Two Challenges That Make Solar Tracker Drive Selection Unique
Solar tracker drives share some characteristics with standard servo positioning applications — but two engineering challenges are specific to solar tracking and are not covered adequately by standard servo drive selection methodology. Both must be understood before any ratio or frame size selection can be made correctly.
A solar panel tracks the sun at 0.375°/minute in azimuth — equivalent to 0.0010 rpm of the drive output shaft. Even through a 320:1 reduction, the motor would run at 0.33 rpm. Standard servo motors lose velocity control stability below approximately 50 rpm — entering a regime where encoder pulses arrive too infrequently for the velocity loop to operate. This means solar tracking speed itself cannot be used as the motor operating point. A completely different drive strategy is required.
Solar farms are typically designed for 25-year operating life with minimal on-site maintenance. A utility-scale solar park may have thousands of tracker drive units spread across a remote site in desert, coastal, or tropical conditions. Each unit must survive: UV radiation degrading seals and lubricant; salt spray in coastal installations; temperature cycles from −25°C night-time to +90°C summer housing temperature; dust and sand ingress in desert sites; and periodic rain-driven pressure washing in agricultural environments. IP65 and sealed-for-life lubrication are not optional — they are the minimum viable specification.
Solar Tracking Motion Requirements — Azimuth, Elevation, and Emergency Stow
Solar tracker drives must execute three distinct motion profiles with very different speed and torque requirements. The gear ratio must accommodate all three simultaneously — which is why the fast repositioning speed, not the tracking speed, determines the practical upper limit on gear ratio.
TRACKING
The sun traverses 180° in approximately 8 hours (equatorial location, clear sky). At the drive output shaft: 0.375°/min = 0.0010 rpm azimuth. Even through i=320:1, the motor speed would be 0.33 rpm — below stable servo range. Engineering solution: intermittent move-and-hold (see Module 3). The torque requirement is wind load torque divided by ratio — typically a modest motor in the 100W–400W range at high ratio.
/ RESET
At dawn, the tracker must move from the previous day’s west-facing stow position back to east-facing start — a 180° azimuth reversal. At 1 rpm output through i=200, the motor runs at 200 rpm — well within stable servo range. This repositioning speed sets the upper limit on gear ratio: at i=320 with n_fast=2 rpm, the motor would reach 640 rpm — still within range. The ratio should be selected such that fast repositioning gives n_motor between 100 and 1,500 rpm.
STOW
When wind speed exceeds the survival threshold (typically 25–30 m/s), the controller commands emergency stow: panel moves to horizontal (minimum wind area) as fast as possible. IEC 62817 recommends stow completion within 3 minutes for most tracker designs. A 90° stow travel at i=200 requires n_out = 90/(3×360) = 0.083 rpm → n_motor = 16.7 rpm — slightly low but adequate for position-controlled stow. Select ratio such that stow motion completes reliably within the time budget at the motor’s rated torque.
The Intermittent Tracking Strategy — Resolving the Motor Speed Paradox
The solution to the motor speed paradox is straightforward once identified: solar trackers do not need to move continuously at tracking speed. They only need to maintain the panel within the required tracking accuracy tolerance. Instead of continuous slow rotation, the drive executes rapid small corrections at repositioning speed, separated by stationary hold periods. During the hold period, the motor is stopped (servo holding position with zero velocity command). During the correction, the motor runs at repositioning speed — well within stable servo range.
Tracking accuracy and energy yield: The cosine effect of tracking inaccuracy reduces panel output by cos(θ_error). At ±0.5° tracking error, the power loss is only 0.0038% — for a 100kW array operating 2,920 hours per year, this is 11 kWh/year, worth less than $1. Tracking accuracy to ±0.5° is more than adequate for flat-panel PV from both an energy yield and gearbox specification perspective. CPV (concentrated photovoltaic) systems are the exception — they require ±0.1° or better because their optical acceptance angle is much narrower.
Wind Load Torque — The Primary Design Load for Solar Tracker Drives
The dominant torque load on a solar tracker drive is not the panel weight — it is wind pressure on the panel surface. Unlike most servo applications where inertia or friction defines the peak torque, solar trackers experience sustained aerodynamic loading that determines both the continuous rated torque and the emergency stow torque. Wind loading scales with the square of wind speed and linearly with panel area, making large multi-panel rows significantly more demanding than single-panel units.
