AC Motor or DC Motor for Solar Water Pumping
Which type of motor AC or DC is best for solar water pumping system?
There are several different types of solar-powered pumps depending on how they have been classified. But primarily there are four types of solar water pumps–submersible pumps and surface pumps, direct current (DC) pumps and alternate current (AC) pumps.
Submersible pumps:
As the name suggests, a submersible pump is located deep below the ground level and remains submerged under water. The suction head of the submersible pump is beyond a depth of 10 metres. The installation of these pumps is done by digging a borewell, which leads to an increase in its installation and maintenance cost.
Surface pumps:
The surface pumps remains out of water and in the open. They are installed where the water table is within a depth of 10 metres. As they need to be on the surface, these pumps are easier to install and maintain. They are, however, not apt for deep water table.
DC pump:
This pump runs on a motor which operates on direct current, therefore no battery or inverter is needed in this type of pump.
AC pump:
The motor of this pump operates on alternating current, which means the direct current produced by the solar panels gets converted to AC using the inverter. The conversion from DC to AC leads to loss of power from generation and consumption.
The selection is based on the groundwater level and the water source. In the case of a borewell with a water table deeper than 10-15 metres, a submersible pump is used. If it is an open well, pond, etc, then using a surface pump is better. Also, the surface pump is installed when the water level is less than 10 metres.
Based on the classification, the following types of motor pump sets are available:
- Surface mounted motor pump set
- Submersible motor pump set
- Floating motor pump set
- Any other type of motor pump set after approval from test centres of the Ministry of New and Renewable Energy.
- Although DC pumps have an advantage over AC pumps in terms of higher efficiency and no requirement of an inverter for operation, the cost of DC pumps is higher. Also, the repair and maintenance of DC pumps are difficult in rural and remote areas due to lack of service centres in these areas.
Detailed Comparison: DC vs. AC Solar Water Pumps
| Feature | DC Solar Pump | AC Solar Pump (with VFD/Inverter) |
|---|---|---|
| System Architecture | Simple. PV panels connect directly to a DC motor, sometimes via a simple controller. | More complex. PV panels connect to a specialized variable frequency drive (VFD) that converts DC to AC, powering the AC motor. |
| Motor Type | Brushed DC (older, less common) or Brushless DC (BLDC – modern, efficient). | Induction Motor (Asynchronous) – most common, rugged, and reliable. |
| Efficiency | Very High for BLDC motors, especially at partial load. Good match for the variable nature of solar. | High. Modern VFDs and motors are very efficient across a wide power range. |
| Cost | Lower upfront cost for very small systems (< 500W). | Higher upfront cost due to the VFD. However, better long-term value for larger systems. |
| Reliability & Maintenance | BLDC: Very High (no brushes, sealed). Brushed DC: Lower (brushes wear out). |
Exceptionally High. Industrial-grade 3-phase AC induction motors are renowned for their durability and can run for decades with minimal maintenance. |
| Performance in Variable Sunlight | Excellent. Speed and power draw are naturally proportional to solar irradiance. They start easily even in low morning/evening sun. | Excellent. The VFD continuously adjusts the motor’s frequency and voltage to match the available solar power, optimizing performance all day. |
| Distance from Panels to Pump | Problematic. DC power suffers significant voltage drop over long wires. Panels must be close to the pump (< 100 feet is ideal). | Major Advantage. Can use high-voltage AC transmission. The VFD can be placed near the panels, and the pump can be thousands of feet away (e.g., in a deep well). |
| Scalability & Power Range | Best for low to medium power applications (e.g., small fountain pumps, garden irrigation, shallow wells). | Highly scalable. Dominates the market from 0.5 HP to tens of HP. Ideal for deep wells, large-scale irrigation, and village water supply. |
| Complexity & Components | Simpler system with fewer components. | More complex due to the VFD, but this is a mature, reliable technology. |
Decision Guide: Which One Should You Choose?
Choose a DC Solar Pump if:
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Your application is small-scale (e.g., garden pond, small garden drip irrigation, livestock watering from a shallow source).
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Your total power requirement is under 500W – 1,000W.
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Upfront cost is the primary concern and the system is simple.
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The solar array can be installed very close to the pump.
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You are comfortable with a system that is less scalable for future expansion.
Typical Use Case: A 200W DC surface pump used to move water from a stream to a small irrigation tank.
Choose an AC Solar Pump (with VFD) if:
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You need to pump from a deep well.
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Your requirement is for higher flow rates or higher pressure (e.g., for large-scale crop irrigation, pressurizing a home’s water system).
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The pump needs to be located far from the solar array (a very common scenario).
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You value long-term reliability, durability, and minimal maintenance.
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You have a larger budget and are looking for the best performance and return on investment.
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You want a system that can be easily integrated with a generator or grid power as a backup (many VFDs support this).
Typical Use Case: A 3HP (2.2kW) AC submersible pump in a 300-foot well, with the solar array located 500 feet away, providing water for a farm or small community.