Snubbers 101

[Beagle Beagle]

Top of the day people, let me ask hypothetical snubber type questions. I read the directions and visited a couple of hyperlinks that Marty had posted on this subject and still came away from it kind of lost. From what I’m reading you can have enough of these things. I got them on the contactor coils on the emergency stop and coolant contactors but should I have a three phase on the coolant contactor also? One of the links also talked about having one on the air solenoid my brake uses. Cool! Now where exactly do you put it? I don’t mind blowing an extra thirty clams if it keeps me from having to do it later on. Thanks in advance, Bob L

[tblough]

First, snubbers go across the coil and not the contacts so single pahse or 3 phase doesn't make a difference. When current normally flowing through a coil of wire (fan motor, relay coil, etc.) is cut off, the current through the wire resists stopping and generates a high voltage spike before it stops (like water hammer in pipes). A "snubber" is usually a capacitor+resistor or a diode across the coil that prevents the voltage spike from travelling back up stream and destroying either the electronic output (if directly connected) or welding the relay contacts of the controlling device.

The "snubber" should be matched to the control voltage which is probably 120VAC, 12 or 24VAC, or 5, 12, 24VDC. Being either a diode or RC network, they should cost around $2-3$ not thirty clams.

[joneg]

you should also use 3 phase r/c snubbers from phase to phase on all motor contactors

[ashesman]

The general rule for snubbers (and flywheel diodes) is that they should be placed anywhere an inductive load is suddenly interrupted. So, anywhere a solenoid or really/contactor coil will be suddenly electrically disconnected.

When a coil is energised by passing current through it, a magnetic field is generated around its winding. When the coil is deenergised the magnetic field wants to collapse. The collapsing of the magnetic field will induce a voltage into the coil. If the coil is open circuit when the magnetic field collapses, the induced voltage is extremely high. The induced voltage acts to keep the current through the coil going in the same direction is was.

A snubber is used to short this “transient” voltage/current and keep it circulating in the coil until the energy is dissipated as heat. Without snubbing, arcing of relay contacts can occur and the rapidly changing current can cause electromagnetic interference. The negative of snubbing is that it increases the turn off time of the coil as magnetic field collapse is slower.

Snubbers should be ideally placed as close to the coil as possible to keep the physical current loop as small as possible. Small current loops are essential for reduced interference. Often this is not practical and they are placed at the terminal block or relay that drives the coil.

For DC driven coils, a flywheel diode is most effective and cheapest. The diode needs to have a forward current rating of say twice the current drawn by the coil. Diodes however cause the slowest turn off time but may not be noticeable in practice, RC snubbers are needed for AC driven coils.

Snubbers should be placed on all outputs of a CNC controller that drive an inductive load such as a relay or solenoid or contractor winding. However, I personally have never seen them fitted on things like motors and pumps driven from contractors etc.

[xr4x4ti]

What about on the relay output of an Allin1DC turning on and off a simple lube pump. It seems like overkill to have one on there, but the Centroid schematic shows one.

Thanks,
Tim

[ashesman]

That is a definite time when a snubber should be used. If the relay is opened while the pump is at it’s peak current then a large voltage will be developed across the pump. This could cause interference or contribute to excessive arcing on the relay contacts.

Although I would really need to see the entire circuit and voltage/current ratings to make a call.

So many times I have seen in snubbed relays cause interference issues like USB disconnect and even chip latch up (failure that requires power cycle). In automotive you will never find a relay without a flywheel diode.

[joneg]

attached is an example of rc snubbers on motors

[ashesman]

Some real world examples of snubbers and flywheel diodes on a machine I decommissioned tonight:

Single phase RC snubber on the electrical cabinet fan.

Flywheel diodes on each of the output drivers. These were connected across each of the 24VDC solenoids. The machine has solid state outputs so essential these are protected from excessive flyback voltage.

RC snubber across the Z axis brake solenoid. This is actuated of 24V DC but uses a snubber rather than a flywheel diode. Most likely to ensure the solenoid is de-energised quickly

A three phase RC snubber for the coolant pump. The pump wired to exactly where the snubber is connected.