Alternative Motor Technologies for Traction Drives of Hybrid and Electric Vehicles

By Dr. Sab Safi, C.Eng. - Consultant - SDT Drive Technology.

The need for efficient and environmental friendly vehicles has pushed significant research in electric vehicles (EVs) and hybrid vehicles (HEVs) since power electronics became reliable at the necessary power levels.

There is now a distinct possibility of limited supply or very high cost of rare earth (RE) magnets that could make rare earth motors unavailable or too expensive for hybrid electric vehicle (HEV) mass production. The demand for RE magnets is also further exacerbated by the growth of wind power, which is projected to consume a significantly greater share of RE magnets than hybrid vehicles Therefore, there are high demands of a rare-earth-free motor for the next generation HEVs.

A primary challenge is the choice of the most efficient and most cost effective electric machine. While induction machines (IMs) are considered mature and permanent magnet synchronous machines (PMSMs) and switched reluctance machines (SRMs) are still evolving for EV and HEV applications. Induction machine designs still follow rules established several decades ago. While there is an opportunity for enhanced induction machine designs that utilize advanced manufacturing technologies, materials, and control methods, research on hybrid permanent magnet machines and switch reluctance machines (SRMs) in addition to comparisons of both is available and sometimes ignores induction machines.

In practice, many advanced EV designs employ induction machines, often accompanied by reports of superior performance. Even so, induction machines in HEVs have not been popular. The issue addressed here is a review of induction machines, as an alternative to PMSMs, and some performance aspects of induction machines are also compared to permanent magnet synchronous and SR machines

Automobile Requirements In an HEV (Fig. 1), an electric traction motor converts electrical energy from the generator and/or the energy storage unit to mechanical energy that can help drive the wheels of the vehicle. Unlike a traditional vehicle, where the engine must "ramp up" before full torque can be provided, an electric motor can provide full torque at low speeds. This characteristic gives the vehicle excellent acceleration from standstill.

Fig1. Main components of a Hybrid Electric Vehicle (HEV)

Ideal Torque-Speed Profile of Traction Motor Drive for EV/HEV: Electric motor drives play the most important role in EV/HEVs. In general, the electric motor drive is the only torque source in a series hybrid vehicle. In parallel and series– parallel hybrid vehicles, the electric motor is the torque source that provides the peak torque (power) to meet the vehicle performance requirement, such as, acceleration. Thus, the development of compact, lightweight, high efficiency and proper torque-speed profile becomes crucial.

The ideal torque (power)-speed profile for traction application is the constant power in all the speed ranges. The constant power profile can maximize the vehicle acceleration performance at a given power rating or minimize the power rating at given vehicle acceleration performance. In an engine (gasoline or diesel)-alone powered vehicle, a multigear transmission is used to modify the engine torque-speed profile to make the tractive effort- speed profile on a driven wheel close to this ideal profile. However, a well-controlled electric motor drive has the torque-speed profile close to the ideal one as shown in Fig. 2. The torque-speed profile of a well-controlled electric motor drive includes two distinguishable regions: constant torque and constant power. The corner speed is usually called base speed. At a given power rating and maximum speed, the lower base speed results in a larger maximum torque. A term, speed ratio, is defined as the ratio of the maximum speed to the base speed.

In vehicle drivetrain design, one of the most important efforts is to reduce the power rating (volume and weight) of the drivetrain with a given vehicle performance. Fig.2 shows the required motor power ratings with different motor speed ratios, which produce the same vehicle acceleration performance. It clearly shows that a traction motor drive with a long constant power range is preferred.

Fig. 2. Ideal Output characteristics of traction motor drive b) Tractive effort and power versus vehicle speed with different speed ratio.

