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Linear motors will drive Musk’s 1,220km/h Hyperloop system

13 August, 2013

Technology visionary Elon Musk has proposed a high-speed transport system powered by linear induction motors that would carry passengers at speeds of up to 1,220 km/h, and could cover the 615km distance between Los Angeles and San Francisco in about half an hour.

Musk, who made a fortune by founding the PayPal online payment company, currently runs the electric car manufacturer, Tesla Motors, and the space exploration venture, SpaceX. He has now outlined his vision for a high-speed transit system called Hyperloop which he believes could have significant advantages over other forms of high-speed travel.  

Up to 28 passengers would travel in 1.35m-wide capsules whisked along inside steel tubes containing air at a low pressure (100Pa) to minimise air resistance. The 3.5-tonne capsules, spaced about 30 seconds apart, would carry on-board axial compressors to create an air cushion to suspend the pod. The compressors would also help to prevent the build-up of air in front of the pod and provide part of the propulsion. Each compressor would be driven by a 325kW motor powered by a 1.5-tonne battery that could keep it running for 45 minutes.

The capsules would be supported by an array of 28 air bearing “skis” producing an air cushion 0.5–1.3mm between the pod and the tube walls. According to Musk’s calculations, the aerodynamic power requirements at 1,130km/h would be about 100kW. He estimates that the total drag generated by the air bearings at 1,220 km/h will be 140N, resulting in a 48kW power loss.

The main propulsion will come from a series of linear induction accelerators at intervals along the length of tube. Musk chose this form of motor rather than permanent magnet machines, partly because of the lower material costs – the “rotor” can be a simple, lightweight aluminium shape, and no magnets are needed. In addition, the lateral forces exerted by the stator are just 13N/m and are inherently stabilising, simplifying the problem of keeping the rotor aligned in the motor’s air gap.

Musk envisages creating the rotors from 15m-long aluminium blades integrated into the capsules, each one being 0.45m high and 50mm thick. Because the current will mainly flow in the outer 10mm of the blade, it could be made hollow to cut its weight and costs. The gap between the rotor and the stator mounted on the bottom of the tube would be 20mm on each side.

The 0.5m-wide and 10cm-high stators will be laid along 4km sections of the tube – long enough to accelerate and decelerate the capsule between 480 and 1,220 km/h and to accelerate at 1g. They would be installed near stations and at booster points about 110km apart along the line. Only about 1% of the tube’s length would need to be fitted with stators.

The three-phase stators, laid out symmetrically on either side of the rotor, will have one slot per pole per phase. A variable number of turns per slot will allow the inverter to operate at a nearly constant phase voltage, thus simplifying the design of the power electronics. The two halves of the stator may need to be braced to resist the 300N/m magnetic forces that will try to bring them together.

Each accelerator section will need two 65MVA inverters – one to accelerate outgoing capsules, the other to capture energy from incoming capsules as they slow down. The inverters will power only those sections of track occupied by capsules, thus improving the power factor, according to Musk.

Power for the Hyperloop will come from photovoltaic arrays mounted on top of the tube for most of its length. They will generate an annual average output of 57MW – more than enough to power the system. Because the peak power needed to accelerate and accelerate the capsules will be up to three times the average power, there will be lithium-ion battery arrays at each accelerator stage. According to Musk’s calculations, launching a capsule will need only 0.5% of the energy stored in the battery.

He estimates that the costs for the linear accelerators will be up to $140m, while the solar arrays and electronics will add a further $210m. The total cost of a complete 40-capsule Hyperloop system linking Los Angles to San Francisco would be around $6bn – less than 9% of the cost of a proposed high-speed rail link between the two cities. With a capacity to carry 7.4 million passengers every year, Musk reckons that one-way tickets could sell for around $20.

Elon Musk says that he does not have the time to develop the Hyperloop concept himself, but he has put a 57-page document outlining the system into the public domain and is asking for feedback on the proposal.

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