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"welding" of relay contacts problem

"welding" of relay contacts problem

"welding" of relay contacts problem

(OP)
Why does my relays stop "switching" after some time of use. I am currently working on controlling a 24v 5A DC motor using a microcontroller and a uln2003a darlington array. Also using a 5v coil rating, 28v contact rating, 10A relay to control the motor. Everything is working perfectly except that after a few minutes of use, each relays stops switching....

i read somthing about using supression circuits, but the uln2003a has, and should have a built in protection... might need to protect the contact too? how? any help would be greatly appreciated.

also, does using an other types of relay with different material helps? i heard about expensive relays which supposedly does not weld... is it worth it?

RE: "welding" of relay contacts problem

Please verify that your contactor is rated for 28vdc and 10a inductive, not 28vac and 10a resistive.

RE: "welding" of relay contacts problem

I agree with Peebee. There should be a VA rating or HP rating of the relay if it has the ability to switch an inductive load. I know AC motors have a large inrush of current at start-up. I would think DC motors do as well so you can not use general purpose relays for inductive loads of this nature.

RE: "welding" of relay contacts problem

So you have a Darlington switching 5vdc to a mechanical relay/contactor, and the mechanical relay contact switching 24vdc to the motor, right?

When the "relay stops switching", does the motor stay on or off (I'm assuming on since you suspect welded contact, but want to doulbecheck).

Maybe you have already done, but I would suggest you do careful measurements/observation of the problem to narrow down it is in fact a problem with the output contact.  Also it seems possible that the relay may be mechanically binding, electrically shorted/opened, or may not be receiving a correct signal from Darlington.  (Maybe you have an intermittent-duty relay which you are energizing for long periods and damaging bey overheating after short use?)

Maybe the contacts can be inspected to confirm your suspicion of welding. (maybe you have already done that?).  If contacts welding than the others have provided good suggestions and you have identified the possible remedies of examining relay contact ratings vs application, possibly adding surge supression, possibly go to new relay.

RE: "welding" of relay contacts problem

One other question -- why is "welding" in "quotes"?

Are the contacts actually welding, or do you just suspect that they may be "welding?"

RE: "welding" of relay contacts problem

I would also ask what is the required duty cycle of the relay?

RE: "welding" of relay contacts problem

(OP)
Thankx for everyone's reply...

anyway, relay is confirmed to be 28vdc, about the 10A inductive ill check that..Ü

Signals from darlington is confirmed to be correct, and reason i suggested it was welding because when i give it a hard tap (the relay) it workd fine again for a while...
also when i check the voltage across the coil during switching, it is of the correct value...

As for the duty cycle, i will check on it, but how does it affect my system? never thought about it....

RE: "welding" of relay contacts problem

If the relay is switching often, the repeated make/break can cause excessive contact heating and therefore welding.
You may need to oversize the relay as you would a motor starter for jogging/plugging operation.

RE: "welding" of relay contacts problem

(OP)
CROWBAR: i think u rightÜ the relay stops switching when i deliberatly switch fast....

does oversizing mean higher contact rating?

no other solution? like supression ckts? if so, how do i connect them, im confused on the configuration of say, diodes polarity with respect to the contact
 ---> (+5v) coil
|_________
X       X |---> 24vdc source
   X -->  |to motor
X       X |---> gnd (normally closed)
__________|
-----> (to darlington array, low activated) coil

so basically when i apply a 0v to the other side of the coil, the relay switches to the normally open side which is the 24vdc source

thankx! you have been much help

RE: "welding" of relay contacts problem

It would seem you would need higher HP (amperage) rated contacts or a change in contact configuation - that would be double break contacts.  

RE: "welding" of relay contacts problem

(OP)
i dont know if it is a "novice" question, but what is a double break contacts?

also can u suggest on a better contact configuration?

another question is: is there any kind of ckt that i can use to protect or lessen the surge currents

thankx againÜ

RE: "welding" of relay contacts problem

the fact that it seems to respond to a hard tap... makes me think of relay mechanical binding more than contact welding.

Mechanical binding might have many causes but thermal comes to mind.  Are you sure the relay is rated for 5vdc continuous and that no more than 5vdc is being applied?

