VFD efficiency: Three best practices

The use of variable frequency drives (VFDs) has many benefits including improved process control, energy savings, higher reliability and reduced wear and tear.


VFD is used for variable demand control of the centrifugal pump. Courtesy: Motion IndustriesThe use of variable frequency drives (VFDs) to control the speed of an AC induction motor has many benefits including improved process control, energy savings, higher reliability and reduced wear and tear. AC motors are very common in manufacturing and processing plants. When a motor is run at half its maximum speed, it consumes significantly less energy than it does at full speed.

For centrifugal loads (fans and some pumps) the power at half speed can be as little as 1/8th the base speed power. When using a VFD, motor speed can be changed almost instantaneously to address load and process changes (temperature, pressure, force, etc.). An added benefit is its ability to increase the precision of process control with the ability to control motor speeds to within 0.1% tolerance.

A VFD can provide a "soft start" capability for a motor (that is, the motor can be ramped up to desired speed instead of being turned on at full RPMs), decreasing the mechanical stresses associated with full-voltage startups. This results in lower maintenance costs and a longer motor life. In cyclic loads, the VFD also helps to avoid motor overheating.

VFDs have advanced over the years in both functionality and high speed switching technology. The output waveform is not a perfect sine wave, which can present some challenges that can be overcome by following some best practices upon installation.

Here are three best practices when looking to improve VFD efficiency:

1. Specify/install an input line reactor

Transient voltages on the AC power lines can cause inrush currents to a VFD drive, resulting in an overvoltage condition of the DC bus. These transient voltage conditions are often caused by utility capacitor switching and will cause VFDs to shut down without warning. The addition of a line reactor will limit the magnitude of inrush current. This current prevents trips and component failures and reduces the amount of potential downtime.

A line reactor will also reduce input line distortion, which is caused by the nonlinear characteristics of drives. The line reactor will limit the inrush current to the rectifier, rounding the waveform, reducing the peak currents, and lowering the harmonic current distortion. High-peak currents may cause distortion of the voltage waveform. The reduction of those peak currents also reduces total harmonic voltage distortion and mitigates harmonics sent back on to the line.

2. Specify/install shaft grounding rings

Due to the high-speed switching frequencies in pulse width modulated (PWM) inverters, variable frequency drives induce shaft currents in AC motors. The switching frequencies of insulated-gate bipolar transistors (IGBT) used in these drives produce voltages on the motor shaft during normal operation through parasitic capacitance between the stator and rotor and discharge through the bearings and can cause pitting, fluting, and premature motor failure.

Shaft grounding rings for AC motors divert harmful shaft voltages to ground and extend bearing life. Most motor manufacturers stock standard motors with grounding rings installed internally. They can also be added externally in the field or installed internally by a motor repair facility.

3. Install an output filter for long motor lead lengths over 100 feet

The inverter section of a drive does not produce sinusoidal voltage, but rather a series of voltage pulses created from the DC bus. These pulses travel down the motor cables to the motor. The pulses are then reflected back to the drive. The reflection is dependent on the rise time of the drive output voltage, cable characteristics, cable length, and motor impedance. If the voltage reflection is combined with another subsequent pulse, peak voltages can be at a destructive level.

Gary Jacott, Motion IndustriesOne IGBT drive output can have reflected wave, transient voltage stresses of up to twice the DC bus. Research has indicated that the fast switching capability of the IGBTs, along with an excessive lead length between motor and VFD, will contribute to reduced motor life. To reduce problems, use an output filter such as:

  • Line reactors at the inverter output (typically protects to about 500 feet)
  • dv/dt filter (RLC-Resistance, Inductance, Capacitance) at the inverter output (typically protects to about 2000 feet)
  • Sine filter at the inverter output (not distance limited)
  • Snubber circuit at motor (not distance limited).

These devices reduce the rise (dv/dt) and reduce the voltage level seen at the motor terminals. VFD-rated cable is also recommended.

These best practices are good rule-of-thumb recommendations and will help prevent premature motor failure and improve system reliability. See the VFD manufacturer's installation manual and the motor manufacturer's guidelines for more specific details.

-Gary Jacott has spent 30 years as an electrical engineer, including 20 years specializing in the industrial automation niche. A graduate of the University of Minnesota, Gary currently works with Motion Industries' Process Pumps & Equipment division in Omaha, Neb.

Top Plant
The Top Plant program honors outstanding manufacturing facilities in North America.
Product of the Year
The Product of the Year program recognizes products newly released in the manufacturing industries.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
October 2018
Tools vs. sensors, functional safety, compressor rental, an operational network of maintenance and safety
September 2018
2018 Engineering Leaders under 40, Women in Engineering, Six ways to reduce waste in manufacturing, and Four robot implementation challenges.
GAMS preview, 2018 Mid-Year Report, EAM and Safety
October 2018
2018 Product of the Year; Subsurface data methodologies; Digital twins; Well lifecycle data
August 2018
SCADA standardization, capital expenditures, data-driven drilling and execution
June 2018
Machine learning, produced water benefits, programming cavity pumps
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
October 2018
Complex upgrades for system integrators; Process control safety and compliance
September 2018
Effective process analytics; Four reasons why LTE networks are not IIoT ready

Annual Salary Survey

After two years of economic concerns, manufacturing leaders once again have homed in on the single biggest issue facing their operations:

It's the workers—or more specifically, the lack of workers.

The 2017 Plant Engineering Salary Survey looks at not just what plant managers make, but what they think. As they look across their plants today, plant managers say they don’t have the operational depth to take on the new technologies and new challenges of global manufacturing.

Read more: 2017 Salary Survey

The Maintenance and Reliability Coach's blog
Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
One Voice for Manufacturing
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Maintenance and Reliability Professionals Blog
The Society for Maintenance and Reliability Professionals an organization devoted...
Machine Safety
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
Research Analyst Blog
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Marshall on Maintenance
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
Lachance on CMMS
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
Material Handling
This digital report explains how everything from conveyors and robots to automatic picking systems and digital orders have evolved to keep pace with the speed of change in the supply chain.
Electrical Safety Update
This digital report explains how plant engineers need to take greater care when it comes to electrical safety incidents on the plant floor.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
Randy Steele
Maintenance Manager; California Oils Corp.
Matthew J. Woo, PE, RCDD, LEED AP BD+C
Associate, Electrical Engineering; Wood Harbinger
Randy Oliver
Control Systems Engineer; Robert Bosch Corp.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Design of Safe and Reliable Hydraulic Systems for Subsea Applications
This eGuide explains how the operation of hydraulic systems for subsea applications requires the user to consider additional aspects because of the unique conditions that apply to the setting
click me