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The heart of electric vehicles is an electric motor that supplies mechanical power and torque to the wheels. As for any other component in an EV, weight, efficiency and compactness of the motor are key parameters. Magnets mounted on electric motors are very often made of rare Earth metals like neodymium that are in short supply, and such scarcity adds to that of other materials needed to build batteries, so optimal design of electric motors is essential.
The deployment of EVs is gaining momentum for countering the effects of global warming, the extreme effects of which are felt at all latitudes. Electric machines are crucial components of any EV powertrain, and the target for increasing power density and efficiency at competitive costs is accelerating the design of new motor architectures.
General scheme of a motor drive
Figure 1 shows the most general scheme of a motor drive. The power converter generates regulated electric energy that is used to drive the motor, whose output is mechanical energy. Figure 2 represents a typical EV architecture with overall well-to-wheel efficiency, which accounts for all of the energy consumed. By comparison, an internal-combustion engine exhibits a well-to-wheel efficiency of 14% and 17%, depending on whether it is based on the Otto cycle or Diesel cycle.
Electric motors can be divided into DC and AC motors. Each motor type consists of two main physical parts: The stator is the stationary part, and the rotor is the rotating component. The main AC motor types use three-phase alternating current to generate a rotating magnetic field, the frequency and power of which are controlled by the power electronic circuitry that reacts to the accelerator.
AC and DC machines are based on similar physical principles. Speed and torque are the functions of voltage, current and magnetic flux. The simple equations of electromagnetic machines can be expressed as Tem= kI, where Tem is torque,2 k is the machine constant depending upon the magnetic material and I is the current supplied to the machine. An analogous relationship exists between the back-electromotive force voltage (E) induced onto the machine windings and the angular speed (ωr) of the machine rotor: E = kωr. The two equations are derived from the well-known Ampère’s force law and Faraday’s law of induction.
Motor requirements
The performance of any EV is affected by the electrical motor specifications and is determined by the torque-versus-speed and power-versus-speed curves of the motor. High efficiency, light weight, power density, small size, reliability and cost are essential requirements. Furthermore, the motor is subject to different operating conditions in response to frequent start and stop, climbing, acceleration and deceleration. This means that high torque at low speed is necessary for enabling proper starting and acceleration.
The EV motor also needs to feature high power at high speed and a wide speed range in the constant power region, as shown in Figure 3. The constant torque region is important at low speeds to provide a good start and overcome sloped routes. The constant power region determines the maximum EV speed on flat roads. When the base speed is achieved, the motor reaches its rated power limit, and the motor torque decreases proportionally to the square of speed. This range is different in different motor models, and it is a key parameter when choosing the proper EV motor.
Electric motor types
A general list of EV motors includes DC motors, asynchronous or induction motors (IMs), permanent-magnet synchronous motors (PMSMs), switched-reluctance motors (SRMs) and electrically excited synchronous motors (EESMs). All have a radial magnetic flux. Flux is perpendicular to the rotation axis. Recently, the axial flux motor has been attracting interest because it can provide substantially more power while having a lower mass density.
DC motors
DC motors are robust and easy to drive. They exist in brushed and brushless versions. The brushed DC motor is a mature technology that provides low cost, high torque at low speed and easy speed control—all important features for traction motors. However, brushed DC motors are not widely used in EVs because of their large size, low efficiency and maintenance requirements due to the brush and collector wear-out. There are many uses on a vehicle for which they are the optimum choice: windshield wipers, seat and mirror adjusters, electric steering, electronic throttle control, exhaust-gas recirculation, window lift and electric pumps, to name several. EV brushless DC motors use an electronic commutator/inverter system in place of the brushes and have a much higher efficiency. The stator coils are energized in a specific sequence by an electronic controller, which creates a rotating magnetic field that interacts with the permanent magnets on the rotor.
Asynchronous induction motors
The IM is very common in EVs because of its simple construction, high reliability even in harsh conditions, robustness, easy maintenance and low cost. IMs can be naturally de-excited if the inverter faults, an important safety feature for EVs. As the name implies, the rotor rotates asynchronously to the stator rotating magnetic field, generated by three coils, equally spaced and supplied by currents whose phases differ by 120°. The rotor is mainly of the squirrel cage type, with long bars running through it and shorted on both ends. The stator rotating magnetic field induces voltages and currents onto the rotor by Faraday’s law. The induced current then interacts with the rotor magnetic field, causing it to generate torque. The disadvantages of IMs are slightly lower efficiency compared with PMSM motors, the higher heating loss generated on the rotor because of induced current and a relatively low power factor. The machines are commonly designed to have multiple pole pairs for increasing frequency by given mechanical speed, which reduces the machine’s size and weight.
