Losing Relays

I've got an issue on a new system that I installed about two weeks ago. I've had to replace 3 of the 10 interposing relays that I have on the outputs of my Micro830. I am utilizing Allen Bradley 700-HLT1U24 terminal block relays. The relays operate DynaQuip 110v motorized valves. The relays stop switching and lock in either the NO or NC position.

I believe the issue stems from neglecting to attach the -24vdc common to the negative common of the PLC. I had not used a wiring scheme like that before and didn't realize it was absolutely necessary.

Also, the valves only draw 1.8a locked rotor and the relays are rated at 3a 110vac. Switching rate is not terribly frequent, >5 minutes, but on state is typically 5-10 minutes. I have ensured that the limit switches on the valves are set correctly so that a locked rotor condition does not occur.

Did I damage my relays by being a buffoon and merely need to replace them? Or is something else wrong?

I believe the issue stems from neglecting to attach the -24vdc common to the negative common of the PLC.

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I do not know what you mean by this. The coil has to have a positive and negative to operate.
Is the coil failng or the contacts failing? I suspect the contacts are failing from inrush. Also, you need to check the specifications of the relays as to what the 3A contact rating entails. There shouild be more information asd to the duty of the relay contacts. Still suspect inrush is you problem and you need bigger relays.

The plc has +Com and -Com terminals. Both are required for the outputs to function properly. The -com is used to bleed off power when the output is off. Without it the relays seem to hold an intermediate state that may be on or off. You can see this by the LED indicator remaining on, dim but on.

I'm not ruling out inrush. The relays have a 30A make/break rating. There was not a duty cycle rating on the electromechanical area of the spec sheet. There is one on the solid state portion but I am not using that relay.

BobB is correct. Contacts are arcing and welding. A snubber across each contact may help, but won't be a long term cure. If you use relays, increase to 10 Amp contacts. You'll still need snubbers.
The best solution is solid state. This one from Automation Direct will work well.

califflash said:

.. The relays have a 30A make/break rating...

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The spec sheet is confusing. This is the part that tells me the relays are too small.

" Electrical Life Operations 6 A Resistive: 100 000 min.
24V DC, 1 A Inductive: 200 000 min.
120V AC 1 A Inductive: 300 000 min. "

6 Amp contacts, with a long life rating at 1 Amp. A 3 Amp inductive load will burn them up, as you are experiencing.

Be careful of solid state relays - if the load is not high enough they will not turn off without a drain circuit across them. Drain circuits can have their own problems as well.

I suspect that BobB is on the right track.

These terminal-block relays are physically very small and are considered "pilot duty", because they are meant to drive small electromagnetic loads, i.e. solenoids in pilot-operated valves or in solenoid coils in motor starters.

You won't find a "horsepower" rating on these, but you'll find designations like "NEMA B300 Pilot Duty" and "IEC AC-15 250V" (which means small electromagnetic loads).

The issue might also be the opposite; you might be experiencing significant back-EMF spikes when you de-energize the motor. Unless there's an RC snubber or MOV in the circuit, the voltage spike when you open the contacts can easily destroy or weld a small set of contacts in short order.

You're not wrong to expect that a contact rated for 3A steady state can handle 1.8A steady-state. But the practical matters of the current surge when you start a motor, and the voltage spike when you stop one, often indicate a much physically larger relay or contactor is necessary.

Does DynaQuip provide any information about inrush current for these valve operating motors, or any advice on relay sizing ? My brief overview of their product literature seemed to show 10A rocker switches in a lot of cases.

That reminds me... the 20A rocker switch in the shop vac is burned out and I've got some cleaning to do.

