What Does VFD Stand For and What Does it Do?

What Does VFD Stand For? 

VFD stands for Variable Frequency Drive. It is a type of motor controller that drives an electric AC motor by varying the voltage and frequency of its power supply. By adjusting the frequency of the AC voltage being supplied to the motor, the VFD is able to control the speed of the motor. This is possible because, Frequency, given in Hertz (Hz), is directly related to the motor speed given in Revolutions per minute (RPM). In other words, the faster the frequency of the supply voltage, the faster the motor speed in RPM.

There are a variety of other names that may be used to refer to a Variable Frequency Drive (VFD), including: 

  • Adjustable Speed Drive 
  • Adjustable Frequency Drive 
  • Variable Frequency Drive 
  • Variable-Voltage Drive 
  • Variable Speed Drive(VSD) 
  • Frequency Converter 
  • Variable Frequency Inverter(VFI) 
  • AC Drive 
  • Microdrive 

Whatever the name used, a Variable Frequency Drive is always a type of motor controller used to drive and control electric AC motors. There are a variety of factory and plant industrial facilities across the globe that utilize different types of high-power electric motors. And due to the high power consumption associated with those motors, such industrial settings end up paying huge amounts of electricity bills. To overcome this challenge and increase the efficiency of industrial AC motors, the VFD was introduced about four decades ago. So, let’s look at what a VFD can actually do.

What Does A VFD Do?

  • As a power conversion electronic device, the VFD converts a basic fixed-voltage, fixed frequency sine-wave line power to a variable-voltage, variable frequency AC output that is used to control the speed of AC motors. 
  • Even though VFDs control the frequency and voltage being supplied to an AC motor, they are often referred to as speed controllers, because the end result is an adjustment of motor speed. As previously stated, frequency is directly related to the motor’s speed in RPM. For example, the higher the frequency of the power supply, the greater the number of motor RPMs. Thus, VFDs allow AC motors to operate at many different speeds. 
  • A VFD is not only capable of adjusting the motor speed, but it’s also capable of adjusting the torque of the same motor. Hence, by varying the frequency of an AC supply voltage, the VFD can control both motor speed and torque to meet the requirements of a given application. Basic VFDs are able to maintain the motor torque by keeping the voltage-to-frequency ratio constant. While advanced VFD systems are equipped with more adaptive and intelligent control algorithms to improve the performance of the motor, in terms of speed and output torque. 
  • The Variable Frequency Drive has the capacity to control ramp-up speed of a motor during start-up. This enables smooth starting and acceleration of the motor to the desired operational speed, at a controlled acceleration rate. A process often referred to as soft starting of an AC motor. Similarly, if you need to increase the motor speed due to changes in application requirements, you can simply use the VFD to turn up the motor speed to the required RPMs. 
  • The VFD is also capable of controlling ramp-down speed of a motor when stopping, thereby providing controlled deceleration. A braking option is also available with the VFD. Therefore, if your application does not require the AC motor to run at full speed, you can use the VFD to ramp down the motor’s supply frequency and voltage so as to meet the speed requirements of the driven load. 
  • Most VFDs have simple toggle commands for forwarding or Reverse motor operation. Hence, the VFD system has the capacity to control the direction of an electric motor. For instance, when you turn ON your asynchronous AC motor you find that it’s running backward. Then you can use the VFD to change its direction of rotation. However, if such a motor is turned OFF and later restarted, it will restart while still running back and you’ll have to use the VFD again for direction control. 

In summary, VFDs allow users to use a single electric AC motor for a variety of application conditions and processes which may require different operational speeds. In contrast to the fixed frequency supplied by line power (utility power) that runs AC motors at full speed all the time, the VFD provides speed flexibility as well as costs-savings in many different industries like manufacturing. Later on in this article, we’ll discuss the various benefits of using VFDs.  

How Does A VFD Operate? 

As previously stated, to adjust the motor’s RPM, a variable frequency drive works by adjusting the frequency and voltage supplied to the motor. To practically achieve this, the VFD converts the supply AC voltage twice. First, it takes in fixed frequency and voltage AC supply into a rectifier bridge which has diodes that convert the AC power supply to DC voltage. Then using a capacitor bank and inductors cleans the DC voltage. Next, the VFD uses transistors acting as electronic switches to convert the DC voltage back to AC and sends it out to the connected motor at the desired frequency.

The use of the transistors as switches enables the VFD to adjust the frequency of the AC voltage it supplies to the motor. And as discussed in the previous sections, this adjustable frequency is what controls the speed of an inductor AC motor. Thus, we can rightfully say that the main function of a variable frequency drive is to vary/adjust the operating frequency of an AC power supply to a motor.

