Is there a Unisaw in my future?

[Gary R Katz]

Whaddaya think?
http://eugene.craigslist.org/tls/5129645608.html

I've had an old Delta/Rockwell contractors saw forever and it's sometimes not quite enough saw. I've been reading SMC for years and it seems like once a month somebody posts a question about upgrading to a Unisaw. They post a pic of a saw for sale and you guys always have good advice. So, whaddaya think? (The price seems unusually low)
Oh yeah, I already have a Biesemeyer style fence on my old saw. I assume it will fit this Unisaw.

[Mike Henderson]

Some of the old Unisaws had pretty small motors (small in HP). Make sure it will be usable before you spend the money. I'd want 3 HP.

The fact that this motor can be wired for 120V would indicate it's less than 2 HP.

Mike

[Gary R Katz]

Mike, it does look like an older saw, the motor looks big, but I know that's no way to judge a motor.

[Gary R Katz]

I just called the guy and he says the motor is 1 1/4 motor. I think I'll pass on this. Thanks Mike for the timely reply!

[Dave Cav]

That seems like a reasonable price, and there may be a little room for negotiation, and it appears to be in decent shape. The picture does not show a fence and I wouldn't pay $500 for a saw with no fence and miter gauge.

The motor appears to be an old Repulsion/Induction motor. They are very stout motors; I had a 1 HP R/I motor on my previous Unisaw and never felt like it was underpowered. Millions of cabinets and projects were made with 1 HP Unisaws.

[Yonak Hawkins]

I just called the guy and he says the motor is 1 1/4 motor. I think I'll pass on this. Thanks Mike for the timely reply!

A motor doesn't make a saw. It looks clean. It looks old enough to be American made. I'd find that out. ..No fence or extensions. Right tilt is a negative in my book. I would test it. It's got a rebuilt motor, new belts and a new blade so you could push some thick hardwood through it and listen to it. If the bearings seem good and everything seems solid I'd ask him why he's selling it and weigh that answer. It may well be worth a counter offer, considering a few hundred for a new motor and fence.. It is, after all, a Unisaw.

[Art Mann]

It is a right tilt saw which would be a deal breaker for me. The photo really looks good though.

[mark kosse]

Gary, it always amazes me when people blow off right tilt saws. Your saw is a right tilt now so you understand it's function. 500.00 isn't that great of a price with no fence and dust door though. If it had any kind of useable fence, yes. As it is if you pull your fence off and put it on this saw it makes the contractor saw of little resale value unless you have another fence to put on it. The dust door is just aesthetic but it would bug the heck out of me not having one. I once painted a 1/4" piece of plywood and put that where a missing door was and that sufficed, sort of.

Now on to the motor. I'm going to suggest if your contractors saw sometimes isn't enough you might have a slight alignment issue and need to tweek it some. Slight alignment issues add stress to the blade making it seem underpowered. The beauty of a cabinet saw is they are so easy to align vs the contractor saw. I have that same one hp motor on one of my unisaws and the only reason I feel it needs a bigger motor is because bigger is always better. It's simply a mental thing. The 1hp motor does everything I ask of it and will out power the 1.5 motor I had on an old Delta contractor saw. That motor is a workhorse.

Here's to making a good decision. Good luck!

[Mike Henderson]

A 1 1/4 HP motor is just 1 1/4 HP. Doesn't matter if it's a repulsion-induction (RI) motor - it's still just 1 1/4 HP. RI motors use the repulsion part to get the motor started. Once it's running, it's just an induction motor, like any induction motor today. Additionally, RI motors have brushes which are a maintenance issue.

If 1 1/4 HP motors were sufficient for cabinet saws, we wouldn't see most modern cabinet saws with a minimum of about 3 HP. And many people recommend getting one with a 5 HP motor. The only cabinet saw with less than 3 HP that I'm familiar with is the SS with the 120V motor (1.75 HP) - but there may be others.

A good cabinet saw should be essentially a lifetime purchase. I wouldn't want to be stuck for the rest of my life with one with only 1 1/4 HP.

Sure, you can do good work with a 1 1/4 HP motor - it's better than nothing - but why limit yourself that way?

Mike

[Also, Unisaws require a motor with special mounting so you can't just swap out the motor for a bigger one with standard mounting. It has to be a motor that fits the Unisaw and those are often expensive - maybe as much as what you would pay for that Unisaw.]

[Allan Speers]

It is a right tilt saw which would be a deal breaker for me. The photo really looks good though.

I used a right tilt Uni for 10 years, and never once wished it was left tilt. The same can probably be said for a few hundred thousand woodworkers, over the years.


As for that R/I motor - I bet there's a vintage tool owner out there that would pay good money for it. Maybe even enough to buy a solid used 3 HP induction motor. FWIW, I think a new Baldor would be a little north of $400.

The other option is to ask him what 3ph motor he has. If that's 3 HP, you could get a $200 VFD for it, which would give you soft-start, mag starter functions, and a dynamic brake, plus 3 ph motors last longer and actually give a little more torque than "equivalent" 1-ph motors. If he does have a 3 HP 3ph, I would jump on that saw.

[mark kosse]

There is no 1.25 hp uni motor.. It's a 1 hp or a 1.5 hp if it is ri. As to rating the rest isn't worth arguing. My 1977 powermatic states in the manual 2hp MAX and that was during the carbide blade era and 20 years after this saw. Hp is overrated and subject to manufacture rating systems.

