Types of Traction Motors for Electric Vehicles
By: Nilay Joshi
Tags: Electric Vehicles, Automotive, Electric Traction Motors, Hybrid Electric Vehicle, Electric Motors

Recently, rising concern over environmental pollution has shifted automotive industry’s focus towards electric and hybrid electric vehicles. These vehicles are predicted to replace conventional Internal Combustion (IC) engines that use numerous parts in their drivetrains. In IC engine vehicles due to the nature of power transfer from combustion of fuel to rotation of wheels, there are a lot of factors that influence performance characteristics; characteristics like power, torque and speed. But in electric vehicles, due to a simpler power train that has fewer parts, most of the vehicle’s performance comes down to the performance of battery pack and motor. Therefore, successful motor selection can improve the overall performance of the vehicle. High efficiency motors will improve the vehicle’s range and performance by utilizing the most out of the battery pack. That is why motors are an important part of EVs. In this article six commonly used electric traction motors are explained.

1. DC series motor –

Stator — electromagnetic windings.

Rotor — Coil windings that are powered by DC current.

DC series motors have high torque at low speeds. This makes them useful in traction applications. These motors were used in the early days of electric vehicles, before power electronics and microcontrollers were developed. The DC series motor is simple and robust in construction and uses a simple speed control method. It can also withstand excessive loads. These reasons make it a good candidate for traction application.

But the use of brushes and commutator decreases its reliability and increases the need for maintenance. Incapability of operating at high speeds, since there is a risk of flash-overs, a bigger size and low efficiency also add up to the drawbacks. Another disadvantage is the difficulty of regenerative braking.

2. Brushless DC motor (BLDC) –

Stator — electromagnetic windings powered by DC current.

Rotor — Permanent magnets instead of rotor windings.

A sensor (usually Hall effect sensor) determines position of the rotor and based on that appropriate coils are powered at the right time by an electronic controller.

By using high energy permanent magnets instead of coil windings these motors have a more compact size in comparison to brushed DC motors. The absence of brushes and commutator also means more reliable operation and less maintenance. Heat generated in the motor is reduced because there is no current in the rotor, making it easier to cool. In addition, high efficiency, low noise and ease of control make this motor useful for electric vehicles.

BLDC motors can come in two configurations; in-runner or out-runner type. The in-runner type has rotor placed inside the stator. This system requires an external transmission system and thus requires more space. In out-runner type, the rotor of the motor is present outside and the stator is inside. These are also known as hub motors as their rotor is directly coupled to the wheel without the need for an external gearbox. These can be easily mounted in small spaces and are commonly used in electric bicycles and scooters.

High cost of permanent magnets and the risk of demagnetization at high temperatures are the main drawbacks of BLDC motor.

3. 3 Phase AC Induction motor (IM) –

Stator — 3 phase windings arranged to create a rotating magnetic field (RMF).

Rotor — Coil windings or squirrel cage.

There are two type of induction motors, namely, wound-rotor and squirrel-cage motors. Squirrel cage induction motors are superior to wound-rotor type motors in terms of cost, maintenance and sturdiness. Hence, they are loosely named as induction motors. Induction motors are very common in electric vehicle applications because of a commutator-less design that ensures smaller size, reduced weight and more reliable operation in comparison to DC motors. This motor doesn’t require rare earth magnets like permanent magnet motors making it cheaper and able to run at higher max speeds. Conversely, due to the use of coils instead of permanent magnets, eddy current losses are increased and more heat is generated. Due to this better cooling system is required for the motor. Another drawback of induction motor is that its torque is adjusted by varying supply voltage and frequency. And a complex inverter circuit is required to do so, making this motor difficult to control.

Conventional Induction motors were used in Tesla’s early model S and model X cars, before they shifted to their Internal Permanent Magnet — Synchronous Reluctance Motors (IPM-SynRM).

4. Permanent Magnet Synchronous motor (PMSM) –

Stator — 3 phase windings (similar to that of induction motors).

Rotor — Permanent magnets.

PMSM is similar to BLDC motor which uses permanent magnets on the rotor. It has similar advantages of being light weight and having high efficiency. The difference is in the back EMF; PMSM has sinusoidal back EMF and BLDC has trapezoidal. Because of this a constant torque, without ripples is produced in PMSM. PMSM is proving to be a better alternative than induction motors for electric vehicles, despite its high cost. It is the best choice for high performance applications.

Electric hypercar manufacturer Rimac uses liquid cooled Internal Permanent Magnet Synchronous Motor that can operate at 95% peak efficiency. Similar motors are found in many hybrid electric cars like BMW’s i8, Toyota Prius, Nissan Leaf and GM’s Chevrolet Bolt. Ather Energy, an Indian manufacturer is also using PMSM motors for their electric two-wheelers.

5. Switched reluctance motor (SRM) –

Stator — coil windings that carry DC current and are switched on and off in order.

Rotor — pieces of laminated steel with no windings.

Reluctance motors convert reluctance torque to mechanical motion and have a simple and robust construction which makes them possible to run at high speeds. Its low inertia rotor helps in high accelerations. Moreover, it is easier to cool, since heat generated is mostly in the stator. The drawback of the SRM is the complexity in control and increase in the switching circuit. It also has some vibration and noise issues.

6. Synchronous reluctance motor (SynRM) –

Stator — 3 phase windings arranged to create a rotating magnetic field (RMF).

Rotor — pieces of laminated steel with no windings.

The principle and construction of SynRM is similar to that of SRM. The difference is in the excitation which in case of SynRM is from a rotating magnetic field as opposed to pulsed DC supply in case of SRM. They use 3 phase AC and an inverter circuit similar to that of induction motors. With further development and successful commercialization, this motor can replace the widely used induction motors in electric traction applications.

Tesla started using Permanent Magnet assisted SynRM motor in their Tesla model 3, shifting from induction motor drives.