The Synchronous Rotary Mechanical Bridge Rectifier.

Synchronous spark gaps are deservedly popular with many people who like to build Tesla coils. So is dc operation of Tesla coils. However, modern semiconductor rectifiers are not altogether happy with high levels of rf and have limits to their current capability and peak inverse voltage tolerance. The old fashioned synchronous rotary mechanical bridge rectifier (SRMBR, the original bridge rectifier) offers a solution to this problem.

Here is a 250kV SRMBR built by Siemens Brothers and Halske for hospital therapeutic x-ray service around the time of the First World War. With suitable tubes it produced x-rays of 0.05 Angstrom for treating tumours. The transformer is composed of two 125kV units in series on a common iron core and the secondaries are oil filled. At the front are two chokes (horizontal circular 'pies' on insulated stands) to help protect the transformer against rf from the sparking at the rectifier. The synchronous motor is mounted in the centre and above the transformer. An insulated shaft extends either side of the motor. Mounted on both sides of the motor are two heavy wires at 90 degress to one another, but one of the total of four wires is invisible owing to the angle and a support which obscures it. The wires in the 10 o'clock - 4 o'clock direction are most easily seen, but one wire at 7 o'clock can be seen just clear of the end support at the left front. The stationary electrodes of the bridge can be most clearly seen at the front of the array, where on the left hand side of the motor, two stationary electrodes are commoned at the top and connected to one of the chokes by a downcoming vertical wire. The action of the bridge should need no further explanation - it's a pretty simple piece of technology, and a semiconductor bridge simply replaces the rotating mechanical switches by solid state ones, so if puzzled you ought to be able to figure it out from there. The sketch below shows the general arrangement.