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Jan 21, 2022 | Blog

An Introduction to Variable Frequency Drives, Their History & How They Work

Variable Frequency Drives (VFD) are electrical components that are used to control the speed of motors. It also explains the recent History of Variable Frequency Drives.

VFD (variable frequency drive) is a method of managing the speed of an induction motor by adjusting the frequency of the AC supply volts.

It is possible to run the motor at varying speeds depending on the requirements by changing the output AC frequency.

This allows significant gains in efficiency and production, as well as lower maintenance costs. Keep reading to find out all should know about VFD and how you can use it to your advantage.

Introduction to Variable Frequency Drives

Electricity exists in two forms that include alternating current (AC) or direct current (DC). DC is electricity that we see in solar panels and batteries that delivers current in a single direction.

Here, electrons flow between sources of positive and negative, which is necessary for completing a circuit. Electricity always attempts to return to its source, which is illustrated by DC voltage being maximized near the positive.  

When the DC link power is cut, the line or circuit is zero. Similarly, if we turned the power on and off in pulses, we would recognize fluctuation from the width of the pulses.

AC voltage is used day-to-day when we plug devices into a power outlet. Here, the load continually flows in waveforms back and forth.

Because of the interaction between positive and negative charges and a magnetic field, a pattern known as a sine wave occurs. AC sine waves have an input frequency based on the number of occurrences per second.

In North America, we use a multiphase system consisting of three supplies. The three phases are linked to a VFD that connects to a rectifier.

The rectifier is composed of diodes, which impact the directional flow of electricity carried to a DC bus. Finally, the current flows to an inverter that is composed of IGBTs.

Variable frequency drives (VFDs) are used in facilities with AC motors or if your VFD output varies using mechanical means. A VFD is a requirement for AC motors regardless of whether a variable output is necessary. 

Critical energy losses would occur in the absence of a VFD and they must be well-maintained through preventative maintenance measures. VFD maintenance is a vital part of a facility’s maintenance schedule that should be properly designed and adhered to.

An AC motor’s speed and other parameters interact with a motor controller and operator interface. The speed of the motors (AC-induction motors) is influenced by the voltage frequency.

The voltage is commonly measured in hertz (Hz) and dictates the synchronous speed. When the frequency is increased, motors will operate faster.

To gain a better understanding of variable-frequency applications, here’s how they started:

History of Variable Frequency Drives

In the late 1800s, Nikola Tesla invented three-phase alternating current motors, which was a significant advancement. This is particularly in comparison with Thomas Edison’s DC motor in terms of both efficiency and dependability.

Controlling AC motor speed was a difficult process that took decades to evolve; therefore, dependence on DC motors continued. Years later, small components were designed that could regulate the voltage supply to a motor.

Variable speed drives (VSDs) were primarily limited for use with large motors necessary for heavy industry. However, as AC technology developed, VSDs became used more widely in a variety of applications.

The control hardware and software and advanced microprocessors of today allow VFDs to operate with tremendous versatility. One of the critical aspects of conserving power involves the ability to limit the pull of a motor when starting.

What Are Variable Frequency Drives?

Variable frequency drives work by using AC motors which operate at a fixed speed. The machine’s speed is determined by the frequency of the power supply, the stator design, and magnetic poles.

Often referred to as alternating current drives (AC drives), variable speed drives handle the speed of AC motors.  AC motors are used across most industries today, particularly in heating, ventilation, and air conditioning (HVAC) systems.

For example, refrigeration systems have compressors that need their speed regulated. This involves coordinating the cooling needs while also attempting to maximize efficiency through energy savings.

HVAC system components including fans and pumps all consume potentially large energy volumes that must be optimized for performance. A VFD unit interacts with the electrical source or supply of the motor to regulate the rotational speed.

Basic Concept of Variable Frequency Drives

For centrifugal pumps and fans, blowers, and HVAC systems, a variable torque load allows variations in horsepower and torque. Significant energy may be conserved by operating at lower speeds.

