What Is The Best Motor For Solar Trackers?

When it comes to specifying electric motors for solar photovoltaic tracking applications, environmental protection is a prime consideration due to their exposure to the elements. While the postal service lets neither rain nor sleet nor gloom of night interfere with its assigned duties, in solar PV tracking you can throw in blizzards, hailstorms, gale winds, torrid heat and, for good measure, add virtually perpetual reliability. Motor designs for solar power applications, therefore, must stand up to extremes in temperature (both absolute and over a broad range), humidity and highly corrosive salt sprays, wind loads and abrasive airborne particulate matter.

As solar power projects become larger in scope, use of motors with integral intelligence capabilities becomes more important. The types of motor drive functionality that can now be built into the motor can permit communication amongst motors over a network, thereby reducing overall system cost and total cost of ownership (TCO). Motor types used in solar power applications run the gamut.

Stepper motors are inexpensive but become complicated and lose some of their economic benefits when components are added to operate in the closed-loop position control schemes that characterize solar tracking. Stepper motors’ air gap is a fraction of the size of other motor types and can lead to the rotor binding against the stator when there are large temperature differences between different parts of the motor, as when one side of the motor sees strong sunlight and the underside is shaded. Typical stepper motor speed range is also limited on the high side to about 400 rpm, which is disadvantageous when stowing trackers quickly when bad storms approach.

Permanent magnet brush dc motors (PMDC) are relatively efficient, easily controllable and, if properly built, can last a long time (up to 5,000 hr continuous duty), despite the brush or commutator wear that is inherent in their design. They also exhibit a wide speed range that is advantageous in stowing situations.

Brushless dc (BLDC) motors today, though, find the widest application in tracking systems because they are truly maintenance-free and have a low TCO. The BLDC motor has no wear-prone brushes, is highly efficient (typically 85 to 90%) and hits 3,000 rpm, a distinct advantage when a short stowing time is important.

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With higher level integral BLDC motor embedded intelligence, a brushless motor can serve as master control to host and run programs in the event of network interruptions, such as returning the tracker to a safe position in a network outage. These motors may also use macro-like commands, wherein simple trigger messages initiate complex functions. In addition, diagnostic functions may take place over the network to report on motor status and health.