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Power transformer snubber
2

Power transformer snubber

Power transformer snubber

(OP)
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

RE: Power transformer snubber

That's not a reed relay for a start. smile

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.
  

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

RE: Power transformer snubber

(OP)
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!

RE: Power transformer snubber

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.
  

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

RE: Power transformer snubber

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.

RE: Power transformer snubber

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.

RE: Power transformer snubber

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.
  

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

RE: Power transformer snubber

(OP)
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.

RE: Power transformer snubber

(OP)
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.

RE: Power transformer snubber

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

RE: Power transformer snubber

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.
  

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

RE: Power transformer snubber

(OP)
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.

RE: Power transformer snubber

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

RE: Power transformer snubber

(OP)
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

RE: Power transformer snubber

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

RE: Power transformer snubber

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

RE: Power transformer snubber

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.

 

RE: Power transformer snubber

Also, if you are custom building a special PCB for this project then you should look at using a triac (~30A rated) and some heatsink and then add a driver circuit. Check these little drivers out.

http://www.fairchildsemi.com/pf/MO/MOC3021M.html

If you don't like that, then google search triac driver circuits and you'll find lots of possibilities.

 

RE: Power transformer snubber

Gate firing becomes more demanding with highly inductive loads. This is one of the problems with many SSRs designed for resistive loads when applied to inductive loads. Keep away from triacs for anything inductive, it opens up a world of commutation problems: anti-parallel thyristors are a better solution.
  

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

RE: Power transformer snubber

(OP)
I think I've ruled out 90 deg. turn-on/off since there are few/expensive options available (although this would be an ideal solution).  Zero-crossing ruled out as well.

So, at this point I'm looking at either random-fire SSR or mechanical relay.  SSR for no moving parts; mechanical for lower power dissipation and smaller formfactor, as well as less trouble with inductive loads.  If I go with an SSR I will probably have to offload it from the PCB since I'm working with a fixed size and running out of room.

Right now I'm not seeing a way to limit in-rush current.  Closest possibility is a thermistor, but this wouldn't perform predictably under repeated and variable cycles.  With in-rush current a reality, how much should the relay be over-rated for a long life?  Will the Panasonic ALFG2PF12 cut it?

Either way I'll add a MOV or RC snubber--no reason not to for 50 cents.  I like the MOV better, any reason to go with RC instead?

Thanks again everyone, I'm learning quite a bit!

RE: Power transformer snubber

An RC snubber works two ways, it may reduce inrush also. The reason is that inrush not only depends on where on the sinewave you switch on. It also depends on what state you left the core in on last switch-off.

If you left the core with (for instance) +20 percent flux and switch on (randomly) at the positive zero crossing, you will get very high inrush.

You can use the snubber to demagnetize the core if you make the snubber so lossless (low R and quite large capacitor) that the output swings a few cycles as flux is taken down at switch-off. You will then switch on with near zero remanent flux and your inrush will never be extremely high - just high. And that is a lot better than extremely high.

I feel a measurement coming. Check in an hour or so. There might be a recording for you.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Power transformer snubber

(OP)
Ah, that makes perfect sense.  I had even seen this oscillation in my SPICE simulation, but hadn't put two and two together.  In this case I would want the snubber parallel to the transformer, not the relay, since I don't want leakage current, right?

RE: Power transformer snubber

Yes! That is exactly what you saw. Yes! Parallel to the transformer.

Did you check different R and C? Perhaps you put your results here. So I don't have to.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Power transformer snubber

(OP)
Well I won't discuss the boring details, but ultimately R and C are determined by a few simple equations.

First I measured open circuit RMS current (Im) on the primary.  From there,

C=2*Im*V/(Vp^2*w), where Vp is desired maximum transient voltage and w=2*pi*f and V is operating RMS voltage.

To determine R, I played around with a few equations to compromise between power dissipation, relaxation time, and number of oscillations.  Relaxation time is approximated by 4 times the time constant (alpha) and number of oscillations is approximated by the inverse of the damping ratio (delta),

alpha=2*R/Lm, Lm=V/(I*w)

delta=alpha/w0, where w0=1/sqrt(L*C)

The above determined Lm=0.6H, C=4.7uF, R=22ohm.  This will limit transients to 270V, oscillate about 30 times in 0.25 seconds, and dissipate about 1W.

RE: Power transformer snubber

Does that minimize inrush, compared to no snubber at all? (I hope it does).

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Power transformer snubber

(OP)
In theory...

Everything's still on paper/computer at this point, and I don't have enough information about the transformer to model hysteretic effects in SPICE.

I'll post an update when I have that information.

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