VFD Question

[682bear]

Please bear with me... this may get long...

Last week I posted in the 'what did you buy today' thread about some machines I bought cheap, including a K&T Model H horizontal mill. In that post, I referred to it as 'junk'...

There is a reason for that. It has had some 're-engineering' done on it. One issue in particular is that someone had previously removed the spindle speed control mechanism and connected the motor directly to the gearbox input shaft with a double v-belt setup.

My idea is to work around this by powering it through a VFD... but I know almost nothing about VFDs.

As it is setup now, the motor (3hp 208v 3ph) is rated at 1725 rpms which by my calculations (with a 4:1 motor to spindle ratio) should be turning the spindle about 430 rpm, give or take...

When I was machining the new gears for my Hendey lathe on my vertical mill, I had the spindle turning about 80 rpm and that worked out well, so that is sort of my goal... to be able to turn the spindle on the K&T at 80 rpm and still have useable torque.

So, my question is whether a VFD will be able to do what I want, or do I need to play with the pulley ratios to get the spindle slower by mechanical means first.

IOW, what spindle speed should I shoot for running the motor at 60 cycles to expect to acheive my goal after adding a VFD?

Also, I'm assuming if I can reduce the rpms with a VFD, I can potentially increase them also? Is there a generally accepted effective range that VFDs can handle?

If I am way off base with my thinking on this, let me know... like I said, I really don't know anything about VFDs aside from what I've picked up reading H-M.

Thanks -Bear

 

[Bob Korves]

A VFD will change the speed over a very wide range, but there are trade offs. You will lose torque at the lower and higher speeds. I suspect that a model H would have a speed lower than 80 rpm from the factory, maybe more like 25 rpm. Slow speeds usually mean heavy loads, and the cutter is likely to stall and/or burn out if you go too slow (or fast.) The 3hp motor will help a lot, but watch out for it overheating at low speeds. Motors not rated for high speeds can also fail when run too fast, especially older ones.

 

[mksj]

You loose the mechanical advantage when lowering the motor speed below 60Hz. Older motors have a constant torque of around 2:1 to 4:1 so motor torque is constant say down to 20Hz, but Hp drops off in a linear fashion below 60Hz. So the short answer is if you are going from 430 RPM to 80 RPM with the VFD your motor is running at 12Hz. So not no power and it will burn up at that low a speed with any type of load. A usable speed range for older (4P 1750 RPM) motor's I would say is 20-80Hz, the performance drops off beyond that and you run into more cooling issues unless it is a TENV or TEBC inverter type motor. If you replaced the motor with inverter/vector type motor say TENV, then you could have a usable speed range of around 20-180 Hz, but to optimize the motor you would need to change the speed reduction ratios significantly. Older motors, are also more likely to have overheating and insulation degradation using VFD's so one usually uses lower carrier frequencies, lower overload settings and in some case a dV/dT filter between the VFD and the motor. Ideally if you can, I would try to change the belting ratios so the motor is running closer to the 40-80Hz range at your desired spindle speed. A VFD is still a good option, be sure to adjust the output voltage and current to you specific motor. Let us know if this covers your query.
Mark

 

[682bear]

You loose the mechanical advantage when lowering the motor speed below 60Hz. Older motors have a constant torque of around 2:1 to 4:1 so motor torque is constant say down to 20Hz, but Hp drops off in a linear fashion below 60Hz. So the short answer is if you are going from 430 RPM to 80 RPM with the VFD your motor is running at 12Hz. So not no power and it will burn up at that low a speed with any type of load. A usable speed range for older (4P 1750 RPM) motor's I would say is 20-80Hz, the performance drops off beyond that and you run into more cooling issues unless it is a TENV or TEBC inverter type motor. If you replaced the motor with inverter/vector type motor say TENV, then you could have a usable speed range of around 20-180 Hz, but to optimize the motor you would need to change the speed reduction ratios significantly. Older motors, are also more likely to have overheating and insulation degradation using VFD's so one usually uses lower carrier frequencies, lower overload settings and in some case a dV/dT filter between the VFD and the motor. Ideally if you can, I would try to change the belting ratios so the motor is running closer to the 40-80Hz range at your desired spindle speed. A VFD is still a good option, be sure to adjust the output voltage and current to you specific motor. Let us know if this covers your query.
Mark

Ok, that explains the relationship between Hz and rpms... so if I can keep it around 45Hz, I should be ok? That would mean adding another level of reduction of @4:1, giving a total reduction of 16:1, or around 108 rpm at 60Hz?

I think I've got it figured out... but I haven't slept in 21 hours, so I may be thinking a little fuzzy...

-Bear