1.4.1 - Electric Vehicle Motors

Version 3

    Direct current motors


    These are "series" type motors known for their high torque on startup, used to equip the first electric vehicles in the late 19th century. Small series of experimental direct current motors with separate excitation were then used in the 1990s for greater speed control flexibility.

    Activated by very simple power electronics by direct use of battery voltage, these motors were nonetheless adversely affected by the thermal loss of joules (RI2) by the rotor and collector/sweep system, as well as by wear on the blades and brushes. An output of 90 % was barely achieved.


    Alternating current motors

     

    This was a veritable revolution in power electronics which enabled alternating current motors to be used for electric vehicles.
    In fact, an alternating current with variable frequency and voltage has to be created from the direct current of the battery, with several tens of amps intensity and three-phased for simpler motor technology. The inverter is therefore completed by a converter that enables the pulse width to be modulated.
    The system also has to be reversible to recharge the battery during deceleration or braking.

    At present, asynchronous or synchronous motors of various types are used
    - The asynchronous motor
    - The permanent magnet synchronous motor
    - The wound rotor synchronous motor
    - The variable reluctance synchronous motor


    The asynchronous motor

    This is a motor, the stator of which, fed by three-phase sinusoidal current, generates a rotating field inducing current into the windings by short-circuiting the rotor and causing it to rotate. As its name indicates, slip dependent on the charge and speed exists between the speed of the rotating field created by the stator and the speed of the rotor. If the slip is high, the output is not as good. More voluminous than other types of motors but more robust, it is well suited to the high horsepower of buses and agricultural or industrial vehicles. Example the 120 kW Solaris bus.
    It is the type of motor most in use today: progress made in the 2000s has made its application to lower horsepower electric vehicles attractive.

    The Permanent Magnet Synchronous Motor (PMSM)

    The stator fed by three-phase current generates a rotating field that causes synchronous rotation of the rotor, the source of a constant magnetic field created by a set of permanent magnets. Therefore there is no slip and its performance and output are both very good but, until the late 2000s, it was reserved for small and medium horsepower vehicles.
    With superior torque/weight and volume specific power, it makes little noise and can be used in wheels, which means mechanical transmission can be replaced with simple electric wires.
    Michelin has therefore developed the "active wheel", which contains a traction motor and an active suspension motor.

    Since the permanent magnets can be subject to reduction in magnetization due to significant heat build-up, it is important to make progress in controlling losses, iron losses due to hysteresis and eddy current in particular.
    In addition, the very powerful permanent magnets currently use rare earth elements, such as dysprosium, neodymium and terbium. China alone produces 95 % of these, half of which is earmarked for its own consumption. Growing world demand, together with environmental concerns in China, led to a significant rise in prices (5 to 10 fold) in the late 2000s.
    Alternative solutions to rare earths that are more abundant and less expensive had to be found. Prototypes were therefore presented in 2012, with comparable levels of performance to the solutions using rare earth magnets.

    Phoenix motor for an electric automobile.
    Photo taken from “images.popular-electric-vehicle”

    Synchronous wound rotor motor

    The rotor-driven magnetic flux previously created by magnets is, in this case, created by current circulating in the rotor which requires the use of brushes, unlike other types of motors known as "brushless". The weight of a wound rotor is greater than that of a magnet rotor. It obviously enables the risks linked to magnets to be avoided and action to be taken on the rotor flux and therefore the torque.

    The variable reluctance synchronous motor

    Reluctance is to a magnetic circuit what resistance is to an electric circuit. The operating principle of this motor stems directly from the so-called "maximum flux" rule, with the mild steel rotor presenting cyclically variable reluctance in opposition to the stator windings. This is a brushless synchronous motor, without any magnets or windings on the rotor. The simplest and most widespread application is obviously the stepper motor in which the rotor consists of a series of slots, fewer in number than the number of stator poles.
    The difficulty is to produce a rotor with a high-accuracy, cyclically variable radius, leaving a minimum air gap. In the case of electric vehicles, these motors are mainly developed for outputs above 20 kW, which tends to put them into competition with asynchronous motors. The market share for these motors is still fairly low.

    Characteristics common to electric traction motors

    Illustrations of types fig 28, 29, 30 pages 62 and 63 A. DOUAUD book 

    High torque on startup, power practically independent from speed over a wide range and output between 80 and 95 % are the main differences between electric motors and combustion engines. In particular, this results in the possibility of connecting the motor to the wheels with one gear reduction ratio, that is to say without a gearbox.

    Which motors will be used to equip electric or hybrid vehicles in the future?

    At present, asynchronous motors are in the majority. Their performance is not as good as permanent magnet synchronous motors and they take up more space but they are not as expensive, although copper prices are also rising. In 2012, their sales increased again by 50 %.

    Variable reluctance motors - less expensive but noisier (whine) - are the most direct competitors, in particular for high horsepower vehicles; their market share is nonetheless the lowest at present.

    Permanent magnet synchronous motors and wound rotor types are already in use on medium horsepower electric vehicles and on hybrid vehicles. The use of other materials for magnets to replace rare earths should foster the development of this type of motor, particularly for insertion into wheels.

    It can therefore be seen that it is not from electric motors that major performance gains will be achieved for electric vehicles.