Author Topic: AC Active Soft Starter for Inductive Load with less energy dissipation  (Read 3865 times)

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Offline BravoV

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Aware of various AC soft starter designs, but most are using resistance at the initial inrush moment and then later by passed by relay switch, or even much more simpler design using NTC to limit the inrush.

Discovered this design -> Active Softstart for Welders



The designer claimed its suitable for inductive load, in this case it was designed for welder.

Quote :

Working principle
It exploits the fact that current in transformer is 90° out of phase of the voltage, which leads to conclusion that best time to switch on the transformer is in voltage peak of mains power line (which was quite counter-intuitive for me at the first time). This circuit detects the peak using 300V bi-directional transil (TVS diode) which is then used to trigger snubberless TRIAC which switches on the power transformer of welder. Once this happens, there is normally open relay with AC coil connected in parallel to transformer which shorts out whole active circuit and therefore prevents any excesive heating caused by high currents through semiconductor components (= No heatsinking needed!). To make circuit more robust and endurable there are two more components to prevent possible damage to semiconductor elements. In series with TVS there is resistor to limit current through TVS and to TRIAC's gate. Also there is 390V varistor across the whole switching circuit which prevents voltage spikes from transformer to kill the TRIAC and partially helps to supress arcing in relay contacts.



What interest me are these features :

- Much less dissipation, especially for load that getting turned on an off often in short period like a drill.
- Less delay or instantaneous power delivery at power on, cmiiw.


Question, is this design decent ?

- Do I need to adjust the 300V TVS (Bi-Diretional) for the triggering, to somewhat lower abit as my mains is 220V while its designed for 230V ?
- What is a good enough wattage for the R1 resistor ?

Appreciate any comment or feedback regarding this circuit.

TIA
« Last Edit: December 30, 2018, 10:24:23 am by BravoV »
 

Online coppercone2

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Re: Yet another mains active soft starter, is this design decent ?
« Reply #1 on: December 23, 2018, 09:06:44 am »
Are you sure this makes sense with using on a power tool? It's not like you turn it off every second.

What I do for my soft start high dissipation designs is solder the resistor upside down on some wires over a little catch net so if it melts itself it breaks the circuit and falls onto a cooling mesh.

I think its safer at high currents because there is nothing to explode, at most a relay won't short it out and the resistor will melt itself free and break the circuit. Just make sure the cooling mesh is low enough that the resistor can't melt on one side, and short out the mesh. it has to be able to fall freely. and the wiring around it must be secure. And out of paranoia i recommend single strand wire.

also fuse and stuff is missing
« Last Edit: December 23, 2018, 09:15:05 am by coppercone2 »
 

Online coppercone2

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Re: Yet another mains active soft starter, is this design decent ?
« Reply #2 on: December 23, 2018, 09:48:25 am »
in terms of shop tools and a delay, I think the most beneficial candidate for a circuit like this would be a chop saw since that is often used fairly rapidly. A table saw or drill press don't really get turned on that much IMO.
 

Online coppercone2

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Re: Yet another mains active soft starter, is this design decent ?
« Reply #3 on: December 23, 2018, 09:58:58 am »
http://class.ece.iastate.edu/ee330/miscHandouts/AN_GOLDEN_RULES.pdf

there is also this beast,

http://www.icbase.com/File/News/download/ON_Reference_1.PDF

I tried to read it before but I got unethused as it made more sense to learn about drives and tubes (for fast switching) then this component (SCR and derivatives) in great detail, because I felt for physics stuff and experiments it would make more  sense to use the tube and for mechanical drives and precision process control you would want to use a drive anyway if you can (even for heaters they have special multi kW drive type circuits), which kinda made me feel like the SCR/TRIAC etc is supposed to be a circuit more for 'economic' use which is not that interesting to me in the home laboratory.

The exception to this that I found is for protection circuits, even for advanced electronics (because of devices like bourns makes, i.e. TSP), due to its speed, but if I was going to do that I would do it to protect the drive not run the saw. The case made for the welder is good though. I also wonder what other electromechanical devices it makes sense to run with.
« Last Edit: December 23, 2018, 10:02:36 am by coppercone2 »
 

Online coppercone2

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Re: Yet another mains active soft starter, is this design decent ?
« Reply #4 on: December 23, 2018, 10:17:53 am »
and I suspect you will thank yourself in the long run if you put a drive on your drill press so you can properly regulate drill speed. Its been on my list for a while. You really are supposed to do it and its too frustrating to mess with the belts IMO. That would be the best mod to go for with the drill.

