Variable Frequency Drive (VFD) Working Principle

Motors are being used worldwide on the industrial or domestic level. AC induction motors are used most commonly and extensively due to their large number of applications. But there is a need to eliminate the problem associated with the AC induction motor and to run it in a very efficient way. For that matter, many devices are used but the best among all the devices is the “Variable Frequency Drive”.

The variable-frequency drive (VFD) is a device that converts a fixed AC voltage and frequency into an adjustable AC voltage and frequency. It is used to control the speed of an AC motor. By adjusting the voltage and frequency the AC induction motor can operate at many different speeds.

A VFD is capable of adjusting both the speed and the torque of an induction motor. Variable frequency drive may be referred to by a variety of other names, such as:

• Frequency converter
• AC Drive
• Frequency inverter
• VFD
• Adjustable frequency drive
• Inverter

Fixed-speed motors serve the majority of applications. In these applications or systems, control elements such as dampers and valves are used to regulate flow and pressure. These devices usually result in inefficient operation and energy loss because of their throttling action.

However, it is often desirable to have a motor operate at two or more discrete speeds, or to have a fully variable speed operation. The conventional control elements can often be replaced by incorporating variable speed operation using a VFD.

How does a variable frequency drive work?

Understanding the basic principles behind variable frequency drive operation requires understanding the three basic sections of the device: The rectifier, DC bus (DC filter), and inverter. 

The voltage on an alternating current (AC) power supply rises and falls in the pattern of a sine wave. When the voltage is positive, the current flows in one direction; when the voltage is negative, the current flows in the opposite direction. This type of power system enables large amounts of energy to be efficiently transmitted over great distances.

The rectifier in a VFD is used to convert incoming ac power into direct current (DC) power. One rectifier will allow power to pass through only when the voltage is positive. A second rectifier will allow power to pass through only when the voltage is negative. Two rectifiers are required for each phase of power.

Since most large power supplies are three-phase, there will be a minimum of 6 rectifiers used. Appropriately, the term “6 pulse” is used to describe a drive with 6 rectifiers. A VFD may have multiple rectifier sections, with 6 rectifiers per section, enabling a VFD to be “12 pulse,” “18 pulse,” or “24 pulse.”

After the power flows through the rectifiers it is stored on a dc bus (dc link or dc filter). The dc bus contains capacitors to accept power from the rectifier, store it, and later deliver that power through the inverter section. The dc bus may also contain inductors, dc links, chokes, or similar items that add inductance, thereby smoothing the incoming power supply to the dc bus.

The final section of the VFD is referred to as an “inverter.” The inverter contains transistors that deliver power to the motor. The “Insulated Gate Bipolar Transistor” (IGBT) is a common choice in modern VFDs. The IGBT can switch on and off several thousand times per second and precisely control the power delivered to the motor. The IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine wave at the desired frequency to the motor.

Motor speed (rpm) is dependent upon frequency. Varying the frequency output of the VFD controls motor speed:

Speed (rpm) = frequency (hertz) x 120 / no. of poles

Variable frequency drive can help to:

• Control the speed of the motor.
• Limit demand and electrical consumption of motors by reducing the amount of energy they consume.
• Decrease mechanical stress due to pressure spikes with direct online start & stop.
• Reduce the starting current of the motor.
• Protect the motor with its internal protection functions.
• Control process better
• Inherent power-factor correction
• Bypass capability in the event of an emergency

Connection diagram