RE: "welding" of relay contacts problem

Suggestion: The double break contact is a bridge type of contact that has a bridge with a contact on each side of the bridge. Electromagnetic contactors often have double break contacts.
Please, could you post the manufacturer and data of that switching arrangement that you have.

RE: "welding" of relay contacts problem

(OP)
i'm not sure wats the difference between relay mechanical binding and relay contact welding, but wat im refering to is the contacts inside the relay does not respond to the applied coil voltage.

i think there is no problem with the coil voltage since it does work and when i take the voltage across it during switching, it does go up to around 5v, problem is the contacts does not switch. so i suspect that the contact  have 'welded' to either the NO pin or the NC pin...

RE: "welding" of relay contacts problem

(OP)
the darlington array is only used for the coil, which has a very minimal current and voltage....

i dont think there's anything wrong wiht the coil since it still produces the right voltage during switching, problem is contacts does not 'switch', which led me to conclude that it has welded to the NO or Nc pin...

RE: "welding" of relay contacts problem

Please explain what you mean by "..it [the coil] still produces the right voltage during switching.."

RE: "welding" of relay contacts problem

(OP)
the coils are controlled by the darlington array and a microcontroller, so when i send a signal to the coil of the relay, it should energize and switch the contacts... but mine does not(after quite some time), so i have to determine whether its the coil not getting enrgized or the contact not switching so i took a probe n luked at the voltage across the coils and determined that it has the right voltage, another thing is that when i tap the relay, it switches again...

anyway, im quite sure that the problem is with the contacts and the high DC current since when the system is not connected to the motor, eveything is perfecctly okay....

RE: "welding" of relay contacts problem

So now I understand you have verified the correct voltage on the input to the relay.

It does not address my suggestion that the relay itself may be mechanically binding. i.e. you apply a voltage and it doesn't move.  

From an input/output viewpoint it may look the same as a contact problem, but it is a different physical failure mechanism.

Can you see any motion during normal operating of the relay?  If yes then check for normal motion during failure to help rule out mechanical binding.

If you can't see normal motion then consider whether the coil is rated for continuous voltage application, otherwise may overheat and bind. Also perhaps you can feel it and subjectively determine whether it might be overheating.

Is it just one relay exhibiting this characteristic or many?  May be design or may be single fault relay.

RE: "welding" of relay contacts problem

(OP)
Im having problems understanding the difference between mechanical binding and contact welding, wat i do know is that the contact or pin inside the relay that is supposed to switch, welds or binds to either the NO or NC pin.

This problem arises when i switch fast, which led me to conclude that transient current produced by the inductor motor heats up the relay contacts, causing them to bind or weld. when i switch slowly, the relay works perfectly okay. when i disconnect the motor and try to switch fast, it still works okay.... only when the motor is connected and switching is fast does the problem occur..

Also the it does not occur on one relay alone but on all four...

i think it is a design fault, since DC motor does produce high EMF, which heats up the relay, but im contrained in using it, so my choices are either design a supression ckt ( i have tried several,but to no avail :(  or buy an expensive relay, which advertises not to bindÜ im still hoping to find a supression ckt, if not what exactly will i look for in a relay, my current relay is already rated at 10A 120VDC with coil rating at 5vDC.

thank you for ur insight, keep it coming, it has been a great help in analyzing the problemÜ

RE: "welding" of relay contacts problem

Hi, when you switch on a dc motor it is allmost a short untill it starts moving. This causes a large current to flow which can weld relay contacts and also weaken the field magnets of the motor. You need some current limiting in the circuit, a low value resistor might work or some active element such as a transistor or surge limiting thermistor.

RE: "welding" of relay contacts problem

yes it's a good point by cbarn.  I certainly don't disagree with the possibility of contact welding.

I don't understand what is the difference between switching fast and switching slow. Are you talking about the longer time between turning the motor off and then turning it back on?

RE: "welding" of relay contacts problem

(OP)
cbarn: i will try thatÜ any recommendatin on the resistor value?

pete: yes switching fast means turning the motor on and off... as far as i know, DC motors have high transient currents, causing the 'melt'

i just read about the double break contacts, thankx to jbartos and crowbar, will also try to luk into thatÜ

RE: "welding" of relay contacts problem

I would put a truck headlight relay there. It is made for 24 V and 30 A.

