Failing Interposing Relays, and Varistors

5 posts in this topic

First post here,

We currently use AB PLC's with relay outputs to control inductive loads.  "Hydraulic Valve Coils"  both 120VAC and 24VDC.  We use interposing terminal block relays 6A to protect the PLC outputs, and the DIN cables we use have MOV varistors in them to protect the interposing relays from voltage spikes.  

The problem i am experiencing is rapid failure of the interposing relays.  Contacts failing closed.  It just cost me a 6000 dollar hydraulic pump:(  

Does anybody have any experience with MOV's or indication of a stuck relay?  I'm trying to figure out what to do to reduce relay failure.  

The PLC valve outputs fire at least one time every ten seconds.


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What you need to do is find out the inrush of the coils. I would contact the manufacturer or use their website to determine this value.

The following information will help you to determine the inrush based on your equipment specifications.

Once you have this rating, I would look for an interposing relay to handle the value. A Solid State could also be used.

Hope this helps you out.

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Thank you for the response!  

I know the wattage and voltage of each coil alog with an estimated power factor, which gives me hold in current values, but not inrush.  

28W 120VAC PF .85= .274A hold in current

30W 120VAC PF .85= .294A hold in current

30W 24VDC= 1.25A hold in current

I know inrush can be a much higher value than hold in current, but my speculation is that a 6A capable relay should be sufficient.  I will check with MFG to verify.

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Yes, you need to get more information about loads but .... I think you are looking at fixing the symptoms rather than trying to determine root cause (and likely that would be the valve). All inductive loads (including solenoids) have very low impedance when "air core" (no ferrous core pulled in). If the hydraulic valve is sticky and fails to shift, solenoids that operate valve will stay energized but they will draw huge current. What kind of overcurrent protection you have for your loads? Individual melting fuse (not a circuit breaker) in series with each solenoid? How are you monitoring valve for correct operation?

Next, are you monitoring solenoid current? If valve is stuck and solenoid(s) draw abnormal current, generate alarm and shut down the pump.

Btw. if your hydraulic connections can lead to pump failure, something is not right... 


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Thank you for your response.  I spoke with a Phoenix contact "Relay Engineer" today.  Very helpful BTW.  The problem is not in the valve coil, but the voltage spike when the coil de-energizes.  The spike is welding the relay contacts closed.  The cables we use for the valves have a built in metal oxide varistor, that should prevent this spike from hitting the relay contact.  I am going to try using a cable with a different "faster acting" shunt or "zener diode" and see if it makes a difference.  

I am not monitoring the valve current or fusing individual valve coils.

As far as the hydraulics are concerned, I have 3 hydraulic pumps that the combined flow is over 900gpm.  The pressure sides of all of the pumps have check valves, and overpressure relief valves.  But none of the individual relief valves can handle the combined flow of all the pumps.  So, if a pump relay sticks on, it can hold the check open, and when pressure starts to build, it can turn the pump with the stuck relay into a motor, "try to spin it backwards" and break it.  

One thing i did not realize is that relay current is rated for a resistive load, and not an inductive load, but the relays are sized large enough "lucky".  

I am also going to try a DPDT relay and go from the common of one contact out the NO to the common of another contact with the load connected to the NO.  This should create a stop gap that should minimize contact arcing.

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