About Solar Pump Inverters

Introduction to Solar Pumping System

A photovoltaic water pumping system utilizes the electricity generated by a solar array to drive a water pump. The whole system mainly consists of a solar array, a solar pump inverter, and a water pump. It is a rapidly developing electro-mechanical integrated system in recent years. It is the most attractive water supply method in areas rich in sunlight, especially in remote areas that lack or have no electricity.

System Components

Solar Array: Also known as solar panels, the main function is to convert solar energy into electrical energy, providing operational power to the water pump motor.

Solar Pump Inverter or Controller: It controls and regulates the operation of the solar water pump, driving the pump using the electricity generated by the solar array. It adjusts the output frequency in real-time based on the intensity of sunlight to make the output power close to the maximum power of the solar array.

Water Pump: Generally refers to a machine that lifts liquid, transports liquid, or increases the pressure of liquid, i.e., converts the mechanical energy of the prime mover into the energy of the liquid to achieve the purpose of pumping the liquid.

What is Solar Pump Inverter?

A solar pump inverter controls and regulates the operation of the solar pumping system (solar water pump system), converting the DC power generated by the solar array into AC power to drive the pump. It also adjusts the output frequency in real-time based on the intensity of sunlight to achieve Maximum Power Point Tracking (MPPT).

A solar pump inverter is an off-grid inverter that does not rely on the grid and can independently carry loads. However, conventional off-grid inverters need to be equipped with batteries to function, and the cost of lead-acid batteries is high, accounting for about 30% of the system's cost. They have a short lifespan of only 3-5 years, affecting the system's return on investment. 

The solar pumping system does not require batteries to operate; it works as long as there is sunlight. Building a water tower at a high place allows for water extraction from the tower when needed. The inverter itself can also be equipped with a water level switch, making it very convenient and practical. Its function is equivalent to the batteries in off-grid systems, but the cost of a water tower is much lower than that of batteries.

The electric motor is the most difficult load to handle in an off-grid system because it requires a large amount of energy to start. The starting power of a conventional electric motor is about three times its rated power, while a water pump motor requires a starting power of about five times its rated power to lift water to a high place. 

If a conventional off-grid inverter is to drive a water pump motor, it needs to amplify by five times; for instance, a 2kW water pump motor would require a 10kW off-grid inverter to start. During normal operation, the DC input needs to be greater than 2kW for the motor to function continuously. 

This increases system costs, while a solar pump inverter introduces a special algorithm, generally only increasing power by 20%. For example, a 4kW water pump motor can be started with a 5kW solar pump inverter. During operation, the photovoltaic input power does not need to be 4kW to keep the pump running; about 1kW can also make the water pump work.

Why does a Pump Inverter have such powerful functions?

This relates to the principles of AC power. AC power has three elements: voltage, current, and frequency. Under normal circumstances, the frequency is a constant 50Hz, and the motor spins at 50 cycles per second, with power varying with voltage and current. However, for electric motors, power is also related to frequency. The typical rated power is the power at a 50Hz frequency. When the frequency decreases, the power also decreases. 

The rated power of the motor = rated torque × rated speed. As long as the rated torque remains unchanged, the motor can operate. Thus, when the frequency and voltage decrease, the rated power decreases proportionally to speed or frequency. A pump inverter incorporates a frequency converter function, allowing it to change the frequency of the AC output. During startup, the frequency is lowered to trade speed for power. After starting, the frequency is raised to increase speed. The frequency can also vary with sunlight, so even with a 1kW photovoltaic input, a 4kW water pump can be operated.