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Saving energy with PM synchronous motors

04 January 2013

A direct product comparison test was recently conducted to see how a Bauer Gear Motor IE4 rated, super premium efficiency permanent magnetic (PM) synchronous motor performed against a standard asynchronous motor. The test was carried out on a Huber disc thickener, at a waste water treatment plant in Germany, in partnership with inverter drive specialist Danfoss.

Bauer believes the market is gradually coming round to the fact that energy efficiency has to be one of the key determining factors when specifying a motor. Energy prices are only going to go one way, so it is important that a motor’s lifelong running costs are considered, rather than simply the cost of procurement. The company developed this new range of super premium efficiency permanent magnetic synchronous motors (PMSM) in anticipation of the new IE4 classification and customers’ needs for higher efficiency drives.

The PMSM series features an efficient design of rotor with embedded rare earth magnets that replaces the squirrel-cage construction of conventional LV induction motors. The design offers a number of key benefits; it eliminates heat losses from the rotor, while total losses are reduced by approximately 25 percent to give an overall improvement in total efficiency of 10 percent or more. This translates into a lower total cost of ownership, a reduction in CO2 emissions, and ongoing savings that buffer against future increases in energy costs.

PM motors have been much in the trade news of late, but there is still reluctance in the market to buy them, as the purchase cost is higher than that of standard motors. In some light duty applications, where the motor is rarely on, it is still more economical to specify a standard motor, but if the duty cycle is high then a PM motor can quickly deliver a satisfactory return on investment and then go on to achieve operational savings for a long time to come.

The test
The disc thickener mentioned in the introduction to this article is in continuous operation for seven hours per day, seven days a week, so it provided an excellent opportunity to test the PMSM against a standard asynchronous motor (ASM). Prior to the test the existing ASM was controlled by a standard inverter, so prior to the PMSM being installed, a Danfoss inverter drive was retrofitted to the original system to achieve a fair comparison.

The frequency inverter was programmed to monitor the loads on each motor to ensure that they ran at optimum efficiency. In order to ascertain that any differences in efficiency could be attributed to the motors, each drive used the same Bauer gearbox with a reduction ratio of 381:5 and an efficiency of 94 percent.

It was found that with the frequency inverter installed the ASM created 2.62Nm torque at 1,350rpm, achieved 61.5 percent efficiency and consumed 0.26kWh. Following its installation, the Bauer PMSM created 3.5Nm at 1,500rpm, delivering 87.7 percent efficiency and consuming just 0.16kWh. Thus the energy savings from installing the PMSM yielded a 40 percent saving in energy use over the ASM, both motors being controlled by the same inverter.

Over a four year period it was estimated that, with an inverter used in both cases, an ASM would consume 2,657kWh whereas Bauer’s PMSM would consume 1,635kWh - a total saving of 1,022kWh.

Bauer head of R&D, Jens Gabel says the aim of this development effort was to give customers very real savings in terms of their energy costs. “We are very happy with the results of the test as they show that super premium efficiency motors do deliver a real life measurable benefit and in many higher demand applications should certainly be considered both by design engineers and maintenance engineers alike.”

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