- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
In all single phase AC induction motors, there is no relative phase rotation so they are not inherently self-starting like 3 phase motors. Something must create that relative rotation first, which generally consists of a phase shift, a delay in torque production between two sets of windings making one chase the other, at least at first. Once spinning, that is no longer necessary because the back EMF of the spinning rotor creates it for itself. So there are 4 main types of Single Phase AC motors; Split Phase, Cap-start / Induction Run (CSIR), Permanent Split Capacitor (PSC), and Cap-Start / Cap-Run (CSCR) ... and Shaded Pole (SP). So there are FIVE, five types of single phase AC induction motors. 
Of those, only 3 of them have capacitors; CSIR, PSC and CSCR.
In PSC, the capacitor in is series with one of the windings to create the phase shift necessary to make it spin and that cap stays in the circuit permanently as a Run cap, and that means the capacitance can't be too high or it will cause problems. So the phase shift it creates and the resultant starting torque that represents, is relatively low; somewhere in the 30-150% range of FLT (the upper ranges are going to have issues with over heating and longevity so are rare). So in general we consider PSC motors as "low starting torque" motors, good for things like centrifugal pumps, centrifugal fans and centrifugal compressors where the actual load doesn't couple with the motor fully until the speed is around 75% or more. The good thing about them is that with the cap in there all the time, they have good power factor and run cooler, so better efficiency compared to others (Split Phase, Shaded Pole and CSIR).
CSIR has only a Starting cap, which is switched out of the circuit at that 75+% speed and can produce 200-400% starting torque, so it is good for harder starting applications, most often the type used in industrials that use single phase because of that. So think of belt drives, conveyors, air compressors, saw motors etc. But they are more expensive than PSC too and the PF and efficiency is lower because there is no running cap, so the long term cost of ownership goes up.
CSCR is kind of a combination of PSC and CSIR in that the starting cap gives it higher starting torque like the CSIR, but the running cap gives it the better efficiency of the PSC. It is also the most expensive so pretty much only gets used over CSIR when the long term running costs will make the higher cost worth it.
Split phase is kind of going by the wayside because they have really high starting current (600-700%) but relatively low starting torque (175%) for it. Their only advantage is that they are the cheapest to make (other than Shaded Pole) because there are no caps at all, the phase shift is just the two sets of windings and a start switch to take one out at 75% speed.
Shaded Pole use a shading ring in the armature to cause a delay in the magnetics, thus the relative rotation. They are very cheap to make, have no moving parts like PSC motors, but also no capacitors at all. The problem is, they are the lowest of low on starting torque (25-75%) and run at high slip so they are very inefficient. PSC motors are generally relegated to really low torque applications like little fans and blowers.
Of those, only 3 of them have capacitors; CSIR, PSC and CSCR. In PSC, the capacitor in is series with one of the windings to create the phase shift necessary to make it spin and that cap stays in the circuit permanently as a Run cap, and that means the capacitance can't be too high or it will cause problems. So the phase shift it creates and the resultant starting torque that represents, is relatively low; somewhere in the 30-150% range of FLT (the upper ranges are going to have issues with over heating and longevity so are rare). So in general we consider PSC motors as "low starting torque" motors, good for things like centrifugal pumps, centrifugal fans and centrifugal compressors where the actual load doesn't couple with the motor fully until the speed is around 75% or more. The good thing about them is that with the cap in there all the time, they have good power factor and run cooler, so better efficiency compared to others (Split Phase, Shaded Pole and CSIR).
CSIR has only a Starting cap, which is switched out of the circuit at that 75+% speed and can produce 200-400% starting torque, so it is good for harder starting applications, most often the type used in industrials that use single phase because of that. So think of belt drives, conveyors, air compressors, saw motors etc. But they are more expensive than PSC too and the PF and efficiency is lower because there is no running cap, so the long term cost of ownership goes up.
CSCR is kind of a combination of PSC and CSIR in that the starting cap gives it higher starting torque like the CSIR, but the running cap gives it the better efficiency of the PSC. It is also the most expensive so pretty much only gets used over CSIR when the long term running costs will make the higher cost worth it.
Split phase is kind of going by the wayside because they have really high starting current (600-700%) but relatively low starting torque (175%) for it. Their only advantage is that they are the cheapest to make (other than Shaded Pole) because there are no caps at all, the phase shift is just the two sets of windings and a start switch to take one out at 75% speed.
Shaded Pole use a shading ring in the armature to cause a delay in the magnetics, thus the relative rotation. They are very cheap to make, have no moving parts like PSC motors, but also no capacitors at all. The problem is, they are the lowest of low on starting torque (25-75%) and run at high slip so they are very inefficient. PSC motors are generally relegated to really low torque applications like little fans and blowers.