And I never saw noise analysis done with those types of circuits on the VARIAC. Not sure if you want that in your lab if its going to make some kind of noise (i even put a disable switch on my variac digital display so its lab worthy).

a chop saw however.. i am not sure if regulating speed on a wood cutting saw for metal cutting is a good hack, I don't think its built right for metal cutting even if you slow it down to the 200 surface feet per minute or whatever it needs  (those cold cut metal saws look more robust, i suspect it might vibrate alot), so the (appropriate single material chop saw) might be a good candidate for this project other then old welders.
« Last Edit: December 23, 2018, 10:33:23 am by coppercone2 »
 

Offline duak

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Re: Yet another mains active soft starter for inductive load
« Reply #5 on: December 23, 2018, 05:46:00 pm »
BravoV, this is an interesting design that should work providing the components are sized correctly.  What are the power ratings for the variac and the motor?  What is the current rating for the circuit breaker?  How long does it take for the circuit breaker to open?  ie., is it when the switch is first flipped on or after some delay?

BTW, I have a spot welder that I want to build a timer into and this has given me some ideas.  Commercial units either have a very large contactor or a complicated SCR based system, either of which are far too large to fit inside the spot welder.

Cheers,
 

Offline BravoV

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Re: Yet another mains active soft starter for inductive load
« Reply #6 on: December 23, 2018, 06:36:45 pm »
@coppercone, thanks.

Offline BravoV

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Re: Yet another mains active soft starter for inductive load
« Reply #7 on: December 23, 2018, 06:42:38 pm »
BravoV, this is an interesting design that should work providing the components are sized correctly.  What are the power ratings for the variac and the motor?  What is the current rating for the circuit breaker?  How long does it take for the circuit breaker to open?  ie., is it when the switch is first flipped on or after some delay?

Thanks for confirming.

About the variac, its 1KVA and I guess the drill is also similar, and it tripped on the flipped of the power switch. But its all my fault and laziness  ::), as these were not supposed to be used at that particular mains outlets as it has only 6A circuit breaker, yeah, I should change it, but please ignore those, as I'm going to build soft starters for my other transformers too. (Edited out those from 1st post)

As hobbyist, cmiiw, the gate resistor, since the current is not that large, 1 watt should be enough ? Again, cmiiw.


BTW, I have a spot welder that I want to build a timer into and this has given me some ideas.  Commercial units either have a very large contactor or a complicated SCR based system, either of which are far too large to fit inside the spot welder.

Great, please, if you decided to build yours, share it please. Also interested on your modifications on this design if any.
« Last Edit: December 23, 2018, 07:26:09 pm by BravoV »
 

Offline BravoV

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Re: Yet another mains active soft starter for inductive load
« Reply #8 on: December 23, 2018, 07:15:03 pm »
As enthusiast with limited knowledge/skill & experiences, my understanding of this circuit is to switch on the relay, hence connecting the inductive load, when the voltage is at it's peak (RMS x 1.414) as the current is at minimal (pointed by red dots).

Of course in the reality, due to the TVS diode voltage will not be accurate as the exact peak, but really near to the peak, and also mains voltage is not guarantee to be stable all the time anyway, but this circuit at least will switch on "close to" those red dot points to have minimal inrush at the switching on period.

Do I understand this correctly ? CMIIW
« Last Edit: December 23, 2018, 07:29:31 pm by BravoV »
 

Offline BravoV

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Re: Yet another mains active soft starter for inductive load
« Reply #9 on: December 23, 2018, 07:37:40 pm »
Another question pop out ...

What if the relay turned on and misfired at the wrong time (e.g.: further from the red spot) caused by the mechanical delay ?  :-//
« Last Edit: December 23, 2018, 07:44:41 pm by BravoV »
 

Online Benta

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Re: Yet another mains active soft starter for inductive load
« Reply #10 on: December 23, 2018, 07:44:59 pm »
You're not understanding the circuit 100%.

