reg double coil syndrome!

[ramanan91]

how to design a ladder for a motor which has to rotate both clockwise and anticlockwise.........i know i cant use a load twice.but the motor(load) is oly one.....any suggestions???

 

[Brijm]

You need to say what type of motor, or more specifically, what is between your PLC and the motor. But normally, you would need at least two outputs, for a forward/reverse motor (or a network conncection if going to a drive.

 

[ramanan91]

consider a normal gear motor.... i accept we need two outputs...the basic concept is tat we need to change the polarity given to the motor to obtain a reverse rotation.but how to do tat with ladder logic.....???

 

[Brijm]

In your first post, you mention double coil syndrome. If you are using the term correctly, then you are trying to scan two or more coils, with the same address. In this case, the output of the last coil, will be your result at the end of the scan. You will need to use internal bits, to eliminate the second coil.

 

[ramanan91]

thank u .ll try doin it n catch ya soon.........

 

[dploof23]

Hello,

I would use 2 outputs. One for forward and one for reverse.

Using a 90vdc motor for example:
1st output energizes a relay to run forward.
2nd output energizes a DPDT relay that will reverse the polarity of the feed.


Hope this helps

-Dave

 

[The Plc Kid]

Use a drive or a revesing contactor your choice. Either way you will use 2 seperate outputs with different addresses. So no double coil.

 

[shooter]

In real you need two outputs.
motorcw and motorccw
you wil need to lock them like start cw and notmotorccw do motorcw.
etc.
hey you just begin so be patient with yourself.

 

[brucechase]

If the motor was a DC motor, you could wire the motor with only 1 output and a DPDT Relay. See this post:

http://www.plctalk.net/qanda/showpost.php?p=458031&postcount=23

 

[daba]

If the motor was a DC motor, you could wire the motor with only 1 output and a DPDT Relay. See this post:

http://www.plctalk.net/qanda/showpost.php?p=458031&postcount=23

I would also go for the "Run" and "Direction" outputs. It makes the PLC logic easier, as you don't have to worry about interlocking that you would use if you used "RunForward" and "RunReverse" outputs. This is true for interlocks in the code AND mechanical interlocks on the motor contactors.

It is also a safer arrangement if you have maintenance people who think along the lines "If it won't go in reverse, I'll force the RunReverse run bit on".

 

[brucechase]

I would also go for the "Run" and "Direction" outputs. It makes the PLC logic easier, as you don't have to worry about interlocking that you would use if you used "RunForward" and "RunReverse" outputs. This is true for interlocks in the code AND mechanical interlocks on the motor contactors.

It is also a safer arrangement if you have maintenance people who think along the lines "If it won't go in reverse, I'll force the RunReverse run bit on".

I don't disagree about being easier, but the point of my post was to show that it can be done with only 1 output. I also stated a small DC motor. There are no interlocks necessary for the contactor - it can be done with 1 relay. Now if we were talking 480 VAC 3 phase, that would be different, but I suspect that the OP homework assignment didn't cover that.

 

[I is a noob]

Don't they make duel rotation motor starters with mechanical interlocks? All you would need is two differnt out puts to the two differnet coils....

 

[bernie_carlton]

Bruce - your motor would only run all the way to each limit, no intermediate stops. Kind of like an air cylinder - all the way extended or retracted.

As for Run and Fwd/Rev outputs, in order to reverse direction on larger motors there should be timing to turn off 'Run', wait for a moment for the motor to stop, change the Fwd/Rev direction output, then turn on Run again. (Anybody for charred relay contacts?)

 

[leitmotif]

consider a normal gear motor.... i accept we need two outputs...the basic concept is tat we need to change the polarity given to the motor to obtain a reverse rotation.but how to do tat with ladder logic.....???

THe short answer is you do not do polarity change with a PLC. For one thing the outputs are not able to handle teh power required by teh motor.

That then forces you to use some other kind of switching of "polarity" It matters not what kind of motor AC DC 3 phase or single phase. You must have a switch for foward and one for reverse.

This can be done with electromechanical relays. You can get these setup for reversing where there is a mechanical interlock preventing both relays from actuating. In addition you can put electrical interlocks in. In old days this was done with a B contact on each relay in series with the otehrs coil.

In a PLC it would be simple the FWD output would have a "B contact" in the rung for the REV output and vice versa.

Even with solid state DC drive or 3 phase VFD you will still use PLC outputs to provide a FWD and a REV signal to the controller.

Dan Bentler