Traction – why it is so important for the Hyperloop energy efficiencyMarch 19, 2019
Traction is the system that converts electrical power into forward motion, and recovers that energy during braking. Hyperloop may use conventional electric motors driving wheels, or may use maglev with linear motors for traction.
For simple constant-speed cruising, traction is required to overcome wheel or maglev resistance, and aerodynamic drag which will be low in the vacuum. But if the route requires a lot of high-speed acceleration, inefficient traction can cause greatly increased energy consumption.
It takes massive energy to accelerate to a high speed, and not all is recovered by regenerative braking. Traction losses in both the acceleration and braking phases are combined to give the overall energy cost. A 70% efficient linear motor will consume 3.4 times the acceleration energy, compared to a 90% efficient rotary electric motor (The linear motor uses 143% on acceleration, and only recovers 70%, for a loss of 73%, compared to 21% for the rotary motor).
Hyperloop using maglev
A Hyperloop system using maglev must use linear motors, as there is no physical contact with the track, and traction efficiency becomes a major challenge.
A linear motor has a rotor and a stator, like a rotary motor, but the stator is built into a track.
Linear motors would greatly increase Hyperloop’s construction costs, as the expensive track runs along the full length of the route.
Linear motors are less efficient than rotary motors for 3 main reasons:-
1) To control the costs, the stator on the track cannot have the expensive materials and precision construction used in modern high-efficiency motors.
2) A much larger air-gap is necessary for a linear motor, to give the clearance necessary for high speed. This reduces the thrust and efficiency of the magnetic fields.
3) There are ‘end losses’ for the magnetic fields, due to the limited width of the track.
Hyperloop using wheels
Using wheels for Hyperloop would give superior traction efficiency, 90% or better. However, the wear and tear of the wheels will increase the operational expenses. It is up to designers to decide whether the projected usage makes sense to invest into linear motor or keep things simple with rotary motors. An added benefit of standard motors is that they can accelerate and recuperate energy wherever and whenever it is needed. The capsule using LIM needs to follow a pre-designed acceleration and braking profile, which greatly reduces flexibility.
Tesla uses a high-performance motor, driving the wheels via a single-speed gearbox. The wheels convert rotary power to linear traction with almost perfect efficiency.