Tubalcain converts a wood bandsaw to metal using a VFD, calls it a failure .

I started down the same path as Mr Pete when I began my bandsaw conversion. The VFD alone wasn't enough to get down to the real slow speeds needed for cutting steel. I used a bolt on planetary-gear reducing pulley and replaced the direct drive motor with a shaft and step pulley arrangement. Works great, heres a link for anyone interested:
www.hobby-machinist.com

Bandsaw speed reducer build.

Hi All, I recently picked up a nice mid 50's vintage, Boice Crane, 14" vertical bandsaw. It has a three phase motor so I figured I'd simply throw a VFD on it and be able to get down to metal cutting speeds, wrong!... The motor is rated at 1150 rpm and running it at 15hz only gets the blade down...
www.hobby-machinist.com www.hobby-machinist.com
 
Agreed on the gearbox requirement. When I converted the wood bandsaw I bought I simply added an add-on gearbox to the setup, bolting it and the motor to a sub-plate which was then bolted to the saw base using slotted holes for adjustment. That gets the speed down to a comfortable if fixed level and is more than adequate to my needs for the sheet and plate work (less than 1/2" plate) that I use it for.

Anything that requires heavy hogging of thick materials gets given to the horizontal bandsaw.
 
cjtoombs said:
If you wanted 1/2 hp at the machine, you could use a 3 hp motor and VFD, which gives a 12:1 speed ratio. This would be from 1/6 the rated speed to twice the rated speed. If set up for the low speed to give 100 fpm, that means the high speed would be 1200 fpm.
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I'm losing you in the conversions. How is making the motor 6 times more powerful affecting the speed? The only way to get a speed ratio is with compound pulleys or gears, right? input rpm/output rpm. What am I missing there?

Running a 3 hp motor at 10 hz to get 1/2 hp at 100 sfm seems to be what you're recommending. And then also I'm thinking in rpm, so maybe that's what's got me in a bind. Does that negate the recommendations from manufacturers stating not to run a VFD at less than 50 %? I guess it's ok outside of industrial applications? I'm not attacking. I really want to understand. And I do know of several motors that have been running 30 hz for at least a couple years with no ill effects so far. So maybe that little nugget of wisdom I picked up somewhere doesn't hold water?
 
jwmay said:
I'm losing you in the conversions. How is making the motor 6 times more powerful affecting the speed? The only way to get a speed ratio is with compound pulleys or gears, right? input rpm/output rpm. What am I missing there?

Running a 3 hp motor at 10 hz to get 1/2 hp at 100 sfm seems to be what you're recommending. And then also I'm thinking in rpm, so maybe that's what's got me in a bind. Does that negate the recommendations from manufacturers stating not to run a VFD at less than 50 %? I guess it's ok outside of industrial applications? I'm not attacking. I really want to understand. And I do know of several motors that have been running 30 hz for at least a couple years with no ill effects so far. So maybe that little nugget of wisdom I picked up somewhere doesn't hold water?
Click to expand...

Running a standard motor at much lower than the rated speed can cause overheating. But unless you are trying to pull near maximum torque this is normally not a problem. The problem is the fan on the motor, as the RPM is reduced, the airflow is reduced. This can be negated by installing a seperate fan. Industrial motors that are designed to run over a wide speed range are equipped with separate cooling fans, spindle motors for instance.

Depending on the VFD used, the torque may drop off as the speed is reduced. Modern sensorless vector VFDs will maintain full torque down to near 0 RPM, and thus constant torque up to the rated RPM, and constant HP above the rated RPM. The older V/Hz VFDs drop off the voltage as the speed reduced and thus the torque is reduced as the RPM is decreased.
 
Yes, at 10 hz you could run into heating issues as stated above depending on the motor you use. If you use one of those antique motors with no fan, it shouldn't be an issue. In the example I spoke of above, you are using the VFD to vary the speed, but using gearing/belts to get the saw into the right speed range. A 12:1 speed ratio on the VFD would give that idealy at 100 to 1200 FPM, but if you changed the gears/belts you could adjust that to 200 to 2400, etc. A 3 hp motor to get 1/2 hp at the blade is a bit of an extreeme example, as the cost for the motor and VFD would be pretty high compared to what you could do with change belts, but it just depends on how anoying it is for you to change belts. Personaly, I think combining a varialbe speed pulley set up with a VFD motor is the way to go for wide speed ranges without changing belts.
 
I have a 30-1 baldmor gear motor I will be selling (cheaper than VFD and LOTS of torque.) It was for my bandsaw project which I never had time for. I ended up buying a Swag table with a variable speed Milwaukee and really love it. Foot switch is great on these.
 
another pitfall at low speed is a motor not designed for a VFD doesn't seem to work as well. In the applications with which I am familiar needing a large speed range, meaning going below 10 Hz, an inverter duty motor is a great asset, maybe even a must. Even unloaded, it takes a change to the normal volt / hertz curve to make it work at all.
 
Yeah, when you run a motor at lower frequency you run the risk of saturating the iron core.
The iron core effectively acts as an amplifier for the magnetic field up to the saturation limit of the core material, and past that point you get much less force increase for a given current increase. The peak magnetic field in the core is proportional to the current, and therefore the torque, so the power output (torque*speed) at the low end is likewise limited.


What that limit is depends on the motor design.

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What a timely topic for me as I have been researching how to convert my recent purchase of a 1939 Delta 14” wood band saw to a metal one.

Over the past several weeks I have searched this forum, Practical Machinist and OWWM for wood to metal band saw conversion's and it seems to me that the consensus is that VFD buy itself is not the best solution due to the torque loss as slow speeds (as Mr. Pete declared). Jack shaft reduction and/or gearbox speed reducer seem to be the best solution along with VFD or not. I purchased a copy of The Home Shop Machinist January/February 2004 for their article “Converting a Wood cutting Band Saw to Cut Metal” and that was done with a jack shaft.

I purchased the band saw along with a Craftsman 10” table saw for $50 and then turned around a sold the table saw back to the PO for $23. So for a $27 investment I can now to spend a little $ to convert it to a metal cutting one. The frame is sturdy enough as Delta also manufactured a wood/metal version of this saw with a gear box reduction. The saw was a little beat up and I have so far I have purchased two missing pieces from OWWM BYOD for about $76.

Not sure at this time as to what my final solution will be, although the Home Shop Machinist article sounds doable as I like the idea of being able to convert from wood to metal with just a belt change.
 
So from what I’m reading here, the damage we try to avoid with regard to running a vfd at below 30 hz is not damage to the vfd, but to the motor. Is that correct? This is assuming a max hz in this example of 60.

I don’t know a thing about sensorless vectorless vfd’s. It sounds pricey though. Is this what we’re getting on EBay for 100 dollars or less? Or are those the old volts /hz models?
 
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