Variable Frequency Drive

Diagram of a Variable Frequency Drive

To optimize productivity and minimize resources, production companies have turned to Variable Frequency Drives or VFD's. This type of AC drive has also been referred to as a variable-speed drive, a micro drive, or an adjustable frequency drive because of the ability to vary motor input frequency and voltage. While it is a slightly complicated process, the end result is much more energy efficient and quality controlled than other options on the market.

About 30% of the world's electrical energy is used by electric motors that run with fixed-speed centrifugal pumps. While these pumps are reliable, they are not energy efficient. Variable frequency drives are relatively new in the oil and gas market, but have been well received because of their obvious energy savings and their ability to regulate quality better than the traditional centrifugal pump.

Variable Frequency Drives are used in a variety of applications from small household appliances to oilfield compressors.

VFD System Description

Three smaller systems come together and create a drive system, in which the variable frequency drive is used. These include a main drive controller assembly, drive operator interface, and an AC motor. Each sub-system allows ultimate control and makes this system as a whole incredibly efficient, and equally productive as a traditional pump.

The controller for the VFD is a solid state power electronics conversion system, meaning it does not utilize gas discharge tubes or a vacuum system. The controller is made up of a rectifier, a direct current, and an inverter.

The operator interface allows the motor to be turned on and off and for the operating speed to be adjusted. Sometimes the interface also allows for other functions such as switching between automatic and manual function, and reverse. Typically there is a keyboard and numeric display, as well as a host of indicator lights or meters and gauges to relay information to the operator. These vary widely depending on the function and type of machine.

The variable frequency drive includes an AC motor that is typically a three-phase induction motor. These are appropriate for most purposes, although some applications require a one-phase induction motor. AC motors are the most economical choice, and fulfill standards as set by the law.

Variable Frequency Drive Efficiency

Fixed speed motors can save energy when they run on VFD. Specifically in pump applications, the variable frequency drive allows for a change in torque and power. Additionally, variable frequency drives can run a motor in a unique pattern than minimizes mechanical and electrical stress.