VFDs: Four Easy Steps to Achieving Quick Payback on HVAC Energy Savings

By Wayne Stebbins, Commercial Industry Consultant, ABB, New Berlin, Wis. May 2, 2006

HVAC variable frequency drive solutions running on bypass can waste 56% or more energy for your HVAC air handling system; here’s how to get back to efficient usage.

Despite the economic advantages and significant energy savings available by using AC variable frequency drives in HVAC applications, many building operators do not repair or replace drives when they fail; this is because the motors can easily continue to run through a bypass contactor. While this is a great solution for short-term outages, continued operation in this mode quickly becomes a very expensive way to operate a fan.

The historic challenge

Budget cuts and competitive pressures have reduced building maintenance staffs in many public and private enterprises. This frequently results in a re-prioritization of activities, forcing concentration on addressing issues which are required to be fixed at the moment. Repair items such as a HVAC drive system operating in bypass mode may be considered only a nuisance, to be dealt with as time permits. And of course, time seldom does so permit.

Consider the rational for installing HVAC VFDs over the past 10 to 15 years to control the airflow in buildings. These drives were originally installed to replace the throttling systems designed to regulate flow of air in the system. While throttling reduced the flow, the motor still ran at nearly full-load speed and, in some cases, worked even harder to overcome the added system restriction. By reducing the speed of the motor, the variable speed drive ensured no more energy than necessary was used to achieve the required flow.

For example, in theory a fan running at half speed consumes only one-eighth of the energy compared to one running at full speed. Field experience has shown that, when the effect of static back pressure is factored in, the relationship is somewhere between 1/4th and 1/8th of the energy consumed at full speed, depending on the applications mechanical application.

Recent studies indicate that 8% to 12% of HVAC drive systems are running in a bypass mode due to a drive fault. The intent of a bypass contactor is for use in case of a drive failure for short time emergency service. It was never intended to be a long-term solution to a drive malfunction. While the misapplication of long-term use is understandable due to increased pressures on typically under-manned building maintenance organizations, there are proven approaches to solving this pressing problem.

Why worry about this now?Energy costs continue to soar. Between 1999 and 2004, electricity costs increased by nearly 15%, according to the U.S. Dept. of Energy. When VFDs were purchased for the application, the additional costs were justified based on saving money and improving profitability by using less energy for HVAC air handling in the building. Here’s an example of how long-term use of bypass contactors affects energy costs: if 10% of the drives are in bypass mode, up to 56% more energy can be consumed by the HVAC air-handling system. This is based on the assumption that all motors being operated are the same size, and all HVAC systems are operating on average at 50% flow.

Four steps to savings

The path forward presents two clear choices. First, do nothing and continue to lose increasing amounts of money every day as additional VFD systems periodically fail and go into bypass operation. Second, develop a program which will change the way drives are proactively maintained. To develop such a program, it’s useful to detail how a typical preventative maintenance program is centered around the following activities:

1. Review your situation. Use either an outside or internal resource person to inventory the drives in the building/complex to gather the following information:

  • the number of installed drives and the make and model of each unit;

  • age of the drives and how long they actually have been in service;

  • the horsepower of each drive;

  • the duty cycle of each drive, noting that load level vs. length of time data may be difficult to determine so this will often be an estimate;

  • the number of drives operating in the bypass mode;

  • existing replacement drive inventory and onhand spare parts to support downtime.

2. Replace or repair all drives operating in the bypass mode, to begin realizing the original energy savings. It is important to work with a supplier that demonstrates the capability to replace or repair drives easily. Select one that will assist in the maintenance and support on an ongoing basis.

3. Create or contract a preventative maintenance program that focuses on the specific issues of drives and how to keep them up and running. These activities typically include, but are not limited to, the following. With the VFD de-energized:

  • inspect environmental conditions on each drive;

  • inspect power components and circuit boards for deterioration;

  • inspect for loose connections;

  • clean interior components of the drives.

With the VFD re-energized:

nsimulation or variation of signals from the control system to verify that the VSD is responding properly;

ncalibration of the drive to original factory settings;

nreview of the drive application for possible upgrades and operational enhancements.

4.Replace older and highly critical drives before they fail. When a drive is over 10 years old, or in a demanding and highly critical application, consideration should be given to replacing it before failure. Even with the cost of a new drive and installation, the benefits will include lower operating costs and improved client comfort. Simple-payback, ten-year life-cycle costing or other financial analysis techniques may be performed to evaluate, formally, the economics for drive changeout.

Drive track record

VFD systems installed in the facility have a proven track record of energy costs and improving client comfort. The ability to keep drives running as designed will assure continued savings and comfortable clients. A number of significant improvements have been made to present-day VFD systems compared to what was available 10 years ago. Drives size and parts count have been reduced along with cost while increasing performance, quality and warranty periods. Commonly available features include embedded PI control functions which eliminate the need for closed loop output signals from the BAS. The PI controller typically includes feedback inverse, square root and differential control functions on board which lower your costs of HVAC control systems installation and wiring.

VFD units now typically combine sophisticated IGBT power switching with advanced microprocessor logic to reduce audible motor noise and meet accepted power quality standards. A number of communication options are available which can be tailored to a wide variety of BAS data and control formats. On board metering of electrical kW and kWh information provides data useful in efficiency and billing calculations.