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Figure 12.2 Single phase alternator

The output of this type of machine will rise to a maximum in one direction, then fall to zero, rise to a maximum in the other direction and then fall to zero again.

Polyphase Circuits

Polyphase or “multi-phase” alternators have two or more single phase windings symmetrically spaced around the stator.

The number of separate stator windings determines the number of phases present in the supply. The currents and voltages generated in this type of machine will have the same frequency but be out of phase with each other.

Corresponding values of voltage or current will be separated by an equal number of degrees. The most common polyphase alternator is the three phase alternator which has become the standard AC distribution system for aircraft. This is illustrated in Figure 12.3.

Note that the phase windings are mechanically arranged to be at 120° to each other in the sequence A, B, C so that the outputs are electrically separated by 120° as shown in the diagram. It can be seen that “A” phase reaches a peak going positive before “B” phase reaches a peak going positive before “C” phase reaches a peak going positive. This is the phase sequence ABC.

The peak values of the voltages induced in the three single phase windings of the three phase alternator shown in Figure 12.3 are 120° displaced from each other. The three phases are independent of each other.

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Figure 12.3 Three phase alternator

The advantages of three phase systems are:

•They have a greater power / weight ratio.

•They are easier to connect in parallel.

Three Phase Alternator Connections

The outputs of a three phase alternator can be connected by either the “Star” or “Delta” method. These connections are shown in Figure 12.4.

Figure 12.4 Star and delta connection for three phase alternators

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The Four Wire Star Connection

A star connected three phase alternator has the three phases joined at one end to form a fourth connection known as the neutral point. Refer to Figure 12.5.

Figure 12.5 Star connected alternator

The neutral point is normally grounded and used as the earth return in modern aircraft. The neutral line will carry any out of balance current. This means that if there is an earth fault on one phase, the neutral will carry an exceptionally high load.

This is the type of alternator that will be fitted to a typical aircraft distribution system because it can cope with different loads on each bus bar, the delta connection can not.

The connection at the opposite end of the phase from the neutral is called the line connection. A voltmeter measuring the potential difference between the neutral and the line lead would read phase voltage. A voltmeter measuring the potential difference between two line connections would read line voltage.

In this type of alternator the phase voltage and line voltage are different because phase voltage is measured across one phase whereas line voltage is measures across two phases and is the vector sum of the two.

Given one or the other of these values, the following formula will enable the student to establish the missing criterion:

Line Voltage = 1.73 × Phase Voltage

Note: (1.73 = √3)

The line voltage of a typical aircraft supply system would be 200 volts, and from the formula above it can be seen that the phase voltage would be:

	200	or 115 volts
	1.73

To be more specific, a modern aircraft power supply would be 115 V/ 200 V/ 400 Hz/ 3 phase.

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While the voltages of line and phase differ in the star connected system, because the windings form only one path for current flow between phases:

LINE CURRENT = PHASE CURRENT

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Figure 12.6 Delta connected alternator

Delta Connected Alternator

As can be seen from Figure 12.6, in this system the ends of the phases are joined together to form a closed mesh and the loads are connected in a similar fashion.

Logically, because the potential measured across the phase is measured between two lines, then:

LINE VOLTAGE IS PHASE VOLTAGE

BUT

LINE CURRENT = PHASE CURRENT × √3

This type of connection will not be used in a practical distribution system because it cannot cope with unbalanced loads as there is no neutral point. However, they may be used for specific purposes e.g. speed sensors or tacho generators.

Practical AC Generators

Rotating Armature alternators suffer from various disadvantages:

•The rotating coils are heavy and centrifugal forces are high.

•Efficient insulation of the rotating coils is difficult.

•The resistance across the brushes to the slip rings is high.

•The rotating coils are difficult to cool.

•They have a poor power to weight ratio.

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