How do they Work?
The basic concept of this motor-generator is that it makes use of regeneration to enhance its gain. Energy is produced by the motor and the output power is managed by altering the field current of the generator. Considering the case of a typical generator, the load brushes are situated perpendicular to the magnetic field flux.
If you want to convert a generator to an amplidyne, you will have to connect what seems to be the load brushes jointly and get the output from another set of load brushes parallel to the field. The brushes that are perpendicular are called the 'quadrature' brushes.
Such a simple modification can enhance the gain by 10,000 times or more. In the above case of the GE Amplidyne 115 to 250 Volt Motor Generator, the characteristics of the generator facilitate generating around 87 ampere of armature current at 115 volts at the terminals for output; the values range accordingly for variations up to 250 volts.
This indicates a power output of around 10,000 watts (P= IE), implying a power gain of 100. In short, 100 watts control 10,000 watts of output, and this is how an amplidyne works in simple terms.
However, today MOSFETs can produce even high power gains of as high as 1,000, and IGBT are also capable of achieving power gains in the range of 500-2000, which is why you don't get to see amplidynes in action anymore.