Old Unisaw Purchase - What will I miss

[Dwain Lambrigger]

I have been active recently shopping for a new table saw. I want a cabinet saw, within my budget of 600 to 800 dollars. I found a 1948 Unisaw. it looks in fair shape, and according to the seller, it runs really well. My question. It has the original motor, but it is only 3/4 or 1 hp. Being single phase, will I miss anything between this, and a more recent two or three horsepowered cab saw? I really like the idea of old iron, but don't want to buy, then refurbish, then find out it is still too weak to do what I need. Besides, I don't know what the market would be like for an old Unisaw refurbished out here in AZ.

Thanks,

[Julian Wong]

Dwain, A uni cabinet saw of that vintage probably 1 hp repulsion induction motor.

Although it is rated only 1hp, but because of its design and being R/I, it puts out the power of probably closer to a 2hp motor of today. It will suffice unless you plan on cutting 10/4 or 8/4 hard maple all day every day with a full kerf blade.

[Robert Conner]

Those are Great Saws. That motor should do every cut you need to do as Julian said. The motor is very searched for because they run so well. The cast Iron and steel in those older models are thicker then the newer ones. Does it have the Goose Egg Motor Cover? If so grab it, someone else will.
Robert

[Ray Newman]

Old Iron is really nice.

I would ask the seller to make a cross out on a piece of wood 'bout 10" or so in width that you bring. Make sure that the edges are true. After making a cross cut, check it w/ a square. If it is off from blade wobble, the arbor bearings could be worn & need replacing.

You can check the bearings by unplugging the saw, removing the blade & placing a 1" diameter dowel about 24 - 36" long on each bearing & then have someone else turn the arbor by moving the belts while holding the dowel to your ear. This poor man's stethoscope will magnify any noise & bad or worn bearings will make a grinding noise.

See below link for info on rebuilding a Unisaw & other ideas as to what to look for --
http://www.sawcenter.com/unisaw.htm

[Sean Kinn]

Just incase you needed another vote...don't get caught up in the power issue. These old motors are very strong despite their rating. I just cut a bunch of 8/4 red oak and hard maple no problem. I do use a thin kerf ripping blade, but that was just because I already had it from my old saw. Fact of the matter is I'd love a 3 or 5hp saw just like the next guy....but unless you are cutting 8/4 or 10/4 stock all day (and in a hurry to do it) you simply don't need a 3 or 5hp saw. Even running several hundred bf of oak in a shot my 60 year old R/I motor never even got more than slightly warm to the touch (note: I do have it wired for 220).

[William Addison]

I used one a lot and I don't think it cut differently than the 3hp I have now. They are fine old saws.

[Mike Henderson]

Dwain, A uni cabinet saw of that vintage probably 1 hp repulsion induction motor.

Although it is rated only 1hp, but because of its design and being R/I, it puts out the power of probably closer to a 2hp motor of today. It will suffice unless you plan on cutting 10/4 or 8/4 hard maple all day every day with a full kerf blade.

I'm afraid that's not true. The repulsion part of an RI motor is only used during start-up. Once the motor is up to speed, the repulsion part is disconnected and the motor operates as a standard induction motor.

You can tell more by looking at the power into the motor at full load (voltage and current). An electric motor is a power conversion device so the power out cannot exceed the power in, and the power out will actually be less because of conversion losses.

Unless that 1HP motor is grossly under-spec'ed or mis-labeled, it will only put out 1HP under full load. If the service factor is greater than one, it will be able to put out more power for a short period of time, while generating excess heat - but you can't get that power continuously.

RI motors have greater start-up torque than capacitor start motors, but for woodworking applications that's not an important factor because most tools start-up without any load.

Mike

[Wilbur Pan]

I always thought that the advantage of repulsion induction motors over was that for a given horsepower they developed more torque under load.

From "Electric Machines":

Repulsion-Induction Motor

The repulsion principle is often used merely to provide a high starting torque at a a relatively low starting current. When the machine has accelerated close to the operating speed, it may be converted to a single-phase induction motor to provide essentially constant speed running characteristics. This is achieved if, in addition to an outer commutator winding, the rotor carries an inner squirrel-cage winding. The torques developed are then additive. Such a motor is called a repulsion-induction motor. At starting, the cage current is limited by its relatively high reactance and the commutator winding develops a major part of the torque, but during running conditions the cage torque increases, compensating for the natural drop in the speed-torque characteristic of the commutator winding.

