Where Kt is the torque constant, J is the moment of inertia, Bm
is the viscose torque constant for rotational losses, Te and TL
are the electromagnetic torque and the load torque
respectively.
2.3 Centrifugal pump load
There are two types of pump widely used in PV pumping
systems: volumetric pump and the centrifugal pump.
However, it was reported in [10] that, a load composed of a
DC motor driving a constant volumetric pump represents a
non-matched load for a PV generator because the motor
driving a constant volumetric pump requires a nearly constant
current. Conversely, it was found in [11] that the PV sourced
energy utilized by the centrifugal pump is much higher than
with the volumetric pump, because the centrifugal pump
works for longer periods even for low insolation levels.
Moreover, the characteristics of the DC motor and centrifugal
pump combination are well matched with the maximum
power locus of the PV generator. Centrifugal pumps are
inexpensive, simple, require low maintenance and are
available in a wide range of flow rates and heads. Therefore a
centrifugal pump is considered in this work.
The centrifugal pump load develops speed-dependant torque.
The speed-torque relationship for a centrifugal pump
including friction torque is given by Equation (8) [12].