Alternative Traction Motor Technology For the past several years, the interior permanent magnet (IPM) motor has been considered the popalur choice for electric traction drive systems. However, with the rapidly increasing costs of magnets and the possibility of a future shortage of rare-earth (RE) metals, the use of IPM motors may not continue to be economically or technically feasible.Therefore, it is timely to consider other options for motor types. The purpose of the the following discussion is to show why we need to consider induction motors and switch reluctnace as alternatives for HEV/EV application (Fig. 3,)

(a) PMSM

(b) IM

(c) SRM

Fig 3. Cross section of the alternative motors designs

A. Brushless Permanent - Magnet Synchronous Machines (PMSM) The permanent magnet synchronous motor (PMSM), also known as the sinusoidal brushless DC motor or PM DC motor is a synchronous motor, where the field mmf is provided with permanent magnets. PMSMs are classified according to the position and shape of the permanent magnets in the rotors. The PMSM configuration is shown in Fig. 4. Three common arrangements of the rotors are surface mounted, inset, and interior or buried. Interior PMSM is the most popular motor technology for HEV applications. The reason for interior PMSM popularity in parallel HEVs is that a relatively low-power motor is sufficient to supply the drive train of a medium sized car, thus, not requiring large, expensive permanent magnets. The brushless dc motor and interior permanent magnet motor (IPM) are two typical motor drives for EV/HEV applications.

An IPM motor is a hybrid that uses both reluctance torque and magnetic torque to improve efficiency and torque. These motors are created by adding a small amount of magnets inside the barriers of a synchronous reluctance machine. They have excellent torque, efficiency, and low torque ripple.

However, PM motors inherently have short constant power range due to its rather limited field weakening capability, resulting from the presence of the PM field, which can only be weakened through production of a stator field component, which opposes the rotor magnetic field. The speed ratio is usually less than two. Other major issues are failure modes and the high cost of the motor.

Fig.4. PMSM view

PM hybrid motors: Recently, the use of additional field windings to extend the speed range of permanent magnet motors has been developed. The key is to control the field current in such a way that the air-gap field provided by PMs can be weakened during high-speed operations. Flux weakening is accomplished by applying a large demagnetising current on the d-axis of the permanent magnets. An adequate field weakening can be achieved with auxiliary excitation winding is needed in the rotor or stator to reduce directly the flux in the machine and reach the high speed.

Due to the presence of both PMs and the field windings, these motors are so-called PM hybrid motors. The PM hybrid motor can achieve a speed ratio of around four.

However, PM hybrid motors have the drawback of a relatively complex structure. The speed ratio is still not enough to meet the vehicle performance requirement, especially for off-road vehicles. Thus, a multiyear transmission is required. SDT are currently working on a combination of these three motor technologies. All of the basic electromagnetic topologies could be implemented with varying degrees of difficulty in the detailed design.

SDT Drive Technology has developed wide range high performance brushless motor drives and a method of control embodied in electronic component architecture and software code (Phase Advance Control) which allows high-speed operation above the base speed which is achievable with constant DC bus voltage when the commutation angle advance is varied.

SDT Drive Technology and partner have introduced integrated high speed permanent motor products to satisfy that demand. Fig.5 is SDT’’s electric drive unit. This employs a planetary gear with reduction ratio in range of 4-6:1 with an integral water cooled PMSM The unit has a high-speed motor (up to 9000 rpm) and supply voltages between 24 and 600Vdc,depending on the available power source and output power requirement.

Also we supply advanced high power controller designed for traction applications for : Electric traction and starter/generators, compatible with DC, 3-phase AC-induction and DC- brushless motors, wide voltage supply range (100-450V) and continuous power output of up to >160kW

Fig. 5a Brushless Traction Motor 60kW Fig. 5b Advanced Traction controller Traction

B. Induction Machines (IM) Drive This technology (Fig. 6) is well known and the motors are readily available. In addition, they are relatively cheaper since they have been mass-produced for a long time. Also, they provide benefits such as low maintenance and weight. Unlike PMSMs, they can be rated for high power levels and, unlike SRMs, their acoustic noise is comparatively lower

Field orientation control of an induction motor can decouple its torque control from field control. This allows the motor to behave in the same manner as a separately excited dc motor. Extended speed range operation with constant power beyond base speed is accomplished by flux weakening. However, the presence of breakdown torque limits its extended constant power operation. A properly designed induction motor & control, e.g., motor, with Field orientation control (retain the current regulation & control capability in the extended constant power range to avoid the breakdown torque) achieve a field weakened range of about three to five times its base speed. The availability of a long field weakened range, obviously, makes the induction very suitable for vehicle application.

