What is a Variable Frequency Drive and difference between VFD and Soft Starter

What is a Variable Frequency Drive

In this article, you will learn more about variable frequency drive(VFD). Specifically, what they are and when exactly do we need and use them. Basically, we will start from how the Variable frequency drive work. Before then, remember that electricity is dangerous and can be fatal. So, you should be competent and qualified to carry out any electrical work.

What is a VFD(Variable Frequency Drive)?

What exactly is a VFD? Well, it stands for a Variable Frequency Drive. Although, they can be sometimes be referred to as Variable speed drives or AC drives. The other names are deduced from their ability to control the rotational speed of an AC motor. However, AC motors are quite common and are used majorly everywhere. Meanwhile, you find its application in commercial, Residential, Industrial, Energy, Utilities, Hospitality, Education, and Healthcare and the list goes on.

The Variable frequency drive as mentioned earlier, they are used for running an AC motor at Variable speed. They ramp up their speed to give the motor a smooth start-up. Some persons prefer to is as just a drive because they work by adjusting the frequency of the motor to adjust the RPM(Revolutions per minute). 

The VFD literally converts the voltage twice. Firstly, it converts the three(3) phase AC to a DC and this is done with the use of a Diode. Meanwhile, the capacitor then clears up DC. Thereafter, the DC is then converted back into an AC using a transistor acting as switches. Utilizing this switch mechanism of the transistor is what allows the adjustment of the frequency that the frequency supply to the motor. This also controls the speed of the motor. 

The VFD unit is connected to a motor electrical supply. You can see it as being in the middle of the electrical supply unit and the AC motor. Well, being in their middle, helps to control the frequency of the electricity being supplied to drive the motor. Since frequency and speed are inter-related. Meaning, we are able to control the frequency of the motor, then we can be able to control the rotational speed also. Therefore, with all these controls, then we have our Variable frequency drive.

Understanding how a VFD works.

Well, all these said could be a lot to take in but let me compare the internal operation of the VFD with a simple plumbing system. But before then, you first need to understand some fundamentals of electricity. In the electrical system, there are basically two types of electricity. There we have the alternating current, also called AC and the direct current also called DC. But firstly, let’s take a good look at the Direct Current. The direct current is the simplest type of electrical current and can be gotten from either a solar panel or batteries. 

You can picture what direct current is like a river with a current flowing in only one direction. Also, remember that current is the movement of electrons. So, in DC these electrons move in one direction, they flow from the negative to the positive terminal. Although, conventional current moves in the opposite direction, that’s from the positive to the negative terminal. 

Meanwhile, for electricity to flow, we need to complete the circuit so that the electricity can flow back to its source. When trying to view the waveform of a DC using an oscilloscope, you will find the maximum voltage at the positive region. However, when the power is cut off, the line drops to zero. A square pattern is formed when a switch turns ON and OFF the supply. While a pulsating pattern is formed at continually switching On and Off. 

Single-phase and Three-phase Electricity 

The other type of electrical current is the alternating current or AC. This type of current is what you find at your home receptacle(outlet). Here, the movement of electrons is in both directions ( forward and backward). Well, if we trace this to the generator. We will find that the wires are connected to a stator, with a rotating magnet in the middle of it. The magnet consists of both a North and South Pole, you can picture it as a positive and negative half.

What is Variable Frequency Drive

Meanwhile, the electrons are negatively charged. Again, from the operating mechanism of a magnet, the same pole polarity opposes while opposite pole polarity attracts. However, when the magnet cut across the core, the negative and positive half are going to, therefore, push or pull the electrons within a couple of coils. This results in a sine waveform of an alternating current with no constant voltage. Moving from zero to its maximum and also the negative. 

For Alternating current, we have both single-phase and three-phase electricity. For single-phase, we have just a single connection to a single phase of the generator. Therefore, for a single-phase, we have just one sine waveform. Although, this is different for a three-phase because, in a three-phase, we have the connection to all three phases. Well, phases are coils of wires inserted into a generator at an angle of 120 degrees from each other. Therefore, each phase coil experiences the peak magnetic field at a different time to get three(3) sine waves. 

Remember that electrons will want to flow back to its source to complete the circuit. Thereby, we can essentially connect all phases together and current will moves between the different phases as the polarity changes. Meanwhile, any excess will flow back to the source through the neutral only if it’s unbalanced. Nevertheless, more power is delivered with three-phase than that of a single phase. To understand more of three phases from a transformer, then click on the link below

Recommended: What is a transformer and learn about its working principle

Illustration with the plumbing system:

Earlier, we said that the Variable frequency drive(VFD) converts the voltage twice. First, from an AC to a DC with the help of a diode. This literally operates as a check valve in a water system. A check valve, like a diode in electricity, only allows water to flow in one direction. The capacitor act as a water filter to keep everything clear and usable. While the transistors act as a valve, they basically turn the flow On and Off when needed. With this, the VFD help to control the frequency of the motor. 

What is Frequency? 

In a simple sense, frequency refers to the number of times that an AC sine wave repeats per second. Although, the frequency at which most electricity operates is different in different countries. Like in North America, the electricity operates at 60Hz. That’s the 60Hz frequency sine wave repeats 60 times per second and each wave has a positive and a negative half. Therefore, the polarity of the system will change 120 times per second. Although, 50Hz electricity is mostly found in the rest of the world. Having the sine wave repeats itself 50 times per second in half with polarity changes 100 times per second. 

