Electrical Machines in Electrified Transportation

The driving factors for transportation electrification include improved efficiency, reduced environmental impact, and replacing petroleum or similar fuels as primary energy sources. Compared with the internal combustion type conventional vehicles, electric vehicles  (EVs) have greater well-to-wheel energy efficiency. The added benefit is no carbon emissions during EVs operation. However, the success of EVs depends on their successful integration with the infrastructure systems that support them. Read on to learn more about the AC motor drives and electric machines used in electrified transportation.

 

Overview

The efficiency and performance of electric machines significantly impact the fuel consumption, acceleration, high-speed performance, and driving comfort of the electrified powertrains. The electric traction motors have stringent operational requirements concerning torque-speed characteristics. It is crucial to have the ability to operate at specific operating points based on the application. For example, high torque at lower speeds is anticipated for quick acceleration, hill climbing, engine auto-start, and reversing at high road gradients. City driving demands operation at medium speed range, whereas highway driving requires high-speed range.

The traction motor must provide high efficiency at its most frequent operating points to improve the overall powertrain efficiency and fuel consumption. In addition to the vehicle platform, engine size, drive cycles, volume, weight, lifetime, and cost constraints, various other parameters including torque-speed characteristics, peak-power requirements, and thermal, structural, and noise vibration harshness (NVH) conditions define the selection of the right electric machine for the application at hand [1]. These selections also affect the machine design process, from selecting the core and insulation material to permanent magnets, the number of poles, winding configuration, and assembly, manufacturing process, etc.

AC motor drives form a crucial part of the transportation electrification sector. The study of electric machines includes the understanding of AC machine operation and modeling of critical electric machines like permanent magnet synchronous machines and induction machines. The other aspect to look into is the DC-AC inverter operation and controls, along with AC drive modeling and simulations [2]. The more efficient and higher performance electric traction motors improve the use of electrical mode. The engine runs closer to peak efficiency, leading to lower fuel consumption and higher all-electric range in EVs.

 

Electric Machines in Electrified Transportation

The use of the right electric machines is inevitable for the effective operation of electrified transportation. There are several types of electric machines used in powertrains, and it depends on the specifications, performance, and application at hand. Some of the key electric machines used in automotive applications include interior permanent magnet synchronous machines (IPMSM), induction machines (IM), and switched reluctance machines (SRM). The typical electric machine types employed for traction applications are shown in Figure 1 [1].

 

Figure 1. Typical electric machine types for traction applications (a) IPMSM (b) IM (c) SRM [1]

 

IPMSM is the most commonly employed electric machine in hybrid and EVs currently available on the market. The IPMSM has permanent magnets embedded inside the rotor, which provides an independent excitation source. Thus, IPMSM can supply high torque density and better efficiency, especially at low and medium speed ranges [1]. The selection and configuration of the permanent magnets have a significant effect on the output torque of the machine. The design has to accommodate for maximum temperature and demagnetization as it defines the size and volume of the magnet such that the overall design can be optimized with respect to cost and performance. In permanent magnet-based traction motors, high-energy rare earth permanent magnets provide higher torque density. However, the key disadvantages of permanent magnet-based machines are the sensitivity of rare earth magnets to temperature and their higher cost.

In an IM, the magnetic field generated by the stator currents induces a voltage on the rotor conductors, and the rotor currents create torque. Compared to IPMSM, IM operates at a lower power factor with lower efficiency at low speeds due to a lack of independent rotor excitation [1]. The high torque and high-speed operation with IM can be achieved by using copper rotor bars and improvements in the mechanical design. The key disadvantage of IM is the inherent rotor copper losses, especially during high torque operation. The heat generated in such cases can be difficult to extract and limits the torque density of IM.

SRM is known to have the simplest, most robust, and lowest cost structure compared to IPMSM and IM. It typically has a salient pole structure made of laminated silicon steel. In this case, the torque production is based on the change of magnetic reluctance. The key disadvantage of conventional SRM is the significant torque ripples and lower power density. Currently, SRM is not used in any major hybrid or EVs on the market as the traction motor. However, advanced design and control techniques can make it a possible choice for such applications in the near future.

 

Key references:

1.   Berker et. al., Making the Case for Electrified Transportation, 2015.

2.   Dragan Maksimovic et. al., Power Electronics for Electric Drive Vehicles, 2013.