A single-phase motor has windings spaced on factors of 180 degree intervals... The catch is, the windings only act upon the armature with torque during a fraction of the rotation. For example, on a single-cylinder internal-combustion engine, the piston fires around TDC, but the piston can't transfer full torque to the crank until the crank is at 90-degree angle. Partial torque is available- It's a sinusoidal function... SIN 45 = .707, which means, when the crank is 45 degrees past TDC, the piston's downward force is translated to the crank by only 70%. At 90, it's SIN90 = 1, and by 135 degrees, it's down to 70% again.
A similar issue occurs in single-phase motors- magnets have only two poles- North and South, so it's all based on 180-degree geometry. For example, A single-pole motor's windings are on 180 degree intervals, a double-pole motor are on 90-degree intervals, a four-pole motor's windings are on 45 degree angle. (That's what makes the base speed of each of these at 3600, 1800, and 900rpm at 60hz). Every factor of those intervals creates a point where the single-phase motor has very limited torque, and when you load the motor, the result is vibration through the driveline.
In a 3-phase motor, there's no 180-degree intervals... they're all 120-degree intervals. That means, every factor of 120 degrees, ONE of the three poles is well within 45 degrees of a field, so instead of getting vibration that varies from Sin45 to Sin90 (70 to 100%) torque, it's going from Sin60 to Sin90 (87 to 100%) torque. That's a substantial drop in vibration... it ALSO means, however, that the 3-phase motor can develop full STARTING torque, where a single-phase motor will stall or burn a starting-capacitor.
If you have a 1/2hp drill-press... put it on the highest ratio, and hold the chuck with a really firm grip... and turn the motor on... you'll be able to hold it back. DON'T try that With a 1/2hp 3-phase motor under ANY circumstances.
There's a reason why high-zoot lathes still run belt-drives- gear-head lathes put vibration in the workpiece, which causes ripples... single-phase motors do the same thing. You'll find that for lathe applications, running a 3-phase drive motor yields a better cut quality than a single phase... and the only thing better than 3-phase... is a DC drive system.
But if you've got a motor that fits, put it on and run it... but don't pitch the 3-phase- sooner or later, you'll want it again.
