Retrotechtacular: DC To DC Conversion, Rotary Style

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.


24 thoughts on “Retrotechtacular: DC To DC Conversion, Rotary Style

  1. This setup of DC motor speed control was used extensively in tower crane hoist machinery in the past. Even up until as recently as the early 2000’s. Look up Ward Leonard Control. Our newest Ward Leonard crane has a 2004 manufacture date.

    A three phase AC motor is mechanically coupled to a DC generator; the DC generator has it’s armature directly wired to the armature of a large (160 HP) DC motor. By varying the field voltage of the generator with a low current supply, you get extremely precise and smooth control of the DC motor. Most of our cranes are also set up with field weakening for high speed. Under no load conditions, the field voltage of the DC motor is dropped below the nameplate rating, dropping the available torque, but increasing the rpm significantly – great for quickly bringing light loads up or down 500 feet the side of a building.

    It’s a super simple system that’s a pleasure to troubleshoot and work on, but ridiculously expensive to maintain.

    1. Rolling mills too. But with the fancy power electronics nowadays, replacing it with a solid-state alternative is probably cost-effective (in Australia at least, but probably not in India).

      1. As an Indian, I’m pretty sure construction (and any other industry that requires heavy duty machinery) only use modern technology nowadays, and it’s probably the only kind accessable to them. I would be very surprised to see any device that uses old technologies like dynamotors, or vacuum tubes over here (in India).

  2. Motor generators were also used to produce the different voltages for mainframe computers. Not only did it do a good job of producing different voltages and frequencies but also did an excellent job of smoothing out transients in the commercial power.

    1. Smoothing various things out seems to be a big part of the care and feeding of mainframes. An organisation I worked for had their beast housed in a multistory building that was underground and the entire structure was “floating” on piles of rubber and steel plates for seismic smoothing. We were not even in an area that has noticeable tremors, I suspect it was actually nuclear blast protection as the doors in the access stair well were arrange to prevent a blast wave propagating through the shaft, every other door opened the other way.

  3. Was there any internal modification to the “generator” washer motor?

    Disappointing that so many YouTube videos title repurposing a motor as a generator as “free electricity”. Had to clear browser history and cache so I could get out of that perpetual motion junk…

    Could use my 120 volt drill press for the motor side, and a ‘broken’ (because no batteries and won’t scrap it) cordless drill as the generator.

    Side note: My spellchecker doesn’t have “repurpose” or “repurposing” in its dictionary. Sad sign of the times.

    1. I think that’s still pretty common. We used a 400 or 500 HP motor-generator set to make 400V 50Hz out of 480V 60Hz to run our equipment before shipping to Europe.

  4. Motor generators were common, and still enjoy some uses for high current applications. For low current applications like the high voltage in tube type car radios, they used mechanical shoppers called vibrators to chop the DC into pulsating DC that could be ran through a transformer.

  5. Increasing the number of poles in the generator increases the frequency of the output.
    Even if you immediately rectify the AC output, the capacitors needed to smooth out ripple can be smaller [and cheaper].

  6. We had MG sets in the Navy to convert 450v three phase 60 Hz power to 450v three phaze 400 Hz power. It was a backup to our solid state ‘static frequency converter.’ That thing had huge capacitors that we had to discharge to do maintenance. Once, the bleed resistors failed; and we had to discharge the caps with a shorting probe. Pow!

  7. Its not DC/DC, but the same principal is still used as one of the most efficient ways to convert single phase to 3 phase (Rotary Phase Converters) when necessary.

  8. They are still used in specialized applications. Saw a massive motor-generator set used to convert power for an electric motor-driven full-scale rotor test stand at a helicopter company (1950s era, still in use as of a couple years ago). The visible part of the drive rotor on the motor it powers is the size of an SUV. The genset is housed in its own dedicated room, and is the size of at least 2 SUVs. Supposedly, until recently they had to notify the power company ahead of time when doing a whirl test. They still use the end of a broom handle to flip the massive switch to energize it.

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