# VFD help please



## notqwik (May 3, 2013)

Hello,  
Trying to set up a VFD with a new Baldor motor and a old Hardinge cataract lathe.

Is it best to set the pulley sizes so at 60 Hz on the VFD the lathe spins at the middle of the range of lathe spindle speed?  Then vary the frequency to vary the upper and lower speed of the lathe?

What is the typical range of rpms I can expect from a 1740 rpm motor w/ a TECO FM50-101-C?  at 120 Hz is the speed double?   What are the limitations of a motor typically? i.e. I would guess it would not be a good thing to make them spin twice as fast as they are rated to, or is there a limit on how slow you should make them spin?

If any one can help that would be great.  Or point me at a thread that can answer these concerns.  Thanks.


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## Tony Wells (May 3, 2013)

I certainly wouldn't be comfortable running a motor at twice the rated frequency. I would calculate the pulley based  on no more than 125% (75 Hz) being 125% of the specified top spindle speed. That way you do gain a little on the top end, taking advantage of the VFD for that. That's the same as saying that at 60 Hz, it matches the speed chart on the machine. Are you leaving a gearbox alive in the headstock, or is this direct to the belt drive?


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## Ray C (May 3, 2013)

What Tony said x 2.

Many newer motors (in the last 5 years or so) are stamped with both minimum and maximum speed ratings.  Below the rating, you risk overheating the motor and they're not guaranteed to be balanced beyond the max rating.  If your motor is older and only has a max RPM rating, the rule of thumb is you can decrease speed by (roughly) 40%.

I'm also firmly of the belief that unless you have a belt driven spindle, you should use the gears as you normally would and only vary the frequency from about 50 to 60 Hz (in North America that is).  This is what I do and only tweak the frequency when I'm threading, facing (to increase RPMs to keep SMF constant) or on finicky pieces (usually stainless steel) that just wants to be cut at a particular SFM.

Yes, the RPMs are roughly linear with driving frequency.  Theoretically, it should be exactly linear but, the motor slips more at lower frequencies (thus the tendency to overheat due to slippage).

Did your lathe originally have a two speed motor?  If so, the best solution is to get a slightly higher HP motor (so you will have enough torque at lower speeds) and just use the VFD to drop the R's down into the -30% range.

One last thing... Jawed chucks are rated for a top speed.  Older chucks usually top-out around 2500 and newer ones can go up to 3000 or 3500.  Keep an eye on that lest you get your head knocked off with a 15lb hunk of jagged semi-steel forging...

Ray


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## notqwik (May 4, 2013)

Ray C said:


> What Tony said x 2.
> 
> Many newer motors (in the last 5 years or so) are stamped with both minimum and maximum speed ratings.  Below the rating, you risk overheating the motor and they're not guaranteed to be balanced beyond the max rating.  If your motor is older and only has a max RPM rating, the rule of thumb is you can decrease speed by (roughly) 40%.
> 
> ...



This is a newer baldor stamped at 1743 (I think) rpm.  Did not notice a range.  This is a Hardinge cataract lathe with a belt, originally with a one speed motor.  I think I will size the pulleys to spin the spindle at about 2000 rpms at 60 Hz.

Thank you for the information


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