Single phase Vs two phase Vs three phase: When one has grasped the concept of a three phase motor with the three point rotating magnetic fields, one has grasped the basic concept of most any induction motor. Be it a two pole, 3600 RPM or a four pole, 1800 RPM, or a larger number of poles with a corresponding lower speed. (60=PS/120, for English/European=PS/100) A two phase motor works the same way as a three phase motor, but with a little less torque. But still requires three wires. As such, three phase has won out over time as the most efficient way to transmit power. Two phase essentially has been relegated to the dust bin of history. So the question then becomes how to generate a two phase supply in a three phase world for a single phase motor.
An induction motor will run on single phase, albeit at reduced torque compared to three phase. But starting the motor takes at least two phases. Without getting into the math and magnetics involved, it falls to one of a very few systems. The first, simplist, method is a "Shaded Pole" motor, where a second phase is simulated with magnetism by the "shading coil", a copper band around a portion of the pole piece. But a shaded pole motor has limited torque, severely limited. It works for small fans and clocks, but not real work. But they're cheap and easy to build in mass.
The second method is to use a second winding which is the divided by "start" and "run" functions. Again, the second phase for a "run" function has limited torque and is generally relegated to larger fans, air conditioning heat exchangers and the like. The offset of the "second" phase may be permanent run or a start function that is then switched out once running. The offset is created by the second winding having a slightly different reactance from the main coil. This creates a slightly earlier or slightly later magnetic field, creating the effect of a second phase. This type of motor is commonly known as "split phase".
Another method is to use a capacitor in conjunction with the inductor (coil) to create this same difference in timing, the second phase. The capacitor type motors are the most common in our field of interest because of their much higher starting and running torque. They are also then divided into "capacitor start" and "capacitor run" types, with the former being the more common. Capacitive run motors have a higher run torque but are much more costly to build and run. They do serve some specific purposes where attendance is lower, meaning long term reliability. But as a rule, seldom show up in a "casual" market.
Technically known as "Capacitor Start, Inductive Run" motors, they are the most common and are then divided into multiple voltage and/or multiple speed motors which are (way!) outside this description. For all practical purposes, they are known simply as "single phase motors". The most common being 3600 RPM synchronous (~3450 real) or 1800 RPM synchronous (~1745 real).
The difference between synchronous speed and real speed is called "slip" and is literally where torque comes from. It's what makes AC motors self regulating for speed. As speed falls off (load), slip increases which increases torque (and current). As the current increases, the motor gets warm, to the point where the magic smoke gets out. It can be put back in, but costs so much it's usually cheaper to replace a small motor than to rewind it. As the speed picks up, torque falls off. As synchronous speed is approached, torque falls, reaching zero(0) at synchronous speed. Hence current falling off as the motor is run without a load.
Now to specifics, the originator's concern about a Craftsman (Sears) motor on a machine. Craftsman may have sold an "inductive start inductive run" motor. But I have NEVER seen or dealt with or heard of one in 50 odd years in the field. I have been dealing with electricity far longer, but the 50 odd years is professionally.
The thing about Craftsman motors is a capacitor in a "flat pack" configuration that is buried in the cast base of the motor. Such a capacitor is, to my knowledge, no longer made. While there are replacements around, they too will have fallen in quality, "Q" as it is known in the electronics field. An electrolytic capacitor, a whole 'nuther subject, will fail over time. A "motor starting capacitor" is simply a "non-polarized" 'lytic, larger than a DC 'lytic. If the motor will start, the capacitor is good, or at least usable.
It can be verified with a generic motor starting capacitor by changing (bootlegging) the two wires that come to the base of the motor. Modern Sears appliances use an after-market motor, Emerson I think, who was a customer of US Steel when I worked there. But that also is way outside this discussion.
The start switch, a centrifugal device, is a whole 'nuther issue. Mounted internally on the motor shaft, it usually has some sort of "fly weight" device that reacts to the speed of the motor. Outside of the labor of winding the motor, it is the most expensive part of a single phase motor. Hence the moves over the years to "cheapen" such devices. Add to that the condition of the internal wiring in respect to the insulation getting brittle, stiffening, makes opening an older Craftsman motor a questionable operation at best.
Add to that, most fractional horsepower motors have sleeve bearings, making alignment of the end bells a slow, difficult process on reassembly. My perspective is simply to replace such a motor on question. I have a preference for Baldor (farm duty) TEFC motors. They are a little expensive, but highly reliable. And the sealed case makes them "chip proof". They will likely be beyond the reach of many casual machinists, but worthy of consideration.
Motors are a very deep subject. And understanding them is dependant on many electrical and magnetic theory details. I have attempted to cover as much as possible without getting into mathematics and magnetic theory. And having consumed several hours in typing and a pot and a half (10 cup pot) of coffee, I must concede that I am not as articulate as I am knowledgeable. Here's hoping you got this far. For small motors, it's costly but simpler to replace rather than to repair.
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