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How to Buy a Variable Frequency Drive

Extensive automation and high production speeds are crucial to production plants. 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 important to match the VFD to the needs of a particular application based on four critical considerations.

Specific Needs and Demands 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

After the application has been specified, it’s time to determine the input voltage, frequency, and power distribution configuration. Applications in North America are generally ac, but in a few cases, dc may be used. Low-voltage input options range from 115 V ac and 690 V ac; typical input frequency ratings are 50 and 60 Hz. This is far from common, but ac drives can function from dc sources, hence accommodating more technologies and applications with more demands.

Required Enclosure

Proper enclosure is a must as it protects the drive and increases its longevity too. Drives are usually components of bigger automation platforms secured in clean, air-conditioned control rooms, shielding the equipment from ambient stress. 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. The drive’s longevity largely depends on where it is installed.


Two crucial issues are involved in this part of the process, and one is whether or not feedback is needed. 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 other issue or question is what type of communication is required. Communication plays a critical role in 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. Or the user may need actual statistics on drive-controlled conditions, for which Fieldbus communications are a favorable option.

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