PLC to inverter connection

Hi all!

I am a noobie to the electronics world trying to learn as
many as possible.Recently the company I work for asked me
to present with a solution for a conveying system for bottles.
I thought about using a plc to start/stop the conveyors with
photocels.This plc will connect to an inverter to provide
a soft start/stop on the various motors of the conveyors.
I have written the program for this application on Step7 since
I intend to use a siemens S7 300 plc and it works correct on
the plcsim that I have tested it.My problem is that I cannot
understand how I am supposed to connect the digital outputs
of the DO card to the inverter(s) so as to start the appropriate
motors each time.I have searched through the net but I couldn't
find any simple aswer.
I would really appreciate it if someone could give me a small
help on this.

If you give us some more information on your drive, we might be able to help you out. What brand,interface,model number? Generally speaking you can take the digital output from your plc and either give the drive enable signal, or a run command to the drive that you wanted to run.

dear nswu1 thank you for your answer.The truth is I haven't ended
up with a particular drive yet.From a small search I did I find
FC 300 from Danfoss to cover my needs.So the basic idea is that
I connect the digital output from the PLC to a digital input
connection on the inverter and then the inverter will have some
kind of option in its interface to configure the start/stop
function?

once again thank you for "opening my eyes"

Danfoss offers Profibus DP, that I would suggest is your best solution.

We had 100's of Danfoss Drives linked to Siemens S7 PLC's for cans and bottles.

Ensure you choose a 2DP option of PLC and purchase your drives with the Profibus option.

The ssd drives range will also have something to cover your needs.

You can control them from digital and analogue outputs from the plc or by devicenet, profibus, ethernet, controlnet, canopen, modbus, link or firewire.

Obviously the S7 being siemens lends itslef to Profibus.

If you control the drives over a network then not only can you perform start/stop, the plc will also be able to read the motor amps, any drive faults, set the speed etc etc

There are many ways to skin the cat. It all depends just how involved(complicated) you want to make it. If all you want is to have a interlock,start/stop,stepped speed outputs, then just use digital inputs. If you want to vary the speed according to process then you would need analog output from plc and a complicated pid loop. Using a certain vendors preferred(owned) communication buss is always a option, but unless you have experience these tend to be a pain.

Generally, drives require a contact closure for Run, Forward, Reverse commands. The easiest way to interface is to use contact outputs on the PLC. If you don't have contact outputs available on your PLC, you can use DC outputs as long as they are at the correct voltage for the drive. You will need to connect the DC common terminal on the drive to the DC common terminal on the PLC's output card and pay attention to details like whether the drive's inputs are sourcing or sinking. You connect a sourcing PLC output to a sinking drive input and vice versa.

If you need the PLC to provide a speed command to the drive, you may also need an analog output from the PLC.

Thank you all for your answers.
The application I m doing is a simple one with no variable speeds
on the conveyors.It will just be soft start/stop depending on
photocell signals on the lines.So I guess the easiest setup would
be to use some digital inputs for the photocells and some digital
outputs 0-10v to connect to the inverter.On some other applications
I have seen, they used one inverter for each motor (but they had
variable speeds on the conveyors).Do you think I can do this with
one inverter only? I have 9 motors to control.

stefanos

Kalimera,
Simatic S7-315-2DP and Vacon-drive connected with ProfibusDP.
You have "everything" You need, +/-10.000 speed or frequence selection, You don't need analog-I/O.
With PPO4 or 5 and you can get up to 9 different process data from drive as. Motor Current, Voltage, Torque, Power, estimated temperature, DC-Voltage, Fault/Warning-Number etc.
"Free" monitoring tool with trend up to 8 process data.
http://www.vacon.com

