Page 242 - Applied Photovoltaics

drop in motor current accompanying a fall in light intensity on the solar
panels affects both the field and armature windings. On the other hand, they
tend to be able to pump more water than shunt DC motors on sunny days
(Cultura, 2004).

2. The permanent magnet DC motor, as illustrated in Fig. 11.14b, overcomes
the above limitations of the series connected DC motor. Here the field
windings are replaced by permanent magnets, therefore producing a constant
flux, independent of the armature current and light intensity. This also greatly
improves the starting torque of the motors, particularly at low light levels, and
gives them excellent performance under reduced load. At half load, such
motors typically lose less than 10% of their efficiency.

a b

electronic
commutation
c d

Figure 11.14. DC motor types. (a) Series DC motor. (b) Permanent magnet DC
motor. (c) Shunt DC motor. (d) Brushless permanent magnet DC motor. (Used
with permission of McGraw-Hill Companies, adapted from Fitzgerald, A.E.,
Kingsley C. Jnr. & Kusko, A. (1971), Electric Machinery, McGraw Hill, Tokyo,
Japan)

3. The DC shunt motor is represented by Fig. 11.14c. With these, the field
current is determined directly by the motor voltage according to

V
I m (11.1)
f
R
f
where I f is the field current, V m is the motor voltage and R f is the field
resistance. Similarly, for the armature:

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