Trying to save energy dollars isn't always as simple as it seems. Installing variable-frequency drives (VFDs) and retrofitting lighting systems are two of the quickest and most widely accepted ways to achieve significant energy savings. Yet, new problems may surface as a result of this work. One of the first steps in solving these problems is to understand that the energy-efficient devices themselves may be to blame (Sensitive Controls on page 15). While most energy-saving electronic lighting ballasts, lighting controls, and VFDs work just fine on modern electrical distribution systems, this equipment can create troubleshooting nightmares in older facilities.
One of the most common energy-saving applications in facilities is to use VFDs on centrifugal pumps, fans, and blowers. Varying motor speeds is a much more efficient way to control flow rates — and thereby maintain water temperatures — than to run the motor and pump at full speed and throttle a valve to adjust flow rates. However, problems will result in the following situations:
The VFD has not been properly sized and selected.
The drive and motor installation was not done with a VFD in mind.
Parameters were not properly set at startup.
The effects of harmonic currents were not considered in design and addressed in maintenance.
VFDs are complex electronic devices that vary their output voltage and frequency to control motor speed. The distribution system supplies voltage; however, current doesn't flow into the VFD until the rectifier circuit in the converter section begins to conduct. Because current doesn't flow linearly as voltage is applied, the equipment is called “nonlinear.”
When the diodes suddenly allow current flow, it creates a “notch” on the sine wave that causes sine wave distortion. The VFD rectifier circuit also causes currents to flow back into the distribution system in multiples of 60Hz. Current that flows back in multiples of the fundamental is called harmonic current. The third harmonic is three times (3÷) the fundamental of 60Hz, or 180Hz. The fifth harmonic current is at 300Hz, and so on.
These harmonic currents not only tend to distort voltage throughout the plant, but certain harmonic frequencies also create problems unique to that harmonic. For example, the third harmonic causes overheating in neutral conductors and transformers. The fifth harmonic can cause motor issues, such as overheating, abnormal noise and vibrations, and motor inefficiency.
Other typical nonlinear loads added during energy upgrades include electronic ballasts, computers, controls (PLCs, etc.), and various components of building automation systems. Let's review four troubleshooting situations you may encounter in the field when working with VFDs.
- Installation checks
To troubleshoot VFD control problems, first review the installation design. Chances are the proper drive, motor, and associated equipment were selected — but verify them anyhow. Walk down and observe the installation. Were correct cable types selected and installed properly? Is the installation suitable for the environment in which it is installed? Are enclosures free of dust, and is adequate ventilation provided?
- Drive parameter checks
Review the parameters programmed into the drive. Does the data entered match the motor nameplate? Has the drive been set for proper operation, such as variable torque for energy-saving pump and fan applications? If the VFD is not controlling the motor as expected, it could be because operational parameters were either set incorrectly or reset by some well-meaning individual attempting to correct other problems.
- Quick measurement checks
Measure VFD input voltage with a true-rms digital multimeter, verifying voltage unbalance falls within manufacturer's specifications. Measure harmonic frequencies and levels at the point where power is supplied into the VFD, using a power quality clamp meter or power quality analyzer. In addition, check for harmonics back at the feeder where the power to the VFD is also supplying other loads.
If voltage unbalance is the problem, shift and evenly distribute single-phase loads. If harmonics are found to be the cause, contact the drive manufacturer or a harmonic filter manufacturer, and install a properly tuned harmonic filter.
- Poor motor speed control and/or nuisance trips
One typical VFD problem encountered in the field is that the drive fails to control motor speed properly and may even experience nuisance trips. The two mostly likely causes of this particular problem are:
Voltage unbalance on the three phases supplying the drive.
Harmonics flowing out of the drive back into the distribution system.
For example, a chiller with a VFD installed may experience temperature control problems at specified locations in the system due to distorted sine waves created by the harmonics. This distortion affects the operation of PLCs, temperature controllers, and other controls in the chiller.
There is also another possibility: If tachometer feedback cables are not properly selected and installed, erroneous motor speed information will be fed back into the VFD, making it impossible for the VFD to control real motor speed.
Run shielded cable for these low-voltage signals and ground only at one end. When routing these low-voltage conductors, ensure they are not installed close to power conductors. Electromagnetic induction from power cables can affect low-voltage control.
Without a doubt, lighting retrofits save money on the monthly electric utility bill. However, many facilities invest in lighting upgrades only to be left with flickering lights or lights that don't operate at all. Seemingly unrelated events also start to occur: 3-phase motors overheat, servers and computers malfunction, and data is lost. Nuisance trips on circuit breakers suddenly begin occurring. Newly installed electronic equipment mysteriously trips on overvoltage or overtemperature — yet the equipment doesn't show any signs of such abnormal conditions.
Such problems are generally associated with harmonics. One IEEE study indicates these harmonics can become a significant issue if fluorescent lighting comprises 25% or more of the facility load.
Electronic ballasts often introduce harmonic currents back into the distribution system. If the facility is older — and only one neutral wire was pulled in for each of the three ungrounded phase conductors to the lighting circuit (sharing neutrals) — the result may be overheating neutral conductors, panelboards, and transformers. The fix for this situation is to pull in additional neutral conductors, one per phase total as needed. Infrared thermography can often identify these issues before failure.
The installation of dimming controls is another energy-saving strategy that is currently being employed. This dimming can be achieved with manual dimmers or with photosensors. If adopting this approach, make sure to match the proper type of dimmer control with the ballast and lamp type to be dimmed. Mismatches can result in improperly operating equipment and damaged lighting system components.
Depending on the type of dimming controls used, additional control wiring operating at 0V to 10V may be installed. However, placing this control wiring too close to power conductors during installation or maintenance can result in erratic lighting control. One way to cut down on problems is to keep control wiring as short as possible during installation.
Automated lighting controls that switch lighting banks off and on erratically after the installation of energy-saving controls should be checked for proper sensor operation. Some photosensors may have a dead band adjustment available to change the time between the on and off cycles.
If banks of lighting are being switched to save energy, phase unbalances could appear on 3-phase systems. This could lead to overheated motors. If your maintenance guys start getting calls to replace motors that have been destroyed by overheating, then you should implement a new procedure to check the voltage supplied to the motor during all phases of plant operation. Maximum voltage unbalance at motor terminals should not exceed 1%. Operation of a motor at greater than 5% unbalance will probably result in motor damage.
A proactive approach
After an energy upgrade project has been implemented, the best tool for solving problems is a well-trained and properly equipped workforce. Knowledge of how the energy-saving application works is the first step in solving associated problems. You should start by reviewing the design, installation, and any startup procedures to isolate and correct any problems. As maintenance on the equipment is required, use a logical, systematic approach with the right tools to isolate problems. Review the Table (click here to see Table) for typical problems encountered after energy-saving upgrades and for ideas on where to get started.
Burnett is an energy auditor with Step1 energy Audit, Englewood, Colo. He can be reached at [email protected].
Sidebar: Sensitive Controls
Modern control systems are quite often sensitive to any quality problems with the electrical power they are supplied. That means nonlinear high-efficiency loads create operational problems not only with other sensitive plant equipment, but also with themselves. Ironically, the very equipment installed to save energy often causes inefficiencies and unexpected maintenance costs. Fortunately, some quick checks and basic electrical troubleshooting can resolve many problems.