PENN (Customer) asked a question.
Hello Everyone,
My second issue that hopefully someone can help me with.
I am using a Click C0-10DRE-D PLC which uses relay outputs. I was using it to advance 110V conveyor every 5 seconds. Looking at the PLC specs, I should get around 300,000 cycles from the relay output or maybe 3 months before they're fried.
I decided to get a contactor (CWC09-10-30L03) with a discrete output (C0-08TD2). The contactor needs 24VDC to energize the contacts. The choice for the contactor was to put it through the cycles rather than the plc relay output. So, using the discrete output, I want to energize the contactor and therefore get millions of cycles without any issues. The problem I have is the discrete output doesn't seem to put out enough current to energize the contactor. If this is the case, it's really silly to use a relay output to drive a contactor since the failure point would be the relay.
My question here is whether I just ordered the wrong type of discrete output or am I missing something all together.
Any help would be appreciated.
RP
Use something like this, https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/electro-mechanical_relays/slim_interface_relays/52000 to interface between the PLC output and the contactor.
Hello, thank you for your feedback.
Help me understand this better please. I thought the 24VDC contactor coil was designed to be energized by a discrete output or am I misunderstanding it?
Depending on the output ratings of the digital output, it may or may not drive a mechanical contactor reliably. Flyback voltages and other transients can also damage the digital outputs. The method bcarlton described is one of the more reliable methods by which to drive a contactor. In the event that relay fails, it can be easily replaced in < 1min with no rewiring involved as opposed to swapping a PLC card. Using the interface relay allows the long life of digital PLC outputs and the ease of swapping a plug in relay.
That makes sense. Thank you both for your help!
RP
Just one more question if I may. Since I'm running a conveyor motor well below 2 amps, I would think just the interface relay would suffice without the need for a contactor?
In general, no you shouldn't use an interposing relay to drive a motor. Motors typically need a switching means (contactor) and some form of overload protection. For most any application, a combination contactor/overload is probably the right choice. Contactors also have the advantage of having auxiliary contacts that make wiring much easier. Motors (inductive loads) typically have huge inrush currents that necessitate the use of a contactor. Contactors are also rated based on their usage. For example jogging applications are much harder on a contactor and require a de-rating of the contactor.
Hello,
Understand. That makes sense. Thanks for your help.
RP
As Homer Simpson says "D'oh". It was a loose wire creating the problem. The outputs work fine now. Thank you everyone for your advice. Even though the wiring was the culprit, everyones advice is appreciated.
RP
" If this is the case, it's really silly to use a relay output to drive a contactor since the failure point would be the relay. "
If one is using an inexpensive socketed (thus easily replaced) relay it is not really that silly, much better that wearing out a relay built into a PLC.
Though I don't understand why you are having problems, your CWC09-10-30L03 contractor; "DC coil low consumption: 1.7–2.7 W", rounding up to 3W and dividing by 24V is 125mA
the C0-08TD2; "Maximum Output Current 0.3 A/point , 1.2 A/common", 0.3A (300mA) is well more than 125mA. Maybe double check your wiring?
Hello, thank you for your answer.
I think you misunderstood me. My point regarding "silly" is using the built in relay on the plc to drive the contactor is counter intuitive since the whole purpose of using the contactor is not to overuse the built in relay but instead use a discrete output. My point was that if the discrete output couldn't;'t drive the contactor coil then by default I would have to use the plc relay output, however I now see there are other options.
As far as using my discrete output to drive the contactor coil, I agree, there should be plenty there to energize it. It wouldn't be first time I screwed up the wiring. I will have to recheck.
Thanks for your help and will advise.
Regards,
RP