Tips for Buying a Variable Frequency Drive
Production plants benefit from greater automation and higher production speeds. A reflection of this is variable frequency drives (VFD’s) controlling three-phase motors not only in automated process plants but also in commercial buildings. However, it is vital to align the VFD with the application based on four key points.
Unique Needs and Requirements of the Application
In the world of VFD’s, applications can be categorized as variable torque, constant torque or constant horsepower. The most significant factor in this scenario is matching the load characteristic to the drive. The drive must meet or surpass the motor’s need for current under a normal load or in an overload state.
VFD Power Source Qualities
Once the application has been determined, the input voltage and frequency should be defined, as well as the power distribution configuration. Applications in North America are generally ac, but in a few cases, dc may be used. Low-voltage input options begin at 115 V ac but can reach a maximum of 690 V ac, while usual input frequency ratings are 50 and 60 Hz. While unusual, AC drives may work from dc sources, thus increasing the possibilities for alt technologies as well as applications that are more demanding.
It is important to ensure a proper enclosure because it protects the drive and lengthens its lifespan. Many drives form part of more comprehensive platforms and are kept in clean, air-controlled control areas, reducing ambient stress on the equipment. Chassis or minimally protected products are available from manufacturers for this exact purpose, but when individual motor controllers are left by themselves in the field, they are exposed to varying conditions. Understanding the location where the driver will be installed is key to its durability.
There are two questions that must be asked around this point of the process, and one is, is feedback necessary? A lot of drive applications regulate process more than speed. Drives can work with a variety of processes through embedded memory, faster processors and inputs/outputs.
These may include a transducer in the drive to maintain a level in a vessel through a PID or winder application that depends on multiple encoders serving different drives to determine the accurate diameters and speeds. Irrespective of the process type or measured signals, they still determine the output of the drive to the motor.
The second question or concern is the type of communication that must be used. Communication is vital to most applications. In certain cases, a display is all that’s required to give the operator a heads up when the process is ready. Communications originating from the drive can be analog and digital outputs. In other cases, what’s needed are statistical details on the conditions the drive controls, and for this, Fieldbus communication is a good choice.