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What is a VFD?

A VFD–Variable Frequency Drive–is a type of motor controller used in electro-mechanical drive systems to regulate the speed and torque of AC motors. It accomplishes this by adjusting the frequency and voltage of the power being supplied to the connected AC motor. VFDs also have the capacity to control ramping up of an AC motor when starting for a smooth start-up or to prevent heavy loads from straining AC motors on start-up. They can also control the ramp-down of the motor when stopping.

Although the VFD controls the input frequency of the AC voltage supplied to a motor, we generally refer to this as speed control, since the end result is an adjustment of the motor speed. This is possible because motor speed in Revolutions per Minute (RPM) is directly related to the frequency given in Hertz (Hz). So, the higher the input frequency of the AC voltage supplied to a motor, the faster the RPM speed of that motor and vice versa. Thus, by varying the input frequency and associated voltage supplied to an AC motor, the VFD is able to control the motor speed in RPM.

VFDs are widely used across different types of industries to control the speed of different equipment/machinery and processes. Many industrial facilities (especially mechanical and electrical industries) make use of high-power AC motors that are characterized by high power consumption. Therefore, it’s of utmost necessity to match the operating speed of such motors to the required process or load to avoid energy wastage and consequent huge amounts of electricity bills. For this reason, these industrial plants use VFDs to increase the efficiency of their high-power AC motors, thereby saving on energy consumption and boosting productivity.

You’ll often come across VFDs in AC motor systems driving pumps, conveyor belts, compressors, fans, and other types of machinery in industrial facilities. These speed regulation applications of VFDs account for 75% of all Variable Frequency Drives operating globally. VFDs are also found in servo systems, power plants for frequency regulation, robotics for speed and torque control, CNC machine systems for improved precision, and VFD-fed welding machines. They are also common in water treatment plants where they regulate a water flow by controlling the pump motors used in such plants.

Note: Variable Frequency Drives (VFDs) are also referred to as AC drives or Adjustable Frequency Drives (AFDs), though other terms like Variable Speed Drives (VSDs) are sometimes used interchangeably with VFDs. However, while VFDs vary the input frequency supplied to an AC motor to adjust the motor’s speed, VSDs vary the voltage supplied to a DC motor to adjust its speed.

Working Principle of a VFD 

A VFD is usually installed between a power supply and an AC motor. So, the supply power first goes into the VFD, where it’s regulated before being delivered to the AC motor. And as previously stated, the VFD controls the motor speed by adjusting the frequency and voltage of the power being supplied to the AC motor. To accomplish this, the VFD circuit consists of four main sections that function together to adjust the frequency delivered to a given AC motor. These sections include:

Rectifier Section: This is the first section of the VFD circuit. It’s usually a bridge rectifier circuit that converts incoming fixed frequency, AC voltage from the mains into unfiltered DC voltage. The rectifier circuit can be bidirectional or unidirectional depending on the VFD application. It utilizes Silicon Controlled Rectifiers (SCRs), transistors, diodes, and other semiconductor switching devices.

If the rectifier circuit makes use of diodes, the converted DC voltage is an uncontrolled output but using SCR provides a controllable DC output voltage that’s varied by the gate control of the SCR. A minimum of four(4) diodes are required for single-phase AC conversion while at least six(6) diodes are required for three-phase AC power conversion and in this case, the rectifier section is considered to be a six pulse converter.

DC Bus Link/Filter Section: The unfiltered DC output voltage from the rectifier unit is fed to the DC Bus Link. This section consists of large capacitors as the standard components and which function as ripple rejection filters. They filter out frequency-dependent AC ripples from the pulsating DC output voltage of the rectifier unit and connect it to the input of the inverter section.

This section may also include inductors and other types of filters such as harmonic filters for smoothening the rectified DC output depending on the type of ripples present and application requirements. In addition to filtering the rectified DC voltage, the DC Bus Link also stores the smoothened DC voltage for retrieval by the inverter section.

Inverter Section: This is the switching section of the VFD circuit and is made up of electronic switches like thyristors, IGBTs (Insulated-Gate Bipolar Transistors), power transistors, etc. It receives the filtered/steady DC voltage from the DC Bus Link and converts it into AC voltage with an adjustable frequency to be delivered to the connected AC motor.

The inverter section makes use of modulation techniques such as Pulse Width Modulation (PWM) to vary the frequency of the AC voltage being supplied to the connected induction AC motor, thereby controlling the motor speed. Note, if three-phase AC voltage is supplied to the rectifier section, the inverter unit will convert the DC voltage from the DC Bus Link into three-phase AC voltage with an adjustable frequency. Similarly, if the AC supply voltage to the rectifier unit is single-phase the inverter will output a single-phase AC voltage with a variable frequency.

