- Category 1: purely mechanical problems, the highest percentage
This category is really ignored by too many people.
Common situations include:
* Guide rail into the iron filings, dirt
* Silk rod local strain, deformation
* Bearing began to bite the dead
* The coupling is mounted crooked
* A part is no longer in the "right position” after the crash.
* The load is already over design, but still want to run at high speed.
In this case, the motor is not not spinning, but it is spinning very hard. The drive is quickly full torque, and then - F7900. if the mechanical resistance is not resolved, you change how many drives are useless.
- Category 2: parameter monitoring set too "tight”
Siemens has always been conservative on the protection.
If:
* The allowable speed deviation is too small
* The monitoring time is too short
* The torque limit is too low
The F7900 can also be triggered during acceleration, heavy cutting, or at low speeds and high loads if there is nothing wrong with the system itself, but it is important to emphasize: don't change the parameters right away. Siemens official also clearly said: parameter adjustment, always ranked after "confirm the health of the machine”. Otherwise you just "cover” the alarm, not to solve the problem.
- Category 3: motor wiring or cable problems
This category is particularly common after repair and replacement.
For example:
* Star / triangle connection is wrong
* Wrong phase sequence
* Oxidized, loose terminals
* Dark wounds inside the cables
These problems will directly lead to the motor theoretically can turn, but the actual output torque does not go up. The driver does the math: "I've given you so much current, why are you still so slow?” So the F7900 appears.
- Category 4: motor parameters or motor identification is not done well
This is a high incidence point in the repair site.
After changing these things, it is most likely to have problems:
* Change the motor
* Change encoder
* Change the drive
* Have done remodeling
If:
* The wrong motor model is selected
* The rating parameters are not correct
* Motor ID has not been run completely
The torque model calculated by the driver is wrong, it will think it "can't push” even under normal load.
- Category 5: abnormal encoder or speed feedback
Siemens is very sensitive to feedback signals.
Common scenarios:
* Poor encoder cable contact
* Plug not fully inserted
* Bad shielding and interference
* Encoder type parameter mismatch
Once the speed feedback is jittery or abnormal, the driver will mistakenly think that "the motor is not moving”, even if the machinery is really rotating, the F7900 will come.
- The last category: the driver itself (the proportion is not high)
does also exist:
* Abnormal current detection
* Power module aging
* Busbar instability
* Long-term overheating
But our experience is: only after the previous categories are all clean, it is the turn to suspect the driver hardware.
