how to energize the contactor

[maheshboobalan]

hello Dear everyone...

I am getting 24 VDC output, from mitsubishi FX5UMT/ES PLC...whether it is enough to energize the contactor of 18 A DC Motor

 

[janner_10]

What is the contactor coil voltage and current?

Usual practice is to use a relay controlled by the plc to switch a coil voltage to a contactor.

 

[maheshboobalan]

24 VDC siemens contactor for 3 phase AC motor
Current Rating 18A

 

[maheshboobalan]

so i need to connect the relay one end to PLC output and
other end of relay connect to contactor coil...yes am i correct

 

[janner_10]

Sort of. The PLC output goes to the relay coil. The 24v to the contactor coil comes from your 24v terminals, through the relay n/o contacts and onto your contactor.

 

[James Mcquade]

maheshboobalan,

look at the contactor specifications for the coil specifications.
you should see inrush and holding current.

the inrush is typically 6-10 times greater than the holding current.

I would have the plc output goto an interface relay and use a reverse bias diode to remove the counter emf.
the interface relay can then power the contactor and use a reverse bias diode again to eliminate the counter emf.

james

 

[jraef]

Several issues here.

1. If the COIL of the contactor is 24VDC, there is no "inrush" value, it will just state the watts and if you need amps for the coil power, divide the watts by 24.
2. If the SOURCE of your 24VDC is the PLC itself, then you CANNOT use that to power the coil. The PLC likely has a LIMITED ability to provide 24VDC to use for the digital inputs and maybe the analog I/O, but typically that power is measured in MILLIAMPS, whereas the contactor coil might be measured in AMPS. The first time you try to drive that coil from that built-in power supply on the PLC, you might fry it. You must use a SEPARATE source of 24VDC running through the Output contact of the PLC. If you already are doing that, then great.
3. In addition, you must also understand the capacity of the OUTPUT device on the PLC. If it is a "Relay Output", that's better; those are often rating 2-5A. But if it is a "Solid State" (transistor) output, it may also be rated in mA and the coil draw of the contactor may damage it. If so, then you use the PLC output to drive a relay, then use the relay to drive the contactor. The concept is called an "interposing relay". Some people like to use interposing relays even if the PLC has a relay output, because it is easier to replace an interposing relay than to replace the PLC Output if something goes wrong.
4. Fuse early and often.
5. Fave you picked out a contactor? You CANNOT equate the AC switching capacity of a contactor to it's DC switching capacity, they are very different, and the difference varies GREATLY with the motor DC voltage. So for example an IEC contactor rated for 20A AC switching can switch 20A at 24VDC, but if the DC motor is 110V DC, is can only switch 6A and at 250VDC it can only switch 1.5A. So when you say "18ADC" without stating the MOTOR voltage, you could be talking about a contactor difference in orders of magnitude depending on the motor voltage, which will greatly affect the coil current rating.

 

[jvdcande]

@OP: asking this kind of question indicates you have an education in programming and not industrial automation or electronics/electrical engineering. If you want to continue with PLCs I strongly suggest you to take a course on electric circuitry.

 

[maheshboobalan]

Thank you sir