Lets talk about wood working motors

[Jeff Bratt]

I'm trying to understand this slip thing ... Maybe someone has a link to a graph that shows all these relationships. I can't find one the has them all on the same page...

From the link I supplied earlier, an illustrative graph for a type B motor...



When an unloaded motor starts, it starts from A and goes on through through to point E very quickly. No-load speed is just slightly less than synchronous speed - 3600 rpm for a 2-pole motor, 1800 rpm for a 4-pole motor. As the load on the motor increases, it slows down - which means the slip increases, the current draw increases, and the torque increases, until you reach point D. The motor rated speed, current, torque, and slip are all measured at point D. Increasing the load slows the motor down more, and increases all the other things, heading towards point C. If you increase the load so the motor slows down past point C, the motor stalls.

[Roger Frazee]

Excellent Jeff...... thanks....

[Chip Lindley]

To add minimally to all that has been offered so far, I will leave the technical aspects to others. In my 25 years experience, most problems with machines run on a 15A, 115V circuit at home concern the fact that the circuit has other draws on it besides that contractor-type TS.

IF the 15A circuit were dedicated entirely to the saw, it still may or may not be sufficient. An old Rockwell CS with 15A (1.5hp) never failed to start for me on 115V, but did dim the lights. The saw shared the circuit with lighting and whatever else was plugged into the garage wall sockets. During heavy cuts, the branch circuit breaker tripped often. The overload button on the motor never tripped!

Just the opposite would occur with an *under-powered* saw. During heavy cuts, a *lite* motor on the minimal side of 1hp would trip the motor's overload before the branch breaker were tripped. Consumer CS-type saws are simply not intended to rip 8/4 oak! Consumers rip pine, plywood and mdf.

Practically speaking, if a dedicated 30A/115V circuit must be run to successfully run a 2hp motor on 115V, that motor might as well be switched to 230V, using existing 12ga wire. 230V circuits opens up many possibilities beyond the 1.5hp *barrier*....including running a rotary phase converter to power dirt-cheap 3-phase machinery!!

[Jeff Bratt]

I'll add a little more about motor overloading. If you look at this graph again:



you can see there is an area between points D and C where the motor will still operate (at a slower that rated speed), but is driving more than its rated load. These motors have a "service factor" that specifies that the motor can be operated 15%, 25% or possibly 50% above the rated horsepower for short periods of time. The graph in this example shows the stall point C at over 200% of rated horsepower. While the motor will run in this area, the ultimate limitation is heat. More current flowing in the motor windings means much more heat is being generated. The class of insulation used in the windings determines how hot the motor can get before it "burns out". And that is literally what happens - the insulation melts, the windings short-circuit, and the motor is toast. For a totally enclosed motor, you can understand why the cooling fan is important part of the design considerations.

The actual performance of each individual motor may vary slightly, but all should be within their nameplate ratings. And while these ratings are not absolute limits, operating a motor in its "overload area" for extended periods of times will eventually cause trouble. This is why some motors have thermal circuit breakers in them to try and prevent overheating the coils. Also starters or contactors for higher horsepower motors often have "heaters" in them calibrated to the horsepower of the motor being controlled. These monitor the current flowing through the motor and shut it down if overload conditions persist for too long.