- •Textbook Series
- •Contents
- •1 DC Electrics - Basic Principles
- •Introduction
- •Electromotive Force (EMF)
- •Current
- •Resistance
- •Factors Affecting the Resistance
- •Units of Resistance
- •Resistors
- •Power
- •Series and Parallel Circuits
- •Kirchoff’s Laws
- •Annex A
- •2 DC Electrics - Switches
- •Switches
- •Proximity Detectors
- •Time Switches
- •Centrifugal Switches
- •3 DC Electrics - Circuit Protection and Capacitors
- •Electrical Faults
- •Circuit Protection Devices
- •Fuses
- •The Cartridge Fuse
- •Spare Fuses
- •High Rupture Capacity (HRC) Fuses
- •Dummy Fuses
- •Current Limiters
- •Circuit Breakers
- •Reverse Current Circuit Breakers
- •Capacitors
- •Capacitance
- •Capacitor in a DC Circuit
- •Capacitor in an AC Circuit
- •Capacitors in Parallel
- •Capacitors in Series
- •4 DC Electrics - Batteries
- •Batteries
- •Secondary Cells
- •Lead Acid Battery
- •Alkaline Battery (Nickel Cadmium, NiCad)
- •Battery Checks
- •Battery Charging
- •Secondary Batteries Summary
- •5 DC Electrics - Magnetism
- •Magnetism
- •Temporary Magnets
- •Permanent Magnets
- •Permeability
- •Magnetism
- •The Molecular Structure of Magnets
- •The Magnetic Effect of a Current
- •The Corkscrew Rule
- •The Magnetic Field of a Solenoid
- •The Right Hand Grasp Rule
- •The Strength of the Field of a Solenoid
- •Solenoid and Relay
- •The Forces on a Conductor Which is Carrying a Current
- •Questions
- •Answers
- •6 DC Electrics - Generators and Alternators
- •Electromagnetic Induction
- •Fleming’s Right Hand Rule
- •Faraday’s Law
- •Lenz’s Law
- •Simple Generator
- •Simple DC Generator
- •Characteristics of the Series Wound DC Generator
- •Commutator Ripple
- •Characteristics of the Shunt Wound DC Generator
- •A Compound Wound DC Generator
- •Flashing the Generator Field
- •Alternators
- •Voltage Control
- •Voltage Regulator Operation
- •Layout of a Generator System
- •Load Sharing Circuits
- •Operation of Load Sharing Circuit
- •7 DC Electrics - DC Motors
- •Electric Motors
- •Fleming’s Left Hand Rule
- •Practical DC Motor
- •Back EMF
- •Slow Start Resistor
- •Commutation
- •Series Wound Motors
- •Shunt Wound Motors
- •Starter-generator Systems
- •Actuators
- •Solenoid Actuators
- •Motor Actuator Construction
- •The Split Field Series Actuator
- •The Split Field Series Actuator Operation
- •Motor Actuators
- •Rotary Actuators
- •Linear Actuators
- •Actuator Brakes
- •Actuator Clutches
- •Visual Indicators Used with Linear Actuators
- •Visual Indicators Used with Rotary Actuators
- •Indicator Lights
- •Electromagnetic Indicators
- •Questions
- •Answers
- •8 DC Electrics - Aircraft Electrical Power Systems
- •Aircraft Electrical Power Systems
- •Dipole or Two Wire System
- •Single Pole (Unipole or Earth Return) System
- •Generators and Alternators
- •Voltage Regulators
- •Overvoltage Protection Unit
- •Generator Cut-out or Reverse Current Relay
- •Rectifiers
- •Inverters
- •The Generator Differential Cut-out
- •Generator (or Alternator) Warning Light
- •Generator (or Alternator) Master Switch
- •Monitoring Instruments
- •Ammeters and Voltmeters
- •The Battery
- •Bus Bars
- •Bus Bar Systems
- •Parallel Bus Bar System
- •Load Shedding
- •Generator or Alternator Failure
- •9 DC Electrics - Bonding and Screening
- •Bonding
- •The Static Discharge System or Static Wicks
- •Discharge of Static on Touchdown
- •Screening
- •Questions
- •Answers
- •10 DC Electrics - Specimen Questions
- •Questions – General 1
- •Questions – General 2
- •Answers – General 1
- •Answers – General 2
- •11 AC Electrics - Introduction to AC
- •Introduction
- •The Nature of Alternating Current
- •Terms
- •The Relationship of Current and Voltage in an AC Circuit
- •Resistance in AC Circuits
- •Inductance in AC Circuits
- •Inductive Reactance
- •Capacitance in AC Circuits
- •Capacitive Reactance
- •Impedance
- •Resonant Circuits
- •Summary
- •Power in AC Circuits
