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, most commonly manufacturing plant operations and the like. 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.

One common use of a VFD would be seen in a factory’s conveyor belt production system, where it would direct 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, a motor can’t be relied upon 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. It’s generally accepted that 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) is a more general tool used to control automation processes by handling a larger variety of inputs and outputs. That said, a PLC can be used 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

Several types of equipment can benefit from upgrading to VFDs, especially if you’re looking to address issues like low voltage in motor applications, but here are five of the most common use cases:

• HVAC systems: In HVAC systems, a VFD is used to dictate the speed of motors that power fans, pumps and compressors by adapting the frequency of the electrical power supplied to them. This allows the system to accurately pair airflow and water flow levels to a facility’s heating and cooling needs — paying off in significant energy savings by operating at the speed required by existing load requirements.
• CNC machine: With a CNC machine tool, a VFD is mainly applied to control the speed of the spindle motor by modifying the frequency of the AC power delivered to it, granting precise speed alterations during machining operations. This is vital for various cutting jobs and material types, as the VFD serves as the “speed controller” for the spindle motor, enabling fluid acceleration and deceleration and sustaining accuracy throughout the machining endeavor.
• Welding machine: The general purpose of using a VCD on a welding machine is to manage the power furnished to the welding process, allowing for precise adjustments in welding settings such as current and voltage. This results in superior weld quality and optimized control over the welding arc.
• Conveyor: A conveyor uses a VCD to control the speed of its motor by adjusting the frequency of the electrical volts supplied to it — providing precise control over the conveyor belt tempo and material flow. This is particularly helpful in circumstances in which the material flow needs to be dynamically modified based on production needs. Basically, the VFD facilitates fluid and regulated advancement of goods along the conveyor line by altering the motor speed as needed.
• Robotic system: A VFD is employed to precisely guide the speed and acceleration of electric motors within a robotic system. This permits steady motions and precise positioning — plus efficient operation — by adjusting the frequency of the power furnished to the motor. Basically, this provides the robot with increased agility and flexibility in its movement. This is invaluable with tasks that require subtle manipulation or differing speeds throughout various segments of a robotic motion succession. 

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. 

Example of a VFD Being Used on 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

So, how do you go about selecting the optimal variable frequency drive for your specific motor type? There are several important factors to take into consideration: 

• Horsepower: It’s wise to choose a VFD equal to (or greater than) the motor’s horsepower rating. Take into mind the motor’s top torque rate and horsepower and weigh the motor’s full load amps (FLA) against the amp rating of the VFD.
• Voltage: When selecting a VFD, the most critical dynamic with voltage considerations is simply making sure the rated voltage of VFD matches your site’s available power supply voltage. In other words, the voltage of the VFD should be equal to the voltage of the motor and the power line you plan to connect it with (208V, 230V, etc.).
• Current: The key factor here is ensuring the VFD’s rated current matches or exceeds the full load current of the motor you plan to use (generally noted on the motor’s nameplate). Then, you can be confident the VFD will have the capability to furnish ample current to handle the motor’s maximum power draw under standard operating environments.
• Acceleration: If your motor’s function necessitates common overloads or fast acceleration, select a VFD with a power rating above that of the motor to manage these circumstances.
• Overload: Make certain the drive can handle any overload conditions you might anticipate during startup or occasional extra loading. Numerous applications face short-term overload conditions because of starting requirements or impact loading. 
• Altitude: Your VFD’s cooling is impacted by the altitude of its operating environment since the air becomes thinner as altitude increases. This drop in air density lowers the air’s cooling properties. While most VFDs are designed to function at full capacity at up to about 3,330-foot altitudes, if you do happen to be at a higher altitude, the drive needs to be oversized to make up for the decrease in cooling.

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.