VFD for bandsaw?

OK, so looking at gearboxes, 30:1 gets me 140 sfm, 20:1 gets me 213 sfm, and 15:1 gets me 290 sfm.

The largest feasible pulley diameter is probably 11", and the smallest 1.5"; that difference gets me down to 569 sfm without using an intermediate pulley (which would mean an axle and a mount, doable but again more complexity means more procrastination).

So, it looks like there are two approaches:
1. gearbox to reduce the top speed to around 300 sfm and a VFD for variable speed from 0 to top speed
2. pulley to reduce the top speed of roughly 600 sfm, and a VFD for variable speed from 0 to 600 sfm
Is #1 correct? Should the gearbox instead be used to reduce the speed to say 140 sfm in order to preserve torque at the low-end speed, with the VFD only used to *increase* speed?

Of these two, the pulley is vastly cheaper. A gearbox, vfd, and motor all cost about the same (~150), so adding a gearbox into the mix increases the cost by 50%.

Is there a low-cost option to have variable speed without a VFD, excluding of course step pulleys?
 
ThinWoodsman said:
OK, so looking at gearboxes, 30:1 gets me 140 sfm, 20:1 gets me 213 sfm, and 15:1 gets me 290 sfm.

The largest feasible pulley diameter is probably 11", and the smallest 1.5"; that difference gets me down to 569 sfm without using an intermediate pulley (which would mean an axle and a mount, doable but again more complexity means more procrastination).

So, it looks like there are two approaches:
1. gearbox to reduce the top speed to around 300 sfm and a VFD for variable speed from 0 to top speed
2. pulley to reduce the top speed of roughly 600 sfm, and a VFD for variable speed from 0 to 600 sfm
Is #1 correct? Should the gearbox instead be used to reduce the speed to say 140 sfm in order to preserve torque at the low-end speed, with the VFD only used to *increase* speed?

Of these two, the pulley is vastly cheaper. A gearbox, vfd, and motor all cost about the same (~150), so adding a gearbox into the mix increases the cost by 50%.

Is there a low-cost option to have variable speed without a VFD, excluding of course step pulleys?
Click to expand...
I'm looking at a brushless DC motor like this

DC MOTOR 1750RPM 3 4HP 56C 90V TEFC PERMANENT MAGNET CA STOCK for sale | eBay

Find great deals on eBay for DC MOTOR 1750RPM 3 4HP 56C 90V TEFC PERMANENT MAGNET CA STOCK. Shop with confidence.
www.ebay.ca
 
gr8legs said:
I have become an evangelist for the VFD / 3-phase motor retrofits.

Most VFDs will have a feedback loop to keep motor speed constant and provide sufficient torque and the benefit of 'dialing in' the best speed on the fly is golden.

AutomationDirect and DealersElectric both sell packages of matched VFD and motor combinations at what seems like very reasonable prices.

see<https://dealerselectric.com/motor-and-drive-packages-vfd-speed-control.asp>

I would not use a DC motor as the brush/armature requires much more maintenance than a 3-phase motor, and as the speed lowers in a DC motor (for the same torque) the current through the brushes / armature increases which leads to more maintenance.

YMMV

Stu
Click to expand...
So one of the things I've not seen in the VFD discussions is heat. Turning this much slower would be ok with the VFD, but the motor won't overheat with this slow fan speed?

I'll admit I bypassed the whole thing because I didn't have a 3ph motor and just did a double belt reduction on my 14" HF bandsaw with the single phase motor it came with. It's just single speed around 120 and works great for all kinds of metal. Also YMMV
 
C-Bag said:
So one of the things I've not seen in the VFD discussions is heat. Turning this much slower would be ok with the VFD, but the motor won't overheat with this slow fan speed?

I'll admit I bypassed the whole thing because I didn't have a 3ph motor and just did a double belt reduction on my 14" HF bandsaw with the single phase motor it came with. It's just single speed around 120 and works great for all kinds of metal. Also YMMV
Click to expand...

Would you post pics of your setup?
I’m trying to be somewhat frugal about this.
I already have a 12” Beaver cast iron bandsaw that I can use as compared to spending $300+for an import horizontal saw or $400+ for a portaband and a Swag table.


Sent from my iPhone using Tapatalk
 
Cooling a 3 phase motor below its base speed is influenced by many factors specific to the motor type used. The typical motor would be an TEFC where cooling becomes an issue below ~15Hz, where a TENV can go much lower. The issue is the drop in Hp, so for a 1 Hp motor you are only getting 0.25Hp at 15Hz. A BLDC motor would also suffer cooling problems at low speeds and is a more expensive option. I think the major point that is being missed is the mechanical advantage imparted by the gearbox to get you into the operating range previously recommended. If you want to use the VFD to slow the motor significantly below its base speed you would probably need to double the motor Hp, you will still have cooling issues below around 15-20Hz and your motor/VFD costs go up. Many metal bandsaws use a reduction gearbox/drive to get a sufficient speed reduction. When using a mechanical reduction gearbox you also need to keep your speeds extremes within the operating specs of the gearbox.
 
mksj said:
Cooling a 3 phase motor below its base speed is influenced by many factors specific to the motor type used. The typical motor would be an TEFC where cooling becomes an issue below ~15Hz, where a TENV can go much lower. The issue is the drop in Hp, so for a 1 Hp motor you are only getting 0.25Hp at 15Hz. A BLDC motor would also suffer cooling problems at low speeds and is a more expensive option. I think the major point that is being missed is the mechanical advantage imparted by the gearbox to get you into the operating range previously recommended. If you want to use the VFD to slow the motor significantly below its base speed you would probably need to double the motor Hp, you will still have cooling issues below around 15-20Hz and your motor/VFD costs go up. Many metal bandsaws use a reduction gearbox/drive to get a sufficient speed reduction. When using a mechanical reduction gearbox you also need to keep your speeds extremes within the operating specs of the gearbox.
Click to expand...
Thanks, that's very helpful. I guess once again my cheap ways worked out as the double belt reduction I did on mine was cheap and it has more hp for cutting slow for metal. But I don't do any wood working just some plastic like UHMW. For that I just swap the wood blade back in and I'm good to go.
 
mksj said:
I think the major point that is being missed is the mechanical advantage imparted by the gearbox to get you into the operating range previously recommended. If you want to use the VFD to slow the motor significantly below its base speed you would probably need to double the motor Hp, you will still have cooling issues below around 15-20Hz and your motor/VFD costs go up.
Click to expand...

Hmm, good point. So even with the gearbox or a pulley lowering the bandsaw into the (say) 400 sfm range, the VFD is going to be running the motor hot when it slows it down to hit the lower range. Plus doubling the motor spec, ouch.

Think I'll just box this up and deal with it later. Doesn't sound like there's a simple drop-it-in-and-go solution, and I can limp by on the portaband+SWAG table for awhile longer.
 
You must log in or register to reply here.
Back
Top