Article and photos by Gregory Havel
The rising energy costs during the last decades of the 20th century led to the development of equipment designed to reduce electrical energy consumption. In factories, electric motors account for the greatest part of energy use. Electronic “variable-frequency drives” (VFDs) were developed so that electric motors would use less energy and use it more efficiently. These devices controlled the use of electricity by adjusting the frequency [Hertz (HZ)] of the alternating current supplied to the motor, based on its required speed and torque.
These early variable-frequency drives worked so well for industrial motors that they are now used on heating, ventilating, and air conditioning motors in schools, clinics, hospitals, and office buildings.
Photo 1 shows a VFD that controls a pump motor at a high school. It is rated at 50 horsepower and 480 volts three-phase.
A modern VFD used for HVAC equipment can be used on fans, pumps, or compressors, and can adjust the frequency of the electricity to be used by a motor from 1 Hz to 120 Hz (60 Hz is supplied by most electric utilities) in response to the actual load on the motor, based on pressure, flow, or temperature conditions. This adjustment reduces energy use and increases the efficiency of the motor.
Photo 1. Click to enlarge
VFDs are available for use on three-phase motors from ½ to 500-horsepower ratings at 480 volts. The VFD enclosure often includes a circuit breaker or fusible switch that can be locked out for worker safety; manual operating controls; a programming keypad and digital readout; a USB port for connection to a programming computer or building automation system; interface connections for temperature, humidity, flow, pressure-control systems, sensors and safety switches; and even surge protection for the microprocessor-controlled electronics.
Photo 2 is a close-up of the control pad on the cover of the unit shown in Photo 1.
Photo 2. Click to enlarge
The top section contains three groups of light-emitting diode (LED) indicators, an LED digital display, and a programming keypad. The two left LEDs in the top row show whether the motor is running forward or reverse. The far right LED in the top row shows whether there is an alarm condition in the system. The frequency in Hertz is the default in the digital display, which has other options that can be selected by the keypad. The far left LED below the digital display shows that the drive is engaged. The two LEDs below the programming keys show whether the drive is running or if it is executing a programmed stop.
The “SYSTEM STATUS” section has LEDs that show whether any safety switches have opened; the damper and building automation system controls are functioning; the smoke purge cycle has been activated by fire alarm; and whether the drive is in transfer or autorun mode. It also shows the status of the control circuit power.
The “DRIVE” section has a button with an LED to show if the drive is engaged; and LEDs that show whether it is ready to run, running, or has a problem.
The “BYPASS” section has a button with an LED to show that the drive has been bypassed and LEDs to show whether the motor is running or overloaded. This section also has buttons for automatic operation of the motor without the drive’s being engaged, for turning off the motor; and for operation of the motor without the controls or drive engaged (hand).
Photo 3 shows two VFD controls for a pair of five-horsepower pump motors that circulate chilled water in the coils of an air-conditioning unit in a museum. These have fewer features and indicators than the unit in Photo 1, but they function the same way and are programmed by a computer or a building-automation system.
Photo 3. Click to enlarge
For more information, Internet search for “variable frequency drives,” or visit the drive manufacturer’s Web sites.
The hazards of fighting fires with water near VFDs are the same as those presented when mixing electricity and water at any time. The enclosures for these controls often have ventilation openings so that they do not overheat. We must use caution in manually turning off the switch on a VFD because it is likely to be electronically interconnected to other motors or monitored by other equipment. Since this may disrupt a critical industrial or life-safety process or activate alarms, we should get directions from building personnel before changing any settings. Better yet, let the system operator run the system shutdown procedure from a remote location.
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for Scherrer Construction Co., Inc. He has a bachelor’s degree from St. Norbert College. He has more than 30 years of experience in facilities management and building construction. |