(if you have any metalworking skills the motor mount issue is easy to overcome. I did with one of mine. http://wiki.vintagemachinery.org/UnisawAlternateMotors.ashx )

[Phil Thien]

A 1 1/4 HP motor is just 1 1/4 HP. Doesn't matter if it's a repulsion-induction (RI) motor - it's still just 1 1/4 HP. RI motors use the repulsion part to get the motor started. Once it's running, it's just an induction motor, like any induction motor today. Additionally, RI motors have brushes which are a maintenance issue.

If 1 1/4 HP motors were sufficient for cabinet saws, we wouldn't see most modern cabinet saws with a minimum of about 3 HP. And many people recommend getting one with a 5 HP motor. The only cabinet saw with less than 3 HP that I'm familiar with is the SS with the 120V motor (1.75 HP) - but there may be others.

A good cabinet saw should be essentially a lifetime purchase. I wouldn't want to be stuck for the rest of my life with one with only 1 1/4 HP.

Sure, you can do good work with a 1 1/4 HP motor - it's better than nothing - but why limit yourself that way?

Mike

[Also, Unisaws require a motor with special mounting so you can't just swap out the motor for a bigger one with standard mounting. It has to be a motor that fits the Unisaw and those are often expensive - maybe as much as what you would pay for that Unisaw.]

The toque-speed performance of a RI motor is similar to a DC compound motor.

Which basically means that if your RPM's drop to 75%, the brushes are engaged and then it is torque city (compared to a capacitor start motor which reintroduces a capacitor which does very little).

Source:
http://www.industrial-electronics.com/elecy4_23.html

[Mike Henderson]

The toque-speed performance of a RI motor is similar to a DC compound motor.

Which basically means that if your RPM's drop to 75%, the brushes are engaged and then it is torque city (compared to a capacitor start motor which reintroduces a capacitor which does very little).

Source:
http://www.industrial-electronics.com/elecy4_23.html
HP is torque times RPM times a factor. No matter how you get the torque, you won't get more than 1 HP out of a 1 HP motor. And if the motor is turning at 75% of rated RPM, the HP will be less or the torque will have to be higher.

Let's say the torque is higher. That means that the motor is drawing more current than it's rated for and that will cause the motor to overheat and eventually fail. The general rule or thumb in motors is that for every 10 degrees C that the motor exceeds the rated temperature, the life is cut in half.

There's no free lunch. A 1 HP motor is not going to produce 2 HP, and HP is the measure of work, not torque. Pushing a motor beyond it's rating (which is actually what Service Factor is) will reduce the life of the motor.

Mike

[If the repulsion part of the motor gave you more (free) power, they'd leave it engaged instead of disengaging it for normal operation. The repulsion part is only used to start the motor.]

[Let me expand a bit on this. Suppose you had a 1 HP motor that you have loaded up such that the RPMs have fallen to .75 times the rated RPM. That motor is overloaded and is drawing excess current. As the RPMS decrease the HP decreases because HP is torque times RPM times a constant factor. So by the time the motor has reached .75 of it's rated RPM, it's only producing .75 HP.

According to your theory, when the Repulsion part of the motor kicks in, it somehow increases the torque and allows the motor to continue to work. Since the replusion part of the motor will disengage at .75 of the rated RPM, let's look at the amount of torque needed at that RPM to keep the motor going at 1 HP. I'll ignore the constant factor and assume that HP = T x RPM. And since the RPM is constant at .75 of the rated RPM, I'll call that K.

So the motor without the repulsion factor is kT1 = .75
When the repulsion kicks in, the formula must be kT2 = 1 because the load is 1HP (or even a bit more) and that's why the motor slowed down.

A bit of manipulation gives .75T2 = T1, or T2 = 1.333T1.

So to generate 1 HP at .75 of the rated RPM, we must increase the torque by 33%. Where is the power for that increase going to come from? From increased current flow. But wait! The motor was already drawing excess current to fall to .75 of the rated RPM. This extra current is going to cause excess heat in the motor, and eventually cause it to fail.]

[Kent A Bathurst]

What is the HP of the original 3ph motor he has? and then, $$ for a VFD to run it?

Those things were horses you could not kill..........I agree that the LH is preferable, but that would not prevent me from a killer deal for a RH...especially, if I only had the room, for a 2d TS to set up with a 3/8" dado stack.......

But - if you want plug-and-play, then a pass is probably better.

[Kent A Bathurst]

.............So to generate 1 HP at .75 of the rated RPM, we must increase the torque by 33%. Where is the power for that increase going to come from? From increased current flow. But wait! The motor was already drawing excess current to fall to .75 of the rated RPM.......]

Mike -

you continue to amaze me, on any number of topics..on this one, I gotta ask: where from you get all this Gandalf-level knowledge?

[Ross Becker]

Mike- you're describing the characteristics of a repulsion start, induction run motor. The Delta rockwell motors were Repulsion-induction motors- which are different than you're describing. The source Phil linked above describes the difference between those. Also, while 1HP is 1HP, there are different ways to measure HP, some more conservative and some less conservative. Do you really think that manufactures haven't altered how they measured motor HP in order to make it look better on paper since the 40s?

