How to Choose a Variable Frequency Drive

As crucial components in industrial automation, Variable Frequency Drives (VFDs) regulate motor speeds and boost energy efficiency. But to optimize your operations and reap these benefits, selecting the appropriate VFD is critical. This decision depends on multiple factors, from motor specifications to operating conditions and beyond.

Our guide simplifies this process, empowering you to make an informed choice when investing in VFD technology. So, whether you’re a plant manager, a machine designer, or a maintenance engineer, this comprehensive guide will pave the way to maximize efficiency and durability in your operations.

Definition of Variable Frequency Drive (VFD)

A Variable Frequency Drive (VFD) is a type of motor controller. It drives an electric motor by varying the frequency and voltage supplied to the motor. VFDs are also known as adjustable speed drives, adjustable frequency drives, or inverter drives. They enable speed control, enhancing energy efficiency in various applications.

VFD Drive Working Principle

The Variable Frequency Drive (VFD) begins its operation by taking in AC power from the mains supply. This is the standard form of power available in most households and industries. The VFD then uses a component called a rectifier to convert this alternating current (AC) into direct current (DC). This is necessary because the speed control mechanism of the VFD operates on DC.

Once the power is in DC form, a component called a DC bus stores and smooths out the power. This prepares it for the next phase, which is handled by an inverter. The inverter is where the magic happens. It uses semiconductor devices such as IGBTs (Insulated Gate Bipolar Transistors) to switch the DC power on and off rapidly, thereby recreating AC power.

What’s special about this recreated AC power is that its frequency and voltage can be controlled. By varying the frequency of the AC voltage, the VFD can control the speed of the AC motor it’s connected to. This means the VFD can adapt the motor’s speed to the demands of the load, offering enhanced control and energy efficiency. This whole process is the fundamental working principle of a Variable Frequency Drive.

Definition of Variable Frequency Drive (VFD)

Components of a Variable Frequency Drive

A Variable Frequency Drive (VFD) consists of several key components:

Rectifier: This component converts input Alternating Current (AC) into Direct Current (DC). It’s the first stage in the operation of a VFD.
Intermediate DC Circuit: This is a DC bus that temporarily stores the DC power. It contains capacitors that smooth out the ripple in the DC current, generated by the rectifier.
Inverter: The inverter takes the smooth DC voltage and turns it back into AC. It uses devices known as IGBTs (Insulated Gate Bipolar Transistors) for this process. This newly formed AC is the output of the VFD.
Controller: This is the brain of the VFD. It controls the operation of the rectifier, intermediate DC circuit, and the inverter. It also receives input signals (digital or analog) and adjusts the operation of the VFD accordingly.
Heat Sink/Cooling System: Since a VFD generates heat during operation, it requires a cooling system to avoid overheating. Heat sinks, fans, or other cooling elements are used.
Display and Programming Interface: Many VFDs have built-in displays and programming interfaces. They allow users to monitor operational parameters and change settings as needed.
Protective Components: VFDs include various protective components like circuit breakers, fuses, and overload relays. These components help protect the VFD and the motor it’s controlling from damage due to overloads or short circuits.

Components of a Variable Frequency Drive

Different Types of VFD drives

There are three main types of Variable Frequency Drives (VFDs), each with unique characteristics and suitable for different applications:

Voltage Source Inverter (VSI) Drives: These are the most common type of VFDs. They convert AC to DC, then back to a variable voltage and frequency AC signal. However, they can cause motor cogging at lower frequencies.
Current Source Inverter (CSI) Drives: These drives invert DC to AC by modulating the output current. They are often used in high-power applications due to their good overload capacity.
Pulse Width Modulation (PWM) Drives: These are an advanced type of VSI drives, using pulse width modulation techniques to create a sine wave output. They offer better input power factors and efficiency, making them the most widely used in the industry.

Each type has its own set of advantages and limitations, making them suitable for different applications. Therefore, understanding these types can help you make an informed decision when choosing a VFD.

Advantages of VFD

There are several advantages to using a Variable Frequency Drive (VFD):

Energy Efficiency: VFDs can optimize motor speed based on the load, leading to significant energy savings.

Smooth Operation: VFDs ensure smooth start and stop of motors, reducing mechanical stress and prolonging equipment life.

Speed Control: By adjusting the frequency of the electrical supply, VFDs provide precise control over motor speed.