The torque formula: T_wind = 0.5 × ρ_air × v² × A_panel × n_panels × Cd × R_arm, where ρ_air = 1.225 kg/m³, A_panel = 2 m² (400W panel), Cd = 1.0–1.5 (depends on array configuration), R_arm = 0.6 m (distance from rotation axis to panel centre of pressure).
| Tracker Configuration | Cd | T @ 15 m/s design wind |
T @ 20 m/s strong wind |
T @ 25 m/s stow trigger |
T @ 30 m/s survival (stowed) |
With SF=2.0 Design torque |
|---|---|---|---|---|---|---|
| Single panel (1×400W) | 1.0 | 165 N·m | 294 N·m | 459 N·m | 662 N·m | 588 N·m @ 20m/s |
| 2-panel row | 1.0 | 331 N·m | 588 N·m | 919 N·m | 1,323 N·m | 1,176 N·m @ 20m/s |
| 4-panel (2×2) ★ typical small farm | 1.3 | 860 N·m | 1,529 N·m | 2,389 N·m | 3,440 N·m | 3,058 N·m @ 20m/s |
| 10-panel row (5×2) utility scale | 1.4 | 2,315 N·m | 4,116 N·m | 6,431 N·m | 9,261 N·m | 8,232 N·m @ 20m/s |
| 20-panel row (10×2) large utility | 1.5 | 4,961 N·m | 8,820 N·m | 13,781 N·m | 19,845 N·m | 17,640 N·m @ 20m/s |
Wind torque T = 0.5 × 1.225 × v² × 2.0 × Cd × n_panels × 0.6m. Design torque = wind torque at 20 m/s × SF=2.0. For survival load (stow check), use T at 30 m/s — gearbox must hold panel stationary with motor de-energised (worm gear) or with servo holding torque (planetary). Note: 10-panel and 20-panel row designs require multiple drive units along the row — these figures are per drive unit assuming equal load sharing.
A 4-panel (2×2) tracker is the most common residential and small commercial solar farm configuration in Korea. At 20 m/s wind with SF=2.0, the design torque is 3,058 N·m — which exceeds the rated output torque of all standard single-stage EP-ZDE and EP-ZDS units. Two options are available: (1) use EP-ZDS-190 at 3-stage with rated output 1,800 N·m — only meets 59% of design torque at single unit; (2) use two drive units sharing the load, each carrying 1,529 N·m, which EP-ZDS-190 handles within its rating. For 4+ panel configurations, multi-drive or dedicated high-torque tracker drives are required. Korea Ever-Power application engineering provides multi-unit configuration guidance for these cases.
Backlash in Solar Tracking — What Matters and What Does Not
Backlash is frequently cited as a critical specification for solar tracker drives. In standard servo positioning — where the drive reverses direction frequently — backlash creates a dead band at each reversal that directly affects positioning accuracy. Solar tracking is fundamentally different: during the tracking day, the drive moves in only one direction (east to west). Backlash, being a direction-reversal phenomenon, has no effect on tracking accuracy during unidirectional motion.
Backlash has zero effect on tracking accuracy. The panel moves continuously in one direction — the gear mesh is always loaded on the same tooth flank. No dead band is engaged. A gearbox with 25 arcmin backlash tracks as accurately as one with 3 arcmin, assuming the drive is under load from wind.
When the drive reverses at dawn to reset from west-to-east, the backlash dead band must be traversed before the output shaft begins moving. At tracking speed (0.375°/min), traversing 8 arcmin (0.133°) of backlash takes approximately 21 seconds. For standard flat-panel PV, this is negligible. For CPV systems needing ±0.1° accuracy, even 8 arcmin = 0.133° may briefly exceed tolerance during reversal.
| Backlash Spec | Dead Band at Reversal | Time to Traverse at tracking speed |
During Tracking | Suitable For |
|---|---|---|---|---|
| <8 arcmin (ZDE/ZDS) | 0.133° | ~21 sec | No effect ✅ | All flat-panel PV, CPV with dawn compensation |
| <12 arcmin (2-stage) | 0.200° | ~32 sec | No effect ✅ | All flat-panel PV applications |
| <25 arcmin (ZDWE/ZDWF) | 0.417° | ~67 sec | No effect ✅ | Flat-panel only; too wide for CPV tracking |
Specification implication: For standard flat-panel solar farms, backlash is not a meaningful selection criterion beyond ensuring a minimum quality level. The standard EP-ZDE/ZDS (<8 arcmin) or even a lower-cost unit with <25 arcmin is technically adequate from a tracking accuracy standpoint. The specification criteria that actually matter for solar tracker drives are: (1) wind load torque capacity, (2) IP65 for outdoor lifetime, (3) gear ratio for servo stability during repositioning, and (4) temperature range for the deployment climate. Backlash is a secondary parameter — specify <8 arcmin for quality assurance, not because it is the limiting accuracy constraint.
Outdoor Environment Requirements — IP Rating and Temperature by Deployment Zone
The deployment environment of a solar tracker determines the minimum IP rating and material requirements for the drive gearbox. Solar farms span nearly every climatic zone — from Korean coastal regions with salt spray to desert installations with extreme UV and dust to tropical installations with high humidity and frequent rain. The 25-year design life requirement (IEC 62446) means no component can be considered “too expensive to specify correctly” at initial design.
EP Series Selection for Four Solar Tracker Configurations
The four primary solar tracker configurations used in Korean and Asian solar installations have distinct drive requirements determined by panel count (wind load), tracking accuracy requirement (flat PV vs CPV), axis count (single vs dual), and deployment scale (residential to utility).
