The Dynamotor Simplified

[Robert Murray-Smith] doesn’t like the price of inverters to convert DC to AC. That led him to build a dynamotor, or what is sometimes called a motor-generator set. These devices are just DC motors driving a generator. Of course, motors can also be used as generators and [Robert] had a stack of brushless motors in the form of PC fans. A two-fan dynamotor was born.

The brushless motors are attractive because, traditionally, the brushes are what usually fail on a dynamotor. The fan that will act as a generator needs some surgery, but it is simple. He scraped off all the control electronics and connected wires to the coils to form a three-phase generator. There’s no need for the fan blades in that configuration, either. If you were using ordinary motors and a generator, getting shafts concentric would be an important task. With the fans, it is simple to just line up the mounting holes and you get perfect alignment for free.

How does it work? [Robert] has a second video showing the output on a scope. You can see both videos below. The dynamotor makes a good-looking sine wave, probably much better than most reasonable-priced solid state inverters. He didn’t mention how much current he could successfully draw, but it probably isn’t much. You’d also need a transformer to replace a commercial inverter that would put out line voltage, so that would be some more loos in the system. On the other hand, if you wanted AC at a lower voltage, you might just replace all the transformers, if you were building a piece of gear yourself.

We’ve looked at how these things work in some detail. There were common in old tube radios, particularly military ones.

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Antique Army Surplus Receiver Restored

If you’ve ever been to a hamfest in the United States, you probably have at least seen an ARC 5 Command set. These were very rugged receivers and there were a ton of them made. Hams have been reworking them for years. In a recent video [Tom N3LLL] shared some of his tips for restoring them.

You might think these are just like a regular old radio, but there are some unique challenges, including capacitors filled with beeswax and strange threaded screws. [Tom] made several custom boards to replace the dynamotor with a solid-state inverter, replace odd capacitors, and provided a faceplate. He also 3D printed some replacement studs to replace the often decayed anti-vibration studs for the dynamotor.

The teardown at the end shows how rugged these things are. Tom’s restoration philosophy is to modernize the set while keeping the outward aesthetics. The receivers perform well, and as you might expect are built like tanks.

If you want to try your hand at restoration, these are not very expensive because there were so many of them made. Often the shipping is about the same price as the radio, but one in good shape can cost a bit more. We think the real fun is getting one that is not in such good shape and making it better.

Everyone has their own style and we know some restorers are more purists, but as a practical matter, [Tom’s] restorations look great, sound great, and preserve these great old radios so that someone might still be using them in another 75 years.

We’ve covered the ARC 5 before, unsurprisingly, and that restoration was a bit more traditional if you prefer it that way. If you need something to listen to on the AM band, try a matching transmitter.

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DIY Tiny Dyno

The geared DC motor has become the bread-and-butter of the modern-day beginner project. Unfortunately, with the advent of vast online catalogs peddling a wide assortment of these mechanical marvels, validating the claim that one DC motor will outperform the others is a challenge.

Such is the dilemma that our own [Gerrit Coetzee] faced as he set out to buy these geared motors in bulk. In his initial teardown, he quickly compares the change in design, from the original which possess the two-part clutch that extends on overloading, to the clones with the feature disabled altogether.

He then goes on to research methods of measuring the motor’s output where he discovers the Prony Brake which leads to the Rope Brake Dynamometer. This is where things get interesting and [Gerrit Coetzee] goes on to hack his own version of the machine. The idea is to have a rope wound to the wheel that is powered by the motor. With one end of the cord attached to a spring scale and the other end to a suspended weight, the motor speed affects the force on the spring scale. This change in force measured by the scale can be used to calculate the power output by the motor.

[Gerrit Coetzee] goes on to replace the weight with springs and the scale with an electronic load cell while using a stepper motor to stretch the cord thereby adding the requisite tension to the string. We thought this was a very elegant solution where the entire experiment could be controlled electronically.

This is a work in progress through the writeup is an excellent example of how to tailor a traditional experiment to the modern times. We have seen similar investigations for larger salvaged motors and dynamometers with lots of sensors.

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.

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