Battery charging Motor-Generator set
Background
In the 1930s, if a DC supply was needed capable of delivering hundreds of amps at a variable voltage, one of the most satisfactory ways of producing it from the fixed voltage electrical supply was using a motor-generator set. Not to be confused with a generator driven by a prime mover such as a steam engine, the motor-generator uses an electric motor to convert energy from the electrical supply into mechanical energy, which in turn is used to drive a generator to convert it back into electrical energy. This may seem a little longwinded and inefficient but there was sound logic behind it. Firstly, the motor could be made for whatever kind of power was available. DC or AC, high voltage or low, the only function of the motor was to turn the shaft at a steady speed. The generator likewise could be made for exactly the type of output desired, be it DC, AC, unusual frequencies or voltages, even multiple independent supplies could be produced by driving two or more generators from a single motor. The static transformer provided a more efficient solution to convert AC from one voltage to another, but the motor-generator set offered the more exotic scenario of frequency conversion, say from the 50 hertz supply network to 400 hertz for instrumentation, which a conventional transformer cannot do. In the field of heavy-current supply for electroplating, the generator could be designed to provide many thousands of amps at just a few volts; at this they were more or less unrivalled. Another advantage was the ease with which a variable output voltage could be achieved, as a suitably wound dynamo can be controlled easily by a small field rheostat to vary the voltage down to nearly zero. Thus they found widespread application for variable-speed motor drive (often called Ward-Leonard drive after the inventors), battery charging, carbon arc supply etc.
The Secret Motor-Generator