Brushless motors achieve quiet efficiency

Tom Shelley reports on the benefits to be obtained by turning to new designs of permanent magnet brushless motors.

Brushless permanent magnet motors and drives are being developed into designs that can act as wheel hub, run almost silent without cogging and achieve speeds in excess of 400,000 rpm. While more expensive than induction and brushed motors, their higher efficiencies normally pay back price differences in months rather than years and can be used in environments where other motor types are problematic. Dr Sab Safi of SDT Drive Technology in Southampton has prototyped integrated DC brushless motor/drive combinations that are almost silent. One such is a 150W continuously rated unit, 57mm in diameter, which produces only 35dB(A) of sound and a 350W continuously rated 87mm unit that can just about be heard running. Both motors are designed to run at up to 20,000 rpm and have integrated controllers so there are only two external wires in each case. Their quietness stems from an avoidance of torque ripple. Dr Safi says: "Measures taken to minimise torque ripple include skewed slots, special shaped slots and stator laminations, selection of the number of stator slots with respect to the number of poles, decentered magnets, selection of magnet thickness and magnetisation distribution." Since the cogging torque is produced by the PM field and stator teeth, Dr Safi explained that one needed to have a slotless winding to eliminate cogging torque. This, however, increases air gap, which in turn reduces the PM excitation field. To keep the same air gap magnetic flux density, slotless brushless motors use more PM material than slotted motors. To eliminate cogging torque without the need to skew or be slotless, he said that one needed to have a number of stator slots close to the number of rotor poles. To further increase the capabilities of proprietary brushless PM motor systems, Dr Safi uses a technique which he and two co authors published in 1995, based on research work they had undertaken at the University of Newcastle. This allows phase current to build up in a motor winding before back EMF reaches any significant level. He said this is "A step beyond the art of influencing electric-motor operating characteristics through various methods of weakening the motor magnetic field". In this way, electrical energy is taken from the supply and stored as magnetic energy as the winding current attains its initial high value. When the back EMF reaches its maximum level, mechanical output energy is obtained from both the electrical energy input from the supply and the magnetic energy stored in the windings. The YASA motors described in Eureka's September 2009 edition as used on the Morgan concept LifeCar and the Riversimple cars use axial magnetic flux motors, with permanent magnets on rotor elements on each side of the stator coils. Dr Safi remarks that the ideal configuration is not clear, but for lower powers up to 1kW, suitable for wheelchair applications, he favours using radial magnetic flux, with the permanent magnets on a hub inside or outside the stator coils. The large amounts of time and effort required to develop new designs of electric motor and make them efficient are a common theme in every conversation with developers. David Baillie of HT Servo displayed his latest frameless, brushless 'IntegratedThruster' at the recent Unmanned Underwater Vehicle Showcase and announced that his 32N thrust, 108mm diameter units are now fully 'productionised' and in service to provide the ability to hover in a new version of the Hydroid Remus 600 autonomous underwater vehicle. The thrusters were originally invented by Dr Suleiman Abu-Sharkh at the University of Southampton and described in a series of papers dating from 2001 to 2004. The motorised thrusters have no shafts, and use permanent magnets in a hub in whose centre is the propeller. They employ a sensorless drive which monitors rotor position from back EMF. Baillie said that this is sufficient to allow good speed control down to low speeds. The company also offers designs ranging from a size 50, 79mm in diameter, with a thrust of 25N up to a size 300, which has a diameter of 384mm and produces thrust of 2500N. Development times would be even longer if it were not for design software. Air Bearings of Poole uses Opera from Cobham Technical Services to allow it to extend the speed of its brushless spindle motors for printed circuit board drilling to beyond 400,000 rpm. Over the last decade, speeds have increased from 80,000 rpm to 350,000 rpm, allowing more than ten vias and holes to be drilled per second. Increasing rotor speed still further depends largely on reducing rotor mass, which has required going from induction motors to permanent magnet types. Opera incorporates integrated thermal modelling, which provides the means to predict temperature changes in the air bearing-mounted rotor. Integrated electromagnetic and thermal solvers help predict and minimise harmonic losses, which can be substantial as a result of the need to use solid, non-laminated rotors. In addition it is possible to calculate and so reduce or eliminate induced eddy currents in metallic components that are in close proximity to the rapidly rotating magnets. The bottom line, according to Neil Russell, the company's R&D manager, is: "The design automation now gives us great confidence that we can improve design throughput substantially, by between five- and ten-fold with the same head count." But as well as permitting greater rotation speeds, the other big advantage that permanent magnet motors offer is greater efficiency than induction motors. Because there are no resistance losses in the rotor cage bars, there is no need to induce current. In addition, permanent magnet motors work efficiently at low speeds, so for low speed applications, there is no need for a reduction gearbox. Julien Ollivier, marketing energy efficiency for Baldor says his company has for some time been selling a permanent magnet motor and drive system specifically targeted at cooling towers. Motor efficiencies are typically 97% over the load range 60% to 100%. Tests on two identical cooling towers at Clemson University in South Carolina in the US showed that the new system achieved input power savings of 11.8% compared to a traditional geared system, with high speed noise reduced from 82.3dB(A) to 74.4 dB(A) and reduced vibration.