VFD Speed control for a BP

I've installed a VFD on my step pully BP. From prior posts it seems like the generally best way to manage speed control for safe bearing life and motor cooling is to program the VFD for X-2 motor RPM (120 cycle), run in the middle two pullys, and use back gear for lower speeds, to keep the motor RPM's about +/- 33% of name plate. Do I have this correct? Thanks...............Bob

Yep thats a commonly agreed safe range. In practice its oftern possible to go a lot faster and a lot slower. But you do have to consider things like cooling at lower speeds. My lathe i oftern run at 10 hz, after a few hours it will be warm but not alarmingly hot. Like most things, a bit of thought and carefull testing is the way to go. higher speeds make more noise and impact things like bearing and belt life. So just consider every thing carefully if you want to push the +-30% approach.

Step pulley...

Hi Bob-

Adam's right on about cooling, but that's usually not going to get you without any warning... you'll know it. You'll notice a pretty awesome wail when running that low speed motor at twice it's original speed... the cooling fan will really be pulling air, so you'll know it's goin' fast. The best way to solve it with ORDINARY TEFC motors, is to pop the shroud, remove the mechanical fan, and install a 'muffin' fan on the shroud to keep a constant, quiet flow regardless of rotor speed.

What you will find, is that when you run the BP's pancake motor, is that it has quite a bit of inertia compared to a conventional motor. When you program your VFD's accelleration curves, you'll probably find on slowdown, that you get an overvoltage trip... because the motor has so much inertia that it'll be going into a dynamic braking state to maintain the coast-down curve. Installing braking resistors will alleviate this, but be extra careful with your tooling- make sure you don't have a situation where something... i.e. a boring head... threads on in one direction, and you slow it down at a high rate... it comes unthreaded, and does a disco-dance across your workpiece, and onto your feet. Happens with thread-on lathe chucks, too.

There's quite a few guys who've documented their PB-VFD setups. I did a conversion on a J-head, using a toothed-belt reduction and a common motor... if you search my handle and "another variation" you'll find the thread.

DK :-)