Module 3: AC Theory and Waveforms

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Questions and Answers

What occurs when a coil moves parallel to the lines of flux?

No EMF is induced as lines of flux are not cut. (correct)

A high voltage is generated across the coil.

A magnetic field is created in the coil.

EMF is induced due to motion.

How is frequency defined in the context of AC?

The number of peaks in voltage during a cycle.

The maximum voltage achieved in a cycle.

The number of cycles completed in one second. (correct)

The total time required to complete one cycle.

What is the period of household power operating at 50Hz?

0.005 seconds

0.001 seconds

0.1 seconds

0.02 seconds (correct)

If a waveform has a period of 100 microseconds, what is its frequency?

10kHz Signup and view all the answers

In the context of AC cycles, what does 'alternation' refer to?

One half of an AC cycle from zero to peak and back to zero. Signup and view all the answers

What represents the complete cycle of a sine wave in terms of degrees?

360º Signup and view all the answers

Which term describes the average value of an AC voltage over a complete cycle?

Average Voltage Signup and view all the answers

What does the vertical axis in an AC sine wave graph represent?

The magnitude and direction of voltage or current Signup and view all the answers

Which of the following waveforms is characterized by sudden changes in amplitude?

Square Wave Signup and view all the answers

In a sine wave, which component varies continuously in both positive and negative directions?

Voltage Signup and view all the answers

What term describes the maximum value of voltage in a sine wave?

Peak Voltage Signup and view all the answers

What is the shape of an AC waveform characterized as symmetrical and sinusoidal?

Sine Waveform Signup and view all the answers

Which term refers to the value of an AC signal measured at a specific moment?

Instantaneous Value Signup and view all the answers

How many cycles does a four-pole generator complete in one revolution?

Two cycles Signup and view all the answers

What is the formula used to calculate frequency in relation to poles and speed of rotation?

Frequency = No of Poles x Speed of Rotation / 120 Signup and view all the answers

Which of the following is NOT a way to express alternating current or voltage?

Continuous Signup and view all the answers

How many times does the peak value of a sine wave occur in one cycle?

Twice Signup and view all the answers

What does the peak-to-peak voltage or current represent?

The difference between the peak positive and peak negative values Signup and view all the answers

What is the induced voltage at the initial position of 0º in a simple AC generator?

Zero Signup and view all the answers

At what value can instantaneous voltage or current be found on a sine wave?

Between zero and peak value Signup and view all the answers

Which value indicates the highest instantaneous value reached during a sine wave?

Peak value Signup and view all the answers

At which angle does the induced voltage reach its maximum positive value in a simple AC generator?

90º Signup and view all the answers

What happens to the induced voltage as the armature rotates from 90º to 180º?

It decreases to zero Signup and view all the answers

What is the relationship of the number of poles to the frequency of a generator?

Increasing the number of poles increases the frequency Signup and view all the answers

What is the average value of a full-wave rectified output of a sine wave compared to its peak value?

63.7% of peak value Signup and view all the answers

Which phase of the sine wave corresponds to the armature cutting more magnetic lines of flux in a negative direction?

180º to 270º Signup and view all the answers

How is electrical phase measured?

In degrees Signup and view all the answers

What is the relationship between the speed of the coil and the induced EMF?

Higher speed yields higher EMF Signup and view all the answers

What does it mean when two sine waves are said to be 'in phase'?

Their maximum and minimum points occur at the same time Signup and view all the answers

How many degrees is one full cycle of a sine wave?

360 degrees Signup and view all the answers

If the green signal leads the blue signal, what can be said about their phase relationship?

Green is leading blue Signup and view all the answers

What takes place at 270º in the rotation of the armature in a simple AC generator?

Induced voltage is at maximum negative value Signup and view all the answers

What is the key characteristic of the sinusoidal waveform produced by a simple AC generator?

It exhibits polarity changes Signup and view all the answers

In an AC circuit, what is the implication of sine waves being out of phase?

There exists a phase difference between them Signup and view all the answers

What occurs to current in a pure resistance circuit when the voltage sine wave increases positively?

Current also increases positively Signup and view all the answers

Which equation correctly represents the average value from a peak value for a full-wave rectified signal?

2 x peak value ÷ π Signup and view all the answers

What does it mean if two waveforms have the same frequency but different phases?

