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Some Notes about three-phase rectification

The following is an explanation into three phrase power as produced with an automobile alternator.

The folowing could also be apply to the multi phase design as per the offset magnet design if scaled up.

Six diodes are required to rectify the current from three stator windings, because each winding requires both a positive and a negative diode, to allow the current to flow to or from the end of its winding at the appropriate time.

The arrangements of diodes for both Y (star) and delta windings are shown in Figures 56.8 and 56.9. The positive diodes shown on the right-hand side are connected to the positive battery terminal; the negative diodes shown on the left-hand side are connected to the negative battery terminal through the earth.

With this arrangement, the end of each stator winding will always supply current to the correct battery terminal. There is always a path to the correct battery terminal through one diode, the other diode being used to prevent current from reaching the incorrect battery terminal.

As an example of how this arrangement operates, consider the brief period during which the winding A (Fig. 56.8) has an e.m.f. induced into it, so that current flows from the upper end of the winding, while at the same time, the winding B also has an e.m.f., so that current flows away from B through the winding to A.

A circuit will then be completed to allow the current to flow from A, to the centre positive diode and so to the positive battery terininal. The circuit will be completed from the neoative btttery terminal, via earth, through the lower negative diode to the end of the winding B.

With further rotation of the rotor, the conditions at A and B will be reversed, so that current then flows from B through the lower positive diode to the positive battery terminal. The circuit is completed via earth through the centre negative diode to the end of the winding A.

Circuits can be traced out for the other windings in a similar manner. There are six separate circuits similar to the one shown. As well as this, there is an overlapping of circuits during certain periods of operation.

The delta connected windings (Fig. 56.9) will function in a similar manner. Circuits can be traced from each winding, or combination of windings, through the diodes to the battery terminals.

From the above, it can be seen that rectification with diodes is simply a means of directing the current to flow to the correct battery terminal at all times and it is the arrangement of the diodes in the circuit that makes this possible.

Rotor field

The rotor field windings must be energised to enable the alternator to operate. Current is provided to the rotor by means of the slip rings and brushes. The current may be obtained from the battery or from the alternator itself. When current from the battery is used, the alternator is referred to as being externally excited. When current from the alternator stator is used, then the alternator is self-excited.

Self-excited alternators use three extra diodes making nine diodes in all. These three diodes are known as field diodes or exciter diodes, and the other six diodes as power diodes. A simplified circuit using exciter diodes is shown in Figure 56.10.

A simplified rotor field coil, located on the right-hand side of the diagram, is energised by direct current. This is a.c. from the stator windings which has been rectified by the exciter or field diodes.

Externally-excited alternators use current from the battery circuit or current from the alternator external circuit to energise the rotor field coil.

Alternator construction

An alternator consists basically of three main parts: the stationary parts forming the stator the rotating parts forming the rotor and the diodes and heat sink forming the rectifier.

These have already been covered in simple form.

The various parts of an alternator.


This comprises a three-phase winding, the coils of which are wound into slots in a ring-shaped lamininated core. The three windings. each constituting one phase. are connected together in either delta or Y (star). Each winding consists of a number of coils to provide a uniform output. The ends of the windings are attached to the diodes.


The rotor consists of eight claw shaped poles. These are mounted on a shaft., with the field coil between them. The ends of the field coil are connected to slip rings on the end of the rotor. This allows the field coil to be excited by means of current supplied by A. pair of brushes in contact with the slip some alternators.

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