The Many Types of EV Motors

Electric vehicles (EVs) are becoming a popular green alternative to internal combustion engine (ICE) vehicles, the dominant driver for over 100 years. EVs operate via an electric motor powered by battery energy.

There are various types of motors used for EVs. This article takes a look under the hood and gives an overview of the most commonly used EV motor types.

Overview of Motor Types

EVs use traction motors that are capable of delivering torque to the wheels. Electric motors can be roughly divided into two types: DC and AC motors. Both types can be used in EV applications.

DC motors are robust and allow simple control. They can be made as brushed and brushless DC motors. Brushed DC motors are a mature technology that provides low cost, high torque at low speed, and easy speed control. These features are very important for traction motors. However, brushed DC motors are not widely used in EVs because of their disadvantages, which include large size, low efficiency, and requirement for frequent maintenance due to the brush and collector structure. Brushless DC motors have a much higher efficiency. These motors use an electronic commutator/inverter instead of the brushes.

Compared to DC motors, the advantages of AC motors are high efficiency, less maintenance, higher reliability, and regenerative capability that enables braking energy to be returned to the batteries.

What Features Should an EV Motor Have?

An EV’s motor and electronics efficiency directly influences the battery weight, because the lost power needs to be compensated. Every 1% lower efficiency requires 1% more power from the battery (meaning more batteries).

The EV’s performance directly depends on the electrical motor specifications. The performance of the motor is determined by the torque-speed and power-speed characteristic of the traction motor.

The grade ability and maximum speed are important parameters in these curves. The desired motor grade ability requires high torque at low speed, enabling proper starting and acceleration. The EV motor needs to have high power at high speed and a wide speed range in the constant power region as shown in Figure 1. The constant torque operating region is important at low speed to provide a good start and up-hill drive. The constant power region determines the maximum EV speed on flat surfaces.

When the base speed is achieved, the motor reaches its rated power limit and the motor torque decreases proportionally to the square of speed. The constant power region starts beyond base speed in the range from base speed up to maximum motor speed. This range is different in different motor types and it is an important parameter when selecting the proper EV motor type. Also, the motor operation range can be adjusted by using the corresponding control drives.

Selecting the proper output characteristic of an EV motor is a challenge because it is necessary to find the balance between acceleration performance and wide speed range in the constant power region. When increasing the constant power region, the power requirement for acceleration performance is decreased. The torque requirement is increased which influences the motor size and its final price.

These are the features we desire in an EV motor:

• High efficiency
• High instant power
• Fast torque response
• High power density
• Low cost
• High acceleration
• Robustness

We will now look at how these features stack up in the following motor types:

1. DC motors
2. Permanent Magnet Brushless DC motors (PM BLDCs)
3. Induction motors
4. Permanent Magnet Motors
5. Switched Reluctance Motors (SRM)

DC Motor

The biggest advantages of DC motors in EVs are robust construction and simple control. DC motors have appropriate torque-speed characteristics providing high torque at low speed. Their main disadvantages are size, low efficiency, low reliability and high maintenance, and limited speed because of the friction between brushes and collectors. There are two DC motor types: brushless and brushed DC motors. The latter are increasingly suppressed because of the advances in power electronics.

Permanent Magnet Brushless DC Motor (PM BLDC)

PM BLDC motors use permanent magnets instead of the rotor windings. Since they do not include rotor losses their efficiency is higher than inductive motors. PM BLDC motors have a short constant power operation region because of their permanent magnet field weakened by a stator field. Since EVs require a wider constant power region, this can be extended by using conduction angle control where the speed range may reach three to four times the base speed.

The permanent magnets also limit the motor torque to be high. The magnets are significantly influenced by the high temperature which reduces the remnant flux density and thus the motor torque capacity. The mechanical forces and magnet prices are the biggest disadvantages of this type of motor. The increased centrifugal forces caused by higher motor rotation speed can cause safety issues due to the possible breaking of the magnets.

Induction Motor (IM)

This motor type is very common in EVs because of its simple construction, high reliability, robustness, simple maintenance, and low cost and operation at different environmental conditions. IMs can be naturally de-excited if the inverter faults, an important safety advantage for EVs. The field-oriented vector control of IMs is industrially standardized.

The disadvantages of IMs are slightly lower efficiency (compared to PM motors), higher power losses (increased because of the cage losses), and a relatively low power factor. The weakening of the flux can be used to extend the speed range in the constant power operation region. This region can be extended by using dual inverters as well. Rotor losses can be also reduced by careful motor design.