Fig. 6. IM view

In addition, the motors use cast copper rotors for superior performance over the Aluminium cast rotor (Fig. 7). The perfection of the casting of copper is a technological advancement.

Depending upon the size of the motor, studies show that the use of copper can increase the efficiency of an induction motor by one to three percentage points. In order to calibrate the significance of this small improvement, consider a 50 kW ac induction traction motor like the rating of the interior PMSM. If an Aluminium rotor was used and the motor efficiency turned out to be 93% and by the use of copper if it increased to 94%, the decrease in rotor losses would be about 570 watts. This copper rotor would be much easier to cool, and the extra 1% would improve the battery driving distance. Further study has also shown that the operation efficiency (for actual driving cycle) of the cast copper induction appears to have efficiency at least comparable to, or perhaps a bit higher than, the permanent magnet machine.

However, if PMSM motors become infeasible for reasons of cost or availability, induction motors would be the next choice, so they should not be forgotten.

Fig. 7a Rotors with aluminium cast Fig. 6b. Rotors with copper cast

Fig. 8. SRM View

C. Switched Reluctance Motor (SRM) Drive SRMs are structurally similar to stepper motors, except that the stator phase currents are switched based on rotor positions. Specifically, SRM (Fig. 8) uses a doubly salient structure with toothed poles on the rotor and stator. Each set of coils is energized to attract a rotor pole in sequence so it acts much like a stepper motor. With current technology, SR motors have inherently high torque ripple. In addition, the high radial forces can create excessive noise levels if not carefully designed. These machines are best suited in high-speed applications where ripple is not an issue.

In comparison with mature motor technologies such as the induction machine, dc shunt motor, and even the more recent brushless PM synchronous machine (PMSM), the SR machine (SRM) offers a competitive alternative to its counterparts despite the relatively young age of SRM technological advancement. Although the basic concept of the SRM has been around for about 170 years, advances in power electronics, digital control, and sensing technologies have completely reinvigorated the capabilities of the SRM which provide some novel design opportunities which are better suited for vehicle propulsion

Unlike most other motor technologies, both the rotor and stator of the SRM comprise salient teeth such that torque is produced by the tendency of its rotor to move to a position where the inductance of an excited stator winding is maximized and reluctance is minimized. This condition generally occurs when the corresponding stator tooth is fully aligned with a rotor tooth. The non-steady state manner in which torque is produced in the SRM introduces the requirement of a sophisticated control algorithm which, for optimal operation, requires current and position feedback. In addition to non-steady state operation, the SRM often operates with the rotor and stator iron in saturation, increasing the difficulty of optimal control and making the machine very difficult to accurately model without the aid of computer processing and modelling techniques. Therefore, since the SRM is very technologically demanding in terms of design, modelling, and control, the evolution of SRM technology has been limited until these demands were adequately addressed. Furthermore, the progression of other motor technologies such as the IM and PMSM have not been as limited by the state of other technologies.

The absence of PM material, copper/or Al in the rotor greatly reduces the requirement of mechanical retention needed to counteract centrifugal and tangential forces. This causes the SRM to be especially well suited for rugged applications or high-speed applications wherein high power density is desired. As there are no conductors in the rotor, only a low amount of heat is generated therein, and most of the heat is generated in the stator, which is easily accessible in regards to thermal management.

They can inherently operate with constant power range. The serial design and simulation results show that the speed ratio 6-8. This long constant power range makes the SRM highly favourable for vehicle traction application. However, high speed operation may cause high mechanical losses due to aerodynamics drag and viscosity losses. To reduce the mechanical losses, carful design optimisation consideration need to be given to the shape of the rotor. These machines are best suited in high-speed applications where ripple is not an issue.

