Efficient Motor Management and Variable Frequency Drives Can Save Money, Reduce Energy Costs and Benefit Productivity

By Jonathan Susser | September 1, 2020

Electric motor-driven systems account for approximately 50 percent of all electricity generated in the world and can often represent some of the highest energy users in a facility. Effectively overseeing them with motor management practices, therefore, is critical to saving money, reducing energy costs and improving productivity.

Motor Basics and Applications

An alternating current (AC) electric motor is a machine that converts electrical energy into mechanical energy. It draws electric current and produces a rotational or turning force (known as torque) on a metal shaft that is connected to end-use equipment.

Many kinds of AC motors exist. The broadest distinction is between induction and synchronous motors. Induction motors are particularly common in commercial and industrial settings. They have a simple design but are tough and rugged and can withstand significant wear and tear. Within induction motors, the next breakdown is between polyphase or three-phase motors and single-phase motors, which are more prominent in smaller applications when less torque is needed or when there are incoming power supply limitations.

Induction motors have three layers of components: rotating components, housing components and fixed components. The rotating components include the shaft, shaft fan, rotor and rotor fans; the housing components include the end bells or bearing housings, the stator housing, cooling fins, junction box and fan shroud; the fixed components include the seals that keep contaminants out, the stator windings, core iron or lamination stack, and bearings on the drive end and non-drive end of the machine.

One of the most important parts of a motor is its nameplate. The nameplate contains data that is essential when it comes to replacing the motor, setting up its parameters, developing a motor management program and more. Information you’ll find on the nameplate includes the motor’s serial number/model, horsepower, speed, voltage, current draw, service factor, frame size, insulation class, power factor, efficiency and National Electrical Manufacturers Association (NEMA) design.

Motors are used in myriad applications and technologies, including refrigerators, dryers, ovens, boilers, pumps, chillers, vacuum systems and much more. These applications can be split into two categories: centrifugal loads and linear loads. Centrifugal loads include equipment such as pumps, fans and blowers; they’re what are known as “spinning” inertial loads. Linear loads are direct loads, such as process lines, crushers and elevators. They may also contain rotational “winding” loads.

Keeping Motors Running

To maximize a motor’s lifespan and reliability, it is important to run the motor at its desired operating conditions. These operating conditions include the following:

  • Phase balance for voltage and current
  • Vibrationally-sound mounting
  • Good shaft alignment
  • Clean environment
  • Max ambient temperature of 40 degrees Celsius (105 degrees Fahrenheit)
  • Constant load of 75-100 percent of rated load

But just as motors have desired conditions, they can also experience adverse conditions. Heat, in particular, is a motor’s worst enemy. It can degrade insulation and break down bearing grease prematurely — over half of motor failures are caused by mechanical failure in the bearings. Heat can arise from high ambient temperatures; blocked fans, vents or blowers; unbalanced voltage; single-phasing; harmonics from variable frequency drives (VFDs); or motor overloading.

To avoid heat issues and prevent catastrophic failure more generally, it is important to follow operations and maintenance guidelines, such as those outlined in the NEMA MG 1 Standard. Here are some preventive maintenance actions to consider:

Basic Preventive MaintenanceAdvanced Preventive Maintenance
Look/feel/smell/listen for worn parts, discolored wire; strong vibration, overheating; burning smells; excessive, unusual noisePerform a vibration analysis
Clean exterior surfaces and ventsConduct infrared thermography
Lubricate bearingsPerform current signature analysis
Perform laser alignmentEngage in periodic online monitoring
Check motor control center bolt/bus bar tightnessConduct remote monitoring

Motor Management

On average, an electric motor’s purchase cost accounts for only 2-3 percent of its total lifetime cost. The remaining 97-98 percent comes from the electricity needed to operate the motor. Therefore, every organization that uses motors should have strategies for managing their motor population, which can help produce energy and cost savings, reduce maintenance needs and downtime, and more.

A motor management program should consist of three broad components: purchasing, operations and maintenance, and repair versus replace.

Purchasing

Develop purchase specifications for obtaining new motors. Consider homing in on just one or two preferred motor manufacturers, and specify model numbers for particular applications. Having a standardized purchasing process makes it quicker and easier to order the products you need.

Also, be sure to consider motor efficiency over price tag. Set minimum efficiency levels for new motors and train your employees on evaluating life-cycle costs in addition to upfront costs. Efficient motors are initially pricier than their less-efficient counterparts (they’re also physically larger), but they can be substantially cheaper to operate.

Operations and Maintenance

Follow the maintenance practices discussed above to ensure your motors operate at peak efficiency. Focus first on critical motors with high usage rates.

Repair and Replace

Make sure you factor in energy costs when making repair/replace decisions, but generally, a large percentage of motors are repaired instead of replaced. When a motor needs repair, use a repair center that you trust. Prioritize ones that have third-party certification that verifies that the repair center can maintain efficiency and reliability through the repair process.

Develop motor repair specifications for your repair center to follow. Repair specifications are important because efficiency can easily be lost. As one real-world example, an electric motor repaired with lower-quality bearings that were not properly greased dropped 1.5 percent in efficiency compared to when the correct bearings were used.

Your motor repair center can also serve as a source of information and guidance. It may be able to help track your motor inventory, provide a failure history and perform maintenance, such as a vibration analysis.

Putting It All Together

Once your motor management program is in place, you can assess its impact by examining motor failure incidents and energy costs per unit of production. But your work shouldn’t end there. Your program is not a static entity — use what you learn to update and improve it regularly.

One tool that can aid your motor management program is a motor survey. A motor survey is a spreadsheet that helps track and evaluate a facility’s motor population to support repair/replace decisions, identify opportunities for energy savings and increase reliability. It can be time consuming and costly to create but highly beneficial to understanding how to move forward.

You don’t need to survey every motor in your facility, so start by grouping your motors into different categories, such as based on size (larger vs. smaller), significance (critical vs. non-critical), usage (high vs. medium vs. low) or reliability (frequent vs. less frequent failures). Typically, you’ll want to survey larger motors and ones that operate at least 2,000 hours annually.

Once you know what motors you want to survey, capture nameplate data and other information. The more data you enter into the spreadsheet, the more helpful and accurate it will be. Then, examine the spreadsheet for any discrepancies or opportunities for energy savings potential.

VFD Basics and Applications

One technology that is commonly used with motors is a VFD, sometimes called a variable speed drive (VSD), adjustable frequency drive (AFD), adjustable speed drive (ASD), or just inverter or drive.

VFDs add features to motors that previously either had to be performed manually or didn’t exist. For example, they provide speed control, offer energy-reduction potential from decreased speeds, enable process control for precise applications and have a high power factor at all motor loads.

VFDs are particularly useful for the centrifugal loads highlighted earlier, processes that are throttled or variable flow processes. Target larger motors with high annual operating hours, moderate to high horsepower and moderate load — motors with very high load will not benefit as much from a VFD.

Potential downsides to VFDs are that they require additional capital investment, can cause power quality issues upstream through harmonic distortion and may actually reduce motor life. There are strategies to mitigate these concerns, but it is important to screen motors, especially those with centrifugal loads, to determine the best candidates for adding a VFD.

Conclusion

Motors are at work all around us. Knowing how and where they are used and designing a program to effectively manage them can save you money and increase your productivity. The next time you’re at your facility, think about your motors. Capture nameplate data from 10 of them, plug the data into a spreadsheet and start exploring your potential savings.