"welding" of relay contacts problem

[archielim22]

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?

 

[peebee]

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

 

[buzzp]

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.

 

[electricpete]

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.

 

[peebee]

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

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

 

[Crowbar1]

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

 

[archielim22]

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....

 

[Crowbar1]

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.

 

[archielim22]

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

 

[Crowbar1]

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

 

[archielim22]

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

 

[electricpete]

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?

 

[jbartos]

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.

 

[archielim22]

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...

 

[jbartos]

Suggestion: Visit

http://www.phanderson.com/printer/allegro.html

for ULN2003a darlington array parameters, 50V/500mA

 

[archielim22]

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...

 

[electricpete]

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

 

[archielim22]

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....

 

[electricpete]

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.

 

[archielim22]

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