Basic Electrical Theory, Chapter 5 & 6 Final Exam Questions

The leakage of the electromagnetic flux lines between the primary and secondary windings represents wasted energy.

true

Voltage induced in the secondary winding of a transformer is dependent on the number of secondary turns as compared to the number of primary turns.

true

The nameplate motor FLA rating describes the motor current rating when it carries its rated horsepower load at its rated ______. power resistance CEMF voltage

voltage

When describing the relationship between voltage and current, the reference waveform is always _____. a. current b. resistance c. voltage d. none of these

voltage

When a motor starts, the current drawn is at least ___ times the motor FLA; this is known as motor locked-rotor amperes (LRA). 0.80 1.25 3 6

6

How many 20A, 120V circuits are required for forty-two, 300W incandescent luminaires (noncontinuous load)? a. 3 circuits b. 4 circuits c. 5 circuits d. 6 circuits

6 circuits

How many 20A, 120V circuits are required for forty-two, 300W luminaires (noncontinuous load) that have a power factor of 85 percent? a. 4 circuits b. 5 circuits c. 7 circuits d. 8 circuits

7 circuits

A half-wave rectifier can be used to convert ac voltage into dc voltage to continuously charge a capacitor.

false

AC inductive or capacitive reactive loads cause the voltage and current to be in-phase with each other.

false

Autotransformers use separate windings for the primary and secondary.

false

The true power of an ac circuit equals the volts times the amperes.

false

As current flows through the transformer, the iron core is temporarily magnetized. The energy required to realign the core molecules to the changing electromagnetic field is called "________ loss." conductor resistance flux leakage eddy currents hysteresis

hysteresis

In a purely resistive ac circuit, the current and voltage are _____. This means that they both reach their zero and peak value at the same time. a. in-phase b. out-of-phase c. a or b d. none of these

in-phase

The _____ of an ac generator contains the electromagnetic field, which cuts through the stationary conductor coils stator rotor coil winding

rotor

As the alternating current through a conductor increases, an expanding and collapsing electromagnetic field through the conductor induces a voltage within the conductor. This is known as "___ voltage." a. applied b. circuit c.self- induced d. none of these

self-induced

The effective value is equal to the peak value ______. a. times 0.707 b. times 1.41 c. divided by 1.41 d. none of these

times 0.707

The peak value is equal to the effective value ____. a. times 0.707 b. times 1.41 c. divided by 1.41 d. none of these

times 1.41

A ___ is used to raise or lower voltage and it has the ability to transfer electrical energy from one system to another with no physical connection between the two systems. capacitor motor relay transformer

transformer

Because of conductor resistance, flux leakage, eddy currents, and hysteresis losses, not all of the input power is transferred to the secondary winding for useful purposes.

true

Current flow in the secondary transformer winding creates an electromagnetic field that opposes the primary electromagnetic field resulting in less primary CEMF. The primary current automatically increases in direct proportion to the secondary current.

true

Efficiency describes how much input energy is used for its intended purpose.

true

Even when power is removed from the circuit, capacitors can store large amounts of energy for a long period of time. They can discharge an arc if inadvertently shorted or grounded out.

true

For most practical purposes, transformer efficiency can be ignored.

true

Three-phase, ____ wye-connected systems can overheat because of circulating odd triplen harmonic currents. 2 wire 3 wire 4 wire none of these

4 wire

What is the nameplate FLA for a 20 p, 208V, three-phase motor with 90 percent power factor and 80 percent efficiency? 51A 58A 65A 80A

58A

The total opposition to current flow in ac circuits is called "______" and is measured in ohms. a. rsistance b. reactance c. impedance d. skin effect

impedance

The movement of electrons caused by an external magnetic field is called "________ current," and the associated potential that is established is called "_____ voltage." a. circuit b. applied c. induced d. none of these

induced

Transformers are rated in _____ VA kW W kVA

kVA

Once a motor begins turning, the rotor windings will be increasingly cut by the stationary magnetic field, resulting in an increasing counter-electromotive force.

true

The energy transfer ability of a transformer is accomplished because the primary electromagnetic lines of force induce a voltage in the secondary winding.

true

The heating from harmonic currents is proportional to the square of the harmonic current.

true

The magnitude of self-induced voltage within a winding is directly proportional to the current flow, the winding, and the frequency at which magnetic fields cut through the winding.

true

The primary electromagnetic field induces a voltage in the secondary. As the secondary current flows, it produces an electromagnetic field that reduces the strength of the primary flux lines. This results in an increase in primary current.

true

Three-phase ac generators have three equally spaced windings, ___ out-of-phase with each other. 90° 120° 180° 360°

120°

If the output is 1,600W and the equipment is 88 percent efficient, what are the input amperes at 120V? a. 10A b. 15A c. 20A d. 25A

15A

Motors are used to convert electrical energy into mechanical work and the output mechanical work of a motor is rated in horsepower. 1 hp =_______.

