Technical World – Electrical Machines Energy Efficiency & Electric Motors

Electric motors to drive pumps, fans and compressors are by far the largest consumer of the 15,000 GW of average power generated across the globe.

Typically these motors run high duty and at high loads, resulting in high annual energy inputs. As is the case in power generation, but even more so here, there are huge opportunities to improve efficiency with the use of new technologies. Over the last couple of decades there has been large-scale adoption of Variable Frequency Drives (VFDs) to a vast number of these applications, typical resulting in annual energy saving of between 20%-50%. The improvements are primarily due to the use of a frequency converter allowing the operating speed of the rotating equipment to be matched with the varied load profiles that are seen.

Whilst large market potential to convert fixed-speed installations to variable-speed still exists, there is now an urgency to introduce higher efficiency motors to replace typical induction motors that are used. The technology of choice is usually a Permanent Magnet machine, particularly as they exhibit much higher part-load efficiencies than any other type of machine, whilst also being more efficient at full load as well. Permanent Magnet machines are also usually more power dense, thus reducing the physical size of the motor. The main drawback is normally the cost of developing and/or producing the motor, so a payback analysis is required. Where the payback can be achieved in a short period of time, say 2 to 3 years, and considering that typical installations will have an average service life of over 20 years, the future benefits are considerable. An example efficiency map of a TPS Permanent Magnet motor is shown below;

Typical Part-Load Efficiency Map for TPS Permanent Magnet Motors

 

Example simple payback analysis:

 

Motor rating = 400 kW Average efficiency improvement = 5%
Annual duty = 6000 hours Annual amount of electricity saved = 100,000 kWhrs
Average load factor = 0.7 Annual cost savings at 10 c/kWhr = $10,000
Payback period = 2 years Affordable premium = $20,000

In many applications the adoption of Permanent Magnet technology has already proven very attractive, especially where there are other benefits which improve the payback analysis. In particular, where a gearbox was traditionally used in conjunction with a VFD and an induction motor, it makes for a much better solution to eliminate the mechanical gearbox and operate the motor at the speed of the pump, fan or compressor. For high-speed turbo-compressors, the use of a direct-drive Permanent Magnet motor has proven to be highly desirable, not just for efficiency reasons, but also for reliability, elimination of maintenance costs and ease of implementation. Where the motor and compressor operate ‘inside’ the working fluid, friction-less bearing technologies are employed to avoid the use of contaminating lubricants and/or wear in the bearings.

Turbo Power Systems have several Permanent Magnet motor products in production which are used for turbo-compressor applications up to 500 kW. Current and future developments will extend this range to several Megawatts. For all of these high-speed machine products, we design and manufacture our own power electronics equipment, ensuring an ideal match between the two parts and increasing the cost effectiveness of the system.

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