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Forum topic by Leviticusk posted 10-10-2015 07:01 PM 672 views 0 times favorited 15 replies Add to Favorites Watch
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Leviticusk

12 posts in 960 days


10-10-2015 07:01 PM

Topic tags/keywords: bandsaw motor repair advice electrical

hi folks,

this is my first post here, i find you all so helpful i’m hoping you can shed some light for me!

i bought a restored 20” rockwell bandsaw recently. it has a 3 ph 1 hp motor, with a secondary 3 ph 1.5 hp motor acting as the static converter / “idler” to make it work in my single phase shop. due to a mishap, i blew up the capacitor and seem to have burnt out the auxillary 1.5 hp motor. possibly damaged the initial motor too but i don’t know. repairing it is going to be expensive, and 1 hp seems small for a 20” saw that i am going to use for re-sawing. so i am thinking to just get a single phase motor, maybe 3hp if i can find one. so my question is can i simply install a new 1 ph 3 hp motor? i know i may need to change switch and wiring. it is on a 2 pole 20a breaker so that shoud be fine. but my main questions are about the motor swap. does the frame size matter or can i just remount a new motor and drill new mounting holes? does shaft dia. matter? as long as i get the same rpm (1725) and same size pulleys? does belt length matter?

admittedly i know very little about motors! lots of research but still can’t find answers to these questions. i gather new pulleys are easily found at any auto store, i could replace belt with link belt but not sure how to calculate belt length from final position? the original motor is a 203 frame which is hard to find used and i have no budget for a new motor… hoping to find something decent on CL (i am in the middle of nowhere canada!)

any and all advice and expertise would be so greatly appreciated! i am just getting started making furniture, but setting up my shop is quite overwhelming, as i know how to use the machines but not set them up or tweek them! and then there s the electrical stuff which is not how my woodworkers brain is wired!

thanks in advance, i am super grateful for anyone’s help here! cheers, tracy


15 replies so far

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MrUnix

4213 posts in 1661 days


#1 posted 10-10-2015 07:14 PM

What capacitor is it that you ‘blew up’? 3 phase motors don’t usually have any capacitors.

As for swapping it out… it can be done, but it’s usually not an easy/straightforward task. You need to make sure the new motor speed, rotation direction and shaft size are the same as otherwise you will need to play around with new pulleys to get the speed correct. Same frame is good, but there is no reason you can’t drill new mounting holes if needed. Belt length has nothing to do with it… doesn’t matter how long or short it is – speed will be the same. Wiring may also need to be modified.

I’d try to fix the setup you have now. 3 phase motors are very, very hard to kill. Alternatively, you might want to consider a VFD to replace your RPC. Would most likely be cheaper/easier than a new motor and would give you all sorts of additional features (soft start, electronic braking, variable speed, ability to wire up an emergency stop, etc…).

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

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Leviticusk

12 posts in 960 days


#2 posted 10-10-2015 07:36 PM

thanks brad!

the guy who restored it added a capacitor to make the static converter from 3 to 1 phase. from all my reading it acts as the third leg. i did “blow it up”, pretty much exploded. there are two on switches and i threw the wrong one first… apparently that was very bad! i replaced the capacitor hoping that was all, but the motor still won’t start. the auxillary “idler” motor was smoking hot at the time of this incident, so since it won’t start i assume i burnt. out. the reaon i am thinking to change it is that with this set up the 1hp motor is only at maybe 75% capacity. i don’t honestly understand it all but i’ve been reading lots to try and figure it out! which makes it less that 3/4 hp, which really is too small for a saw of this size. so rather than spend $ fixing this i thought it might be best to just upgrade to a bigger motor which might cost the same in the end.

i can find a single phase motor in almost the same rpm for pretty cheap, i can get a new pully to match the old saw rpm (i read the whole pulley/rpm calculation). but you are saying the shaft size is also a factor? that would be trickier to match. i assume there is probably some calculation so i can adjust the pulley size to accomodate the different shaft size? as for rotation direction is there a way to know that? there doesn’t seem to be anything marked on the motor… i assumed it had something to do with how it is wired?

thanks for your help brad, i really appreciate it!