Permanent-magnet synchronous motors
PMSMs have permanent magnets in the rotor. They have a simple construction, higher efficiency than IMs and high power density. In a PM AC machine, the rotor contains from one to several pairs of PM poles. The current fed to the stator interacts with the PMs on the rotor to generate torque. The drawbacks of this type are high costs, eddy current loss in PMs at high speed and a reliability risk because of the possible breaking of the magnets. There are two variants of PMSM motors: surface-mounted permanent magnet (SPM) and interior permanent magnet (IPM) synchronous motor drives. IPM motors, largely used in EVs, have better performance than SPMs, but the downside is their complex design.
Torque is generated in the interior permanent-magnet machine by two mechanisms: the interaction between the PM flux and the supplied current, and the interaction between the supplied currents and the iron material to create a reluctance torque. This happens when a ferromagnetic object, placed in an external magnetic field, gets magnetized and aligns with the external field. The torque is generated between the two fields twisting the object around the line with the magnetic field.
The picture in Figure 5 represents the 220 PMSM model from BorgWarner running at 18,000 rpm, capable of delivering a torque from 250 to 450 Nm. Power ranges from 135 to 300 kW, and the operating voltage is 300–800 V.
Switched-reluctance motor
An SRM exploits the magnetic reluctance3 of the rotor and stator to generate torque and control the speed. The rotor is made up of salient poles (poles extending outwards), while the stator contains concentrated windings. The rotor and stator are separated by an air gap, and the rotor poles align with the stator poles when the motor does not spin. The number of rotor and stator poles determines the motor’s torque and speed characteristics.
One of the key advantages of SRMs is their high torque density, i.e., a higher amount of torque for the size, an important feature wherever space is limited, as in EVs. Another advantage of SRMs is their ability to provide accurate control of the rotor position and speed. The absence of magnets, by avoiding issues with mechanical forces, enables the motor to operate at a higher speed. Because the phases are not connected, SRM motors can continue to operate even when one of the phases is disconnected. The drawbacks of this motor type are increased vibration and acoustic noise. In addition, the salient-pole rotor and stator construction cause a high torque ripple.
Electrically excited synchronous motor
PMSMs exhibit the best efficiency of all, but rare Earth materials are a problem. Some manufacturers, such as BMW and Renault Groupe, thus use a hybrid motor design, i.e., a synchronous motor without rare Earth materials. In fact, instead of using permanent magnets in the rotor to create currents, these motors use brushes and slip rings. The EESM stator is supplied with alternating current, creating a rotating magnetic field. Direct current is concurrently supplied to the rotor to generate a magnetic field that aligns synchronously with the stator’s rotating field. This synchronized interaction between the rotor and stator magnetic fields produces torque to propel the car. Plus, one can vary the rotor’s magnetic field strength to optimize peak power output. According to BMW, this kind of motor provides an efficiency of up to 93%, very close to that of PMSMs. Despite such advantages, this type of motor uses brushes that must be carefully designed for a sufficiently long service life. An alternative to the excitation system of the EESM has been proposed, which is based on a wireless-power–transfer system.
Axial flux electric motors
Unlike radial flux motors, in an axial flux machine, the magnetic fields created by the rotor and stator align parallel to the rotation axis. This configuration allows for a more efficient use of magnetic flux and reduces power losses, improving overall motor efficiency. Quantitatively, in axial flux machines, the relationship between torque and diameter is proportional to the third power of the diameter. In radial flux machines, by contrast, the torque varies proportionally to the second power of the diameter and linearly with the length. This leads to a reduction of overall dimensions on the axial flux machine for the same torque developed, translating into high power density. One disadvantage is their relatively high manufacturing cost due to the precision required in aligning the rotor and stator. Furthermore, as the technology is still evolving, its use is not widespread. Figure 6 shows the P400 R axial flux e-motor from Yasa, a company owned by Mercedes-Benz.
References
1Hayes, J. G., & Goodarzi, G. A. (2018). “Electric Powertrain: Energy Systems, Power Electronics and Drives for Hybrid, Electric and Fuel Cell Vehicles.” Wiley.
2Torque is the extension of the linear force concept to rotating bodies and can be expressed as the product of the perpendicular component of force and the distance of the applied force from the rotation axis.
3Magnetic reluctance (R) is used in magnetic circuits and plays the same role as electrical resistance in electrical circuits. It is a measure of how much it opposes the magnetic flux (F). It can be expressed as R = F ÷ F, where F is the magnetomotive force that for a current I circulating in a coil of N turns is NI.
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