Scott Swaaley On High Voltage

If you were to invent a time machine and transport a typical hardware hacker of the 1970s into 2018 and sit them at a bench alongside their modern counterpart, you’d expect them to be faced with a pile of new things, novel experiences, and exciting possibilities. The Internet for all, desktop computing fulfilling its potential, cheap single-board computers, even ubiquitous surface-mount components.

What you might not expect though is that the 2018 hacker might discover a whole field of equivalent unfamiliarity while being very relevant from their grizzled guest. It’s something Scott Swaaley touches upon in his Superconference talk:  “Lessons Learned in Designing High Power Line Voltage Circuits” in which he describes his quest for an electronic motor brake, and how his experiences had left him with a gap in his knowledge when it came to working with AC mains voltage.

When Did You Last Handle AC Line voltages?

If you think about it, the AC supply has become something we rarely encounter for several reasons. Our 1970s hacker would have been used to wiring in mains transformers, to repairing tube-driven equipment or CRT televisions with live chassis’,  and to working with lighting that was almost exclusively provided by mains-driven incandescent bulbs. A common project of the day would have been a lighting dimmer with a triac, by contrast we work in a world of microcontroller-PWM-driven LEDs and off-the-shelf switch-mode power supplies in which we have no need to see the high voltages. It may be no bad thing that we are rarely exposed to high-voltage risk, but along the way we may have lost a part of our collective skillset.

Scott’s path to gaining his mains voltage experience started in a school workshop, with a bandsaw. Inertia in the saw kept the blade moving after the power had been withdrawn, and while that might be something many of us are used to it was inappropriate in that setting as kids are better remaining attached to their fingers. He looked at brakes and electrical loads as the solution to stopping the motor, but finally settled on something far simpler. An induction motor can be stopped very quickly indeed by applying a DC voltage to it, and his quest to achieve this led along the path of working with the AC supply. Eventually he had a working prototype, which he further developed to become the MakeSafe power tool brake.

Get Your AC Switching Right First Time

The full talk is embedded below the break, and gives a very good introduction to the topic of switching AC power. If you’ve never encountered a thryristor, a triac, or even a diac, these once-ubiquitous components make an entrance. We learn about relays and contactors, and how back EMF can destroy them, and about the different strategies to protect them. Our 1970s hacker would recognise some of these, but even here there are components that have reached the market since their time that they would probably give anything to have. We see the genesis of the MakeSafe brake as a panel with a bunch of relays and an electronic fan controller with a rectifier to produce the DC, and we hear about adequate safety precautions. This is music to our ears, as it’s a subject we’ve touched on before both in terms of handling mains on your bench and inside live equipment.

So if you’ve never dealt with AC line voltages, give this talk a look. The days of wiring up transformers to power projects might be largely behind us, but the skills and principles contained within it are still valid.

29 thoughts on “Scott Swaaley On High Voltage

    1. Yeah, this is an OK primer on high power (compared to what most fresh hackers use) but I’d just come back from my lab, where I was working with a 50 kv, 2 kw DC power supply and a 1kw RF amp putting out 3kv PP into my vacuum system’s plasma device….So I had a chuckle…on the other hand, Jenny does find the good ones.

      Yes, you do have all the problems he mentioned with mains voltage, one of the biggies is as he said, the enormous peak amperage available if you screw up – seriously high power (moreso than high voltage) and yes, if you don’t handle kickbacks correctly, your semiconductors and even relays aren’t going to live long. Failing to handle that well can also kick enough of a spike back into the line to bother other things….

      I really doubt that you would fry a motor’s windings with a sudden stop unless things were really close to going up in smoke already. We’re talking about a second’s worth power to spin it up – or down…maybe put in a shorter time if you stop fast enough, but by definition, fast means it’s not there for long and copper does have thermal mass.

  1. >”An induction motor can be stopped very quickly indeed by applying a DC voltage to it”

    Permanently, just as quickly.

    You’re dumping all the inertia as heat in the rotor, which is then left without cooling as it stops spinning. The magnet wire insulation gets a terrible heat shock.

    1. Actually many power tools just short the motor out to stop the blades fast. I am sure that it heats the motor up a bit but the amount of self eating goes down with the speed of the motor, until both are stopped. I have not seen a power tool that uses DC to stop a motor. I have worked with CNC mills decades ago that used saturable core reactors to vary the drive to motors, but that is entirely different.

    2. I’ve done quite a bit of testing on this, actually, with thermocouples on the core, the windings, and the chassis. With proper braking, temperatures don’t get anywhere near the temperature rating of most windings (155 C). In fact, the power entering the motor during braking is significantly less than the startup power of most induction motors.

    3. There is no magnet wire in the rotor of an asynchronous motor, it’s a cage of aluminium (usually) with iron laminations. The rotor isn’t damaged by heat easily, the thermal capacity is huge, and with a direct grid connection (no inverter) there is a limit to the inertia the motor can drive anyway.

