I'm currently using a 15HP ELTE spindle and driving it with a GS3-4015 VFD from Automation Direct.

My problem is that I can't get the spindle to run below about 100 RPM.  The VFD just can't get the job done.

Anybody out there know of a VFD that will actually drive a 15HP spindle down to 10-15 RPM?

I am not sure the reason you cant go below 100rpm, all motors I have run via a VFD can crawl round an 1rpm or less, sounds like it may be a parameter you are needing to adjust. Having said that I dont like VFDs much as your torque drops off rapidly when you slow things down too much, or that has certainly been my experience with them.
Hood

I've been through this with the guys at Automation Direct.  They tell me that it is nearly impossible to turn a large spindle below about 1%, and that's what I'm getting.

What type of spindles / VFDs do you run?

Its always been small compared to you, just 4KW the biggest, have servo on the lathe which is 12.5Kw. Might be worth a PM to panaceabeachbum and ask him as I think he has a 20Hp spindle via VFD on his lathe.
Hood

I'll give it a go.  The guys at Siemens can't tell me (?), so I'm asking around.