Interfacing PLCs to inverters......
There are many ways.
The ways that I have used in the past, with different brands of PLC are:
1, simple digital IO. Wire the required number of outputs of the PLC to the inputs of the inverter, and vice versa. This can be via interposing relays if the voltages don't match or we want to separate the power supplied for some other reason. Generally, inverter inputs consist of an enable input, forward and reverse, and several speed selection bits for a few selected speeds. Inverter outputs generally consist only of a run feedback and a fault output. If you only need one speed and one direction, then you may be able to get away with only a single output. All inverters I know of can be setup this way.
2, Analog IO. An analog signal can be used to set the speed instead of digital signals. I tend to prefer this method to the one outlined above. Every type of drive I have used has the capability of analog control, although you have to check what type of analog control is available for the drive. The most common ones are 4-20mA or 0-10V, with many drives supporting both methods.
3, Bus system. For Siemens you should use Profibus. This has the advantage of the drive appearing as standard IO on the PLC. You can set a wide range of values within the drive, including speed, ramps etc. Some drives also allow parameter setup, for example the SEW drives we have can have the hundreds of parameters required to be setup stored within the PLC. It takes a few seconds to upload this information, but it increases flexibility. Most European drives and many other manufacturers have Profibus cards for their drives. I always try to use this method as my first preference.

digitron said:

Thank you all for your answers.
The application I m doing is a simple one with no variable speeds
on the conveyors.It will just be soft start/stop depending on
photocell signals on the lines....
stefanos

Click to expand...

If you do not need to vary the speed, an Inverter is overkill. Use a soft starter, they are 1/5th the price and much much simpler. From a control standpoint they would be no different than any other motor starter.

I have seen, they used one inverter for each motor (but they had variable speeds on the conveyors). Do you think I can do this with one inverter only? I have 9 motors to control.

Click to expand...

No, with a drive or a soft starter, you cannot start individual motors using one device. You will either need separate controllers or one large controller and start them all at once.

You can use 1 inverter to start more than 1 motor at once. The inverter must be larger than the sum total of the motors horsepower. An example I know of that works well is 20 one hp motors are being started by one 30 hp drive. Each motor has its own individual thermal overload. They are tied to an announciator board to let the operator know if any are tripped.

The drive was sized at 150% so that if any one motor had problems, the fault currents would not affect the drive and allow the individual overload to trip the motor off.

Each motor gets the same V/F pattern and ideally runs the same speed. I have seen pipe handlers with 10 motors on the same drive work with minimum problem. Again each motor had individual overload protection.

Leadfoot said:

You can use 1 inverter to start more than 1 motor at once. The inverter must be larger than the sum total of the motors horsepower. An example I know of that works well is 20 one hp motors are being started by one 30 hp drive. Each motor has its own individual thermal overload. They are tied to an announciator board to let the operator know if any are tripped.

The drive was sized at 150% so that if any one motor had problems, the fault currents would not affect the drive and allow the individual overload to trip the motor off.

Each motor gets the same V/F pattern and ideally runs the same speed. I have seen pipe handlers with 10 motors on the same drive work with minimum problem. Again each motor had individual overload protection.

Click to expand...

I believe jraef was referring to that fact that he could not indepentantly control more than one drive from a single inverter, that is start and stop each drive independantly, they all start at once or they all stop at once.

Not good to have contactors after the inverter, they would not ramp after the first has gone through the ramp sequence etc, in any case.

PeterW said:

I believe jraef was referring to that fact that he could not indepentantly control more than one drive from a single inverter, that is start and stop each drive independantly, they all start at once or they all stop at once.

Not good to have contactors after the inverter, they would not ramp after the first has gone through the ramp sequence etc, in any case.

Click to expand...

Yes, that is exactly what I meant.

I would go on to say that switching a load onor off with a contactor AFTER a VFD is already operating is one of the fastest ways to damage the transistors of the VFD.

If you do the logic so drive ramps the motors up there will be no issues. If a motor has a fault, it can be disconnected with little or no impact on the inverter. Remember, big inverter, small motor.

Now, I heartily agree once an inverter is running, one should NOT close a contactor to a motor trying to start the motor on the inverter.