snubber in contactor coil

A common AC (you didn't say whether the contactor coil is AC or DC operated, so I guessed) coil snubber consists of a resistor in series with a capacitor, connected across the coil. When the driving contacts opens, the coil current detours to the capacitor, which limits the rate of change of voltage as it absorbs the energy that was stored in the coil. The lower the value of the resistor, the better this works. Except that when the driving contact closes at peak voltage, there is a large inrush current as the cap is popped up to that peak voltage, while the contact is bouncing. Bad for both RFI and contact life. So a compromise must be struck on the resistor value. Since the coil draws essentially no current during the bounce time, because of its inductance, I usually size the resistor to limit the peak current to no more than the contact current rating. The capacitor value and voltage rating must be large enough to contain the peak stored coil energy without exceeding its voltage rating. But way too much capacitance causes the series resistor to get hot while the coil is energized.

In some cases, I have used a smaller capacitor than could safely store the peak coil energy, and limited its peak voltage by paralleling it with an MOV.

So to design an effective snubber that doesn't get too hot, or damage the contacts, you need to start with information about the contact rating (that is driving the coil) the peak driving voltage, the coil's peak current and its inductance (to come up with the (I^2*L)/2 energy that gets transfered to (V^2/C)/2 in the capacitor.

Or you but an off the shelf RC snubber rated for the coil voltage and hope it works and survives.

John Popelish wrote: (snip)

Sorry. The energy stored in the capacitor is (V^2*C)/2

It's hard to tell whether you're talking about snubbing the coil or the contacts or both? And the OP is classically conclusory: has no idea in hell what he's doing but still *knows* a snubber is the answer. You are a glutton for punishment it seems.

Either that or John's just a natural teacher.

Cheers Chris

Step 1) Look up contactor in catalogue. Step 2) Order the matching supressor/snubber/whatever it is called in catalogue.

Then fun starts:

A snubber cross the coil will keep the flyback energy from the inductance from exceeding the breakdown voltage of the driving device, if properly designed, but the coil side should not be causing the sort of noise that would hang the DSP. I'd guess more likely the very fast risetime transients from the contacts opening and/or closing are the culprit. You can also apply a snubber to them. The snubber design depends on the load they are driving: the voltage, the current, and whether the load is resistive or inductive (or possibly capacitive). Beware that optimal design of a snubber for contacts is different than optimal snubber design for a solid-state switch driving a relay or contactor coil. You should be able to find web references for both; I have in the past, and some are quite good.

Cheers, Tom

Hi Tom,

Back in the old days, a laser company moved in next door and all our sensitive equipment went belly up. It turns out they used 10KW heaters to soften the glass, and the temperature control was a simple thermostat.

Every time the contacts opened the arc resonated with stray inductance and capacitance in the wiring, and generated a burst of 40MHz noise, measured with my HP spectrum analyzer. The RFI went everywhere and was impossible to filter or shield against.

Hertz would have been proud:)

I talked to the owner and he allowed me to add simple snubbers across all the contacts. I believe it was something like 47 ohms in series with 0.1uF, but this was a very long time ago. Anyway, it was pure guesswork.

Since there was no way of figuring out the wire inductance in the ratsnest, there was no calculation involved, and the first try solved the problem. I guess the trick was to prevent the arc from starting in the first place.

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

Yep, exactly: for opening contacts, keep dv/dt low enough that the contacts spread fast enough that no significant arc develops. The arc itself should not be the problem; it's the very fast dv/dt when the arc finally quits. And to do the job, you need capacitance, with just enough series resistance to keep the current pulse in the capacitor when the contacts close under control. But the snubber across a contactor coil driven by a solid-state switch such as a triac will typically use a much larger resistance, too large to work well for a contact snubber.

Arcs/sparks have a bad reputation for causing EMI, but it's not the arc itself, it's that the arc can act like a very fast switch. Spark transmitters used the spark to connect a capacitor in parallel with a coil, with the capacitor pre-charged to typically a rather high voltage. Though the spectrum was fairly broad, most of the energy was concentrated at the resonance of the capacitor and inductor. Spark transmitters generating up to a significant fraction of a MEGAwatt output were built and put in service to communicate roughly half-way around the world at what we'd now call VLF frequencies: in the tens of kHz.

Cheers, Tom

...

I heard or read somewhere once something to the effect that the resistance should equal the inductive reactance, anthough they didn't say at what frequency, and as you've said, there was no way to measure the inductance, so a WAG, if it worked, was obviously the right answer. ;-) Also, the guy that made that reactance remark just said to use a large enough capacitor that its reactance is negligible; I'd think "critically damped" would be the value that would resonate at the inductance you used for the resistor calc, at the frequency of interest.

At least that's a starting point - I tried to learn about snubbers once, but it turned out to be as much black magic as science, much like ferro- resonant transformers. :-)

Cheers! Rich