AC Systems, Level 1 - 3rd Ed. Lesson 3

At what degrees on the sine wave is the voltage equal to zero?

0°, 180°, 360°

The most common frequency in the United States is ? Hz, which means the voltage waveform completes ? cycles in ? second(s).

60 / 60 / 1

What is the frequency of a waveform that has gone through 60 positive and negative alternations in one second?

60 Hertz

When a conductor cuts the magnetic flux lines at ? , the maximum voltage will be induced into the conductor.

90°

A(n) ? occurs each time the AC generator finishes one complete rotation (360° travel).

Cycle

Skin effect is caused by induced ? from the AC current flow, which cause the electrons to be repelled toward the outer surface of the conductor.

Eddy Currents

An AC waveform will cross the zero axis, but a DC waveform will not cross it.

True

Skin effect increases the effective resistance of the conductor.

True

Another name for the effective value is the ? .

the RMS value

The plotted resulting waveform as the conductor rotates in a magnetic field follows ? .

the sine of the angle of rotation

AC is converted from one level of voltage for transmission and then reduced to a different level of voltage for utilization by the end-user by using ? .

transformers

When is the voltage on an AC sine wave at the maximum positive and maximum negative value?

90°, 270°

Skin effect is smaller at high frequencies.

False

The rules of Ohm's Law, as learned in DC Theory, do not apply to AC circuits.

False

The power loss of a transmission line can be reduced by either reducing the ? of the wire or the amount of ? .

resistance / current

When distributed by transmission lines, power loss is due to the ? of the wire that generates heat in the conductor. The heat is generated as a result of ? flowing through the conductor.

resistance / current

Each time the generated voltage within a generator alternator reverses polarity, the current in an external circuit connected through slip rings to the generator alternator ? .

reverses direction

Alternating current is used to supply over 90% of the world's electrical power requirements.

True

What would be the ratio of power loss of a transmission line carrying 1,000 amperes at 1,000 volts as compared to 10 amperes at 100 kV? Assume a resistance of 1 ohm on the transmission line.

10,000 : 1

Give the RMS value for 120 VAC effective.

120 V RMS is effective voltage

As a generator armature makes one full revolution, it goes through ? of mechanical rotation.

360°

Reducing the resistance of the wire by one-half will reduce power loss by ? , but reducing the current by one-half will reduce the power loss by ? .

50% / 75% P = I² x R 160 = 4² x 10 (base equation) 80 = 4² x 5 (if resistance is reduced) 40 = 2² x 10 (if current is reduced)

Give the average value for 120 VAC effective. (Round the FINAL answer to the nearest whole number.)

Eᵃᵛᵍ = 2 x Eₚₑₐₖ / π Eᵃᵛᵍ = 2 x 170 / π Eᵃᵛᵍ = 340 / π Eᵃᵛᵍ = 108.225 = 108 V

If the AC voltage input to a full-wave rectifier is 100 volts peak to peak, what is the average DC voltage value? (Round the FINAL answer to the nearest whole number.)

Eᵃᵛᵍ = 2 x Eₚₑₐₖ / π Eᵃᵛᵍ = 2 x 50 / π Eᵃᵛᵍ = 100 / π Eᵃᵛᵍ = 31.83 = 32 V

The peak voltage is measured and found to be 170 volts. What is the effective voltage? (Round the FINAL answer to the nearest whole number.)

Eᵉᶠᶠ = Eₚₑₐₖ / √2 Eᵉᶠᶠ = 170 / 1.414 Eᵉᶠᶠ = 120.208 Eᵉᶠᶠ = 120 V

A sine wave has a maximum voltage (peak) of 144 V. What is the voltage at 45° of rotation? (Round the FINAL answer to one decimal place.)

E₁ = Eₚₑₐₖ x sinΘ E₁ = 144 x sin(45°) E₁ = 144 x 0.707 E₁ = 101.8 V

Give the peak-to-peak value for 120 VAC effective. (Round the FINAL answer to the nearest whole number.)

