Capacitor and AC Circuit Principles

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Questions and Answers

What happens to the rate of current flow as a capacitor approaches full charge?

It reduces until it becomes zero. (correct)

It remains constant.

It increases steadily.

It fluctuates regularly.

If the capacitance in a circuit is increased, how does it affect the capacitive reactance?

Capacitive reactance remains unchanged.

Capacitive reactance decreases. (correct)

Capacitive reactance increases.

Capacitive reactance becomes negative.

What is the effect of frequency on capacitive reactance according to the relationship described?

Reactance equals frequency multiplied by capacitance.

Frequency has no effect on reactance.

Reactance is inversely proportional to frequency. (correct)

Reactance is directly proportional to frequency.

In an AC capacitive circuit, how does the phase relationship between current and voltage compare to that of an AC inductive circuit?

They are exactly opposite. Signup and view all the answers

What does the time constant in a capacitive circuit determine?

The time it takes for the charged current to decay. Signup and view all the answers

At which position does the induced EMF reach its maximum value?

Position B Signup and view all the answers

What happens in position C regarding the electrical output of the DC generator?

No EMF is generated. Signup and view all the answers

What is the role of the commutator during generator operation?

To maintain a constant current direction in the circuit. Signup and view all the answers

In which position does a direct short circuit occur due to the brushes contacting two segments?

Position A Signup and view all the answers

What is the effect of the brushes after the loop rotates past the 180° point?

They switch contact to ensure consistent current flow. Signup and view all the answers

What does commutation in a DC generator closely resemble?

Transformation of AC voltage into DC voltage. Signup and view all the answers

Why is there a risk of an arc causing damage to the commutator at certain positions?

High current flow due to short circuits. Signup and view all the answers

What happens to the current direction in the loop during generator operation?

It alternates but is redirected by the commutator. Signup and view all the answers

What is the purpose of binding steel wire around the armature coils?

To prevent displacement by centrifugal force Signup and view all the answers

How is sparking at the brushes of a generator primarily mitigated?

By employing screening and suppression techniques Signup and view all the answers

What is the role of the vacuum-impregnation with silicone varnish in the armature windings?

To maintain insulation resistance in various conditions Signup and view all the answers

What connects the ends of the coils to the commutator in the generator?

Silver brazing Signup and view all the answers

What component in the generator is responsible for carrying the drive shaft through the armature shaft?

A splined drive shaft Signup and view all the answers

What is the primary function of the brush holders in a generator?

To secure the brushes and limit wear Signup and view all the answers

What type of terminals do the free ends of the pigtails from the brushes typically terminate in?

Spade or plate type terminals Signup and view all the answers

In the context of generator design, what is the importance of steel laminations in the armature?

To reduce eddy current losses and enhance magnetic flux Signup and view all the answers

What happens to the instantaneous values of the power wave compared to the current and voltage values in a resistive circuit?

They can be less than the instantaneous voltage and current values. Signup and view all the answers

How is average power in a resistive circuit determined from the maximum power value?

Average power is half of the maximum positive power value. Signup and view all the answers

What relationship exist between voltage and current waves in a resistive circuit?

They are in phase and pass through zero at the same time. Signup and view all the answers

What is the approximate product of RMS values of voltage and current if V = 0.707V and I = 0.707A?

4.0 W Signup and view all the answers

What is the primary function of the series winding in a starter generator unit?

To provide initial torque for starting Signup and view all the answers

When the starter generator unit operates as a starter, which of the following windings is primarily unused?

Shunt field winding Signup and view all the answers

In the context of power calculation, what does multiplying two numbers less than one result in?

A value less than either of the original numbers. Signup and view all the answers

What defines the axis of the power wave in a resistive circuit?

The average value of power. Signup and view all the answers

In the context of a starter generator unit, which windings are utilized when it operates as a generator?

Shunt, compensating, and commutating windings Signup and view all the answers

What is the typical voltage and current required for starting in a starter generator unit?

24Vdc and 1500amperes Signup and view all the answers

Which statement is true about inductive reactance in relation to frequency?

Inductive reactance increases with lower frequency voltages. Signup and view all the answers

What effect does multiplying an RMS voltage value by an RMS current value have in an AC circuit?

It calculates the average power consumed in the circuit. Signup and view all the answers

What waveform characteristic can be expected when the changeover switch is operated at 2-second intervals in an AC circuit?

The voltage and current vary sinusoidally Signup and view all the answers

What is the purpose of the compensating and commutating windings in a generator?

To ensure sparkless commutation from no load to full load Signup and view all the answers

What type of electrical current alternates its direction at regular intervals?

