Resources:
Bauer Gear Motor, member of the Altra Industrial Motion Group
Danfoss Group

The difference between a synchronous and an asynchronous motor

Regulations in many countries now mandate a minimum energy efficiency for electric motors that are at least 1 hp. But does it make sense to replace motors smaller than 1 hp in the interest of energy efficiency?

That was one of the questions a German wastewater-treatment plant tried to answer with some recent tests. Workers there took measurements to gauge the economics of replacing an ordinary 0.37-kW (0.55-hp) induction motor with a more-efficient design for powering a disc thickener, a rotating sieve that filters water from sludge and mud. In use, the motor rotates the sieve with a constant torque because the sludge has a constant consistency. The new motor, a synchronous permanent-magnet design, ended up using about 40% less energy than a conventional induction motor.

The original induction motor created 2.62 Nm of torque at 1,350 rpm and operated with 61.5% efficiency, and dissipated 0.26 kW/hr. The new motor, from Bauer Gear Motor, Somerset, N. J., created 3.5 Nm of torque at 1,500 rpm and operated with an efficiency of 87.7%, dissipating 0.16 kW/hr. Both motors operated from an inverter from Danfoss, Baltimore, and used the same Bauer 381:5 gearbox which was 94% efficient.

Estimates are that over four years, the induction motor would consume 2,657 kW-hr compared to 1,635 kW-hr for the synchronous PM motor.

Bauer personnel involved in the test note that there are moreefficient induction motors available and that comparisons with one of those motors would not be as dramatic. Nevertheless, the synchronous PM motor would have still saved energy and been less expensive to boot. An induction motor with premium efficiency for this application would have demonstrated at least 82.5% energy efficiency, weighed 88 lb, and cost about $905, say Bauer personnel. The 87.7%-efficient synchronous PM motor used in the test weighed 55 lb and cost about $879.

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