Axial Flux Induction Motor For Automotive Applications
Existing hybrid as well as electrical vehicles, such every bit Toyota Prius, Chevrolet Bolt, Nissan Leaf, as well as BMW i3 all utilization high-energy-density permanent magnet (PM) machines for electrical propulsion. The magnetic fabric is commonly sintered neodymium–iron–boron (NdFeB).
Squirrel cage induction motors (IM) convey been successfully used inwards electrical vehicles (GM as well as Tesla) as well as commercial vehicles (buses as well as trains). They are much cheaper as well as to a greater extent than robust although they tin create make to become the same torque density.
Currently, the interior permanent magnet synchronous motor (IPMSM) is the well-nigh mutual machine inwards use, but manufacturers are swell to develop drive motors alongside much lower magnet content. High torque density as well as efficiency are required, every bit good every bit a broad constant powerfulness range.
In an effort to ameliorate the torque density of automotive induction motors Evans Electric convey developed a double stator, copper rotor, axial flux induction motor. (AFIM) Like a typical squirrel cage induction motor, the AFIM eliminates the require for rare-earth permanent magnets entirely, yet matches the IPMSM for torque density as well as release energy efficiency.
Axial Flux Torque Density (Nm/L)
The earliest electrical machines were axial flux motors alongside the commencement paradigm built past times Michael Faraday inwards 1831. Axial flux machines [Figure 2] (aka. Pancake motors) offering high torque as well as powerfulness density values. Influenza A virus subtype H5N1 double stator architecture offers the highest torque density coupled alongside balanced axial forces. Further, the curt axial length as well as solid-core copper rotor structure (patented past times Evans Electric) farther heighten torque density values when compared to radial flux motors.
Conventional radial flux motor geometry [Figure 3] requires azimuthal move of magnetic flux through the rotor. Azimuthal flux path volumes final result inwards substantial parasitic eddy electrical flow losses unless laminates are used. Influenza A virus subtype H5N1 double stator, unmarried rotor axial flux geometry [Figure 2] has a shorter flux provide path that inherently avoids parasitic eddy electrical flow losses as well as results inwards a much smaller sum rotor volume. Radial flux machines weigh an average of 40% to a greater extent than than axial flux machines at the same output.
For example, the Telsa Roadster air conditioning induction motor outputs similar torque values but is dimensionally 3x the book of the Evans Electric AFIM which was designed to accomplish output torque density of 100 Nm/L.
AFIM torque density tin go farther increased past times reducing the impedance of the machine as well as increasing the bar number. Typical pole numbers are vi to 8 piece the utilization of high rotor bar publish increases torque as well as decreases torque vibration.
The utilization of copper inwards the disc rotor lowers rotor resistance which improves efficiency but tin restrain starting torque.
To obtain high efficiency the machine should go running at depression sideslip hence that higher pole publish is commonly required along alongside depression rotor resistance. However, this tin restrain the starting torque. This agency that the input impedance every bit a whole needs to go depression which does Pb to high input current. However, to become high powerfulness constituent the input reactance should go depression compared to the input resistance. This leads to the require for compromise.
In industrial motors deep bars or double cages tin go used but these options are non available inwards an axial flux design. However, the company steel rotor aids the starting torque since this appears every bit an effective high resistance rotor. The maximum torque tin go increased past times reducing the impedance of the machine as well as increasing the bar number.
Efficiency – Comparing IM as well as PM Machines
Efficiency is of import for hybrid electrical vehicle drive motors. With energetically ‘free’ excitation, depression key reactance as well as the powerfulness to convey a high pole count, permanent magnet machines tin go extremely low-cal inwards weight as well as highly efficient. This is specially truthful for industrial applications that involve a restricted speed range, pregnant a fixed in conclusion voltage.
In such applications the pattern strategy is to build inwards a high internal flux (which permanent magnets make admirably), hence that torque is produced alongside minimal input current. Then efficiency is high, as well as the motor tin go made low-cal inwards weight as well as physically small.
Traction motors for applications such every bit automotive, are unlike inwards that they must piece of occupation over a broad speed range. Automotive traction motors require a sure as shooting torque (‘base torque’) at depression speeds, upwards to what is called the ‘base speed’, as well as hence a roughly constant powerfulness over a speed hit from the base of operations speed to a maximum speed that mightiness go several times the base of operations speed. In traction motor applications using injure plain DC machines, this torque-speed characteristic is accomplished using what is called ‘field weakening’, or precisely reducing plain electrical flow at high speed.
The requirement for performance over a relatively broad speed hit tin forcefulness precisely about compromises inwards permanent magnet motor design. It is non possible to plow downward the permanent magnet flux to command in conclusion voltage at high speed. It is possible, however, to counter the permanent magnet flux alongside armature current, as well as this tin go done inwards permanent magnet fields if the permanent magnet flux is non likewise strong.
So permanent magnet machines built for broad speed hit functioning by as well as large convey relatively weak permanent magnet excitation. Then it is necessary to build a high score of saliency into the machine hence that torque tin go produced past times interaction of in conclusion electrical flow alongside that saliency. Thus the permanent magnet machines built for automotive traction functioning make non convey all of the attractive features ane would await of permanent magnet machines.
At depression speed as well as high torque they make non actually piece of occupation every bit permanent magnet machines: they are to a greater extent than akin to synchronous reluctance machines, using the interaction betwixt saliency as well as (large) in conclusion currents to make torque. At high speeds they employ much of their armature electrical flow capability to counter the permanent magnet flux, as well as this negatively affects efficiency at high speed. Such machines tin go made to go quite efficient at relatively depression torque flat as well as intermediate speed.
The Evans Electric AFIM achieves maximum efficiency of 96%.
Effective Efficiency of Traction Motors
To empathise the impact of motor losses, including PM drag loss on actual machine operation, nosotros straightaway endeavor to evaluate the effective efficiency of a machine alongside a realistic operating scenario.
Recognise that losses inwards machines come upwards from 2 sources. First, acceleration forcefulness requires electrical flow inwards the windings, hence resistive losses occur. Second, rotational speed produces loss from friction, windage and, well-nigh important, gist loss.
In permanent magnet machines at that topographic point is ever flux acquaint hence that at that topographic point volition ever go rotational losses. Induction motors tin go de-excited hence that gist loss tin go ‘turned off’ when the motor is non producing torque.
In actual functioning every bit a traction motor the induction machine is to a greater extent than efficient as well as has a substantial payoff because it tin go de-excited when it is non producing torque, eliminating electrical loss inwards that condition. Since those losses are acquaint only when, as well as to the extent, the induction motor is producing torque, hybrid vehicles are expected to go to a greater extent than efficient when induction machines are used for the drive motor.
Cost Analysis
Rare-earth fabric costs tin go upwards to several orders of magnitude to a greater extent than expensive than mutual steel as well as copper typically flora inwards IM [Figure 1]. The reduced book of an AFIM farther lowers fabric costs compared to a conventional radial flux IM. Influenza A virus subtype H5N1 reduction inwards motor fabric costs non only improves Hybrid as well as electrical vehicle profitability but also facilitates the long term tendency towards multi-motor EV powertrain architectures ie. dual as well as quad motor AWD.
Source: Evans Electric 
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