Variable Frequency Drive or VFD

Variable Frequency Drive or VFD

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If you’re an HVAC engineer, then you must have heard about the VFD. But do you know what is the need for VFD or how VFD work? In this article, you’ll learn in detail about the working and benefits of using Variable Frequency Drive or VFD.

What is VFD?

VFD is an abbreviated form of Variable Frequency Drive. There are other terms similar to VFD like Adjustable Frequency Drive, Variable Speed Drive(VSD), Variable Voltage/Variable Frequency (VVVF) Drive, Inverter Drive, AC Drive, etc.

But as per my opinion, one shouldn’t call it Adjustable Speed Drive or Inverter Drive because Adjustable Speed Drive refers not only to VFD but to many other drives like a belt or gear drives, Variable Pitch Sheaves, Fossil Fuel Engines, Eddy Current Clutches, Hydraulic Couplings, DC Solid State Controllers, etc.

Variable frequency drives are adjustable frequency controllers used in electro-mechanical drive systems to control the speed and torque of an AC induction motor by varying the input motor frequency and voltage.

What is the difference between VFD & VSD?

Variable frequency drive (VFD) refers to AC drives only and Variable speed drive (VSD) refers to either AC or DC Drives. VFDs vary the speed of an AC motor by varying the frequency to the motor. VSDs referring to DC motors vary the speed by varying the voltage to the motor.

Principle of Operation of AC Induction Motor:-

First of all the AC line provides power to the motor & this power induces a voltage in the rotor. Due to this induced voltage, a current flows into the rotor. This flow of current in the rotor creates magnetic fields in the stator with north and south poles.

The synchronous speed of an AC induction motor depends upon the number of poles in the stator and the electrical frequency of the line power supplied.

Synchronous Speed = 120 x Frequency / Number of Poles


                                       Frequency = Electrical frequency of the power supply in Hz.
                           Number of poles = Number of electrical poles in the motor stator

Motors’ speed can be changed either by altering the electrical frequency or the number of poles, or both.


  • VFD has the capability to provide a voltage at a frequency from less than 1 Hz to about 120 Hz.
  • If the frequency is increased above 50 Hz, the motor will run faster than its rated speed, and if the frequency of the supply voltage is less than 50 Hz, the motor will run slower than its rated speed.

What happens to the Horsepower when we lower the speed and torque using frequency?

Horsepower = Speed (in RPM) x Torque (in pound feet) /5,252

Hence, you can see if we lower the speed and torque of a fan motor by decreasing the frequency, the power requirement of the fan will get reduced. Now, the question which may strike your mind is that;

Why do we need to lower the Speed/RPM of a fan?

So friends, as an HVAC engineer, you might be aware of the fact that we design fan of AHU at final static pressure and not at initial static pressure. Now, for the beginners, what is static pressure? Static pressure simply is the resistance to the airflow and this resistance is created by the components (like filters, cooling coil, mist eliminator, hot water coil, etc.).

When a component is new, it works on its initial static pressure, and since you’ve designed the whole system at final static pressure, this will deliver more air. Now, look at the graph.

This graph represents the relation between static pressure and airflow rate across a fan.

       Here,                            P= Static pressure;       Q= Airflow Rate;       N= Frequency

If you reduce the airflow, then the pressure head will increase, increasing the power requirement. Therefore, if you want to save some energy here, then you have to provide the fan with VFD so that it can ramp up or ramp down the fan frequency according to the requirement. This means the whole frequency curve shifts below, as you can see from the graph, there are two frequency curves. So, you can easily reduce the airflow rate without raising the static pressure.

Fan Laws:-

Since power input to the fan changes as the cube of the airflow rate (as per Fan Law), this will be the most efficient form of energy saving.

How does VFD alter the Frequency of the voltage supplied to the AC motor?

A basic VFD system generally consists of a rectifier, a DC filter, and an inverter. According to the variable frequency drive working principle, its electronic controller specifically designed to change the frequency of the voltage supplied to the induction motor.

Rectifier – Rectifier is usually multiple diodes connected in parallel. The function of diode is to allow the current only in one direction. Therefore, rectifier is used to rectify the AC voltage into DC.

DC Filter – DC filter uses a capacitor or indicator to smooth out the rough DC into smooth constant DC voltage.

Inverter – It consists of a number of electronic switches known as IGTBs (insulated-gate bipolar transistor) which controls the direction and frequency of output AC voltage.

Working Principle of VFD:-

So, when the 3-phase AC current enters into the rectifier, it allows the current to flow only in the forward direction by blocking the backflow of current., thus, producing a rough DC output with ripples. Now, this rough DC output enters into the DC filter.

This DC filter uses a capacitor or indicator. A capacitor works like a storage tank means it absorbs the electrons when there is an excess and at the same time it releases the electrons when there is a shortage. These electrons fill the gaps to smooth out the ripple and thus convert the rough DC output into smooth constant DC voltage.

This smooth DC now enters into the inverter which has multiple IGTBs. These IGBTs open and close like switches but very fast and in pairs in order to control the flow, voltage, and frequency of electricity. The output voltage and frequency are controlled by controlling the opening and closing times of switches. Therefore, the motor will experience AC voltage even though it is coming from DC.

In the diagram given below, you can see how the 3-phase AC current converts into DC & then again into AC with altered frequency.

Thus, variable speed operation involves reducing the speed of the fan to meet reduced flow requirements. There are various ways to achieve change in flow are pulley change, damper control, inlet guide vane control, variable speed drive, and series and parallel operation of fans. Then,

Why do we prefer VFD?

The answer to this question lies within the graph provided below:-

Although variable speed drives are expensive, they provide infinite variability in speed control. However, variable speed control may not be economical for systems, which have infrequent flow variations.

So, friends, this was all about Variable Frequency Drive or VFD. I hope you must have understood the need for VFD now. This is the simplest way that I could have explained.

Now, if you’ve read my earlier posts, you know that this is the time for my question. So, guys, today’s question is related to the Building Management System (BMS) and Variable Frequency Drive (VFD).

Average velocity sensor in the supply duct near AHU or Differential pressure transmitter across the blower; which input parameter is better to control and regulate the VFD and why?

Let me know the answer in your comments. For any queries, please comment in the comment section below.

Thank you very much guys for your precious time.

Stay Home! Stay Safe!


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1 Comment

  • VFD is one of the most complex issues for HVAC engineers. Most complex questions about VFDs found meaning, thanks to the clear and simple narrative. Thank you

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