VFD Circuit and Its Operations 

The operation of a VFD circuit- that is converting AC voltage into DC and then into an AC voltage with adjustable frequency, makes use of four sections or blocks. These sections are: (i) Rectifier Section; (ii) DC Bus/Filter Section; (iii) Inverter Section; (iv) Control Unit Section. The diagram below shows a standard VFD circuit with the four sections:

Figure 1: A Circuit Diagram of a Standard three-phase VFD

Each one of the four components of a VFD circuit is explained below: 

A) The Rectifier Section 

The input AC power supply is connected to the full-wave rectifier section of the VFD, which converts the AC voltage into unfiltered DC voltage. The rectifier section consists of either 6 or 4 diodes, but in other cases, thyristors are used. For three-phase AC power conversion, six diodes are used while four diodes in a bridge configuration are used for the conversion of single-phase AC power. The diode full-wave rectifier provides uncontrolled power conversion but the thyristor rectifier is used for controlled power switching that varies the rectifier’s DC output voltage.

The VFD circuit in Figure 1 represents a three-phase AC power rectifier. This rectifier consists of six diodes, with each phase of the three-phase AC power supply being connected to a pair of diodes for rectification. The six diodes then act as a diode bridge rectifier that converts the three-phase AC power signal into a single DC power rail. Three of the six diodes are connected in forwarding Bias with the positive (+ve) DC output rail, as shown in Figure 2. The other three diodes are connected in Reverse Bias with the negative (-ve) DC output rail, also shown in Figure 2.

The VFD rectifier section thus converts the sinusoidal AC input signal into a pulsating DC output signal which oscillates between a maximum positive peak and a minimum zero volts. Depending on the polarity of the AC power sinusoidal wave, the diodes can get forward biased or reverse biased to provide positive or negative pulse in both positive and negative DC rail, respectively. For a visual representation, the AC input and DC output waveforms are given with the VFD circuit diagram, as shown below:

Figure 2: Internal Circuit Diagram of a 3 phase Variable Frequency Drive

B) The DC Bus and Filter Section

The standard diodes in the rectifier section only convert the AC power signal into a DC output, but the resulting DC signal is not smooth enough as it’s associated with frequency-dependent AC ripples. To eliminate the AC ripples and smoothen the DC output, ripple rejection filters are required. Large capacitors are used as the standard components in this section to filter out the AC ripples from the pulsating DC output of the rectifier section.

In addition, inductors may also be included in this section depending on the type of ripples present. Some applications also require the use of other types of filters like harmonic filters to reduce/eliminate input AC noises and harmonics. Moreover, the DC Bus/Filter Section stores the filtered DC power to be retrieved by the inverter.

C) The Inverter or Switching Section

The inverter section converts the steady DC voltage into alternating AC voltage with adjustable frequency. Various types of electronic switches may be used in the Inverter section, from high power transistors or IGBTs (Insulated-Gate Bipolar Transistors) such as MOSFETs (Metal–Oxide–Semiconductor Field-Effect Transistors) or MOS (Metal–Oxide–Silicon Transistor). These transistors rapidly switch ON or OFF thereby providing alternating voltage at the output. The switching frequency of the IGBTs determines the frequency of the AC output. While the switching rate of those IGBTs is controlled via the control unit in the last section of the VFD circuit.

The frequency of the AC output from the Inverter is directly proportional to the switching rate of the transistors. For instance, a high transistor switching rate will provide a high-frequency AC output while a low transistor switching rate will provide a low-frequency output. Three-phase AC voltage conversion uses 6 IGBTs, whereas single-phase AC voltage conversion uses 4 IGBTs. Figure 1 shows a three-phase power conversion VFD circuit in which six IGBTs have been used.

D) The Control Unit Section

This section is responsible for coordinating the transistor switching operation, output power, and output frequency. It has an integrated monitoring and controlling user-friendly interface, and sensors for acquiring the necessary operational data. Through the interface or Programmable Logic Controller (PLC) the operator can start and stop the motor, adjust the motor speed, change its direction of rotation, etc.
The control unit section also includes an embedded microprocessor or Digital Signal Processor (DSP) which communicates with the PLC and user interface (such as keypad or HMI) to monitor motor operation and check for any fault conditions. The embedded microprocessor is programmed to control the rectifier and inverter section. It is designed to react in microseconds (𝜇s) in the event of any fault conditions. Speed sensors provide feedback data which is used to monitor the motor speed and adjust it accordingly as per the application requirements.

Advantages of Using VFDs 

A) Precise Motor Speed Control: VFD systems allow tighter control of operational speed and acceleration of induction AC motors. For example, applications like bottling lines which include easy-to-tip products considerably benefit from VFD controlled motor acceleration that results in a gradual increase in power. This allows conveyer belt systems in such industries to smoothly ramp up rather than abruptly jerking to full power. Also, VFDs allow controllers like PLCs to remotely adjust motor speeds. 

B) Reduced Energy Consumption and Costs Savings: Electric AC motor systems consume over 60% of the total electric power consumed in today’s industrial facilities. Optimizing the control systems of these motors with VFDs is capable of reducing the overall energy consumption in a factory facility by as much as 70%. In addition, in applications where an electric AC motor is required to operate at less than full speed, then you can use the VFD to match the motor speed to the speed requirements of the connected load. This cuts down energy consumption and energy costs over similar motor-driven equipment running at a constant speed for the same duration of time.  

