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

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Parts list (affiliate links):

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!

Thanks to JLCPCB for sponsoring this video.

Music:
2011 Lookalike by Bartlebeats
Killing Time, Kevin MacLeod

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
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Комментарии
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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 :-)

greatscottlab
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Tripping circuit breaker? Hah, I don't have such weakness with my 30-years-old house wiring. The circuit resistance is high enough to limit any inrush current.

Asu
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The current into an inductor equals the integral of the voltage accross it, in formula: i = 1/L.integral(Vdt). If you switch it on at the (rising) zero crossing the voltage remains positive for 180 degrees. This causes the magnetizing current to rise to approx. 2x its normal value. However the inductor core is not made for this 2x magnetizing current and thus the core will saturate. This causes the coil inductance to drop, which increases the current even (a lot) more. If you switch at the maximum voltage, the current also starts to rise, following the integral relation, but after 90 degrees the voltage polarity reverses and the current starts to go down again. In this way the current starts of nicely lagging the voltage by 90 degrees, as it should be in the stationary situation, with peaking and tripping the breaker.
Best regards, Ben.

beneindhoven
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On the large DC power supplies we design at work, we just use a 20 ohm NTC inrush current limiting resistor, and a contactor that bypasses it after the main bulk caps are charged. Which could be done with an RC timer + Comparator

Andymank
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I had a similar issue with a 3000 watt transformer for reducing power from 220 to 120 volts. I later learned that the type of circuit breaker used makes a difference. There are some breakers rated for inductive loads and some for resistive loads. Most home breakers are of the resistive loads found in most homes. Replace breaker with an inductive rated breaker. Did this and no problems now.

Subgunman
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not gonna lie but this is exactly something I need to make for about 10-15 past days and here you go, guess a trip to DIY electronic store is required!

TonyStark-kbgr
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Another much easier and cheaper way which I used succesfuly in the past was a 1s timed relay in parallel with a high power resistor with just enough ohms to limit the current to under 16A.

Both of that in series to the load will limit the current for 1s and then the relay shorts the resistor.

Worked like a charm :-)

uwejunghanns
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Thanks for admitting the error. Thats news to me, since I don't normally work with inductors, so zero point made sense originally. Thanks to all the Commenters for the additional explanations!

boots
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Easier and cheaper to change the breaker to type C time curve which is for high startup loads.

mattsan
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Great project! For softstart I used a "Finder DPDT Multi Function Timer Relay" set to 1 second with a thermistor over the "off" position. The first second you power up the thermistor heats up then the relay clicks to the "on" position so the attached device is powered directly. The switching time seems fast enough so far.

oarendo
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Thermistor with relay latching. Monitor voltage drop across thermistor and latch when it falls low enough. Your solution of course works and is very creative.

jeffm
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I have a 15A variac behind a 15A capable isolation transformer (isolated and adjustable voltage is sometimes handy). All in parallel with a second lower power isolation transformer.
I use a high power resistor and a timer relay. The inrush current is limited by a simple resistor in series on the mains side and after a second or two, the resistor is bypassed by a timer relay. Works a treat.

hoggif
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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

engaad
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You know the day is gonna be great when GreatScott uploads

arnolaurie
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Your programming has become much better over the years. You started off writing terrible code and now your code looks quite nice.

I'm glad to see you improving that skill!

soranuareane
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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.

InssiAjaton
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Thanks so much for posting this! I've needed an AC soft starter for quite some time but was hesitant to spend the $300+ that most places want for a decent one. Thought for sure the circuit would be much more complicated than it is! I'll be using it for an air conditioner instead of an autotransformer.

KGJYS
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i'm always amazed by your penmanship

gglovato
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Getting better and better. It's explained well, and the whole lesson is demonstrated by a working device. I also like how this guy goes to great lengths to show the flow between the problem and the solution. I mean he hauled a big and heavy electric motor just to show that it applies to them as well. Well worth the subscription.
P.S.
What I don't like is the fact that he doesn't want to admit that he took the "Great Scott" name from the movie "Back to the future" (the mad scientist). Yet, I figured it out! Got him!

liviuconstantin
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Only feedback I'd have for you would be to move those LEDs, put the red one near the back, as it indicates power in, move the other one to out outlet side and make it light up red when the button is pushed and go green when the sequence is done (those looked to be three or 4 legged LEDs, so either dual color or maybe RGB?). This way it is much easy to know what the state of the thing is by a quick visual glance, and therefore a bit safer to use.

JarrodCoombes