How to make an AC Softstarter because my autotransformer keeps tripping my circuit breaker!

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In this project video I will show you why my new autotransformer keeps tripping my circuit breaker and how I built an AC softstarter in order to “softly” start the transformer so that it does not trip my circuit breaker. Along the way we will find out how exactly this problem gets created and how you can fix it easily. BTW, such softstarters are also useful for motors 😉 Let’s get started!

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Music:
2011 Lookalike by Bartlebeats
Killing Time, Kevin MacLeod
(incompetech.com)

0:00 Showing off the autotransformer
1:04 The problem with the transformer
1:40 Intro
2:13 Functional Principle of an autotransformer
4:27 Finding the problem
5:52 The reason for the problem
7:10 DIY Solution 1 (Simple)
8:15 DIY Solution 2 (Softstarter)
10:33 Final Test & Assembly

source

26 thoughts on “How to make an AC Softstarter because my autotransformer keeps tripping my circuit breaker!

  • March 28, 2021 at 8:58 pm
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    Seems like I made a rather big mistake in the video. My transformer should not be switched on during the zero crossing point but at the voltage peak in order to decrease the current flow. While recording the current waveforms on the oscilloscope, everything appeared good to me and I am not sure why my circuit breaker stopped tripping when I switched at the zero crossing point. But many viewers pointed me in this direction and after doing research, it seems to be correct. Sorry about this. But if you want to build the softstarter you can still try it out whether it works for you and if not then you can always change the Arduino code in order to switch at a different time point. Sorry again for the inconvenience. I do make mistakes from time to time since this is a one man production. Stay creative 🙂

  • March 28, 2021 at 8:58 pm
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    I don't think the inductive reactance magically disappears on power up leaving only the resistive component. The sum of both is just too high for that type of circuit breaker. 🙂
    You need to build a circuit breaker with an Arduino in it. Oh, and a TCP/IP connection to the Web.

  • March 28, 2021 at 8:58 pm
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    As you mentioned, even a zero volt point initial turn-on is not always curing the problem. The reason is the remanent magnetism in the transformer core. If the magnetism left in the core at previous turn-off matches the direction of the start time voltage growth slope, there still is a big inrush. The theoretical analysis considers the fact that there is a 90 degree phase shift between the voltage and the current (on the primary). So, at the positive going zero crossing of the voltage, the usual transformer operation condition would have actually the negative peak current at this moment. You don't have that, if you make the primary connection at the voltage zero crossing moment. Your gradual increase of the voltage is indeed the way to prevent the breaker popping inrush. As funny as it may sound, if the transformer was turned OFF gradually, you would get near zero remanence and then the optimum time to turn it again on would be near the voltage peak (either positive or negative) !!! Too bad, you generally cannot know the state of the remanence, nor force it to any known state. I hope this explanation can serve other people who may suffer from the same issue and are wondering why.

  • March 28, 2021 at 8:58 pm
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    Glaubst du, so ein Softstarter könnte mir bei meinem Problem auch helfen?
    Manchmal wenn ich die Steckdosenleiste, an welcher mein Fernseher und meine Spielekonsolen hängen einschalte, fliegt die Sicherung und manchmal nicht.
    Dieses Problem tritt nämlich nicht immer auf. Manchmal fliegt die Sicherung und manchmal nicht.
    Da mein Zimmer und das meiner WG Kollegin an der selben Sicherung hängen, gehe ich davon aus, dass wenn zu viele Geräte eingeschaltet sind, und ich dann die Steckdosenleiste meiner Konsolen einschalte, dass durch die Spannungsspitze beim Einschalten die Sicherung rausfliegt. Wenn ich dann die Sicherung wieder einschalte, kann ich zocken und alle anderen Geräte laufen auch wieder.

  • March 28, 2021 at 8:58 pm
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    If you decide to use your small variac again you might want to use ring terminals and nuts instead of directly soldering wires. It should be safer and more durable.

  • March 28, 2021 at 8:58 pm
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    I had this problem with a audio PA (power amplifier) close to 800wrms, it has a really heavy chunky toroidal transformer with an added problem, huge value capacitors. To make the things even worse it is connected on a multiple output socket controlled by a relay in order to switch the complementary systems like an AV amplifier, equalizer and other stuff. Sometimes the breaker tripped, to solve this i just added a 75ohm 100w resistor (what i had on hand) in series with the amplifier transformer, then i added a pcb with just an rc timing circuit to activate a relay for bypass the resistor. Problem solved, quick and simple.

  • March 28, 2021 at 8:58 pm
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    I used a 220V relay after a serial resistor to softstart an electric motor for a circular saw. The delay is just a second but it works well. The relay shorts the resistor. There are solutions with PTC or NTC resistors but they need to be big for 3000W.

  • March 28, 2021 at 8:58 pm
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    That was very cool that the SSR has a zero crossing bult-in, however make sense when you think about what it does. Very cool solution.

  • March 28, 2021 at 8:58 pm
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    May I suggest that you add a relay to bypass the triac after completion of the load soft starting in order to limit the power loss in triac and saving it from overheating. And you might want to add a variable resistor to enable the user to set the soft starting phase duration.
    Great job

  • March 28, 2021 at 8:58 pm
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    I hope you got a really good autotransformer. Tech Tangents recently got one (a European model as well, you could have plugged it in to your outlets without difficulty, although it would have been a bad idea). Turned out to be badly made, with lots of crappy solder joints, as well as connections that were soldered when they should have been crimped.

  • March 28, 2021 at 8:58 pm
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    Why make it so complicated? I made softstarter to my 10kVA transformer by pre magnetizing the coil through power resistors for some milliseconds before directly powering the coils. Very simple and works.

  • March 28, 2021 at 8:58 pm
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    you may end up melting the enclosure near full load with that heatsink unable to breathe. you have ~1 watt per Ampere dissipation on that triac, so that box is going to get hot.

  • March 28, 2021 at 8:58 pm
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    In my country indonesia to solve the problem inrush current and make tripping circuit Breaker, we use a cheap soft starter with a wire and the wire mould with cement and it works we never complaining about tripping MCB

  • March 28, 2021 at 8:58 pm
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    1:05 The bad quality of all Schuko connectors is mind blowing. Worst mains connector EVER! The worst, it is not polarized. For this applications critical.

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