For applications including mixers, compressors, and others a fixed torque load is ideal with variations only in horsepower. Here, the energy savings is again the result of operating at lower speeds.

Basic Control Mechanisms

The motors of today are generally designed for optimal efficiency – however, systems being upgraded must be assessed. The option is either to assume the cost of a new motor or use an existing, which meets the specifications.

Conditioning equipment for handling incoming power is another consideration. For example, harmonic filters for stabilizing currents and voltages returning to a power supply.

The electrical system may potentially need to be modified to correspond with various control mechanisms. Examples include enhancing the protective systems and supply transformer redundancy.

In addition to ensuring that the electrical system has the infrastructure to support an alternate system, space is a concern. Transformers, reactors, and other equipment might need dedicated outdoor space, which also applies to drives positioned indoors.

System Description

Appliances and compressors of various sizes work with VFDs to maximize efficiency by optimizing voltage and frequency. We will review some of the key system components that are involved.

1. AC Motor

Generally, the motor for a variable frequency drive uses a three-phase induction, which is largely based on affordability. Less commonly used are wound rotors, permanent magnets, and synchronous motors.

2. Controller

The sub-systems include a converter, DC capacitor, and inverter. Here, a converter is used for transitioning AC to DC voltage.

3. Operator Interface

Operators have control of the VFD using an interface that allows for the adjustment of speeds that are of varying complexity. 

For example, operator interfaces might be housed on a network featuring digital control capabilities.

4. Speed Control

The highest level of performance is often achieved using an option that changes the levels of torque and speed. 

Quadrants loads or quadrant “operations” range from one, two, or four loads for optimizing force and direction.

What Are the Different Types of Variable Frequency Drives?

Current source inversion (CSI) VFDs are capable of recapturing power and are used in a variety of industrial applications. Some of the power that is generated by the motor can be transferred back to the power supply with CSI.   

The construction of a CSI VFD requires inductors for operations and an SCR converter for handling DC voltage. The SCR regulates the voltage, which is needed for generating a variable frequency output necessary for the motor.

CSIs are partly favored because of the relative simplicity associated with the circuitry system. Other advantages include the ability to operate reliably in most cases and to regenerate power.

Voltage source inversion (VSI) is not capable of regenerating power like the CSI models do and is largely considered inefficient. Due to the instability of the power supply, the motor tends to struggle when operating below 6 hertz.

The symptom that occurs when operating at lower hertz ranges is typically referred to as cogging. The cogging is described as being a rotational shift that can be unpredictable as the shaft moves.

The CSI and VSI models do still have a commonality in terms of using a converter to handle DC power. However, the current is not regulated or transferred back to the power supply as a CSI unit would.

Pulse-width modulation (PWM) VFDs have emerged as the most commonly used in the market today because of several advantages. The PWM units will not experience cogging when operating in the lower ranges of hertz and deliver fixed DC bus voltage.

PWM VFDs have proven to offer a combination of trouble-free operation with greater efficiency and reduced operating costs. The motors that work well with a PWM unit range from up to 500 horsepower, allowing tremendous versatility.

At the same time, PWM drives are considered very reliable and generate a minimal number of harmonics. Output frequencies are estimated to range from 2 Hz to 400 Hz and have a rating of typically 460V or 230V across most manufacturers.

When looking at how voltage interacts with the convertor, we see how the capacitors establish and maintain fixed (stable) DC. Both the voltage and frequency are partially controlled by insulated gate bipolar transistors (IGBTs) that generate significant output.

In each half-cycle, the width of the pulses will vary to some extent, which is described as pulse width modulation. The motor’s frequency is dictated based on how many transitions occur in a second.

These pulses are optimized to yield a viable wave. Moving forward, injection-enhanced gate transistors (IEGTs) are going to be used to optimize the process.

Requirements of Variable Frequency Drives?

Professionals that are responsible for VFD selection must consider a host of variable factors based on their unique needs. For example, simply selecting a motor based on the stated horsepower alone is not always the best decision.

