Conventional vehicles that rely on internal combustion engines (ICE) could become a rarity in the next few decades. Electric vehicles (EVs) are growing in popularity, replacing petrol-driven transportation faster than expected. Despite a higher initial price tag, EVs typically offer savings over time. The environmental benefits aside (which are noteworthy), EVs cost half as much to maintain and repair as gas-powered vehicles over time.
Instead of an ICE, electric vehicles have a powertrain comprising of a battery pack, an inverter, a transmission, and electric motors. When a driver accelerates, the battery pack supplies electricity to the stator of the electric motor. This powers the rotor, which turns the gear mechanism and the wheels. The speed and torque of the engine are controlled by a power electronic controller (PEC) and gear motor.
The battery pack and electric motor are the key components of an electric vehicle. Although different EVs (such as cars compared to buses) use different types of electric motors, the features of the motor should provide high starting torque, strong traction, power density, and high efficiency and reliability. Ideally, it will also offer low cost and maintenance.
Types of electric motors
If you think induction motors are the go-to choice for EVs, you’d be correct for the most part. Induction motors are typically used, but they’re not the only choice. Choosing the ideal motor for manufacturers is a complex process that depends on the type of vehicle and involves R&D.
Other electric motors in EVs include:
- Brushed dc
- Brushless dc
- Permanent-magnet synchronous
- Three-phase ac induction
- Switched reluctance motors
Electric vehicles can use dc or ac motors, but the latter is more common in everyday vehicles. Sometimes asynchronous and synchronous motors are used, depending on various factors. Let’s explore some of these choices in greater detail.
Dc versus ac motors
To be effective, an electric motor must also be a traction motor. This means it must have a high starting torque to move forward and manage hills. Dc motors offer the best torque even at a low rpm. The control circuitry for dc motors is also simple.
However, despite these advantages, dc motors (whether brushed or brushless) are not widely used in EVs because they’re are big and heavy, which is far from ideal in a battery-powered vehicle. The lighter the EV, the less strain on the battery. Reliability and maintenance are also concerns.
Therefore, dc motors are typically limited to e-rickshaws and some two-wheelers, which can be driven at low speeds. High torque at low speed enables dc motors to run at a constant speed, regardless of the load.
EVs used for day-to-day mobility typically use ac induction motors, which offer high efficiency and low maintenance at a reasonable cost — and they’re lightweight, which is important. Ac motors also have regenerative capabilities, allowing the battery to benefit from the energy expelled during braking and recharge.
Traction motors and self-relieving property
One important characteristic of traction motors is self-relieving. This means speed and torque are inversely proportional to one another. When the speed increases, the torque reduces, and vice versa.
The power output of the motor is equal to the product of torque and speed.
The self-relieving property of a traction motor protects it from overloading. When a traction motor begins rotating, it should deliver a constant torque while its speed increases gradually. Eventually, it’ll reach an optimum driving speed, but the torque must be maintained. This is called the base speed of the traction motor.
Beyond the base speed, the torque drops while maintaining a constant power output. However, this implies that the speed increases at the cost of torque. The traction motor has the maximum torque and the highest power output at the base speed. This is the point where an electric motor is at maximum stress. So, at any rate past the base, the motor recovers stress by increasing the speed while reducing the torque.
The overall performance of an electric vehicle depends upon its motor’s torque-speed and power-speed characteristic curve.
A traction motor performs two distinct types of operation:
Figure 1. Torque/power – the speed traction motor characteristics.
1. Constant torque. A healthy motor provides the maximum torque when it begins rotating and should maintain this same torque until it reaches the base speed. This is the region where an EV accelerates after stopping.
2. Constant power. When the motor accelerates at the cost of torque, its maximum speed is determined. The constant power determines an EV’s speed range.
The constant torque and constant power region of a traction motor operate inversely. Typically, engineers attempt to balance and maximize the acceleration performance and speed range of an EV.
Ideally, the motor delivers maximum torque at a low speed. Once the base speed is reached, the engine maximizes the vehicle’s speed range at a constant power — without compromising the torque.
As mentioned, every manufacturer’s goal is a lightweight electric vehicle with high starting torque and power efficiency. But no electric motor is perfect. There will be compromises made between the torque, acceleration, and speed.
Brushed dc motors
Dc motors ensure high torque at low speeds. They can withstand variable loads and rotate at a constant speed despite sudden changes in load. They offer excellent acceleration performance but limited speed capabilities. This is because dc motors use brushes and commuters, limiting the maximum speed attainable.
