A Review of Variable Frequency Drives

Variable frequency drives (VFD), which are also known as AC drives, frequency drives, inverters, speed drives, and micro-drives, are motor controllers capable of driving electric motors. The way this is accomplished is that the frequencies and voltages going to electric motors are varied.

Brief Introduction to Variable Frequency Drives

This type of adjustable speed drive is used primarily in electro-mechanical drive systems as a means of controlling torque but also AC motor speed by varying input voltage and frequency of the motor. There are several applications that use these drives to include smaller appliances to massive mine mill compressors.

Today, roughly 25% of electrical energy in the world is consumed by electric motors within various industrial applications. Using a variable frequency drive in centrifugal load service, motors save energy. Although these drives are extremely popular within North America, they hold a very small market in other parts of the world. The bottom line, installation of variable frequency drives could significantly improve energy efficiency.

Throughout the past four years, the cost and size of variable frequency drives has been dramatically reduced thanks to power electronics technology. In addition, overall performance has been improved upon through advances made in semiconductor switching devices, control and simulation techniques, control software and hardware, and different drive topologies.

Taking a Closer Look

The drive systems in which variable frequency drives consist are main drive controller assembly, drive/operator interface, and AC motor. For the drive controller assembly, this drive serves as solid-state power conversion systems that consist of a direct current link, inverter, and rectifier bridge converter. Because of advances in controller, more significant increases in current ratings, switching, and voltage have occurred.

The drive/operator interface is what provides an operator the ability to stop and start a motor but also adjust the speed of operation. Additional functions such as switching between automatic and manual control, as well as reversing are also available.

Initially, a variable frequency drive applies low voltage and frequency when starting a motor. Once started, voltage and frequency increase using a controlled rate or by ramping up to accelerate. For stopping, the sequence of starting a motor is reversed. At the point where frequency hits zero, the motor shuts down.

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