The profile of the operation must be considered to avoid unwanted consequences including abruptly starting or stopping based on variables. One strategy for making these determinations involves calculating the maximum current and torque.

VFD sizing considers more than only horsepower based on the variable environments. For example, having extended capacity might be needed to prevent overload during short periods, while maintaining stable performance.

In rare cases where installations exist in areas of high altitude, a larger-sized VFD may be needed. This is needed to compensate for reduced abilities to air cool the VFD.

Where are Variable Frequency Drives Used?

It might be more appropriate to identify the applications where VFDs are not an option than vice-versa. When demand exists for changes in torque and speed, a VFD is a logical solution.

This involves applications that have fluctuating speeds i.e., pumps and blowers, as well as tools necessary for production. The speed of a conveyor system might be changed remotely using a potentiometer for manual adjustment.

The operator tasked with regulating the speed of a conveyor will adjust it according to frequency drive parameters (minimum or maximum).

Where Variable Frequency Drives Are Not Used?

What are some settings or applications that are inappropriate for using a variable frequency drive? The majority of these include areas deemed as hazardous, such as where conditions are highly volatile or explosive.

These determinations should always be based on the recommendation of the product manufacturer that has completed the needed testing.

Advantages and Disadvantages of Variable Frequency Drives

VFDs have a host of possible advantages and disadvantages based on a variety of factors including application, setting, and more. A VFD has proven to be an effective option for those seeking to reduce energy consumption in pumping stations.

Often, VFDs are used to extend the serviceable lifespan of a pump. However, often these well-intentioned efforts will backfire when personnel lacks the proper tools for monitoring these configurations.

VFDs have also proven to allow for better management of industrial operations by improving certain metrics. For example, AC motors may operate with improved control of speed by optimizing flow rates.

VFDs also can allow for enhanced automation by eliminating the need for what had been manual tasks. Today’s VFDs can be automatically programmed to respond according to various conditions.

Motors that are linked to a VFD can often operate at slower speeds to limit the overall consumption of energy. In these applications, a facility manager could potentially create a forecast that will determine the potential savings.

Here, the management team could create a preliminary cost analysis that illustrates how purchasing a VFD can represent an investment. Utility companies are among those who are most cognizant of the energy savings that a VFD may deliver.

In certain traditional configurations that involve AC motors, the PWM can generate increased noise and winding that stresses the motor.  In time, the insulators of the motor unit may prematurely break down and cause the failure of the unit.

PWM also may create problems for motor shafts, particularly when the insulation and grounding in insufficient for handling current. Often referred to as “electrical discharge machining,” this is a problem that adversely impacts the bearings of pieces of equipment.

PWM also involves transforming AC to DC and results in distortion, typically in drives with considerable horsepower. The distortion has the potential to harm sensitive parts of an electrical system; therefore, harmonic filters may be necessary

VFDs often are susceptible to extreme temperatures (hot or cold) and sometimes implementation of climate control measures is necessary.  Also, the drives are vulnerable to dust, which creates a need for a structural enclosure.

Key Terms & Components of Variable Frequency Drives

There are a variety of terms that apply to VFDs, which should be clearly defined.

Acceleration refers to a fluctuation in motor speed that is the result of a VFD that is expressed relative to time. Acceleration time is the time that elapses between a change in speed, which is often continual.

Auto tuning describes the process where a VFD evaluates or measures a needed motor constant. “Online” auto-tuning is when the process of tuning occurs at a start, which allows for superior accuracy of operation.

Auto tuning in real-time involves a servo-driven machine that optimizes stability and responsiveness. Here, tuning refers to the adjustments needed to enhance the performance of a machine for ideal operation.

Frequency response is an expression of speed describing the response to a command. Frequency response generally increases with a rise in speed loop gain; however, it must not reach a level that creates vibration.  

Power regeneration could be explained as an energy that the system recycles by returning it to the power supply. The process of regenerating the energy requires a separate unit that facilitates the movement of power.

Proportional control or P control is used for minimizing torque while positioning suppresses current. P control might also serve to limit vibration, which can occur immediately upon detection of a deviation.

