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Solution for Relay burning Issue

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Mohan.N

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Required solution for Relay burning issue

Hi All,
We have a product to control 3phase agriculture motor.
We have deployed many units in the field and during rainy seasons our product is getting damaged.

I have shown device connection diagram in attached image.
Damage is happening to Relay and all the semiconductor components on the board.

Relay contact tracks on the board is melting out and all the GND and VCC tracks on the board are damaging.

Images of the damaged boards are attached for reference.
You can find the data sheet of the relay in below link

Link : https://voron.ua/files/pdf/relay/JQC-3F(T73).pdf

I suspect during thunder or lightning condition, very high voltage is striking the relay contacts.

Isolation between relay contact and coil is breaking down and this heavy voltage is passed to board through relay coil.

Please provide suggestions to solve this issue and protect relay from high voltage surges.

IMAG1.jpgconnection diagram.jpgIMAG2.jpgIMAG3.jpg
 

Your PCB layout is wrong, it does not have the spacings to work on 440VAC mains. :cry:

Your relays, not enough clearance/creepage. So it will always arc there and blow up your board. At the DC jack, at the pcb traces to the contacts.
The 440VAC line you run to your PCB terminal block will always have overvoltage transients and lightning etc. up to 6kV.

The motor contactor needs MOV across the coil. Otherwise, back EMF spikes with be greater than your start/stop relays 250VAC contacts.

Where is the fuse?
 
Re: Required solution for Relay burning issue

high voltage is striking the relay contacts.

Do you have long wire runs? A parallel lightning bolt can induce heavy current in long wire runs. I believe that happened to our long building when lightning stuck nearby. Several sensitive appliances were permanently knocked out. Including a low voltage intercom system.

Also does DC or AC energize your relay coils? If it's DC then a freewheeling diode is typically necessary.
 
Thanks for the Reply..

Your PCB layout is wrong, it does not have the spacings to work on 440VAC mains.
--> What should be the minimum spacing required to work with 440VAC. Can you help with any reference layout or documents.

Your relays, not enough clearance/creepage.
--> Can we get any reference design or application note to understand about it and follow the same.

The 440VAC line you run to your PCB terminal block will always have overvoltage transients and lightning etc. up to 6kV.
--> Please help with suggestions to avoid this transients and prevent the damage from this over voltage transients.

The motor contactor needs MOV across the coil
--> Please suggest recommended MOV spec to try it out.

Where is the fuse?
--> We dont have any fuse on board. Fused will be there for 3 phase supply as indicated in connection diagram.
Do we need fuse for relay contacts ?

Do you have long wire runs? A parallel lightning bolt can induce heavy current in long wire runs.
--> 3 phase supply line wires will be taken from transformer to starter input. These wires will be longer.

Also does DC or AC energize your relay coils? If it's DC then a freewheeling diode is typically necessary.
--> DC 5v is energizing relay. We have used free wheeling diode across coil.


Both relay and other components on the board are powered by single 5v DC supply.

Do we need separate supply for relay to isolate Relay part from rest of components and control relay through optocoupler?
Due to High voltage spike or lighting, if relay goes bad, other components should be safe.

Presently we are using sugar cube relay.
Do we need to go for rectangular or other type of relay to have more isolation between contact and coil ?
 
Last edited:

That design is scary.

  • The relays are not rated for 480 volts. The contacts may arc, burn out, or weld.
  • The traces are too close together for 480 volts.
  • There are no fuses.
  • The way that circuit is set up, if the controller fails while the contactor is closed, the contractor will remain closed, with whatever it controls still running.

Suggestions:
  • The "relay to stop the motor" should be normally open, and the controller should have to keep it turned on to keep the motor running.
  • Fuse the wires to the contactor. The fuse rating is whatever the contactor coil needs.
  • Everything on the 480V side has to be rated for 480V. Preferably 600V.

Solid state relays are an option, but it's hard to get good ones in China. Most of the cheap Fotek solid state relays are fakes; they can't handle the current their markings claim.
 

Don't forget to use Varistors and TVS diodes for transient voltage suppression. this is so important.
The relay you used is not good for 480V.
 

Adding bidirectional TVS (in series if necessary) across motor coils is good to prevent too much arcing at contactor switch off.
Littelfuse do some that can be put in series.
 

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