PLEASE NOTE:
There isn’t a single universal physics formula to calculate the optimal Sleep and/or WakeUp Voltages because PV I-V curves are nonlinear, and behaviour depends on specific panel temperature coefficients, Solar VSD efficiency curves, pump load curves, and wiring losses. This percentage-of-Voc method is a practical standard which might require some adjustment in the field. Please also evaluate the below practical considerations in the field:
- Temperature Effects: Voc drops with heat (~0.3-0.4%/°C above 25°C). Test/adjust values in the field under real conditions. Cold mornings increase Voc (good for wakeup), hot afternoons decrease it.
- Hysteresis: Generally start (WakeUp Voltage) at 75-80% Voc (higher) and stop (Sleep Voltage) at 65-70% Voc (lower). Aim for 8-15% difference between Sleep and WakeUp Voltages (or 40-80V on typical 500-700V arrays). Too small → chattering. Too large → lost pumping time.
- Field Tuning: Monitor voltage/current under varying irradiation. If the system wakes/stops too often or runs inefficiently (high current, low speed), adjust up/down by 5-10V increments.
- Protection: These settings protect against undervoltage stalls more than it optimizes every last watt. Oversizing the array (1.3× minimum) already helps stability.
- MPPT Behaviour: The Solar VSD tracks the MPP, so operating voltage hovers near Vmp when the power is sufficient. The sleep threshold acts as a “not enough power” guard.
Recommended Base Rule
| Sleep Voltage | WakeUp Voltage |
0.65 to 0.75 × Array Voc (at STC or expected conditions) | 0.78 to 0.88 × Array Voc (at STC or expected conditions) |
| At this point on the I-V curve (near or slightly below Vmp under load), the array can typically deliver meaningful power. Below this, voltage collapses quickly under load as irradiation drops. | This is typically 5-15% higher than your Sleep Voltage setting to ensure the array voltage recovers sufficiently (indicating better irradiation and available power) before attempting to restart the motor. |
Adjusting for Oversizing Ratio (Array kW / Motor kW)
The oversizing ratio (Array kW ÷ Motor kW) has a significant influence on how much the array voltage sags under load, particularly when irradiation is low. A higher oversizing ratio provides more reserve power, resulting in less voltage drop, which allows the sleep and wakeup voltage thresholds to be set higher. Conversely, when the array is only marginally oversized, the voltage drops more noticeably, requiring more conservative (lower) threshold settings.
- kW Ratio = Array kW / Motor kW
- This should be minimum x1.3 for 3-Phase Motors and x2 for Single-Phase Motors (which tend to be less efficient and typically still experience a higher starting surge compared to three-phase motors due to the charging of the capacitors which creates an additional surge).
Sleep Voltage | WakeUp Voltage |
High kW Ratio (≥ 1.8 ~ 2.0+) | |
| With a higher oversizing ratio, the array has excess capacity, so under partial irradiation the system can still supply the required motor current while operating closer to the array’s Vmp. This results in less voltage sag compared to a minimally oversized array, where voltage drops more sharply as irradiation decreases. With a higher oversizing ratio, the array voltage sags less under declining irradiation, and the Sleep and WakeUp Voltages can thus be adjusted slightly higher. | |
| The Sleep Voltage can be set higher (typically 72~75% of Voc) so the drive stops before the system enters a very low-efficiency operating region with high motor current and poor performance. | The WakeUp Voltage can be set higher (typically 85~88% of Voc) to wait for stronger sunlight to ensure the motor only starts when sufficient power is available, reducing failed starts, high inrush current, and inefficient operation. |
Low kW Ratio (1.3×) | |
| The Sleep Voltage can be set lower (typically ~65% of Voc) to allow the drive to continue running longer as irradiation drops, preventing frequent sleep/wake cycling while still protecting the motor from operating in a very low-efficiency region with high current draw. Going too high would cause frequent unnecessary sleep/wake cycling. | The WakeUp voltage can be set lower (typically ~78% of Voc) to allow the drive to start earlier under moderate irradiation. With a low oversizing ratio, array voltage drops more significantly. A higher wakeup threshold would cause frequent failed starts and excessive cycling, while a lower setting helps maximise operating hours. |
Incorporating Motor Voltage (AC) and Array Voltage (DC)
The minimum DC bus voltage required by the Solar VSD is determined by the peak AC output voltage needed for the motor. This is calculated as approximately 1.414 × AC RMS voltage (the √2 factor for peak voltage after rectification).
- For a 400V AC motor, this equates to roughly 566V DC. However, the Solar VSD requires additional headroom (typically 30~60V above this value) to maintain a good modulation index, compensate for voltage drops, switching losses, and internal voltage ripple. This brings the practical minimum DC operating voltage to around 590~620V for reliable full-voltage operation.
- For a 230V AC motor, the equivalent calculation is 230V × 1.414 ≈ 325V DC. In practice, the Solar VSD needs headroom of around 20~40V, so the minimum recommended DC bus voltage is typically 345~370V. Because single-phase motors often have higher inrush currents due to capacitors, it is also advisable to maintain slightly more headroom on the wakeup voltage for stable starting.
The Sleep Voltage should thus be set well below the array’s nominal operating DC voltage – typically near the Maximum Power Point (Vmp), which is commonly between 75% and 85% of Voc under standard conditions. This ensures the Solar VSD continues operating only while the array can still deliver meaningful power near its optimal point. However, the Sleep threshold must also remain safely above the absolute undervoltage fault level of the Solar VSD and Motor. Setting it too close to the drive’s minimum DC bus limit can trigger nuisance faults, cause the drive to trip, or force the motor into an unstable operating region with excessively high current draw. A good balance keeps the system protected from inefficient low-voltage running while avoiding unnecessary shutdowns.
To ensure reliable operation, the sleep voltage should be set at least 10~20% higher than the minimum DC voltage required by the motor at reduced speed. This provides sufficient headroom for the Solar VSD to maintain stable AC output voltage and prevents the motor from operating in an undervoltage condition that could cause excessive current draw, overheating, or potential damage.
Full Suggested Formula/Process:
- Determine Array Voc (from panel specs, temperature-adjusted if needed – cold temperatures increase Voc).
- Calculate the kW Ratio = Array kW / Motor kW.
- Determine Base Multiplier based on above kW Ratio – see table below:
| Base Multiplier | ||
| kW Ratio | Sleep | WakeUp |
| 1.3 | 0.650 | 0.780 |
| 1.4 | 0.664 | 0.794 |
| 1.5 | 0.679 | 0.809 |
| 1.6 | 0.693 | 0.823 |
| 1.7 | 0.707 | 0.837 |
| 1.8 | 0.721 | 0.851 |
| 1.9 | 0.736 | 0.866 |
| 2.0 | 0.750 | 0.880 |
- Calculate as follows:
- Sleep Voltage = Array Voc x Sleep Base Multiplier
- WakeUp Voltage = Array Voc x WakeUp Base Multiplier
- Cross Check:
- Sleep Voltage: Should be > ~1.414 × Motor AC Voltage × 0.85 (some headroom for low speed) and also be 10~20% below expected Vmp at STC for your array configuration.
- WakeUp Voltage: Should comfortably exceed the minimum DC needed for reliable AC output: > 1.414 × Motor AC Voltage × 1.05~1.10 (extra headroom for modulation, losses, and initial ramp-up).
*Please see our EMHEATER Excel Solar Array Calculator HERE to assist with the Sleep and WakeUp Voltage calculations.
