What is a Variable Frequency Drive (VFD)?

A VFD (Variable Frequency Drive) is a power electronic device used for controlling the operation of electric motors, especially induction motors. It is a type of motor drive whose working principle is based on the variation of supply voltage and frequency. It is also known by many other names such as variable speed drive, AC drive, adjustable frequency drive, adjustable speed drive, micro-drive, etc.

VFD is primarily used for controlling the speed of an induction motor by varying the frequency and voltage of the input supply. In this article, we will discuss the basics, working, advantages, disadvantages, and applications of variable frequency drive (VFD).

But before discussing about VFDs, let us first know how the speed of an electric motor is dependent on frequency.

Relation between Speed and Frequency

For an electric motor, the speed (RPM) of the motor is directly proportional to the frequency of the supply voltage i.e.,

$$\mathrm{N_S\:\varpropto \:f}$$

The relation between frequency and speed is given by the following equation,

$$\mathrm{N_S\:=\:\frac{120f}{P}}$$

Where

• $\mathrm{N_s}$ is the synchronous speed of the motor in RPM

• f is the supply frequency in Hertz

• P is the number of poles in the motor

From this equation, we can state that the higher the frequency, the higher will be the speed of the motor. Hence, if we need to control the speed of an electric motor, we can do this by controlling the frequency of the supply voltage. To adjust the frequency, we can use a variable frequency drive or VFD.

What is a Variable Frequency Drive (VFD)?

As stated above, a VFD or Variable Frequency Drive is a motor controller device or motor drive used for changing the speed of an electric motor or an induction motor. From the basics, we know that the speed of an induction motor is directly proportional to the frequency of the applied voltage. Therefore, by adjusting the supply frequency, we can regulate the speed of the motor.

VFD is one of the most popular motor controllers used in various industrial applications where precise speed control is required.

A VFD changes the frequency and voltage of the supply to meet the speed requirements of load condition of the motor. The block diagram of a typical variable frequency drive is shown in the following figure −

It consists of three main components namely, an input converter to convert AC into DC, a filter to filter out the AC component from the DC, and an output inverter to convert the DC into AC with a different frequency and different voltage value.

Let us now understand the working of a variable frequency drive in detail.

Working of Variable Frequency Drive

A variable frequency drive operates by varying the frequency and voltage of the input supply to the induction motor. This variation is done according to the load and speed requirements of the motor.

We can understand the working of a variable frequency drive from the above shown block diagram. From this block diagram, it is clear that a VFD performs the following three main functions −

• Rectification

• Filtering

• Inversion

In the first stage, a rectifier is employed for converting the input AC voltage into DC voltage. This converted DC voltage is then filtered by using a capacitor filter circuit to remove the AC pulses. Finally, the filtered DC supply is given to an inverter that converters the DC into AC of the desired frequency. At the end, this variable frequency supply voltage is given to the electric motor whose speed it to be controlled.

From this discussion, it is clear that the working of a VFD or variable frequency drive is quite straight forward.

Advantages of Variable Frequency Drive

A variable frequency drive is widely used for speed control of AC electric motors because of the following main advantages −

• Variable frequency drives improve the energy efficiency by regulating the speed of the motor according to the load requirements.

• Variable frequency drives provide an efficient and precise way of controlling speed of electric motors.

• VFDs reduce the mechanical jerks and strain on motor during speed control.

• VFDs are also equipped with protection devices to protect electric motors from faults like undervoltage, overvoltage, short-circuit, phase imbalance, etc.

• VFDs also reduce the vibration and noise during operation of the motor.

• Variable frequency drives allow for soft starting and stopping of electric motors.

• VFDs also reduce the inrush current taken by the electric motor during its starting phase and hence protect it from overheating and insulation failure.

• Variable frequency drives smooth the operation of motor and reduce the wear and tear. Consequently, they increase the lifespan of the motor.

• VFDs reduce the reactive power taken by the motor and thus improves its power factor.

• The use of VFDs reduce the need for capacitor banks for power factor improvement.

Disadvantages of Variable Frequency Drive

Variable frequency drives have several advantages as explained above, but these drives also have certain disadvantages. Some of the major disadvantages of variable frequency drives are given below −

• The initial cost of variable frequency drives is very high as compared to other types of motor drives.

• VFDs require installation and programming for desired operation which adds additional cost of professional required for this purpose.

• The design, construction, and maintenance of variable frequency drives are relatively complex than other types of motor control drives.

• Technical expertise is required for operation and maintenance of variable frequency drives.

• Variable frequency drives can inject harmonics into the electrical supply system. This can interfere the operation of other sensitive devices and can damage them.

• Variable frequency drives are vulnerable to harsh environmental conditions like dust, moisture, temperature variations, etc.

• Since variable frequency drives involve conversion of AC into DC and then DC into AC back. This results in some sort of power loss and reduced efficiency.

Applications of Variable Frequency Drive

The main use of variable frequency drives is to control the speed of AC electric motors. Therefore, VFDs are widely used in the following applications for speed control of motors −

• Large industrial fans

• Water and liquid pumps

• Compressors

• Process control systems

• Paper and textile mills

• Conveyors

• HVAC (Heating, Ventilation, and Air Conditioning) systems

• Lifts and elevators

• Lath, drill, milling machines, etc.

Conclusion

A variable frequency drive or VFD is a type of motor controller used for speed control of AC motor, mainly induction motors. It is used in a wide range of applications because of its reliability and efficiency.

A variable frequency drive is nothing but a motor drive that controls the speed of an electric motor by varying the frequency and voltage of input supply to the motor. VFDs are used because of their energy saving mechanism, wide range of speed variation, soft starting of motor, extended motor lifespan, etc.