Typically, the process of driving applies to the delivery of power from an amplifier to a motor. A regenerative brake describes the inverse of this process where energy is returned to the amplifier upon deacceleration.

The motor’s rotational energy flow is initiated when the load speed is reduced and brought to the amplifier. The servo amplifier receives the energy via a resistor or capacitator that facilitates the brake torque.

Regenerative overvoltage is a situation resulting from a large surge of regenerative energy, often exceeding the system’s capabilities. During regenerative operations, the variable frequency drive is unable to handle the voltage and often results in capacitor failure.

Different from the previously mentioned overvoltage condition, Undervoltage means that the power supply voltage is too low compared to the servo amplifier. For devices operating at 200V, the value is below 160V.

Mechanical Applications
  • A traction drive is a VFD with two adjustable metal rollers, which creates variable output speeds that are expressed as a ratio.
  • A variable-pitch drive uses a system of pulleys and a belt that adjusts and creates variable output speed
  • A hydraulic hydrostatic drive relies on manipulating hydraulic fluid to generate variable speed output using a pump
Electric Applications

Electric applications of VFDs include eddy current drives, which control speed through direct current (DC) coils on a magnetic rotor. Eddy current drives are rotational machines that are largely driven by pumps and electric motors.

Fortunately, most experienced maintenance technicians find these units to be simplistic enough to repair.   Eddy current drives have traditionally been among the most commonly used for controlling pump and fan speeds.

In the 1980s, VFDs replaced older eddy current machines because of the reduction in energy consumption. Eddy’s current variable speed drives have proven good for low overall ownership costs, ease of repair, and more.

Eddy current coupling is a VFD positioned between a motor with a fixed speed and the machine being driven. Here, one fixed speed rotor and one variable speed rotor are positioned adjacent to one another with a small gap in between.

A magnetic field is developed that merges the rotors and generates torque. Generally, as the rotors increasingly operated at different speeds, the efficiency declined.

Computer Applications

VFDs are increasingly being adopted to the digital technology that is most popular in the market today. Trends have emerged involving the ability to remotely program, monitor, and otherwise operate VFDs from some mobile devices.

Commands can be entered on a smartphone or laptop which allows for often quick and easy setup and programming capability. Maintenance technicians often will reference the manufacturer’s troubleshooting guide and watch instructional videos to guide them through these processes.

In some VFD configurations, programming tasks may be performed while the main power is inactive. VFDs can be linked with PLCs to allow for digital operation with Modbus ASCII and Modbus RTU or some Ethernet options.

Two-way digital communications may be used along with discrete and analog solutions in real-time for more expansive control options. These two-way communications are dependent on a PLC and allow users to monitor various parameters, codes, and variables.

Signals are communicated including error codes when conditions such as excessive current levels or temperature extremes are detected.  Multiple vendors have introduced PC-integrated software options that allow users to generate customized control panel settings.

Some of the leading options are easily downloaded from a PC to a drive via USB or other connection. Smart drives have proven to operate pumps, mixers, machines used for packaging, fans, and more.

Other, Non-Electrical Component Considerations

Many facility maintenance departments face considerations regarding whether to repair or replace a VFD. One of the challenges involved with repairs includes the likelihood of finding available parts for the specific model or type.

Replacement components are often hard to find for some of the older models that have not been produced in recent years. In many cases, the problems are the result of basic environmental conditions that are placing additional stress on a machine.

Examples might include extreme temperatures, excessive moisture or humidity, the presence of dust, and many others. The majority of engineers will agree that these decisions should be made only after identifying the root of the problem.

In the market today, there are various types of filters that represent an affordable means of limiting harmonics. For example, passive harmonic filters are often placed in a motor center that is supplying multiple VFDs.

Schematic Symbols for Variable Frequency Drives

Schematics are diagrams used to illustrate the configuration of all components and connections within a circuit. American National Standards Institute (ANSI) and the International Electrotechnical Commission (IEC) are the two creators of the standard symbols used.