- •Power in a Purely Resistive Circuit
- •Power in a Purely Inductive Circuit
- •Power in a Capacitive Circuit
- •Power in a Practical AC Circuit
- •Power Factor
- •Power Factor Resume
- •Questions
- •Answers
- •12 AC Electrics - Alternators
- •Introduction to Aircraft Power Supplies
- •Generators / Alternators
- •Rotating Armature Alternator
- •Rotating Field Alternator
- •Alternator Output Rating
- •A Single Phase Alternator
- •Polyphase Circuits
- •Three Phase Alternator Connections
- •The Four Wire Star Connection
- •Delta Connected Alternator
- •Practical AC Generators
- •Brushed Alternators
- •Brushless Alternators
- •Frequency Wild Alternators
- •Obtaining a Constant Frequency Supply from a Frequency Wild System
- •Constant Frequency Alternators
- •Constant Speed Generator Drive Systems
- •CSDU Fault Indications in the Cockpit
- •The Drive Disconnect Unit (Dog Clutch Disconnect)
- •Variable Speed Constant Frequency Power Systems (VSCF)
- •Self-excited Generators
- •Load Sharing or Paralleling of Constant Frequency Alternators
- •Real Load
- •Reactive Load
- •Parallel Connection
- •Before Connecting in Parallel
- •Layout of a Paralleled System
- •Real Load Sharing
- •Reactive Load Sharing
- •Load Sharing General
- •Alternator Cooling
- •Generator Fault Protection
- •Bus Tie Breakers (BTBs)
- •Discriminatory Circuits
- •Differential Fault Protection
- •Synchronizing Units
- •Generator Failure Warning Light
- •Load Meters
- •Voltage and Frequency Meters
- •Generator Control Unit (GCU)
- •Emergency Supplies
- •The Ram Air Turbine (RAT)
- •The Auxiliary Power Unit (APU)
- •The Static Inverter
- •Ground Power Constant Frequency Supply System
- •Typical Controls and Indications
- •Questions
- •Answers
- •13 AC Electrics - Practical Aircraft Systems
- •Power Distribution
- •The Split Bus System
- •Parallel Bus Bar System
- •Questions
- •Answers
- •14 AC Electrics - Transformers
- •Transformers
- •Transformation Ratio
- •Power in a Transformer
- •Three Phase Transformers
- •Autotransformers
- •Rectification of Alternating Current
- •Half Wave Rectification
- •Full Wave Rectification
- •Three Phase Rectifiers
- •Transformer Rectifier Units (TRUs)
- •Inverters
- •Questions
- •Answers
- •15 AC Electrics - AC Motors
- •Alternating Current Motors
- •The Principle of Operation of AC Motors
- •The Synchronous Motor
- •The Induction Motor
- •The Squirrel Cage Rotor
- •The Induction Motor Stator
- •Slip Speed
- •Starting Single Phase Induction Motors
- •Fault Operation
- •Questions
- •Answers
- •16 AC Electrics - Semiconductors
- •An Introduction to Semiconductors
- •Conductors and Insulators
- •Semiconductors
- •N-Type Material
- •P-Type Material
- •Current Flow
- •The P-N Junction
- •Reverse Bias
- •Forward Bias
- •The Junction Diode
- •The Bipolar or Junction Transistor
- •Summary
- •17 AC Electrics - Logic Gates
- •An Introduction to Logic Gates
- •Binary Logic
- •Truth Tables
- •Gate Symbols
- •Positive and Negative Logic
- •The ‘AND’ Gate
- •The ‘OR’ Gate
- •The ‘INVERT’ or ‘NOT’ Gate
- •The ‘NAND’ Gate
- •The ‘NOR’ Gate
- •The ‘EXCLUSIVE OR’ Gate
- •Questions
- •Answers
- •18 Index
AC Electrics - Introduction to AC 11
Summary
•The Voltage and Current phase relationship in reactive circuits can be remembered using the following mnemonic:
C I V I L
In a Capacitive circuit, I current leads Voltage leads I current in an L inductive circuit.
•The effect of frequency variation on inductive and capacitive reactance is shown in the following graph.
AC Electrics - Introduction to AC 11
Figure 11.14
Power in AC Circuits
The power absorbed in a DC circuit, according to Ohm’s Law, is the product of the Voltage and the Current. So it is in AC circuits. However, due to the change in phase relationship between voltage and current in reactive circuits, the actual power absorbed is not necessarily the same as the power apparently supplied.