In regards to mass-transportation vehicle propulsion, the primary problems with SRM technology are the torque ripple and acoustic noise that is associated with the fundamental manner in which torque is produced. Considerable research efforts to eliminate or sufficiently reduce the torque ripple and noise problems have been applied but they often bring about important sacrifices of efficiency and/or power density and cost

Design & Performance Considerations PMSMs offer the highest torque density, however, they fail to present an acceptable power density. Therefore any compactness gained at Low speed is undermined by power demands at higher speed where the motor mostly works.

For traction application, the torque density is the most important criterion of the electric motor, which reflects the volume and weight of machines at given torque demand. Table 1 lists the typical torque density values for all three type of motor technology. This Table shows that the PMSMs enjoy the highest torque density and therefore will potentially have the lowest weight for given torque and power ratings. However, the fixed flux magnets limit its extended speed range. The induction machine and switched reluctance machine have similar torque density. More details of the operation characteristics of these three machines are described as follows. Various comparison studies have also shown that the efficiency and power density of the SRM and ac induction machine with copper rotor bars are roughly equivalent

Table 1. Example of machine characteristics in the 20kW -60kW range Motor Torque Density Power Density Max. Speed

Induction 4.0 Nm/kg 1.5 kW/kg 15, 000 rpm PM 5.0 Nm/kg 1.0 kW/kg 9, 000 rpm SRM 4.0 Nm/kg 1.5 kW/kg 20, 000 rpm

Efficiency for Dynamic operation: In a EV/HEV system the overall machine-operating point is not defined. So unlike industrial drives, traction drive efficiency cannot be defined in one operating point. Efficiency of traction motors should be maximized in their drive cycle (frequently operating regions).

Operation efficiency for a drive motor is total energy delivered to the drive train divided by total efficiency consumed by the motor, which is energy delivered plus losses. In this respect, we approximate losses as having two components: one of these is proportional to speed squared and the other component is proportional to torque squared. Using these approximations and data from a ‘drive cycle’, we could estimate dynamic efficiency. This dynamic depends on the correlation between torque and speed. Study has shown that in terms of effective drive cycle efficiency, an induction machine appears to be the superior solution.

In the induction motor the excitation is provided by stator currents and so produces some loss. There is also core loss present and that loss is roughly proportional to the square of terminal voltage. This increases with speed up to the ‘base’ speed. But with appropriate controls the excitation of the induction machine can be set to produce ‘optimal’ losses for any given operating point. In PMSMs, excitation is provided by the magnets, which are lossless. There is excitation related loss from eddy currents and hysteresis in the core iron. This loss is roughly quadratic in speed.

Load loss results from the currents required to produce torque. Load current is inversely proportional to excitation flux and directly proportional to torque. In PM machines, then, torque related losses are proportional to the square of torque. In induction machines the same is true but torque must be produced on the rotor as well, so the induction machine has higher torque related loss.

The torque (power)-speed profile for traction application reflects the common sense notion that the more highly correlated the force required to accelerate a vehicle and the speed of that vehicle the higher will be the dynamic efficiency. Since there are losses associated with both speed and acceleration but real power is the product of force and speed, when force and speed are not correlated the losses will be higher relative to output power.

In actual vehicle operation (with a typical driving cycle), the induction machine has a substantial advantage because it can be de-excited when it is not producing torque, eliminating idling electrical loss. The impact of the rotational losses on vehicle efficiency depends on the speed profile of the vehicle.

The results understates the dynamic efficiency advantage of the induction machine because it does not take into account the possibility of de-exciting the induction machine when it is rotating but not producing drive effort, something that is very important in hybrid vehicles that cruise on engine power only, using the drive motor for acceleration and braking.

Summary (1) By considering, in detail, the actual drive cycle of the vehicle, IMs can have dynamic efficiencies higher than those of PMSMs, even if the single point efficiency of the induction machine is lower than the single point efficiency of the PMSM. Therefore, dynamic (operation) efficiency for drive cycle must be evaluate, but static efficiency maps miss dynamic effects and under predict induction machine efficiencies.