RE: "welding" of relay contacts problem

Hi, you will need to experiment with the value, get a pack of 1/2ohm 10W resistors.

RE: "welding" of relay contacts problem

archielim22,

1) The driver, ULN2003A, does have a suppression circuit.  However, unless that suppression circuit has been provided externally, pin 9 must be connected to the positive side of the relay.

2) Much has been said about the relay's output, but what is the relay's coil resistance?

3) Much has been said about the microcontroller driving the ULN, but what is the voltage at the ULN's input pin?

4) Are any of the other drivers in the ULN being used?  If so, what is the combined coincident load, in mA?

5) What is your "switching-rate?"

6) If the other drivers aren't being used, then, parallel 1 or 2 others to the one being used!

RE: "welding" of relay contacts problem

(OP)
cbarn: ok ill do that. problem is the relays might already be busted, and i have to keep busting relays to see what value worksÜ
But i have to do it, got no other choice...


Shortstub:
1.) Yes, pin 9 is connected to the positive side of the relay.
2.) I think coil resistance is 63ohms
3.) Voltage at input is also 5V, using the darlington array to increase current.
4.) Although 4 of the arrays ports are being used, only one port is being used at a time.
5.) Switching rate varies depending on the user, but the maximum rate would be around 5 switches per second.
6.) Connecting them in parallel would increase my driving/sinking current right? And this would increase the 'power' to unweld the contacts? or yuo have a different reason in suggesting thisÜ pls explain if yes, thankx!

Any relay experts out there?Ü pls suggest what relay to use, including manufacturer and model, and why Ü

RE: "welding" of relay contacts problem

Archielim22,

Reur pt 2.  Sinking current is 380 mA, which is within the chip's rating.

Reur pt 6.  That is correct.  Two drivers will reduce the current to 1/2 of the above, or 190 mA!

Based on your measurements, I don't believe the problem is the chip, but instead, the switching frequency.  Contact welding is not usually a problem of "making" DC current.  However, it is a severe problem for "breaking" the current.  Some visible arcing is normal, but is there considerable "visible" arcing?  Arcing is more of a problem if the current that is being interrupted is long in duration, i.e., high L/R.  Double-break contacts will increase the interrupting capacity by about 3 times over single-break.  But, in the end, contact "life" is very, very, dependent on the L/R of the "load!"

The automotive-type may be worth a try. You might even try "breaking" both the neg and pos lead, but frankly, I believe that the application is much to severe for the relay you have chosen.

Relay selection is very much an "art", therefore I suggest you contact a relay "specialist!"
 

RE: "welding" of relay contacts problem

Hi shortstub, could you explain how a contact can weld as it breaks.

RE: "welding" of relay contacts problem

Archielim22,

Am I wrong? Or, are you using this circuit for PWM control of the DC motors' speed?

If so, you'd be far better of using a power semiconductor, like a MOSFET, in place of the relay!  An addtional benefit it can be driven by the microcontroller, eliminating the ULN2003A!

RE: "welding" of relay contacts problem

Cbarn24050,

If I interpreted Archielim22's comments correctly, then the relay is being energized to operate the motor thru "making" of its contact(s).  The coil is not being energized to "break" the motor circuit.  I'm not suggesting contact "welding" occurs only because of "breaking" of the current circuit, but, it is the greatest contributing factor.  The L/R ratio determines the duration of the "arc!"  Extinquishing of the arc is a function of the speed of separation of the contacts.  The required speed is not possible with a standard relay.

Arc duration contributes to the melting of the contact material, contact pitting, and distribution of the arc products.  All play a part in the "welding" phenomena.  Contact bounce upon "making" could also be part of the problem, as well as "chatter!"  However, the latter is more of a problem with AC circuits, but that's a topic for aother thread!

Of course, a sluggish movement could also be a contributing factor.  Proof of the latter would be arcing upon "make!"  In any case, 5 contact operations per second is excessive.  I am certainly not suggesting the use of high-speed photograpy, although I did use a fast-frame cam-corder to analyze a similar mechanical function.

