5 Reasons to Install Energy-Saving VFD’s

Posted on July 17th, 2018

5 Reasons to Install Energy-Saving VFDs

Call it a variable speed drive, adjustable frequency drive, adjustable speed drive, AC drive, inverter drive or something else, a variable frequency drive (VFD) can reduce the energy usage of an electric motor up to 90%, extend motor life, deliver a payback in as little as 6 months and often qualify for energy-saving financial incentives. Since HVAC consumes the most energy in a building and motors account for the majority of HVAC energy usage, using VFDs on pump and fan motors can result in substantial energy cost reductions. Following is a short primer on VFDs and how to start saving money with them.

How VFDs Save Energy

Imagine a hypothetical car without an accelerator pedal and an engine that always runs at a constant 2,500 RPM whether parked or cruising down the highway. The car starts, the engine races, it’s shifted into “Drive” and it rockets down the road. The only way to control the car’s speed is to constantly apply the brakes. This is analogous to the way electric motors in your HVAC system may be working…not very efficiently.

The basic AC induction motor is designed to run at a constant speed. This is fine so long as the driven equipment (e.g. pump or fan) needs to run at the same speed most of the time. The fact is, however, that many pump and fan motors run at full speed less than 5% of the time. Moreover, pump and fan systems are frequently oversized to account for design uncertainties, future expansion or unusual conditions. Oversizing further decreases the likelihood a motor will run at full speed.

Like applying the brakes to control the hypothetical car’s speed, HVAC systems use mechanical means such as dampers, valves and vanes to reduce air and liquid flow rates while the electric motors continue to run at full speed. In both the car and HVAC cases, engines and motors running at a constant speed simply waste energy, increase wear and tear and create a less-than-optimum situation.

VFDs save energy by enabling electric motors to operate at less than full speed. The Laws of Affinity for centrifugal pumps and fans states. Reducing motor speed by 25% decreases energy consumption by nearly 60% while reducing motor speed by 50% decreases energy consumption by nearly 90%. Even with the 2% to 3% energy loss in the VFD, the energy savings can be very significant.


How VFDs Work

The modern VFD appeared during the latter stages of the Digital Revolution (1980s & 90s) as power semiconductors capable of controlling industrial-level voltages and currents increased in availability and reliability. There are several types of VFDs, but the most commonly used VFD is the Pulse-Width Modulation (PWM) type. Simplicity, reliability and value are the advantages of the PWM VFD.

A VFD for a pump motor, for example, might have a 480Vac input. The VFD rectifies the incoming alternating current (AC) to direct current (DC) because it’s much easier to “manipulate” DC than AC. Electronic switches (insulated-gate bipolar transistors or IGBTs) capable of turning “on” and “off” up to 15 thousand times per second output pulses of direct current of varying duration (pulse-width modulation) in order to replicate a three-phase AC sine wave of the desired voltage amplitude and frequency for controlling motor speed.

There are numerous ways that VFDs can be controlled to optimize the operation of motors and driven equipment (pumps, fans, etc.) from simple constant torque control to complex torque-limiting control with system feedback. In certain applications, a single VFD can control multiple motors.

Installing VFDs

The reasons for installing VFDs include:

  1. Energy Savings. Allowing a VFD to reduce motor speed can reduce motor energy consumption up to 90%.
  2. Reducing Wear. VFDs start motors slowly by ramping up the voltage rather than slamming motors with full line voltage to start. This reduces wear and potential overheating.
  3. Offset Oversizing. In many HVAC applications, centrifugal fans and pumps are oversized to account for various uncertainties. VFC speed control minimizes energy wasted by oversized motors that may never run at full speed.
  4. Reduce Demand Charges. Most commercial and industrial facilities are subject to utility Demand Changes based on the highest current demand occurring during a year. VFDs can help reduce Demand Charges by limiting motor inrush currents that can be 5 to 11 times higher than full-load current.
  5. Financial Incentives. Significant credits, rebates and tax deductions are available from many local utilities and governments agencies to incentivize the installation of energy-saving VFDs.

Start Saving Money with VFDs

Simply installing a VFD on a motor is no guarantee of energy savings. Optimizing the application of VFDs requires a thorough understanding of HVAC systems in order to maximize energy savings.

We understand.

Our engineers and technicians work on HVAC, VFD applications and energy efficiency measures every day. Moreover, ENGIE MEP Services takes a holistic approach to saving you energy and money including smart building controls, distributed energy solutions and energy and energy storage-as-a-service (EaaS and ESaaS).