Variable-Frequency Drives

Variable-frequency drives (VFDs)—also sometimes referred to as variable-speed or adjustable-speed drives—allow induction-motor-driven loads such as fans and pumps to operate at rotational speeds ranging from 10 to 300 percent of a motor’s nameplate speed rating. By controlling motor speed to correspond with varying load requirements, retrofitting electric motors with VFD controls can increase motor energy efficiency—in some cases by as much as 50 percent. VFDs can also improve power factor and process precision, and they can deliver other performance enhancements and nonenergy benefits such as motor soft starting and over-speed operating capabilities. Using VFDs can also help eliminate the need to use expensive, energy-wasting mechanical throttling devices like control valves or outlet dampers. The wide range of VFD benefits includes:

  • Energy savings for some fan and pump applications
  • Improved process control and regulation
  • Precision process control for motors used in industrial applications
  • Built-in power-factor correction
  • Bypass capability to protect equipment from damage caused by power outages and emergencies
  • Protection from overload currents
  • Safe, precision-controlled motor acceleration

A number of helpful guides have been written over the years to support business decisions around VFD upgrades. One resource we have found to be particularly applicable is a comprehensive report published by Natural Resources Canada, Variable Frequency Drives—Energy Efficiency Reference Guide (PDF), which remains a highly-relevant resource for estimating the energy savings and cost-effectiveness of VFD installations in different market sectors and end-use applications. The guide also presents three case studies describing facility upgrades that incorporated VFD motor controls retrofits (see sidebar).

Case Study: Eddy-Current Drive Replaced with Variable-Frequency Drive

A stainless steel tubing company produces tubes on a drawbench that enables it to reduce tube diameter and wall thickness to match customer requirements. This facility would run its drawbench using a standard-efficiency, 150-horsepower (hp) motor with a rated speed of 1,800 revolutions per minute (rpm), coupled to a speed reducer that incorporated an eddy-current clutch—offering reliable but inefficient drawbench operation. The eddy-current clutch was replaced with a variable-frequency drive (VFD), and the original motor was replaced with a 200-hp, 1,200-rpm unit (the lower-rated motor speed was selected to deliver greater torque).

The eddy-current coupling system required 190 hp to reduce the diameter on a typical steel tube, while the VFD-controlled motor required less than 90 hp to drive the same process. Annual drawbench operating time was reduced by 623 hours following the upgrade, since the increased torque enabled the drawbench to reduce the tubes to the desired size using fewer draws for each tube. The energy consumption of the drawbench was reduced from 440,000 to 290,000 kilowatt-hours—with motor efficiency gains of 34 percent and an estimated simple payback period of only six months. Table 1 shows a breakdown of the energy costs and savings.

Table 1: Steel tubing facility improves operations and saves energy with VFD upgrade
Depending on your facility’s processes and demands, variable-frequency drive (VFD) upgrades can provide substantial savings with short payback periods.
Steel tubing facility improves operations and saves energy with VFD upgrade

VFDs tend to operate at high efficiencies, with a typical VFD operating efficiency of around 97 percent when a motor is fully loaded. VFDs for controlling motors larger than 10 horsepower (hp) commonly have efficiencies over 90 percent for loads greater than 25 percent of rated capacity, which is often considered the practical lower limit for motor loading using VFDs.

Appropriate VFD Applications

Ideal candidates for VFDs are loads where torque output increases with motor speed. As a result, the large majority of VFDs are installed on centrifugal pumps, fans, blowers, and most types of compressors.

Loads that require the same torque output for all motor speeds can also be appropriate for VFDs. However, the VFD must be carefully sized to ensure adequate starting torque, and the duty cycle fraction represented by low-speed motor operation will be limited. Examples of constant-torque loads are reciprocating compressors, positive-displacement pumps, conveyers, center winders, and drilling and milling machines.

Motor loads requiring that torque decreases as motor speed increases are the most difficult, though not impossible, to control with VFDs. These usually involve high-inertia applications, such as accelerating and decelerating large vehicles (for example, to deliver railway traction) and drives that control motors incorporating flywheel loading. In these applications, less torque is required to keep the loads spinning than is required to accelerate them. Custom-engineered solutions are often required to handle the extra heat generated in starting and stopping these loads.

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