# Run and Start Capacitors

#### edrodriguez4962@gmail.com

Joined Dec 6, 2011
1
i need to understand about run and start capacitors

#### Georacer

Joined Nov 25, 2009
5,182
Greetings! And welcome to AAC!

Your post, originally found here (http://forum.allaboutcircuits.com/showthread.php?t=62844), has been moved into its own thread, where it will draw more responses.

Please refrain from "hijacking" existing threads with tangent or off-topic questions. You can create new threads for new questions using the "New Thread" button on the upper left of the page.

#### strantor

Joined Oct 3, 2010
5,636
I think of single phase AC as it relates to single phase motors like a steam train. as in the way it turns linear force into rotational. Single phase pulses directly back and forth. imagine if you stopped a steam train with the push rod extended straight out; without some help ( something pushing on the wheel from a different angle, maybe from the top, or 90 degrees out of phase hint hint), it would never start spinning. once it got spinning (inertia), it wouldn't need any more help. same thing with a single phase motor. The capacitor shifts the phase 90 degrees to get the motor spinning and then once it's up to speed, a centrifugal switch removes the capacitor from the circuit. (that's for a capacitor start motor). A capacitor run motor does the same thing, except without the centrifugal switch. the cap stays in the circuit the whole time.

#### crutschow

Joined Mar 14, 2008
27,727
I think of single phase AC as it relates to single phase motors like a steam train. as in the way it turns linear force into rotational. Single phase pulses directly back and forth. imagine if you stopped a steam train with the push rod extended straight out; without some help ( something pushing on the wheel from a different angle, maybe from the top, or 90 degrees out of phase hint hint),
..................
Train locomotives are built with the cylinders on opposite wheels 90 degrees out of phase for that very reason -- so the train can always start even if one cylinder is at top or bottom dead center.

#### strantor

Joined Oct 3, 2010
5,636
Train locomotives are built with the cylinders on opposite wheels 90 degrees out of phase for that very reason -- so the train can always start even if one cylinder is at top or bottom dead center.
yup. I should add that fact next time I bust out my canned start/run cap analogy.