Permanent Magnet Synchronous Motor (PMSM)

PMSMs, similar to BLDCs, have permanent magnets in the rotor. Unlike BLDC motors that have a trapezoidal back electromotive force (EMF) waveform, PMSMs have a sinusoidal back EMF. They have a simple construction, high efficiency, and high power density, thus they are suitable to be used as traction motors (common in hybrid vehicles, EVs, and buses). PMSM motors have a higher efficiency compared to IMs. 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 varieties of PMSM motors: surface-mounted permanent magnet (SPM) and interior permanent magnet (IPM) synchronous motor drives. IPM motors have better performance than SPMs, but the downside is their complex design.

Switched Reluctance Motor (SRM)

The benefit of SRMs is their high torque component, enabling their use in many applications such as wind energy, generator starter systems in gas turbine engines, and high-performance aerospace applications. Moreover, their advantages in EVs include their robustness, simple control, high efficiency, wide constant power operation region, fault tolerance, and effective torque-speed characteristics. Since they do not contain brushes, collectors, or magnets, the maintenance of SRMs is very simple and effective and their price is very competitive.

The absence of magnets eliminates the problem with mechanical forces, enabling the motor to operate at a high speed. Since the motor's windings are not used, there are no copper losses in the rotor ensuring the rotor temperature is lower than other motor types. Since the phases are not connected, SRM motors can continue their operation even when one of the phases disconnects. SRM rotors have a lower inertia than other motor types. The drawbacks of this motor type are increased vibration and acoustic noise. In addition, the salient-pole rotor and stator construction cause high torque ripple. The high rotor inductance ratio allows sensor-less control to perform.

Proper motor design enables the wide constant power operation region, which in turn allows operation at high speeds. SRMs have a suitable torque/power speed characteristic for EV applications.

Comparison and Evaluation

Dorrell et al. [1] evaluated IPM, IM, and SRM motors and compared them at a rotating speed of 1500 rpm and 6000 rpm and their maximum power. Parameters under consideration included torque, iron loss, copper loss, efficiency, and current density.

For the case of 1500 rpm, the torques of IPM and IM are higher than SRM. IM has higher copper losses. It is shown that IPM has the highest efficiency (91.3%).

At 6000 rpm, SRM provides the highest torque value. The IPM has again the highest efficiency and it is higher at higher speed (SRM 96.1%, IM 95.2%, SRM 88.2%).

Mounir et al. [2] analyzed DC, IM, PM, and SRM types by comparing their power density, efficiency, controllability, reliability, and price in EV applications.

Table 1: Comparing the relevant features of different motor types [2].

It is noticeable that the IM motor type has all the characteristics suitable for EVs. In this application, safety is one of the most important considerations and the SRM and IM types provide driving safety. However, the rated speed of IM is relatively low. PM has a higher power factor and efficiency in low-speed region.

The SRM type does not use brush collectors and magnets and thus has fewer maintenance requirements. This type also has lower power losses than other types. This is because of the short winding ends and their total length. The rotor does not contain conductors enabling low rotor temperature and easy cooling, which is one of the main advantages of SRM type motors. SRM operates at high speeds in a wide constant power region and allows the extremely high-speed operation. Besides this, the motor is lightweight, competitive and has high efficiency. If all characteristics are considered, SRM is the most suitable motor type for EVs.

Even with their relatively high power density and efficiency, BLDC motors are not commonly used in EV applications, mostly because of their limited constant power range.

Motor Types Used by Popular EVs

The Tesla Model S and Model X use conventional IMs. The Model 3 uses an SRM with internal permanent magnets in the stator, called an internal permanent magnet switched reluctance motor (IPM-SRM). Dual-motor versions were also introduced by Tesla—the Model 3 uses an IM in the front and an IPM-SRM in the back. It is the opposite case for the Model S and Model X.

The GM Chevrolet Bolt uses a PMSM where the magnets are placed inside the rotor. This motor type is also used by the Toyota Prius, Nissan Leaf, and BMW i3.

Every manufacturer utilizes their approaches and technologies to make their propulsion as efficient as possible and produces many varieties of the same motor type.

References:

[1] Dorrell, D.G., Popescu, M., Knight, A.M., Evans, L., Staton, D., A., "Comparison of Different Motor Design Drives for Hybrid Electric Vehicles", IEEE, pp. 3352-3359, 2010.

[2] Z., Mounir, El Hachemi Benbouzid, M., Senior Member, Diallo, D., "Electric Motor Drive Selection Issues for HEV Propulsion Systems: A Comparative Study", IEEE, vol. 55, no. 6, pp. 1756-1764, November, 2006