Optimizing Energy Efficiency in Factory Production Systems

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FAULHABER Antriebssysteme

ROHM Semiconductor GmbH

MEV Elektronik Service GmbH

Analog Devices SAS

Maximum efficiency is achieved when there is maximum rotor-stator field misalignment. Motor efficiency also depends on motor construction and in particular the rotor field structure. In asynchronous induction motors (AIM) current flows in both rotor and stator winding and some minimum stator current is consumed in magnetizing the core. Permanent magnet synchronous motors (PMSM) are more efficient because they do not require any current to magnetize the rotor field. Ultra-high efficiency interior permanent magnet (IPM) motors generate additional torque by virtue of their salient magnetic core structure.

All of the above motors are used in industry depending on power and application requirements but the asynchronous induction motor is by far the most common by virtue of its simple construction and ease of use. Permanent magnet synchronous motors have a higher torque to weight ratio and the low inertia rotor structure makes it very suitable for high dynamic control in automation equipment. However, the AIM can be started when connected directly to a three-phase ac line and speed can be controlled using a simple frequency inverter. Before the present day focus on efficiency, it was typical to connect a fan, pump or compressor motor directly to the ac line and control the process using dampers, valves or a simple on/off control.

Open loop frequency control drops the power consumption of a centrifugal pump to less than 20% of full power when running at 50% speed while consumption is 50% of full power using an on/off control to reduce flow rate to 50%. The availability of such gains in system efficiencies encourages factory operators to retrofit inverters to existing fixed-speed motor applications.

These days, advanced algorithms can tune the stator voltage to regulate the rotor field and so optimize the efficiency. Drive manufacturers now supply standard drive “boxes” that can be configured for a variety of motor models and types. The latest analog and digital signal processing devices enable the introduction of advanced control to even cost sensitive inverter applications. Estimation algorithms calculate the angular position of the rotor field from stator current and voltage measurements alone.

Efficient Motion Control: Isolation & Communications

These sensorless control algorithms ease the adoption of highly efficient IPM motors to maximize process efficiency in applications such as compressors and conveyers. Higher power applications such as winding machines or large pumps still tend to use induction motors but the typical efficiency of 500kW motor can be as high as 96%. These drives typically include algorithms to optimize the motor efficiency and monitor the drive health. Traditionally, these drives have serial field bus connections that allow operational and diagnostic data to be logged by a local PLC. A growing trend is to connect the drives to the factory network using industrial Ethernet protocols such as Ethernet/IP or Modbus TCP to improve efficiency by coordinating the operation of multiple drives.

The combination of precision in motion control and communication timing enables shorter machine production cycles and reduces the amount of energy consumed to manufacture each part.

This is critical in automation systems where machine productivity and quality often has a higher weighting than the motor energy efficiency because of the capital invested in the equipment. Drive manufactures support automation applications with PMSM servo motors and drives designed for fast response and high precision in the speed and position control. A fast control processor coupled with precision voltage drive and current feedback delivers smooth dynamic torque control.

The high voltages and currents in the power inverter presents a challenge to the circuit designer because the isolation circuits must meet stringent electrical safety standards. High-speed magnetic isolation technology supports safe isolation of analog and digital signal voltages without compromising speed or precision. Precision A/D converters embedded in the encoder position provides up 24-bit resolution position feedback which enables high dynamic velocity control at speeds as low as 1 RPM.

This performance level supports automation applications such as multi-axis milling of precision machine parts, assembly of fine geometry integrated circuits or injection molding of cellphone parts. In addition to control precision, the motion timing of the motors needs to be tightly synchronized since a timing error translates directly into a trajectory error in multi-axis position control.

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