I will add a little here - some years ago I was building a new power station for the RAAF at a new air base. The generation was at 415VAC and step up transformers were used to step up to 11kV. There were about 10 ACBs at 24VDC switching and 6 HV circuit breakers with 110VDC switching.
The 415VAC ACBs were not a problem at all and small relays were utilised to switch the opening and closing coils at 24VDC - as we normally used without problems.
The coil current on the HV circuit breakers was alledgedly 2 amps. We used 15amp inductive load rated relays expecting them to be OK. First time I switched an HV circuit breaker I sat there watching the relay arc and burn in the control panel and then melt before my eyes! Finished up using 20A inductive contactors with the contacts in series to get some air gap to quench the 110VDC arc.
Admittedly is was 110VDC and DC can be quite nasty after you exceed 32VDC but it is an indication of what an inductive load can do to contacts and relays.
I firmly believe you need to use larger relays. I only ever use those smaller relays for switching contactor coil loads - nothing else.

califflash said:

The plc has +Com and -Com terminals. Both are required for the outputs to function properly. The -com is used to bleed off power when the output is off. Without it the relays seem to hold an intermediate state that may be on or off. You can see this by the LED indicator remaining on, dim but on.

Click to expand...

Is there possibly a problem with the outputs wiring or power supply? Where does your 24vdc supply come from?
Can you post a wiring diagram of your setup?

Are the outputs transistor or SSR? I always use transistor outputs and relays with flywheel diodes and never, ever have this problem. Never use SSR ouitputs in a PLC - too much trouble. Never use relay outputs either - easier to change an external relay in the middle of the night is one fails.

I think I need to explain the motorized valves a little more. The valve does not have a hp rating because its a short duty application. The valve turns 90 degrees over 15 seconds at which point a cam engages a limit switch to disengage the power to the motor. The rating of the motor is 1.8 amps LOCKED ROTOR not FLA. I would have sized differently if it had given me typical nema ratings. A locked rotor conditions produces current draw several times higher than the FLA of a motor, correct?

My plc outputs are transistor. I have a xle-120 power supply calibrated to 24v

I went back to work to investigate a little more. I ohmed the coils of good and bad relays. Good relays show around 300-400 ohms. My bad ones produce ol on my meter. Additionally I cut open the opaque housing of several of the spent relays and found nice clean new looking contacts that were free to move.

I plan on upsizing my relays now however I need to determine if my original wiring was to blame for these coils to go out or if there is another issue. I need to keep everything running until I can get different relays in and would hate to be burning the coils on those as well.

This statement indicates welding contacts.

califflash said:

... The relays stop switching and lock in either the NO or NC position...

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The limit switches are handling the off spike, so the contacts only need to handle the startup spike. The relays are probably sized correctly.
We need more info on how you are wiring the coils.
Is the O/L on your meter indicating open or shorted?

I will post a diagram later this evening when I get home. If anyone cares to look before then see the micro830 manual under wiring. I have +24vdc wired to the +com and -24vdc wire to the -common the plc. Relays are wired to -24vdc on a2 with a 14ga signal wire between the plc output and the a1 of the relay.

The relay output commons are supplied 120vac the no and nc contacts power the valve open and closed respectively. The a2's and commons of all output relays are tied together with 700-tbj20r jumpers. Ihave varified that the jumpers are inserted fully. Resistance from the a2's to the - on the power supply is either 0 or 1 on all terminals.

All wires in the cabinet are 14 ga thhn stranded. The - output of the 24v power supply IS NOT grounded. The power supply is grounded on the frame only. Valves are wire through conduit approximately 15 feet to the control using 12 ga thhn.

You keep saying "-24vdc". There are power supplies out there that do if fact have a +24vdc and -24vdc which would give you 48vdc. If that is what you are doing then it could be your problem.

What is the exact catalog number of your 24vdc power supply? Or test it with a meter.

It appears we are now able to talk about the problem - burning out the coil. Seems to me
1. Verify the power supply AC or DC and 24 or 48 V
2. Verify whether coils as rated on the relay match the power supply
3. Install one and make sure POLARITY is correct ie put the + wire on the coil + terminal.
4. Energize to ensure you really ahve the correct polarity and voltage and if really curious measure inrush and holding amperage.

Uhhh if I practiced what I preach
and did it ALWAYS
I would NEVER have burned out a coil.

Dan Bentler