Preventing Relay Contacts from Burning

What voltage & current are you switching? AC or DC? What is the load? If inductive (as I suspect), what is the inductance?

Also, do you have coil suppression? The wrong sort can slow down the relay action, and make arcing worse.

How about using a zero-crossing solid-state relay?

Look like over loading of the contacts.

How does blowout coil problem or a dirty or improperly mounted arc chute . both which will effect arc quenching .

Another thing that might happen is contact opening slowly due to a brass shim been worn or missing at the pivot on the coil armature .

Just a couple of thoughts of what I have run into at 3 in the morning........r

If you are in a environment with high levels of RF (for example very close to large transmitters or antenna systems) this can also lead to contact burning. Alternatively if the relay is in a corrosive atmosphere it will accelerate contact erosion. However as others have said, it is most likely due to inadequately suppressed inductive loads.

Hi

relay contact is used for swithcing of DC load having R of 43 ohm, inductance is about 300 mH, and supply voltage is 135 V dc

Arc chute can not be provided as it is very small PCB mounted Relay,

Best regards

Divyesh patel

What is the make/part number of your relay? As I've found recently, getting a relay or contactor rated for DC inductive loads can be quite a problem.

Check the rating of the relay to make sure it is properly rated for your application. The PCB mount may make it difficult to upgrade. Some manufacturers have spacial alloy contacts for inductive or cold lamp inrush loads. The 135 VDC supply is about 70% more erosive to the relay contacts than AC because it is always interrupting the full 3A load current @ 135V. A suppression diode across the inductive load will minimize the back EMF voltage spike as the contacts open, but this alone may not be enough to obtain the desired life. If you can not upgrade the relay, you may want to make it a socket mount to al least make it easy to change. Otherwise - back to the drawing board!

I have a similar problem with the relays fitted in petrol pumps. They switch 240Vac motors and solenoid valves at about 6A. Although rated at 30A ac the contacts burn out or weld together after a few months and I have a large box of scrap relays as a result. A more important side effect is the 'splash' of RF noise caused by the arcing which is picked up on the nearby 5vdc tracks and this often corrupts the CPU memory! The only way to overcome this problem is to fit a good solid state relay. They are more expensive but work out cheaper when you consider the cost of 'down time', engineer's travel/work time, repair cost and the cost of holding spare boards to carry out the replacement at first line.

I wish equipment designers would sometimes listen to the maintenance people rather than the Company bean counter.

Bill's suggestion for a solid state relay having zero volt switching is not only a good idea for relay life, but also for safely keeping a 6A 240V arc away from petrol pumps - yikes!

Zero-crossing is O. K. for AC but doesn't apply for DC, although as kilowatt0 suggested in another post, this SSR from Futurelec should work.

Many years ago, and for low current applications,
we used a "contact oil" to prevent arcing.
This was extensively used to good effect.

I have no further info. on the oil, and would suggest a
little research and/or contacting some oil suppliers for
this type of oil as a possible solution?

e.g. http://tetra.jp/eci/ecieng/05eciseihin.htm

Hope this helps.

jt.

Freedom is in the mind; don't let anyone take that away.

Three amps on a little relay could easily be the single factor that is your problem. As others here have identified, check your relay specifications. Many relay manufacturers state a maximum interrupt current specification for their relays. Make sure that your reverse biased diode is across the inductive load side of the circuit and rated for not just the reverse breakdown voltage but the three amperes that it will briefly handle. I also recommend that this diode be a Schottky diode to minimize the turn on time. (This is not the diode across the relay coil.) Lastly, during the diode turn on lag, a snubber capacitor in parallel to the diode will instantly handle the inductive current. A rule of thumb hand wave will be to double the diode turn on time for dT, double the diode turn on voltage (1.2V) for T and calculate C by C=I*V*dT. However, adding a capacitor for snubbing in some configurations can actually hurt your relay contact because depending on the source impedance, on contact closing the surge current to charge the capacitor can now exceed the current rating while the contacts bounce.

Some additional information for the discussion, the large box of burnt relays I've collected are from PCBs which do have snubbers, diodes, etc. They are made by Potter&Brumfield, Schrack, Omron and IMO so the only common factor is the repeated switching of an inductive load. Probably petrol pumps are the worst case as the motors switch on and off every few minutes, day after day! I had one relay which 'torched' and burnt all the components off the board within 6" of the relay, fortunately just outside the explosive zone area!

If you read the specs for solid state relays they recommend Zero-volt switching for resistive loads or motors where the power factor is greater than 0.8 and Random switching versions for inductive loads with power factor less than 0.8.

The only draw-back with SSRs is the large amount of conducted noise they make which spoils the EMC compliance somewhat. Even this can be overcome as I have developed a new SSR which is well within the EMC limits at 240Vac 80A, (full patent and EMC certification). A 440V version should be ready for sale at the end of March.

Tom,

I presume that you are not the guest who originally posted this problem. For your case, running AC motors a capacitor or better a resistor and series capacitor in parallel with your motor load will doing the snubbing quite well. The snubbers do not have to be mounted on the relay board or near the motor so space should not be too critical of a problem. Since you have an explosive zone, I recommend fixing this before things go bang.

After 30 years running a service operation with over 4,500 busy petrol stations I think I have seen every type of petrol pump design ever made since the last war and even earlier. Every manufacturer has used an electric motor to drive the pump and they all use a relay of some sort to switch the load. Some motors have relays built inside plus capacitors and additional centifugal switches, some have the relay on the power supply board in the top of the pump. They use snubbers, diodes, electromechanical and solid state relays. All I can say, in answer to the original question is electro- mechanical relays have a much shorter lifespan and are prone to contact arcing. If the motor is used infrequently, switched on for long periods or has a resistive load there should be no problem, but for repetitive switching of inductive loads you really need to consider an electronic switch such as the solid state relay.

There are every kind of electronics components for this and may be a RC circuit needs a so big capacitor to be useful (.5*L*I**2=.5*C*V**2,where V is voltage you accept on ),a very small component used i think was varistors its name,no any electronic supplier,just power electronic supplier for industry should offer you the best for this problem.-