[Mike Henderson]

Mike- you're describing the characteristics of a repulsion start, induction run motor. The Delta rockwell motors were Repulsion-induction motors- which are different than you're describing. The source Phil linked above describes the difference between those. Also, while 1HP is 1HP, there are different ways to measure HP, some more conservative and some less conservative. Do you really think that manufactures haven't altered how they measured motor HP in order to make it look better on paper since the 40s?
Well, the definition of a HP has not changed and the engineers of the '50's were well able to measure (and calculate) HP. We're not talking about a motor made in the 1800s.
But beyond that, you can look at the power input to a motor and and see if the rating is reasonable. Those old motors were less efficient than the induction motors we have today so take the voltage and current into the motor, multiply it together, multiply it by about .8 to .85 for efficiency and then divide by 748 watts per HP.

None of this stuff has changed. I really don't think those old motor were underrated. A HP is still a HP.

And it doesn't matter what you call it, there's no free lunch with any kind of repulsion-induction motor. If the motor starts to stall in the induction part of the envelope the repulsion portion is not magic. To restore the required HP requires power in the form of current and that current will overheat the motor. If those RI motors were so great we'd still be using them. But we aren't so that should tell you something.

Mike

[Those old motors may be even less efficient than .8 to .85. Smaller HP motors are less efficient than larger motors so the efficiency on those old motors could be .7 or so.]

[Phil Thien]

There's no free lunch. A 1 HP motor is not going to produce 2 HP, and HP is the measure of work, not torque. Pushing a motor beyond it's rating (which is actually what Service Factor is) will reduce the life of the motor.


Just pointing out an important difference. RI motors are capable of delivering significant HP at lower RPM, as compared to cap start/cap run motors.

I'm not saying they will do it forever or that it is wise to bog any motor down.

The RI motors are typically substantially overbuilt and a local lumber yard pushed their 1-1/2-HP Unisaw very hard for decades and it still worked great (on the original motor) until the day they closed shop.

Their saw may have even been a 1-HP. I can't remember, I just remember it was the same HP as my contractor saw motor but substantially outperformed my saw.

[Art Mann]

I used a right tilt Uni for 10 years, and never once wished it was left tilt. The same can probably be said for a few hundred thousand woodworkers, over the years.


Well, I have used both left and right tilt saws too. After using a left tilt for a while, I lost interest in owning a right tilt saw. My experience is it is more trouble to use and use safely than a left tilt, especially when cutting bevels. I know of some other people who feel just like I do. Grizzly Tools is one company that holds that opinion. They have dropped right tilt from their lineup. I just wanted to point this out to the original poster, because he might not realize there is a difference. His opinion is the one that matters.

[Phil Thien]

If those RI motors were so great we'd still be using them. But we aren't so that should tell you something.

Mike

Modern cap start/run induction motors are substantially less expensive to produce on a per-HP basis. So the clear solution was to outfit cabinet saws with 3-HP and 5-HP induction motors. Less expensive, and any advantage of the RI is essentially moot.

[Phil Thien]

Well, I have used both left and right tilt saws too. After using a left tilt for a while, I lost interest in owning a right tilt saw. My experience is it is more trouble to use and use safely than a left tilt, especially when cutting bevels. I know of some other people who feel just like I do. Grizzly Tools is one company that holds that opinion. They have dropped right tilt from their lineup. I just wanted to point this out to the original poster, because he might not realize there is a difference. His opinion is the one that matters.

I've used both LT and RT and my current saw is RT and I find that I prefer the RT because while using a sled to cut a bevel, I get to stand to the left of the blade and find that works a little better for me.

[Mike Henderson]

Modern cap start/run induction motors are substantially less expensive to produce on a per-HP basis. So the clear solution was to outfit cabinet saws with 3-HP and 5-HP induction motors. Less expensive, and any advantage of the RI is essentially moot.
Absolutely. The only advantage of an RI motor is that it has greater staring torque than a capacitor start motor. And since our woodworking tools start essentially unloaded, the RI motor is useless in woodworking applications.

Mike

[Mike Henderson]

Just pointing out an important difference. RI motors are capable of delivering significant HP at lower RPM, as compared to cap start/cap run motors.

I'm not saying they will do it forever or that it is wise to bog any motor down.

The RI motors are typically substantially overbuilt and a local lumber yard pushed their 1-1/2-HP Unisaw very hard for decades and it still worked great (on the original motor) until the day they closed shop.

Their saw may have even been a 1-HP. I can't remember, I just remember it was the same HP as my contractor saw motor but substantially outperformed my saw.
A motor in a woodworking tool should not be bogged down to the point where it is running at .75 of the rated RPM. And even if you did that, you have not explained where the power comes from the maintain the rated HP at that RPM. Those old motors were pretty inefficient which means they were physically bigger than motors of today so they may tolerate short periods of excess current better than the more efficient, and smaller modern motors. But that has nothing to do with RI.

Mike

[Mike Henderson]

Mike -

you continue to amaze me, on any number of topics..on this one, I gotta ask: where from you get all this Gandalf-level knowledge?
I am an old man and I've lived an interesting life.

Mike

[Mike Henderson]

Just an added comment on motors. Ever notice that a 1725 RPM motor is bigger than a 3450 RPM motor for the same HP? The reason is that the 1725 RPM motor must produce twice the torque to produce the same HP as the 3450 RPM motor. And to produce that greater torque, everything has to be bigger.

If you could get a universal motor with a real HP rating, that motor would be smaller than the 3450 RPM motor for the same HP. The reason? Because the universal motor runs at 10,000 RPM to 12,000 RPM so it can produce less torque to generate the same HP.

Mike

[Phil Thien]

A motor in a woodworking tool should not be bogged down to the point where it is running at .75 of the rated RPM. And even if you did that, you have not explained where the power comes from the maintain the rated HP at that RPM. Those old motors were pretty inefficient which means they were physically bigger than motors of today so they may tolerate short periods of excess current better than the more efficient, and smaller modern motors. But that has nothing to do with RI.