The idea is that the Triac fires close to the peak mains voltage. This enables the relay to pull, which will take a little time (10s of ms). When it does, it takes over the current flow through the Triac.
The result is a balanced current flow through the transformer as desired.
 

Online coppercone2

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Re: Yet another mains active soft starter for inductive load
« Reply #11 on: December 24, 2018, 05:17:41 am »
there is another thing you can do, which is use a GFIC.

I think that the electromagnetic pulse generated from the inrush current will trip a GFIC and it will trip faster then braking the breaker so you can just reset an outlet 1/10 times rather then going to the breaker. There might be enough coupling in the system to trip it by exceeding the ground current.

I am pretty sure my garage used to trip the breaker from power tools before I got a GFIC. I think the GFIC trips slightly more then the breaker but you just poke it and it ends up being fine, but I don't think I ever had as bad as a 10% trip rate, more like 1%. I have not had to reset the circuit breaker in years.

*may be bad for the GFIC to interrupt it, I got the 20A rated ones with dust proof covers. But its better then going into the basement with a flash light.

My hunch is that the electromechancial response time of the GFIC is much faster then the thermomechanical response of the breaker, so it breaks the circuit before the breaker gets to its trip point. May be worth a try. More like a discriminator based approach then a control based approach.

But again 10% sounds like ALOT. I would be pissed if I had to reset it every 10 uses. More like once in a hundred or more. I keep a electricians screw driver near by to poke the GFIC switch. It's also a long ass wiring run to where the saws are however. The peak current is probobly much smaller.
« Last Edit: December 24, 2018, 05:23:11 am by coppercone2 »
 

Offline BravoV

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Re: Yet another mains active soft starter for inductive load
« Reply #12 on: December 24, 2018, 01:11:17 pm »
You're not understanding the circuit 100%.

The idea is that the Triac fires close to the peak mains voltage. This enables the relay to pull, which will take a little time (10s of ms). When it does, it takes over the current flow through the Triac.
The result is a balanced current flow through the transformer as desired.

Thanks for pointing that out.

Say working at mains with 50Hz freq, hence once cycle (2 peaks) at 20ms, and each peak distance is 10ms.

So are you saying as long of the relay switching mechanism under 10 ms, it will be just fine ? cmiiw


EDIT :

I get it now, again thanks for reminding my error.

The TRIAC will conduct 1st at the (near) peak voltage, and then follow by the relay by-pass as "parallel" connection, so a bit of delay of the relay contact won't matter too much, brain fart.  :palm:

The needed components ordered, will update once I got all and assembled, and of course testing it.  :P
« Last Edit: December 26, 2018, 10:13:38 am by BravoV »
 

Offline BravoV

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Re: Yet another mains active soft starter for inductive load
« Reply #13 on: December 30, 2018, 09:22:13 am »
As I'm waiting for the components to arrive completely, redraw the circuit as to my version attached below.

The triac I'm going to use is ST's BTA24-800BW, 800V and 25A rated (250A peak), and its called "Snubberless TRIAC" by ST.

As this for inductive load, currently my biggest load at my lab equipment is the 220V 1000VA isolation transformer, with primary winding specification = 301 mili Henry and DCR 1.2 Ohm. For now, just assume this soft-starter circuit will be used for this transformer.

Few questions :

1. The original circuit doesn't protect the relay's solenoid winding against the inductive load kick back, cmiiw ? Is it ok that I put the extra "U5" 390V MOV to protect it as in my schematic ? My relay spec abs. max voltage for the solenoid winding is 110% rated (462V RMS).

2. As my ST's TRIAC has the built in feature called "Snubberless", does this mean I don't need the RC snubber if using standard TRIACs in this case ?

3. As my updated schematic pointed, I put two optional RC snubbers A and B, if I use standard non snubberless TRIAC, which one should be placed ? Or both ?  ... or I don't need any snubber at all as original schematic ?  :-//

4. Did I miss anything else ? or made mistakes ?

Appreciate any comments or advice, TIA.
« Last Edit: December 30, 2018, 10:38:17 am by BravoV »
 

Offline duak

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #14 on: December 31, 2018, 09:42:34 pm »
BravoV, just thinking out loud about the snubbers.  I don't know if they are needed but here are the situations they could be helpful in.

