ARC Quenching - Single-Pole Double-Throw Relay

Refreshing to see a well-framed question in these days when many 'wot dat?' posts are seen daily.

You should search for a Tyco document called 'Tyco_v4bg_4.pdf' which i can't find anymore, pity. If you can't either, send me a private mail message using the CR4 pm system, giving your email id, and i will send the document. It may give some helpful tips to you.

Is the client using both NO and NC of one set of contacts? If so, it is asking for trouble. He must use 'potential-free' contacts. Just my two cents.

Thank you Mr. Sridhar for your suggestion.

But in our case it is imperative to avoid the 2 potential situation as the relays are used to switch-over the voltage from one stage to another by changing the input-winding contact point.

Each relay requires upto 8ms energising time and the change-over action takes about3-8ms. So the system is so designed that at any point in time only one relay be operated to cause change in voltage. If 2 individual relays were used to cause the make and break, there could either be discontinuity in the Voltage//Current AC or it may cause bucking of the windings by temporarily shorting a winding of the auto-transformer.

Hence we are moving forward to triacs for their new design, but triacs also have their own inherent problems (lets leave triacs alone for another day of reckoning ;) ).

The client would like to know if the arcs can b reduced to aid them in reducing the problem in their old design.

Does it become difficult to control the arc when both NO and NC are carrying a potential?

regards,

Vishal...

Firstly, yes, if you use both NC and NO of a changeover contact, you must ensure that there is no inductive load, since persistent arcs can cause havoc. So the US insistence on potential-free contacts makes sense usually.

Arcs can be controlled to some extent by using (a) RC networks, (b) freewheeling diodes (DC only) and (c) MOVs across the break. Explained in that Tyco document. Please give me your personal mail id if you want that document... i am repeating myself, sorry.

I am not sure if this information is relevant, forgot to mention earlier. The common point is the potential carrier/supplier. The moving contact establishes the contact to 2 winding segments at different points of time.

The windings are part of an auto-transformer whose one end is connected to the Neutral line of the mains input supply. The phase is linked and provided by the choice of relay, similar to a daisy chain arrangement.

During transition(when moving contact is physically not in contact with the NC or the NO) the potential of the winding end, the NO and the NC becomes same as the Neutral(drops to 0) while the moving contact is held at phase potential. This is what causes me to observe a 150 to 250V AC odd voltage between the NO and C and also NC and C. So I guess providing Varistors was the worst idea ever as it would cause the winding connected to the NO and NC to buck or short-circuit. This is my assumption from the observation made. I'll post the images soon.

So I guess the varistor//thermistor based snubbing is out of the window for me.

Rather should now try the RC combination snubbing... But I am more concerned about the diode current rating selection as discussed earlier in my posted question. It would be great help if someone could guide me in determining the diode current for use in the snubber circuit.

regards,

Vishal...

I couldn't find "Tyco_v4bg_4.pdf" either, but since Tyco Electronics is now TE Connectivity there may be something on this website:

http://www.te.com/en/home.html

I found some devices of the type here. Don't give up searching online. You never know when lightning will strike.

YES! You hit it !

http://relays.te.com/schrack/pdf/c0_v4bg_4.pdf

Thank you both for the document... Will read and get back..

You tried magnetic arc suppression?

We did speculate that option but could not come to terms with the location of the magnets or how to ensure an attraction or deflection of the arc.

My BAD.. I should have explained the relay in-detail as well.

The relay is a standard 40 to 50A relay, (Wires can be screwed on the terminals. Not a PCB mount or Panel mount type.)

The distance between (not referring to the terminals) the static contactors and the moving contact in either position is 2-3mm. The relay dimension is 30mmx40mmx60mm.

There doesn't seem to be a provision for fixing the magnet close enough with out endangering electrical safety of the equipment.

The rough cost of each relay is about USD1.

Thanks for the suggestion though.. And if possible do suggest how I may be able to successfully mount the magnet and what strength would be required to deflect upto 4A to start with. And the actual current flowing in the circuit can reach upto 45A.

with thanks and regards,

Vishal...

Snubbers and arc suppression are now ancient history to me and my input there would be out of date.

However, I may be able to give some suggestion on what might be the arc source even with resistive load. You describe this as a "tap changing" system and so the source may be either the transformer coils themselves, depending on the structural configuration you have.

Hi. thank you for ur responce.

The source is not a worry at all. I am more concerned with the snubber design.

Though outdated as per ur consideration, ur outdated information could still b new information to me. So dont bother and bring it on.. :)

thanks again...

The implication that you are switching the taps of a transformer is a big hint. Regardless of the final load being a resistive load, on transition there will always be a reactive component. Since you are switching an AC signal, the standard practice of a Flyback diode across the inductive load will not work. I suspect your best solution will be a careful consideration of relay contact opening and closing times. The phrase "make before break" instead of the traditional "break before make" should be for one of your relays in this switching process. This will likely require more relays/contactors than you're using now but this is a common solved problem using these relays.

I cannot remember which CR4 member recently explained a schematic diagram here that had these types of relays. But I think this approach solves any current lag switching of reactors.

I think Redfred is on the right track. It is worth looking into the principles used in power transformer on-load tap changers. There is lots of info online about how they work, and you can probably adapt their principles to your smaller-scale application. See the following link in Wikipedia for an animated example:

http://en.wikipedia.org/wiki/Tap_(transformer)#Mechanical_tap_changers

By overlapping the operation of your relays and using a snubber of some sort on the main switching relay(s), you can significantly reduce the stress on at least most of your relay contacts. Since your relays are operated electrically rather than mechanically, some of the (loss of) timing issues that usually cause failure of mechanical tap changer contacts can be eliminated.

If you are switching transformer coils, it's likely an inductive kickback issue. The first line of defense is a diode across the inductor coil.

http://www.coilgun.info/theoryinductors/inductivekickback.htm

But your coil gun link refers to DC applied across an inductor being removed not AC across an inductor. With an AC voltage the diode will be forward biased and by passing the inductor for half of the cycle.

A possible configuration is discussed in my question itself. Also another way of using it is 2 back to back TVS diodes of a voltage rating that we need to clamp to.

The bigger issue is the rating selection of these diodes.

vishal...

The exact configurations do not help but diodes can be used in parallel to the switching point along with a voltage limiting circuit. but the bigger issue for me is in determining the current requirement or rating of the diode.

regards

Vishal...

As posted in the question i do intend to use the diode but am not able to determine of what rating it should be to perform the operation without burning or burning a hole in the pocket ;)..

Thanks..

Vishal...

IMHO this design will never work good enough. The steps are pretty big and the switching lacks intelligence, it is depending on "luck" only (read switch- response time, reaction time or name it)

Connect an oscilloscope and measure what "really" happens. You will have spikes that can reach to 2 times the voltage. (I try to translate, we call it reverse voltage, because of the frequency vs. alternance) With relays, you cannot switch cutting through the zero points of a period like is done with power controls. (too slow)

The make before- break theory is correct, - look at a rheotor autotransformer with a sliding contact on the circumference. The "brush" is made to overlap part of the next (or previous on the way back) winding too, and here the steps are a lot smaller.

We used to use voltage stabilizers on this principle, with a sliding contact, driven by a servo motor

Make break with relays is also difficult to make it work down up and also up down.

If you ever succeed, please let us know,

Success,D.

Is it safe to shortcircuit the winding for a short period (or long depending on how u consider 2-8ms). I thought that would cause o/p voltage to buck.

Any suggestions...

Depends on what you call safe:

Relays that last a lot less than what is expected? Contacts that are burning quickly and the metal that melts down? Do the relays also stick?

I have no idea on how big the steps are and

Is a ladder safe with cut steps of 1 meter ?

Or better as example: one with at random placed steps between 5 cm and 100 cm.

Doing this properly, requires a different switching device, that controls the situation and not a gamble clicker how long it lasts system.

Switching only when the O- point is reached can help. Think of Triacs, or other electronic devices. properly controlled.

We are designing a triac based system for them, but they require a solution if possible, for their existing systems.

The voltage and turns in between steps are approx 30V, 25 turns...

I think the effect of shorting the windings will be visible in the o/p line. that is not a good thing..

In general it is safe because of two factors. First, the voltage differential across the two contacts is relatively small. So the short circuit of connecting two transformer taps together will be a smaller voltage difference. Second, the period of time when two contacts are connected together are brief. Thus a finite number of joules will be heating this closed loop of a transformer windings, cable wiring, and two contacts. Usually this brief interval of added thermal loading will be an insignificant number of joules compared to the joules generated in normal operation.

Remember the transformer is not really an ideal voltage source. There is a source resistance of each tap winding along with the wiring and contact resistance that is connecting these two correlated voltage sources together.

Now one can certainly misapply this approach of make before break relay contacts by any of a number of bad ideas. The two that maybe relevant here are how large of a differential voltage will exist between two adjacent contacts and how frequently will these contacts be switched.

The contacts may be switched evry 200ms (worst case) and every 30 seconds (nominal condition), every 30-60 minutes (most expected), not at all (best case)..

After all is said and done, its a voltage stabiliser ;)

Have you experimented with a resistor-capacitor contact protection scheme?

http://www.industrologic.com/mechrela.htm

Large high-voltage circuit breakers use oil immersion, a puff of dry air, or a pressurized Sulfur Hexafluoride atmosphere to suppress contact arcing. Those methods would also work for lower voltages and currents if the working environment gives you enough elbow room to implement them.

there is absolutely no room to alter anything inside the relay. I have been trying RCs. but not able to guideline a value based on my req.

The arcing happens on all relays. The test//analysis was done on one.

So the said relay is not a faulty relay or line, rather the arcing exists as an inherent part of the system. Our endeavour is to reduce the current transitions and also the arcing to a minimum.

I have not read all posted comments, but had past experience manufacturing voltage regulators having up to 5% regulated outputs.

What I had done was exactly the opposite of what you described. I also used multi-tapped auto-transformer type, but the tap-switching mechanism was done on the secondary side or the output side of the regulators. By doing this approach I minimized most of the arcing that otherwise will be always presented by the transformer inductive windings, and therefore prolonging the contact life of the relays.

The only time I will one relay on the primary side is when a customer put a special order to increase the range of voltage fluctuations covered, like anywhere from 130 to 270 volts range while still maintaining the 230 +/- 10% output voltage!

I believe one key advantage of my approach, switching the secondary side, since most of the relay contacts are not going to encounter pure inductive load of the transformer!

Have you considered "gas discharge tubes"?

When starting a compact fluorescent lamp you actually try to cause the high voltage spike that is creating your arc. For noise suppression, a capacitor is used but it sounds like you are trying to suppress the voltage spike caused by the interruption of current in an inductive circuit. The gas discharge tubes mentioned above may work out very nicely for your application. They are fairly small and cheap.

Seems to be an interesting option. Will read more on them. If it seems to satisfy the requirements, will try them.

Thanks.

Vish_al210 -

To minimize contact arcings on the relay, I had done exactly the opposite of how the relays are connected as you described .

The tap changing was done on the output or load side of the transformer - not on the primary and not presenting the transformer as the load! By doing this approach I greatly minimized the magnitude of arcing caused by the transformer reactances which is mostly inductive in nature.