Mike

When the RI motor slows significantly, the motor's starting mechanism is engaged and that starting torque comes back. It isn't unlimited but isn't insignificant, either.

I've stalled modern induction motors like the one on my 1.5-HP contractor saw. It seems that once the motor drops in RPM, you stand a pretty good chance of stalling it altogether. That is, it is often difficult to moderate your feed rate quickly enough to prevent stalling the motor altogether.

That doesn't happen so much on RI motors. You obviously can't feed stock like it is a 5-HP motor, but at the same time it is much easier to push the motor to its limits without risking seeing the blade come to a full stop. You have enough time to react to a drop in RPM's.

I've cut LVL's on a contractor's saw as well as a Unisaw equipped with a bullet motor with the same HP rating. The motors act quite a bit differently at the margins.

[Harold Weaver]

I got to go with Dave Cav on this one. The Unisaw will be a far superior machine than your contractor saw. The proof is inside the cabinet. The 1 horse RI motor will do anything you need to do, if you find that it does not, then you need a 12/14 saw and not a Unisaw. Just my 2 cents worth.

[Mike Henderson]

When the RI motor slows significantly, the motor's starting mechanism is engaged and that starting torque comes back. It isn't unlimited but isn't insignificant, either.

I've stalled modern induction motors like the one on my 1.5-HP contractor saw. It seems that once the motor drops in RPM, you stand a pretty good chance of stalling it altogether. That is, it is often difficult to moderate your feed rate quickly enough to prevent stalling the motor altogether.

That doesn't happen so much on RI motors. You obviously can't feed stock like it is a 5-HP motor, but at the same time it is much easier to push the motor to its limits without risking seeing the blade come to a full stop. You have enough time to react to a drop in RPM's.

I've cut LVL's on a contractor's saw as well as a Unisaw equipped with a bullet motor with the same HP rating. The motors act quite a bit differently at the margins.
If you're going to make such claims, please tell me where the power comes from to generate that increased torque. Saying that the repulsion mechanism kicks in is not sufficient. That mechanism cannot produce power without increased current and that increased current will generate heat, which will affect the life of the motor. This stuff is basic electricity.

If the repulsion mechanism was some magic torque enhancer, the engineers would have designed the motor with it always engaged.

A 1 HP motor is still a just a 1 HP motor. A RI motor is no better than a regular induction motor when the motor is operated in its normal operating RPM range. And when you operate it such that the repulsion mechanism kicks in, you're in an area where the motor is drawing excess current.

Perhaps what we're seeing is that those old RI motors are very low HP motors and people actually do bog them down because the machine is so under-powered - and the motor can take it because it's so physically large that it takes a lot to get it hot. But that hardly makes RI motors a good choice for a table saw. It would be much better to have a motor that can do the work without bogging down in the first place. I think those old motors were small HP so that they could be run on 110 volts. And because motors were pretty inefficient - and physically large - they might not have been able to fit bigger HP motors into the units. Bigger motors may have also been very expensive back then.

Mike

[Gary R Katz]

The least I can say about this thread is that I learned a lot! I owned a bakery for 37 years and changed out a lot of single and 3 phase motors but I never had to delve into the details of which motor was best or adapting a motor to a different use.
I'm going to pass on this saw for now. I'm used to a left tilt and I'm an old dog who doesn't want to learn a new trick. The motor has no more power than my contractors saw and I'm not that interested in chasing after another motor or messing with the 3 phase motor and an adapter. I just want to saw wood!
I know I've got a slight alignment issue with my belt. I've got the pulley extended out almost to the end of the shaft and I've never looked beyond that easy fix. As I said, I just want to saw wood. I had 37 years of maintaining and fixing with old bakery equipment and that's enough!
Thanks for you help with this. I'll be much better informed when the next saw comes around.

[Phil Thien]

If you're going to make such claims, please tell me where the power comes from to generate that increased torque. Saying that the repulsion mechanism kicks in is not sufficient. That mechanism cannot produce power without increased current and that increased current will generate heat, which will affect the life of the motor. This stuff is basic electricity.

I have stated a bunch of times that the increased torque is coming from the starting circuit. I agree that it causes increased current and heat.

I hope you will agree that about the worst thing you can do to a motor is lock the rotor. I'd rather kick that starter in and have it give me a chance to back-off the feed to prevent locking the rotor.


If the repulsion mechanism was some magic torque enhancer, the engineers would have designed the motor with it always engaged.

I didn't call it a magic torque enhancer, but if it helps me prevent locking the rotor...


A 1 HP motor is still a just a 1 HP motor. A RI motor is no better than a regular induction motor when the motor is operated in its normal operating RPM range. And when you operate it such that the repulsion mechanism kicks in, you're in an area where the motor is drawing excess current.

I agree.


Perhaps what we're seeing is that those old RI motors are very low HP motors and people actually do bog them down because of that - and the motor can take it because it's so physically large that it takes a lot to get it hot. But that hardly makes RI motors a good choice for a table saw. It would be much better to have a motor that can do the work without bogging down in the first place.

Mike

I don't disagree with that at all. At the same time, it is what it is. Old Unisaws came with RI "bullet" motors. They work pretty well, certainly better than any modern 1.5-HP induction motor I've tried. While I'm not going to say a 1.5-HP bullet is the same as a 2+ HP modern induction, I also would go so far as to say it is the same as a 1.5-HP modern induction, either. The extra moving mass, the extra torque before a stall, the heavier construction all kind of help make them an okay compromise, IMHO.