C) Improved Motor Control Efficiency: Variable frequency motor speed control is a highly efficient method, which enables users to operate their motors at the most efficient speeds for a given application. This significantly increases production levels by reducing errors. For example, VFDs provide smoother action for conveyors and belts which eliminates jerks during start-up, allowing higher production throughput. 

D) Return on Initial Setup Investment: Using VFD systems to control industrial motors can improve products quality, eliminate the need for mechanical drive components, and reduce overall production costs. The result is a return on the initial setup costs of the VFD system. 

E) Less Power Required during Start-Up: Starting AC motors across the line voltage requires a substantial amount of electric power than when a VFD is used. And if industrial operators start such motors during peak hours of electricity consumption, they’re likely to incur surged prices. However, industrial AC motors with VFD controllers demand lower power during start-up.

 F) Limited Inrush Current: Inrush current is the huge amount of current drawn by an electric motor during start-up. It’s usually five to eight times higher than the motor’s rated current. Large amounts of inrush currents cause motor winding stress, voltage dip on connected DC bus, and winding overheating. But when the VFD is used to start large induction AC motors, it safely limits the starting current. This reduces the chances of winding or winding insulation damage. 

G) Reduced Maintenance: Starting a motor load with the VFD protects both the motor and the driven load from “instant shock” associated with across the line starting; this smooth starting eliminates a great degree of wear in gears, conveyor belts, and gears. Also, the VFD system eliminates water hammer effects through the smooth acceleration and deceleration speed cycles it provides. This smooth starting, operation, and stopping of AC motors using VFD reduce mechanical stress and equipment breakdown, eventually reducing maintenance and downtime-related costs. 

H) Extended Equipment Life: The VFD has the capacity to start an AC motor at zero voltage and frequency, this keeps the motor winding in check for flexing and heating. Also, with gradual changes in speed, the VFD is able to safely start and stop an AC motor, thereby eliminating mechanical jerks. This extends the mechanical service life of the motor. In addition, the VFDs protect motors from issues such as phase changes, thermal overload, overvoltage, under voltage, etc. Moreover, the optimal motor control provided by the VFD helps equipment to last longer. 

I) Limits and Adjusts Motor Torque: VFDs are capable of limiting and adjusting the amount of operational torque, and ensuring that the AC motor does not surpass this limit. This protects motor-driven machinery from damage and it also protects the product or process in question. 

J) Power-Factor Improvement: Induction AC motors usually have a low power factor. A low power factor brings about reactive power losses; whereby energy is wasted in terms of heat. When a VFD is used, it inherently improves the power factor enabling induction motors to utilize power more efficiently. 

Drawbacks of Using VFDs 

A) High Initial Setup Costs: The primary drawback of using a VFD system is that it requires a considerably high initial setup investment. Compared to across-the-line or direct-on-line motor starters, VFDs are expensive. Therefore, for a large factory or plant with multiple high-power motors that need to be controlled using VFDS, the initial installation costs would be fairly high.  

B) Special Motor Design Requirements: The Pulse Width Modulation (PWM-based) AC output of a VFD is not purely sinusoid. It can thus create mechanical stress in the windings of an induction AC motor, this could result in heating up and degradation of the winding insulation. To prevent this, a special motor construction with an improved winding insulation design specified for use with PWM inverters will be required to run with the VFD. 

C) Power Line Harmonics: Since the rectifier section of the VFD circuit draws current non-linearly, then it can create notching harmonics in the power supply line. And during the operating condition, these harmonics create distortion in other devices and equipment connected in parallel with the VFD or in the same source power line. Thus, VFDs require additional harmonic filters. 

D) Complex Operation: The operation and settings of a VFD are quite complex, as compared to across-the-line or direct on-line (DOL) motor starters. Modern VFDs include an integrated user-friendly interface, but still, their operability cannot compare to that of a DOL starter with a simple pushbutton operation.  

E) Extreme Environment: Compared to a direct online starter circuitry, the VFD electronic circuitry is highly sensitive and its operation is affected by extreme environmental conditions such as too high or too low temperatures. Thus, additional measures are required to enable the VFD system to cope with harsh environments.   

Applications of Variable Frequency Drives

Here are some applications of VFD systems: 

  • The adjustable-frequency feature of the VFD is used to control the speed of inductor AC motors. The speed of these motors depends only on frequency. These motors are widely used in manufacturing processes. 
  • VFDs are used to precisely control motor speed, with smooth starting and stopping in conveyor belt systems. The result is increased production and fewer accidents. 
  • Cooling systems use the precise motor speed control provided by VFDs to maintain the required temperatures. 
  • Escalators and Lifts benefit greatly from the smooth motor starting and stopping feature of the VFD. 
  • To create energy consumption savings, applications where the load’s power and torque vary in a non-linear way use VFD-based motor control systems. Such applications include variable torque fans and water pumps, and crushers in the mining industry. 
  • For precise speed control and positioning, cranes and hoists use the VFD system. 
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