Other drawbacks of brushed dc motors include low power efficiency, poor reliability, and their bulk and large size. They were initially used in e-rickshaws and electric three-wheelers but have since been replaced with brushless dc motors.
Brushless dc motors
Rather than brushes and commuters, brushless dc motors have permanent magnets, providing a power efficiency as high as 95 to 98% while delivering similar torque. They also offer a high starting torque and variable load capacity.
Brushless motors can be found in two main configurations: out-runner and in-runner.
In an out-runner motor, the rotor is positioned outside the motor, while the stator is inside. This setup, known as a hub motor, eliminates the need for an external gear mechanism. The motor directly drives the wheel, ensuring maximum power transfer. But this limits the use of the motor in terms of a maximum power rating. The power output cannot be compromised with vehicle stability. As a result, an out-runner motor often has built-in planetary gears for speed variation.
For the in-runner motor, the reverse is true — the rotor is located on the inside while the stator is on the outside. This configuration requires an external transmission to transfer mechanical energy to the wheels. The biggest drawback of in-runner motors is their size and weight.
Brushless dc motors are ideal for EVs that drive at a low speed with variable loads. Dc motors provide the best torque, but using permanent magnets limits speed. Permanent magnets are also subject to high maintenance and can break in harsh conditions.
Permanent magnet synchronous motors
Permanent magnet synchronous motors (PMSM) are brushless dc motors, but a permanent magnet replaces the rotor winding. These motors offer higher efficiency than induction motors because there are no rotor losses. They also offer high power density, but the constant power region is relatively short, and the torque is limited.
Adjusting the conduction angle control typically elongates the PMSM’s short constant power, enabling it to achieve a maximum speed of three to four times the base speed.
One unique advantage of PMSM is a sinusoidal back electromotive force (EMF). This differs from the trapezoidal back EMF typically found in brushless dc motors. Sinusoidal back EMF is a voltage generated by the rotation of the motor, which opposes the applied voltage. It’s useful because it lets the battery recharge (when braking) using a regenerative ability.
The disadvantage of permanent magnets is their poor durability in withstanding variable temperatures. Safety is a concern, as permanent magnets can break under harsh conditions. They’re also not cheap. However, thanks to their high efficiency, PMSMs are widely used in EVs that carry a limited load but require a high power output.
Induction motors in EVs
Induction motors are the most commonly used motors in today’s EVs. They have a simple design, low maintenance requirements, low cost, and — compared to dc motors — offer reliability in different environmental conditions.
The torque of an induction motor is adjustable using the voltage/frequency or field-oriented control method. This enables a high starting torque in three-phase induction motors. Since the torque is adjustable, a maximum load capacity can still be guaranteed over a wide speed range. The speed range is usually increased by using dual inverters. The motor can also be de-exited even if the inverter fails, providing additional safety.
Induction motors have a slightly lower power efficiency of 92 to 95% than PMSM. This means a larger battery pack would be needed to meet the exact power requirements.
Other drawbacks of induction motors include higher power losses, lower power output, and the need for more complex power electronics and inverter circuits. But because of their reliability, environmental robustness, and low cost, these are the preferred choice in most EVs and e-scooters.
Switched reluctance motor in EVs
The switched reluctance motor (SRM) uses a piece of laminated steel without any copper winding as the rotor. In fact, it doesn’t use copper winding or permanent magnets. This leads to a more straightforward construction with several advantages.
These motors have no copper losses in the rotor and lower inertia, so they do not easily overheat. The lack of inertial losses also means they achieve high acceleration performance. This motor provides high power density, a wider constant power region, and a greater speed range. The lack of permanent magnets adds mechanical reliability at high speeds and temperatures.
SRMs provide an ideal combination of acceleration performance, high power efficiency, wide speed range, and low maintainability. The biggest drawback of these motors is their complex control circuitry and noise — they can be loud. Once these cons are rectified, SRMs will likely take little time to replace induction and PMSM motors in EVs.
Conclusion
Brushless dc motors are preferred for low-speed electric vehicles due to their high starting torque and variable load capacity. Heavy EVs like buses generally use PMSMs. Induction motors are commonly used in electric cars and electric scooters.
SRMs provide the ideal combination of acceleration performance, speed range, power efficiency, reliability, and low maintenance. However, engineers still must resolve issues with SRMs in terms of complex control circuitry and noise control before they replace induction motors and PMSMs commercially.
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Filed Under: Electric Motor, FAQs