Contact "welding" is never the result of a single break/close cycle.  For the given "switching-rate" I am sure if the contacts were examined periodically, then, the worsening condition would be observed.  Anyway, in my experience, the problem is not the electronics, but the electromechanics.  Typically, contact "life-expectancy" is expressed in millions of operations.  Of course, I'm unaware of the actual number in Archielim22's case, but, it could be as high as 400,000 operations per day!

RE: "welding" of relay contacts problem

Hi shortstub, am I to take it that you cant explain it. I cant say i'm suprised.

RE: "welding" of relay contacts problem

Cbarn20405,
And what part wasn't explained to your satisfaction?

RE: "welding" of relay contacts problem

Suggestion/question: Is ULN2003A properly applied? Have you tried to replace it by another one, known to be good one?

RE: "welding" of relay contacts problem

hi shortstub, you didnt explain how a contact can weld as it breaks.

RE: "welding" of relay contacts problem

Cbarn24050,

Now I understand where you're coming from... the resistors!  I wondered why you were so enraged at my reponse that you decided, albeit, anonomously, to throw a "barb!"  Now, don't get me wrong, it's your perogative to question anything I provide as a technical view.  But do it in professional manner.  Say, it's B... S..., and then explain your position!  This forum, afterall, is  supposed to be about "give & take!"  But, more on this later.

I agree that "welding" can occur on "make!"  Obviously, it can only occur while the contacts are "mated."  It's a DC motor, about 1/6 Hp, and it is reasonable to assume that inrush can be about 2 to 3 times FLA.  This value is not too large given the rating of the contacts, i.e., 10 Amp.  But, before mating surfaces can fuse, there must be a large current-density, i.e., current thru a small area of contact.  Archielim22's observation was that the contacts' operated for some time, so its also reasonable to assume that the contacts were"good" at the start.  Further, it was my position, that the degree of stress on "make" was far less than that at "break!"  I didn't say the contacts fused, while parting.  Also, Archielim22 did not observe a drop in the coil voltage, which could have led to "chattering!"  Furthermore, I've "played" with quite a number of ULN's, although 2001's instead of the 2003's, for many years!  

That said, then, as far as this thread is concerned, it is clear to me the problem is the relay switching-rate, at least until proven otherwise.

Now, cbarn, if you're questioning my credentials I'm more than happy to provide them to you, or anyone...on list... or off!  My real "gripe" with you, cbarn, is, that I perceived the "barb" as questioning my veracity.


 



RE: "welding" of relay contacts problem

(OP)
Wow so much replies since i last read this....

1.) Shortstub maybe right that switching rate is a problem since if i 'switch' the relay at lower rate, evreything works okay....

2.) i dont know if this will help, im using a SPDT relay, and looking at my schematic above, the motor's EMF during break sinks current at the NC pin.... which is a very high current, which can lead to a melt, and consiquently, a weld. i have already provided a diode at this pin for supression.

3.) For the 'make' part, im providing a 3.3OHM series resistance for the initial moment that the NC and NO are shorted, producing high current at startup.

4.) i will now be testing a power PCB relay with a 16A 125VDC contact rating. If this does not work, i dont know what willÜ

5.) A side note, the OEG relays lasted for about a minute of high rate switching before welding, the OMRON lasted for about 3 minsÜ i believe the MATUSHITA ralys are considered the best, but they are so f*@&$ expensive..Ü

RE: "welding" of relay contacts problem

Hi shortstub, I think you need to read again what you posted, if your retacting it now thats fine. You seem to be 1 of those hotheads who posts before reading, may I suggest more use of the "preview post" button.

RE: "welding" of relay contacts problem

Archielim22,

Now that the mechanism of contact "welding" has been explained, at least to the satisfaction of one, would you like to pursue the "art" of contact protection?

RE: "welding" of relay contacts problem

(OP)
Shortstub,

i think i have solved my problem, at least i think so... i did the following:

1.) supplied a diode across NC and Common contact to supress incoming EMF current during 'turn-off'

2.) connected a 1 ohm resistor between relay and motor to limit the current during startup

3.) decreased switching rate

4.)  supplied a relatively expensive relay for robustness of contacts

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