Case 1: the triac stops conducting when its anode current drops below the holding current (a few mA?).  I expect this to happen some degrees after the zero voltage crossing since current lags voltage in an inductor and it will tend to keep the triac conducting.  Since there is little current flowing thru the transformer & relay coil, there should be a minimal inductive kick voltage generated.

Case 2: the line switch opens at a significant transformer current.  Initially, the inductive stored energy starts and maintains an arc between the switch contacts.  At some points the current either drops enough or the gap opens enough to cool the arc and the switch opens.  Snubber B has been absorbing some of the energy but now it has to absorb it all.  Snubber A doesn't do much because either the triac is on or the relay is.

Case 3: a differential voltage spike comes down the line between the conductors when the triac is off.  Snubber A handles some of the current and, if it happened at the right time, the triac could be triggered.

Will you breadboard this circuit vs go to a PCB right away?  If the latter, there is a small chance that the relay will not pick up in time and it will chatter.  This isn't good for AC relays especially because their coils can burn out.  Good thing is, AC relays pick up quite quickly.  If it doesn't you might have to make provisions for a DC relay driven from a rectifier and capacitor.

Best wishes for a Safe & Happy 2019
 

Offline BravoV

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #15 on: January 02, 2019, 06:52:29 am »
BravoV, just thinking out loud about the snubbers.  I don't know if they are needed but here are the situations they could be helpful in.

Really appreciate your response and thought here, thank you.  :-+


Case 1: the triac stops conducting when its anode current drops below the holding current (a few mA?).  I expect this to happen some degrees after the zero voltage crossing since current lags voltage in an inductor and it will tend to keep the triac conducting.  Since there is little current flowing thru the transformer & relay coil, there should be a minimal inductive kick voltage generated.

At the on state (switched ON), once the relay is fired, will the triac off instantly ?



Case 2: the line switch opens at a significant transformer current.  Initially, the inductive stored energy starts and maintains an arc between the switch contacts.  At some points the current either drops enough or the gap opens enough to cool the arc and the switch opens.  Snubber B has been absorbing some of the energy but now it has to absorb it all.  Snubber A doesn't do much because either the triac is on or the relay is.

So the arc on the relay contacts will still happen even there is a U3 MOV (390V) across it ?



Case 3: a differential voltage spike comes down the line between the conductors when the triac is off.  Snubber A handles some of the current and, if it happened at the right time, the triac could be triggered.

Ok, I'm abit lost, so there is a chance that this circuit won't turn off ?


Will you breadboard this circuit vs go to a PCB right away?  If the latter, there is a small chance that the relay will not pick up in time and it will chatter.  This isn't good for AC relays especially because their coils can burn out.  Good thing is, AC relays pick up quite quickly.  If it doesn't you might have to make provisions for a DC relay driven from a rectifier and capacitor.

Best wishes for a Safe & Happy 2019

Happy new year 2019 to you too.

I haven't decide it, cause I haven't receive the relay, but for final it will be on PCB, but for early prototyping, I'm thinking of dead bug style (yes, I'm aware this is dangerous voltage), it will be build with alot of precautions and shielded (shrink wrapped) at exposed metal parts.

Again, thanks.

Offline duak

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #16 on: January 02, 2019, 08:39:03 pm »
G'day BravoV, some answers for you

Q: At the on state (switched ON), once the relay is fired, will the triac off instantly ?

  Yes, when the relay contacts close they take the current from the triac and it goes non-conducting.  If the contacts open with current flowing thru them, say during a bounce, an arc will be struck between them.  The voltage across the arc shouldn't be more than 20 V so the triac is not likely to be retriggered and even if it is triggered, the contacts will close again shortly.

Q: So the arc on the relay contacts will still happen even there is a U3 MOV (390V) across it ?

  I was referring to the switch contacts opening, not the relay contacts as the relay contacts should never open with current thru them.  The voltage needed to strike an arc between contacts that are opening is on the order of a few volts and a MOV is practically useless.  The voltage is so low because when contacts are opening electrons can jump the gap when it's still much less fractions of an mm.  If enough current is flowing the electrons vaporize the contact metals making them conductive ions and forming an arc.    An RC snubber is much better because if sized properly it can carry some of the current and extinguish the arc.

