Hey jarrfarr,
it’s not the VFD that limits the lowest speed of the spindles, most VFDs offer 0–400 Hz, some do 580 Hz like mine. It’s the spindle and the way it is built that limits the lowest speed.
Since the rotational speed of the spindle is calculated this way:
SPINDLE_RPM = $VFD_FREQ × 2 × 60 / $SPINDLE_NUMBER_OF_POLES
e.g. 400 Hertz × 2 × 60 seconds / 2 poles = 24,000 rpm
this means with a spindle with two magnetic poles a VFD can deliver 0–24,000 RPM spindle speed.
Note that most 24000 RPM spindles with magnetic 2 poles have a minimum speed allowed by manufacturer of something between 8000 and 6000 RPM and running them slower can damage them, it would be good to set “Lower limit of frequency” inside the VFD to something between 133 and 100 Hz.
Those spindles with 2 magnetic poles usually are rated 6000–24,000 rpm.
Some are rated at 8000–24,000 RPM. See datasheet/manual of your spindle.
If you want to mill steel or plastic, it would be better to buy a spindle with four magnetic poles. This means that with the same frequency coming from the VFD, those run at half the speed but double the torque.
Since the rotational speed of the spindle is calculated this way:
SPINDLE_RPM = $VFD_FREQ × 2 × 60 / $SPINDLE_NUMBER_OF_POLES
e.g. 400 Hertz × 2 × 60 seconds / 4 poles = 12,000 rpm,
those spindles with 4 magnetic poles are usually rated 3000–12,000 rpm,
so good for the slower speeds needed for milling steel and plastic.
Most spindles with 2 magnetic poles are rated with max. 24,000 RPM. Higher speed of up to 60,000 RPM is offered for the industry but I don’t think many here need this, and it becomes very expensive then. Mechatron offers some variants of its 24,000 RPM model with 30,000 RPM and 42,000 RPM. But in fact what is demanded more often here is to have a spindle that runs slower than the usual that min. out at 6000 RPM.