Once again the Resistive, Inductive and Capacitive circuits need to be examined separately and then a practical circuit having a combination of all three is considered.
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11 AC Electrics -Introduction to AC
Power in a Purely Resistive Circuit
AC to Introduction - Electrics AC 11
The power in a resistive circuit is the average value of all of the instantaneous values of power for a complete cycle. The instantaneous power value is found by multiplying the instantaneous values of current and voltage. If this process is carried out over a full cycle, it will give the power curve shown in Figure 11.15.
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6 |
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5 |
|
Voltage |
4 |
Average power |
Current |
3 |
(True power) |
Power |
|
RMS Volts × |
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2 |
RMS Amps |
|
1 |
Watts or kW |
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0 |
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Voltage and Current |
-1 |
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-2 |
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|
‘in phase’ = Real power |
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-3 |
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-4 |
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-5 |
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-6 |
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Figure 11.15 Power in a purely resistive circuit
Notice that the power curve is always positive because the voltage and current are in phase and its frequency is twice that of the voltage and current.
This positive power is known as the True Power, Real Power or Wattfull Power and its value is the product of the RMS current and the RMS voltage. It is measured in watts or kilowatts (kW).
The average power over a complete cycle is the average value of the power curve and can be represented by a line drawn halfway between the minimum and maximum values.
Power in a Purely Inductive Circuit
Figure 11.16 shows a purely inductive circuit where the current ‘lags ‘the voltage by 90°. It can be seen that by plotting instantaneous values of current × voltage we can obtain the waveform of instantaneous power.
The axis of that power waveform is the same as that of the voltage and current but its frequency is double.
If the axis of all the waveforms is the same, then the positive power is equal to the negative power. The positive cycle represents power given to the circuit to generate the magnetic field, and the negative cycle is power given back by the circuit in generating the Back EMF.
Thus in a circuit that contains only inductance, the true power is zero and only the power required that is necessary to overcome the inductive reactance is absorbed. This called reactive power and is the product of the voltage and current that is 90° out of phase. It is measured as Volts × Amps Reactive VAR or kVAR.
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AC Electrics - Introduction to AC 11
The product of the RMS voltage and the RMS current in this circuit is known as the apparent power and is measured in VA or KVA.
Positive power
Phase angle 90° current lags voltage
True power = 0
Negative power
Power = Volts × Amps Reactive. VAR or kVAR (No real power generated when current 90° out of phase with voltage)
Figure 11.16 Power in a purely inductive circuit
Power in a Capacitive Circuit
Power in a purely capacitive circuit is very similar to the inductive circuit, because the current is also out of phase with the voltage, but this time leading. Refer to Figure 11.17, once again the positive power is equal to the negative power thus no real power is absorbed. The power required is only overcoming the capacitive reactance. When the voltage and current are 90° out of phase the power required is all reactive power (VAR or kVAR).
As before the RMS volts × RMS amps is apparent power (VA or kVA)
Power = Volts × Amps Reactive. VAR or kVAR (No real power developed when current 90° out of
phase with voltage)
Positive power
True power = 0
Phase angle 90° current leads voltage
Negative power
Figure 11.17 Power in a purely capacitive circuit
AC Electrics - Introduction to AC 11
175
11 AC Electrics -Introduction to AC
Power in a Practical AC Circuit
AC to Introduction - Electrics AC 11
A practical AC circuit will always have some resistance and some inductance, and the amounts of each will depend on the construction of that circuit. An AC circuit may also have capacitance if capacitors are fitted.
Calculating power, therefore, depends on the ratio of resistance in a circuit to the inductance or capacitance (remember that inductance has the opposite effect to capacitance so if both are present in a circuit, the effects of one will cancel out some of the other leaving the circuit more inductive or capacitive depending on which one is more dominant, the resistance will always be there).
Figure 11.18 shows a circuit having equal resistance and inductance; notice the phase angle is 45° and that the amounts of positive power and negative power are not equal.
A line dividing the power curve into two equal areas would show the average power consumed in that circuit. The “average power” in a circuit with both resistance and inductance is the true power (kW) consumed in that circuit.
The apparent power (kVA) is the RMS volts × amps and the reactive power (kVAR) is the amount of power required to overcome the inductive reactance.
More positive power than negative power. True power axis now above the zero axis.
Phase angle of 45° current ‘lags’ voltage
Positive power |
True |
power |
(kW) |
Negative power |
true power (kW) Power factor = apparent power (kVA)
Figure 11.18 Power in a circuit having equal amounts of resistance and inductance
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