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Author Topic: Bridgeport 2J Vari-Speed Head, Sensorless Vector Drive, Mach3, cheap tachometers  (Read 1191 times)

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Hi I have a Hitachi WJ200 VFD hooked up via Modbus through RS485 to take advantage of SLV drive. I have a cheap $15 tachometer (AGPtek DT-2234C) that came with some reflective tape on the spindle. My vari speed crank is labeled up to 4000 RPM but stops doing anything past the 3600 RPM label, which turns out to be around ~3200 RPM @ 60hz, according to the cheap tachometer.

If I synchronize the head to the Mach3 at 1500 RPMs, then when I set Mach3 to 3000 RPMs the tach shows 2960 RPMs, and Mach3 @ 4000 RPMS -> 3950 reading. Is this considered acceptable? I tried going to Spindle Calibration page but couldn't find any documentation on it. It seems to be only for closed loop spindles.

A few other questions:

How accurate are these cheap digital tachs? They seem to be quite precise and consistent when I barely crank the variable head to adjust the RPMs manually.

Is there a difference between setting the Variable speed head lower and upping the frequency on the VFD to achieve the same RPMs versus setting the variable head at max? I don't think I would want to run it past 5000 RPMs anyway.


Thanks for your help!
The cheap tachs work well they only have to count over a fixed time span which is easy. Your VFD puts out a steady frequency in hz which is what your input is telling it to do. However an induction motor must have some slip to magnetize the armature which appears to be what you see. That drive has a tach input I believe so you could close the loop within the drive. I can't see any benifit to that, repeatabilty of setting is always more important. If you want higher speeds though you can program a higher top frequency in the drive. Be careful though, top frequency is 400 hz or higher.
So if I set the variable head at the 1000 speed mark and going to 300Hz to get 5000 RPM, it's not gonna put extra load on the spindle or on the VFD? Just making sure stuff doesn't fail prematurely, thanks!
NO, that would be real bad!  Your motor is probably rated 1750 rpms at 60 cycles. So the gear reduction is reducing that down to 1000 rpms with proportionally greater torque at the spindle.  Setting the VFD for 300 Hz would make your motor run at 8,750 rpm, and it would likely self-destruct at that speed in a very short time. Also the torque would fall off badly as well.  Now if your motor is a 1750 rpm motor the maximum safe speed would likely be 3450 rpm like a standard 2 pole motor would give you.  The bearings and all would be fine. So you could set your drive to give 120 Hz for 3450 rpm and then adjust your gear reduction to produce 5,000 rpm, and that would be okay, assuming your spindle is okay with that speed.  The torque of course will be lowered proportionally  as well.  So no free lunch with a VFD.  People who really like them simply do work where full motor horsepower is seldom needed.  If you need the most power at the spindle gearing is always better.
Yikes, I will keep that in mind. OK so if it's safe to run 120 Hz continuously, then I will set to 2500 RPM @ 60Hz.