T_design ≤ 600 N·m (1-panel at 20m/s, SF=2.0)
n_output_fast ≤ 2 rpm
Tracking: ±0.5° adequate
IP54 (inland) or IP65 (coastal)
T_motor = 588/(200×0.90) = 3.3 Nm
→ 400–750W servo motor
i=200: n_motor@2rpm = 400rpm ✅
→ i=160–200 recommended
EP-ZDE-160, 160:1 (IP54)
or EP-ZDS-115, 160:1 (IP65, coastal)
T_ceiling: 450/210 N·m ✅ vs 588 Nm design
T_design 1,176–3,058 N·m (SF=2.0 @ 20m/s)
IP65 recommended (outdoor 25yr)
n_output_fast ≤ 1.5 rpm
Tracking: ±0.5° adequate
2-panel: T_design=1,176Nm → EP-ZDS-142 ✅
4-panel: T_design=3,058Nm → 2× drives
i=160–200, [email protected]=240–300rpm ✅
2-panel: EP-ZDS-142, 160:1, IP65
4-panel: 2× EP-ZDS-142 sharing load
or 1× EP-ZDS-190 (1,800Nm ceiling, 3-stage)
Azimuth + elevation axes (2 drives)
Tracking accuracy: ±0.1° (CPV optical acceptance)
Correction every 16 seconds
IP65 mandatory (outdoor, high-value target)
T: 200–800 N·m per axis at design wind
Azimuth i=200–256 (motor stable @ 200–256rpm fast)
Elevation i=120–160 (lower speed range)
BL <8 arcmin → CPV reversal OK with controller compensation
Azimuth: EP-ZDS-115/142, 200:1, IP65
Elevation: EP-ZDS-115, 120:1, right-angle (ZDWF)
Both: BL <8 arcmin, FKM seals for outdoor
Very high torque: 500–1,800 N·m per drive unit
Azimuth only (trough tracks E-W daily)
Housing temperature risk: proximity to hot trough may exceed +90°C
IP65 essential; FKM seals mandatory
Verify thermal isolation from collector structure
Design torque: 500–1800 Nm
EP-ZDS-142 ceiling: 910 Nm ✅ (medium CSP)
EP-ZDS-190 ceiling: 1,800 Nm ✅ (large CSP)
Ratio: 100–160:1 (higher output speed needed)
Medium CSP: EP-ZDS-142, 120:1, IP65, FKM
Large CSP: EP-ZDS-190, 100:1, IP65, FKM
⚠ Verify housing temp ≤ 90°C in proximity to collector
Solar Tracker Drive Specification Checklist — Six Parameters Before Ordering
Calculate T_wind at design wind speed (typically 20 m/s for operating, 30 m/s for survival). Apply SF=2.0. Determine panel count per drive unit. Use the wind torque table in Module 4 to find T_design. Verify against EP series output torque ceiling for selected frame and ratio.
Set i such that n_motor at fast repositioning (1–2 rpm output) gives 100–600 rpm. Check n_motor at max repositioning ≤ 3,000 rpm. Use intermittent tracking strategy — do not attempt to run motor at tracking speed continuously. Recommended range: i=120–256 for most solar tracker configurations. See the high-ratio guide for detailed analysis.
IP54 minimum for inland temperate (standard Korean inland site). IP65 mandatory for coastal, desert, tropical, and agricultural sites. Specify FKM seals for any outdoor installation — standard NBR degrades under UV and ozone exposure. IP65 = EP-ZDS series only.
Verify housing temperature ≤ +90°C. In direct sun with poor ventilation, dark-coloured gearbox housings in desert can approach 85–90°C in summer. For CSP/trough systems, thermal isolation from hot collector structure is essential. EP series rated −25°C min; for colder climates, specify cold-start protocol.
For flat-panel PV: ±0.5° tolerance is adequate; backlash up to 25 arcmin acceptable from tracking accuracy standpoint. For CPV: ±0.1° or better; specify <8 arcmin and implement controller backlash compensation at dawn reversal. Accuracy during tracking (unidirectional) is not backlash-limited for either type.
EP series sealed grease has L10 = 20,000 hours ≈ 7 years at 2,920 hours/year operating time. For a 25-year solar farm design life, plan for two gearbox replacements (at year 7 and year 14). Include replacement cost in LCOE calculation. Stocking spare units in bulk reduces per-unit replacement cost; confirm Korean distributor availability before commissioning large solar farms.
Korea Ever-Power provides solar tracker drive specifications including wind load torque calculation for your specific panel configuration, gear ratio recommendation for stable servo operation, IP rating assessment by deployment zone, and housing temperature verification. Provide panel count, wind speed design point, deployment location, and tracker type (single/dual axis, PV/CPV) for a complete EP series recommendation.
編集者: Cxm
