Variable Frequency Drive (VFD) 101: A Beginner’s Guide

A variable frequency drive (VFD) is essentially an electric motor controller used in industrial engineering settings, especially in manufacturing plant operations. The device dictates the speed and torque of an AC motor by altering the frequency and voltage of the power supply. A VFD can additionally control the acceleration and deceleration of the motor during start-up and stop.

A factory’s conveyor belt production system provides a good example of VFD use, in which it directs the AC motor driving the pace of the belt’s progress. This pays off for maintenance teams in a number of ways, including energy savings, extending the equipment’s lifespan by not overworking it and reducing maintenance costs since the efficient running of the motor and belt means less unnecessary wear and tear.

“Industries such as manufacturing, HVAC, water treatment, oil and gas, and mining benefit significantly from VFDs,” says Taner Caglar, global head of product portfolio and customer services for General Motion Control at Siemens Digital. “Companies in these sectors use VFDs to control motor speed and improve energy efficiency, leading to cost savings and enhanced process control.”

How VFDs Work

VFDs serve their function by changing the voltage and frequency of an electric motor’s power supply and by making it possible for a motor to run at different speeds and directions (backwards and forwards). 

Three main parts comprise the operations of a standard variable frequency drive:

Rectifier

The key significance of the rectifier is its capability to convert AC power to DC power. This furnishes the needed electrical groundwork for the VFD to operate. Lacking an effective rectifier, the VFD simply cannot supply the stable DC power necessary for sufficient modulation. The dependability and effectiveness of this component are absolutely vital to the proper functioning of the VFD — as it makes certain that established and regular power is provided to the motor.

DC Bus

The DC bus is indispensable for maintaining a steady DC voltage, essential to the inverter’s smooth operation. A reliable DC bus minimizes the threat of fluctuations that could impact motor performance by making certain the inverter receives unwavering power. This consistency is crucial in avoiding both voltage sags and surges, which often result in inefficient operation at best — or even harm to the motor and other associated equipment. The DC bus’s function as a buffer serves to safeguard the VFD and maintain strong performance.

Inverter

The inverter section is the true core of a variable frequency drive, taking the stable DC power from the DC bus and converting it back into AC power with measured frequency and voltage. The inverter — by adjusting the frequency of the AC power — can regulate the motor’s speed and torque. This ability to allow pinpoint motor control for various purposes is the main purpose of the VFD. Without precise and efficient use of the inverter section, you can’t rely on a motor to operate at the necessary pace and torque. It’s also worth noting that a key element of the inverter is the insulated gate bipolar transistor (IGBT), which provides the inverter’s switching elements.

Variable Frequency Drives vs. Variable Speed Drives vs. Programmable Logic Controllers

While variable frequency drives and variable speed drives serve similar purposes, there are key differences between the two. VFDs are deployed to control the speed of AC motors by varying the output frequency directed to the motor, while variable speed drives (VSDs) can dictate the speed of either AC or DC motors by varying the frequency or voltage sent to the motor. VFDs are optimal for situations in which close precision is needed, and VSDs are best for applications in which broad speed adjustments are acceptable. 

Both VFDs and VSDs focus on controlling the speed of an electric motor, while a programmable logic controller (PLC) acts as a more general tool used to control automation processes by handling a larger variety of inputs and outputs. That said, you can use a PLC to send control signals to an VFD about which motor speed to use based on sensor readings from other tools.

Equipment That Benefits From the Use of VFDs

Upgrading to VFDs can solve issues like low voltage and high energy consumption. Here are five common applications:

• HVAC systems: VFDs adjust the frequency of electrical power to fans, pumps and compressors. This aligns airflow with a facility’s actual heating or cooling load, resulting in significant energy savings.
• CNC machines: These drives control the spindle motor’s speed for precise machining. They enable fluid acceleration and deceleration, ensuring accuracy across different materials and cutting tasks.
• Welding machines: VFDs manage the power supplied to the welding process. Precise adjustments to current and voltage result in superior weld quality and better control over the welding arc.
• Conveyors: By adjusting motor frequency, VFDs provide exact control over belt tempo and material flow. This allows for dynamic speed modifications based on real-time production needs.
• Robotic systems: VFDs guide the speed and acceleration of electric motors to ensure steady motion and precise positioning. This increased agility is essential for tasks requiring subtle manipulation or varying speeds.