Alternating current (AC) Signup and view all the answers

Which component is not used during the operation of the starter generator as a generator?

Series winding Signup and view all the answers

What is the RMS value of an AC current compared to its peak value?

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

Which statement correctly describes the average value of sine wave AC?

It is 0.637 times the maximum value. Signup and view all the answers

How does the calibration of DC meters for measuring AC values impact their readings?

They respond to average values but are calibrated to read RMS. Signup and view all the answers

What characteristic differentiates a triangular waveform from a sinusoidal waveform in AC applications?

Triangular waveforms are used primarily as electronic signals. Signup and view all the answers

What is the relationship between the RMS current and the peak current in a sine wave AC circuit?

I RMS equals I PEAK divided by 0.707. Signup and view all the answers

In what instance is the average value of a DC waveform equal to its maximum value?

When the waveform is entirely linear. Signup and view all the answers

Which of the following statements about AC waveforms is true?

Different AC waveforms can have varying shapes. Signup and view all the answers

If a sine wave AC with a peak value of 1 amp is applied, what is the approximate temperature rise if compared to a 1 amp DC current?

It raises the temperature by 70.7°C. Signup and view all the answers

Study Notes

Magnetism and Shielding

Sparking from generators and motors creates electromagnetic waves, interfering with electronic and radio equipment.
Braided screens are grounded to the main earth system to prevent ground loops.
Screening using metallic casings eliminates interference effectively.
Output supply cables are screened to prevent direct radiation.
Filters (chokes and condensers) are used to prevent interference conducted along cables.
Filters are crucial in minimizing electromagnetic interference.
Coaxial cables and screened cables are used for effective signal protection.
Skin effect at higher frequencies reduces the efficiency of screened cables.
Bonding systems and static dischargers improve the effectiveness of screening for electronics.

DC Motor/Generator Theory

Electrical equipment in airplanes relies on energy from generators, which convert mechanical into electrical energy.
A DC generator converts mechanical energy to direct current (DC) electricity.
Basic principles involve a coil rotating in a magnetic field, inducing a voltage in the coil's conductors.
Slip rings in a simple generator connect the rotating coil to an external circuit, whereas a commutator in a DC generator reverses the current direction, creating DC voltage.
The rotating parts (coil and commutator) are called the armature.
Commutation converts alternating current (AC) to direct current (DC).
The commutator ensures consistent DC flow in the external circuit.
Arcing can occur between brushes and the commutator, potentially damaging the device, so brushes are carefully placed in neutral planes for DC voltage/current generation.

Generator Classifications

Generators are classified based on how their magnetic circuits are energized.
Permanent magnet generators, separately excited generators, and self-excited generators.
Self-excited generators are powered by current produced by the machine itself, categorized by how the fixed windings (electromagnetic field and armature windings) are connected (shunt, series, or compound).
Aircraft DC power systems often use self-excited shunt-wound generators.

Self-Excited Shunt-Wound Generators

Common type in aircrafts; high-efficiency is needed;
They are described by the configuration of the field windings connected in parallel with the armature.
The field winding has high resistance relative to the armature, which helps produce consistent voltage changes. The arrangement is highly efficient.
The physical structure of a typical four-pole generator includes a yoke (housing), armature (rotating parts), and end frames.
Components like the yoke, armature, and end frames are carefully assembled for effective operation.

DC Motors

DC motors convert electrical energy to mechanical energy, reversing the function of a generator.
Types include series, shunt, and compound.
Series motors provide high starting torque, useful for starting heavy loads quickly.
Shunt motors have relatively constant speeds under varying loads, making them suitable for applications with a constant demand.
Compound motors combine the features of series and shunt motors, suitable for variable load conditions.

AC Theory Introduction

Alternating current (AC) periodically reverses its direction.
AC current can change over time, as shown in a waveform.
AC voltage changes with time in a sinusoidal pattern and is indicated by a waveform.
Resistor circuits use AC just as easily as DC given the same frequency requirements.

The Elementary Generator

Electricity is produced when a conductor moves through a magnetic field.
An armature loop rotating within a magnetic field generates voltage, picked up by a slip ring/brush arrangement.
The waveform of this generated voltage is a sine wave.
The generator's output voltage changes throughout a cycle of 360 degrees.

Period

The period of an AC waveform is the time required to complete one full cycle.
Measured in seconds, often represented as T.
Related to frequency, a measure of the number of cycles occurring in 1 second, related to 1/T.

The Cycle

The term "cycle" represents one complete variation of an AC voltage from positive peak to negative peak then back to positive peak.
Frequency measures the number of cycles per second, in units of Hertz (Hz).
Standard domestic supply frequency is 50 Hz.
Aircraft electrical supplies have a standard frequency of 400 Hz.

