Speed range for VFD and three phase motor

[RotarySMP]

I have a nice Simens 370W Micromaster vector VFD which can ouput 0-650HZ (way cool device), and an italian 370W three phase motor rated at 1380rpm @50Hz. They work (ebay specials).

They will be used todrive a Sherline ER-16 spindle:
http://www.wrathall.com/Interests/CN...nkenDeckel.htm

I have set up the head to allow for four step sheaths, but am wondering what the practical frequency/RPM limits are for a standard motor. It was built in 1986, and there is no mention of Invertor duty etc on the motor.

The lower RPM limit is set by motor cooling, and the upper limit by armature winding strength right?

I have set the VFD to a setting which automatically reduces current progressively to 50% set current at 50% RPM. What is a practical upper limit for a standard motor? 100Hz? 200Hz? So far I only tried up to 75Hz to make sure the inverter works.

At this stage I have almost no milling experience, and am working on a small model radial (edwards radial 5). I have followed Swedes learning experience with hi rpm spindles, but was only aiming for a speed range of about 300-3000 so far.

If the VFD/motor would survive 25hz to 100Hz range, then I could probably get away with using the Sherline two step sheath and just make one more for the motor.

What frequency ranges are you using for your invertors?

[Al_The_Man]

For standard 4 pole motors (1380) etc, you can usually use them up to 120hz to get twice the rpm. Vector rated motors have high grade insulation and are well balanced, but I believe that most motors for the last several years have been wound with high insulation enamel wire, I have so far not had a problem with several 'standard' motors, you may find all you need is no more than 1:1 and 5:1
Al

[RotarySMP]

Thanks Al. That is good to hear. Is 25Hz a good lower limit, or can a low power motor like this 370W reject heat well enough to go lower?

[Al_The_Man]

You will find the motor current higher & higher temperature on lower rpm, if you are going to run it for extended periods around 700rpm, monitor the temperature and if necessary fit a ducted fan on the end.
Al

[Swede]

Al is correct. You can make a very simple unit from a 3" to 6" dia section of PVC pipe and install within it a muffin fan, or better yet a squirrel-cage air mover.

I was going to do this with my 2HP Leeson 3P motor which I fitted to my VFD/mill retrofit, but I've found that even with relatively long operations, the motor simply doesn't get that hot. I think it will vary with load, motor, a whole bunch of factors, but just realize that adding forced cooling air to a motor is pretty easy to do, and can be done AFTER the installation of everything else if you find it's necessary.

I really like my VFD and would never go back to step pulleys!

[FLUTE HEAD]

What belt position, if any, is a good rule of thumb with the vfd's. Or are you bipassing these pulley's? Concerned about torque. I thought a motor was only capable of achieving torque at low speeds from the pulley ratio's. I have three step pulleys and two belts (typical harbor freight round column mill). Maybe I am supposed to only have two pulley's one on motor one on spindle, with a vfd drive.

[Al_The_Man]

I have a knee mill which has a two speed gearbox, I run the 4 pole motor up to 120hz with the VFD so I double the original motor speed, I replaced the vari-speed pulley it had with direct timing belt drive, so basically I run from 620 to almost 3600 with direct drive and with the gearbox in low gear gives 640 to 75rpm, I have found this works for me.
Al

[RotarySMP]

I don't have much milling experience and the motor I got is pretty weak (370W) two pole so I am making the head with 4 pulley steps plus the VFD. Time will tell whether that is overkill.