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Old August 18th, 2013, 10:49 PM   #1
arunprackash
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Wiring of an AC motor to a DC PLC with relay outputs

Hello,

I am a final year diploma student doing an automation project.

Currently, I am finding how to connect an AC motor to DC PLC with relay output.

The AC motor is single phase 230Vac. I'm powering it from AC mains. (in Asia, the AC main 230V)

The PLC's power supply is 24Vdc. It has relay type outputs. I have a 24Vdc power supply.

Powering them looks straight forward. Right now, I am finding how to use the PLC output to switch the motor on/off and how to reverse it's direction. I have attached the wiring diagram specified in the motor catalogue.

I apologise if I sound very nooby.

Thanks in advance for your replies.
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Old August 18th, 2013, 11:38 PM   #2
leitmotif
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PLC output relays may not (quite probably) be large enough to handle motor current - especially so the starting current.

You will need a heavier relay (interposing relay). THe coil of the interpose relay is controlled by the PLC and the motor is energized by the interpose relay contacts. Make sure the voltage rating of coil is same as PLC --
AND be sure to install protector to protect the PLC output from "high" voltage produced by the collapsing relay field.

Dan Bentler
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Old August 19th, 2013, 12:04 AM   #3
Viks
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As Leitmotif suggested, you'll need external interposing relays (you can use 24vdc coils as you already have a power supply). You'll need two PLC outputs and two interposing relays if you want to change direction of rotation also. Connect the coils of the interposing relay to the PLC output and use the NO (normally open) contact of the relay to connect the motor. I've attached a picture of an interposing relay if you dont understand. Hope this helps.
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Old August 19th, 2013, 03:18 AM   #4
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Do not use relays for such a task. Use contactors !
Socalled 'heavy-duty' relays are also not suitable.
It doesnt matter that it is 'small' and only single phase.
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Old August 19th, 2013, 03:50 AM   #5
arunprackash
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Thank you Dan Bentler and Vikash Kumar!

You guys are unbelievable. I never expected replies this quickly.

I understand it now.
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Old August 19th, 2013, 03:57 AM   #6
arunprackash
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Hi Jesper,

Thanks for letting me know about contactors.

I researched about it. I am dealing with lighter loads. So, I've decided to use relays. My project supervisor is also okay with that.

Your reply is much appreciated.

Arun.
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Old August 19th, 2013, 04:08 AM   #7
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It is not the load, it is the inductance that will destroy the relay over time.
If you just have to make a setup for a very short time, relays may be OK. But for a real application, do consider contactors. There are miniature contactors that are almost as small as relays, and dont cost much more.
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Old August 19th, 2013, 04:26 AM   #8
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when I was doing my apprenticeship we were shown a 'Continuity Tester'
this consisted of a 6V dc Battery a 6V Bell (coil type)
and bare copper leads
when you hold the leads - no problem
then touch them together - no problem and The Bell rings
NOW open the two leads away from eachother - I Bet you wont do that again
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Old August 19th, 2013, 04:27 AM   #9
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that Back EMF burns contacts and is the reason JASPER is teling you to use contactors - for a training application shoud be ok
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Old August 19th, 2013, 06:19 AM   #10
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Quote:
Originally Posted by iant View Post
that Back EMF
I did my apprenticeship on the railways maintaining rolling stock. The lighting was often controlled via a 24VDC contactor. One day whilst applying vaseline to the stitch contacts of the control curcuit I inadvertantly switched the lights off. The resulting shock through me across the train carriage to land in an embarrased heap on the floor.

I learned a lesson that day!

Nick
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Old August 19th, 2013, 06:39 AM   #11
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I think we all have stories like that.
as a 4th Year apprentice - I was working on a tailshaft ballancer. (old Valve control system)
I had traced the problem to the speed contoller - Isolated the power
- Pulled the valve out lying on my side on the floor.
the frame was at the back and up the top so pulled the valve down and found the static capacitor underneath.
threw the valve as my arm pulled itself away.
AND
the valve shattered
the only part of the valve that did not break...
the part with the number on it
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Old August 19th, 2013, 06:52 AM   #12
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Fella's, don't we need some thermal overload protections in the circuit somewhere? Or, can we assume it is internal in the motor?
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Old August 19th, 2013, 06:59 AM   #13
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No - so long as the smoke doesnt come out it will be fine
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Old August 19th, 2013, 07:09 AM   #14
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I believe the wiring diagram posted above is for an Oriental brand motor. Most of these have an internal self-resetting thermal switch.

OP didn't provide the size of the motor but Oriental makes small induction motors from 1/750 to 1/4 hp. I think a relay would be sufficient for a good part of this range.
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Old August 19th, 2013, 07:30 AM   #15
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Quote:
Originally Posted by kolyur View Post
OP didn't provide the size of the motor but Oriental makes small induction motors from 1/750 to 1/4 hp. I think a relay would be sufficient for a good part of this range.
Maybe for the 1/750 hp size. Apart from that I do not agree.
Sorry for reiterating what is maybe not relevant to the OP, but just stating it so that no-one reads this thread and thinks relays are OK for small motors.
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