The Use of Variable Speed Drives (VSD) with Solar Water Pumping Systems in Zimbabwe

If you are interested in getting Expert advice on Borehole Drilling and Borehole Installation in Zimbabwe, we provide that information for FREE here at Borehole Experts Zimbabwe so please get in touch to discuss your project by calling or sending a WhatsApp message to +263 77 389 8979 or +263 71 961 3479.

The use of solar photovoltaics (PV) technology as an alternative to traditional energy sources such as petrol and diesel generators to power electric water pumps has become increasingly apparent. 

Solar PV technology is used extensively by remote villages and farmers as the power source to pump water for domestic consumption, sanitation, livestock and irrigation.

A variable speed drive (VSD) is a piece of equipment designed to regulate the speed and rotational force (torque) of an electric motor. VSD’s are becoming a standard inclusion as part of the solar water pumping system controller. This article explains is the role of VSD in solar pumping.

Solar Pumping Systems

A typical solar water pumping system contains the following equipment: a solar array, which converts sunlight into electricity; system controllers, which control the array and the pump; an electric motor, which drives the pump; and a water pump, which moves the water from a source to its delivery point.

Electric motors

Two main kinds of electric motors are available: DC motors and AC induction motors. While DC motors can be used for solar pumping applications, they are not as widely manufactured or used and are therefore a more specialised and expensive product, whereas AC induction motors are widely available and a cheaper product.

Currently the most widely available motor for industrial purposes is the “squirrel cage induction motor”. This product has low manufacturing costs, robust construction, adaptability to submersible and flameproof applications, and its ability to accept a wide range of voltage / power ratings. Given these advantages, the use of AC induction motors to drive water pumps is an obvious choice.
The Use of Variable Speed Drives (VSD) with Solar Water Pumping Systems in Zimbabwe
The Use of Variable Speed Drives (VSD) with Solar Water Pumping Systems in Zimbabwe
Historically AC induction motors had distinct disadvantages compared to the DC motor. On start-up, the current and running speed of a DC motor can be readily controlled by use of a variable resistance in series (older technology) or a Pulse Width Modulated controller (modern). The AC induction motor relies on the supply frequency

There may be other components in the pumping system, such as a battery or an AC source connected to the system controller. A variable speed drive (VSD) is a type of motor controller that controls the output of motor by varying frequency and voltage. 

VSD in Solar Pumping systems

A VSD can provide a reduced starting voltage to the motor windings, thus reducing the starting current of the motor and giving it time to gain momentum before the full load is supplied with power. The VSD also allows the user to start and stop the pump at a controlled, programmable rate (e.g. accelerate or decelerate over a time period) while putting minimum strain on the motor. This reduces the mechanical wear of the motor as well as the start- up loads on the PV array.

Reduce energy consumption

A VSD with a variable torque load can be used in water pumps to reduce the input energy requirement. As described by the affinity law, the power consumption of the pump can drop significantly with a small drop in the speed of the motor. Provided that the flow rate is acceptable, running a pump at a lower speed over a longer period of time can deliver considerable energy saving. 

The VSD can reduce the operating speed of the motor, allowing a smaller PV array to be installed to deliver the pumping requirement.

The ability of the VSD to control pump operation also means that in case of cloudy conditions or times of reduced solar irradiation, the motor could still run to provide deliver pumping work.

A VSD can be programmed to run the motor at a certain speed to obtain a desired flow rate from a pump. Most VSDs come with an integrated proportional–integral–derivative (PID) controller which allows the drive to derive a set point based on actual feedback from the process e.g. from a flow transducer or tank level sensor and operate in a closed loop mode if desired.

VSD with a closed loop control offers better system control and increases the system's reliability.

Cost implications Of The  Variable Speed Drives (VSD)

The variable speed drive reduces the cost of the system in two ways:
  1. Increased system efficiency and
  2. Reduced operational and maintenance costs. 

The system efficiency is increased with the capacity to run the pump at lower speed. This can translate to a smaller solar array or the elimination of the need for batteries, inverter or a diesel generation set, which can significantly reduce the capital cost of the system. 

By allowing the pump to run at a lower speed, system wear and tear is reduced, increasing the lifespan of the pump and delaying replacement and accompanying costs.


The variable speed drive as used with a solar pumping system means that the solar PV array is more effectively matched to the pumping system in terms of installed capacity and therefore cost. The inclusion of the VSD also provides more flexibility in the control of the pumping system, allowing the pumping regime to be varied to suit the application with the least energy use.

Variable speed drives can also be retrofitted into existing solar pumping systems with an AC induction motor after evaluating the suitability of the motor, particularly its insulation rating.

Variable Speed Submersible Borehole Pumps from Borehole Experts Zimbabwe: 

Contact Borehole Experts Zimbabwe on 077 389 8979. Borehole Experts Zimbabwe are specialists in providing Variable Speed Submersible Borehole Pumps, featuring a wide range for all of your Variable Speed Submersible Borehole Pumps requirements. 

Feel free to check with us on our Variable Speed Submersible Borehole Pump alternatives; if you have any questions about our Variable Speed Submersible Borehole Pump range or queries our support team will be happy to help.

Borehole Experts Zimbabwe aims to make sure that each job is done correctly. At Borehole Experts Zimbabwe, we Drill and Install Boreholes and Water Systems in and around Zimbabwe. 

For All Your Solar Systems and Borehole Drilling Services Contact Us:

If you are looking for a Drilling Company in Zimbabwe, look no further! We have been drilling boreholes all over the country for many years. Our team of experts knows what it takes to get your job done right. With our expertise and experience, we are confident that we will be able to find the perfect location where you want us to drill your water well. Contact us today so that one of our representatives can answer any questions you may have about borehole drilling or pump installation.

Get In Touch With Us: If you are interested in getting Expert advice on Borehole Drilling and Borehole Installation in Zimbabwe, we provide that information for FREE here at Borehole Experts Zimbabwe, so please get in touch to discuss your project by calling or sending a WhatsApp message to

Request A Quote:

If you have further questions, contact our friendly team today they will be more than happy to assist.

Business Office: No. 7 Frank Johnson Avenue, Eastlea, Harare, Zimbabwe.

Phone: +263 77 389 8979 and +263 71 961 3479.
Borehole Experts Online
Borehole Experts Zimbabwe Are Experts In The Design, Installation, Ongoing Maintenance and Emergency Repair of Borehole Pumps, Providing Bespoke Solutions To Suit Your Requirements. We Are Able To Specify, Drill, Supply and Install Borehole Pumps As Well. Borehole Experts Sales
Connect via WhatsApp