Frequency Ranges

Frequencies in electronics span a wide range, from a few hertz to millions.
High frequencies often used in signal transmission.
Frequencies are represented numerically with factors such as kilo (10³), mega (10⁶), etc.

Max or Peak Value

The maximum value of a waveform is the highest positive or negative value reached. This is also called peak-to-peak.

Effective or RMS Value

RMS (Root Mean Square) value refers to the effective value of an AC waveform.
It's related to the heat generated by a current.
RMS values are commonly used for AC measurements.

Average Value

The average AC value is obtained by averaging the instantaneous values over a half cycle.
Average value is about 0.637 of the maximum value.
An average value is 0.637 of the peak value.

Types of AC Waveforms

Common AC waveforms include sinusoidal, square, triangular, and sawtooth.
Their shapes and properties differ, suitable for different applications.

RLC Circuits

Circuits with resistance, inductance, and capacitance; these components affect how the circuit functions.

Resistance in AC Circuits

AC current and voltage rise and fall together, in phase in a purely resistive circuit.

Power in AC Resistive Circuits

The power used in an AC circuit is the average of all the instantaneous power values over a full cycle.
In purely resistive AC circuits, true power and apparent (volt-ampere) power are equal, thus the power factor is 1 or unity.

AC Inductive Circuits

Inductance creates opposition to current flow, known as inductive reactance, creating a delay.
Inductive reactance is proportional to frequency and inductance.

Effects of Frequency on Inductive Reactance

Lower frequencies result in more current in an inductive circuit.
Higher frequencies decrease current.

True and Apparent Power

True power (Watts) measures the actual power consumption.
Apparent power measures the product of voltage and current.
Power factor describes the ratio of true power to apparent power.

Capacitors (Review)

Capacitors have conductive plates separated by a dielectric material.
Factors determining capacitance are plate area, distance between plates, and dielectric material.

Series Connected Capacitors

To find the combined capacitance of capacitors connected in series, take the reciprocal of capacitance. Values for each capacitor need to be placed at 1/C1 + 1/C2 + ...

Parallel Connected Capacitors

Calculating the total capacitance of capacitors connected in parallel is achieved by summing the capacitance values.

Capacitive Reactance

Capacitive reactance is the opposition to AC current flow offered by a capacitor.
Current leads voltage by 90 degrees in a capacitive circuit.
Frequency influences capacitive reactance, higher frequencies mean lower capacitive reactance.

Effects of Frequency on Capacitive Reactance

In a capacitive circuit, current leads voltage, with higher frequency meaning a higher current.
Low frequency results in less current flow.
Capacitive reactance varies inversely with frequency.

Phase Relationship—Current and Voltage in Capacitive Circuits

In a purely capacitive circuit, current leads the voltage by 90 degrees.

Transformers—Introduction

Transformers are used to change voltage levels in AC circuits, often in aircraft systems for instrument power supplies.
They are commonly found in lighting systems and various avionics equipment.
Transformers can be used extensively in aircraft for changing voltages for different uses.

Transformer Principle

AC current in a coil creates an alternating magnetic field.
This field induces a back EMF in the coil.
By placing another coil next to it, alternating current in the first coil can induce a corresponding current in the second.

Turns Ratio

The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding in a transformer.
A step-up transformer gives a larger voltage in the secondary winding by increasing the turns and a step-down transformer reduces the voltage in the secondary coil by reducing the turns.
The relationship between primary and secondary voltage is directly proportional to the turns ratio (Vp/Ns = Vs/Np).

Phase Relationship

A 180-degree phase shift occurs in a transformer between primary and secondary voltage due to back EMF generated in the secondary winding.

Transformer Construction

Transformer cores are typically made of ferromagnetic materials to allow efficient magnetic flux transfer between windings.
Core types such as single-phase and three-phase cores are used in different transformer designs.

Transformer Losses

Losses in transformers include copper loss (resistance in the windings) and iron loss (hysteresis and eddy currents in the core).
Frequency of the supply affects transformer performance due to inductive reactance. Higher frequency leads to higher current and possible overheating.

Servicing

Proper maintenance of transformers, including regular checks and cleaning to avoid damage or overheating.

Filters

Filters discriminate against frequencies, attenuating frequencies based on their design (band-pass, band-stop, low-pass, high-pass).
Filters are used to remove/attenuate noise or unwanted frequencies.

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Description

Test your understanding of capacitors and AC circuits with this quiz. Explore concepts like current flow in capacitors, the impact of capacitance on reactance, and the phase relationship between current and voltage in different types of circuits. Challenge your knowledge of electrical principles with practical applications.