[Allan Speers]

Well, I have used both left and right tilt saws too. After using a left tilt for a while, I lost interest in owning a right tilt saw. My experience is it is more trouble to use and use safely than a left tilt, especially when cutting bevels. I know of some other people who feel just like I do. Grizzly Tools is one company that holds that opinion. They have dropped right tilt from their lineup. I just wanted to point this out to the original poster, because he might not realize there is a difference. His opinion is the one that matters.

No argument from me, Art. If I were to buy a TS today, I'd prefer a left-tilt.

- but it would't be the main factor by a longshot, and again, I never ever felt like my rt-tilt was holding me back in some way.

[Allan Speers]

Perhaps what we're seeing is that those old RI motors are very low HP motors and people actually do bog them down because the machine is so under-powered - and the motor can take it because it's so physically large that it takes a lot to get it hot. But that hardly makes RI motors a good choice for a table saw. It would be much better to have a motor that can do the work without bogging down in the first place. I think those old motors were small HP so that they could be run on 110 volts. And because motors were pretty inefficient - and physically large - they might not have been able to fit bigger HP motors into the units. Bigger motors may have also been very expensive back then.

Mike

I think they were only 1.5 HP because R/I motors were very expensive and very heavy.

As for the power, I am mostly guessing here, but weren't the old Uni motors 1725 rpm? AFAIK, a 1.5 HP 1725 rpm motor has a lot more torque than a 1.5 HP 3540 rpm motor (no?) and torque is much more important than HP in a woodworking machine. (unless you lose that torque via pulleys that bringgn the arbor speed back up, I guess.)

Another possible factor is that repulsion & R/I motors used up to 8 poles, and a very heavy shaft. I'm pretty sure that also contributes to extra torque.



- but I could definitely be wrong. :o

[Allan Speers]

Mike- you're describing the characteristics of a repulsion start, induction run motor. The Delta rockwell motors were Repulsion-induction motors- which are different than you're describing.

I think that's incorrect.

AFAIK, there are R/I motors, and there are repulsion motors. The term "repulsion induction" (which the Uni 1 ph bullet motors were, as you state) MEANS "repulsion start / induction run."

[Mike Wilkins]

I had that same machine many years ago; 1 1/2 HP single phase 240 volt motor, purchased with no fence for $525.00. Did not know any better then but after the additional of a Unifence it performed great for several years until a slider slid into the shop. I made a lot of furniture/house projects with that machine, even with the smaller HP motor. Even with no fence, the price is not too bad. Maybe you can negotiate a better price.

[Mike Henderson]

I think they were only 1.5 HP because R/I motors were very expensive and very heavy.

As for the power, I am mostly guessing here, but weren't the old Uni motors 1725 rpm? AFAIK, a 1.5 HP 1725 rpm motor has a lot more torque than a 1.5 HP 3540 rpm motor (no?) and torque is much more important than HP in a woodworking machine. (unless you lose that torque via pulleys that bringgn the arbor speed back up, I guess.)

Another possible factor is that repulsion & R/I motors used up to 8 poles, and a very heavy shaft. I'm pretty sure that also contributes to extra torque.



- but I could definitely be wrong. :o
HP is the measure of work. When you're cutting wood you're doing work (work in the physics sense) so HP is what's needed to keep the blade moving when you're ripping, for example. A 1725 RPM motor will have twice the torque of a 3450 RPM motor for the same HP, but both will do the same work.

A 1725 RPM motor will have four poles, for 60 Hz power. If the motor had eight poles, it would run at about 863 RPM (for 60 Hz power).

The heavy shaft could contribute inertia, not torque.

Using pulleys will not change the HP of the unit - except that you will lose a small amount of HP due to the losses in the pulley system. Since the HP is the same, the torque will change (up or down) based on whether you slow down or speed up the RPMs. HP is basically torque times RPM (times a factor).

Mike

[David Kumm]

Look for a used Delta Rockwell 12-14 or a PM 72. Many 72 were left tilt although a Beisemeyer fence makes tilt direction irrelevant. In addition to the larger motor, the guts of the machines are far heavier and overbuilt in comparison to the Uni. They take up the same space and the price isn't much higher. The build is way better. Dave

[Mike Henderson]

I have stated a bunch of times that the increased torque is coming from the starting circuit. I agree that it causes increased current and heat.

I hope you will agree that about the worst thing you can do to a motor is lock the rotor. I'd rather kick that starter in and have it give me a chance to back-off the feed to prevent locking the rotor.



I didn't call it a magic torque enhancer, but if it helps me prevent locking the rotor...



I agree.



I don't disagree with that at all. At the same time, it is what it is. Old Unisaws came with RI "bullet" motors. They work pretty well, certainly better than any modern 1.5-HP induction motor I've tried. While I'm not going to say a 1.5-HP bullet is the same as a 2+ HP modern induction, I also would go so far as to say it is the same as a 1.5-HP modern induction, either. The extra moving mass, the extra torque before a stall, the heavier construction all kind of help make them an okay compromise, IMHO.
I thought about your position last night and I think I now understand it. See if I'm correct here: If you're going to have a table saw that is under powered, and you're going to bog the motor down, a RI motor will not stall as easily as a capacitor start induction motor.