Q3: Ok, I'm abit lost, so there is a chance that this circuit won't turn off ?

 No, it will shut off because the input is AC and the instantaneous current will sooner or later drop to zero after the mains switch is opened.  The the arcs extinguish and the voltage isn't high enough to re-establish them.  Good thing this isn't a DC system where arc can and does go on, and on, and on...

 I'm referring to the case where the switch happens to be on but the triac hasn't yet been triggered.  If a voltage spike comes down the line at just the right time, the triac could be triggered too soon and orginal problem occurs, ie. the transformer core saturates and the fuse blows or the breaker trips.   This shouldn't happen often but since you are using this in an electrical lab, you could very well have voltage spikes on the mains that a snubber could help with.

  If you find this still happens, then a slightly more complicated trigger circuit will be needed.   I would exchange D1 & R1 then add provisions for a capacitor between their junction and the cathode of U1 (right hand side).  This will form an RC network to attenuate any spikes from the line.  I'd probably remove snubber A.   R1 would then be something like 470, 1/2 W and the new capacitor 100n, 630 V.   I'd also move U3's right hand terminal to the net connecting pin 4 of the power switch. 

Hope this is a bit clearer,

Take care,
 

Offline MrAl

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #17 on: January 02, 2019, 11:09:14 pm »
Aware of various AC soft starter designs, but most are using resistance at the initial inrush moment and then later by passed by relay switch, or even much more simpler design using NTC to limit the inrush.

Discovered this design -> Active Softstart for Welders



The designer claimed its suitable for inductive load, in this case it was designed for welder.

Quote :

Working principle
It exploits the fact that current in transformer is 90° out of phase of the voltage, which leads to conclusion that best time to switch on the transformer is in voltage peak of mains power line (which was quite counter-intuitive for me at the first time). This circuit detects the peak using 300V bi-directional transil (TVS diode) which is then used to trigger snubberless TRIAC which switches on the power transformer of welder. Once this happens, there is normally open relay with AC coil connected in parallel to transformer which shorts out whole active circuit and therefore prevents any excesive heating caused by high currents through semiconductor components (= No heatsinking needed!). To make circuit more robust and endurable there are two more components to prevent possible damage to semiconductor elements. In series with TVS there is resistor to limit current through TVS and to TRIAC's gate. Also there is 390V varistor across the whole switching circuit which prevents voltage spikes from transformer to kill the TRIAC and partially helps to supress arcing in relay contacts.



What interest me are these features :

- Much less dissipation, especially for load that getting turned on an off often in short period like a drill.
- Less delay or instantaneous power delivery at power on, cmiiw.


Question, is this design decent ?

- Do I need to adjust the 300V TVS (Bi-Diretional) for the triggering, to somewhat lower abit as my mains is 220V while its designed for 230V ?
- What is a good enough wattage for the R1 resistor ?

Appreciate any comment or feedback regarding this circuit.

TIA

Hello again,

I took a look at the "peak turn on" feature and it looks like it works.  Here's why.

If we normalize the time functions for everything except time for both possible ways of doing it (turn on at peak vs turn on at zero) we get two different functions:
1.  [turn on at peak] sin(t)
2.  [turn on at zero] 1-cos(t)

They have interesting characteristics.
#1 starts at zero then ramps up to some peak value which here is 1.  So it's a regular sine current.
#2 also starts at zero, but then ramps up to a higher peak value which here is 2.  It too is a sine wave, but now has an offset of 1 so it actually peaks at 2.  That's twice as high as #1.

Now in a real circuit with some small resistance as well as the inductance, #2 will eventually damp out to a regular sinusoid.  The speed at which this happens will depend on the value of both the resistance and the inductance and may actually limit that '2' to a lower value, but it will most likely be higher than #1 anyway.  This means that it makes sense to switch at the peak rather than at zero.

Of course this is quite easy to test too.  If you change the circuit a little you can get it to switch at zero, but beware the peak could go as high as 2 times normal if the exponential part of the response is slow.

