Motor Dimensioning Worksheets

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The earliest steps into the design of this conversion project were taken towards selecting the right triphase electric motor. Our approach was to collect into a single document all the necessary knowledge to get to reliable results.

Such knowledge included all those automotive engineering fundamental laws, formulas and calculation aimed to match the wheel torque delivered by a certain motor with the drag torque applied to the wheels at certain driving conditions characterized by the car aerodynamics, speed and level.

This resulted in a workbook which represents the design workflow from the key car characteristics up to the graphic cross-evaluation between the available wheel torque curves and the needed drag torque curves which represents the needed torque for a car in order to keep similar performance after the conversion to EV

The workbook structure

Each workbook contains the following sheets:

  • The Key Parameters sheet: It takes in input the key characteristics of the chosen vehicle. It is the process starting point.
  • Acceleration Force: Nominal force to be applied to the vehicle for each of the gear ratios
  • Hill climbing Force: The force applied to the vehicle at different hill incline percentages for each gear ratio
  • Coefficient of Drag: shows a study of the Aerodynamic Force and the Rolling resistance as a function of speed and vehicle weight
  • Drivetrain Efficiencies: Calculate the losses introduced by the trasmission drivetrain and lists the gear ratios applicable for the chosen vehicle
  • Torque Required: Is a synthesis of the previous sheets and calculates in tabular form the overall resistent torque to the wheel as a function of speed and incline
  • Torque Available: It takes in input the chosen Motor Torque-to-Speed characteristics and converts them into torque at the wheel as a function of speed and incline

Clearly, the last two sheets can be cross-checked in order to evaluate the theoretical performance of a certain motor applied to a give vehicle and the ability to provide torque at different gear and incline conditions.

The Kevin dimensioning worksheets

Here below you may find the dimensioning workbooks customized with the technical data of the car we have used for the first conversion, a Ford Fiesta Titanium 1.4, and the torque-to-RPM characteristic curves from two candidate engines, the HPEVS AC-50 @ 144V and 108V, and the AC-51 @ 144V. The sheet name indicates whether the motor characteristics have been taken at Continuous regime regime conditions (cont) or at peak power conditions (peak).

These books are a practical example of the basic dimensioning approach. Note that only the sheets with the results expressed within the International Measures System (IS) have been used, but the same could be done using the Imperial system (the "US" sheets)