A better approach, however, is to have a motor that you do not operate in excess of its capabilities, both because of the potential damage to the under powered motor and the ease of doing the work.

Mike

[Phil Thien]

I thought about your position last night and I think I now understand it. See if I'm correct here: If you're going to have a table saw that is under powered, and you're going to bog the motor down, a RI motor will not stall as easily as a capacitor start induction motor.

A better approach, however, is to have a motor that you do not operate in excess of its capabilities, both because of the potential damage to the under powered motor and the ease of doing the work.

Mike

Well I'm not going to start with the premise that the saw is underpowered, that would require knowledge of the intended use. Underpowered for cutting 8/4 slabs all day in a production environment, absolutely. Underpowered for cutting sheet goods and 4/4 hardwood lumber, and occasional 5/4 and 6/4 and 8/4 material, in a hobby shop, no way.

I've made a ton of projects with my Ryobi BT3000 saw (universal motor). I cut 8/4 lumber with the thing (on occasion). Sheet goods and 4/4 and 5/4 stuff routinely. I'm not saying the BT3000 is all anyone needs. But saying everyone needs a 5-HP cabinet saw is similarly bad advice.

For many people, a $500 saw with one of those 1.5-HP RI ("bullet") motors is likely a very good fit.

Keep in mind there are still a great many of them in use and you don't see many (any?) threads like this where an actual user of one of those saws complains that the bullet motor is standing between them and the projects they'd like to achieve.

[Mike Henderson]

Well I'm not going to start with the premise that the saw is underpowered, that would require knowledge of the intended use. Underpowered for cutting 8/4 slabs all day in a production environment, absolutely. Underpowered for cutting sheet goods and 4/4 hardwood lumber, and occasional 5/4 and 6/4 and 8/4 material, in a hobby shop, no way.

I've made a ton of projects with my Ryobi BT3000 saw (universal motor). I cut 8/4 lumber with the thing (on occasion). Sheet goods and 4/4 and 5/4 stuff routinely. I'm not saying the BT3000 is all anyone needs. But saying everyone needs a 5-HP cabinet saw is similarly bad advice.

For many people, a $500 saw with one of those 1.5-HP RI ("bullet") motors is likely a very good fit.

Keep in mind there are still a great many of them in use and you don't see many (any?) threads like this where an actual user of one of those saws complains that the bullet motor is standing between them and the projects they'd like to achieve.
Well, I see two situations:

1. The motor HP is properly matched to the work, in which case you never get into the performance area where the repulsion mechanism is enabled. In that case, the motor performs the same as any other induction motor and the repulsion function has no advantage.

2. The motor is under-powered (HP) and the motor becomes bogged to the point where the repulsion mechanism is enabled. In that case, you should have a larger motor. Operating the motor in the bogged state (where the repulsion mechanism is enabled) increases the current through the motor significantly above rated current. You are then definitely in danger of causing damage to the motor.

It has to be one or the other.

Mike

[Allan Speers]

Thanks for the elaboration, Mike. But 3 followups:



The heavy shaft could contribute inertia, not torque.


Isn't torque and inertia pretty much the same thing, resistance to a counter-force? - And isn't THAT what we need when cutting wood, so the machine doesn't bog down? Like a race car vs a tractor. One goes really fast, but the other can pull stumps out of the ground.



Using pulleys will not change the HP of the unit - except that you will lose a small amount of HP due to the losses in the pulley system. Since the HP is the same, the torque will change (up or down) based on whether you slow down or speed up the RPMs. HP is basically torque times RPM (times a factor).


That's exactly what I said (asked about) - the different pulley brings the torque back down. But now I know for sure, so again thanks.
---------------

Now for the complicated question:

You wrote, "A 1725 RPM motor will have four poles, for 60 Hz power. If the motor had eight poles, it would run at about 863 RPM" which of course is true. I knew that part. HOWEVER, it seems to me that the more poles an induction motor (or a repulsion motor running at more than 75% speed) has, the less space there is where the shaft is rotating only due to inertia.

SO: Take an 2 pole motor and an 8 pole motor, and pulley them so that both are turning an arbor at the same speed. Surely the 8 pole motor has more torque, or if that's not the right term (I think it is) the 8 pole motor has more resistance to being bogged down.

No? Isn't that why older, quality machines tend to have slower motors, with their modern cost-cutting counterparts using 3450 rpm whenever possible?

[Phil Thien]

Well, I see two situations:

1. The motor HP is properly matched to the work, in which case you never get into the performance area where the repulsion mechanism is enabled. In that case, the motor performs the same as any other induction motor and the repulsion function has no advantage.

2. The motor is under-powered (HP) and the motor becomes bogged to the point where the repulsion mechanism is enabled. In that case, you should have a larger motor. Operating the motor in the bogged state (where the repulsion mechanism is enabled) increases the current through the motor significantly above rated current. You are then definitely in danger of causing damage to the motor.

It has to be one or the other.

Mike

You forgot #3: The motors were substantially overbuilt (by today's standards), have a lot of thermal and rotational mass, and seem to tolerate being driven to the margins quite well.

[Mike Henderson]

You forgot #3: The motors were substantially overbuilt (by today's standards), have a lot of thermal and rotational mass, and seem to tolerate being driven to the margins quite well.
Margins? By the time the repulsion mechanism kicks in the motor is way into overload. And even if the motor doesn't fail today, you're working with a table saw that is, by definition, significantly under-powered - which makes it more difficult to do the work it was intended for.

