Correct installation of variable frequency drives demands attention to vital factors such as location and environment, electrical connections grounding, supply voltage, and protective devices.
Over 50% of drive failures are a result of improper installation and start-up. As such, you can avoid many problems by carefully planning your VFD installation prior to the actual work beginning. There are several key factors that must be considered to ensure a proper VFD installation.
The drive installation procedure begins with a thorough reading of the drive's manual. (That's the book we always put aside until we have problems and need answers). This manual will give you important information about receiving, storage, and planning for the installation before the equipment arrives. It's a good practice to make several copies of the manual; it seems they always end up in that great void with all the other lost manuals. Better yet, specify additional spare manuals for the drive with your order.
Cooling and line power quality considerations
There are many important considerations for all industrial electronic equipment; however, the two most important are cooling and line power quality.
Cooling. VFD control units should be installed in locations where the maximum ambient temperature does not exceed 40 [degrees] C (104 [degrees] F). This is a common temperature rating for most units. If higher ambients are expected, derating of the VFD may be required.
You should avoid installing units in mezzanines, direct sunlight, or near external heat sources because these locations usually have unpredictable temperature rises. If you must install VFDs in such areas, take steps to provide necessary cooling.
Proper cooling of the VFD's heat sink is essential for successful operation. Wall-mounted units employ a "chimney effect" for cooling of the heat sink and must be mounted against a smooth, flat, vertical surface. If a wall-mounted unit is to be installed in a free-standing position, then plywood or a sheet of metal should be fastened to the back of the unit to ensure that chimney-effect cooling will be achieved. Larger free-standing VFDs require minimum clearances around them for sufficient air flow to circulate through them, assuring adequate cooling.
Altitude affects the heat dissipation capability of the heat sink and units in locations at 3300 ft (1000 m) above sea level must be derated. A common rule of thumb is a 2% derating for every 1000 ft above the 3300 ft altitude level.
Supply line power quality. The line voltage (supply) to the drive input should not vary plus or minus 10% because most drives will trip via a protective fault. This voltage stability should be considered when running conductors to the drive and voltage drops should be calculated for long runs.
Very often external control signals are used to start/stop and control the speed of a VFD. Control signal types can vary depending on the application. It's not unusual to have a 115V or 24V control circuit for start commands and a 4-20mA or 0-5VDC signal for speed control. These control signals must be run independently of each other as well as separate from any power wiring. Induced noises can produce erratic drive behavior that is very difficult to troubleshoot.
Other VFD installation considerations
There are other installation considerations for VFDs. These include electrical connections, grounding, fault protection, motor protection, and environmental parameters such as humidity and moisture.
Electrical connections. The sizing and installation of VFD line and load conductors should conform with the NEC and other appropriate local codes.
Grounding. For safe and dependable operation, all VFDs must be properly grounded. This normally calls for a grounding conductor to be brought back to a single-point grounding location, usually selected to be at the service. In addition, a grounding conductor must be brought back from the motor to the VFD's internal grounding terminal. This direct motor ground to the VFD is required to minimize interference and for the ground-fault protection function of the drive to operate properly.
Fault protection. Many VFDs have short-circuit protection (usually in the form of fuses) already installed by the manufacturer. This is usually the case on larger hp units. Smaller units (1/3 to 5 hp) normally require external fuse protection. In either case, the selection and sizing of these fuses is critical for semiconductor protection in the event of a fault. The manufacturer's recommendations must be followed when installing or replacing fuses for the VFD. Be sure to torque-bolt fuses in place according to the manufacturer's specifications to assure fast operation of fuses in case of a fault.
Motor protection. All motors require overload protection. The most common practice is the use of a motor overcurrent relay system that will protect all three phases and protect against single-phasing. This type of protection will respond to motor overcurrent conditions of an overloaded motor, but will not detect over-temperature conditions.
A motor operating at reduced speeds will have reduced cooling; as a result, it may fail due to thermal breakdown of the motor windings insulation. Thus, the optimum protection for a motor is thermal sensing of the motor windings. This sensing is then interlocked with the VFD's control circuit. This is highly recommended for any motor that is to be operated for extended periods of time at low speeds.
Humidity and moisture. As is the case with all electrical and electronic equipment, high humidity and corrosive atmosphere's are a concern. Drive units should be installed in a noncorrosive location whenever possible, with ambient humidity ranging between 0 to 95% noncondensing.
Completing the installation
Installation of a VFD is no different than any other type of motor control. Common sense, along with the manufacturer's guide lines and adherence to the appropriate code requirements, are all that is needed.
At this point in time you'll probably want to power-up the VFD and give it a test. STOP...DON'T DO IT! Most fatal VFD failures occur about now.
A start-up procedure must be followed for the safe and correct start-up of the VFD. If the installation is performed by outside contractors, a notice should be placed on the VFD reading: Start-up by Facility Personnel Only.
Solomon S. Turkel is Senior Instructor and Course Author for ATMS (Advanced Technologies Marketing and Service), Inc., Baltimore, Md.