And from "Electric Motor Repair":

[Repulsion-induction] motors are also made with compensating coils to increase the power factor of the motor circuit.

If an RI motor can develop torque that would be equivalent to a higher horsepower induction motor, I can see how that might lead one to conclude that a RI motor has "different HP" than a modern day motor. The HP is exactly the same, but different 1 HP motors can generate different amounts of torque.

There's a parallel in looking at cars. HP ratings is what everyone focuses on, but more information on how a car's engine will perform can be gotten from looking at the torque band.

[Jim Andrew]

My experience with old tools is, old horses are bigger than new ones. Jim

[Jim Andrew]

Should also say, if they are in good shape. Sick horses don't pull. Jim

[Mike Henderson]

If an RI motor can develop torque that would be equivalent to a higher horsepower induction motor, I can see how that might lead one to conclude that a RI motor has "different HP" than a modern day motor. The HP is exactly the same, but different 1 HP motors can generate different amounts of torque.

The problem with that theory is that HP is force times velocity. In a motor, it's torque times angular velocity, essentially RPM. That's why universal motors generate such high HP in a small physical size - they rotate at a much higher speed than an induction motor even though they have less torque.

If an RI motor generated more torque than a capacitor start induction motor, at the same RPM, it would have a greater HP rating. So if you start with two motors with the same RPM, say 3450 RPM, they will have the same torque at rated load if they have the same HP rating.

Note that a four pole induction motor, which runs at 1725 RPM (in the US) has to generate twice the torque compared to a 3450 RPM motor to get the same HP rating, and thus is larger physically than a two pole induction motor of the same HP rating.

There's no magic.

Mike

[Also, you can't load an induction motor down to the point where the slip gets really high (and the RPM goes way down). If you did, the current into the motor would exceed the rated value and the motor would overheat fairly quickly. So induction motors run in a fairly narrow RPM band between no-load and full-load. This is different from an engine on a car which has a very wide RPM band.]

[David DeCristoforo]

FWIW, I would buy the '47 uni in a hot second assuming that it is in good working order and is complete (original base, motor cover, switch, etc). If any of the original parts are missing I would try and beat the price down a bit but don't get too carried away because some old iron buff will snap it up, esp if it had the "four legged" base.

[Seth Poorman]

I always thought that older motors ran better because motor windings were made from larger gage wire ,causing less resistance,,,,less resistance (under load) keeps the motor rotating..
Newer motors of today are still rated the same but since they use smaller gage wire,resistance is higher and once under load the power table drops........ I think...

[Wilbur Pan]

Also, you can't load an induction motor down to the point where the slip gets really high (and the RPM goes way down). If you did, the current into the motor would exceed the rated value and the motor would overheat fairly quickly. So induction motors run in a fairly narrow RPM band between no-load and full-load. This is different from an engine on a car which has a very wide RPM band.

I don't quite understand this statement. I think we've all seen a piece of woodworking machinery that was fed a piece of, say, 10/4 white oak and seen/heard the motor slow down, hopefully without burning out.

Your point about motors of equal HP generating equal amounts of torque at equal RPM is true, if the motors are operating at the same RPM. But that isn't always true.

The graph at the bottom of this page of "Electrical Machines" illustrates what I was saying poorly above. The lower curve on the left graph illustrates torque developed over the speed range of an induction motor. The diagram on the right shows torque developed over a range of speeds of a RI motor. Picture both motors powering identical table saws going at the same speed without a load, and toss some 10/4 white oak at them. Both motors will slow down. In the middle range of the graph, the RI motor will have more torque as the speed decreases than the induction motor, and will be able to recover faster. In fact, you can clearly see a region in the middle of the speed range where an induction motor continues to lose torque as it slows, while the RI motor actually gains torque as it slows. I think this is where the perception that an RI motor is "stronger" than a modern day induction motor of the same horsepower rating.

If I'm off about my interpretation of these graphs, I'd be glad to be shown where I went wrong.

[John Cooper2]

Can't speak to the one your looking at, but I bought one of the same vintage for $500.00 and spent a few $$$ on link belts added a few hours of cleanup/tuneup. Now I have a wonderfull old saw that has plenty of power and it's something you don't see everyday.