MOV size

[cxc2007]

Hi,
Hope you all are fine.

I have a question that someone migth know the answer.

How to calculate the correct MOV, to put in parallel with a 380VAC coil, to avoid the spark in a series Normal Open contact that it used to activate the coil ?

Im trying to purchase the correct accesory, but Its hard to find, so I want to just connect a MOV across the coil.

Thanks you all.

Br.

Christian.

[T3sl4co1l]

One rated for 380VAC (or 530VDC / peak), of course.

Shouldn't have to be very big, it's not absorbing lightning strikes.

Size, voltage and form factor (wire leads, screw terminals, terminal blocks, etc.) are the only differentiating characteristics of MOVs, as far as I know.  (There is a "sharpness" parameter, but I don't think it's usually very well if at all specified, controlled or selectable.)

Also very common to use an R+C snubber, usually 0.1uF + 100 ohms or something like that.  These are available pre-packaged for panel wiring purposes.

Tim

[moffy]

MOVs have a tendency to explode catastrophically when overloaded. I have used them and when they fail they can do a lot of damage. The R/C snubber idea sounds better.

[cxc2007]

Hey, thanks you.

Any Idea how to calculate the snubber ? (resistor power, etc)

Thanks again.

One rated for 380VAC (or 530VDC / peak), of course.

Shouldn't have to be very big, it's not absorbing lightning strikes.

Size, voltage and form factor (wire leads, screw terminals, terminal blocks, etc.) are the only differentiating characteristics of MOVs, as far as I know.  (There is a "sharpness" parameter, but I don't think it's usually very well if at all specified, controlled or selectable.)

Also very common to use an R+C snubber, usually 0.1uF + 100 ohms or something like that.  These are available pre-packaged for panel wiring purposes.

Tim

[SeanB]

There are formulas, but they typically are a 47n to 330n capacitor rated for the expected mains voltage ( typically a 400 or 630V X2 capacitor) with a series 47R to 100R 0.5W fusible resistor in series.

You can buy them ready made, or DIY. I did so as the price from RS was a little steep, even for RS, so I went out and got some 47n 2200V polypropylene capacitors ( replacement TV flyback tuning capacitors) and used some series 100R 0.5W resistors. 5 caps and resistors along with some heatshrink cost less than 10% of ONE snubber from RS. They are surviving, having done around 15 million AC switching cycles, though the plug in relay they are protecting has had to be replaced already from contact wear. They do not totally stop the arcing, but do reduce it a lot.

[T3sl4co1l]

You can calculate the snubber from the coil inductance and resistance, but you'd have to know that first, which isn't usually specified.

Tim