Control Unit: The main function of this unit is to generate the appropriate pulse to control the output of the electronic/semiconductor switching devices of the inverter section in the proper sequence. It consists of an embedded microprocessor used for all decision requirements and internal logic.

The control unit also performs other functions like controlling and configuring the VFD settings, detecting and diagnosing fault conditions, as well as interfacing communication protocols. It also receives a feedback signal regarding the motor speed in real time, and using the signal as the speed reference it regulates the ratio of the voltage output from the inverter to output frequency to control the motor speed accordingly.

A Standard Three-Phase VFD Circuit

Simply put, to control the speed of an AC induction motor, the VFD takes in fixed frequency, AC input voltage from the main power supply line into its Rectifier unit. The rectifier bridge circuit in this unit converts the AC supply voltage into a DC output voltage that’s not smooth enough. Next, the rectified DC output is sent out to an intermediate DC Bus Link with a capacitor bank and inductors, where it’s cleaned by filtering out frequency-dependent AC ripples and other AC harmonics or noises. The steady DC voltage then flows through the Inverter section, where electronic switching devices are used to convert it back into AC voltage but with a variable frequency. The resulting AC output voltage from the inverter is then delivered to the connected AC induction motor at the desired frequency.

This process allows the VFD to adjust the frequency and AC voltage supplied to the connected induction motor depending on the demands of the process or connected load. If the output AC voltage from the VFD is at a higher frequency, the speed of the connected AC motor will increase. Thus, using an operator interface linked to the control unit of the VFD circuit, operators can control the speed of a VFD-driven AC motor by setting the desired frequency manually or automatically with a Programmable Logic Controller (PLC).

Types of VFDs 

The different types of VFDs are classified depending on how the VFD circuit converts the incoming AC supply voltage to DC voltage and the type of rectification used. There are three main types of VFDs currently available in the market. They include:

Voltage-Source Inverter (VSI) Type: This is the most common type of VFD. It makes use of a simple diode bridge in the rectifier unit to convert AC input voltage into DC output voltage. In its DC Link Bus section, capacitor banks are used to filter and store the rectified DC voltage. Next, the inverter switching circuit makes use of the stored DC voltage to provide an AC output voltage with an adjustable frequency.

Current-Source Inverter (CSI) Type: The construction of this type of VFD depends on input current rather than supply voltage to provide a smooth output voltage in contrast to the way VSI VFD types are designed to depend on the variable frequency range for the same. Also, instead of using a diode bridge rectifier in the rectifier section like the VSI type, the CSI type VFDs use an SCR (Silicon Controlled Rectifier) bridge converter to convert AC signal into DC signal.

The rectified DC voltage is filtered by a series of inductors in CSI VFDs as an alternative to the capacitor banks used in the VSI type. In addition, the CSI-type VFDs can provide a square wave of current rather than a square wave of voltage.

Pulse Width Modulation (PWM) Type: This VFD type is a modified and enhanced version of the VSI type VFD. It makes use of the PWM technique to provide a stable output voltage with an improved frequency ratio and power factor; in which fixed DC voltage pulses of various duty cycles of different signals are used to simulate a sinusoidal waveform. PWM-based VFDs use a simple diode bridge rectifier like the VSI type to convert AC signal into DC signal. In addition, the switching circuit of the inverter section of PWM VFDs controls the signal duty cycle in a variable frequency range like the VSI type.

The control unit of the PWM VFDs is programmed to control the duty cycle of the output AC voltage from the inverter by adjusting the switching speed of the electronic switches in the inverter section. Also, an additional regulator is used at the output of the PWM VFD to regulate the output PWM voltage pulses so as to provide a stable and proper voltage and current waveform to the connected AC motor or load. PWM-based VFDs are more preferred due to their numerous advantages over the CSI and VSI VFDs.

Advantages of Using VFDs  

Allen Bradley VFD PowerFlex 755

There are many benefits of using VFDs to control AC motors in different industrial applications, some of which include: 

Energy Consumption Optimization: A VFD can be used in an application in which an AC motor is not required to run at full speed so as to reduce energy consumption and associated energy costs. In such an application, the VFD is used to vary the power supplied (input frequency and voltage) to the AC motor to match the speed of the equipment being driven by that motor to the load requirements. This way, the VFD is able to optimize and reduce the energy consumption of the AC motor in comparison to direct-on-line (DOL) motor operation, where AC motors run at full speed regardless of the load demand. It’s common to achieve up to 40% energy savings when using VFDs to control AC induction motors.

Fans and pumps driven by AC motors benefit significantly from VFDs. Because these AC drives offer optimized motor control systems superior to dampers and ON/OFF motor controls, such that by controlling the rotational speed of a fan or pump motor they can reduce the motor’s energy consumption by 20% to 50%. This is similar to reducing the speed of a car by lightly pressing on the accelerator, instead of using brakes that waste a lot of energy.

