The solar converter
Grundfos RSI
The RSI converts the DC power from a solar array into three-phase AC power to drive a submersible pump, without batteries. It integrates maximum power point tracking (MPPT), has an IP66 protection rating, covers a power range of 1.5 to 250 kW and can, depending on the configuration, be backed up by the mains or a generator. These characteristics are the essential elements to take into account for its sizing.
A frequency converter built for the sun
The RSI converter provides the interface between the solar array and the submersible motor. Photovoltaic modules deliver a DC voltage that varies with irradiance and temperature, while the motor requires a three-phase supply whose voltage and frequency must be precisely controlled. The RSI performs this conversion while integrating a maximum power point tracking (MPPT) algorithm. It continuously adapts the output frequency, and therefore the pump's rotation speed, to the power instantaneously available. The pump thus operates across the whole irradiance range, with a flow rate that naturally follows the energy supplied by the solar array.
Low voltage or high voltage: a defining choice
The RSI converter is available in two input voltage ranges, and the choice between them governs the sizing of the solar array. The high-voltage version is intended for medium and high-power installations, allowing the use of high-voltage module strings. The low-voltage version, limited to a maximum power of 15 kW, is designed to work with lower DC voltages and matching solar array configurations.
| 3 × 208-240 V | 3 × 380-440 V | |
|---|---|---|
| Power range | 1.5 to 15 kW | 2.2 to 250 kW |
| Minimum voltage at the maximum power point | 230 V | 450 V |
| Recommended voltage | 290 to 336 V | 530 to 615 V |
| Maximum input voltage | 400 V | 800 V |
| Output frequency | 5 to 160 Hz | 5 to 160 Hz |
| Protection rating | IP66 | IP54 or IP66 |
The minimum MPPT, recommended and maximum input voltages define the sizing constraints of the solar array. The module string must keep a voltage above the minimum operating threshold when cell temperature is high, a condition for maintaining maximum power point tracking. Conversely, its open-circuit voltage must never exceed the converter's maximum input voltage under the most unfavourable conditions, particularly in extreme cold, when module voltage is at its highest. Sizing must therefore guarantee that both limits are respected at the same time, to ensure reliable, continuous operation of the installation throughout the year.
The RSI is selected on the motor's current
Converter sizing is determined by the motor's rated current. For the same power, a motor designed for a lower rated voltage has a higher rated current. It is therefore common for the required converter to belong to a power class higher than that of the motor, in order to cover the current demanded under all operating conditions.
| RSI size, 3 × 380-440 V range | Rated output current | Size |
|---|---|---|
| 2.2 kW | 5.6 A | A |
| 4 kW | 9.6 A | A |
| 7.5 kW | 16 A | B |
| 15 kW | 31 A | B |
| 22 kW | 46 A | C |
| 37 kW | 72 A | C |
| 55 kW | 105 A | MR7 |
| 110 kW | 205 A | MR8 |
| 250 kW | 460 A | ED |
The same principle applies when replacing or upgrading an existing installation: the converter is chosen from the rated characteristics shown on the motor's nameplate, not from its apparent power alone. The RSI's commissioning wizard indeed relies on these parameters, namely rated voltage, frequency, rotation speed, rated current, power factor and power, to configure the converter. To simplify this step, it includes a library of the main Grundfos motors, allowing the installed model to be selected directly.
What the RSI can drive
The technical brochure lists the compatible Grundfos families: the SP submersible pumps, the CR vertical multistage pumps, the end-suction NB and NK, the MTR, the CM and the in-line TP. Only two conditions: a rated frequency of 50 or 60 Hz, and a motor able to run on 3 × 380 V or 3 × 220 V.
What an RSI installation really includes
A complete installation includes, in addition to the converter, several items of equipment listed in the instructions. Two of them are design-related and must be planned from the project study stage onward.
| Item | Role | Status |
|---|---|---|
| DC disconnect switch | safely isolate the solar array | essential |
| DC surge protection | protect the converter input | essential |
| Dry-run sensor | dedicated input on the converter | essential |
| Sine-wave filter | protect the motor insulation from voltage spikes | depending on voltage and cable length |
| Combiner box | bring together the module strings | depending on array size |
| Level sensor | stop on full tank | optional, input provided |
| AC disconnect switch | disconnection between output and motor | optional |
This constraint is decisive for the choice of converter range. The 3 × 380 to 440 V models require a minimum voltage of 450 V at the maximum power point, which necessarily implies an open-circuit voltage above 400 V. Installing a sine-wave filter is therefore systematically required with this range. Conversely, the 3 × 208 to 240 V converters are designed to work with a solar array whose input voltage stays below 400 V, which makes it possible, under the conditions specified by the manufacturer, to do without this filter.
The built-in protections, and the sensor that completes them
The RSI converter integrates a complete set of protection functions designed to safeguard the motor, the converter and the solar array. It notably monitors overcurrent, overvoltage and undervoltage, earth faults, motor phase loss, short- and long-duration overloads, rotor lock, underload operation, and the converter's internal temperature. It also includes a protection specific to photovoltaic applications: when the power supplied by the array becomes insufficient to guarantee stable operation, the converter automatically stops the pump drive to prevent degraded operating cycles.
Commissioning includes a second, decisive setting: the minimum delivery frequency. The procedure consists of starting the installation in sunny weather, progressively increasing the frequency until flow actually appears, then storing this value in the converter. This setting captures the pump's cut-in threshold, below which no flow is delivered at the outlet.
Heat and altitude
Two derating factors must be taken into account when sizing the converter, particularly in the hot, high-altitude regions where solar pumping systems are frequently deployed. Up to an ambient temperature of 40 °C, the converter can deliver its rated current. Beyond that, and up to 60 °C, the permissible output current must be reduced in accordance with the manufacturer's specifications. Similarly, full-load operation is guaranteed up to an altitude of 1,000 m. Beyond that, a derating of 1% of rated current is applied for every 100 m, up to the maximum altitude of 3,000 m.
Five decisions, one sizing
Choosing an RSI means simultaneously choosing a pump, a motor, a converter size, a module string length and a number of parallel strings. These five decisions are linked: the voltage window dictates the string length, the string length dictates the array's power step, and the pump sets the cut-in threshold below which no flow is delivered at the outlet.
Where this information comes from
- RSI 1.5 to 250 kW installation and operating instructions, Grundfos: technical specifications, voltage ranges, output sizes and currents, drive module efficiencies, start-up wizard, fault codes, minimum delivery frequency setting.
- RSI AC Drives instructions, Grundfos: types of motors driven, ambient temperature ranges and derating, altitude derating, switching frequency.
- RSI technical brochure, renewable solar energy converter for pump control 1.5 to 37 kW, Grundfos: general description, installation components, compatible pump families, accessories.
- SP and SPE installation and operating instructions, Grundfos: voltage and cable-length limits with and without a sine-wave filter, temperature sensors on converter-driven motors.