Hello Experts,

We use a crane with s7-300 having 3 stations connected remotely and controls 4 drives.

We are unable to figure out the reason for a communication loss between the stations which occurs only when the hoist is brought down. The system trips with the diagnostic buffer attached in the post.

The HMI shows overweight and over-speed signals during the fault. This is consistent with loss of signals from one of the PLC racks(no:23) since over-speed and overweight are NC contacts which trip in case of fault.

We have done the following already

1. Exchanged the PLC racks with already working cranes. 

2.Exchanged/verified all sensors connected with the system concerning hoist working.

3. Checked all connectors in the PB connectors for shielding and short circuit  

4.Routed an external wiring for the network to eliminate harmonics.

There have been no improvements in the system till now. Kindly suggest possible solutions from experiences which you have had in the past. Also please suggest if I am missing something. 

Hello,

relative big Slave address numbers on bus, how much Slaves on bus now with S7 CPU? What distances between each of them and also Master S7 CPU? What comm speed? Maybe I'll be start here now.

hello Duris_I,

1. There are 3 slave stations connected to the CPU (A SLD has been attached with the post for your reference)

2. The distances have been mentioned in the SLD attached also with an error margin of +/- 1 mt.

3. Comm speed is 1.5Mbps(from what i remember)

Any help you can offer is much appriciated.

Hello Duris_I,

1. We have completely changed the rack ,interchanging it with another crane. So i think that eliminates the bus connectors.

2.OB86 and OB122 are already included in the program in the CPU.

Any other suggestions you can think of? 

Regards,

Anandh 

Anandh86

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communication loss between the stations which occurs only when the hoist is brought down.
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 Casual observation, no solution suggestion ->

It sounds as if there is a power surge when the load is lowered. When load is lowered the motor becomes a generator. The generated power needs to be channeled somewhere - a common DC bus and/or external resistance.

The drive needs to be configured for this operation. Some filter devices may be required to dampen the backfeed EMF. Earthing plays a big role too. Even on the drive circuit board can push energy into the communication cables.