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Fred Hargis

3932 posts in 1955 days


#3 posted 10-10-2015 07:46 PM

The shaft is only important if you want to use the old sheave (pulley). If you replace it (with one the same size) and it fits the new motor, it should work just fine. Don’t overlook the other point Brad made: the rotation must be correct (the motor may be reversible).

-- Our village hasn't lost it's idiot, he was elected to congress.

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MrUnix

4213 posts in 1661 days


#4 posted 10-10-2015 08:08 PM

First off.. what you have now, if it has an idler motor, is a rotary phase converter (RPC), not a static phase converter. Static converters will reduce your motors HP by roughly 1/3, but an RPC (or VFD) will not – so you were getting the full 1HP out of your motor with that setup.

Shaft diameter only matters if you want to re-use the existing motor pulley… but if you are going to buy a new one anyway, you just need to make sure it has the same bore as the shaft diameter of the new motor. If you have a 1725 RPM motor now, I’d stick with that instead of going to a 3450rpm motor just to keep the same pulley sizes. Rotation direction should be easy to figure out, and many (not all) motors are reversible.

For your existing 1HP 3-phase motor, a VFD would cost you about $100 more or less and would only require you to wire it up (no motor swap, no new pulleys, etc..). And if it makes any difference, you can get one that would take 120vac input (standard wall outlet) instead of 240vac, which would give you additional flexibility in where you can use the machine (along with the other benefits of a VFD).

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

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Leviticusk

12 posts in 960 days


#5 posted 10-10-2015 08:41 PM

oh boy, so much to learn! so, looking into VFDs i am rather overwhelmed by more info that i am unfamiliar with. if i am going to just keep the system i have for the same $100 i can get another 3 phase idler or possibly fix the one i may have burnt out? if the set up currently is a rotary conveter and i am getting full hp, i suppose i could stick with it. if i replace the idler it looks like any hp will work, even though the main motor is only 1 hp? is that accurate? as for the capacitor, everything i read says 50-100 micro farads per hp of idler motor. what is on there is 430-516 which according to everything is way too big for this 1.5 hp idler! so i should get a smaller one… i know there are many ways to go about this, i am trying to find the least expenive and least work, as i’d really like to use the saw and not spend extra time fussing with the motor! i figured if i am sending money anyhow a bigger motor might be nice…

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TopamaxSurvivor

17657 posts in 3137 days


#6 posted 10-10-2015 09:44 PM

Without any knowledge of testing, any appropriate test equipment or what you are doing, the first thing I suggest you do is disconnect the capacitor, turn on the idler motor and give it a spin to see if it works. If it does, try the saw. If the saw is running backwards, you spun the idler the wrong direction.

The capacitor just tell the idler which way to spin on stating.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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TopamaxSurvivor

17657 posts in 3137 days


#7 posted 10-10-2015 09:50 PM

BTW, considering the saw motor may have experienced considerable trauma, I suggest you have it tested by a motor shop before you just buy a VFD. The insulation in older motors may or may not tolerate the electronic power provided by a VFD. Some will trip the protective circuits of a VFD even though they will operate normally on conventional power.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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Leviticusk

12 posts in 960 days


#8 posted 10-11-2015 12:32 AM

The idler motor doesn’t turn on, that is why I am asking all these questions trying to figure out how to go about fixing it. I live on an island in the middle of nowhere so it’s not easy to just get an answer, that is why I thought I’d try the friendly people on here. I’d prefer not to go the VFD route as it is much more to learn, replacing the idler or just replacing the 3 ph set up with a single phase motor seems easier, there are just a few variables I was trying to get help understanding.

