How to make an AC Softstarter because my autotransformer keeps tripping my circuit breaker!
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For those of you more chronologically challenged as myself, do you remember such a device? If so, what was it called?
Date: 2021-03-28
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Comments and reviews: 9
funkynicco
Another funny thing is capacitors in switch mode power supplies. I have a lot of SMPS devices, computers and servers in the house and I came across a terrible annoying thing with power failures. When the power comes back on the SMPS devices all charge up their capacitors and almost always trips the inrush breakers. This has happened to two different houses with modern wiring, which means it is more likely my devices causing the issue.
Petition for requiring (adequate) NTC thermistors in commercial SMPS to limit inrush current to capacitors, some of the computer PSU's have them but I don't think they limit enough when you have several SMPS devices. Alternatively a precharge circuit, I don't mind that the power supply is charging up over 5-10 seconds after applying power and then it just keeps that power in capacitors. There is no need for instant charging as far as I can think.
Also, with adequate NTC thermistors I also imagine there should never be soundable sparks when connecting power plug to the power supply, but most often there is quite an audible spark (with computer PSU's mainly, and sometime even a flash.
With all that said, I've had an interest of soft-starting groups of devices in my home after potential power failures (doesn't happen that often in new house though. Basically all I would need is to have a timed relays clicking on at different set times when power comes back which should not overload the breaker(s.
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Another funny thing is capacitors in switch mode power supplies. I have a lot of SMPS devices, computers and servers in the house and I came across a terrible annoying thing with power failures. When the power comes back on the SMPS devices all charge up their capacitors and almost always trips the inrush breakers. This has happened to two different houses with modern wiring, which means it is more likely my devices causing the issue.
Petition for requiring (adequate) NTC thermistors in commercial SMPS to limit inrush current to capacitors, some of the computer PSU's have them but I don't think they limit enough when you have several SMPS devices. Alternatively a precharge circuit, I don't mind that the power supply is charging up over 5-10 seconds after applying power and then it just keeps that power in capacitors. There is no need for instant charging as far as I can think.
Also, with adequate NTC thermistors I also imagine there should never be soundable sparks when connecting power plug to the power supply, but most often there is quite an audible spark (with computer PSU's mainly, and sometime even a flash.
With all that said, I've had an interest of soft-starting groups of devices in my home after potential power failures (doesn't happen that often in new house though. Basically all I would need is to have a timed relays clicking on at different set times when power comes back which should not overload the breaker(s.
reply
44R0Ndin
I was thinking about this and my solution might be different than yours. I'd use a pretty simple designed-for-this-purpose power NTC thermistor.
Initial resistance like 10 ohm, hot resistance less than an ohm.
Should sort the issue out rather nicely, and the advantage is that it really doesn't care about what the size of the initial current draw spike is, it will adapt to nearly all situations.
If less hot/on resistance is needed, put two or more in parallel until you get the current carrying capacity and/or on-resistance you need.
Of course, for safety's sake the container for such a device should be of metal, the input and output grounding terminals should be connected together, and then there should also be a connection from the power input earthing terminal to the metal case. Then again I bet that such a thing is available commercially, so it becomes a case of DIY or Buy? but my mind is on buy because you should be able to get one with several relevant safety certifications (UL for US mains-powered devices, or equivalent EU/UK certifications) whereas you can't get that with a home-built one even if it's built to the same standards (your home insurance might not cover you if the thing starts a fire and it DOESN'T have those certifications.
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I was thinking about this and my solution might be different than yours. I'd use a pretty simple designed-for-this-purpose power NTC thermistor.
Initial resistance like 10 ohm, hot resistance less than an ohm.
Should sort the issue out rather nicely, and the advantage is that it really doesn't care about what the size of the initial current draw spike is, it will adapt to nearly all situations.
If less hot/on resistance is needed, put two or more in parallel until you get the current carrying capacity and/or on-resistance you need.
Of course, for safety's sake the container for such a device should be of metal, the input and output grounding terminals should be connected together, and then there should also be a connection from the power input earthing terminal to the metal case. Then again I bet that such a thing is available commercially, so it becomes a case of DIY or Buy? but my mind is on buy because you should be able to get one with several relevant safety certifications (UL for US mains-powered devices, or equivalent EU/UK certifications) whereas you can't get that with a home-built one even if it's built to the same standards (your home insurance might not cover you if the thing starts a fire and it DOESN'T have those certifications.
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the
Your explanation of the inrush current on an inductive load isn't quite correct; what you describe is more like how a capacitive load acts than anything. For motors, the inrush current is due to the mechanical inertia; when first turned on, the motor isn't spinning and so there's no back EMF to limit current flow, but once it gets up to speed the back EMF lowers the effective voltage across the coils. For transformers, it's actually caused by connecting at the zero crossing, which can even saturate the core and cause its inductance to fall dramatically, further spiking the current upward. I'm not sure how your zero-crossing switch fixed the problem, actually. The ideal moment to switch a purely inductive load on is actually at the peak of the AC supply voltage. Your transformer may have a significant capacitive component to it, perhaps? Could be a parallel capacitor added for power factor reasons; I doubt it's parasitic winding capacitance.
