How to select motor, controller and battery parameters for an electric vehicle • EVreporter

The selection of the right powertrain plays an important role in a vehicle’s performance. The vehicle manufacturers need to review the characteristics like vehicle weight, speed requirement, gradeability, acceleration, loading capacity & any specific road conditions (e.g. Off-road conditions) for selecting the right powertrain for their electric vehicle. In this article, Varun Rai – Business Head at EMF Innovations, discusses how to go about selecting the powertrain parameters for an EV.

The electric motor used in an electric vehicle must produce the right amount of power required for traction purposes. The important factor is to select an appropriate rating of the motor based on the load to be carried. To arrive at the required motor power, we should first figure out the force required to move the vehicle as per desired specification/requirement.

The force required to drive the vehicle is known as tractive force.

Traction Force (Ftr) = Ftr = Fr + Fad + Fg + Fi

Fr = Rolling Force, Fad = Force due to Air-drag, Fg = Gravitation Force & Fi = Inertial Force

Rolling Force (Fr) = It is a resistive force in the motion of the vehicle. This is also known as the Rolling friction force.

Fr = m* g * Cr * Cos α

M = Mass of the vehicle

g = Gravitation Force (9.81)

Cr = Rolling co-efficient. It depends on the contact area between the tire and the road surface. If the coefficient of friction is high, then the force required to move the vehicle will be more.

Force due to Air drag (Fad) – The force which is faced by the vehicle as it moves through the air.

Fad = 0.5 * Ad * Cd *Af * V2

Ad = Air density

Cd = Coefficient of Drag

Af = Front Area of vehicle

V = Velocity of the vehicle

The formula above indicates that the Speed and the front area of the vehicle play an important role in Air Drag calculation. The front area of the vehicle should be optimised to reduce the Air drag force.

Gravitation Force (Fg) –

Fg = M*g* Sinα

M = Mass of the vehicle

G = Gravitation Force (9.81)

α = Gradient Angle

While selecting the motor power, we should consider the gradeability for which we are designing the vehicle. More gradeability has a higher torque requirement.

Inertial Force (Fi) – Force required to overcome the inertia of moving parts at a given acceleration.

Fi = ma

M = Mass of the vehicle

A = Acceleration.

Acceleration plays a major role in selecting the peak power of the motor. This is the force that major acts while we are starting the vehicle from 0 kmph and continuously once the vehicle comes into motion and catches speed. While cruising, this force becomes zero.

After calculating the Tractive force, we can calculate the torque on the wheel.

Torque on wheel = Tractive force / Radius of the wheel.

Torque of motor (T) = Torque on Wheel / Gear-Ratio

Rpm on Wheel = Vehicle Speed requirement / Circumference of the wheel.

RPM on motor (N) = Wheel RPM x Gear-Ratio

Motor RPM can be directly calculated from the speed of the vehicle.

In the case of the Hub motor, the RPM and Torque on the wheel are the same as on the motor. Whereas in the Gearbox/Chain-drive/Belt drive system RPM on the motor = Wheel RPM x Gear-Ratio.

After getting the torque and RPM required, we can calculate the motor power and Peak power.

P = 2πNT/60 [P = power]

Click here to refer to the calculations and also calculate the motor specifications for your use case.

Conclusion – We should select motor power based on vehicle characteristics like Weight, Front area, Maximum Speed requirement, Maximum Torque, Maximum Power, and Gradeability. Other parameters which we need to consider during the selection of a motor are Efficiency, Weight, Size, and Cooling requirement. Also, we should consider the operating temperature of the motor during the selection of the Motor for the vehicle.

Selection of the right controller for the motor is critical to derive efficient performance from the motor. Motor controller unit interfaces between the motor, Battery and other electronics (Throttle, Display, brakes etc) of the vehicle. It controls the speed and acceleration of the vehicle based on throttle input.

The selection of the controller is majorly based on the Motor power, System operating voltage, and Function requirement.

Controller Peak DC current = (Peak Power Requirement / System Voltage) x System efficiency during peak power.

The peak phase current of the controller is around 3 times the peak DC current. The battery operating voltage range should match with controller operating range.

There are other parameters which need to be considered during the selection of the controller, like control method – Trapezoidal or Field oriented control, Speed control mode or torque control mode, communication protocols, and operation control (Like manual or computer-controlled).

Battery voltage is dependent upon majorly vehicle manufacturers’ preference regarding the voltage. Generally, for a higher-power motor, a higher voltage is preferable. The selection of battery parameters is based on the range required for the vehicle and the capacity to provide peak discharge current and the duration for the peak current.

Battery capacity (Ah or KWh) = (Mileage Requirement / Avg speed) x Avg current or power consumption.

Peak Discharge current depends upon the capacity (C) of the battery and the chemistry of the battery or even the quality of the cell used. Other parameters like energy density, charging time, lifecycles, and operating temp range need to consider during the selection of the battery.

The author can be reached at [email protected].

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