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AC Theory
Lesson 2
| Question | Answer |
|---|---|
| In a direct-current(DC) circuit, the current flows in ______ direction(s), but in an alternating current(AC) circuit, current flows in _____ direction(s). | one two |
| An AC waveform _____ (will/will not) cross the zero axis, but a DC waveform _____ (will/will not) cross it. | will will not |
| The most common frequency in the US is _____ Hz, which means the voltage waveform completes 60 cycles in _____ seconds(s). | 60 1 |
| It is easier to transform _____ (AC/DC) voltage. | AC |
| Why is AC used in power distribution systems more often than DC? | AC can be transformed from one voltage to another. The transformation ability of AC power makes it less expensive and more versatile then DC. |
| I order to lower power loss by reducing the current, you must raise the _____ by the same proportion to deliver the same amount of power to the user. This is expressed mathematically as P=_____. | voltage P= E x I |
| When power is distributed by transmission lines there is power loss due to the _____ of the wire that generates heat in the conductor. The heat is generated as a result of _____ flowing through the conductor. The mathematical expression of this loss is? | resistance current P=I²R |
| The power loss of a transmission line can be reduced by either reducing the _____ of the wire or the amount of _____. | resistance current |
| Reducing the resistance of the wire by one-half will reduce power loss by _____ percent, but reducing the current by one-half will reduce the power loss by ____ percent. | 50% 25% |
| 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 |
| What would be the difference in power loss of a transmission line carrying 1,000 amps at 1,000 volts as compared to 10 amps at 100 kilovolts? | P=1,000 Amps x 1,000 Volts=1,000,000 Watts P=10 Amps x 100,000 Volts= 1,000,000 Watts 10:1 Ratio |
| 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 |
| AC is used to supply over ____ percent of the world's electrical power requirements. | 90% |
| The variable of ____ in the production of electrical current has a major influence on the effects of ____(AC/DC) current. | time AC |
| Do the rules of Ohm's Law, as learned in DC theory, also apply to AC circuits? | Yes. Several variations of the law can be applied to AC circuits under various circumstances. The voltage dropped across a coil of wire with AC current flowing is only partially calculated by using a strict interpretation of Ohms Law. |
| What is skin effect? | Creates the same consequence as reducing the cross-sectional area of the conductor because the electrons are forced to flow in a smaller area concentrated near the surface of the conductor |
| What causes skin effect? | Voltage causes the current flow in the conductor to be repelled away from the center of the conductor toward the outside of the conductor. The current is the forced to travel near the surface of the conductor. |
| Skin effect____(increases/decreases) the effective resistance of the conductor. | Increases |
| Skin effect is ____(smaller/greater) at high frequencies. | greater |
| 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____ direction. | reverses |
| In an AC generator, what changes to cause polarity changes? | The coil(armature) passes under one pole it has one electrical polarity. It creates an opposite polarity when it crosses the opposite magnetic pole. |
| As a generator armature makes one full revolution, it goes through____ degrees of mechanical rotation. | 360° |
| 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 the waveform? | sine wave |
| At what degrees on the sine wave is the voltage equal to zero? | 0°, 180°, 360° |
| When is the voltage on a sine wave maximum positive? Maximum negative? | 90° 270° |
| When a conductor cuts magnetic flux lines at ____ degrees, the maximum voltage will be induced into the conductor. | 90° |
| 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 |
| Each time that the AC Generator completes one complete rotation(360° travel) it is called a(n)_____. | cycle |
| 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 a waveform has gone through 60 positive and negative cycles in one second, what is the frequency of this waveform? | 60Hz |
| The time required for one cycle is called the ____. | period |
| What is the period for the following frequencies? 60Hz_____ 1MHz____ 100Hz____ | t=1/60Hz=0.0167 second t=1/1,000,000Hz=.000001second t=1/100Hz=.01 second |
| A sine wave has a maximum voltage (peak) of 144 V. What is the voltage after 45° of rotation? | E inst=144 x sin(45°) 144 x .707=101.8 V |
| What are the four forms of measurement of voltage and current associated with sine waves? | peak peak-to-peak root mean square (RMS) average |
| How is the peak voltage value measured? | from zero to the highest value obtained in either the positive or the negative direction |
| The peak-to-peak value is equal to ____ time(s) the peak value. | 2 |
| Another name for the effective value is the ____ value. | RMS |
| Explain RMS or effective value. Why is it used? | The RMS of an AC waveform is the square root of the average of all the instantaneous values squared. It gives the effective value of the AC. This RMS value has the same heating effect as DC of the same value. |
| The peak voltage is measured and found to be 170 volts. What is the effective voltage? | E rms= 170/1.414= 120 V |
| The effective voltage is 480 volts. What is the peak voltage? | E peak= 480 x 1.414=679 V |
| What is the average AC voltage for a full cycle of a perfect sine wave? | zero |
| An AC voltage that will produce the same amount of power as a like value of DC voltage is called the ____ value. | RMS |
| A pure AC sine wave has _____ DC value because it is ____ the zero axis exactly as much as it is ____ the zero axis. | no above below |
| When referring to values of AC voltages or currents, what do the letters RMS stand for? | Root Mean Square- for either the voltage or current the RMS value s calculated by using the peak of the waveform divided by 1.414(the square root of 2) |
| Give the following values for 120 VAC effective. Effective, Average, Peak, Peak-to-peak | effective =120 E avg= 2 x 120 x 1.414/3.1416=108 E peak= 120 x 1.414=170 E peak-to-peak= 170 x 2=340 |
| The average voltage value of one alternation can be found using the formula ____ or by multiplying the peak value by ____. | E avg= 2 x E peak/π, .637 |
| Power in a circuit is the amount of ____ done per unit time. | work |
Created by:
SM76502