unisaw information/manual

[fredhargis]

There's a TECO VFD I see mentioned often that somewhere in the $200 range, if I recall. Someone else will have to give you the details. A new replacement motor can run from $450 to over $600, but you might find a used one somewhere (check e bay and you might even find one at OWWM, you could probably sell the old one there as well). One thing about the Unisaw motors is that bracketing that's just a little unique so you can't just go buy any old motor.

[TDKPE]

Based upon a little research so far, it seems the 3 phase offers less draw on electrical resources in use. Does it translate to any more actual power when actually putting saw blade through wood? 

Lotsa misinformation out there about motors.  To drive a 3 hp motor with 3 hp of actual load takes 746 x 3 watts of energy, divided by the efficiency, and by the power factor.  Typical 3 hp motor would be 3 hp x 746W/hp / (0.8 x 0.8)* = 3497 volt-amps.  Dividing by operating voltage (230V is standard motor design voltage) gives 15.2A on a 240V supply (120V and 240V are nominal voltages is North America).

A three-phase motor with the same values would be the same, except that you divide by the square root of 3 to get the current in each of the three supply legs.  That would be about 8.8A at 240V, which is where the idea of lower draw comes from.  But it’s not - it’s just divided among three phase conductors instead of two.  

Three-phase motors tend to be smoother running, however, since both input and output power is constant rather than the pulsing input power of single-phase, where current goes to zero every 1/120th second while reversing direction, but with only two conductors, that means it’s essentially shut off.  Kind of like how a piston engine works - kinda.

With three-phase, while each winding is doing what a single-phase winding is doing, there are three of them, and the way they’re spaced out in time (120 degrees), the total power from all three of them is constant.  Which is why three-phase was invented in the first place - the total power in the three phase conductors is constant, as is the input power to the generator, and the output power from any load using it.

But having said all that, 3 shaft hp is 3 shaft hp, whether it’s from single-phase, 3-phase, direct current, or three horses (or 3000 hamsters in a wheel).

So no, you don’t get anything extra from a 3-phase motor other than perhaps smoother operation and easier maintenance (one moving part, with no centrifugal start switch and no capacitors).

*80% efficiency and 80% power factor.  No energy conversion is 100% efficient, and motors this size usually run in the 80% neighborhood. Motors in the hundreds and thousands of hp get into the upper 90’s.  Power factor is the percentage of the current that’s in phase with the voltage, which won’t be 100% in just about any inductive load, like an induction motor or transformer, but with a run capacitor like in larger single-phase motors, it gets real close, like 98%.  The higher the efficiency and/or power factor, the lower the current for the same output power, which is one reason they’re used on larger single-phase motors.

[Bob Vaughan]

Just occurred to me this was one of the pictures he sent me. Does this offer a clue for single-phase or triple-phase?

FWIW,  a mag starter of that quality level is around $400.00 these days.  That stye is one of my favorites.
Note that the overload is set for 7 amps.  Clearly it is three phase.

[kurt18947]

FWIW,  a mag starter of that quality level is around $400.00 these days.  That stye is one of my favorites.
Note that the overload is set for 7 amps.  Clearly it is three phase.

How does a VFD figure into that? Would he need a mag starter AND VFD? If not it'd make a VFD look pretty cheap.

[Bob Vaughan]

How does a VFD figure into that? Would he need a mag starter AND VFD? If not it'd make a VFD look pretty cheap.

I was pointing out that the mag starter is a good one.  I don't see where I said it went with a VFD.

In my single phase home workshop I have machines run with a VFD and I also use a rotary phase converter for other machines.  If I were setting up a Unisaw, I'd get a single phase motor and rewire that starter for the motor and get on with woodworking.