PLC not following program set

[John Lee]

Hi I have a glass making machine I have a simatic op17 in main and in the back plc s7-300

The issue the screen shows the RPMS correctly and is running the program but the spindle motors are NOT following the correct RPMS running slower ?

No errors are showing anywhere other than the RPMS are not correct

Please help I appreciate the info

 

[robertmee]

Is this on a new install or did the RPMs suddenly start running slower?

 

[John Lee]

It just started suddenly , it’s been running the same program for years no issues.

I can shut the whole thing and still the same not following programmed RPM’s on screen.

It is running the correct cycle rotations but not the RPM’s

 

[robertmee]

Well, software doesn't change on its own. Are the Spindles driven by Servo, Stepper or VFD drives? Has anyone changed or adjusted a parameter in the drive? Or changed the drive itself. What about the voltage source to the drive? Is each Spindle on its own drive or commonly driven from a single drive?

 

[John Lee]

Hi not sure where to find where it is driven by I just know we have 8 motors / 8 stations

they all run by the Program installed over 10 years ago

4 of them run a program that is set certain RPMS

Nothing was changed just started doing its on thing sadly

 

[robertmee]

Almost certainly it's not the PLC...more likely the drive, the signal to the drive, the voltage to the drive, etc...something external has failed. The PLC is likely taking an RPM setpoint and scaling it to your drive. That wouldn't have changed unless someone changed the scaling in the program, or a scaling constant if available on the HMI. Really would need to see detail of interconnections between PLC, drive and motor to troubleshoot further.

 

[John Lee]

Ok I will send some pics tomorrow and videos of the back of the Main system it is from Germany and no one knows here how to resolve it

Thank you for helping

 

[John Lee]

Almost certainly it's not the PLC...more likely the drive, the signal to the drive, the voltage to the drive, etc...something external has failed. The PLC is likely taking an RPM setpoint and scaling it to your drive. That wouldn't have changed unless someone changed the scaling in the program, or a scaling constant if available on the HMI. Really would need to see detail of interconnections between PLC, drive and motor to troubleshoot further.

What’s voltage supposed to be? Can it be read with a multimeter or is there another procedure for reading voltage?

 

[John Lee]

What’s voltage supposed to be? Can it be read with a multimeter or is there another procedure for reading voltage?

And you mentioned the drive and signal to the drive ? How do I test that

 

[alan_505]

If it is an analog signal then you can use a meter, if it is a networked signal then you would need to look at the VSD to see what its commanded frequency is and does that match what you expect.

 

[drbitboy]

What is the physical hardware (e.g. encoder wheel, hall effect sensor, etc.) that is supplying the RPM signal?

Is it possible that hardware has become disconnected from, or is sliding relative to, the spinde, and is not longer rotating in exact synchrony with the spindle?

 

[John Lee]

I do not see anything disconnected it tends to speed up and slow down and actually one of the spindles does stop and restart on its own after a couple rotations

But still the following the program set on the HMI

 

[drbitboy]

@John Lee seems to be saying that the rotation speed (RPMs) displayed in the HMI is correct, but that the spindle is actually moving at a slower rotation speed.

We don't know where the rotation speed (RPM) displayed in the HMI is coming from, but it is probably a measured value (e.g. encoder, etc.). But we do know that the actual spindle rotation speed (RPM) is slower than that HMI-displayed value. So I think the problem is somewhere between the measurement system and the HMI display. Unless someone changed some scaling parameters in the PLC and/or HMI, the problem is unlikely to be downstream of the RPM input on the PLC. So I would start at the sensor and transducer and whatever else is between the sensor and the PLC. That is why I suggested looking for a physical disconnect between the spindle and the encoder (or whatever) device.

 

[Steve Bailey]

Don't overlook mechanical issues. The HMI could be displaying the commanded speed and the motor can't meet the command. The drive could be at current limit, although prolonged operation in current limit should generate a fault. Check for slippage between the motor and the driven load. Has there been any routine maintenance recently? I've seen cases where things did not get put back together properly following maintenance.

 

[John Lee]

No maintenance has been done , it errors or faults show up it continues to run the program set from the PLC, The HMI display shows RPMS, we have 3 set stations spinning but all a slower rotation then what displayed

And only one of them tends to stop completely almost like shutting off but after 2-3 rotations comes back to life and starts to work again but not at the proper RPMS still