No, there's really only the two alternatives I outlined. Much better to get a saw with a motor that is sized to the work it is intended to do.

Mike

[Phil Thien]

Margins? By the time the repulsion mechanism kicks in the motor is way into overload. And even if the motor doesn't fail today, you're working with a table saw that is, by definition, significantly under-powered - which makes it more difficult to do the work it was intended for.

No, there's really only the two alternatives I outlined. Much better to get a saw with a motor that is sized to the work it is intended to do.

Mike


If the motor doesn't fail today? This isn't some sort of theoretical question, Mike, there have been Unisaws with 1.5-HP bullet motors in hobby and commercial shops for many decades now.

Are they all on borrowed time?

Where are all the users complaining about their underpowered bullet motors?

So many threads here and other forums with people suffering blown caps on their newer saws with 5-HP motors, and those Unisaws with bullet motors just keep plugging away, day-in, day-out, for decades.

[Mike Henderson]

If the motor doesn't fail today? This isn't some sort of theoretical question, Mike, there have been Unisaws with 1.5-HP bullet motors in hobby and commercial shops for many decades now.

Are they all on borrowed time?

Where are all the users complaining about their underpowered bullet motors?

So many threads here and other forums with people suffering blown caps on their newer saws with 5-HP motors, and those Unisaws with bullet motors just keep plugging away, day-in, day-out, for decades.
There's no way to escape the facts. If you use an under-powered motor and push it to the point where the current is grossly in excess of the rated current, the motor will eventually fail. You also have a less than optimal experience using the table saw. I had a 1.5 HP table saw and then moved up to a 3 HP SS. It was like night and day.

Why would anyone want to use an under-powered tool?

Of course, some of those users may use the saw within the HP ratings of the motor and that would not cause it to go into current overload.

I would suggest that you keep using a 1 HP RI table saw if that's what you like. I'll use the 3 HP saw that I like.

Mike

[Although there are many reasons RI motors are not used in woodworking equipment today, one is that they use brushes that require maintenance.]

[Phil Thien]

There's no way to escape the facts. If you use an under-powered motor and push it to the point where the current is grossly in excess of the rated current, the motor will eventually fail. You also have a less than optimal experience using the table saw. I had a 1.5 HP table saw and then moved up to a 3 HP SS. It was like night and day.

Yep, they're all going to fail, every single one of them. All those vintage bullet motors, now in use for 75+ years. Eventually it is going to happen, we just haven't given it enough time.



Why would anyone want to use an under-powered tool?

Apparently the users don't feel they're underpowered. You seem to think so, but it doesn't sound like you've ever used one.


Of course, some of those users may use the saw within the HP ratings of the motor and that would not cause it to go into current overload.

Certainly. OTOH they were very popular for commercial shops and a great many of them were ridden hard and put up wet, and don't seem any the worse for it.



I would suggest that you keep using a 1 HP RI table saw if that's what you like. I'll use the 3 HP saw that I like.

Mike

I mostly use a saw w/ universal motor, as stated above. And still I routinely cut 8/4 hard maple and walnut (the corner of my basement is filled with 8/4 walnut). This saw has way less power than the Unisaws I've used with 1.5-HP bullet motor. And still it works fine for me.

[Art Mann]

I won't presume to answer questions directed at Mike, but as a 30 year practicing electrical engineer, I can assure you that torque and inertia are only distantly related quantities.

[Mike Henderson]

So why do we not use RI motors in woodworking equipment? Here are some of the reasons: Comparisons are made to equal HP motors.

1. They are significantly more expensive than capacitor start or three phase induction motors.
2. They are physically larger than standard induction motors.
3. They have brushes and moving parts that are not in standard induction motors. Essentially all motors are going to brushless solutions for reliability.
4. The brushes require maintenance. Standard induction motors only have bearings that require maintenance - as do RI motors.
5. There are no three phase RI motors, only single phase.
6. When operated in the motor's normal operating range, there is no advantage to the RI motor. It is just an induction motor like the standard induction motor. The repulsion mechanism is only used to get the motor up to speed.

The only advantage of an RI motor is that it has higher starting torque which is of value when the load is attached at startup. This is generally not true for woodworking tools.

What Phil is trying to put forward as an advantage is that if the motor is significantly under-powered for the load, the motor will get a "kick" as the RPMs decrease, as the expense of significantly excess current flow through the motor. In reality, the motor should not be operated in such a fashion - its HP should be matched to the load so that it never stalls in that fashion. It's trying to take a problem and turn it into a "feature".

Mike

[Phil Thien]

What Phil is trying to put forward as an advantage is that if the motor is significantly under-powered for the load, the motor will get a "kick" as the RPMs decrease, as the expense of significantly excess current flow through the motor. In reality, the motor should not be operated in such a fashion - its HP should be matched to the load so that it never stalls in that fashion. It's trying to take a problem and turn it into a "feature".

Mike

Feature is your word, not mine. I'm just pointing out functional differences in the motors.

My point is, those old bullet motors can be pushed to their limits and once you hear them losing RPM's you don't have a fraction of a second before they stall (as you would with a modern induction motor). You have some time to adjust your feed rate and finish your cut.

I'm not suggesting they be overloaded on purpose. OTOH, it has to have been the single best-selling cabinet saw in history, with the great majority of them being used by employees in a commercial setting. Point being, they were put in environments where they were likely routinely pushed to extremes and yet held up very well.

You can try twisting this however you like, history is on my side.

[Mike Henderson]

Feature is your word, not mine. I'm just pointing out functional differences in the motors.

