Page 245 - Applied Photovoltaics

centrifugal pumps, unless appropriately modified to increase the torque generated at
high slip frequencies.
11.4.4 Motor losses

Support bearings
Friction in the support bearings has a load-dependent term and a load-independent
term, which jointly contribute significantly to the losses in high efficiency motors.

To achieve high reliabilities and low maintenance, lubrication with grease is essential.
Although expensive, a high quality lubrication grease that has a temperature-
independent kinematic viscosity is recommended, and has been demonstrated to
reduce frictional losses by up to 60%.

Magnetic circuit

Imperfections in the magnetic circuit will always contribute losses although, if
properly designed, these should be quite small. Low efficiency motors generally have
losses in the magnetic circuit as the dominant cause of poor performance. Permanent
magnet motors generally need to be carefully designed to ensure operation at the
maximum of their BH product.

Motor heating
Motor heating can be a serious loss mechanism and can lead to lower reliabilities and
shortened lifetimes. As the temperature increases, the resistance of windings
increases, thereby increasing the resistive losses, which in turn act to further increase
the motor temperature. It is therefore necessary to keep motors cool, both to achieve
high performance and to increase reliability and lifetimes.
Submersible motors are easily kept cool, but surface-mounted motors may need
special attention paid to cooling, such as by a heat pipe or ventilation.

11.4.5 Integrated pump/motor machines

Integrated pump/motor machines, where the pump and motor are matched and
interconnected within the same housing by the manufacturer, have become popular.
Such configurations act to simplify systems and provide high efficiencies when
operating at or near their design point. However, careful attention should be paid to
performance losses and mismatch that results from using these machines away from
their design point, such as with a different head or flow rate.
It is feasible to integrate permanent magnet brushless motors with pumps to such an
extent that the motor also serves as the impeller, within an hermetically-sealed,
submersible unit (Divona et al., 2001).

11.5 POWER CONDITIONING CIRCUITRY

The role of power conditioning circuitry (Ross, 2003; Schmid & Schmidt, 2003) is to
provide the motor/pump with the most suitable voltage-current combination, while
ensuring the solar panels operate at their maximum power points. In effect, it alters

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