How much torque does a solar tracker slew drive require?

Selecting the right slew drive for a solar tracker is crucial for the reliability and safety of the photovoltaic (PV) tracking system. One of the most important parameters is output torque. But exactly how much torque does a solar tracker need?

The answer depends on various factors, including the size of the PV array, total moving weight, center of gravity, wind loads, mounting structure, and tracking configuration. Selecting a slew drive based solely on the weight of the solar panels is usually insufficient.

1. Why is torque important for solar trackers?

A slew drive converts the rotational power of a motor into controlled movement for the solar tracker. During normal operation, the slew drive unit requires sufficient torque to overcome:

The weight of the moving structure

Friction in bearings and mechanical components

Wind loads acting on the PV modules

Imbalance caused by a shifted center of gravity

Start-up and stopping loads

Dynamic loads during tracker movement

For outdoor solar tracking systems, wind load is often one of the most significant factors influencing the required torque.

In simplified mechanical calculations, torque is a function of force and the effective lever arm. Therefore, the further the applied load is from the axis of rotation, the greater the required torque.

2. Basic torque calculation for solar trackers

A simplified calculation formula is: T = F × r

Where:

T = Required torque

F = Applied force

r = Effective distance to the axis of rotation

However, for solar trackers, the actual calculation is more complex because the load varies with the tracker's angle and the wind direction.

For example, when wind acts on a large PV array, aerodynamic forces generate a moment (torque) on the tracker's axis of rotation. This moment must be taken into account when selecting the slew drive unit.

Therefore, actual engineering calculations should consider:

Required Torque ≥ Static Load Torque + Wind Torque + Friction Torque + Dynamic Load Margin

Consequently, providing wind data for the project site is vital for proper model selection.

3. Wind load can be more significant than module weight

A common mistake is calculating torque based solely on the weight of the PV modules. For instance, a tracker may support hundreds of kilograms of components and steel structures. However, when strong winds act upon a large array, the resulting aerodynamic torque can far exceed the gravitational torque.

Therefore, when selecting a slew drive for a solar tracker, it is essential to specify both the operational wind speed and the survival (safety) wind speed.

4. Center of Gravity (CoG) Location is Crucial

The center of gravity (CoG) of the tracker assembly is another critical factor.

If the CoG is close to the axis of rotation, the gravitational torque can be relatively low.

If there is a significant offset between the CoG and the axis of rotation, the required torque increases.

Consequently, two solar trackers with the same total weight may require vastly different slew drive torque ratings.

5. Single-Axis vs. Dual-Axis Trackers

5.1 Single-Axis Solar Trackers

Single-axis trackers typically rotate the PV array around a single primary axis.

The following factors should be considered when selecting a slew drive:

* Total array weight

* Array dimensions

* Center of gravity location

* Wind speed

* Axis of rotation orientation

* Required tracking angle

* Mechanical support structure

5.2 Dual-Axis Solar Trackers

Dual-axis trackers typically feature two independent axes of rotation:

Azimuth axis

Elevation axis

These two axes may be subjected to vastly different load conditions.

For example, when the PV array is tilted, the elevation drive may experience significant gravitational torque, whereas the azimuth drive might be subjected to substantial wind torque, depending on the array's position.

Therefore, torque requirements for the two slew drives should be calculated separately.

6. Do Not Select Slew Drives Based Solely on Torque

While torque is certainly important, it is not the only parameter to consider.

Key Considerations:

Holding Torque

The slew drive must possess sufficient holding capacity to maintain the position of the photovoltaic (PV) array when subjected to external loads.

Overturning Moment / Load-Bearing Capacity

The slew drive must be capable of withstanding the overturning moments generated by the PV array and wind loads.

Backlash

Low backlash is crucial for applications requiring precise solar tracking.

Self-locking Capability

Worm-gear slew drives offer high gear reduction ratios and—depending on design and operating conditions—provide effective resistance against back-driving (i.e., a self-locking function).

Ingress Protection (IP) Rating

Outdoor solar trackers are exposed to rain, dust, humidity, and temperature fluctuations. With proper overall assembly design, an IP66-rated slew drive is an ideal choice for harsh outdoor environments.

Operating Temperature

Motors, gearboxes, seals, lubricants, and other components must be suitable for the temperature range at the project site.

7. What information is needed to select a slew drive?

To perform an initial selection of a solar tracker slew drive, the following information is recommended: