Power transformer snubber 2

[RyreInc]

Greetings,

I wish to switch a power transformer on the primary side using a reed relay, but am worried about voltage transients during switch-off. The XFMR will draw 15A RMS @ 60Hz when on. Is a snubber necessary to protect the relay's 300V maximum contact voltage? If so, is an RC or zener snubber (or other) best? Or, should I consider a zero-crossing SSR?

The relay is a Omron Electronics G2RL-1A-E DC12.

Thanks!
Ryan

 

[ScottyUK]

That's not a reed relay for a start.

Switching a transformer I'd look for one with a bit more rating in hand, although that one is theoretically ok. I'm just a bit conservative, and would choose one in the 25A - 30A range to ensure a long and trouble-free life. Transformers are fairly awkward loads to switch - contactors designed for DOL motor starting are de-rated when switching transformers, just to give you an idea of how severe the duty can be.


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If we learn from our mistakes I'm getting a great education!

 

[RyreInc]

Thanks for the advice. I was already thinking about improving the relay actually, and I've already selected a 25A model (Panasonic ALFG1PF12) and drawn it in my PCB software. It still doesn't change my original question though!

 

[ScottyUK]

I wouldn't use an SSR for this application, but an RC network would certainly do no harm and probably a little good. A Transzorb (which is what I assume you mean by 'zener snubber') wouldn't bring much improvement in relay life. Best bet is a decent quality relay run below its rating, which you seem to be on course with.


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If we learn from our mistakes I'm getting a great education!

 

[sreid]

I like zero crossing SSR's especially for PCB Mounted applications. Not only do you get a soft turn on but a soft turn off. Look at the offerings from Crydom.

Mechanical Power Relays, in my opinion, never have a long trouble free life.

 

[bobmcbride]

I agree with sreid however, if cost is your main priority AND if the relay would not be operated very much over your intended life cycle... Then a relay still could work.
As for you question "Is a snubber necessary to protect the relay's 300V maximum contact voltage?" when switching inductive loads with relays it's always helpful to put a small cap (line rated) in parallel to the relay. This helps reduce the arc that forms as the relay contacts pull away from each other. If the load you are switching is highly inductive you can even go as far as using a RCD snubber instead.

 

[ScottyUK]

SSRs rarely survive through-faults, usually burning out to protect the fuse...

One of the big problems with relays switching transformers is the huge inrush: relays don't have enough thermal mass to prevent to contacts overheating. Normally a contactor would be a better choice, failing that at least pick the relay based on it's motor switching rating and over-size it a bit. If you pick a contactor - which at 15A would be worth thinking about - then look for the AC-6a rating.


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If we learn from our mistakes I'm getting a great education!

 

[RyreInc]

Thanks for the replies everyone, very helpful.

The load on the transformer secondary is a full-wave rectifier that charges a capacitor bank up to 3000VDC. (An inductive choke on primary or an active current limiter post rectifier will be used to limit current.) The relay's duty cycle is approx. 5sec on, 25sec off, for 8 hours per day.

With that in mind, wouldn't a zero-crossing SSR solve the in-rush problem as well as the voltage transient problem? I do like the idea of minimizing moving parts!

In the mean time I'll look into contactors.

 

[RyreInc]

Also, my original thought for the snubber was two anti-series zener diodes, perhaps 200V each. Is this not a good solution?

And wouldn't and RCD snubber be unidirectional? I'm working with an AC signal.

 

[waross]

There are millions of transformers of all sizes being controlled with nothing but a properly rated switch or contactor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RyreInc]

Duly noted, waross.

Will this relay cut it? I can't get any bigger without moving it off the PCB. With or without snubber?

ALFG2PF12

http://www.panasonic-electric-works.com/catalogues/downloads/relays/ds_61B12_en_lfg.pdf

 

[ScottyUK]

From the duty cycle an SSR might be a good choice after all, but watch the peak current if you're feeding a rectifier / doubler type load. A lot of SSRs are primarily designed for resistive heating type loads, and many are explicity not suitable for motor or transformer loads. This loads sounds nearly as bad as a discharge lamp load. Speak to the apps guys at Crydom, but have some detail about your load available.


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If we learn from our mistakes I'm getting a great education!

 

[RyreInc]

bobmcbride, shouldn't the cap be parallel to the transformer, not the relay? I don't want any leakage current when the relay is open.

 

[Skogsgurra]

Zeners and tranzorbs have no place here. Just forget.
A MOV could work. But voltage is not your problem. Inrush is. If there is any problem at all. You are new to power circuits, I assume?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[RyreInc]

Skogsgurra, yeah fairly new to this.

I've been more and more concerned with in-rush the more I read.

For SSRs, this datasheet indicates zero-cross is for resistive loads, while random turn on is for inductive loads... I thought zero-cross was meant for inductive load side effects?

http://www.crydom.com/en/Products/Catalog/c_kr24.pdf

 

[Skogsgurra]

It is 90 degrees that are best for inductive loads. That keeps flux at a minimum and avoids saturation at switch-on.

0 degrees is best for capacitive loads. Resistive loads - doesn't matter - current is always U/R. So no difference at switch-on.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[RyreInc]

Nevermind about the last question, I found an article explaining it:

http://www.newark.com/pdfs/techarticles/crydom/OutputSwitchingSSRs.pdf

 

[Skogsgurra]

That article is of limited use since it avoids to mention the specific problem with inrush and how to avoid it. You had better look for an article on inrush and 90 degree switch-on.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Made one for you:

http://gke.org/arcus/files/Transformer_inrush_single_phase.pdf

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Look for SCR based SSR's. These will only switch AC loads. SCR's are fairly rugged devices and they should work fine switching a transformer. Something like a Crydom CW48 series rated 25A should work fine. Put a ~300V MOV or 0.1uF/100ohm RC snubber across the relay. Make sure you use a suitably sized heatsink.