(2) It is clear that the choice of the highest efficiency machine depends on the torque- speed relationships imposed by the driving cycle, and whether the machine is operating in the field-weakening region or not. The high-efficiency area for an induction machine is shown to be between the high-efficiency areas of a PMSM and an SRM, which implies that induction machines could be favourable for a wide range of driving cycles. A complete dynamic driving cycle to evaluate machines in these applications. Evaluation of the dynamic (which consider transients and dynamics in a vehicle) efficiency is essential in EVs and HEVs. .

(3) There is a possibility that RE magnets may become unavailable or too expensive, and this justifies efforts to develop innovative designs for PM motors that do not use RE magnets. Possible other magnets that may be substituted for RE magnets include Sm-Co, Alnico, and ferrites. Alternatively, efforts to develop motors that do not use PMs but offer attributes similar to interior PMSMs are also encouraged.

(4) In our opinion induction motors (as mature technology and by considering the above) would be the preferred choice if PMSMs are not available. Although if others assume that RE magnets will continue to be available, we also recognize the importance of developing backup technologies.

(5) Any of the above choices are going to require some form of effective liquid cooling and high copper slot fills for the phase windings to miximise efficiency.

(6) SRM motors are durable and low cost, and they contain no magnets and inherently operate with extremely long constant power range. This long constant power range makes the SRM highly favourable for vehicle traction application. However, high speed operation may cause high mechanical losses due to aero dynamics drag and viscosity losses. Significant concerns about SR motors are torque ripple and noise. Efforts are currently being directed to solve those problems through rotor design, modified electronics, and stiffening of the case. SRM motors are durable and low cost, and they contain no magnets and inherently operate with extremely long constant power range. This long constant power range makes the SRM highly favourable for vehicle traction application. However, high speed operation may cause high mechanical losses due to aero dynamics drag and viscosity losses. Significant concerns about SR motors are torque ripple and noise. Efforts are currently being directed to solve those problems through rotor design, modified electronics, and stiffening of the case. (7) SRM motors are durable and low cost, and they contain no magnets and inherently operate with extremely long constant power range. This long constant power range makes the SRM highly favourable for vehicle traction application. However, high speed operation may cause high mechanical losses due to aero dynamics drag and viscosity losses. Significant concerns about SR motors are torque ripple and noise. Efforts are currently being directed to solve those problems through rotor design, modified electronics, and stiffening of the case. (8) SRM motors are durable and low cost, and they contain no magnets and inherently operate with extremely long constant power range. This long constant power range makes the SRM highly favourable for vehicle traction application. However, high speed operation may cause high mechanical losses due to aero dynamics drag and viscosity losses. Significant concerns about SR motors are torque ripple and noise. Efforts are currently being directed to solve those problems through rotor design, modified electronics, and stiffening of the case.

By Dr. Sab Safi, C.Eng. - Consultant at SDT Drive Technology

In our opinion, fundamental research into materials will bring about the biggest changes in electric motors. Superconductors transmit electric power with almost no ohmic losses and can carry power densities up to 100 times higher than conventional conductors. Superconductive wires are now commercially available which can be cooled to their low operating temperatures using off the-shelf systems. Until recently, superconducting materials have been relatively expensive because they contain silver. However, superconductive wires costing less than copper litz wire and with similar current carrying capacities are now at an advanced stage of development. The availability of such materials will be a decisive factor in the widespread application of superconducting technologies.

High flux steels that won’t saturate) when driven hard, high coercivity magnets that are good over a wide temperature range, and low cost bearing and sensors. Any improvement in these areas will bring about change in a positive direction.

Of course, motor topologies will enable the application of improved materials. There is no one prefect or ideal machine topology, each topology has its unique integration advantage. SDT Engineering Capabilities & R&D focuses on new topology areas to reduce size, weight, and cost and to improve reliability