What is a variable speed drive (VSD), and do I need a well pump with one?

A variable speed drive (VSD) is a device used to control the speed or torque generated by an electric motor. When used with well pumps, VSDs monitor the pressure of the water and adjust the speed of the pump to match the water demand, creating a constant pressure. The use of VSDs are becoming more popular as they help a motor to run more efficiently. The VSD will gradually ramp up pump speed, minimizing wear and tear and increasing the life of your pump motor.

A smaller pressure tank is often preferred when using a VSD pump compared to a conventional private well water system. Since the VSD adjusts the speed of the pump to match water demand, the pressure is controlled more precisely without the need for a large pressure tank. Tanks ranging from 2 to 4 gallons are typically large enough as the drawdown capacity, or the amount of water stored before the pump needs to be turned on, is greatly reduced with a VSD pump.

VSD pumps can be very beneficial if your water usage has large fluctuations throughout the day. A VSD can adjust the speed of the pump motor to meet the demand ensuring that you have a constant water pressure regardless of the usage. If space for your water system is a concern, choosing a VSD pump with a small pressure tank may be a good option as well.

There are some downsides to having a VSD system though. One of the biggest downsides is the cost. While the price of installing a VSD pump has dropped and is more comparable to a conventional system, the replacement of the control box is still expensive. In addition, not all water system are compatible with VSDs. You will need to consult with a professional well contractor or licensed electrician to find out if it is feasible to add one to your current system.

Also, one of the drawbacks to installing a smaller pressure tank is it provides less backup water in case of pump failure, other system issues, or power outages. If space allows, consider installing a larger pressure tank. You will need to ensure a larger one is compatible with your VSD system though.

It is recommended that you do your research before installing a well pump with a VSD. Search the internet and talk to a professional well contractor about the pros and cons of these devices.

As you research for information about VSDs, you will notice that the terms, variable speed drive and variable frequency drive (VFD), are often used interchangeably. However, VSD is a broader term that refers to all types of drives used to control motor speed, while VFD specifically refers to an electronic device used to control the speed of an AC motor by adjusting the frequency of the electrical supply.

Butts, Ed. Variable Speed Pumping Systems. Part 2. Variable speed drives and their application. Water Well Journal, April 24, pg. 32, 34-37.

Mott, Andrea. Variable Speeds Drives. Pacific Northwest National Laboratory.