My point is, those old bullet motors can be pushed to their limits and once you hear them losing RPM's you don't have a fraction of a second before they stall (as you would with a modern induction motor). You have some time to adjust your feed rate and finish your cut.

I'm not suggesting they be overloaded on purpose. OTOH, it has to have been the single best-selling cabinet saw in history, with the great majority of them being used by employees in a commercial setting. Point being, they were put in environments where they were likely routinely pushed to extremes and yet held up very well.

You can try twisting this however you like, history is on my side.
Ah, yes, and the facts are on my side.

Mike

[Phil Thien]

So why do we not use RI motors in woodworking equipment? Here are some of the reasons: Comparisons are made to equal HP motors.

1. They are significantly more expensive than capacitor start or three phase induction motors.
2. They are physically larger than standard induction motors.
3. They have brushes and moving parts that are not in standard induction motors. Essentially all motors are going to brushless solutions for reliability.
4. The brushes require maintenance. Standard induction motors only have bearings that require maintenance - as do RI motors.
5. There are no three phase RI motors, only single phase.
6. When operated in the motor's normal operating range, there is no advantage to the RI motor. It is just an induction motor like the standard induction motor. The repulsion mechanism is only used to get the motor up to speed.

The only advantage of an RI motor is that it has higher starting torque which is of value when the load is attached at startup. This is generally not true for woodworking tools.

What Phil is trying to put forward as an advantage is that if the motor is significantly under-powered for the load, the motor will get a "kick" as the RPMs decrease, as the expense of significantly excess current flow through the motor. In reality, the motor should not be operated in such a fashion - its HP should be matched to the load so that it never stalls in that fashion. It's trying to take a problem and turn it into a "feature".

Mike

Oh and #5 is sort of silly, don't you think? That is like me implying that a lack of any starting caps on modern three-phase induction motors somehow makes cap-start motors inferior.

[Mike Henderson]

Oh and #5 is sort of silly, don't you think? That is like me implying that a lack of any starting caps on modern three-phase induction motors somehow makes cap-start motors inferior.
Yes, because of the cap, a capacitor start induction motor is inferior to a three phase induction motor. The cap is a point of failure that the three phase motor does not have.

The only reason we use capacitor start induction motors is because we don't have three phase. If we had three phase, we'd use three phase motors. More reliable.

Three phase motors have good starting torque, also.

Mike

[Phil Thien]

Yes, because of the cap, a capacitor start induction motor is inferior to a three phase induction motor. The cap is a point of failure that the three phase motor does not have.

The only reason we use capacitor start induction motors is because we don't have three phase. If we had three phase, we'd use three phase motors. More reliable.

Three phase motors have good starting torque, also.

Mike

Yeah but we are not discussing three phase and yet you listed the fact that there are no three phase RI motors as some sort of proof that RI motors are unsuitable for woodworking.

Again your position is that a Unisaw with 1.5-hp RI motor is inadequate and everyone should have at least a three hp motor.

[Kent A Bathurst]

Phil & Mike -

You are both highly respected, and highly knowledgeable resources. We all look forward to reading your thoughts and advice on many topics.

However, at this moment, on this topic, you are in a death match, determined to wrestle each other to the ground. And you are wrestling in an arena that very few of us understand.

Please keep up the contributions to the Creek.

And, consider taking this match to PM or a local bar. If there is a local bar, I will join in, buy a couple rounds, just for the pleasure of watching.

Very best regards to both of you.

Kent

[Mike Henderson]

Okay, Kent. But it was interesting.

Mike

[@Alan Speers - I'm sorry I didn't address your posting. It happened to fall at the end of one page on my computer and I didn't see it. I'd have to think about the number of poles and whether that might affect the torque. Off the top of my head, I don't think so. I don't think that the rotor is turning between poles, at any time, due to inertia - there is always force on the rotor. If that were not true, there would be rotor positions where the motor would not start.

Also, while a 3450 RPM motor only has two poles, each pole covers close to half of the stator. The poles are just bigger than the poles in a four or eight pole motor. Look inside a 3450 RPM motor and you won't see much empty space on the stator. Remember, in an induction motor, the field is rotating and the rotor is following the field (a bit slower than the field in an induction motor - the rotor is chasing the field and the difference in their RPMs is known as "slip"). In a four pole motor, the field just rotates at half the line Hertz.

But whatever the torque, it will get factored into the HP since HP and torque are related.

And don't get confused by torque and HP. When cutting wood, it is HP that's important, not torque. For the same HP, a slower RPM motor will have greater torque but it will do exactly the same work as a higher RPM motor.

And if you'll permit me, I made a mistake in my comments in post #13. I did the calculations based on a 1HP motor and assumed that the recovery would only have to generate 1 HP at 75% of the rated RPM. However, induction motors will not stall at 100% of the rated HP. They will provide greater than 100% of the rated HP, albeit at excess current flow. Service Factor captures this. So if you have a service factor of 1.15 on a 1HP motor, it means that the motor will provide 1.15 HP but the increased current flow will cause the temperature of the motor to increase by 10 degrees C, and operation at that level will reduce the life of the motor by approximately half.

The calculations should have been based on a recovery HP of about 1.20 HP (the assumed stall HP level), so the torque required at 75% of rated RPM is about 160% of rated operating torque, which would require current significantly above rated, with the consequent heat. If the motor designers had intended for the starting circuit to be used that way, they would have not designed a disconnect into the system - they would have left it in all the time.]