controling conveyer belt

Hi, I'm writing a simple ladder diagram for practice that is supposed to control conveyer belt. I must have option to change direction of conveyer moving. In PLC simulator from thelearningpit.com I saw example of controling garage door. There are two output: one for one direction and other for opposite direction. I thought to do same wiht conveyer belt., for example output Q4.0 moving belt in one direction and output Q4.1 move belt in opposite direction.
I wonder how this is done in practice in real world problems. Does this solution to use two outputs represent good modeling of real world program?

Can you explaine me in general if how PLC programs are written to change directions of motor?

Thanks

This is a very common automation design question. In general you have the following states to take care of:

1. No Movement

2. Move Forward

3. Move in Reverse

a total of 3 states. You will need 2 boolean flags to handle these 3 states. There are two common ways to allocate the bits:

Method 1:
Bit 1 = Drive Forward
Bit 2 = Drive Reverse

resulting in:

Bit 1 & 2 OFF = No Movement
Bit 1 ON & Bit 2 OFF = Move Forward (Drive ON Forward)
Bit 1 OFF & Bit 2 ON = Move Reverse (Drive ON Reverse)
Bit 1 & 2 ON = Not valid (No Movement)


or Method 2:
Bit 1 = Enable Drive ON
Bit 2 = Define Direction

resulting in:

Bit 1 & 2 OFF = No Movement (Drive OFF, Direction Fwd)
Bit 1 ON & Bit 2 OFF = Move Reverse (Drive ON, Direction Fwd)
Bit 1 OFF & Bit 2 ON = No Movement (Drive OFF, Direction Rev)
Bit 1 & 2 ON = Move Reverse (Drive ON, Direction Reverse)

In the real world the method you choose will usually be determined by the type of motor drive you are using. Both will work just fine, although I prefer Method 2 as it is a little more rigorous because there is no possible invalid state.

Two outputs to control two contactors, one for each direction. Thats OK.

You may want to make a hardwired interlock so that both contactors cannot be activated at the same time.

For the program, just make sure that there is a pause when reversing the direction.

Great explanation PhilipW

Thanks for replies, guys, and JesperMP I have two questions:
1. How hardwired interlock is build in practice? (I'm still student and pretty much learn PLC programming by myself)
2. Should I use timer to make pause when reversing?

1. In most cases it is sufficient to wire an auxilliary N.C. contact from one contactor in series with the coil of the other contactor, and vice-versa.
There are also contactor combinations with a mechanical interlock.
Havent seen any of those in real life though.

2. Yes, you will need at least one timer.
If you want to make it easy, then use two delayed-off timers.
The timers shall activate when a contactor in one direction is activated. Use the timer from one contactor to inhibit the activation of the other contactor.
Because it has a delayed off action, it will make sure that there is a pause when reversing the direction.
You CAN make it with only one timer. It will take a little bit more programming and testing. However, it is a good excercise for a beginner.

Why do you need the timer if you have interlocked them with N/C contacts ?

The N.C. contacts are only there to catch a programming error or a defective (hanging) output.

There are few mechanical systems that are happy with an instantanous reversal. Also the electrical current will jump to the locked rotor current and stay for a longer time than if the mechanical system is allowed to get to a standstill first.
With some systems just a single instantanous reversal can cause a catastrophical damage to the mechanical parts.

Pictures are worth....

MSR and MSF are the hardwired interlock Jesper mentioned. Personally I believe reversing starters should also be mechanically interlocked. Jesper is correct in stating a timer should be used when changing direction, this picture doesnt have that. The main reason to delay is too provide time for the starter to open before closing the other AND allow the motor time to slow or stop before reversing. Too sudden of a change in direction can cause damage...shear keys, break shafts.....IT DEPENDS.

i thought it was code to mechanically interlock forward and reversing contactors so they could never be energized at the same time.

In my factory every motor wired up has one of these (close to 150), ranging from small conveyors to large press motors the size of my office.

They are really inexpensive and you can get them everywhere (AB ect.)

Everything is about redundancy after the mechanical interlock you can also wire in through the N.C contacts adding anouther means of safety and even after that it should be programmed in the plc so that it could never happen.

regards, mark

interlocking is an accepted (preferred) practice in electrical installations, but I've never read that it's mandatory in regards to the NEC. I usually interlock F/R starters on the contactor & in the program.

Pandiani,

You asked how this is done in practice in real world problems.
Just to expand on the other posts in relation to real world
conveyor controls. In my world there would be a 10 sec. audible
alarm that sounds before the conveyor starts in either direction.
So, if you stopped the conveyor and then restarted it in either
direction the alarm or signal would sound for 10 sec. before
the conveyor actually started. This is in addition to the hard
wired interlocks and mechanical interlocks used in reversing
starters. The start up signal is a safty issue and may not
apply in some applications. However, it prevents the possibility
of someone plugging the motor and warns anyone in the area that
the conveyor is going to start.

Wilcotton

The reason we now specify mechanical interlocks on our reversing starters is because we had an instance where a single contact welded, but, somehow, the electrical interlock contact closed and the other starter closed. It shorted L1 to L3 and let the smoke out of a main fuse in our power distibution room. 1/3 of production was shut down for an hour co$ting thou$and$. We interlock in the PLC as a general practice, and electrically and mechanically by plant standard. It's cheap insurance.

I believe (or so I was told in class) that the mechanical interlock on reversing starters is a NEMA (Natl Electric Manufacturer Assoc) requirement and not from NEC (Natl Electric Code).

Regardless of who requires it - a darn good idea - on top of hardwired interlock - on top of virtual interlock in PLC program.

Time delay should almost always be used before reversing motor. Not waiting draws a whole bunch of current
- real hard on contacts BUNCHES of arcs and sparks (kinda neat to look at unless you are the guy who replaces them)
- the motor will do it BUT if done frequently it will overheat.
- the driven load may not be able to reverse instantly - as previously stated twisted shafts, broken set screws, burning belts etc etc.

Dan Bentler

Pandiani,

Now put all the great suggestions that you have received together in one ladder program, and it would look something like this:

If you try to reverse a motor that is running forward (or vice-versa), you need to give the motor time to stop. Otherwise, the starting current for the opposite direction will be excessive... possibly tripping the overloads.

Give the Motor and Starter a break... how much can a few seconds hurt?