Electric motor definitions

what does split phase mean?

what is a capacitor?

What's the difference between a capacitor start and a capacitor start/run motor?

What kind of motor has the the most torque?

Does a capacitor help maintain torque levels after the load is started?

Thanks.

All AC motors require a rotating field to start. This includes induction and synchronous motors. (Brush-type motors are really DC motors, for all intents and purposes.) In the case of three-phase motors, the three-phase power creates a rotating field. But with a single-phase motor, some sort of starting circuit must be added to create this rotating field.

The cheapest single-phase motor is the split-phase motor, which just has an extra winding that shifts the phase of the incoming power to create a rotating field. These motors have very low starting torque, but are good choices for many applications such as fans.

For applications that need more starting torque, a capacitor-start motor is used. It has a centrifugal switch that applies power to a starting winding through a capacitor. The capacitor shifts the phase of the incoming power to create a rotating field. When the motor gets up to speed, the centrifugal switch kicks out so that power is applied only to the run winding.

A capacitor-start/capacitor-run motor has a second capacitor connected to the run winding. This improves running torque and increases the motor's power factor.

Most AC motors are of the induction type. Induction motors "slip" a little bit from their no-load speed. Some applications use synchronous motors, which run locked to the phase of the incoming power. Synchronous motors are used for applications that require a precise speed, such as clocks.

Q what does split phase mean?

Single phase motors are NOT self starting like a 3 phase, they would just sit there n hum n not know which direction to rotate if voltage were applied to a run winding. If one gave em a good spin in one direction they could start up n then run that way. Therefore 2 (splt) winding phases are utilized, a start and a run winding/phase. The start winding is physically offset from the run so the motor begins rotating in that direction and at a certain RPM a centrigugal switch drops out and she runs on the Run winding ONLY in the direction the start winding started it. Split phase can be regular split phase (lower starting torque) or Capacitor Start split phase (more starting torque)

Q what is a capacitor?

A capacitor is a passive energy storage device consisitng basically of 2 conductive plates electrically insulated from each other. It passes AC (but has frequency dependant impedance, sorta like AC resistance) but once charged its like an open circuit to DC. If a capacitor is placed in series in the start winding/phase it alters the electrical phase angle since its current is leading while its voltage is lagging (due to capacitive affect described above) and it sort of alters the starting characteristics/phase yielding more starting torque.

Q What's the difference between a capacitor start and a capacitor start/run motor?

A capacitor start simply is a spit phase having the capacitor in series in the start winding to increase starting torque while a capacitor start n run also has a capacitor in the run winding.

Q What kind of motor has the the most torque?

A Capacitor start split phase motor has more starting torque then a split phase and a capacitor start n run (has a capacitor also in run winding) has the motor windings inductive affect balanced somewhat by the capacitance. An inductor (motor winding) sort of behaves the opposite from a capacitor at AC and the two used together can help cancel out each others effects.

EXTRA INFO In addition to split phase and capacitor start split phase, the methods used to start single phase AC motors, if the start requires extreme low torque (say a small fan motor) they can use a "shaded pole' start motor in which the run winding stays in all the time n dont drop out at RPM like a regular split phase motor.

If extreme high starting torque is required, they use a "repulsion start induction run" motor that has brushes n a commutator type of starting circuit and at a certain RPM the brushes lift away from the commutator and then she runs like a regular AC induction motor.....

Q Does a capacitor help maintain torque levels after the load is started?

Once its started a capacitor in the run winding improves the torque characteristics since as described above the windings inductance can be offset/balanced/counteracted by the capacitance.

NOTE AND DISCLAIMER

Its impossible to describe motor theory and a subject that whole books n courses are written about in a few paragraphs here so the above is an oversimplification and CERTAINLY NOT perfect n accurate, its the best I can do in a short time so before anyone jumps down my throat think about it lol. My old engineering buddy Gerald J is deeper in motors especially DC then I am so believe him over meeeee eeeee if we disagree.

Thats my story n Ima stickin to it

John T (Long retired EE n a lil rusty but I gave it my best shot in the short time available)

Your dissertation is most informative.....I think I could spend a few semesters under your tutelage!

My problems/interest began when I started picking up the odd motor here and there for various projects and learned the hard way that split phase motors 'ain't got it' if the application requires starting under any load at all. I also learned that direct loads pull them down too. I bought a capacitor motor next and the difference was night and day.

I like power and I want it to be applied from the start and during the task. I needed this information so if and when I go to buy motors, I will know what type to buy. Thank you for your help.

John T

Im runnin outa smarts here lol but love the subject, need a lil helppppp ppp

John T

What I have learned is that sellers are mighty proud of their property when they go up for sale and manufacturers are downright disgusting!

I'd like to hang a really stout motor on my table saw...which I don't use much, but when I do.....I want it to walk right through a piece of oak like it was pine. I can't begin to express how it irks me to have the damn thing bog and bind!

My bench grinder is an arbor set-up and that weak-kneed 1/2 horse has got to go as well. It'll do for sharpening/buffing....but when you want to lay into something..it goes south in a hurry.

If I wouldn't have to mortgage my cojones, I'd have a cap start-run motor on everything!

Thanks again, all of you have contributed great info.

I also have a 1hp three phase grinder I picked up nearby off Ebay. I was kind of hesitant, as it is a 10" grinder and only runs 1750rpm. I had been using a 3/4hp 8" Craftsman at work and didn't know if the 1hp would swing those larger wheels. Uhhh, it does just FINE. I have yet to even bog it down when sharpening lawn mower or bush hog blades.

I know a man who runs a 3 phase milling machine off single phase. He took a motor....I don't know if it is 3 phase or not..maybe it is.....and he somehow wired it ahead of the milling machine motor. To start the mill, he flips on the juice and gives the slave motor a spin by rolling it's pulley with his foot (maybe he spins it and then turns on the electric)...anyway once the slave motor takes off, the mill will too! Maybe some of you will explain this to me as well. I have heard that the 3 phase motor running on single phase will not generate as much power as rated. One thing for sure.....you can buy 3 phase motors for nothing compared to single phase.

anyway, Tom told me the single phase power line induces the other 2 phases in its windings and the motor is then wired to the various 3 phase shop equipment motors

he said it wouldn't have as much oomph as real 3 phase but got er done.