Eₚₑₐₖ = Eᵉᶠᶠ x √2 Eₚₑₐₖ = 120 x 1.414 Eₚₑₐₖ = 169.7 Eₚₑₐₖ₋ₜₒ₋ₚₑₐₖ = Eₚₑₐₖ x 2 Eₚₑₐₖ₋ₜₒ₋ₚₑₐₖ = 169.7 x 2 Eₚₑₐₖ₋ₜₒ₋ₚₑₐₖ = 339.4 = 339 V

Give the peak value for 120 VAC effective. (Round the FINAL answer to the nearest whole number.)

Eₚₑₐₖ = Eᵉᶠᶠ x √2 Eₚₑₐₖ = 120 x 1.414 Eₚₑₐₖ = 169.7 = 170 V

The effective voltage is 480 volts. What is the peak voltage? (Round the FINAL answer to the nearest whole number.)

Eₚₑₐₖ = Eᵉᶠᶠ x √2 Eₚₑₐₖ = 480 x 1.414 Eₚₑₐₖ = 679 V

What is the peak voltage value of an AC power source with an effective value of 50 volts? (Round the FINAL answer to the nearest whole number.)

Eₚₑₐₖ = Eᵉᶠᶠ x √2 Eₚₑₐₖ = 50 x 1.414 Eₚₑₐₖ = 71 V

What is the peak-to-peak value of the AC voltage wave?

Eₚₑₐₖ₋ₜₒ₋ₚₑₐₖ = Eₚₑₐₖ x 2 Eₚₑₐₖ₋ₜₒ₋ₚₑₐₖ = 50 x 2 Eₚₑₐₖ₋ₜₒ₋ₚₑₐₖ = 100 V

The waveforms will have the same heating effect on a resistor.

False

Why is AC used in power distribution systems more often than DC?

I. AC can be easily transformed to raise or lower levels of voltage and current for a given amount of power. IV. AC transformers allow very high voltage values to deliver large amounts of power at low current levels.

The power loss in a conductor can be expressed mathematically as ?.

P = I² x R

The time required for one cycle is called the ? .

Period

Sine waves are so named because the voltage at any point along the wave form is equal to the maximum, or peak, value times the ? of the angle of rotation.

Sine

The AC voltage waveform is in the positive direction for some period of time and in the negative direction for some period of time. What is the name given to this waveform?

Sine Wave

The tendency of AC current to flow in the surface area of a conductor and not evenly throughout the conductor is referred to as ? .

Skin Effect

In an AC generator, what changes to cause polarity changes?

The direction at which the conductor cuts the flux field changes as it rotates.

Peak-to-peak, Peak, RMS, and Average are four forms of measurement of voltage and current associated with sine waves.

True

The average DC voltage value of a full-wave rectified AC sine wave can be found by multiplying the peak value by 0.637 or by using the formula Eᵃᵛᵍ = 2 x Eₚₑₐₖ / π.

True

The variable of time in the production of electrical current has a major influence on the effects of AC current.

True

When comparing AC to DC, it is easier to transform an AC voltage.

True

When referring to values of AC voltages or currents, the letters RMS stand for Root Mean Square.

True

The peak-to-peak value is equal to ? time(s) the peak value.

Two

The current waveform shown is an example of a(n) ? .

a 2-amp positive DC current

In a direct current (DC) circuit, the current flows in ? direction, but in an alternating current (AC) circuit, current flows in ? direction(s).

a single / both

The measure of the number of cycles per second of the voltage produced by an AC generator is called the ? of that voltage and is measured in ? .

frequency / hertz

When the current and the voltage sine waves are ? , they both have the same frequency, crossing the reference line at the same time and going in the same direction.

in-phase

What is the period for a frequency of 1MHz?

t = 1 / 1,000,000 t = 0.000001 seconds

What is the period for a frequency of 60 Hz? (Round the FINAL answer to four decimal places of the base unit.)

t = 1 / f t = 1 / 60 t = 0.0167 seconds

What is the period for a frequency of 100 Hz? (Round the FINAL answer to two decimal places.)

t = 1/100 t = 0.01 seconds

The ? is the value of AC voltage that produces the same amount of power as a like value of DC voltage.

the RMS value

A measurement taken from zero to the highest value obtained in either the positive or negative half of a sine wave would determine ? .

the peak value

Power in a circuit is the amount of work done per unit of ? .

time

In order to lower power loss by reducing the current, one must raise the ? by the same proportion as one lowers the current to deliver the same amount of power to the user. This is expressed mathematically as P = ? .

voltage / E x I