746W

Motors must be protected against excessive winding heat by a properly sized overload protection device based on the motor ____ current rating. FLA FLC LRC any of these

FLA

Because apparent power (VA) is greater than the true power (W), more loads can be placed on a circuit , and fewer circuits and panels, and smaller transformers might be required.

False

If the rotating part of the motor winding is jammed so that it cannot rotate, no CEMF will be produced in the motor winding. Result-the motor operates at ___ and the windings will be destroyed by excessive heat. FLA FLC LRC any of these

LRC

______ motors are fractional horsepower motors that operate equally well on ac and dc and are used for vacuum cleaners, electric drills, mixers, and light household appliances. AC Universal Wound-rotor Synchronous

Universal

The opposition offered to the flow of alternating current by a capacitor is called "capacitive reactance," which is expressed in ohms and abbreviated ______. a. Xc b. Xl c. Z D. none of these

Xc

Self-induced voltage opposes the change in current flowing in the conductor. This is called "inductive reactance" and is abbreviated _____. a. Xl b. Xc c. Z d. none of these

Xl

The abbreviation for impedance is_____. a. Xl b. Xc c. Z d. none of these

Z

For ac circuits, the ac _____ of a conductor must be taken into consideration. a. eddy currents b. skin effect c. resistance d. all of these

all of these

The induced voltage in a conductor carrying alternating current opposes the change in current flowing through the conductor. The induced voltage that opposes the current flow is called "_____." a. CEMF b. counter-electromotive force c. back-EMF d. all of these

all of these

If you measure voltage an current in an inductive or capacitive circuit and then multiply them together, you obtain the circuit's ______. a. true power b. power factor c. apparent power d. none of these

apparent power

Self-induced voltage is 180 degrees out-of-phase with the _______. When the applied voltage is at its maximum in one direction, the induced voltage is at its maximum in the opposite direction. a. applied current b. applied voltage c. induced voltage d. induced current

applied voltage

A nonsinusoidal waveform is created when _____ loads distort the voltage and current sine wave. a. linear b. resistive c. inductive d. nonlinear

nonlinear

Dual-voltage ac motors are made with two field windings. The field windings are connected in ________ for low-voltage operation and in _____ for high-voltage operation.

parallel, series

Swapping ______ of the line conductors can reverse a three/phase ac motor's rotation.

two

The motor FLA rating is used when sizing motor conductor size or circuit protection.

False

The actual motor current is dependent upon the load on the motor and the operating voltage at the motor terminals.

True

The movement of electrons themselves does not produce any useful work. It's the effects that the moving electrons have on the loads they flow through that are important.

True

When alternating current increases, the polarity of the CEMF within the conductor tries to prevent the current from increasing.

True

Wasteful circulating _____ in the iron core cause(s) the core to heat up without any useful purpose. conductor resistance flux leakage eddy currents hysteresis losses

eddy currents

_____ can be reduced by dividing the core into many flat sections or laminations. conductor resistance flux leakage eddy currents hysteresis losses

eddy currents

What does it cost per year (at 9 cents per kWh)for ten 150W recessed luminaires to operate if they are turned on for six hours per day? a. $150 b. $300 c. $500 d. $800

$300

What size transformer is required for a 100A, 240V, single-phase noncontinuous load that has a power factor of 85 percent? a. 15kVA b. 25kVA c. 37.5kVA d. 50 kVA

25kVA

In a purely inductive circuit, the CEMF waveform is _______ out-of-phase with the applied voltage waveform. a. 90° b. 120° c. 180° d. 360°

180°

The waveform of the CEMF is _____ out-of-phase with the applied voltage. a. 90° b. 120° c. 180° d. 360°

180°

What is the nameplate FLA for a 5 hp, 230V, single-phase motor, with 93 percent power factor and 87 percent efficiency? 10A 20A 28A 35A

20A

What size transformer is required for a 100A, 240V, single-phase noncontinuous load (unity power factor)? a. 15kVA b. 25kVA c. 37.5kVA d. 50 kVA

25kVA

If the primary phase voltage is 480V and the secondary phase voltage is 240V, the turns ration is____ 1:2 1:4 2:1 4:1

2:1

A motor must have two opposing magnetic fields in order to rotate. The stationary field winding is mounted on the stator, and the rotating part is referred to as the armature.

True

In an ac induction motor, the stator produces a rotating magnetic field that induces current in the rotor windings. The rotor current generates a magnetic field in opposition to the magnetic field of the stator, thereby causing the rotor to turn.

True

In dc circuits, the only property that affects voltage and current flow is: a. impedence b. reactance c. resistance d. none of these

resistance

The expanding and collapsing magnetic field within the conductor induces a voltage in the conductors (CEMF) that repels the flowing electrons toward the surface of the conductor. This is called "______." a. eddy currents b. induced voltage c. impedance d. skin effect

skin effect

In a(n) _____ motor, the rotor is actually locked in step with the rotating stator field and is dragged along at the speed of the rotating magnetic field. wound-rotor induction synchronous squirrel cage

synchronous