Recently we were contacted by the GravityLight™ team regarding a very interesting project they are working on, an alternative to kerosene lamps that is powered only by gravity. Kerosene lamps are still commonly used in the developing world, despite the numerous financial and environmental repercussions.
The concept of the GravityLight is elegantly simple. A weight is manually raised and as it falls it turns the shaft of an electric motor via a gearbox, creating a voltage across the motor’s terminals. This straightforward act provides the system with enough potential energy to provide approximately 30 minutes of lighting.
As they are conducting a rigorous and thorough design process, GravityLight asked if Precision Microdrives could assist them in their attempts to fully characterise their DC motor and subsequently gain a better understanding of their overall system. As Precision Microdrives has a wealth of equipment and expertise we performed a number of tests in order to characterise the DC motors used, including a 132-100 Miniature Core 32mm DC Motor which will be installed in Gravity Light’s own testing equipment.
We currently have facilities in Hong Kong, London and Shenzhen, capable of tests such as Mean Time to Failure (MTTF), Acceptable Quality Level (AQL), motor burn-in time, dynamic torque profiling, geometric characterisation, and more!
Our Hong Kong office offers the most advanced testing using a newly constructed torque dynamometer. The DC motors were not only characterised under normal operation in both directions, but the test equipment was also adapted to characterise the electric motors when operated as generators to simulate the intended real-world use.
The first step was to fully characterise the motors used in the Gravity Light equipment, both the driven motors/generators and the 132-100 test motor.
The industry standard for characterising electric motors is typically to take measurements at a no-load condition and a nominal loading greater than the rated load. However, as the GravityLight team were after a greater level of accuracy we conducted a more thorough analysis.
While dynamically adjusting the torque, three different relationships were observed:
- Current vs Torque
- Efficiency vs Torque
- RPM vs Torque
The second testing procedure involved characterising the motors when used as a generator. Two identical motors were used, one to act as the driving motor (simulating the falling weight) while the other was the Device Under Test (DUT). By measuring the input and output voltage and current it is possible to calculate the input and output power, of course, this makes it possible to calculate the overall efficiency of the system based on the change in power.
Precision Microdrives has been proud to be involved with the GravityLight project, all of the motors tested were found to perform very closely to their specifications. The output power of the GravityLight might be relatively low (you would need over 20 million of them on the highest power setting to produce the same amount of power as a 2MW wind turbine!) but it’s important to remember it is being used to light only a single room at a fraction of the cost of a kerosene lamp with no cost of fuel. This feat of engineering is especially advantageous because the user needs to manually lift the weight, the lower the power consumption the less frequently they need to lift it. The latest version of GravityLight has just been launched on Indiegogo, be sure to check it out!