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Basics of Variable Frequency Drives

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Variable Frequency Drives (VFDs) are used to adjust the speed of motors. There are many different names for VFDs including adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter drive. VFDs adjust the speed of motors by changing the frequency being transferred to the motor. For example, when the VFD is receiving power at 60 hz, it can change the frequency to 20 hz, resulting in a slower rotation of the motor.

The Different Parts of a Variable Frequency Drive:

The three parts of a VFD that allow the drives to change frequency are the converter, dc bus, and inverter sections.

  • Converter: AC power enters the VFD’s converter which converts the power from AC to DC power.
  • DC Bus: After it is converted, the power travels through the DC Bus.
  • Inverter: The power then enters the inverter section which is made up of small circuit board components called insulated gate bipolar transistors (IGBT). The IGBT act as triggers to take the dc power to spit out the dc power in short bursts, then flips the polarity. The IGBT continues spitting out the power in short bursts and flipping the polarity.

The outcome is Pulse Width Modulation (PWM) which is a simulated AC power at a specific frequency.

 

 

Variable Frequency Drive Diagram

 

Some benefits of using VFDs include:

  • Mechanical stress and belt wear on equipment is far less when using a drive versus across the line starting of a motor.
  • Motors running on drives are protected from incoming power surging and single phasing.
  • Reducing the speed of a motor by 20% will reduce the power consumption of a motor by 50%, calculated using the affinity law.
  • High efficiency at low speed
  • Speed Variation

Selecting and Installing the Right VFD

Choosing the correct VFD for your application is critical to getting optimal performance and long-term reliability. Factors like motor size, load type (variable or constant torque), environmental conditions, and control requirements all impact drive selection. Proper installation is equally important—ensuring the drive is wired correctly, configured for your motor, and protected from external factors like heat, dust, and vibration. At Dreisilker, we help customers every step of the way, from selecting the right drive to professional installation and troubleshooting support. Whether you’re managing the install yourself or need a full turnkey solution, our team is here to ensure your system runs efficiently and reliably.

field service drive install

What are the Different Torque Requirements?

There are two types of torque requirements found in VFD applications, including variable torque load and constant torque load. Variable torque loads are typically found in applications like fans, blowers, spinning compressors and spinning pumps which move easy loads such as air or (clean) liquid. The application requires more torque as you are moving more of the air or fluid. As the motor or pump speeds up, the amount of force required to move the load increases. The increase in the need of force is why they are considered variable torque loads.

Conversely, Constant Torque loads are found in the applications that require the same amount of torque to move the loads at slower and faster speeds. Common constant torque loads are found in conveyors, elevators, shredders and extruders.

Both types of torque loads need to be able to provide the application with their full load amps (fla) which is 100%. Variable torque loads can provide an application with low overload capability of 110% fla for instances when the load changes or if it encounters extra loads momentarily. Constant torque loads are designed to allow 150% fla which is considered high overload capability. The high and low overload capabilities allow for the application to handle the temporary overload without the motor shutting down.

Are there any downsides to VFDs?

Although VFDs allow for improved motor efficiencies, they also present a problem for motor bearing life. The VFDs can emit damaging currents that go to ground through a motor’s bearing. Eventually, this can lead to pitting, fluting, frosting of bearings, lubrication break down and overall destruction of the bearing. The harmful currents can be stopped by installing CoolBlue Inductive Absorbers and Aegis Grounding rings. To learn about these products, you can read our blogs CoolBlue Inductive Absorbers and Aegis Shaft Grounding Rings.

 

Aegis Shaft Grounding Rings Protect your Motors and Bearings

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Aegis Shaft Grounding Rings protects bearings and electric motors from damaging shaft voltages caused by variable frequency drives (VFDs). The voltages emitted by VFDs find their way to ground typically through the motor’s bearing. Some signs of these damaging currents include pitting, frosting, fluting and loss of bearing lubrication. Installing Aegis Shaft Grounding Rings to your motors can prevent bearing and motor failure caused by the shaft voltages.

The Aegis Shaft Grounding Rings work by diverting the damaging voltages away from the bearings by sending it to ground through the motor frame. The rings are made of conductive microfibers that encircle the motor shaft and is installed on the drive end of the motor. For motors 100HP and higher, it is recommended to use an insulated bearing on the non-drive end with the Aegis Ring installed on the drive end.

 

Aegis Shaft Grounding Rings Kit
Aegis Shaft Grounding Ring Kit.
Aegis Shaft Grounding Rings Diagram 2
Conductive Microfibers.
Aegis Shaft Grounding Rings Diagram
Diagram of an Aegis Shaft Grounding Ring.

 

Even though a motor may be rated inverter duty, you need to keep in mind that this does not mean that it comes with grounding rings preinstalled. If you need an inverter duty motor with grounding rings pre-installed, you will need to select the option for grounding rings. Most motors now have the option to come with grounding rings preinstalled. If you already have a motor in the field that needs protection, you can order an Aegis Shaft Grounding kit and install it on your motor.

There are different types of Aegis Rings that allow for installation on many applications. They are available in solid and split-ring designs, configured for NEMA and IEC motors, and include four mounting bracket styles and hardware. The different types of Aegis Rings can be found in our online store. If you need assistance finding the proper Aegis Rings for your application, we are able to help you source the rings.

Whenever you have a motor with grounding rings installed, you should add visually examining the Aegis Rings to your preventative maintenance checklist. Like most parts of a motor operation, grounding rings can be negatively affected by contamination. While checking your motors, inspect the rings for any sign of contamination that can cause ineffectivity of the ring. Aegis now offers a 2 year warranty against fluted bearings when you register your rings with Aegis.

Although the Aegis Shaft Ground Rings are a trustworthy solution to protect your motors, it is not the only solution to prevent damaging currents emitted by VFDs. Another solution that we highly recommend you utilize in conjunction with the Aegis Rings is CoolBlue Inductive Absorbers. CoolBlue are common mode chokes that are installed by placing the CoolBlue rings around the motor’s lead wires. To learn more about CoolBlue and its benefits, read “How CoolBlue Protects Your Motors and Bearings.”

If you are utilizing VFDs in your operations, you should consider protecting your bearings from damaging currents through the use of Aegis Shaft Grounding Rings and CoolBlue.