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
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
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!
RE: Power transformer snubber
RE: Power transformer snubber
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If we learn from our mistakes I'm getting a great education!
RE: Power transformer snubber
Mechanical Power Relays, in my opinion, never have a long trouble free life.
RE: Power transformer snubber
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
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!
RE: Power transformer snubber
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
And wouldn't and RCD snubber be unidirectional? I'm working with an AC signal.
RE: Power transformer snubber
Bill
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"Why not the best?"
Jimmy Carter
RE: Power transformer snubber
Will this relay cut it? I can't get any bigger without moving it off the PCB. With or without snubber?
ALFG2PF12
http://
RE: Power transformer snubber
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If we learn from our mistakes I'm getting a great education!
RE: Power transformer snubber
RE: Power transformer snubber
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
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power transformer snubber
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?
ht
RE: Power transformer snubber
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
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power transformer snubber
RE: Power transformer snubber
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power transformer snubber
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power transformer snubber
RE: Power transformer snubber
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
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If we learn from our mistakes I'm getting a great education!
RE: Power transformer snubber
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
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
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power transformer snubber
RE: Power transformer snubber
Did you check different R and C? Perhaps you put your results here. So I don't have to.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power transformer snubber
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
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power transformer snubber
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.