Variable frequency drive or gearbox
The original PM machine is used as a direct drive in the oil well. Getting a gearbox in between really is not feasible. That is the reason I try to see if a high pole count induction motor can do the job. When the motor is installed in a new well, we will start it running at 30 to 50 RPM for a few weeks, than increase the RPM gradually to 300 to 450 RPM over the next year or so. The torque requirement is constant, so the tough part is for the IM to produce the full torque at 30 RPM for weeks. It is both a motor design and drive issue. The IM has to be efficient enough at 3Hz so that it is cool. The drive (I am using Yaskawa A1000 VFD) running close loop field oriented control and my experience with Yaskawa been good.
Any vector control VFD should be capable of 100:1 turn down. But whether the motor even with external cooling fan will not be over heated at 1/100th speed.
Here power requirement of the variable frequency drive is also very less at low speed. But since the application is for continuous operation, energy loss may be significant. Probably higher pole nos should be given a chance.
Gear box will be a high maintenance overhead with higher downtime.
Any vector control VFD should be capable of 100:1 turn down. But whether the motor even with external cooling fan will not be over heated at 1/100th speed.
Here power requirement of the variable frequency drive is also very less at low speed. But since the application is for continuous operation, energy loss may be significant. Probably higher pole nos should be given a chance.
Gear box will be a high maintenance overhead with higher downtime.
