Motor strength for VFD powered bandsaw

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Aug 5, 2014
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Hey guys I'm planning on converting my makita bandsaw to VFD powered so that I can slow it down and use it for cutting steel as well as wood. Since there is some loss in torque when running at slower speeds, what HP would you recommend for a motor that would be running at less than half its designated hz for cutting steel? I can't say exactly what hz will be ideal for steel yet but I'll be running the motor between 0 and 50hz. Thanks guys
 
Makita bandsaw? What size of saw is it?
When I converted my 20" Powermatic model 81, first I just slapped a VFD on there... even at like 3 Hz the blade was not slow enough. Of course 3 hz is also completely not viable in terms of motor power or efficiency or the motor being able to cool itself.

So, I put the smallest pulley I could find that would work, onto the motor- and increased the driven pulley size at the lower wheel shaft.
Still, not slow enough with the VFD turned way down.

What I ended up doing was adding a jackshaft in the saw base, with big primary and secondary speed reduction by pulleys. Now, with the VFD turned down to 12.6 hz (the low end of workable) I'm at the 90-100 sfm speed that the blade needs to be traveling for cutting the steels I use. I did all the calculation by paper, and when I wanted to be dead sure, I borrowed a digital tachometer which confirmed closely my speed calculations.

The original motor for the saw is still on there, which is only 1.5 hp. At the time I thought I'd eventually put a 2 or even 3 hp motor on when I found the right one, but with the resultant torque increase from the slow-down mod, I've never needed more power. I've ripped inch-thick aluminum, tall hardwood blocks, big micarta, 1.5" thick mild steel- it handles it all.
 
Hey Salem thanks for the good info :) it's an upright bandsaw with (if I remember correctly) a 1hp motor, it's surprisingly not listed on the body of the motor. It has about 6.5" cutting clearance. I don't think they sell these in the US - here's a pic of about how big it is.

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My idea was to put a bigger motor on it and use the smaller pulley inside of the machine with the ability to double the hz if I'm cutting wood or anything requiring higher speeds.

Apologies for the crappy pic, but here's what the pulleys inside look like. Seems to roughly double the speed when changed to the bigger pulley.

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Do you think it's possible to use a VFD with a 1.5hp motor on this one while running on the lower pulleys and get it slow enough? Probably useful to know the fpm calculations - I'll find the manual and see what I can dig up :)
 
Yeah, see what the manual says the fpm is in slo mode, then since your VFD is 60 hz (50? where you are at?) at this speed, quarter your Hz to 15 and correspondingly quarter your fpm. It's not quite that linear, but that will get you well into the neighborhood of calc-ing the speed. If it's almost slow enough, you can take your VFD down a few more Hz but below 10-12, it's not gonna be good for the motor especially over long periods of cutting, and your torque is gonna be quite reduced.

I've definitely found VFD control to not be a panacea for speed control- many machines need a wider range than from quarter to double the motor plate.
I still sometimes use the pulleys on my mill, and on my lathe I keep it in low all the time, so I can thread and do one-off turnings with HSS cutters; if I turned things in a modern style at higher cutter speeds using carbide, I'd have to use the manual speed ranges of the lathe to some extent. With the bandsaw, I find it cuts wood well enough for my needs at 400-800 spm, so I just leave the pulleys in metal mode.
 
What information, if any, is there available from the motor rating plate?

I'm guessing the bandsaw is an LB1200F, or maybe there's another designation it goes by wherever you bought it? If it's an LB1200F, all I can find out about the motor is that it's 900 Watts; about 1.2 HP. Cutting speeds are given (on 50 Hz mains) as 6.7 and 13.3 metres/sec. I make that about 1300 and 2600 ft/min. They'll be about 20% higher on 60 Hz mains.

You are going to want to get down to around a tenth of that and preferably a bit less.

There's a pretty good chance the motor is an IEC metric-frame unit, rather than one of the NEMA-frame motors generally used in North America.

I'd be looking to identify the frame size to see what options are open to you simply by changing the motor. Whilst I can't find any details on the motor itself, the screws that hold it on seem to be M6, which suggests either a 71-frame or an 80-frame.
My guess would be an 80-frame as the power ties up quite nicely with a 2-pole 80-frame.

http://www.electricmotorservice.net/iechart.pdf

Brook Crompton list 80-frame motors with different pole counts and power outputs. http://www.brookcrompton.com/upload/20112E__ISS3_W_Alum_IE2_IE3.pdf

The 2-pole (about 3000 RPM on 50 Hz mains) is rated 1.1 kw, 1.5 HP.

