Re: [CAD_CAM_EDM_DRO] Spindle RPM control

Peter wrote:

> Guys, I know this is an idea exchange and that's fine, but controlling
> speed via an axis o/p seems to be getting awfully complicated to
> implement both in hardware and software (g-code). I for one really like
> the KIRSS principle (keep it "relatively" simple...).
>
> I have a Hitatchi SJ100 VFD on my BP clone - I've found that for 99% of
> my routine metal munching, I only use two speeds out of my VFD: fast
> (for a BP spindle) at 4800RPM and moderate, around 2500RPM. I just use
> one output line from my flashcut controller to select speed and another
> to select on or off. Then I programmed the VFD to watch two of its 5
> input lines accordingly. In my flashcut config, I created two M codes
> (I arbitrarily chose M50 for low speed and M51 for high speed) which my
> CAM software can also generate, based on programmed speed. Later I may
> assign another i/o line and M52 & M53 to get a total of 4 programmable
> speeds - if the need arises.
>
> Question: Has anyone fitted external braking resistors to an SJ100 VFD?
> I find that I can turn the spindle faster than 4800RPM but above 4800,
> I get E07 errors due to overvoltage generation when decellerating the
> spindle motor. i.e. I need to brake the motor faster - to prevent the
> error.

Does your VFD support a braking resistor, or is it not an available option
on that model? If it is supposed to support it, and you just need to figure
out how to connect it, that may not be too hard. The labeling isn't really
standard, like the XYZ and UVW labels for the power in and motor
out terminals. If you have a pair of large terminals with B and BR or similar
markings, that is a good indication. Most of the power-brick inverter
modules have one extra IGBT for the braking circuit. They also have
2 DC terminals to connect the filter cap bank to. So, you need to find that
extra power terminal. Normally there will be 3 input terminals, 3 output
terminals, the 2 DC, and that extra braking terminal. The external (user
available) braking terminals should be connected to the - of the cap
bank, and the braking terminal. If you can trace these back to those
locations, you've found the braking terminals. For safety reasons, they usually
need an overtemp sensor that shuts the inverter down if the braking resistor
overheats. I don't think the kind of external mechanical energy sources that
could cook a braking resistor (ie one VFD trying to stop a motor while other
VFDs are applying power to a multi-motor conveyor belt) apply to the milling
machine. A couple hundred watt braking resistor should work fine.

I figure a resistor value that allows the resistor to extract energy at the same
current as the motor run amperes is a pretty good guess. So, a 1 Hp 3-Phase
motor at 240 V runs something like 3.3 A. The DC bus is something around
340 V when running from 240 single phase mains. So, a resistor about
100 Ohms would draw 3.4 A. This needs a resistor of 1200 W if it were to
be run continuously. So, you make some sort of compromise, depending
on the duty cycle. Four each, 100 W 25 Ohm resistors wired in series, and
mounted vertically to develop a little convection should do quite well.

Jon