Solar PV water pumping with variable frequency drive (VFD) or Adjustable Speed Drive(ASD)

Of all the ways solar electricity benefits the people on Earth, none makes as much difference in the daily lives as pumping water. By providing water for irrigation, or potable water has obvious benefits for people in rural areas and especially for those in developing nations.There are 3 different ways that water pumps can be connected to PV modules. The proper choice is determined by application specific factors like how much water is required and whether the water should be available at night or just during high sun hours .

For high amounts of water, a large AC pump is usually required. These pumps require well regulated 3 phase AC voltage and current. By using an AC variable speed controller called a Variable Frequency Drive (VFD) or Adjustable Speed Drive(ASD) the pump motor will have the proper voltage and current. The trick is to supply DC from the PV array directly into the DC bus inside the VFD. The normal AC input is not used. As the sun rises and PV voltage and current increase, some VFD products will accept the input and when the power is high enough, it will start the pump. The PV array must be large enough to provide enough power to start the pump with including the head of water. The size of the PV array required for this method can be very expensive.

As one of the main advantages of using a VFD is the energy cost savings associated with speed control. When using belts/sheaves or gearboxes to reduce speed, the motor still runs at full speed; however, a VFD reduces the actual motor speed, which reduces the amps drawn by the motor. This reduces the amount of energy used—and saves you energy costs. By ramping up the motor speed slowly, VFDs also help customers save energy by reducing the in-rush and mechanical issues associated with starting motors across the line.Solar-VFD-based-solutions

The first and most obvious way a VFD can save you money is through increasing the efficiency of your AC motors and decreasing electricity use. The amount of energy that a VFD can save depends on the average load of the motor and the number of hours per day it is turned on. While average energy savings varies, it’s safe to say that most AC motors do not need to be running at full capacity for most hours of their use. Furthermore, the initial cost of a motor or VFD is generally only a fraction of the cost of operating the motor over its life span. This means that a VFD system usually pays for itself in a matter of months or years.

Added advantages of VFD:

A VFD may be used for control of process temperature, pressure, or flow without the use of a separate controller. Suitable sensors and electronics are used to interface the driven equipment with the VFD.

  • Lower maintenance costs, as lower operating speeds result in longer life for bearings and motors.
  • The motor does not require a starter.
  • The ability of a VFD to limit torque to a user-selected level protects driven equipment that cannot tolerate excessive torque.
  • Users can utilize multi-motor applications, such as pumps or fans, with one control unit.
  • High-speed applications for the wood-finishing industry.
  • approx 30-40% energy is saved 

worksheet for  VFD payback calculatorsavings



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