Benefits of Using VFDs

“VFDs are incredibly helpful in maintenance,” says Siemens’ Caglar, “because they reduce energy consumption by adjusting motor speed to match demand and extend equipment lifespan through soft start and stop functions, minimizing mechanical stress. Enhancing process control with precise speed and torque adjustments leads to consistent product quality.”

There are other pluses to using a VFD, including: 

• Improved energy efficiency: This is likely at the top of the list of VFD benefits, given its ability to reduce energy consumption by furnishing only the power needed based on the current load, resulting in major energy savings when compared to operating a motor at full speed constantly.
• Increased production with better process control: Another huge plus, increased production is the result of VFDs allowing for fine-tuned speed adjustments, which can be vital in maintaining consistent product quality in the manufacturing process.
• Extended motor lifespan: Using a VFD to govern motor speed precisely reduces mechanical stress on motors and other components, leading to a longer equipment lifecycle.
• Reduced maintenance required for motors: By lowering duress on motors and diminishing wear and tear, VFDs decrease frequent maintenance needs and lower maintenance expenses overall.
• Flexible system control: of motor speeds is a huge benefit of VFDs. Varying speeds and loads are a frequent requirement of industrial processes, and VFDs showcase peerless flexibility. This frees up operators to tweak motor parameters instantaneously to match fluctuating states of affairs and provides easier adaptation to changing requirements. 

Disadvantages of VFDs

While a variable frequency drive may seem like a magical solution in many ways, it can have its drawbacks. Here are a few examples of the disadvantages that could come with implementing this piece of equipment into your workflow: 

• Knowledgeable operator required: A VFD is a somewhat complex piece of machinery, so you can expect a learning curve with any operator not already practiced in using this tool, or you may even want to consider hiring someone with VFD expertise.
• Complex installation: Variable frequency drives entail a challenging and time-consuming configuration process and can be more complex to install and maintain than other motor control system solutions. 
• Higher upfront costs: While VFDs can save money in the long run by increasing efficiency and reducing maintenance, they are also normally more expensive to purchase than other motor control systems such as gearboxes or belt drives. 
• Can introduce harmonic current distortion if not used properly: This can cause power decreases and lessen a motor’s efficiency. It can also have fallout on other equipment and power systems. 

VFD Example With a Conveyor Belt

A popular application of VFDs is with conveyor belt operations in industrial settings. Here’s a closer look at exactly how they function in that specific setting. 

Picture a food processing plant, where a conveyor belt is used to move materials through various stages of processing. The speed at which the conveyor belt moves must be revised based on various factors that include:

• The type of product being processed
• The capacity of the processing machines downstream
• Energy efficiency
• The alignment of the belt for optimal functioning
• Set-up or cleaning (which requires lower speed)

So, how exactly does a VFD help with industrial conveyor belt operation?

• Speed control: The VFD guides the speed of the motor that drives the conveyor belt, so when the plant needs to increase production, the VFD can boost the speed of the conveyor belt.
• Energy savings: With a conveyor belt powered by a fixed-speed motor, the belt moves at full-speed regardless of the need, resulting in unnecessary energy use.
• Reduced wear and tear: By ramping up and down gradually (instead of sudden starts and stops), the VFD minimizes mechanical duress on the motor and conveyor components. This assists in extending the lifespan of the system, leading to fewer repairs and maintenance costs.
• Process optimization: The VFD can also integrate with the plant’s control systems to elevate the overall production process. Such as, if a packaging machine downstream is temporarily halted, the VFD can decelerate or stop the conveyor belt to avoid product overflow or jams.

3 Types of VFDs

There are essentially three main types of VFDs. These include:

Pulse Width Modulation (PWM) Drives

Pulse width modulation applies transistors that turn the DC voltage on and off in a planned sequence to yield the AC output voltage and frequency. One of the advantages of using a VFD with PWM technology is the ability to control the amount of current going through the motor windings, which when running a rotary industrial motor, translates into torque control at the motor shaft.

