PDA

View Full Version : 3-Phase motor control using a PLC


Shohadawy
September 10th, 2008, 03:04 AM
I need a circuit diagram that explains the connection of a 3-phase AC motor to a PLC

briancr
September 10th, 2008, 03:51 AM
boom

http://www.plctalk.net/qanda/uploads/explosion.jpg

Ken Moore
September 10th, 2008, 03:55 AM
A PLC interfaces to a motor just like traditional start/stop buttons do, they are often both used.

This smells like a homework question, so that's all I'm going to say for now.

khalil
September 10th, 2008, 04:32 AM
You interface a 3 phase motor to A pLC by connecting the three motor leads to INPUT1 , Input2 and Input3..if the PLC inputs dont light out..swap any two wire of the motor leads..if fire works occure like fourth of July..then buy another PLC and try again.

ps: dont try that at home!

I am joking ofocurse, so please dont try above! however please explain more what you are trying to do and we will help.

best regards,k

Jeev
September 10th, 2008, 05:56 AM
boom

http://www.plctalk.net/qanda/uploads/explosion.jpg

Hahahaha... Love it.

You're not likely to find a PLC that has that kind of relay switching capability. I'll add by saying you need the PLC to actuate some other piece of hardware to start and stop the motor.

FXPert
September 10th, 2008, 06:21 AM
I need a circuit diagram that explains the connection of a 3-phase AC motor to a PLC

Would this be for a BBQ Rotisserie (http://www.google.com/search?hl=en&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=Rotisserie&spell=1) ?

bkottaras
September 10th, 2008, 06:27 AM
Would this be for a BBQ Rotisserie (http://www.google.com/search?hl=en&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=Rotisserie&spell=1) ?
I would set up temperature control (PID maybe??) first in the PLC prior to hooking up the motor!

Jeev
September 10th, 2008, 06:30 AM
Stop talking about food, you're making me hungry =(

Tom Jenkins
September 10th, 2008, 07:02 AM
I'm feeling magnanimous, so I'll get you "started".

You would not connect a three phase motor directly to the PLC. You would use the PLC to operate the coil of a motor starter, and use the starter to operate the motor. If you are in industry and this doesn't get you going, you need more help before you let the smoke out of a system. If you are a student that should be enough to send you to your text for more info.

milldrone
September 10th, 2008, 07:29 AM
Shohadawy,

Here is a student project that has some of the same features as the question you are asking
http://www.plctalk.net/qanda/showthread.php?t=33433&page=1&highlight=sogo

Electrically_Bound
September 10th, 2008, 08:19 AM
I have some old AB plc2s that I wish I could wire up to a 3-phase motor to get rid of them.

Goody
September 10th, 2008, 08:32 AM
It is possible to run a low wattage 3 phase motor direct from a plc.
You would never do it - but its possible.
The motor would not have to take no more than 2 Amp per phase
and you need 3 isolated relay outputs (one per phase)

Program the three outputs to come on together and maybe wire the overload to an input.

Im am being frivolous (like a lot above have been) - just saying it could be done.

Jeev
September 10th, 2008, 08:44 AM
It is possible to run a low wattage 3 phase motor direct from a plc.
You would never do it - but its possible.
The motor would not have to take no more than 2 Amp per phase
and you need 3 isolated relay outputs (one per phase)

Program the three outputs to come on together and maybe wire the overload to an input.

Exactly like that. The only thing you'd have to be cautious of is welding and wearing the relay contacts out themselves. PLC relay outputs were never designed to handle inrush or stall currents and aren't build with the safety factors contactors/DOLs are.

JesperMP
September 10th, 2008, 08:49 AM
Why not try to switch the PLC outputs real fast so that in effect you change the speed of the motor.
I am sure this is perfectly possible.
Team up with CB from this thread:
http://www.plctalk.net/qanda/showthread.php?t=13451

Disclaimer: I will take no responsibility of what happens if you actually go and try this.

TConnolly
September 10th, 2008, 08:50 AM
http://www.plctalk.net/qanda/uploads/a091008.JPG

Tom Jenkins
September 10th, 2008, 08:58 AM
LMAO, Alaric.

I've used a lot of PLCs to run motors, but I never thought of using a motor for running a PLC!

taz3m
September 10th, 2008, 10:35 AM
Alaric this os too good a motor with its shaft directly connected to the PLC.. is it to the input or proving a source to the power supply...???
hahaha

Too good man...

wildswing
September 10th, 2008, 10:47 AM
Alaric...question about your ingenious design...what happens when the PLC's wiring harness runs out of slack ????

khalil
September 11th, 2008, 07:19 AM
Exactly like that. The only thing you'd have to be cautious of is welding and wearing the relay contacts out themselves. PLC relay outputs were never designed to handle inrush or stall currents and aren't build with the safety factors contactors/DOLs are.

unless you use Solid State Relays. !! but I generally use VFDs to control a motor from PLC.

Paulus
September 11th, 2008, 08:45 AM
Alaric...question about your ingenious design...what happens when the PLC's wiring harness runs out of slack ????

Reverse the motor??? :whistle:

Jeev
September 11th, 2008, 09:32 AM
unless you use Solid State Relays. !! but I generally use VFDs to control a motor from PLC.

I use VFDs in open or closed loop or DOLs where I can get away with it.

