Diodes, suppressors and, RC networks oh my!

Going down the list on wiring best practices for VFD installations inductive circuits should be protected against transient interference. I understand the diode on DC, conceptually get the RC network, when I see suppressor I think of a condenser on point ignition systems.

I need to come up to speed on the RC network and suppressor schemes for relays, switches and other inductive loads. Also mentioned as a solution on AC circuits are varistors.

Steve

This is one of those details that can cause all kinds of grief with automation systems and drives if not properly dealt with.

Very simple rule -

Whenever you have an inductive load in your system (motor starter coil, solenoid valve coil, shaft brake coil, relay / contactor coils, electromagnet, etc.) you need to put a (snubber / suppressor) on the coil as close to the coil as you can reasonably get.

DC Inductive loads - Item # RDS, DC Applications Transient Voltage Filter On R-K Electronics, Inc.

AC Inductive loads - Item # RCS, Single Phase 120 VAC & 240 VAC Transient Voltage Filter On R-K Electronics, Inc.

You just get use to using the items above as the kickback faults are hard to trace.
They do slow down the switching which can get in the way.
That little coil in a relay or big coil in a brake turns into a generator when it moves, the power has to go somewhere....., hopefully not back into your driving circuit.
You have to dump this power somewhere.

Switches seem simple but they never "make and stay". They all bounce and you have to get past this first contact or you get false triggers.
This can be done in software or signal conditioning. Now that the processing end is so fast it shows up more.

Adding the above is the simple and maybe standard way. A few dollars and much saved grief.
You can of course "build your own" but the pennies saved are worth???
The exception would be knowing the load and needing very tiny pieces of second in your timing loop on DC circuits.
Any AC switching takes forever to respond.
Bob

Steve in SoCal said:

Going down the list on wiring best practices for VFD installations inductive circuits should be protected against transient interference. I understand the diode on DC, conceptually get the RC network, when I see suppressor I think of a condenser on point ignition systems.

I need to come up to speed on the RC network and suppressor schemes for relays, switches and other inductive loads. Also mentioned as a solution on AC circuits are varistors.
Steve

Click to expand...

Be careful. You mentioned the magic VFD word/acronym...

It would be quite normal to use a ready-built snubber network of about 0.1µF+100Ω across switch or relay contacts to protect them from arcing damage. at 240V 60Hz, that'll dissipate about 8mW of power i.e. nothing to notice. If the supply is a VFD with a switching frequency of 8kHz, it'll dissipate 120W i.e. bang!


I've got a few dozen such ready-built snubbers still in their bags. When I did the sums, I decided not to fit them across the crappy drum switch in my Hardinge HLV after having to make a new set of contacts for it (it's got the motor switched by the drum switch instead of by the two contactors in the cabinet and all of my workshop 3-phase comes from a single large invertor)

Well that IS a topical post, I was just getting back to ask about resistor values on the RC devices. In reading about these it seems that a WAG on the value should be half the resistance of the protected circuit. Specifically I have 3 motor starters, a main power contactor, 3-4 24VAC relays, 2 solenoids and a small 1/8HP 120 motor. In addition i have 2 transformers in the machine 480:120 350Va and 480:24 200Va. None of these is time sensitive in milliseconds, all of them are AC.

I am really trying to do this right the first time and I don't want to let the magic smoke out of anything. I have a good idea of things like running the power, logic, brake and, motor wiring in different locations, not running loops and so forth, the details on stuff like this is what I am just coming up to speed on.

Steve

Don't put snubber diodes on ac-powered contactor coils......

=)

jim rozen said:

Don't put snubber diodes on ac-powered contactor coils......

=)

Click to expand...

I don't think it was ever a consideration, asking what value RESISTOR to use in an RC network.

Steve

My whole shop is a museum pieces! Everything in the place save my welders and hand tools were built before Jimmy Carter came to Washington. All the contactors and relays are just plain old dumb magnetic switches.

Trying to decipher the rest of your post is a whole nuther thing, in plain Mercian not Billisms can you post a link or two? You must have some Yorkshire in you somewhere the way you write!

Steve

I spent some time perusing the bay for solid state relays and contactors, 300 volt class is plentiful and reasonable the 600 volt stuff, out of my price range! Back to banging magnets and RC network specs. In my previous post I mentioned a rule of thumb for choosing the resistor value, half the coil resistance. Is that a good constant to follow for the average contactor coil, solenoid and, ice cube relay? None of these will be switched with a great degree of precision timing, the only thing switched often would be a 120 watt single phase lube pump, it will run when the spindle moves.

To recap the controlled devices; a couple of solenoids 120 VAC, 2 ice cube relays, 7 contactors and or relays, these are all 120 or 24 VAC. Excepting the lube pump the rest will cycle a few time per machine cycle. These are on a manual lathe that does prototype work, it is not running an 8 hour shift daily, it is probbally operating at 5 percent utilization. I want to do this right but, it is not some high speed machine running 24/7. A two million cycle contactor would out live the machine by magnitudes of life cycles.

Steve

No mention of the downside of SSRs? ... they leak like a sieve, hard contacts do not.
Bob

SSRs DO have problems that contacts do not.

Low breakover voltages, for one, they have a voltage rating, and it normally isn't a lot higher than the spec, or they'd be the next spec up. A good relay or contactor is good for a voltage corresponding to the gap between contacts (which can obviously be large or small, according to design).

SSRs also have current ratings, which it is considered rude to exceed for very long. A decent relay when solidly closed will CARRY 5 to 10 times it's rated "make/break" current without problems. Indefinitely. The SSR may have a surge current rating 30x the RMS, but it is usually for one cycle. It's all about heating.

And the SSR, if subjected to a high rate of change of voltage across it, may decide to turn "on" for a half cycle. A relay will not do that, absent a genuine arc.

The SSR will, in almost every case, turn off not when the drive signal is stopped, but whenever the current falls below the holding current. (There may be Gate Turn Off type SSRs, I just don't know about them, most use regular SCRs or triacs). The turn ON will occur whenever triggered, unless you have a "zero-crossing" type.

Relays are not particular, closing and opening whenever. Their response time is normally far slower, and does not correspond to any part of the AC cycle.

The SSR is essentially immune to mechanical shock, but the relay could be mechanically shocked in such a way as to make the contacts close.

Both have their place, both are useful. Each has circumstances that may demand, or may forbid, its use.