Motor and drive efficiency: a collective approach
01 April 2015
Efficiency, reliability and total cost of ownership are all terms that a project engineer will be familiar with; however, looking at items of equipment on an individual basis may not deliver the maximum benefits. Far better to look at the wider picture says Markus Kutny.
In recent years the emphasis has been on improving electric motor efficiency, in part due to the changing regulations which have focussed both manufacturers and customers alike. However, in terms of overall efficiency within drive systems powered by electric motors, the mechanical system also needs to be considered as it offers very significant potential for optimisation.
Between the electric motor and the final driven process is a train of mechanical components - gears, couplings and bearings that transmit the motor torque and moderate the output speed. By examining each component within this mechanical power transmission chain and optimising its design, further efficiencies and savings can be made beyond that of the motor itself.
It is generally accepted that electric motors account for a significant majority of the energy consumed by industry. Perhaps a lesser known fact is that 96 percent of the lifetime costs of an electric motor is associated with its energy consumption. Efficiency is thus more important in terms of total cost of ownership (TCO) than the initial purchase cost of the motor.
High efficiency motors
PMS (permanent magnet synchronous) motors already fulfil the requirements of the soon-to-be-implemented IE4 (Super Premium Efficiency) classification. This is evidenced by their potential to achieve energy savings of as much as 40 percent over that of an IE2 inverter-driven squirrel cage motor.
PMS motors offer considerably improved efficiency when compared to induction motors even under partial load conditions; and extremely high efficiency under rated operating conditions. They also have considerably higher power density, which, for geared motors, yields higher system efficiency with minimal installation volume. Moreover, a PMS motor will maintain constant speed independent of the load and voltage variations, as long as the mains frequency is kept constant.
The low operating and maintenance costs of PMS motors mean that they provide optimum energy savings when driving fans, pumps and compressors, as well as being ideal for constant torque applications such as elevators and conveyors.
From January 2015 onward, energy efficiency class IE3 (Premium Efficiency) became the standard for motors with rated power between 7.5kW and 375kW; from January 2017 onward, this will pertain to motors with rated power between 0.75kW and 375 kW. Inverter-controlled motors are exempt from this regulation, and for such installations, IE2 is sufficient.
Of course, this improved technology comes at a price; but while efficiency translates into a higher initial purchase cost, this is recovered quickly as a result of operational gains. Supporting evidence for this was provided by a trial conducted by Bauer Gear Motor in partnership with Danfoss (reported in DPA January 2013 page 32).
Drive train design
The design of modern drive trains requires specialist knowledge and expertise in order to provide a system that not only meets the specifications, but also delivers high reliability, high efficiency, smooth running characteristics and low noise output. However, further benefits of a good design can be gained by ensuring that the drive train is properly integrated into the application, which requires an in-depth understanding of the industry for which it has been designed.
Less efficient designs such as worm geared motors can be replaced with integral helical bevel geared motors, which offer longer service life in addition to greater efficiency. This approach may also remove the need for a coupling, thereby providing even greater transmission efficiency.
In some cases it can be beneficial to streamline this process and specify a bespoke system built from existing components but integrated seamlessly into the machine in order to deliver an optimally performing arrangement. Closer attention to these issues at the outset of a project will ensure that greater efficiencies are gained, both in terms of cost and energy.
That crucial link between the electric motor and driveshaft - the power transmission coupling - can have a significant impact on the performance of the machine or equipment. Choice of coupling will depend on the application, maintenance requirements and torque capacity; specifying a coupling incorrectly can lead to inefficient power transmission as well as increased maintenance costs.
Couplings are designed to accommodate shaft misalignment, which may be present when one of the connected shafts is located by a self-aligning bearing, or when an unsupported, intermediate shaft is placed between the driver and the driven load. Couplings capable of overcoming true angular misalignment include the single universal joint with its capacity to handle large offsets and torsional damping.
If both shafts are assembled in self-aligning bearings then zero misalignment can be achieved, allowing the use of a solid coupling which simply supports the shaft in perfect alignment. Before installing a solid coupling, however, an interesting test is to try a flexible coupling first.
With the machine at normal operating temperature, measure the speed and/or the current drawn by the motor. The difference between these readings and those with the solid coupling indicate the losses generated by the flexible coupling – a good demonstration of the extra savings that can be made by spending some additional time attaining proper shaft alignment.
The basic construction of flexible couplings consists of two flanges or hubs, which attach to the shafts being coupled and a connecting element that may be metallic, such as in disc couplings; it may also be a sleeve made from elastomeric material, such as EPDM rubber, neoprene, Hytrel or urethane, or it may be mechanical, as in a u-joint or gear coupling.
A joined-up approach
Combining the efficiencies of individual drive train components into one application can yield impressive results. Moreover, further efficiencies may be obtained when the overall design is undertaken by a single, specialist supplier who is able to provide these components matched with, and designed to operate seamlessly with, one another. A single point of supplier contact may also prove beneficial in terms of training or maintenance.
Mechanical efficiencies aside, let’s not forget the motor. Making significant improvements to the mechanical drive train in terms of efficiency, can result in a reduction in overall power requirement from the motor itself. Installing a smaller motor on a more efficient mechanical drive train comprising matched components will result in further energy consumption reduction and improve reliability.
Markus Kutny is product manager, Energy Efficiency Solutions at Bauer Gear Motor
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