The homopolar generator


In principle a homopolar generator consists of a conducting disc spinning about an axis in a magnetic field parallel to axis. When the spinning disc stopped suddenly, all its kinetic energy can be used to generate large current surges.

In order to spin the disc up to speed, a DC power supply is connected. The magnetic force on the current crossing from the axle to the rim of the conducting disc provides the necessary accelerating force. As the conducting disc speeds up, however, there is an increasing voltage generated between the terminals. When the power supply is disconnected this voltage can be used to drive a current through a resistor connected between them.

The main purpose of homopolar generators is as research tools to produce huge surges of current when their terminals are suddenly short- circuited. Apart from increasing the magnetic field, higher generated voltages can be obtained by increasing the speed of rotation or the diameter of the disc. The speed cannot be increased indefinitely as the speed of the edge of the disc is limited to a maximum of about 200 ms-1  by the mechanical properties of the material, usually steel, from which it is made.

One large homopolar generator in Australia, which is designed to produce huge current surges, measures 3.6 m in diameter, rotates  at 15 Hz and is so massive that the kinetic energy it stores at this speed is 580 MJ. When it is short-circuited, the current surges are used to produce short-lived, but extremely high magnetic fields in order to study the properties of matter under extreme conditions. Such fields, it is proposed, could be used in an electromagnetic gun to project a small mass at speeds of over 7km s-1 .This speed is of the order of the speed of satellites in low orbit and hence the projected masses could be used to study the problems encountered by missiles re-entering the atmosphere.