The 4-pole (about 1500 RPm on 50 Hz mains) is rated 0.75 kW, 1 HP.

The 6-pole (about 1000 RPM on 50 Hz mains) is rated 0.55 kW, 0.75 HP.

Almost all (metric) motor manufacturers make motors to the standard power/pole count/frame size combinations. One or two are also making some "compact" motors with frames one size down from the standard for the pole count/power output. These tend not to be ideal for use at low speed with VFDs because the cooling is limited by the frame size and I'd avoid them in this application

I'm about to make some pretty big assumptions here, so you'll need to check the actual motor details very carefully.

Changing the (assumed) 2-pole motor for a 6-pole would reduce the speed range by a factor of 3. The power will be halved, but as the speed will only be one third, so the torque at low speed will be increased by 50%.

This would give speeds of 435 and 870 ft/min at 50 Hz; still too fast, but pretty much any VFD should be able to run the motor down to 10 Hz for speeds of 87 and 175 ft/min.

The VFD should also be able to run the 6-pole motor to 120 Hz for speeds of around 1050 and 2100 ft/min. Above the motor's rated speed, it operates at constant power (torque reduces as the speed increases).

The speed reduction at constant torque will mean that you are stuck with the torque provided by the motor: you won't get the torque multiplication that you'd get by gearing down to a similar speed.

If the design of the bandsaw would let you drill new mounting holes to accept a larger frame-size motor and bore out the drive pulley to accept its larger shaft, you could fit a 4-pole motor on a 90-frame (24mm shaft), perhaps even a 100- or 112-frame (both 28mm shaft).

Going for a larger frame size would open up the possibility of significantly more powerful motors. Also, if you use a "Sensorless Vector" VFD, the low-end speed range can usually be extended downwards. My experience is that motors start to feel "coggy" much below about 10 Hz on a standard V/Hz (or V/F) drive, but that they will run smoothly down to 3 Hz or less on an SV drive.
 
Hey Timgunn you nailed it. I was trying to find the manual for this but apparently it's tucked away somewhere deep, but yes that's the model for sure. I'm on 50hz so it should run right at what you listed, but wow, a tenth is going to be a big jump to get down to.

I also have experienced the "cogginess" of the motor running below 10hz on the vfd, especially around 3-5hz it runs a little bit unevenly. I think we're using the same Vfd too based on our past discussions. Still ok for some things requiring super slow speed that don't need any significant torque, but I can stop the drive with my hands on a 2hp motor with my grinder when it's down around the very slow range so I'm not sure it would hold up to a bar of steel so well.

Great suggestion on switching to 6 pole motor. Use the higher torque at the low end for cutting steel with the small pulley inside of the machine, then double the hz and use the larger pulley for cutting wood.

I'm not so sure about mounting plates for the face-mount motors but i really like the Mitsubishi 3 phase motors out here the best, and they have face mount varieties coming in 19, 24 and 28mm drive shafts. As for mounting holes they probably would not line up, but I could find a way to mount it and re-bore the hole for the main drive pulley to match the motor shaft.

Currently I'll be sticking with the VFD setup that I have (the huanyang 2.2kw), as it's setup to be able to switch machines so it's easy to plug in whatever machine I need to use when I want to use it, so I'd just be adding the bandsaw to that lineup.

The motor frames REALLY jump up in size after 2hp so if 2hp is doable, that'd seem to be easier to install.

Salem I gotta admit I've been wanting to VFD everything in the shop including my drill press [emoji51]but may do that in conjunction with having interchangeable pulleys. If it looks like the VFD is still gonna be running the bandsaw a bit fast I can potentially have a custom drive pulley machined locallywith a tiny wheel for the steel side. Hopefully between the 6pole motor and maybe machining a smaller drive pulley I can get some effective speed and torque for cutting steel while still being able to use this for wood when needed.
 
Here's a screenshot of a spec sheet for the 2hp 6 pole face-mount style motor that Mitsubishi sells here. 24mm shaft and frame type is listed as 100L

The face for mounting this motor is about 10" across. Damn that's huge! I think because it's 6 poles they use the larger frame size?

They have some 2hp motors that are smaller but they're not TEFC, just the drip proof type.

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And then I found this one, horizontal style, in 4 poles, only about 6.5" in diameter compared to the 10" of the previous. Much more compact overall. Would just need a good way to mount it.

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