I'm designing a schematic, i plan on building PCB. Im using relays to control 120vac pumps 100-800 watts. I currently use MOV's rated at about 130vac for suppressing the noise from the pump motors. this seems to work very well. However I have been thinking about other snubbing circuits. The standard RC snubber appears to leak voltage into switched wire even when the contact would be open.
notice the use of the varistors. Pumps could be in water, probably not on GFI so the thoughts of the capacitor of the RC snubber leaking voltage don't sound good
Makes little difference. The snubber has nothing whatsoever to do with the ground, so whether a RCD is involved is immaterial.
In fact, whether the pump is powered on by the relay or not at any moment, it clearly must be adequately insulated and waterproof.
It is true that the snubber capacitor will leak some current into the pump motor, but compared to the running current of the pump, this is negligible and in fact, the resulting voltage across the pump will be trivial. You would certainly not be performing any alterations to the wiring unless it was completely isolated at the switchboard.
How much current would leak into the motor? if i put my multimeter on the neutral and common of the pump would it read the mains voltage?
The varistor is not suppose to activate until around 130vac but does it leak current more/less than the cap? Would the snubber perform better than the varistor?
I have switched the relays in complete darkness and tested with/without the 130v MOV's. i can notice quite a different in relay contacts arcing when combined with the MOV's.
It looks like it requires calculus and microhenrys to compute the cap values. this rules me out. i probably need a general use snubber if i use one but what parts should i use?
for pump connections, i have neutral and ground connected at all times, the relays open com when off. the MOV is currently connected between neutral and com of the pump wire before the relay. i thought about adding another one to the NC side of the relay.
I'm not worried about the motor. The pumps would be inside solutions of nutrient water, sometimes HOCL. im worried that leaking power into the water could cause electrolysis of the liquid solution.
My question is what difference does it make if i use a varistor or a RC snubber. to me it sounds like one is potentially dangerous and wasteful. the other i know works. I never used an RC snubber before or really know what it does.
I'm also not an engineer. so that's why id like to get your opinions on the subject.
A GFI monitors hot-neutral not ground, so i wouldn't want any problems there either "at least in USA"
I guess MOV are good for the occasional spike and probably should be in a metal enclosure or other fireproof device. It sounds like the snubber is the way to go after also discovering that the MOV breaks down for every transient that it suppresses.
What would be a good general parts list for a RC Snubber? i need to know what type/size cap and resistor. the motors are between 150-800 watts.
A few tens of milliamps, possibly a couple of hundred. Not enough to move it.
Depends on what you man by "the neutral and common". If you actually mean the neutral and live, then has it occurred to you that the pump would be running at full power if it had the mains voltage across it?
"microhenrys" are a very small unit of inductance. I cannot see what that has to do with this situation where there is no inductor. As to calculus, I don't think many if any of us use calculus in electrical engineering though it is theoretically involved. Calculations, yes,
So MOVs - or snubbers - are a good idea.
Which will only happen if there is faulty insulation. That has - as I pointed out in #2 - nothing to do with snubbers.
Well, MOVs are only slightly dangerous. Capacitors on the other hand, are not wasteful because they are a reactance and not a resistance, so they dissipate no power.
Probably a very small fraction of a mF, about 1/2000 mF (which would be 0.5 µF).
This unit for example appears to include a 220 Ohm 2 W resistor and what looks like a 220 nF 600V capacitor. At least, 600 V DC or 220 V AC is what it should be.
As pictured, it also incorporates a MOV!
Operating on mains voltages, it is generally advised to either use a capacitor specifically rated for AC mains use at the maximum voltage to be encountered - whether it be 120 or 240 V - or substantially overrated to a DC voltage, so 630 V sounds reasonable.
How a capacitor is rated takes into account things such as how much current it will encounter, both continuously and on transient, so a mains rated capacitor is clearly expected to be connected across that mains voltage and carry whatever current results, continuously.
The DC rating of a capacitor on the other hand, is generally for a capacitor to be charged to something near that voltage and carry only trivial currents as it smooths a supply or conveys a signal from one part to another (as in "valve"/ "vacuum tube" circuits). The capacitor in the snubber will only carry a quite small current with the AC mains across it but - like the resistor - a very brief transient of a larger current when a switching event occurs.
It should manage. Similarly, the reason the resistor is generously rated is not that it is expected to continuously dissipate a lot of power as the capacitor is strictly limiting the current, but that it is rated for the actual mains voltage when the transient occurs.
The final consideration for the capacitor is what would happen if the capacitor actually fails - as a short circuit. Capacitors intended for mains use are generally required to be "self-healing" such that if an internal short occurs, that part of the foil burns away and clears the short. Otherwise they may have to be fused.
Being a curious type, I brought in a commercial snubber from an unknown piece of equipment. IT is rated for 110 to 280 VAC. And is a 470 Ohm resistor in series with a 0,15uF capacitor. Other snubbers will be similar.
And I might add, all snubber caps must be AC rated, not DC. The AC peak voltage is the problem.
As to calculus, I don't think many if any of us use calculus in electrical engineering though it is theoretically involved. Calculations, yes, 