I think that is what you were most interested in but if you have any questions no problem.

 

Offline BravoV

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #18 on: January 03, 2019, 05:04:32 am »
Q: At the on state (switched ON), once the relay is fired, will the triac off instantly ?

  Yes, when the relay contacts close they take the current from the triac and it goes non-conducting.  If the contacts open with current flowing thru them, say during a bounce, an arc will be struck between them.  The voltage across the arc shouldn't be more than 20 V so the triac is not likely to be retriggered and even if it is triggered, the contacts will close again shortly.

So this is no different than ordinary contacts bounce right ?


An RC snubber is much better because if sized properly it can carry some of the current and extinguish the arc.

Just assume this will be used only at the transformer I mentioned before, appreciate any help on finding the right RC values for the snubber, as hobbyist, tried read many documents from the net and I'm still lost & confused.  :'(


If you find this still happens, then a slightly more complicated trigger circuit will be needed.   I would exchange D1 & R1 then add provisions for a capacitor between their junction and the cathode of U1 (right hand side).  This will form an RC network to attenuate any spikes from the line.  I'd probably remove snubber A.   R1 would then be something like 470, 1/2 W and the new capacitor 100n, 630 V.   I'd also move U3's right hand terminal to the net connecting pin 4 of the power switch.

Point me any error if any, sketched your propose change below.

Once all components gathered, I will start prototyping the original circuit 1st, and then try yours too as its easy to change and see the results.

Thanks for your assistance.


Offline BravoV

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #19 on: January 03, 2019, 05:18:00 am »

...

Of course this is quite easy to test too.  If you change the circuit a little you can get it to switch at zero, but beware the peak could go as high as 2 times normal if the exponential part of the response is slow.

I think that is what you were most interested in but if you have any questions no problem.

Hi MrAl, thanks for the reply.

What kind of "little change" are you talking about ? Modification for resistance type of load ?

I'm curious, and yes, definitely I will ask more.  :P


Offline MrAl

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #20 on: January 03, 2019, 02:32:11 pm »

...

Of course this is quite easy to test too.  If you change the circuit a little you can get it to switch at zero, but beware the peak could go as high as 2 times normal if the exponential part of the response is slow.

I think that is what you were most interested in but if you have any questions no problem.

Hi MrAl, thanks for the reply.

What kind of "little change" are you talking about ? Modification for resistance type of load ?

I'm curious, and yes, definitely I will ask more.  :P



Hi again,

Well when i said 'change' i meant that would be JUST for testing, to see the real difference between doing the peak turn on and doing the zero point turn on.  I just wanted to be clear about that.

The simplest way to test would mean a simple change, and that would be to get rid of the peak turn on circuit.  Just make it so that it is 'turned on' all the time.  That way when you plug the unit in, the inductance load gets powered by whatever phase the line voltage happens to be at.
Now when you monitor the input line with a scope and monitor the current level in the inductor load, you will see what happens at various start phase angles.  This allows you to plug in, note the scope display, then unplug, then plug in again, etc.  So you would be plugging and unplugging until you see the voltage on the scope start at around zero degrees.
For the current display on the scope, you will be looking for the maximum before it damps down.  This will look like a sine wave that is above the zero line, then it will work it's way down until it looks like a regular sine wave.  However, the point you want to note is the highest  peak and that will normally occur at the first peak of the current wave sine.  THAT peak could theoretically go up to 2 times the normal run peak.  So if the current normally went up to 20 amps, that first peak could go as high as 40 amps.  That's what you dont want and that is what makes switching at the peak effective because then the max is 20 amps (which is the normal run current peak).

When you do this you could also watch for a time when you happen to plug it in at the positive peak and at the negative peak.  You should see a normal sine then.  If you happen to plug it in at zero degrees but the input sine is about to go negative, then the first peak will be a maximum negative peak, which following the above example could be as high as -40 amps.

It doesnt take long to see the different phase angle responses this way, although it sounds kind of strange to do it this way :-)
Hopefully your setup can handle that first 40 amp peak though.  If not, you may have to add more resistance in series with your load to keep it lower just for the random plug in tests.

Please let me know what happens if you do.