Increased Productivity: VFDs allow the operation of motor-driven equipment at the most efficient speed for a particular application, this results in fewer operating errors due to tighter process control, and thus, increased production levels. For example, an operator can set the deceleration and acceleration of a VFD-equipped conveyor belt at any time to avoid the dropout of loads, which allows for high production throughputs. Moreover, today’s Variable Frequency Drives integrate both diagnostics and networking capabilities to better control motor performance and increase process productivity.

Reduced Maintenance: A VFD starts the connected AC motor by supplying power at a low frequency. It then increases the input frequency and motor speed gradually until the desired operating speed is attained. This provides the motor with a soft start-up. So, if you start a motor load with a VFD, neither the motor nor the driven load will be subjected to “instant shock” associated with across-the-line starting. Instead, the load will start smoothly, thereby eliminating wear and tear of the belts, gears, and bearings in the motor-driven equipment.

In addition, VFD-based motor control is an excellent method of reducing and/or eliminating mechanical jerks or water hammers during equipment start-up and stopping, since VFDs allow smooth deceleration and acceleration cycles. All these factors combined minimize mechanical stress and breakdown of the motor-driven equipment, reducing the need for frequent maintenance and unnecessary downtime.

Extended Service Life: VFDs are capable of starting AC motors at zero input frequency and voltage, this helps keep the windings of the motors in check for overheating and flexing. Also, VFDs offer optimal control of the frequency and voltage supplied to a motor, thereby providing your motor with better protection against thermal overload, under-voltage, overvoltage, phase changes, etc. This extends the electrical service life of the VFD-equipped AC motor.

Controlled Motor Starting Current: High-power AC induction motors can draw up to seven or eight times their rated full-load current during start-up. This huge amount of current drawn by a motor when starting is known as inrush current, and it can cause electrical stress and overheating of the motor winding, and/or voltage dip on any linked DC bus. However, using a VFD to start a large induction AC motor safely limits the motor’s starting current to a permissible range. This results in fewer motor rewinds due to reduced chances of windings and winding insulation damage, and the life of the motor is also extended.

Ease of Change of Direction of Rotation: VFDs can handle applications with frequent start and stop operations, and continually changing directions of rotation. As the VFD only requires a small amount of current to change its direction of rotation once the rotation command is changed. For example, when used in a stand mixer the VFD can control the number of revolutions while the mixer produces the output in the right direction of rotation, regardless of the many times the direction of rotation changes.

Simple Installation and Configuration: VFDs are usually pre-programmed for specific applications. While control power for motor leads, communication lines, and auxiliaries are factory wired. Hence, when you purchase a VFD, you only need to connect it to the power source and to the AC motor whose speed needs to be controlled.

Adjustable Motor Torque Limits: A VFD can be set to limit and adjust the amount of input torque applied to an AC motor so as to ensure that it does not exceed the motor’s torque limit. This protects the motor and the driven equipment from damage. For example, if a piece of motor-driven equipment has jammed, the VFD will allow the motor to continue rotating until the overload device in the circuit is triggered to open.

Elimination of Components for Mechanical Drives: VFD-based motor controls can deliver low-speed or high-speed regulation depending on the load requirements without the need for mechanical drive components such as gearboxes and speed-reducing or speed-increasing devices.

Drawbacks of VFD Systems

Despite the numerous advantages cited above, VFD systems have a few drawbacks. They are as follows:

High Capital Investments: Compared to Direct-On-Line (DOL) or Across-the-Line motor starters, VFD systems are considerably expensive. Hence, setting up a VFD system in a factory or an industrial plant where high-power AC motors need to be controlled using VFDs requires high initial setup investments.

However, they’re a worthwhile investment because controlling industrial AC motors using VFDs leads to energy cost savings as well as improved product quality and high production levels. This results in a return on the initial setup investment of the VFD system.

Power Line Notching Harmonics: The main power supply line of a VFD system is highly prone to distorting, line notching harmonics. This is due to the fact that the rectifier unit of the VFD circuit draws input current non-linearly. So, when the VFD is in operating condition, the devices connected to the same power line as the VFD also get hampered by those notching harmonics. For this reason, VFD circuits require additional harmonic filters.

Low Starting Torques: VFDs are not capable of providing very high starting torques since the maximum torque of a given VFD is limited to what the connected motor can output. Also, VFDs require a high-frequency AC power supply to operate.

Special Motor Winding Insulation: VFDs are known to produce a PWM-based AC output that is not purely sinusoidal. When this output is delivered to an induction AC motor it can create mechanical stress on the motor’s windings, which can result in overheating and degradation of the winding insulation. To avoid this, it’s recommended that VFD-based motor control systems be used with AC motors that have special types of winding insulations designed for use with PWM (Pulse Width Modulation) inverters.

This entry was posted on August 22nd, 2022 and is filed under Uncategorized. Both comments and pings are currently closed.

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