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AC Active Soft Starter for Inductive Load with less energy dissipation
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Benta:
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.
coppercone2:
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.
BravoV:

--- Quote from: Benta 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.

--- End quote ---

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
BravoV:
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.
duak:
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
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