 
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Offline duak

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #21 on: January 03, 2019, 11:17:39 pm »
MrAl, the peak current for a zero voltage switched transformer can be quite a bit more than 2X the rated current.  A transformer actually has a varying inductance that depends on the currents flowing in the windings.  What we see as a single inductance is actually an average of all the instantaneous inductances  that are presented throughout a cycle.  You probably knwo it's better described as a differential equation, but fortunately I've forgotten anything more than that.  I'm at the age now when I've forgotten more than I'll ever remember :o)

Here's a link to another explanation with some waveforms: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3206_AppNote&DocType=CS&DocLang=EN

I mentioned winding currents above.  If the transformer is connected to a resistive load, the current leaving the transformer can prevent the core from saturating during initial connection to the mains.  It's counter intuitive but true.  The linked paper demonstrates this.  Perhaps BravoV, try using an incandescent lamp on the transformer during startup to see if the breaker trips.

BravoV, some answers:

Q: So this is no different than ordinary contacts bounce right ?

 Correct.  Most any contact bounces or at least has a ragged make and break action.  I believe only mercuy wetted contacts get around this.

Q: Snubber design

 here's a link to something on snubber design for triacs:  https://www.st.com/content/ccc/resource/technical/document/application_note/38/88/44/b8/2c/26/44/b8/CD00004096.pdf/files/CD00004096.pdf/jcr:content/translations/en.CD00004096.pdf

 I know what you mean - I'd like to just have the answer too.  I've been a hobbyist since the time of slide rules and after college and a career I can now explain why some designs are bad and won't work or work well or work for long :o
 I think of snubbers this way: I have a current that's trying to pass thru a high impedance eg., an open switch.  If I let it, I'll see a high voltage across the switch that will probably damage it.  Therefore, I let the current go thru a lower impedance such as a resistor (or transient suppressor).  The series capacitor limits the amount of time that the resistor is across the switch and generally for a circuit like this, the bigger, the better.  However, for repetitive signals, the capacitor will have an impedance that will be in parallel with the switch so if it were really large (microfarads) it'd be a dandy snubber but would also bypass much of the current around the triac.  It could also resonate with the transformer's inductance leading to very high voltages in the circuit.  It could also damage the triac should it be charged with a high voltage when the triac fires.  I'm going to say to start around 100nF.  Good thing is that the triac is not used to control the current, rather just when and only for the length of time needed for the relay to pull in.  Please note that the mains switch actually has to do the heavy switching, especially when interrupting the mains current.

 BravoV, the schematic is exactly what I had in mind.  The original design is quite sound and hopefully mine is not worse.  Please note that some of my changes were to solve problems that may not be there and that I may not be aware of some problems that are.

Best o' Luck!



 
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Offline MrAl

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #22 on: January 03, 2019, 11:39:39 pm »
MrAl, the peak current for a zero voltage switched transformer can be quite a bit more than 2X the rated current.  A transformer actually has a varying inductance that depends on the currents flowing in the windings.  What we see as a single inductance is actually an average of all the instantaneous inductances  that are presented throughout a cycle.  You probably knwo it's better described as a differential equation, but fortunately I've forgotten anything more than that.  I'm at the age now when I've forgotten more than I'll ever remember :o)

Here's a link to another explanation with some waveforms: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3206_AppNote&DocType=CS&DocLang=EN

I mentioned winding currents above.  If the transformer is connected to a resistive load, the current leaving the transformer can prevent the core from saturating during initial connection to the mains.  It's counter intuitive but true.  The linked paper demonstrates this.  Perhaps BravoV, try using an incandescent lamp on the transformer during startup to see if the breaker trips.

BravoV, some answers:

Q: So this is no different than ordinary contacts bounce right ?

 Correct.  Most any contact bounces or at least has a ragged make and break action.  I believe only mercuy wetted contacts get around this.

