|
Some other challenges:
Some other challenges:
I blew up an IGBT module by slamming the motor with full output - it requires a soft start to spool up the motor slowly and keep the current low. So, the program now starts at a low frequency and spools up voltage and frequency slowly. The output current is constantly monitored and the voltage & frequency is reduced if the current goes too high.
Another issue with starting is dealing with the start capacitor. Initially, I would try to spool up the motor without any boost, so just 150V. However, as the motor spooled up and voltage was increased, the current rose really fast and caused the control loop to fight itself. The motor would never reach full speed and would eventually trip some other fault and shut down. This was solved by limiting the output voltage to 75V peak using PWM while spooling up to 60hz. Once 60hz is reached, the voltage is increased to the full desired output voltage.
This actually lead to another problem - something odd happens when the motor runs unloaded and the voltage is boosted beyond 310V. I think the motor begins to saturate which causes some weird current behavior and trips some fault and the motor stops. What's more weird is that it appears to lock up the micro. I'm not positive on that yet, but any fault should open the power relay which doesn't happen during this event. I also haven't let this happen since i've coded the diagnostic LED feedback which indicates which fault has tripped.
This also highlighted the need to spool the motor down slowly on shutdown. Another downside to the circuit is that all of the IGBTs cannot be shut off at once. Thus, when all the PWM output is stopped, one upper leg is still ON which allows current to flow in the motor and it screeches to a stop instantly. This doesn't seem good as there are likely high currents occuring. Fortunately, the modules survived this.
|