Page 243 - Applied Photovoltaics

V I R K ĭ N (11.2)
m a a
where I a is the armature current, R a is the armature resistance and KɎN is the
back emf generated (where K is the motor constant, Ɏ is the magnetic flux,
and N is the motor speed). However, on start up, N is zero, which means, if
the motor is directly coupled to the solar panels, the output voltage will be
pulled down by the small armature resistance, owing to the limited solar array
current output. Hence I f will be reduced to extremely small values. The
consequence of this is an extremely low starting torque, thereby limiting start-
up to extremely high light fluxes, if at all. This feature alone makes this type
of motor unsuitable for direct coupling to photovoltaic panels, with the result
that these can only be used in conjunction with appropriate power
conditioning circuitry.

4. The brushless DC motor, as illustrated in Fig. 11.14d, has the permanent
magnets in the rotor and electronically commutates the stator to remove the
need for brushes. The electronic commutating circuitry constitutes a parasitic
power drain, but no more than the series resistance losses of conventional
brushes, and this motor type offers superior performance and long life
(Divona et al., 2001). Applications include electric vehicles, solar racing cars,
electric mopeds, variable speed fans, computer hard disk drives, consumer
electronics, machine tools, sheep shears and aerospace (Gieras & Wing,
2002). They tend to be more expensive but have the significant advantages of
highest efficiency and enhanced reliability, owing to the avoided need for
brush replacement. Magnet materials in use are alnico (Al, Ni, Co, Fe),
ceramics (ferrites) including barium ferrite and strontium ferrite, and rare
earths, including samarium cobalt and neodymium iron boron (Gieras &
Wing, 2002). The commutating circuits derive their timing from Hall-effect
or optical sensors around the shaft.
The first three DC motor types have the severe limitation of requiring brushes. In
many situations, the presence of brushes is not a problem. However, for photovoltaic
applications where system reliability must be extremely high and maintenance low,
their use may be considered unacceptable. Brushes require periodic replacement
(every 1–5 years) and the carbon dust from wearing brushes may cause arcing,
overheating and considerable power loss (Halcrow & Partners, 1981). If replacement
does not take place when required, serious damage may result. It is therefore essential
that a failsafe brush design be employed to stop motor operation, before such damage
eventuates. The other limitation of brushes is that they prevent the motors from being
submersed. This restricts their use with submersible pumps except via undesirable
long transmission shafts. This in itself is quite unfortunate since conventional
centrifugal pumps are normally submersible and, because of their torque-speed
characteristics, are the most suitable of all pumps for direct coupling to photovoltaic
panels.
Fig. 11.15 gives the performance curves for a typical low cost, low efficiency DC
motor. Higher efficiency motors have similar shaped curves. General advantages and
disadvantages of DC motors include:
1. Advantages:

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