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Your explanation of the inrush current on an inductive load isn't quite correct; what you describe is more like how a capacitive load acts than anything. For motors, the inrush current is due to the mechanical inertia; when first turned on, the motor isn't spinning and so there's no back EMF to limit current flow, but once it gets up to speed the back EMF lowers the effective voltage across the coils. For transformers, it's actually caused by connecting at the zero crossing, which can even saturate the core and cause its inductance to fall dramatically, further spiking the current upward. I'm not sure how your zero-crossing switch fixed the problem, actually. The ideal moment to switch a purely inductive load on is actually at the peak of the AC supply voltage. Your transformer may have a significant capacitive component to it, perhaps? Could be a parallel capacitor added for power factor reasons; I doubt it's parasitic winding capacitance.
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GreatScott!
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: -)
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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: -)
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Power
7: 11 that explanation makes no sense to me. The transformer should be a an impedance which is an inductor (Lm) in series with another leakage inductance (Lr) finally in series with a resistive component being the resistance of the wires. The current waveform at steady state is of course going to lag the voltage, the amplitude of which (apparent current) easily found with phasor analysis.
But with a step voltage in to an inductor should result in i=0 initially and build up from there, an inductor current is proportional to volt-seconds, the integration of volts with respect to time. So why is there inrush to an unloaded transformer? Is the inter-winding capacitance playing a significant role or something? Motors are quite complex loads so I can understand inrush for them
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7: 11 that explanation makes no sense to me. The transformer should be a an impedance which is an inductor (Lm) in series with another leakage inductance (Lr) finally in series with a resistive component being the resistance of the wires. The current waveform at steady state is of course going to lag the voltage, the amplitude of which (apparent current) easily found with phasor analysis.
But with a step voltage in to an inductor should result in i=0 initially and build up from there, an inductor current is proportional to volt-seconds, the integration of volts with respect to time. So why is there inrush to an unloaded transformer? Is the inter-winding capacitance playing a significant role or something? Motors are quite complex loads so I can understand inrush for them
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Emanuel
Great engineering solution for a problem. Here in Brazil we would just put a bigger breaker (in the best case cenario! lol.
Anyway, I would love to see you trying to use this motor as a generator =) I tryed here once, and managed to power up some 6 fluorescent tube lamps with a 1/6th HP motor. I used a driller to turn the motor as generator and a capacitor to somehow start generating, but I don't know exactly how this worked. Would be great to see your great engineering analysis of the engineering and waveforms of this.
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Great engineering solution for a problem. Here in Brazil we would just put a bigger breaker (in the best case cenario! lol.
Anyway, I would love to see you trying to use this motor as a generator =) I tryed here once, and managed to power up some 6 fluorescent tube lamps with a 1/6th HP motor. I used a driller to turn the motor as generator and a capacitor to somehow start generating, but I don't know exactly how this worked. Would be great to see your great engineering analysis of the engineering and waveforms of this.
reply
Crk
Sorry GreatScott, your theory behind switching inductive loads at voltage zero crossing is wrong,
it should be switched at current zero crossing (that happens at peak voltage,
look around for documentations, the reason behind breaker tripping should be elsewhere.
(Maybe Arduino and/or SSR are taking up time and they are switching correctly? Or is it simply because of the soft-start that it works)
Try to capture also the mains voltage with the oscilloscope in sync with the surge current and we will see. Thanks!
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Sorry GreatScott, your theory behind switching inductive loads at voltage zero crossing is wrong,
it should be switched at current zero crossing (that happens at peak voltage,
look around for documentations, the reason behind breaker tripping should be elsewhere.
(Maybe Arduino and/or SSR are taking up time and they are switching correctly? Or is it simply because of the soft-start that it works)
Try to capture also the mains voltage with the oscilloscope in sync with the surge current and we will see. Thanks!
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Flederratte
Great problem solving!
Could you come up with a timed load resistor for the purpose of keeping a power bank awake? I am having the problem of my power banks switching off after about 30 sec if the current is below a certain threshold. Some electronics projects need too little current to keep the power bank on. So here a resistor which draws about 150 mA every 30 sec could help. However I currently do not have the knowledge how to realize such a device. I was trying with a NE555 timer IC.
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Great problem solving!
Could you come up with a timed load resistor for the purpose of keeping a power bank awake? I am having the problem of my power banks switching off after about 30 sec if the current is below a certain threshold. Some electronics projects need too little current to keep the power bank on. So here a resistor which draws about 150 mA every 30 sec could help. However I currently do not have the knowledge how to realize such a device. I was trying with a NE555 timer IC.
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Paul
Great Video as usual and I will definitely give this project a try
A Question, Triacs tend to heat up a lot when they are under a heavy load. Wouldn't that heat up and create a powerless of its own?
And if yes, can we attach a relay to the Dome Green LED which shorts the anodes of the Triac basically bypassing the triac (Just like your DC Soft Starter) to avoid power losses of the triac used for the Soft Starter sequence.
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Great Video as usual and I will definitely give this project a try
A Question, Triacs tend to heat up a lot when they are under a heavy load. Wouldn't that heat up and create a powerless of its own?
And if yes, can we attach a relay to the Dome Green LED which shorts the anodes of the Triac basically bypassing the triac (Just like your DC Soft Starter) to avoid power losses of the triac used for the Soft Starter sequence.
reply
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