Automation Optimizing Energy Efficiency in Factory Production Systems
The steep energy demand of electric motors forces legislative bodies and manufactures to support energy efficient systems. How can a motor drive ecosystem contribute to this industrial trend?
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International energy reports1 show that in 2006 electric motors consumed more than 46% of the world’s electricity corresponding to 6040 Mt of CO2 emissions. This has led to demands on motor drive manufacturers to add advanced control functions and tools to support system energy efficiency optimization. A new motor drives ecosystem for optimizing energy efficiency in factory production systems is gaining widespread acceptance worldwide. The good news for Europe is that thanks to energy conservation policies industrial electricity consumption is now on a downward trend at rate of 1% per year2.
The most common uses of motors in factories are in pumps, fans, compressors and conveyers with a majority of these motors available as standard catalogue products. Small motors (under 0.75kW) used in small machines and appliances are 90% of the total global stock but account for less than 10% of motor energy consumption. However, medium size (0.75kW-350kW) industrial motors mostly account for almost 70% of the world’s motor energy consumption.
In a typical application only 50% of the electrical energy consumed by the system is converted to useful mechanical energy and this has attracted the attention of both energy regulators and factory operators across the world. Regulatory agencies in the US, China, Europe and elsewhere have introduced minimum energy performance standards (MEPS) over the past number of years. Premium motor-rated efficiency levels now range from 80% at the lowest power range to 96% in the largest motors so factory operators are looking beyond the motor to secure energy savings.
Figure 1 describes the main elements in an automated machine or process found in a modern factory. This approach provides a new capability to optimize the operation of machines and processes to add new levels of efficiency in energy and productivity.
Efficient Torque Production: Algorithms and Ethernet
There are multiple control layers that determine the efficiency of the motors and the overall production process. The first control layer adjusts the power inverter switching sequence to control the motor voltage and current and maximize torque production efficiency. Next is the position and speed controller that operates the machine efficiently. In process equipment, this could be driving the optimum pump flow rate while in automation equipment this could be a sequence of speed or position commands to execute an assembly function.
In the latter case, the response time of the speed control can be more critical to the machine controller than the efficiency of torque production. The communications and systems layer is growing in importance as multiple motors are now synchronized over high-speed-data networks also connected to the factory network. Process managers can start machines in sequence as they are needed rather than having them waiting in idle mode. Networked safety functions enable efficient starting and stopping of equipment minimizing downtime. Plant managers track motor drive operational and diagnostic data to improve process energy efficiency and reliability.
Motor efficiency is a function of the torque produced per amp supplied at any given speed and terminal voltage. Electric motors produce torque through forces that tend to pull their internal magnetic fields into alignment. In the ac motors in figure 2, these forces are generated through the interaction of stator and rotor magnetic fields. AC motors produces a constant torque when the stator currents are synchronized with the rotor motion to maintain continuous field misalignment. The ac motor speed is directly related to the frequency of the motor currents and speed control requires a variable frequency voltage source.