Application of a VFD(Variable Frequency Drive)

You can find the Variable frequency drive in controlling the compressor’s speed in a refrigerator system. Using the Variable frequency drive compressors in HVACR( Heating Ventilation Air Conditioning and Refrigeration), gives us better control as well as energy savings. The VFD here helps in a close match of the cooling demand, which would result in significant energy savings. Meanwhile, traditionally, the fixed peak compressor is usually used but this just turns ON and OFF. Thereby, resulting in poor control of high inrush current. 

We also find its application in controlling things such as pumps and fans in Heating Ventilation Air Conditioning and Refrigeration system. This is usually to improve the performance and the energy savings of the system. The Variable frequency drive(VFD) is adopted in the industry for a water treatment facility, to control the speed of the water pump. A water treatment plant typically has a constant flow of water coming into the plant. However, if the water demand existing the plant is more than the flow of water coming into the plant. When this happens, the operator will need to slow down the supply. Meanwhile, this is achieved by using a variable frequency drive(VFD) on the AC motor. 

With the help of the VFD, the operator can manually control the flow of the water. Another advantage of a VFD is that it can be controlled with a PLC(PROGRAMMABLE LOGIC CONTROLLER). This is achieved using a basic communication protocol such as RS-485 and RS-232. This device connected to the VFD makes the water control more easier and efficient. 

Basic Parts of a VFD(Variable Frequency Drive) 

The VFD consists of various parts. Firstly, is the Rectifier for which the three(3) phase electricity from the supply is connected to. The rectifier consists of multiple diodes connected in parallel. Meanwhile, diodes only allow electrons to flow in one direction and block its flow in the opposite direction. Since the incoming supply is an AC, then with the help of the diodes, the AC is converted into a DC. 

The output of the rectifier, which is a DC, now flows into a DC bus. The DC bus is the second part and it’s a filter that uses a capacitor to filters ripples to smooth out the rectified DC into a clear constant DC voltage. 

The smooth constant DC voltage from the DC bus then flows into an inverter. The inverter consist of a number of Electronic switches knows as IGBT(Insulated-gate bipolar transistor). Used to control the direction and frequency of the output and with the help of an inverter, we can produce AC electricity from a DC source. 

Internal connection of a VFD 

What is a Variable Frequency Drive
image illustration from TheEngineeringMindset.com


We earlier saw the parts that basically make up a Variable frequency drive. Starting from the rectifier which consists of diodes connected in parallel. Well, the internal connection consists of the first segment of six(6) diodes connected in parallel pairs. Meanwhile, each of the three(3) phase electrical power supply is connected to one pair of diodes. As we know, for there to be a complete circuit, there should be a return path to the supply. So, here a current will flow through the load and then back to source using another phase, which is in the negative polarity of its sine wave cycle. 


However, the load can be anything. It could a motor, a lamp, or an entire circuit. At the supply phases, the electricity will continue to alternate but the diodes will only allow the peak phases to pass and will block the others. The load will see the current flowing through it as a pulsating DC. These repulses can be cleared out and make smooth with the help of a capacitor. The capacitor is connected across the positive and the negative wire going through the load. 

This connected capacitor is like a storage tank. It absorbs electrons when there is an excess and then releases electrons when there is a reduction. This will, therefore, smooth out the repulse in the DC electricity and produce a perfectly straight line on the graph. Nevertheless, now that we have gotten a smooth DC, we then need to convert this smooth DC into AC. Then vary the frequency. Meanwhile, this can be achieved with the use of an inverter. 


Now the load is then replaced with an inverter. Literally, an inverter is basically the number of IGBT(Insulated-gate bipolar transistor) which act here as switches that can turn ON and OFF at a very fast speed. Meanwhile, these IGBTs are connected in pairs parallel with each output phase connected to one pair IGBT. Although, a controller is adopted here to rapidly open and close the switches at multiple times per cycle in a pulsating pattern. Meanwhile, each pulse varying in width, and this is referred to as Pulse Width Modulation.

The cycle is broken up into smaller multiple segments. Each segment has a total amount of current that could flow. But by rapidly pulsating the switches, we control the amount of flow occurring per segment. Moreover, this will results in an average per segment, and the increasing and decreasing then result in a wave pattern. The load then experiences a sine waveform. Therefore, we can control the output voltage by controlling how long the switches are closed. While the frequency can be controlled by controlling the timing of the switches. You should recur that, by controlling the frequency of the motor, you can control the rotational speed of the motor. To learn more about Electric Motor, click on the link below.

Recommended: Electrical Drive System( Electric Motor )

Difference between VFD and Soft Starter 

Both the VFD and the Soft Starter are similar. However both have different characteristics as stated below:

  • A soft starter is generally used when there’s a lot of inrush current that could cause damage to a motor. While the VFD controls and varies the speed of the motor. 
  • VFD consists of three components, which are the rectifier, the capacitor(filter), and the inverter. While Soft Starter consist of just thyristors, which are silicon controlled rectifiers. 
  • A VFD is basically a Soft Starter with speed control. However, the decision of which device to chose from is factored on how much control those your device requires. 
  • The VFD is quite much expensive compared to the Soft Starter. 
  • Soft Starter is mostly smaller in size than most of the VFDs 
  • When it comes to ramping up or down a motor, a VFD and a Soft Starter can do similar functions but the main difference is that the VFD can vary the speed and frequency of the motor. While a Soft Starter can only control the starting and stopping of that motor. 
  • A VFD can easily replace a Soft Starter and the opposite is also true as well. A Soft Starter can also replace a VFD in some cases.

Well, that’s all we have for you on Variable Frequency Drive and we hope that this article helps to proffer solution to your search. Again, you can also check out some Electrical Related Articles by clicking on the link below:

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