Electric Motor Systems
Electric motors are everywhere in industrial applications. They represent around 46% of total electricity consumption, or 7200 TWh/year. Around 64% of motors’ electricity consumption happens in the industry sector. The buildings sector accounts for 33% and the remaining 3% is consumed in other sectors.
In the industry sector, electric motors’ consumption tops 69% of total electricity consumption. Electric motors are the main source of savings in the non-electrointensive segments, which account for the biggest share of the energy consumption in the industry sector (© OECD/IEA, Motors 2011; © OECD/IEA, WEO 2012) (Fig. 5.4).
Electric motor systems are thus critical to optimizing energy efficiency, in particular within the industry sector.
Fig. 5.4 Share of electric motors in total electricity consumption (© OECD/IEA, Motors 2011; © OECD/IEA, WEO 2012)
Fig. 5.5 Electric consumption of various motor types (© OECD/IEA, Motors 2011; © OECD/ IEA, WEO 2012)
Around two billion electric motors are in use worldwide. Electric motors are highly standardized and have different power ratings. Motors with rated power below 0.75 kW have the largest share but consume only 9% of all electricity used by electric motors. The most electricity-intensive group of motors rates below 375 kW (and above 0.75 kW). There are around 230 million such motors in the world and they consume 68% of the total electricity consumed by motors, or around 31% of total worldwide electricity consumption. Finally, the largest motors (with rated power above 375 kW), consume 23% of total electricity of motors; 600,000 are in use (© OECD/IEA, Motors 2011) (Fig. 5.5).
Electric motors are used in four main applications: compressors (32% of electricity consumption), mechanical movement (30%), pumps (19%) and fans (19%).
To understand the efficiency of electricity consumption of a motor, it is important to consider the full system the motor is part of. Electric motor systems are made of the electricity supply system, the motor, and the mechanical equipment associated (transmission, gears, flow reducers, etc.). Electric motor systems can be extremely inefficient from an electricity consumption standpoint. According to the International Energy Agency (2011), 21% of electricity consumption could be saved between a traditional system and an energy-efficient system. The yield of the electric motor system could rise to 63%, compared to 42% for standard applications.
An energy efficient electric motor system is first made of high performance components. Mechanical parts can be selected with high-performance efficiency. For instance, transmission and gears can be selected accordingly. As an example, worm gears have high losses, other systems present better efficiency. Traditional transmission V-belts also have high friction when flat belts have a better yield. The efficiency of electric motors has considerably improved in the last few decades. High-performance motors (IEC60034/IE3 class) operate at around 90% efficiency for loads that vary between 50 and 125% of the rated power; while standard motors may yield as low as 50% efficiency when power is low. The quality of electricity also can impact the overall efficiency of the motor’s operation as the electricity supply produces the magnetic field that helps convert electrical power into mechanical power.
An energy-efficient electric motor system has thus to be first designed for efficiency. This means avoiding intermediary parts (gears, flow reducers, etc.), and designing the motor system to the actual nominal operation of the process. In many applications though, it will remain impossible to design exactly such a system since the speed and torque requirements vary over time. This is the case for applications such as fans, pumps, escalators, and cranes. In these applications, the use of variable frequency drives helps tune the electric power that is supplied to the motors. This can yield an additional efficiency as high as 30%. Variable frequency drives are thus critical to raising process and energy efficiency.