Professor Jerry Connor from the Civil Engineering Department at MIT gave a seminar at the Institute on the topic of “Electromagnetic Damping Devices”. Jerry is a member of the Institute’s Board of Directors and a frequent visitor to Ashurst Lodge. He also gives advice to the Boston office.
Jerry referred to the need to structural damping in many buildings in order to control their motion. Traditional dampers are based on viscous fluids contained in cylinders, such as those used for the Millenium Bridge in London, as well as many other structures around the world. They are expensive, require special care due to leaks and are manufactured by only a few companies around the world.
As a result, Jerry started to investigate the possibility of using electromagnetic dampers. Currents can be created in a fixed magnetic field by the movement of a coil. The electromagnetic force is proportional to the magnetic field and the area of the coil within the field.
Lenz’ law gives the value of the force F opposing the motion which is a linear function of the velocity v, ie F=Cv, where the coefficient c is proportional to the square of the magnetic field and the length of the coil transverse to the field. It is also inversely proportional to the electrical resistance.
The main difficulty of these devices is to have a sufficiently large number for C as to be able to dissipate the energy during motion. A simple way of increasing C is to work with a large number of coils rather than just one.
Another type of damper results from having a fixed coil and a magnetic piston-like component with its corresponding field.
Jerry presented several calculations to see if the scheme was feasible in engineering practice. The results demonstrated that the electromagnetic forces are not sufficiently large for the case of structures subjected to earthquakes. One could try to increase the resistance but this would create a problem to dissipate the heat produced. Another problem of these devices is the need to guarantee the integrity of the coils when subjected to large earthquake forces. It has been proposed to embed them in a composite.
Jerry calculated the power required by the system to balance the force of the earthquake and the same problem as for the first type of dampers occurs, ie the number of dampers needed would be uneconomical.
The systems of electromagnetic dampers for buildings are still the subject of further research by Jerry’s group. What is needed is an efficient way of combining sufficient numbers of coils and magnets to produce devices that can generate the high magnitude forces needed in practice.
There were numerous questions from the audience which resulted in lively discussion