U.S. motor-driven system energy savings
Efficiency of motor-driven systems: Motor-driven system regulations are changing in the U.S. to improve efficiencies beyond individual motors. Considering the motion control system can deliver more than six times the power savings compared to motor efficiencies alone, according to NEMA committee members. Power electronics are helping.
Electric motor system efficiency and power electronics are impacting future energy saving opportunities as attention moves from motor efficiency to motor-driven system energy savings. In the U.S., the first move was to apply motor efficiency regulation. Next is addressing the system. Focusing on the motion control system can deliver more than six times the power savings compared to looking at motor efficiencies alone.
Stage 1: motor as a component
U.S. motor efficiency regulation began in 1992. At this time, the motor already included efficiency data on each name plate. The U.S. Congress drafted legislative language that included definitions of products to be included as well as referencing test methods and efficiency levels. The motor community began a journey into federal regulation that continues today.
In 2001, motor manufacturers created a premium efficiency level that raised the efficiency levels above the federal requirement (NEMA Table 12-12). In 2010, this new level was added to a second round of regulation bringing the markets greater savings to a select category of motors know as Subtype 1, required to meet NEMA Table 12-12, and Subtype 2, required to meet table 12-11. These categories impacted approximately 40% of the units sold each year. In 2014, a third round of regulation was released that will take effect June 1, 2016. This latest round of regulation essentially covers all polyphase motors from 1 hp (0.75 kW) to 500 hp (375 kW). This latest round of regulation was carefully constructed to expand the scope of products to be covered while paying particular attention to unintended consequences that would defeat the energy saving goal of the rule. The motor community worked diligently with the energy advocates and the U.S. Dept. of Energy to create a regulation that took into account mechanical and electrical issues that would undermine the regulation if an efficiency level were to be increased to a point that triggered any or all of the following 10 consequences.
Higher efficiency risks
There were 10 areas considered by the group as "potential consequences" or risks to raising efficiency levels beyond the NEMA Table 12-12 (IE3) levels in existing rules.
- Torque necessary to start loads may be compromised potentially requiring a jump to a higher motor horsepower rating.
- Power factors may diminish, forcing utility or plant modification to correct.
- Inrush currents may increase, requiring cable and switch gear replacement.
- Mechanical sizes may increase, eliminating retrofit ability.
- Repair of failed motors as an alternative may increase, placing previously unregulated product back in service for many more years.
- Motor supplier resources will be used to overcome design and application issues that are better applied to power management solutions.
- The remaining efficiency gains of the motor are extremely small when measured under controlled conditions. Under actual uses that include power quality and varying load conditions, the efficiency gain in industrial and commercial applications are questionable.
- Motor testing and lab variance may be outside their ability to accurately measure such small gains with necessary repeatability.
- The amount of active material such as copper, magnetic steel increases dramatically for an extremely small gain in efficiency (such as 11 kW motor needs about 60% to 70% more material to gain 1.3% efficiency increase).
- The investments needed to redesign and reconfigure production facilities would be better applied to the power drive system (motor + variable frequency drive) which delivers much greater energy savings than regulation in isolation of the motor as one component.
The Figure 1 graph from Michael Turner, Nidec U.K. Technology Center, provides a visual that helps explain motor energy loss (red) versus useful motor energy (blue). By continuing to peruse efficiency of the motor, the challenge would be to reduce losses, which in this example, are less than 5% the kW loss, exposing the motor user to one or more of the 10 issues discussed above. The greatest opportunity for energy saving (reduction in kWh) is achieved by adding the power converter to the motor-driven system to optimize the system and reduce the kW loss by 30%, 40%, 50%, or more. When seen in this perspective it becomes clear that the addition of the power converter to the motor-driven system has distinct advantages.
Avoid dampers, throttling
In the past many motors ran without the benefit of electronic speed control devices. Therefore, the control of the output volume of the pump, fan, or compressor had to be throttled by closing the valve, dampers, or vanes. Though this will adequately control the flow, this is by no means an efficient method. There are significant losses generated in the throttling mechanism and the motor's efficiency drops significantly at the lower operating speeds.
Next generation of savings, called Stage 2 here, for sake of discussion, will change the current metric from efficiency, expressed as a percentage, to energy savings (kWh) by employing power electronics within the power drive system to eliminate mechanical flow control devices. The European Union (EU) began looking at the system in EN 50598, which was published in December 2014. This was an excellent start in establishing a guideline for energy savings as it relates to the entire system.
This work is taking place today in an international standard Working Group (WG) 18 within the International Electrotechnical Commission (IEC). WG 18 is creating the international standard IEC 61800-9-1, 2, which establishes the needed methodology and necessary metrics. The work continues on establishing energy efficiency levels for entire systems. Figure 2 shows all the components that make up the system.
Learn more about stage 2 and see additional information about the authors.