The centrifugal
pumps have relatively high efficiency, but it decreases at lower
speeds, which can be a problem for a solar water-pumping
system at times of low light levels. The single-stage centrifugal
pump has just one impeller, whereas most borehole pumps are
multistage types [2]where the outlet from one impeller goes
into the centre of another and each one keeps increasing the
pressure difference. From Fig. 1, it is quite obvious that the
load line is located far away from the Pmax line. It has been
reported that the daily utilization efficiency for a dc motor
drive is 87% for a centrifugal pump compared to 57% for a
constant-torque characteristic load. Hence, centrifugal pumps
are more compatible with PV arrays. The system operating
point is determined by the intersection of the I V characteristic
of the PV array and that of the motor, as shown in Fig. 1. The
torque-speed slope is normally large because of the armature
resistance being small. At the instant of starting, the speed and
the back emf are zero. Hence the motor starting current is
approximately the short-circuit current of the PV array.
Matching the load to the PV source through a maximum-
power-point tracker increases the starting torque.