If you want to convert one voltage to another, what do you do? Well, if you are talking DC voltages today, you’ll probably use a DC to DC converter. Really, these converters generate some sort of AC waveform and then use either an inductor or a transformer to boost or buck the voltage as desired. Then they’ll convert it back to DC. If you are talking AC voltages, you could just use a transformer. But think about this: a transformer has two sides. The primary makes an alternating magnetic field. Just like rotating a shaft with magnets on it could. The secondary converts that alternating magnetic field into electricity just like a generator does. In other words, a transformer is just a generator that takes an AC input instead of a rotating mechanical input.
That’s a bit of an oversimplification, but in the old days, a lot of mobile radios (and other devices) took this idea to its logical conclusion. A M-G (Motor Generator) set was little more than a motor connected to a generator. The motor might take, say, 12V DC and the output could be, for example 300V AC that would get rectified for the plate voltage in a tube radio.
In many cases, the motor and generator were together in a unit known as a dynamotor. These were often seen in old GE two-way radios (like those used in police cars). The dynamotor only ran during transmit, so keying the transmitter would cause a high-pitched whine to emit from the vehicle’s trunk (the radio was in the trunk; only the control head was in the cabin).
You can see a video of a working 1950’s-era dynamotor, below. Notice how you can gauge the amount of current produced by the whine of the motor.
This is a high-powered solution. If you didn’t need much power — for example, for a regular AM car radio — you’d probably use a vibrator. This is the same idea of converting DC to AC so it can go through a transformer. But the conversion is done with a relay-like device.
In addition to old radios, the dynamotor or motor generator also found use in elevators (because of the need for high-current DC), and trains (DC voltage conversion). Also, if you absolutely need total isolation between input and output, a non-conducting shaft between the motor and generator can get you there.
Still, radios were probably the most common use, including military gear like the ARC 5. You can see a demonstration of troubleshooting the ARC 5 dynamotor in the video, below.