Triac and short circuit. - Page 1
Forum section: electronics / projects

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 29, 2020, 12:18:02 am
content: Hi, friends.Since childhood, I remember that the thyristor does not survive a short-circuit load and any means of protection do not have time to save it.But that was a long time ago.Now I'm trying to design an asynchronous motor to be started by thyristors. The protection system will have time to detect a short circuit and remove the control signal from the triac, but it will remain open until the end of the half-wave.I.e., it can experience a giant current for 10ms.Can modern thyristors survive in such conditions?P.S. Fuses are not suitable for protection, it must be recoverable.

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2020, 01:00:35 am
content: How much is the normal power and how much is the short circuit current? It's just a matter of overbuilding a bit..The Littlefuse alternistor triacs for inductive loads have a 40A rating and a 500A surge rating for one cycle, although there's a note about losing gate control after a surge.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 29, 2020, 01:27:42 am
content: You conflate ( I mean you need to differentiate) triac and thyristor.Triacs generally had lower I_squared_T ratings.Also Triacs could have a failure mode to become shorted in one side, remain working in the other.I think you would need to use anti-parallel thyristors in the situation of a larger induction machine that can have short circuits.Then consider the circuit inductance:If a short circuit occurs in (+) half wave, how long will it carry into (-) half wave?Maybe you could put the circuit into a time -transient simulator. Parameters can be obtained from thyristor and induction motor data sheets.Also the supply impedance should be included. Use you own function blocks to calculate integral of I_squared_dt.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 29, 2020, 01:55:00 am
content: Quote from: bdunham7 on November 29, 2020, 01:00:35 amI have 2 different control units: a miniature one for 2.2 kW power, and a full-size one for 30kW power.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 29, 2020, 02:14:08 am
content: Quote from: mag_therm on November 29, 2020, 01:27:42 amI'm definitely talking about the triac. We often call a symmetric thyristor or simply a thyristor/simistor, meaning that the triac behaves exactly the same, but in both directions.A short circuit will occur as a permanent (damage to the wire or motor winding), it is not a short circuit by the starting current of the motor.The protection system is able to detect a short circuit very quickly and remove the opening voltage from the triac control. The short circuit in one half-wave will only, to end of current half-wave, no more, in the next half-wave the triac will no longer have control to open.The whole point of the question: whether a single half-wave is capable of destroying the triac.The supply resistance can be low enough to allow the current to reach 1000A or even 1200A. This is a powerful 1 MW power station, the line length can be only a few meters and the cable of large cross-section up to.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 29, 2020, 02:33:01 am
content: Quote from: S. Petrukhin on November 29, 2020, 02:14:08 amIf your information is correct, you are on the way to answering your question, with some triac datasheets.But I would still recommend a simulation, or more detailed calculations.Nobody will be able to better answer your query.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 29, 2020, 02:40:45 am
content: Quote from: mag_therm on November 29, 2020, 02:33:01 amYes, this is the obvious and correct way. But there may be a person who has done this and can tell me.

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2020, 03:27:48 am
content: Quote from: S. Petrukhin on November 29, 2020, 02:40:45 amI've never done what you're doing, but I do know that your statement about triacs not tolerating surges is often true unless they are designed and rated so, and also that you need specific triacs for inductive loads. Also, you can use 2 SCRs back-to-back in place of the triac and I think it's a lot easier to get them with the current and surge ratings you need. In fact, I think I have the appropriate SCRs in my garage, but I only have 4 and I'm a long ways away.You still need a fuse in case they run away, which I think is less likely with an SCR but you never know. Power SCRs and Triacs list a fusing energy so that you can select a fuse and then make sure your available short circuit current is low enough that your protection circuit acts before the fuse does. If your gate and control circuits are already based on a triac design, you can use SCR optocouplers like the 11HC4/5/6 to adapt a dual-SCR setup to that circuit.https://www.mouser.com/datasheet/2/427/vs-st110spbfseries-1147815.pdf

author: Circlotron
author_rank: Super Contributor
timestamp: November 29, 2020, 05:32:02 am
content: Quote from: S. Petrukhin on November 29, 2020, 12:18:02 amI have professional experience doing this during the period 1985-1988. Feel free to ask questions.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 29, 2020, 05:52:37 am
content: Diodes, thyristors and triacs can be effectively protected by fuses.These are special ultra fast fuses.The diodes, triacs and thyristors datasheets specify the I²t of the component.To protect it effectively, you must choose an ultra-fast fuse with an I²t at least 30% lower than that of the semiconductor to be protected.

