Designing with solar tracking motors

There is an easy way to boost the output of a solar panel by as much as 35%: Mount it on a framework that lets it track the sun. The electric motors that move these tracking systems are typically small fractional horsepower models (less than 745 W) whose own energy consumption is inconsequential. The same can be said for motors powering concentrated solar power (CSP) projects ranging from solar power towers to Fresnel collectors, and parabolic dish or trough systems that concentrate solar energy and convert it into electrical energy. For example, in one case a 470-ft-long Fresnel collector generating 50 kW is driven by a single 75-mm brushless dc motor and planetary gearbox actuating a slew drive with a total gear reduction of 20,000:1. The system collects 2.5 kW of power for every watt consumed by tracking.

However, conventional off-the-shelf industrial electric motors won’t cut it in such solar applications because the environmental constraints can be quite demanding. Many people have the impression that most solar power applications reside in warm, arid regions. That is not the case. The third-ranking state (behind California and Arizona) in megawatt capacity of PV installations is New Jersey. The world’s largest installed base of solar PV capacity, with some 25 GW, is in Germany, whose climate could be likened to that of the Midwestern U.S.

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. It is particularly challenging to specify a motor that will survive such adverse environmental elements, let alone do so for the lengthy lifetimes called out in today’s solar project plans.

In addition, environmental concerns aren’t the only consideration. As solar power projects become larger in scope, use of motors with integral intelligence capabilities becomes more important. The features these motors can provide, such as communication amongst each other over a network, can reduce overall system cost and total cost of ownership (TCO).

Motors used in solar power projects primarily move large, heavy objects, but they do so at a slow pace. A motor most often mates to a gearbox which reduces the speed of the output shaft and boosts torque. In turn, the motor-gearbox provides the driving force to either slew drives or linear actuators. The overall high gear reduction involved not only optimizes shaft speed for tracking the sun but also acts to resist wind loading effects which can potentially be quite high. Use of a high reduction gearbox is why the motor rating can be 50 W or even less. In all cases, though, designers size motors based on the actual speed, torque and power requirements necessary to follow the sun daily and seasonally.

Both the motors and the gearheads attached to them must be optimized for the low speeds and high torque that characterize solar tracking. Gearing considerations include such elements as engineered lubrication, low-friction gear design, and special sealed ball bearing designs, along with appropriate treatment of output shafts and housings.

AC induction motors were a natural choice for early solar tracking systems because they could draw power directly from the grid. However, it is tough to control ac motors at slow speeds. And to track the sun, the induction motor must turn on and off in a step function. This mode of use did not allow the most efficient continuous tracking and collection of solar energy.

Stepper motors are another alternative for powering tracking systems. They can be inexpensive but become complicated when operated in the kind of closed-loop position control schemes that characterize solar tracking. They just lose some of their economic benefits when components are added to close the loop.

By design, stepper motors have an air gap between the rotor and stator that is a fraction of the size of that for permanent magnet brushed and brushless motors. This small gap can potentially 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. The typical stepper motor speed range is also limited on the high side to about 400 rpm. This limit can be disadvantageous when it becomes necessary to stow trackers quickly when bad storms approach.

Permanent magnet brushed dc motors (PMDC) are relatively efficient, easily controllable and, given the duty cycle for solar tracking applications, can be set up to last a long time (up to 5,000 hr continuous duty). This 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.

Today, however, brushless dc (BLDC) motors probably find the widest use in tracking systems. They are truly maintenance-free and have a low TCO. The electronically commutated 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.

Intelligent BLDC motors

Modern solar power applications of any practical size have evolved to a distributed control model. In the past, developers of solar tracking systems had to design large, complex master control units to track the sun. The controller would synchronize the various trackers in an array by talking to the tracking motors through an expensive, complex cabling system. Today, though, numerous suppliers provide economical off-the-shelf PLCs having solar tracking function blocks.

The typical approach is to combine the industrial controller with BLDC motors having an integral control and CANopen network interface. This makes it possible to daisy-chain-control up to 127 motors 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.

Integral control BLDC motors are also available with Profibus or EtherCAT interfaces or can use Modbus via a gateway.

In some solar trackers an inclinometer can attach directly to integral control motors via CANopen or Synchronous Serial Interface (SSI). The point is to make fine adjustments in the panel position based on mechanical wear. The integral motor control, therefore, also eliminates bus load that would otherwise arise between the master control and inclinometer. In some sufficiently stiff systems, an integral absolute encoder on the motor can eliminate the need for an inclinometer.
Integral control BLDC motors can also serve as master controls that can host and run programs in the event of network interruptions. For example, the motor’s integral control can be programmed to return the tracker to a safe position if the network goes down. The motors may also use macro-like commands, wherein simple trigger messages can initiate complex functions. In addition, diagnostic functions may take place over the network to report on motor status and health.
Environmental factors are a big part of motor design criteria. The temperatures encountered in solar applications can be twice those found in normal industrial settings. It is not unusual to plan for an operating temperature range from -40 to +80°C .

Because they are exposed to the elements, motors should be designed to IP65 (protected against dust and water jet spray) or IP67 (protected against dust and effects of immersion in water to a depth between 15 cm and 1 m). In some cases, motors may need special UV-resistant jacketing.

It is also necessary to design motors with special venting to eliminate condensation which could make the motor fail prematurely. This measure is important because tracking motors are designed to seal out dust and moisture and can see wide and/or rapid temperature changes.

Of course, reliable long life is important for solar installations. So it is not unusual for the integral motor controls to be designed with up to 650,000 hour MTTF. Storms are inevitable, so tracking motors should also be designed to meet shock and vibration standards such as EN61373:1999.

Motor sizing

Engineers size motors for solar panel trackers based on two primary considerations. First, they must calculate the center of gravity (CG) point of the panel, then measure the distance from the pivot point, using the mass of the panels at the CG, to give the torque required for normal operation.

Next to be considered is wind loading. There is also a frictional load situation, because many panel manufacturers prefer to design their equipment so the CG is over the axis, such that the only torque needed is frictional and counters wind loading.

The second consideration is the inertia of the load about the rotating axis. This can usually be calculated through most CAD programs and then taken back to the motor as a reflected inertia. In most solar tracking applications, the torque needed is more critical than the inertia of the system because of the high gear ratios these systems use. However, both must be calculated to ensure the system is correctly sized.

Of course, this discussion is a simplification of all the conditions and effects taken into account when producing a torque value for a solar tracking motor. In most cases, the manufacturer of the panel either has empirical data from previous iterations or detailed calculations from CAD programs that reveal how much torque is required. ”

True-life expectation is another point to consider. Project developers should be able to work with the motor supplier when arriving at a figure for expected motor life. At its simplest this can be a life expectancy projection based on past experience and the project at hand. A greater level of assurance may come from a highly accelerated life test (HALT) or a highly accelerated stress screen (HASS), which is a more destructive test protocol.

Finally, from the motor designer’s perspective, solar power generation is truly exciting because it is an application where one truly gets much more power out than was put in.