PLC s7 200 problem conrolling inverter [VFD]

HI everyone, I have come across problem with PLC s7- 200 ,which has got two modules em 221 and em 232. This plc system controls various conveyors and speed regulation of the products between differnet conveyors.
The main problem is that the PLC is not able to output the inverters (VFDs) that control the conveyors. The voltage across the terminals in the PLC module [ EM 232 CN] that controls[ control signal ] inverters is constant,i.e, 10 Volts DC and hence drives are running at high constant speed instead of varying speed. Also, I have checked the wiring connection and the inputs of the plc; they are fine.
It was fine for almost 2 years + and now when the operator started machine, the drives are running speedly. I would appreciate if someone can help in how to get around the problem.
If there is anymore details need , I would surely help.

DETAILS :
- Delta inverter [ VFD - m ]
- s7 - 200 [ cpu 224 xp cn ]
-plc module : em 221 cn
- em 232 cn [ This is the one that outputs control signal to vfd ]

- On EM 232 CN module , The power is 24 VDC, there are two VFDS that are connected to it as follows :
- VFD1 on M0, V0 [on VFD it is, AV1 and +10V terminals ]

- VFD2 on M1, V1 [on VFD it is, AV1 and +10V terminals ]

Can you connect to the PLC and monitor the program, to see what the Analog output should be? Sounds like it may be something in the program that is telling the output to be 10V.

Are you sure the system worked OK for 2 years +?
I think the connection to the VFD is wrong.

M0 goes to ACM
V0 goes to AVI
and SW2 = AVI (0-10V).
You are connecting the analog output to +10VDC (you will always get 10V)??

HI, Firstly thanks for quick reply. There is a problem connecting placement to laptop. Im working on to resolve that as well.
As the drive was not working normally since the problem started two days back, I think the connection from plc anolog output module to vfd could be messed up. And the other thing I was wondering, has output module gone bad.

First disconnect the two wires that go from the analog output card to the VFD, and see if you have 0-10VDC. Of course you have to change the analog output by the software, HMI etc. This will tell you that the analog output card is OK.

To test if the VFD is ok, you can connect a 4.7K Ohm pot
to the VFD analog input and see if the motor speed changes.
One side of the pot to +10V, the other side of the pot to ACM and the midle of the pot to AVI. This will tell you if the analog input of the VFD is OK.

I hope it's not stuxnet...

I don't know the 200, but on 300 UAQ-card there is a 4-wire system, where 2 additional wires are used to sense the voltage on the load; this to compensate for voltage losses in the threads (kind of like resistance measurement). If this isn't needed, the sensor signals still must be connected on the card terminals. Else an output value > 0 will most likely be compensated to max.

10V is very rarely used in 'our' industry, so I hope I remembered this more or less correctly.

Kalle

Hi Kalle,
"10V is very rarely used in 'our' industry, so I hope I remembered this more or less correctly."

We also use 4-20mA about 90% of the time. The S7-200 only has 2 wires for each channel output.

I have noticed on the anolog output module, that only power LED is on , where as other leds are off. Previouly when the machine was working fine , all the leds were on.

nonuke said:

First disconnect the two wires that go from the analog output card to the VFD, and see if you have 0-10VDC. Of course you have to change the analog output by the software, HMI etc. This will tell you that the analog output card is OK.

To test if the VFD is ok, you can connect a 4.7K Ohm pot
to the VFD analog input and see if the motor speed changes.
One side of the pot to +10V, the other side of the pot to ACM and the midle of the pot to AVI. This will tell you if the analog input of the VFD is OK.

Click to expand...

Hi Nonuke,
We have found the analog output to be at fault.The problematic plc analog module is chinese made [em 232 CN] , and we will be replacing it by Indian make em 232. So , will there be any changes to be done to PLC program.

I can't help you because I never used the Chinese or Indian card. I always use the German model made in China. LOL

Thanks all for help. We have replaced analog module,it's all fine now.