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Old July 20th, 2009, 08:36 AM   #1
halla26
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Book Control of a 3 phase motor using a PLC

Hello.

Pretty new to electronics as I'm currently a apprentice in study. Been assigned a project in which I have to wire a 3 phase 0.46kw motor 400v controlled via a 24vDC PLC.

Now I think off the top of my head, I will be using a isolater with a stop start button with a overload, and in this the coil will be linked to a auxilliary contact with a 24VDC coil, so when when the 24v acitvates the auxilliary, the relay will cause cause the isolater to be live incoming.

Is this right?
i.e.
24v coil controlling a 400v 3 phase 0.46kw motor DOL starter/isolater with overload

Any help be great cheers
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Old July 20th, 2009, 08:47 AM   #2
halla26
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After reading more, I think I have a bigger issue at hand.
The 24v is DC, the motor is AC. How will this be controlled?
i.e. using a DC auxilliary 24v to control the DOL 400v AC?
I have a diagram in my head to how it will work, but im a bit unsure on some bits i.e. the details.
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Old July 20th, 2009, 08:53 AM   #3
mellis
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Go ahead and draw it. Post the drawing.

Your description sounds OK so far. Show us the diagram you have in your head, and you will get comments.
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Old July 20th, 2009, 09:00 AM   #4
halla26
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Thanks.
Well was drawing it in word, but managed to find one similar on the internet.



Now in my head I was thinking of having the 24VDC going to a 24v coil auxillary contact that would then go to the isolator coil and trigger that on causing the incoming supply of the motor to become live.
Is this correct? Or am I still missing something?
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File Type: jpg wiringdiagram1.jpg (27.4 KB, 258 views)
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Old July 20th, 2009, 09:27 AM   #5
OkiePC
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Quote:
Originally Posted by halla26 View Post
Now in my head I was thinking of having the 24VDC going to a 24v coil auxillary contact that would then go to the isolator coil and trigger that on causing the incoming supply of the motor to become live.
Is this correct? Or am I still missing something?
So, you are thinking of using a smaller relay driven by the PLC to control the larger contactor?

If the current required to operate the motor starter coil is relatively high with respect to the PLC output rating, then what you are suggesting is a good idea.

If the PLC can easily handle the coil rating of the contactor, then why add extra components?

JMHO
Paul
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Old July 20th, 2009, 09:36 AM   #6
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You seem to have the general idea. However, if the assignment is to do this with a PLC, then you will want to wire the start/stop push button and the auxilary contact of the motor starter contactor (terminal 52,54 in the schematic you posted) into the PLC inputs. Then wire a PLC ouput to the coil (termials A1, A2) of the motor starter contactor, making sure to include the overload contact (terminals 95,96) in series. If the overload also has a second contact, some people like to wire the second contact into a PLC input. Then you will program the PLC to perform the same start/stop/seal in logic that the circuit you posted is hard wired to perform.

A word of caution: Check the current draw of the motor starter contactor. You are using a small motor so the contactor is probably small, but as you move into controlling very large motors you may find that the contactor coil current draw is greater than the current rating of the PLC output. When this is the case you will need to use an interposing relay between the two.

edit: I see Paul covered the interposing relay already.
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Old July 20th, 2009, 09:55 AM   #7
halla26
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Thanks for the detailed responses, extremely big help so far.
Well the motor according to it's label draaws at 400v 0.86amp at 50hz.

So to apply this, I'm looking into the cabinet, I'm placing a 400v contactor in there with a supply to it, next to this will be a 24VDC auxilliary with 24v going to it's coil that will also link to start/stop. In turn the outgoing of the 24v will go to the contactor A1,A2 including overload contacts.

However to pauls point, I may not require this 24V contactor as the motor contactor coil may be ok to energise off a 24v input?

Thanks again for input and aiding my understanding
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Old July 20th, 2009, 10:31 AM   #8
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There is a problem with using a seperate 24 VDC supply as you show. What happens of someone opens the 3 phase isolator, you must make certain that the contacor drops out otherwise as soon as the isolator is closed the motor will re-start.
If you can use a contactor coil fed from the load side of the isolator with an interposing relay between the 24 VDC PLC output when the isolator is opened the contactor drops out. The contactors auxillary contacts are wired to a PLC input so when they open the logic turns off the PLC's output.
Roy
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Old July 20th, 2009, 10:35 AM   #9
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Have a look at the attached. In that particular example two 380V three phase motors are controlled by a single set of start/stop buttons. All of the PLC IO is 24V. The attached file has snapshots of the drawings with everything omitted except whats needed to illustrate the start/stop/motor starter wiring, it doesn't show the 24V power supply.
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File Type: doc A072009IECSAMPLE.doc (199.5 KB, 117 views)
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Last edited by TConnolly; July 20th, 2009 at 10:38 AM.
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Old July 20th, 2009, 03:10 PM   #10
halla26
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hi Roy.
Yes your totally right didnt notice that.
So with a relay coming off the isolator on to another relay coil, and the 24v going into the relay, then off into the contactor coil of the motor. This in turn would stop the motor running with the isolator being shut, but also having to have the isolator on to run the motor, while before this was not the case. Ahh I understand!

Big thankyou for the diagram Alaric, never thought of looking at schematics as such at work
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Old August 3rd, 2009, 06:05 AM   #11
halla26
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Hello
So far this job has gone well, got it all wwired to the above.
However I have been told to introduce a auto/man toggle.
Got the manual side all wired up from the toggle NO which is fine, however I'm very confused how to wire the auto.

The 24v signal for manual goes to toggle manual NO -> Stop(latched) NC -> Push Start NO -> Coil and NO
So the stop stops the 24v signal going to the push start

However I have the signal for auto going such as Auto NO -> Stop.... then unsure, as if I go from the stop to contactor then im still sending 24v to one side of the push start.

So I have a wire for NO on the contactor and a wire for the relay, and for auto sequence I'm unsure where to put these wires without losing stop function.

(The auto is basically the PLC just sends a run signal and motor just starts)
(Manual is the user can start/stop the motor via DOL)

Thanks in advance
Shaun
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Old August 3rd, 2009, 07:14 AM   #12
ojz0r
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Just use a two state switch to power a parallell start and stop with 24vdc
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