Each set of standards will have a compilation of symbols that represent specific components. If a drawing is created without using either of the symbol formats, others will struggle to interpret the design.

VFDs have emerged as common elements within the industry of machine design and engineering. The VFD unit is often simply symbolized by a small rectangular box with the letters VFD or VS (variable speed) written inside.

A directional short line is typically used that connects a VFD to a motor. In many cases, the motor is also labeled using an “M” symbol or might be symbolized by the device that it delivers torque.

What Are the Main VFD Control Technologies?

Equipment driven by AC motors often operates at a constant (fixed) speed with a load of pumps, fans, etc. To have some control over the speed, a VFD is added to change voltage frequency using several different methods.

Scalar control methods involve maintaining and stabilizing the magnetic field to optimize the motor and generate steady torque. The scalar control methods are often noted as V/Hz or V/f control, with voltage (V) and frequency (f) maintaining speed.

A V/Hz ratio signifies how the rated voltage of the motor is divided by a rated frequency in an open loop. This can be transitioned to a closed-loop configuration by altering and creating a more complex implementation.  

Scalar control methods do not allow for very precise speed control and equate to only about 3% of motor frequency. V/Hz controls operating in an open-loop are very commonly used because it allows for a single VFD to handle several motors.

Vector control methods operate in ways more closely reflecting the methods of DC control; however, in a more complicated way. Often referred to as field-oriented control (FOC), vector control dictates an AC motor’s speed or torque through stator current space vectors.

Vector methods use mathematical calculations to execute a d and q (two-coordinate) time interval system. The (d) signifies the magnetic aspect and the (q) represents the component responsible for generating torque.

Using FOC, the components each have a PI controller, thus they operate independently to achieve the ideal speed. FOC methods, similar to scalar configurations, also may be open or closed-loop, with open-loop being “sensorless.”

Controllers use a mathematical formula or model to assess the current and voltage generated by a motor and adjust accordingly.  In closed-loop configurations, the controller receives information and increases or decreases torque by supplying voltage.

Vector methods are considered to be of greater complexity compared to scalar methods but they deliver enhanced benefits. For example, motors may generate significant torque at low speed in open-loop vector arrangements and may hold loads when paused.

What Can Happen When a VFD Fails?

VFD technology is critical to support processing, manufacturing, warehousing, logistics, and many other operations today. Professionals are tasked with properly configuring and sizing VFD systems to enhance performance, conserve energy, and prevent failures.

Prevention of faults and failures is critical to avoiding potentially costly downtime; therefore, maintenance, monitoring, and development are necessary. VFD failures may adversely impact pumps, fans, motors, and other equipment that facilities depend on.

As digital controls, real-time monitoring, and enhanced types of automation enter the VFD market it should help to limit failures. Increased emphasis is being placed on preventing faults and failures associated with voltage, heat, environmental conditions, and other concerns.

What’s The Future of VFDs?

Market forecasts project that the international VFD market will rise from 19.2 billion in 2020 to 24.3 billion in 2025.  This roughly 20% surge is believed to largely be the result of demand in countries including China, Japan, India, and others.  

Experts suggest that these markets are also going to be home to the highest volume of VFD manufacturing and development. Various foreign markets are expected to emerge from the recent pandemic and resume industrialization, construction, and other activity.

Energy is a vital component for the feasibility of growth, particularly involving electricity and existing or emerging renewable sources. The automotive sector is expected to be very active–an industry that is receptive to new technology and subject to environmental compliance.

Other areas that should continue to use VFDs include oil, mining, irrigation, and more.

Experienced and Reliable Provider of Facility Maintenance and Engineering Services

The trained professionals with Servi-Tek Facility Solutions understand the importance of diligently performing preventative maintenance, repairs, and support services. 

Our clients operate in environments that include industrial, manufacturing, and others that depend on maintaining equipment, devices, and systems. 

Our staff creates customized solutions that save energy, prolong serviceable operations, minimize downtime, and bolster productivity.

Contact us today for a free consultation!

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