Page 61 - Energize July 2022

TECHNICAL

Electricity is generated by the oscillatory motion of the rotor arrangement is now commonly used in linear generators. A further
within the stator coils. The output waveshape is determined by the method, transverse flux magnetisation is also sometimes employed.
speed of motion of the rotor, which varies during the oscillatory For linear generators, the permanent magnet rotor is usually
stroke, the length of the stroke relative to the stator, and the longer than the stator such that the whole stator winding is active
arrangement of the magnets on the rotor relative to the stator. during as much of the stroke as possible. A three-phase stator
winding configuration is usually used, as shown in Figure 3.
Magnet arrangements and magnetic fields
While the actual arrangement of coils and magnets varies Output waveform and frequency
considerably, there are several standard arrangements of coil The voltage developed in the stator coil depends on two things:
and magnets. The configuration of permanent magnets used in • The magnetic field intensity, and
permanent magnet linear alternators can generally be classified • The rate of change of magnetic field. This depends on the
into radially magnetised, axially magnetised and Halbach velocity of the rotor.
magnetised, according to the magnetization direction.
Radial and Halbach magnetisation are common methods. The rotor is not continuously in motion but moves from a
Radial flux and Halbach magnetisation are compared in Figure 2. stationary position at one end of the stroke to a stationary position
The Halbach arrangement of magnets concentrates the flux on at the other end of the stroke. A typical piston displacement is
one side of the magnet row, increasing the magnetic field strength shown in Figure 4.
with the same magnet size. For this reason, the Halbach magnet Each cycle of the machine will produce a series of AC voltages
of varying amplitude. The number will depend on the number of
magnet pairs on the rotor and the number of coils on the stator,
as well as the stroke length. The shape of the wave is determined
by the magnetic flux variation pattern, while the amplitude is
determined by the velocity of the rotor (see Figure 5).

Figure 4: Typical piston motion and displacement in permanent magnet
linear alternators 3

Figure 2: (a) Radial and (b) Halbach magnetisation 5

Figure 5: Typical single phase output waveform from a permanent magnet
Figure 3: Three phase linear permanent magnet alternator 6 linear alternator 1

energize | July 2022 | 59