Chapter

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AC Electrics - Introduction to AC

Introduction							161
The Nature of Alternating Current							161
Terms							163
The Relationship of Current and Voltage in an AC Circuit . .	. .	. .	. .	. .	. .	.	165
Resistance in AC Circuits . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	.	165
Inductance in AC Circuits							166
Inductive Reactance . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.169
Capacitance in AC Circuits . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.169
Capacitive Reactance . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.171
Impedance							172
Resonant Circuits . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	.	172
Summary							173
Power in AC Circuits							173
Power in a Purely Resistive Circuit . . . . . . . . . . .	. .	. .	. .	. .	. .	.	174
Power in a Purely Inductive Circuit							174
Power in a Capacitive Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .175
Power in a Practical AC Circuit . . . . . . . . . . . .	. .	. .	. .	. .	. .	.	176
Power Factor . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	.	177
Power Factor Resume							177
Questions . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.178
Answers . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	.	184

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Introduction

Alternating current (AC) is used in most large modern transport aircraft because of the following advantages that it holds over direct current (DC) supplies:

•AC generators are simpler and more robust in construction than DC machines.

•The power to weight ratio of AC machines is better than comparable DC machines.

•The supply voltage can be converted to a higher or lower value with almost 100% efficiency using transformers.

•Any required DC voltage can be obtained simply and efficiently using transformer rectifier units. (TRUs).

•Three phase AC motors which are simpler, more robust and more efficient than DC motors, can be operated from a constant frequency source, (AC generators).

•AC machines do not suffer from the commutation problems associated with DC machines and consequently are more reliable, especially at high altitude.

•High voltage AC systems require less cable weight than comparable power low voltage DC systems.

The Nature of Alternating Current

If the electrons flowing in a circuit move backwards and forwards about a mean position then the current produced is known as alternating current, AC. The simple AC generator shown in Figure 11.1 shows that a loop of wire (armature) rotated in a magnetic field experiences a continuously changing flux through it so that a voltage will be induced as long as rotation continues.

Figure 11.1 Simple AC generator

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AC to Introduction - Electrics AC 11

The magnitude of the voltage depends on the speed of rotation and the field strength (i.e. rate of change of flux).

When an armature is connected to a load (resistor) in a closed circuit through slip rings and carbon brushes a current will flow around the circuit in proportion to the induced voltage.

If this armature is rotated as in Figure 11.2 then the flux is constantly changing. In positions 1, 3 and 5 the two sides of the loop are moving parallel to the field and so there is no voltage induced as there is no rate of change of flux. In positions 2 and 4 the two sides of the armature are moving at right angles to the field and the maximum voltage is induced as there is maximum rate of change of flux. In between these positions the induced voltage is between maximum and zero.

The polarity of the induced voltage changes as it passes through zero because the direction that each side of the armature moves through the field is reversed. If the polarity reverses then so must the current through the external circuit, and current flowing backwards and forwards about a mean position is alternating current. The direction of current flow through each side of the armature at any point can be determined by using Fleming’s Right Hand Rule for generators.

Figure 11.2 shows one complete revolution of the generator armature and the associated rise and fall of induced voltage.

Figure 11.2 Production of AC

Figure 11.3 illustrates the production of AC. The blue vector arrow OP represents one half of the coil of the generator, pivoted at O and rotating in an anti-clockwise direction. The EMF induced in the coil is proportional to the ordinate ON, or can be calculated by multiplying the max value by the sine of the Phase Angle at that point.

Successive ordinates plotted to a time scale corresponding to the rate of rotation of OP produce a sine wave which represents an alternating current or voltage.

162