Contactor

hello all,

I am working on siemens 1211c DC/DC/RLY CPU. It has relay output. I wish to connect a AC contactor with the output since I am dealing with high power dissipation. Please guide me which AC contactor (model of the contactor) will be okay to be used here?

Thank you and best regards.

the coil has to be 24 Volt, the contact rating is dependent of the power needed.

but i wanted to use AC not DC.

the coil can be 24V AC. but certainly you may use 120/240 VAC coil as PLC relay contacts are voltage free contact. as i remember amp on 240VAC coils should be max 3A but i doubt that you would be even close to this with contactor coil current.

The PLC type is 1211c DC/DC/RLY. The "RLY" is for relay outputs.
So definitely any AC or DC contactor can be driven from the outputs.
Just be sure to have a surge suppressor over the contactor coil. For DC a freewheling diode. For AC an RC or a varistor.

rk155 said:

hello all,

I am working on siemens 1211c DC/DC/RLY CPU. It has relay output. I wish to connect a AC contactor with the output since I am dealing with high power dissipation. Please guide me which AC contactor (model of the contactor) will be okay to be used here?

Thank you and best regards.

Click to expand...

There's thousands of contactors that could be used. You need to go through this process:

What is the contact rating of the relay output module? Usually listed in Amps or VA.

What is the coil rating/holding amps of the contractor you want to use? Usually listed in VA. Convert to Amps for the coil voltage you're using.

If the Coil is less than the module contact rating, you're good to go.

Rahma,

I see here that you've received some help from the Siemens Technical Forum. Ste39 linked you to the SIMATIC S7-1200 Programmable Controller System Manual for an example wiring diagram for your relay outputs. If you also look at the Digital inputs and outputs section, specifically Digital Outputs, you'll see the permissible voltage ranges and current rating for the CPU 1211c relay outputs:

Voltage range 5 to 30 VDC or 5 to 250 VAC
Current (max.) 2.0 A

rk155 said:

...Please guide me which AC contactor (model of the contactor) will be okay to be used here?...

Click to expand...

We are only advising you as to the possible voltage and current you can switch to a contactor coil with these particular relay outputs. It would be very difficult to specify an exact contactor model without knowing what the contactor itself is switching.

rk155 said:

...I wish to connect a AC contactor with the output since I am dealing with high power dissipation...

Click to expand...

Are you referring to AC for the coil voltage, or for the voltage the contactor needs to switch?

If the latter, are you referring to "high power dissipation" as a high make/break arc current on the contactor's contacts as a result of an AC load?

If so, this has no bearing on which coil voltage you select, but does have a bearing on which contactor model is selected.

While it's important to use surge suppression across the coil, this only helps to protect the PLC relay contact from degradation. The coil voltage has nothing to do with arc suppression across the contactor's contacts.

Depending on the manufacturer you use, a coil surge suppressor is often available. You may also have a preference for a particular contactor manufacturer?

You are attempting to select a power contactor from the control side of things. It's more important to select a contactor to suit your application's switching load, duty cycle and fault withstand current. Then select the coil voltage to suit your control circuit. Most contactors can be ordered with different coil voltages, so this should come last.

G.

A little more info.

I consider it almost a MUST to use an interposing relay between a PLC output and a large contactor coil. The reason is, the PLC relay output contact almost ALWAYS has a very low current capacity. In your case, we now know it is only 2A, likely less if the load is inductive, as an AC contactor coil will be. It is FAR FAR easier and less expensive to replace a small cheap interposing relay than it is to replace a PLC, no matter how cheaply you can buy the PLC. It's the cost of down time, not the cost or parts that you need to be concerned about.

So get yourself a small ice cube style relay, rated at 16A for example, and use your PLC output to drive that, then use that relay to drive your contactor coil. With that, I don't think there is any limit to contactor size you can use..

Also, many large contactors offer what is called a "PLC Interface Module" or words to that effect. This is essentially the same thing, except it mounts directly to the contactor coil terminals and uses the Line voltage of the contactor as the coil power, but uses a very low voltage low current signal from the PLC. The main caveat to that is, if you are in the US and using 480V line power, those devices usually cannot handle that voltage. Most are designed for IEC systems where the Line to Neutral voltage is under 250VAC. You are not in the US so I don't think that will be an issue for you, I bring that up only for the benefit of other readers