contains 20 turn of wire, how much was the induced voltage increased?
20 times greater than that for a single-loop conductor
An open delta transformer is assembled from two 150 kVA transformer. The output power of the open delta is _______.
259.8 kVA
A transformer has a 240 V primary and 60 V secondary. With a 5 ohm load connected, what is the primary current?
3 A
A transformer has a 240 V primary and a 120 V secondary. With a 30 ohms load connected, what is the primary volt-amps?
480
What type of core is often found on high-voltage distribution transformers?
H-type
What does EL (Load) represent?
Line voltage of the load
When testing a transformer, what will reveal problems of insulation breakdown that an ohmmeter will not?
MEGGER
Which of these formulas is used to calculate circuit power factor?
PF = W/VA
In which connection are the secondary windings of each transformer providing the phase of a two-phase system?
Scott connection
What is used to convert three-phase power into two-phase power using two single-phase transformers?
Scott connection
Which should be disconnected first if it is necessary to remove one of the transformers for service?
Secondary
What type of connection involves the use of two transformers to supply three-phase power?
T connection
if three-phase load is pure reactive load, then the power calculated has units of ________.
VARs
Which of these formulas us used to calculate three-phase VARs of three-phase power circuit?
VARsc = square root 3 X E line (capacitive) X I line (capacitive)
What is a magetically operated machine that can change vaules of voltage, current, and impedance without changing frequency?
a transformer
The speed regulation of DC motor is proportional to the __________.
armature resistance
What is used to indicate a direction of additive and subtractive polarity on a schematic?
arrows
The current of the secondary induces a __________ in the secondary windings that is in opposition to the countervoltage induced in the primary.
countervoltage
A compound generator has both series and shunt field windings. When those windings are connected so they aid each other in the production of magnetism, that connection is called _______.
cumulative-compound
A three-phase connection in which three separate inductive loads have been connected together is a(n)
delta
Line voltage and phase voltage are the same in the __________ connection.
delta
The compound motor connection that should generally be avoided is the ________.
differential
