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.
Perhaps one of the most unsual motor generator sets we’ve seen is using washing machine motors. If you have nothing to do for four minutes, you can watch the video, below.