      You are absolutely correct that all the kinetic energy gets converted into heat in the rotor, but provided you don’t start and stop the motor every few seconds, it’ll be fine.

  2. A spinning motor is a generator. I would have dropper a constant current load across the supply lines. Even a fixed load like a resistor would do but it may slow down slower as it get slower.

        1. Actually, I took a class on motors and one lab we actually spun up an induction motor then kicked it into generation mode. As long as the rotor doesn’t slip too far, the induced voltage on the rotor will be maintained by back emf from the stator windings.

          1. Nope, single phase. Get two motors coupled, say a separately excited DC and our induction motor, preferably with pulleys but straight up coupled rotor to rotor works too. Apply power to the induction motor until it’s spun up. Then, cut power to the induction motor (you have a measurable amount of time to delay here), and apply power to the DC motor. Provided the speed is closely enough matched with the induction’s rated speed, you will have a loadable AC signal from the induction motor. Once loaded too much, the voltage in the rotor collapses, taking the magnetic field with it, and the signal will attenuate to a mere few volts.
            Has anyone tried plugging in a small hand tool into the plug of a grinder that’s spinning down? I’m guessing a brushed motor would spin happily away the whole time. Or, maybe nothing or maybe burn up windings.

          2. About three decades ago, So I’ve long since forgotten the specifics involved.
            I personally did this with a 5hp compressor motor and a gasoline engine.
            Using various power tools, I guesstimated the setup provided a bit under half of the rated amperage draw of the electric motor, when used as a prime mover.

            Seems like there were some David Gingery books that covered such things.
            Probably just find the info online nowdays.

          1. I’ve built the same thing, it works fine. Once the induction starts it seems to self-excite with positive feedback. I’ve started an induction motor generating just by spinning it. In the video there’s no claim for any kind of energy saving, they are just burning waste vege oil to electricity.

          2. @light beams

            Fair enough, I’ve never tried it myself. The free energy/over unity comment was based on the results from searching the topic, not specifically the actual video.

    1. Agreed…. as a 30+ yr lineman, this stuff is ‘nuthin’ – relatively speaking.

      Try replacing circuit breakers on switch gear that operates at 400kv+ / kilo-amp levels.
      Where an arc flash blast/plasma ball can turn you into crispy ash (you die before you can react).

      1. After one of our “Sparkies” was blown up pushing a disconnect button on a switch gear panel, the system was replaced with a delayed switch so the worker can exit the area. He did survive, and was promoted to avoid a law suit. Small panel actually, only 66kv. Engineering had reduced switch gear maintenance. and a bit of dust built up. Recommended maintenance schedule was restarted.

    2. I laughed when I saw that too. When I was in my teens in the ’70s, I worked with 500V to maybe 1000V power supplies using mains transformers. These power supplies were used to power RF amplifiers and a tube-type Tesla coil. some of them could supply an amp or more current at those voltages. Hams back then really had to take safety seriously…

      1. Microwave oven transformers are the last reliable source to that kind of (terrifying) voltage and amperage combination but they are quickly being replaced by inverters. If you haven’t started, it’s time to stockpile.

        1. On the other hand, there’s no better time than the present for getting high power output small transformer inverters, so perhaps you won’t care for your stash of heavy fixed output current limited transformers when all your friends are using their $10 off ebay 10kW inverters :)

  3. This is an interesting point, as I refer somewhat frequently to Graf’s most excellent Encyclopedia of Electronic Circuits, which dates from the 1970s. The first few volumes adorn the shelf in our makerspace’s electronics lab, and make great lessons because the circuits specify as few component values as possible. But anyway, that’s beside the point.

    The point is that I was struck by the same thing, flipping through them for the first time. Lots and lots and lots of line-powered circuits, 120V as far as the eye can see, in certain chapters. They’re a treasure trove when you want to work with AC, but a bit lacking in the microcontroller department! Thankfully there’s more stuff on the bookshelf…

  4. Since nobody really uses the term medium voltage, the dividing line is 48 volts. It’s code as far as I know. Lower than that unless your are showering and tongue tasting the wire it’s low voltage. Higher up, it’s gotta be in conduit etc. Low voltage DC can bite, but it’s inductive kickback that does it.

  5. To continue flogging dead mules. Technical nomenclature errors are troublesome and should be of significant concern where coming from educators. And more so disconcerting when coming from a licensed P.E. Much on my day-job is wasted on educating managers, educators, and fellow engineers that have been exposed to half-truths devoid of engineering rationale and formally recognized descriptors.

    The definitions for low, medium, and high voltages are well defined per the local and national code, regulations, standards, and directives. ‘Low’ voltage, depending on the end-use scope of the equipment and the jurisdiction, ends between 600V and 1500V.

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