How to Select Motor Power and Torque for EV

The determination of motor power and torque to be used in an electric vehicle (EV) is one of the most important steps when designing an EV. An undersized motor will lead to poor acceleration, overheating, and decreased reliability, while an oversized motor will increase costs, weight, and energy consumption.

The purpose of this article is to help engineers and manufacturers of EVs to accurately determine the appropriate amount of power and torque to use for their EV, as well as to cover the various factors involved in determining these amounts.

Understanding the Functional Differences between Power and Torque

Very Important: In order to accurately select a motor for an EV, it is important to understand the functional differences between POWER and TORQUE as they relate to motors. In the simplest sense:

(1) TORQUE refers to the amount of rotational force that can be produced by a motor. It directly affects the following:

Acceleration (the speed at which a vehicle can reach its maximum speed)

Hill-climbing ability or "Gradeability"

Ability to carry a load.

(2) POWER refers to how quickly (over time) that TORQUE can be produced.

It primarily affects:

Top Speed

Sustained driving performance

Ability to operate in high-speed highway driving conditions.

The most common application for torque and power in EVs is that torque primarily affects performance at lower vehicle speeds; whereas, power affects performance at higher vehicle speeds.

Phase 1: Determine Vehicle Application and Duty Cycle

When selecting a motor for an EV, the first step is to clearly define the intended application of the EV for which you intend to use the motor.

To do this, you should answer several key questions:

Will this EV primarily be used for urban commuting or for long-distance travel?

Will the EV carry heavy loads or will this EV primarily operate only when unloaded?

Is this EV going to experience a large number of start and stop operations?

Does the EV have to operate on flat surfaces only, or have to climb steep slopes?

All EV types (passenger cars, electric forklifts, golf carts, AGV's, utility vehicles, etc.) have different torque and power requirements, despite being operated at similar speeds.

Phase 2: Calculate the Requirement for Wheel Torque

Motor torque is primarily determined by the resistance forces that will act on the vehicle's wheels, which can include:

Rolling Resistance

Aerodynamic Drag

Grade Resistance (Slope)

Acceleration Force

On low-speed starts, the demand for wheel torque is at its highest. The motor must provide the required amount of torque (after any required reduction through the use of a gearbox, if applicable) to overcome these forces under worst-case conditions.

The majority of EVs operating in industry (such as in manufacturing or distribution applications) need to provide a greater amount of starting torque than other EVs, such as golf carts and AGV's (automatic guided vehicles).

Phase 3: Calculate Required Targets for Acceleration and Climbing Ability

Both acceleration performance and climbing ability have a major impact on torque selection when using Earth-emitted torque to determine torque performance capability in an EV.

When determining acceleration and hill climbing capabilities, you should consider the following criteria:

Desired time to achieve maximum acceleration (i.e., 0–30 km/hr)

The maximum slope that must be climbed by the EV

Vehicle's mass when fully loaded.

When using a higher magnitude of torque provides the following benefits:

Greater Acceleration Response

Stable Operation on Ramps and Slopes

Reduced Stress on Drivetrain (Drive Train Components) of the EV

In general, when designing commercial and industrial EV's, continuous torque capability is more important than short-duration peak torque capability.

Speed and continuous operational conditions of the vehicle. Power requirements increase with the speed of the vehicle because:

• Aerodynamic drag increases with speed

• The vehicle's sustained load at cruising speed

The highest power demand coincides with high speed, whereas the highest torque demand coincides with low speeds.

Factors That Determine Most Important Factors

When calculating the required motor power and torque, there are multiple factors to consider including:

• The maximum speed of the vehicle

• Duration of time at maximum speed

• Thermal limits of the motor

To be considered properly sized, the motor should operate most efficiently at the most frequent driving speed of the vehicle and not only at its peak output.

Gear Ratio and Driveline Layout

The power and torque of the motor cannot be determined until all driveline components have been considered. The driveline layout design must take into account the following:

• Whether to use a single-speed or multi-speed gearbox

• Whether to have a direct drive or reduction

• The efficiency of the differential and axle

When calculating the gear ratio, a properly sized motor provides sufficient wheel torque and can be better utilized at every operating range. By optimizing gear ratios, EV designs can reduce the physical size of the motor while still maintaining performance.

Continuous versus Peak Ratings

Most EV motor types can perform well in both peak (short-term) and continuous (thermal limited) aspects. An analysis of continuous ratings is essential for determining the reliability and durability of a motor when operating normally. The continuous power and torque rating will provide assurance of long term performance; peak power and torque values are only typically applicable during events of acceleration or rapid changes in operation.

If an electric vehicle designer uses only the peak rating of the motor when selecting, the designer may miscalculate the continuous ratings. This can lead to overheating and in some cases extensive damage or shorter than normal life.

Motor Specifications Matched With Control Strategy

Motor controllers and control strategy will directly affect how usable torque and power are derived from the motor. Items to take into account are:

• Field weakening capabilities

• Precision of the torque control

• Regenerative braking capability

Electric Vehicles (EV's) most commonly use wide speed range motor designs and advanced control algorithms to manage torque, power, efficiency, and thermal performance.

Common Motor Selection Errors

Common errors made by EV designers when selecting the electric motor include:

• Over-sized or not sized for the power of the motor; this will lead to not accounting for duty cycles.

• Ignoring the continuous torque requirements.

• Using the peak torque numbers as opposed to the usable torque at the wheel.

• Failing to accurately determine what type of driveline the motor is being attached to.

By avoiding these types of mistakes, designers can enhance the efficiency of the electrical system and in turn decrease the total cost of the vehicle.

Conclusion

Deciding on an electric motor's power and torque is an engineering decision at the system level and requires more than just the selection of a single parameter. The correct motor power and torque selections must take into account:

• How the vehicle will be utilized and its operating environment

• What level of torque will be needed for both low-speed operation and load capacity

• What quantity of power will be required to sustain cruise speeds

• Overall operation including all driveline components, control strategy, and thermal limits.

Through the balance of these factors, the electric vehicle designer can best utilize the variables to create an optimal performing electric vehicle that has superior efficiency, reliability and cost characteristics.