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2020, 06:24:56 am
content: Quote from: akimpowerscr on November 29, 2020, 05:52:37 amHe's saying that his circuit can't be fused as primary protection because the design requirements are such that the protective circuit must be self resetting. There can still be a fuse AFAIK, but the whole combination must withstand a short circuit long enough to give the primary protection time to shut off, which could be up to 1/2 of a power cycle.

author: Circlotron
author_rank: Super Contributor
timestamp: November 29, 2020, 06:34:18 am
content: Maybe a set of harmonic reducing inductors upstream of the thyristors might make the AC supply soft enough?

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 29, 2020, 06:41:58 am
content: Electronic protection of triacs or thyristors: This is only possible if there is sufficient inductance in the circuit to limit the current of the first half cycle to a value compatible with that which the semiconductor can withstand. (see datasheet).Be careful with the temperature of the silicon wafer because if it is too close to fusion, you lose control of the scr's or triac which remains in conduction even without a gate pulse.If there is sufficient inductance, the peak current must be detected and if a certain value is exceeded, the protection circuit must block the scr's/triac pulses.A triac does not behave exactly like 2 scr's in anti-parallel: a triac has a dV / dt far much lower than thyristors (except new 3 quadrants triacs) and is much less reliable with inductive load than a set of 2 head to tail thyristors.Using a triac with inductive load is not a power INDUSTRIAL good option.A dead short circuit on an asynchronous motor? this must be an extremely rare defect, I have never heard or seen such a defect.Electronic protection and ultra-fast fuses protection have different functions and are complementary.For exemple, in the case of a thyristor variable speed drive controlling a DC motor, the fuses protect the thyristors in the event of a flash at the DC motor collector.The electronic protection protects the thyristors in the event of a control fault (release of erroneous pulses) or of a mechanical problem in the armature contactor causing a delay in closing (actual case)NB: soft start for a 30HP asynchrone motor I made in 1989

author: Miyuki
author_rank: Frequent Contributor
timestamp: November 29, 2020, 01:32:24 pm
content: Just wonderIf it is a new design why not use IGBTs, power level is not so highBut even SCR is not a problem to choose with few kA withstand capability

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 29, 2020, 02:13:30 pm
content: Starting asynchronous motors by applying progressive voltage through phase control is outdated technology.It has the particular drawback of not providing a high torque on starting and of absorbing a large current that can reach 5 to 6 times the nominal current.The use of this principle is therefore limited to starting a motor that does not require high starting torques such as pumps, fans, escalators, etc.In the case of the soft starter that I supplied in 1989, the application required a very progressive torque at start-up, but also a very high torque.The asynchronous motor was a wound rotor motor with the insertion of resistors into the rotor circuit at start-up.Currently, it is preferred to use a frequency converter (VFD) which allows the nominal torque to be obtained from start-up without overloading the electrical network.

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2020, 06:46:28 pm
content: Quote from: Circlotron on November 29, 2020, 06:34:18 amAre you referring to limiting the short circuit current or to reducing dV/dt across the triacs? I think that might require some large parts at his power level. Given that using phase-control soft start is a bit outdated and electrically noisy, this is probably intended to be a quick-and-dirty sort of project, not intended to have a large BOM or even a large enclosure. Sort of one step up from a contactor? But the OP would have to elaborate on that.

author: Circlotron
author_rank: Super Contributor
timestamp: November 29, 2020, 10:33:29 pm
content: ^^ Limiting the short circuit di/dt and consequently the max current reached during the half cycle to something the scr, triac or whatever can tolerate for a single half cycle.

author: Circlotron
author_rank: Super Contributor
timestamp: November 29, 2020, 10:39:20 pm
content: Back when I was doing soft starters, a variable frequency drive cost an arm and a leg. Nowadays for smaller drives at least, a VFD costs less in 2020 dollars than an equivalent sized SCR soft starter did in 1985 dollars!