Electricity + Control May 2019

DRIVES, MOTORS + SWITCHGEAR

crew wants to be limited in finding replacement motors due to special or non-standard motor di- mensions. Instead, the safe approach is to specify motors with standard IEC or NEMA dimensions from the outset. It will then always be possible to find alternative motors which physically fit the in- stallation. New technologies pack more power into the same motor frame size. This means that the high-efficiency motor keeps the normative dimen- sions but is often one frame size smaller. Similarly, to win the greatest benefits in mainte- nance management, it is important to have spec- ified from the outset that the ac drive connected to the motor has the compatibility to adapt to any standard motor technology. This factor is key for straightforward motor interchange. Adaptability for difficult locations A fast and easy motor retrofit, even in difficult en- vironments and awkward, inaccessible installation areas, also places particular demands on the ac drive specification. Ideally, the drive should be suitable for every installation environment. It should comply to high temperature operability without derating, and su- perior ingress protection at the level of IP55/UL Type 12 or IP66/UL Type 4X, allowing for it to be mounted without a cabinet. Another useful quality is that the drive can be mounted decentrally in the area close to the motor. When decentral mounting is not possible, the best ac drives can be mounted with as much as 150 m motor cable length and still deliver full electromagnetic compatibility. This fa- cilitates flexible installation when, for various loca- tional reasons, the drive cannot be installed close to the motor. Due to the flexibility in motor technology, the most adaptable of the drives available support far more than just increased motor efficiency. They also open the door to using motor technologies that come in smaller motor frame sizes, when this is acceptable in an exchange situation, and this opens up possibilities for solving awkward instal- lation challenges. The energy efficiency discussion in the new millennium has pushed the development of new motor technologies and the reinvention of exist- ing, older technologies. Because industry now has

variable speed drives at its disposal, the older tech- nologies can be optimised for energy efficiency.

Motor- independent drives offer the simplicity of having one drive type installed in the entire plant, with the advantage of automatic motor adaptation to deliver optimal performance of any motor.

Advantages and drawbacks Each motor type has its advantages and draw- backs. Optimisation of the induction machine to improve its energy efficiency (that is, to improve its IE rating) can result in an increase in physical di- mensions. As outlined above, in retrofit situations, its size and weight can become problematic. When comparatively assessing the advantages of different motor technologies, the following fac- tors should be taken into account. - IE class is not a characteristic of a particular technology. - Technologies differ in the way that they behave under part loads. A motor using one particular technology is not intrinsically more efficient than a motor based on another technology. It depends on the application. - Different motor technologies influence the in- rush currents. Sometimes new circuit breakers are required when changing motor type and not using variable speed drives. - Designers need to consider the cos phi/power factor of the selected motor technology when dimensioning the drive. This may result in hav- ing to over-dimension the drive. Despite the higher initial cost, the return on investment may still be attractive due to the lower energy consumption. Technologies like synchronous reluctance motors (SynRM) require a different sizing of the drive. For SynRM, the lower cos phi/power factor often results in the need to oversize the drive Best fit Which applications and motor control strategies are best suited to which motor technologies? Almost every motor technology can run with the correct voltage/frequency ratio applied. How- ever, this basic control has its limitations and ad- vanced control is a superior means of harvesting the optimal motor efficiency under varying load situations and fast load changes. For induction machines where sizing depends on current (amps), an automatic energy optimisation (AEO) control strategy is best to get

Electricity + Control

MAY 2019

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