Raise excitation on the rotor, then create a slowly rotating magnetic field by appropriate switching of the stator phases. The old CSI based drives had a noticeable 'cogging' to the motion until it picked up speed.
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If we learn from our mistakes I'm getting a great education!
This is a 13.8KV 15MW machine which is being stated using a Siemens 4MW VFD which will be switched out when the motor is running. Is there any chance that "cogging" could do any damage
Can you post a little more about the drive and whatever it is driving? There are two Siemens drives guys who frequent this forum who may have specific information on the type you're using.
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If we learn from our mistakes I'm getting a great education!
Sounds like a pulp refiner to me. I did one such thing about eighteen monts ago. We used a low voltage (500 V) VFD (Siemens 6SE70)rated 850 A to feed the primary of a transformer. We had several problems with that one before we actually got it running.
The problems were mostly due to the fact that there was no position feed-back (encoder) and that the manual said it couldn't be done. We got the machine running up to about half synch speed and then got heavy pole angle oscillations that eventually tripped the drive. We tried several modes, the sensorless vector did not work (as the manual had told us), scalar mode (U/f) didn't work either. It was only by using the current source mode that we finally got that thing up and running. Took us two very full days to get there.
"Cogging" is a thing of the past. The old CSI inverters that were used had some cogging due to the block shape of the output current. Today's PWM inverters do not cause cogging and that should not be a problem in your case. It wasn't a big problem with the CSI inverters either - it just seemed strange to see the torque oscillate heavily just before the motor was transferred to the mains.
What could do some damage to a "virgin" and unprepared motor is the PWM, which is known to produce ozone (O3) and that kills insulation if the motor is run continuously. But, if you use the inverter just to start the motor, the time will usually not be more than a minute or two. Not much will happen in that short time.
Do you have an actual problem with this installation? Or is it just an on-going discussion?
I would imagine at 13.8kV and 15MW, it is probably the SIMOVERT S which is a Load Commutated Inverter (current source) and therefore uses thyristor control so not brilliant for torque but generally very reliable in operation, but very big. The only one I ever came across was starting a blast furnace blower.
This is a new installation. The motor is used to drive a gas compressor. The drive is used to start the motor and to provide a lower inrush current during starting
