Eco-marathons are engineering competitions where students build vehicles to travel as efficiently as possible. To engineer an efficient electrical drivetrain, test equipment is necessary to test and improve the drivetrain in a lab setting.

Bristol Racing is the University of Bristol's student society electric race team. We are principally interested in electric vehicles to compete in competitions such as the Shell Eco-Marathon and the Greenpower Challenge.

This project describes the construction of a dynamometer which is a device that can measure a motors torque and speed under a variety of load conditions. The dynamometer will allow the batteries, power electronic converter and motor in the car to be thoroughly tested in a lab setting. The dynamometer is designed for the official Greenpower motor however the design can be easily modified to work for other motors.

The dynamometer design was inspired by Jeremy Fielding's design (video #077). The design works by suspending the motor about its rotor. A torque arm extends from the base of the motor to a load cell. The motor exerts an equal and opposite force on its base due to Newton's Third Law, the force is piped to the load cell which can be used to calculate torque.

The rotor of the motor is connected to a car alternator via a belt connection. The alternator can provide a variable mechanical load to the motor, to simulate race conditions. The alternator load can be programmatically adjusted using a programmable power supply.

The speed of the rotor is detected by attaching a magnet to the rotor and a hall effect switch is used to detect the rotating magnetic field. An Arduino can count the speed of the hall effect switching which can be used to calculate the angular velocity of the rotor.

The speed and torque measurements are piped from the Arduino to a Python Jupyter via a serial connection. The Jupyter notebook script graphs and stores the data for post processing.

Experimental Setup

Motor Step Response

Currently the data is looking promising however we have hit significant issues with the pulleys. The original pulleys were 3D printed from PLA. A significant amount of heat is generated form the belt resulting in the PLA melting. To overcome these issues, we are looking into chain connections or modifying commercially available metal pulleys to fit the motor and alternator shafts. Alternately we may adapt the dynamometer to act against a brake however this reduces the programmability of the load.

Melted Pulley

Once the mechanical braking has been solved, we are aiming to thoroughly test the motor and surrounding electrical drivetrains. Planned experiments include thermal testing and optimisation, sweeping the motors operating range and testing the whole electrical drivetrain in race simulation.

It’s been a fun project so far! Please let me know if you have any ideas or feedback on the project!