EXRO TECHNOLOGIES INC. Electric vehicle adoption is accelerating because of technological improvements catalyzed by government policies and incentives, in pursuit of net-zero emissions.
In 2020, the global number of electric vehicles reached 10 million, yet they only represent 1% of all vehicles. It is projected that by 2030 more than a quarter of all vehicle sales will be electric worldwide. The barriers facing electric vehicle adoption which include charging infrastructure, range anxiety, and the cost and availability of EV models are well known. However, the fundamental challenge facing the electrification of mobility is the tradeoff that exists between speed and torque.
The fundamental challenge of e-mobility is easy to describe but has proven difficult to solve cost-effectively. A vehicle must be able to move around, go up and down hills, stop at a traffic signal, and accelerate on highways, but it also needs to do all this in a reasonable amount of time. A vehicle's ability to get the wheels into motion, go uphill and accelerate on the highway is dependent on torque. How quickly it can move and the time it takes to travel from one point to another is dependent on speed. So, the fundamental challenge that all EVs must solve is how much torque and speed are required to fulfill the applications that the vehicle is designed for. This is complicated by the fact that speed and torque are inversely related.
How Speed and Torque are Defined in an Electric Motor
Speed: The speed of a motor is defined as the rate at which the motor rotates and is measured in revolutions per minute, or RPM. In other words, the speed is defined as how many times the motor spins in 1 minute.
Torque: The torque output of a motor is the amount of rotational force that the motor develops and is measured in Newton-meters, or Nm. Simply put, torque is the twisting force of the motor but can be more difficult to understand than its counterpart speed, so let us take a deep dive.
Three Factors Govern How Much Torque is Required
Rolling Resistance
Rolling resistance is the friction/opposing force that the vehicle must overcome due to the rolling motion between the wheels and the surface on which the vehicle is moving. The rolling resistance is dependent on the material of the tires and the roughness of the surface. The higher the resistance between the tire and the road, the more force/torque that the motor needs to be able to move.
Grade Resistance
Grade resistance is the gravitational force that pulls the vehicle back when it is climbing an inclined surface. The steeper the incline/hill, the more force/torque the motor needs to make the climb.
Acceleration Force
Acceleration force helps the vehicle reach a predefined speed from rest in a specified period of time. The motor torque has a direct relationship with the acceleration force. Better the torque, the lesser the time required by the vehicle to reach a given speed.
Total Tractive Effort
The total tractive effort is the total force required to move the vehicle with the desired performance metrics and is the sum of the three forces listed above. So essentially, the amount of torque a motor will need to put out is determined by how much friction a vehicle has with the road, how much uphill and downhill travel is required, and how quickly a driver would like to go from zero to full speed.
Torque and Speed Relationship: An Analogy
Imagine a scenario where an individual has been asked to hammer a long nail into a wall. How many times the individual strikes the nail on the head in one minute is the speed, how hard each contact is with the nail, is the torque. However, the individual has a limited amount of energy, and increasing the force on each strike, or increasing the number of strikes per minute would result in higher energy consumption and would cause the individual to tire out faster.
Now, let's define two hypothetical situations, where in the first scenario the objective is to maximize the number of strikes per minute and a second scenario where the objective is to strike the nail once, as hard as possible. In the first scenario the individual would have to maintain a short distance between the nail and the hammer, and essentially wiggle their arm to record the highest number of hits. In the second scenario, the individual would have to give a full swing to achieve peak force.
The two scenarios illustrate the inverse relationship between speed and torque and the limitations that exist when the objective is to maximize both. Based on the individual's overall strength, or the power rating of the motor, speed and torque must be balanced to achieve the designated applications.
Vehicle manufacturers can also overcome this fundamental challenge by incorporating multiple motors into the system. With a two-motor design, there is often an induction motor in the back which is optimized for delivering high torque, and a permanent magnet motor in the front which delivers high efficiency at speed. There are also three-motor designs with two motors placed in the back for better traction, gradeability, and to overcome the disproportionate load on the back tires when the vehicle is accelerating. There are also four-motor designs in which each wheel is driven by a separate motor for ultimate performance and traction at a premium price.
Multiple motor designs have their caveats. Additional motors cost more and increase the system weight. Additionally, multiple motors consume higher energy, which translates to reduced vehicle range, and efficiency losses as energy is transferred through multiple components. The combination results in lower vehicle range and higher total cost of ownership.
Mechanical Gearbox
Another solution for overcoming the right balance of torque and speed for a given application is to incorporate a mechanical gearbox. A mechanical gearbox is a device used to increase the output torque or to change the speed of a motor. A gearbox consists of a series of integrated gears that alter torque and speed between the electric motor and a load, based on gear ratios.
A mechanical gearbox has a long service life and supports enormous strength; however, gearboxes have a high cost and are heavy relative to other components in a vehicle. There is also a loss of efficiency when power is transformed through mechanical components. These factors lead to lower overall system efficiency and higher cost associated with additional components.
Accepting the Torque vs. Speed Tradeoff
Another alternative to the fundamental challenge of balancing speed and torque is to accept the system limitation when it comes to certain applications. A racing motorcycle, for example, is designed for performance and thus can achieve extraordinary speeds but is incapable of hauling an RV, as it does not have the required torque for that application. A semi-truck on the other hand, is optimized for high torque and can pull an excessive amount of weight but cannot reach the high speeds that a passenger vehicle can.
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