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TopamaxSurvivor

17657 posts in 3137 days


#9 posted 10-11-2015 03:03 AM

When it is turned on give it a spin and see if it starts. It can spin either way, doesn’t matter. Just see if it runs if gets a spin. If it does, it is probably OK.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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Leviticusk

12 posts in 960 days


#10 posted 10-11-2015 07:45 AM

As I said Bob, when turned on the idler motor does nothing. I was really hoping for a pretty simple answer – can I just put a single phase motor on? I gather now the shaft size is irrelevant, the frame not really relevant, speed should be same, direction same. Alternately I could just replace idler with another 3 ph motor, any HP, capacitor 50-100 micro farads/hp?

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fatman51

335 posts in 1298 days


#11 posted 10-11-2015 08:49 AM

Honestly, while these guys seem to know more than I about three phase motors and phase converters, I have done my share of millwright work and I understand what can happen when people are trying to work with things that they do not understand. I support your idea of converting the thing to single phase because it will be cheaper, and I understand that there is a lot to understand about your setup the way you bought it and even if you do fix it, you still have the double switch problem, which is hokey. Were you to keep it you would need to wire up some kind of relay circuit or you might order an expensive switch, to get rid of that problem too. On the other hand you can buy a single phase motor. Go with 220 volt, 3 horsepower is probably excessive, 1 1/2 to 2 will probably work just fine. It should have a decent starting and braking capacitor, so two capacitors, and it should spin the correct direction. When you purchase a motor, CL,amazon, granger, Northern industrial, Harbor Freight, you will want to get a 220v safety switch, Ganger, Amazon, Ace hardware online, and the correct pulleys and keys, if you need them, just about any hardware store. I agree that the arbor size and mounting brackets are subjective, as long as you make it all fit and mount your motor soundly, but not solidly, it should have some adjustment. I have usually found that I can build an adaptor plate to fit the new motor plate assembly to the existing mounting structure without too much trouble, but, like everyone else, I see no problem with drilling holes and starting over.

I could calculate the belt length for you easier than I could try to explain it, or you could just follow this link. http://www.calculatoredge.com/mech/vbelt%20length.htm. You can also wait until you set it up and measure it with a fabric tape measure like people that sew use, a piece of bailing twine and measure it with your ruler, or just buy a link belt and figure it out. What is important is that you keep the same pulley ratio. Of course, I have been flirting with the idea of putting step pulleys on my little saw so that I can adjust the speed.

-- The Constitution only gives people the right to pursue happiness. You have to catch it yourself. Benjamin Franklin

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TopamaxSurvivor

17657 posts in 3137 days


#12 posted 10-11-2015 06:49 PM


As I said Bob, when turned on the idler motor does nothing. I was really hoping for a pretty simple answer – can I just put a single phase motor on? I gather now the shaft size is irrelevant, the frame not really relevant, speed should be same, direction same. Alternately I could just replace idler with another 3 ph motor, any HP, capacitor 50-100 micro farads/hp?

- Leviticusk

I think you are missing the point. If it gets a spin when turned on, it may run. A 3 phase motor cannot start with single phase power applied. That is what the capacitor does, give it a kick in one direction or the other electrically. If it gets a spin mechanically with power applied it may run.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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TopamaxSurvivor

17657 posts in 3137 days


#13 posted 10-11-2015 06:59 PM

If the 3 phase motor or motors are shot, I’d just go to single phase. Assuming the 3 phase is set up with some kind of motor starter, it can be reused with proper overload load protection. Just use 2 of the 3 poles. Easiest way to protect the motor is get one with internal overloads.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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Fred Hargis

3932 posts in 1955 days


#14 posted 10-11-2015 08:25 PM

You really could use more power on a saw that size, it just seems to me that converting to a larger motor (go single phase while you’re at it) solves more than one problem.

-- Our village hasn't lost it's idiot, he was elected to congress.

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groyuti

45 posts in 419 days


#15 posted 10-12-2015 02:53 PM

-- Spammer in the process of being removed.

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