Peter Nachtwey
September 11th, 2008, 09:57 AM
http://eu.renesas.com/fmwk.jsp?cnt=ac.htm&fp=/applications/motor_control/child_folder/algorithms/&title=AC%20Motor%20Control%20Algorithms

This is just the basics or AC motor control 101. Most people here are not up to the math that is required to implement this. Nor are the PLCs unless you buy a card designed to rotate the phases. Sure you may understand these equations but like I said this is just the simple stuff.

Renesas is a chip manufacturer. Chip manufacturers have excellent application notes. They want people do use their chips in your designs.

TI and Freescale have good application notes too.

Brian123
September 11th, 2008, 12:47 PM
This is just the basics or AC motor control 101. Most people here are not up to the math that is required to implement this. Nor are the PLCs unless you buy a card designed to rotate the phases. Sure you may understand these equations but like I said this is just the simple stuff.

I think what Peter is saying is to use one of his controllers. They are much better at motion control than a PLC. Maybe he could even provide a wiring diagram. Peter?

Brian

leitmotif
September 11th, 2008, 03:03 PM
Reverse the motor??? :whistle:

Use sliprings to let the PLC spin in place.
OR you can do it at one of the poles of the earth provided PLC turns at same rotational speed (RPM) as earth. Check for proper rotation direction in toilet bowl (hmm does the water spin in the same direction as earth rotation?)) This may however be in utter defiance of Einsteins Theory of Relativity. Some study may be required.

So much for humor. Now pragmatic. Could you run a motor directly off a PLC -- YES
IF the PLC contacts have correct ampacity and dieelectric strength.
Motors (small) can be turned off and on with a simple switch. Larger ones require heavier contacts in olden days ie live front switchboard this was done with knife switches now we use a relay. You would do same with PLC for the same reasons.

Dan Bentler

MASEngr
September 11th, 2008, 03:33 PM
I've got a PLC connected to and controlling a 3-phase motor.

PLC -> MODBUS -> VFD -> 3-phase motor

You'd be damned crazy to connect a PLC directly to a 3-phase motor.

MASEngr
September 11th, 2008, 03:37 PM
OR you can do it at one of the poles of the earth provided PLC turns at same rotational speed (RPM) as earth. Check for proper rotation direction in toilet bowl (hmm does the water spin in the same direction as earth rotation?)) T



Actually, that's a common misconception. The Coriolis effect doesn't change the direction of water flow. It's really just hydrodynamics.

http://www.badastronomy.com/mad/1996/coriolis.html

Peter Nachtwey
September 11th, 2008, 09:04 PM
I think what Peter is saying is to use one of his controllers.

Not yet. Nothing was mentioned about performance requirements. In this case I am assuming the application is simple.


They are much better at motion control than a PLC.
I think it is clear that controlling an AC motor directly is not possible. Our controller can't control an AC motor directly either. A drive is required. Yes, a motion controller is much better than a PLC for motion control but there are simple solutions for simple problems. The key here is that a drive is required.

For simple problems, the best solution would be to directly connect to a drive using Ethernet or device net or some other network.

Our controller gets the call when axes must be synchronized, execute complex motion profiles, or the PLC scan is too slow or not deterministic so our user programs are needed instead. There was no mention of needing anything to demanding.

kbpatton
September 11th, 2008, 09:42 PM
http://www.plctalk.net/qanda/uploads/a091008.JPG
I'm at the end of my shift and this pic gave me a good chuckle.

bkottaras
September 11th, 2008, 10:38 PM
I'm at the end of my shift and this pic gave me a good chuckle.
This is some awesome stuff.
Question for Alaric though!
If we go with a Control Logix instead of the Compact Logix do we need a bigger motor????

Dua Anjing
September 11th, 2008, 10:55 PM
no just a suitable gearbox

Lancie1
September 12th, 2008, 01:15 PM
Shohadawy,

What they are trying to say:

1. Normally, a 3-phase industrial motor is NOT connected directly to a PLC. If you want to control the speed of a 3-phase motor using a PLC, you must have some additional equipment, such as a Variable Speed Motor Drive.

2. You may have an instructor that has some type of small toy motor that can be connected in some fashion to a PLC, but this is not very useful in the real world. If that is the case, you should know that it is only for experimenting and has no useful function except for learning about PLCs.

MASEngr
September 12th, 2008, 03:36 PM
Note that if you're connecting a toy motor to a PLC, the transient voltage can be enough to damage your PLC.

If you're running a 24V system and turn off your motor, you could easily be looking at 200+ V across the output. You may not notice until you get premature failures.

Treat every connection as a long-term connection, and make sure that you put in the proper transient suppression systems.

TConnolly
September 13th, 2008, 01:11 AM
OK, I think we've had enough fun with this.

Basic IEC wiring diagram for a three phase motor starter with PLC and start/stop buttons. The PLC does not wire to the motor, it operates K1M, which switches line voltage to the motor.

http://www.plctalk.net/qanda/uploads/A091208A.JPG

bkottaras
September 13th, 2008, 01:25 AM
Are we feeling a bit guilty this late at night??
Personally, I enjoyed the thread!
And will apply the rotating PLC setup soon I hope!

Protiusmime
September 13th, 2008, 03:17 AM
Well, it's early morn for me. And yet, this charade has given me some energy to make it till the end of shift. I can't draw very well nor quickly in Visio. But I do envision waving a PLC quickly up and down in front of and perpendicular to a motor stator. I'm actually beginning to feel some torque. This moves me http://www.plctalk.net/qanda/images/icons/icon4.gif


Now for the more serious side... while we all have been chuckling, this feller has probably gone about inventing exactly the PLC techno that can spin up a 3p 50h mover... and chuckle right along with us, on his way to the bank :)