Motor Capacitors

http://en.wikipedia.org/wiki/Capacitance

Energy: Capacitors are energy storage devices, similar to springs. Just as a compressed spring stores enery, a capacitor stores energy, but in the form of an electrostatic charge.

Steady state DC: Capacitors appear to be open circuits to DC.

Time domain DC: When you apply a voltage to a capacitor, it initially appears to be a short circuit, then becomes an open circuit over time.

AC: Current through a capacitor leads the voltage by 90 degrees.

I could cite a few more examples and give you formulas, but you get the idea. The simplest explanation is that they are energy storage devices, and whatever energy a capacitor stores must eventially be released.

Here's what happens. If I apply a voltage across a capacitor, the voltage (electrical pressure) will push electrons onto one plate and pull them off the other, until enough charges have moved such that the electrostatic voltage between the plates is equal to the applied voltage. One plate now has a negative charge (extra electrons) and the other has a positive charge (missing electrons), and I have stored energy in the form of the electrostatic field between the plates.

Now, lets look at the case where I apply a DC voltage to a capacitor. When I first apply the voltage, such as closing a switch, there will be current flowing across the capacitor until the voltage between the capacitor plates reaches equilibrium with the applied voltage. Once the voltages reach equilibrium, the applied voltage cannot push any more electrons onto one plate or pull any more off the other, and current stops flowing. The number of electrons required to charge the capacitor to a particular voltage is a function of the value of the capacitor, measured in farads (or micro-farads), named after Michael Faraday who first described how a capacitor works.

This is why you can test a capacitor with an ohmmeter. An ohmmeter works by applying a known voltage (usually only a volt or two) across the resistance your're trying to measure, and measuring how much current flows. When you first connect the meter, the voltage applied to the capacitor is different than the voltage between the capacitor plates, and current flows and the capacitor shows a low resistance. As soon as the capacitor charges to the test voltage, current stops flowing and the meter reads a high resistance.

Now consider the case where I have a constantly changing voltage, such as the 120 volt AC in your house, applied across a capacitor. Because the applied voltage is constantly changing, electrons are constantly flowing onto and off of the plates trying to keep up. And yes, the direction of the charge changes 120 times per second.

Capacitors are used to induce a phase shift (change in time relationship between current and voltage) to create a rotating magneitc field in a motor. In a pure resistive load, the flow of electrons is directly proportional to the applied voltage. However, in a capacitor, the flow of electrons is constantly trying to keep up with the CHANGES in the voltage, which means that peak current flows at the point in time during the alternating cycle where the voltage is CHANGING the fastest, not where the voltage is highest.

In a simple example (and ignoring a bunch of other stuff for you purists) putting a capacitor in series with part of the winding in a motor causes the current through the winding to be shifted in time compared to the current through the remainder of the winding. By physically offsetting the second winding in the motor, I can create a rotating magnetic field.

Realize this is kind of long, but hope its understandable.

Keith
(practicing electrical engineer, 30+ years)

PS...a pure capacitor does not have resonance. Resonance occurs when a capacitance and inductance are paired, and the phase shift induced by the inductance exactly cancels the phase shift induced by the capacitor. Real capacitors may show a resonance at high frequencies due to stray inductance in their construction, but thats an undesireable effect that circut designers try to avoid.

Take care n God Bless

John T

Capacitors don't "pass" anything. Think of 'em as an electrical shock absorber with each side or "plate" filling and then discharging back out the same way the juice came in.

So, when they are hooked in an A/C circuit, by thier very "shock asorber" nature they will pass the "effect" of A/C without actually passing any current.

Allan

A capacitors (still depends on its capacitance of course) hardly even noticeable at high frequency but youre right, at DC at which it will NOT pass current through after the charges have built up on the opposite plates. If you place an ohmmeter on the R x 10,000 scale across a discharged capacitor it will show low resistance and will conduct current until the charges are built up, but then the meter will swing/kick back towared infinity ohms i.e. an open circuit to the meters DC current.

Your "shock absorber" analogy isnt all that bad cuz a capacitor stores energy and opposes and resists changes in voltage across it i.e. it can be said to shock absorb I reckon cuz it will absorb or dump energy in an effort to maintain its voltage GOOD MECHANICAL ANALOGY

Hope this helps yall but it just cant be explained in a short paragraph what whole books are devoted to grrrrr r

Yall take care now n best wishes n God Bless

John T Long Retired Elecrical Engineer

A capacitor is made up of two conductive plates that are seperated by an insulator dielectric. Its a totally passive energy storage device which opposes changes in voltage across it unlike an inductor which opposes changes in current through it. Following the initial current conductive charge up of + charges on one plate and - charges on the opposite plate IT BECOMES LIKE AN OPEN CIRCUIT AND DOESNT PASS MORE DC. However, it will allow AC current to flow through it due the the alternating charge up n discharge of opposite charges on the opposite plates and as the AC frequency increases the capacitors impedance decreases i.e. the higher the frequency the easier it is (less impedance) to pass current through it. I always got the L and C mixed up lol but I believe the formula for capacitave reactance is like 1 over j omeca C and for an inductor its j omega L. Impedance is sort of like frequency dependent resistance i.e. as the frequency increases the resistance to AC current flow decreases but as its lowered closer to DC, the resistance increases i.e. it blocks DC but passes AC.

Soooo whay are they used on split phase motors????? Its to increase starting torque over non capacitor split phase motors. They are used to change the phase angle of the current in the start winding (current leads the voltage in a capacitor) which sort of fools the start winding in to where its physically located which is similar to giving it more of an angle/leverage relative to the run winding SOMETHING LIKE THAT, AGAIN, I CANT EXPLAIN AN ENTIRE EE COURSE IN A PARAGRAPH AND AM OVERSIMPLIFYING TO HELP A LAY PERSON UNDERSTAND THE BASICS.

It increases starting torque and is a passive energy storage device which opposes voltage changes across it and its Impedance increases as frequency decreases.

If you need more check out a book and delve into it more I reckon I gotta go eat supper so hope this helps some at least

John T Long retired EE

• Hobby farm

Becareful with the new ones. I keep an inventory of the old brands. Stay away from the run caps. known as "PACKARD".

Thanks. John,PA

Sooooo ooooo , Its not so much it stores any voltage to increase starting torque, its cuz it changes the starting currents phase angle

Clear as mud huh lol

Yall take care n God Bless

I enjoy your website old

John T

• Hobby farm

Take care

John T

That's about all they do is suck up small amounts (or large, if big enough) and release it over and over.

They can be built to resonate, or work best at different specific frequencies.

And can have specific preset amounts of storage. mfd's

and most will have a maximum working voltage attached to them also.