Reduction in Power Surges: VFDs limit the starting current, thereby reducing power surges and enhancing overall system efficiency.

Extended Equipment Life: By controlling the speed and torque of the motor, VFDs can extend the life of mechanical systems.

Cost Savings: The energy savings and reduced maintenance costs associated with VFDs can result in significant cost savings over time.

In conclusion, the use of VFDs can lead to improved system performance, reduced energy consumption, and cost savings.

How to Choose the VFD

When choosing a Variable Frequency Drive (VFD), consider the following factors:

Motor Requirements

Gather basic information from the motor nameplate, such as horsepower (HP), full load amps (FLA), voltage, RPM, service factor, and whether it is inverter-duty rated. This information will help determine the appropriate VFD size and specifications.

Load Type

Determine if the load requires constant torque or variable torque control. For applications like fans or pumps, a variable torque drive is suitable as it provides energy savings by adjusting torque to match load requirements. Constant torque VFDs are needed for applications like conveyors or extruders that require constant torque at all speeds.

Speed Range

Consider the desired speed range of the motor. Ensure that the VFD can accommodate both under-speed and over-speed operations. It’s generally recommended not to run a motor below 20% of its maximum allowable speed to prevent overheating.

Control Methods

Decide on the preferred method of control for the VFD. This could include options like 2-wire control or 3-wire control, speed potentiometer, digital programming/display unit, analog signal follower, selector switch speed selection, serial communications, or Ethernet communications. Choose the control method that aligns with your specific application needs.

Voltage and Phase

Match the VFD’s voltage and phase to that of the motor and available power supply. For three-phase input, ensure the VFD and motor voltage match the field’s voltage (e.g., 208V, 230V, or 460VAC). For single-phase input, consider the need for a phase converter if the motor is three-phase but only single-phase power is available.

Special Enclosure Needs

Determine if the VFD will be mounted indoors, outdoors, or in a specific environment that requires special enclosure ratings (e.g., NEMA types). Consider factors like temperature, humidity, and dust levels when selecting the appropriate enclosure.

Drive Specifications

Pay attention to drive specifications such as Full Load Amps (FLA). Ensure the VFD’s current rating matches or exceeds the motor’s FLA to ensure proper operation. Additionally, consider an oversized VFD if dealing with high inertia loads or loads that are difficult to start for improved performance and reliability.

Additional Tips

Additionally, here are some tips for specifying VFDs:

Determine if a VFD is suitable for your application.
Clarify the reasons for choosing a VFD and identify the desired outcomes.
Size the VFD appropriately based on the maximum current requirements of the load.
Consider braking requirements and select the appropriate braking system.
Determine the input/output (I/O) needs for your specific application.
Choose the proper control mode based on the desired control profile.
Explore communication options and select the suitable protocol for integration.
Pay attention to installation and operating requirements, including factors like harmonics and environmental conditions.

Before making a final selection, consider these points:

Over-sized units may be necessary for hard-to-start loads or applications requiring quick starts or emergency stops.
Motor overheating should be considered for reduced speed or constant torque applications.
Efficiency improvements are more noticeable in smaller motors, but even small efficiency gains in larger motors can lead to significant long-term savings.
Ensure compatibility with volatile environments, particulate-filled atmospheres, and high temperatures.
Understand how the VFD handles load downtimes and fault analysis.
Address power factor correction and consider the need for isolation transformers or line reactors.
Account for power source switching and evaluate the VFD’s ability to handle brief power outages.

Lastly, when following a variable frequency drive buying guide, ask five essential questions:

What is the application type in terms of torque requirements?
What are the power source characteristics, including voltage options?
What enclosure type is needed for adequate protection?
Is the VFD compatible with the process, and what feedback and communication requirements are necessary?
What level of support and service can be expected from the manufacturer, including phone support and local professionals?

Considering these factors and tips will help you choose the right VFD for your application, ensuring optimal performance and compatibility.

Conclusion:

Selecting the right Variable Frequency Drive (VFD) is a vital decision, that impacts safety, efficiency, and performance. From understanding VFD basics to considering load type, motor specifications, and specific application requirements, every aspect plays a part.

Evaluating different types of VFD drives, their pros, and cons, application-specific considerations, and understanding VFD’s role in energy efficiency can guide your selection. Prioritizing safety, installation, and maintenance considerations ensures long-term, sustainable operation.