Voltage Source Inverter (VSI) Drives

VSI VFDs convert AC power to DC and then back to AC at the desired frequency, allowing for precise motor control. They are thus regarded for both their flexibility and high-performance efficiency. These devices have the ability to deal with a variety of voltage levels, a huge benefit in many settings but especially the manufacturing and automation sectors. They can also be a boon to facilities dealing with space concerns as they offer a comparatively compact size.

Current Source Inverter (CSI) Drives

CSIs (sometimes referred to as current-fed inverters) act like a constant current generator. These are often used rather than VSIs for large VFDs due to their uncomplicated nature, regeneration aptitude and trustworthiness. Although more rugged and reliable than VSIs, CSIs do suffer from a poor power factor at lower speeds and also have the downside of possible erratic start and stop actions. 

All three types of drives can be used in both single-phase and triple-phase VFDs; however, the three-phase VFD is typically more common with larger machinery since it accommodates higher power applications.

Key Factors When Choosing the Right VFD for Your Motor

Selecting the right variable frequency drive  is critical for protecting your motor and optimizing energy use. Follow these guidelines to ensure a proper match:

• Horsepower and current: Select a VFD with an amp rating that meets or exceeds the motor’s full load amps (FLA) listed on its nameplate. It is best practice to choose a VFD equal to or greater than the motor’s horsepower rating to handle maximum power draw.
• Voltage: Ensure the VFD’s rated voltage matches both the motor and your site’s available power supply (i.e., 208V or 230V).
• Load and acceleration: If your application requires rapid acceleration or frequent overloads, choose a VFD with a higher power rating than the motor to manage these surges safely.
• Altitude and cooling: Standard VFDs lose cooling efficiency above 3,300 feet due to thinner air. If operating at high altitudes, you must oversize the drive to compensate for decreased heat dissipation.

How to Keep Your VFD Maintained

Siemens’ Caglar suggests the following guidance to best maintain a variable frequency drive. Effective maintenance of VFDs involves: 

• Keeping them clean: Regularly remove dust and debris to prevent overheating.
• Maintaining proper temperatures: Ensure adequate cooling and ventilation. 
• Preventing moisture: Keep VFDs dry to avoid corrosion. 
• Checking connections: Regularly inspect and tighten electrical connections.

As with many types of sophisticated machinery, frequent visual inspections of VFDs is crucial to noticing issues like dust buildup, wear and tear, overheating and loosened connections. And, as is the case with almost all electronic devices, VFDs simply are unsafe in moist or wet environments. They require a cool, dry setting with minimal dirt and dust.

It’s also good to stay ahead of the curve by considering replacement parts sooner rather than later to avoid a sudden performance drop-off. The main bus capacitors should be refreshed about every seven years, for example, while cooling fans should be swapped out every three to five years. Sustaining a vigilant preventive maintenance schedule and correct record of replaced parts is invaluable. 

VFD Trends Today & in the Future

“The most significant current trend is the integration of VFDs with IoT and Industry 4.0,” says Caglar. “This allows for real-time data collection, remote monitoring and predictive maintenance, improving operational efficiency and reducing downtime.”

Use of Compact AI for analysis to predict lifetime of key parts as well as to identify signs of damage is a hot current wave with VFDs, as is the rising appeal of their ability to optimize energy efficiency. As we look toward the near future, we can expect to see the following developments in VFDs of the future:

• Smaller sizing: VFDs are becoming more compact and modular, which has a number of benefits that include making them easier to relocate and providing better ability to fit well in small spaces.
• Safety first: Safety is a focus area for VFD innovation moving ahead, so you can expect to see enhanced safety features being worked into their design.
• Semiconductors on the rise: Wide-bandgap semiconductors are being used more and more often in VFDs to realize substantial energy reduction, lower weight and reduced volume of drive electronics.
• Application-specific solutions: VFDs are being customized for specific applications, which should only increase their effectiveness and efficiency.