Record-breaking motor is ‘five times more powerful’
Researchers at Siemens have developed an electric motor that weighs just 50 kilograms, yet delivers a continuous output of about 260kW – about five times more than conventional drive systems of a similar weight.
The motor has been designed especially for use in aircraft. Its record-setting power-to-weight ratio will allow aircraft with takeoff weights of up to two tonnes to use electric drives for the first time.
To develop the new motors, Siemens' experts scrutinised all of the components used in earlier motors and optimised them to their technical limits. New simulation techniques and sophisticated lightweight construction have resulted in a drive system with a power-to-weight ratio of 5kW/kg. Conventional industrial motors of a similar power rating deliver less than 1kW/kg, while drive systems used in electric vehicles achieve about 2kW/kg.
The new motor, which is scheduled for flight-testing before the end of 2015, is designed to operate at 2,500 rpm, allowing it to drive aircraft propellers directly, without needing gears.
As part of the development programme, experts from Siemens’ Large Drives and Corporate Technology divisions cut the weight of the motor’s end-shield from 10.5kg to 4.9kg. This aluminium component supports the motor bearing and the propeller, which is fixed to a continuous drive shaft. “It’s subject to very large forces whenever the nose of the aircraft moves up or down, so it’s an absolutely vital component for the safety of the aircraft,” explains Frank Anton, head of Siemens’ eAircraft operation. “That’s why, in the past, it was always pretty solid and therefore correspondingly heavy.”
The researchers have now have come up with a prototype end-shield made of carbon fibre-reinforced polymers that weighs just 2.3kg – less than a quarter of a conventional component.
The use of a cobalt-iron alloy in the motor’s stator results in high magnetisability, while the permanent magnets in the rotor are arranged in a so-called Halbach array. Four magnets are positioned next to each other so that the orientation of each field is in a different direction. One result of this is that magnetic flux can be directed optimally with a minimal use of material.
A new cooling concept has also helped to cut weight. “Because of the high current density, we needed a smart way of dealing with the waste heat,” Anton explains. “We use direct-cooled conductors and directly discharge the loss of copper to an electrically non-conductive cooling liquid – which in this case can be, for example, silicone oil or Galden.”
The new motor is powerful enough to propel a four-seater hybrid-electric aircraft and not far short of being sufficient for a regional airliner, which will need 500kW–2MW of power. Such aircraft will reduce not only CO2 emissions but also noise.
“We’re convinced that that the use of hybrid-electric drives in regional airliners with 50–100 passengers is a real medium-term possibility,” Anton says.