10 things to consider when choosing an electric motor

May 17 2016

Selecting the right electric motor to fit a particular vehicle isn’t always straightforward. There are so many variables to consider that it can be difficult to know where to start. Given the price of batteries and electric motors, in order to find the most economical solution, you should look for a powertrain that will fit required vehicle performances as close as possible.

In this article, we will overview 10 basic questions that you need to answer before attempting to find the right motor for your project. Basically, you need to determine the most demanding requirements of your vehicle as well as evaluate  how various  road conditions will impact performance of the powertrain:

1. Vehicle characteristics

The properties of the vehicle such as size, weight, overload and aerodynamics are crucial vehicle characteristics that will ultimately determine speed, torque and power requirements of the electric motor. These aspects will help understand the effects of the operating conditions of the vehicle and are essential to the selection of the right powertrain. Have them within reach for the next steps.

2. Driving cycles

How is the vehicle being used is also very important. What will be the usual driving cycles of the vehicle? Will it be driven in an urban area with many stops? Will it be driven on long distances with only a few stops? All of this will help to determine the vehicle configuration (series hybrid, parallel hybrid, all electric) and battery pack size and ultimately impact the choice of the powertrain.

3. Vehicle configuration (electric, hybrid)

Is the vehicle hybrid or full electric? If hybrid, is it parallel hybrid or series hybrid? As a rule of thumb, if the vehicle routes are not predictable or if it will be driven on long distances, usually the hybrid architecture is preferred.

The full electric configuration is well suited for in city driving where the distance is not too long between charging points, the speed is low and the amount of stops is high.

 

vehicle-configurations-tm4

TM4 can offer most of these configurations.

4. Maximal speed

What is the targeted maximum speed of the vehicle ? How long does it have to be sustained, maybe it is used only for passing?

What are the gearbox ratios available (if using a gearbox) and the differential ratio? What is the rolling radius of the wheel? All of these questions must be answered and used in the calculations to determine the maximum speed the electric motor has to reach in your application.

5. Maximal torque

The maximum torque enables the vehicle to start in a given slope. You need to find the highest grade the vehicle will need to ascend. Using that grade, it is possible to calculate the highest torque required by the electric motor considering the differential and gearbox (if using a gearbox!). Maximal weight is also to be taken into consideration.

grade

 

6. Maximal power

Some grades need to be climbed at a minimum speed some others don’t. Sometimes the maximum power is found simply at maximum speed (this is the case where the vehicle as a large frontal area or goes at very high speed). This translates to having a motor powerful enough to go through all the different conditions the vehicle can be submitted to!

The maximum power enables the vehicle to reach and maintain a constant speed under stringent slope and speed conditions. To calculate the maximum power, you need to have a simulator that takes in account the drag and friction coefficients of the vehicle in addition to the forces needed for the climb.

Again, the duration of the condition also matters:  contrary to combustion engines, the peak power of the electric motor cannot be sustained continuously and it would be over engineered to select the electric motor to be able to do the worst hill climb conditions with no time constraints.

7. Battery Capacity

The battery capacity is typically calculated using a simulator to go through a reference cycle typical of the usage of the vehicle. The simulator can output the consumption of the vehicle in kWh/km. From that value, the capacity of the battery can be calculated by multiplying it with the desired range.

8. Battery Voltage

The battery voltage is dependent on the size of the vehicle. As the battery voltage increases, the current output is lowered. So in the cases where the vehicle continuous power is high like in bigger vehicles, you want to keep the size of the conductors at a manageable level by increasing the battery voltage.

There are normally two ranges of voltages: 300-450Vdc and 500-750Vdc. This is because of the voltage limitation of IGBTs used in the motor controller and the two main standard voltages available for them: 600Vdc and 1200Vdc.

9. Gearbox or direct-drive?

Will the powertrain architecture require a gearbox? Do you want to save the costs related to implementing a transmission or/and simplify your system?

TM4’s SUMO electric powertrain offer a direct-drive approach: the high torque/low speed of the motor allows it to directly interface with standard axle differentials without the need for an intermediate gearbox. While improving system reliability and reducing overall maintenance costs, removing the transmission in an electric vehicle also increases the powertrain’s efficiency considerably, allowing optimal use of the energy stored in the battery pack.

10. Cost

Last but not least, what is your budget? In a previous blog post, we reviewed the different electric motor technologies available on the market, their pros and cons and their relative usage in electric vehicles.

 

To sum up

Once you gather all the information mentioned above, you need the right tools to allow you to calculate the components requirements from the vehicle performance. TM4 can help you make an enlightened choice in your motor selection. Contact-us with the above information at hand.

Christian Pronovost

Christian Pronovost

Sr. Product Manager at TM4 Inc.
Christian Pronovost received a B.S. degree from École Polytechnique de Montréal in Canada in 1992 and has been at TM4 since 1998 as senior electrical engineer. He has been involved in the design of the core technology of current motors and inverters for automotive electric powertrain technologies. He is currently working as Product Strategy Manager for TM4’s product line; he is acting as lead product engineer defining market needs and innovative solutions.
Christian Pronovost

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