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Technologies for high efficiency

21 October 2014

With the next phase of regulations coming into effect in January 2015, motors with a rated output of 7.5-375kW shall not be less efficient than the IE3 or will meet the IE2 efficiency and be equipped with a variable speed drive (VSD). With this in mind, fluid-flow application engineers are reviewing different technologies available to improve efficiencies further, says Siegfried Kreutzfeld, managing director at WEG Motors

Induction motors are currently the most popular for fluid-flow applications, including pumps, compressors and blowers. One reason for this is their robustness, reliability and easy operation. They can be connected to the mains and do not require power electronics. When compared to IE3 motors, the recently introduced IE4 induction motors offer the highest efficiency for fixed-speed applications, with a reduction in energy losses ranging from 10% to 24%. However, when fed by frequency converters, they present a significant decrease in their efficiency, even in constant torque applications. In some applications, they may also need to be over dimensioned or equipped with forced ventilation.

Increasing use of VSDs

 

Significant advances in the power electronics and control techniques have led to an increasing use of VSDs as a way of increasing energy efficiencies in fluid-flow applications. In such applications, permanent magnet (PM) and synchronous reluctance motors can deliver IE4 or even higher efficiency levels in the same frame sizes as induction motors. In particular, WMagnet IE4 PM motors can be up to two frame sizes smaller than induction and reluctance motors, while WMagnet IE5 PM motors can be in the same frame sizes as induction motors, but weigh less, because they contain less active material.

Thus, PM motors deliver higher efficiency than induction motors at low speeds and do not need over dimensioning or forced ventilation. Additionally, they offer high torque-to-current and torque-to-volume ratios, compactness, and fast dynamic response. They also present the highest efficiency of all motors with high power factor, due to the absence of joule losses in the rotor and the excitation flux of the permanent magnets. All these advantages have led to a significant increase in the use of PM motors in a variety of applications – from appliances (washing machines) to industrial drives (compressors, pumps and blowers), servo drives and electric vehicles. However, PM motors have a higher capital cost compared to induction motors because they are equipped with rare-earth permanent magnets. Ferrite magnets could be used in low power applications where low cost is mandatory.

Switched reluctance motors have concentrated windings in the stator and laminated steel with salient teeth in the rotor, with no aluminium cage or permanent magnets. One clear benefit of this design is that – similar to PM motors – it ensures no joule losses in the rotor. Additionally – similar to induction motors – switched reluctance motors offer mechanical simplicity, robustness and reliability. However, they require more sophisticated electronic control than induction motors. These characteristics make them particularly suitable for a variety of applications - from appliances and industrial drives to electric vehicles – as well as wide speed ranges and high speed operation, including compressors and machine tools. For such applications, these motors could offer higher efficiency and power density than induction motors of equivalent power. However, their significant torque ripple could translate into high vibration and acoustic noise.

Reduced torque ripple

Contrary to switched reluctance motors, synchronous reluctance motors can be more easily designed to give reduced levels of torque ripple and acoustic noise and sinewave ac operation (rotating field). They use a conventional polyphase ac stator and their rotor features flux barriers rather than an aluminium cage or permanent magnets. Given these characteristics, synchronous reluctance motors – which are electronically controlled - can be used in a wide range of industrial and commercial applications where variable speed is needed. Similar to switched reluctance motors, they are well suited for wide speed ranges. However, maximum speed operation and performance are greatly dependent on the geometry of the flux barriers in the rotor lamination.

These motors are attractive because the cost of active material is comparable to that of induction motors and lower than that of high energy PM motors. Additionally, they offer easiness of rotor skewing, flux weakening capability (which is important for attaining high speed ranges) and suitability to large overloads. Synchronous reluctance motors can achieve similar energy efficiency levels to those of induction motor, however their power factor is relatively poor, with current being up to 40% higher than in an equivalent induction motor.

Key Points

  • Robust, reliable and easy to operate, induction motors are a popular choice for fluid-flow applications
  • Permanent magnet motors deliver higher efficiency than induction motors at low speeds
  • Switched reluctance motors offer mechanical simplicity, robustness and reliability

 


 
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