Q: Snubber design

 here's a link to something on snubber design for triacs:  https://www.st.com/content/ccc/resource/technical/document/application_note/38/88/44/b8/2c/26/44/b8/CD00004096.pdf/files/CD00004096.pdf/jcr:content/translations/en.CD00004096.pdf

 I know what you mean - I'd like to just have the answer too.  I've been a hobbyist since the time of slide rules and after college and a career I can now explain why some designs are bad and won't work or work well or work for long :o
 I think of snubbers this way: I have a current that's trying to pass thru a high impedance eg., an open switch.  If I let it, I'll see a high voltage across the switch that will probably damage it.  Therefore, I let the current go thru a lower impedance such as a resistor (or transient suppressor).  The series capacitor limits the amount of time that the resistor is across the switch and generally for a circuit like this, the bigger, the better.  However, for repetitive signals, the capacitor will have an impedance that will be in parallel with the switch so if it were really large (microfarads) it'd be a dandy snubber but would also bypass much of the current around the triac.  It could also resonate with the transformer's inductance leading to very high voltages in the circuit.  It could also damage the triac should it be charged with a high voltage when the triac fires.  I'm going to say to start around 100nF.  Good thing is that the triac is not used to control the current, rather just when and only for the length of time needed for the relay to pull in.  Please note that the mains switch actually has to do the heavy switching, especially when interrupting the mains current.

 BravoV, the schematic is exactly what I had in mind.  The original design is quite sound and hopefully mine is not worse.  Please note that some of my changes were to solve problems that may not be there and that I may not be aware of some problems that are.

Best o' Luck!

Hello there and thanks for the comments.

Yeah funny what we can forget when we arent even trying :-)

As to the 2x current, that was a discussion meant to keep it down to the simplest possible terms which means an ideal situation where we have a perfectly linear inductor and perfectly linear resistance.  This is done so that we dont have to use any advanced techniques in order to reason this out.  2x the current is reason enough to use peak switching, and if it is higher than that then so much the better for us if we use peak switching.  So just knowing, at least for now, that 2x the current is a possibility even with ideal conditions, is enough to guide our decision on how effective the given circuit will be.
So that discussion was not meant to be perfectly complete, just enough to give good reason why the peak switching circuit is a good idea.  Note how simple the expressions were even void of any inductance value, and even those as simple as they are were enough.
Also, for testing as described i did mention that more resistance may have to be introduced into the circuit to prevent really bad current surges.

 

Offline duak

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #23 on: January 04, 2019, 04:47:59 pm »
MrAl, I learned something in this exchange that I hadn't had right before.  I encountered this problem many years ago with an induction motor and a solid state relay with a colleague's project and helped solve it by getting rid of the zero voltage switching SSR.  It wasn't my project so I didn't look into the nuts n' bolts of it.  I looked at this again because I have a spot welder at home that I want to build a timer for.  I knew about the general problem but my mental model wasn't accurate - the app note clarified it and showed that your approximation was more correct in predicting the current waveform during the first few cycles.  I had understood that the motor or transformer would saturate on the first cycle but it appears it may not.  Time for the current probe & scope.

Cheers,
 

Offline BravoV

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Re: AC Active Soft Starter for Inductive Load with less energy dissipation
« Reply #24 on: January 04, 2019, 06:10:23 pm »
Hi again,

Well when i said 'change' i meant that would be JUST for testing, to see the real difference between doing the peak turn on and doing the zero point turn on.  I just wanted to be clear about that.

The simplest way to test would mean a simple change, and that would be to get rid of the peak turn on circuit. 

...<snip>...

  That's what you dont want and that is what makes switching at the peak effective because then the max is 20 amps (which is the normal run current peak).

When you do this you could also watch for a time when you happen to plug it in at the positive peak and at the negative peak.  You should see a normal sine then.  If you happen to plug it in at zero degrees but the input sine is about to go negative, then the first peak will be a maximum negative peak, which following the above example could be as high as -40 amps.

It doesnt take long to see the different phase angle responses this way, although it sounds kind of strange to do it this way :-)
Hopefully your setup can handle that first 40 amp peak though.  If not, you may have to add more resistance in series with your load to keep it lower just for the random plug in tests.

Please let me know what happens if you do.

Well, for start, once my prototype is built, I will start with smaller inductive load 1st and later to my 1KVA isolation transformer.

I get it, you want me to see & experience what happened with and without the softstarter ? CMIIW

Yep, definitely will do that, still waiting for my relay to arrive.  :'(

Btw, I only have one HV 20Mhz differential probe.


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