They are out of phase Signup and view all the answers

What is the formula for calculating instantaneous voltage?

Vinst = Vpk x Sin Ø Signup and view all the answers

What is the sine value of 150º when calculating instantaneous voltage with a peak value of 100 volts?

0.866 Signup and view all the answers

In the context of AC voltage, what does RMS stand for?

Root-Mean-Square Signup and view all the answers

What is the average value of a full cycle of a sine wave that is perfectly half-wave rectified?

Peak value ÷ π Signup and view all the answers

What is the instantaneous voltage at 45º of rotation with a peak value of 100 volts?

+70.7 volts Signup and view all the answers

How does the RMS value relate to the peak value for a sine wave?

RMS value is 0.707 times the peak value. Signup and view all the answers

If the peak value is 12 volts, what is the average value of a half-wave rectified signal?

3.82 volts Signup and view all the answers

What characterizes a sine wave when plotted against electrical rotation?

It forms a perfect sine curve. Signup and view all the answers

Study Notes

Module 3: Electrical Fundamentals II - AC Theory

This module covers Alternating Current (AC) theory.
Students should be able to identify sinusoidal waveforms and describe associated terms like phase, period, frequency, and cycle.
Students should be able to describe voltage, current, and power relationships, including instantaneous, average, root mean square (RMS), peak, and peak-to-peak values, and perform calculations for these.
Students should be able to identify triangular and square waveforms.
Students need to understand single-phase and three-phase AC power principles.

AC Waveforms - Sine Waves

A sine wave is a symmetrical waveform that varies equally around a fixed level. It's commonly used to represent voltage or current in AC circuits.
Sine waves are alternating, meaning they swing both positive and negative.
Sine waves have a direct relationship to circular rotation.
The vertical axis of a sine wave graph represents magnitude and direction of voltage or current.
The horizontal axis represents time or angle of rotation in degrees.
A waveform above the time axis is positive; below, it's negative.
A full cycle is 360 degrees.

Alternating Current (AC)

Electrons flow first in one direction and then in the other with AC.
Current and voltage in AC circuits vary continuously.
AC is graphically represented by a sine wave.

Simple AC Generator

Voltage varies as an armature rotates through a magnetic field.
At 0 degrees, conductors are parallel to the magnetic field, so no voltage is induced.
As the armature rotates from 0° to 90°, conductors cut through more lines of flux, inducing a maximum voltage in the positive direction.
From 90° to 180°, the induced voltage decreases to zero.
From 180° to 270°, voltage builds to a maximum in the negative direction.
From 270° to 360°, voltage decreases to zero.
A full sine wave is produced during a 360° rotation.

AC Signal Types

Sine waves, square waves, triangular waves, and sawtooth waves represent different AC signal types.
These waveforms alternate between positive and negative values (above and below the X-axis).
A key feature is that the areas enclosed above and below the X-axis are equal for alternating waves.

AC Values

Instantaneous values: Voltage or current at any point on the sine wave. V_inst = V_pk x Sin Ø, where Ø is the angle of rotation.
Peak value: The maximum positive and negative values of the waveform.
Peak-to-peak value: The difference between the maximum positive and maximum negative value of the waveform.
RMS value (root mean square): The equivalent value of a DC voltage or current, calculated using the root mean square method. For a sine wave, the RMS value is 0.707 times the peak value.
Average value: The average value of a full cycle of a sine wave. For a half-wave rectified sine wave it is peak/π.

Frequency and Period

Frequency: The number of cycles completed in one second, measured in Hertz (Hz).
Period: The time required to complete one cycle, measured in seconds (s). The relationship is: Frequency=1/Period.

Phase

Phase: A term describing the relative position of two or more sine waves.
Signals are in phase when they cross the zero line at the same time.
Signals are out of phase when they don't cross the zero line at the same time. Phase difference is measured in degrees.

Single and Three-Phase Power

Single-phase: One conductor/winding rotates in a magnetic field.
Three-phase: Three conductors/windings (120° apart) create three phases of AC power.

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Description

This quiz focuses on the principles of Alternating Current (AC) theory, including the identification and analysis of various waveform types like sine, triangular, and square waves. Students will explore essential concepts such as phase, period, frequency, and power relationships, enhancing their understanding of single-phase and three-phase systems.