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.
AC induction motors have been used in early solar tracking systems because they can draw power directly from the grid, but it is difficult to control AC motors at slow speeds necessary in most tracking applications. When an induction motor turns on and off in a step function to track the sun, it does not permit the most efficient continuous tracking and collection of solar energy.
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.
In today’s distributed control design of PV tracking arrays, brushless DC motors with embedded intelligence can be networked with economic off-the-shelf PLCs having solar tracking function blocks. For example, with a motor integral CANopen network interface up to 127 BLDC motors can be daisy-chain-controlled over a 500-m (1,640-ft.)-long bus running at 125 kBd (5 km at lower Baud rate) using simple, inexpensive yet robust twisted pair cabling with a shield. Each motor can then close current, position and velocity loops on its own.
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.
Obtaining affordable insurance in the relatively new PV power sector of the alternative energy industry is still a challenge. Since insurers are “risk averse” the adoption of motor and drive components that are not considered “prototypes” (i.e., operating less than a year or not yet commercially manufactured) and that have a track record may be the only way for PV power developers to minimize the challenge of expensive insurance premiums.