capacitors
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scott thompson
Senior Member
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- Anaheim, California
Re: capacitors
HID Ballast kits have Capacitors mostly to correct a poor Power Factor.
The Capacitor "Keeps" the VARs (Reactive Power) at the Ballast by "Storing It" in the Capacitor/Winding assemblidge (mostly in the Capacitor!).
HID Ballasts also use a Capacitor to perform additional Lamp operating tasks (High Reactance Ballast kits, for example).
On Induction Motors, the type of Motor is more of a factor than anything.
If the Motor is 1?, the Capacitor(or Capacitors if 2 or more used) is used in the Starting Circuit of a Split Phase Motor.
1? Induction Motors are not "Self Starting", like their Polyphase counterparts are, so they require some means of starting.
Some will have "Start/Run" Capacitors, which use a large value Capacitor for Starting, and a small value Capacitor for Running. These Motors keep the "Auxillary Winding" (The Split Phase Start Winding) in operation throughout the use of the Motor, whereas Split Phase Start type Motors only have the Aux. Winding connected during the initial starting of the AC Motor (centrifugal switch opens the aux. winding at apx. 80% full rotor speed).
PSC (Permanent Split Capacitor) type Motors - commonly used in Ceiling Fans, keep the Aux. Winding connected at all times - plus it is connected via a small value Capacitor.
Shaded Pole Motors do not require this stuff, as the embedded shading pole does the job!
The reason behind Polyphase Induction Motors being "Self Starting" and 1? not being Self Starting, is when the Rotor of the Motor is stationary, a Polyphase circuit will produce a Rotating Magnetic Field - which will make the Rotor begin to move.
A 1? circuit is Stationary, so in order to make the Rotor move, one "Side" of the Stationary Magnetic Field needs to be Reduced.
This is achieved by the Auxillary Winding (or shaded pole).
If a Polyphase Motor has Capacitors connected to it, they would be either for Power Factor Correction, or part of an AC line Filter.
Sorry to toss out such basic explanations, so feel free to ask for more info!
I am sure others have more to add.
BTW, take a look at items at the Technical Reference area of ECN, for some additional information.
Scott
HID Ballast kits have Capacitors mostly to correct a poor Power Factor.
The Capacitor "Keeps" the VARs (Reactive Power) at the Ballast by "Storing It" in the Capacitor/Winding assemblidge (mostly in the Capacitor!).
HID Ballasts also use a Capacitor to perform additional Lamp operating tasks (High Reactance Ballast kits, for example).
On Induction Motors, the type of Motor is more of a factor than anything.
If the Motor is 1?, the Capacitor(or Capacitors if 2 or more used) is used in the Starting Circuit of a Split Phase Motor.
1? Induction Motors are not "Self Starting", like their Polyphase counterparts are, so they require some means of starting.
Some will have "Start/Run" Capacitors, which use a large value Capacitor for Starting, and a small value Capacitor for Running. These Motors keep the "Auxillary Winding" (The Split Phase Start Winding) in operation throughout the use of the Motor, whereas Split Phase Start type Motors only have the Aux. Winding connected during the initial starting of the AC Motor (centrifugal switch opens the aux. winding at apx. 80% full rotor speed).
PSC (Permanent Split Capacitor) type Motors - commonly used in Ceiling Fans, keep the Aux. Winding connected at all times - plus it is connected via a small value Capacitor.
Shaded Pole Motors do not require this stuff, as the embedded shading pole does the job!
The reason behind Polyphase Induction Motors being "Self Starting" and 1? not being Self Starting, is when the Rotor of the Motor is stationary, a Polyphase circuit will produce a Rotating Magnetic Field - which will make the Rotor begin to move.
A 1? circuit is Stationary, so in order to make the Rotor move, one "Side" of the Stationary Magnetic Field needs to be Reduced.
This is achieved by the Auxillary Winding (or shaded pole).
If a Polyphase Motor has Capacitors connected to it, they would be either for Power Factor Correction, or part of an AC line Filter.
Sorry to toss out such basic explanations, so feel free to ask for more info!
I am sure others have more to add.
BTW, take a look at items at the Technical Reference area of ECN, for some additional information.
Scott
rcwilson
Senior Member
- Location
- Redmond, WA
Re: capacitors
Think of a capacitor as a tank for electrical charge, just like a surge tank in a water system. It stores up charge and releases it like a battery. (Just smaller and faster and it can?t hold a charge for long.) In some applications, like the furnace air cleaner, or your computer power supply, the capacitor is used to smooth out the HV DC power that is choppy from being rectified from the AC line. The capacitor smoothes the DC voltage (pressure) the same way a surge tank on a water system smoothes out the water pressure.
On AC, the capacitor is charging and discharging 120 times a second, filling and emptying its tank on every half cycle. It stores up energy in one quarter cycle and puts it back in the next quarter cycle and starts storing up energy with the opposite charge in the next quarter cycle, etc. (Not quite technically correct but close enough for an example.) Think of a tank getting filled and emptied.
An inductor or anything that creates a magnetic field (like a transformer, a motor, a lighting ballast or a conductor carrying a current) also stores up energy in its magnetic field. This also happens 120 times a second on AC. The inductor/transformer/cable/motor builds up its magnetic field by pulling energy from the line and then puts it back as the magnetic field reduces and then builds back up in the opposite direction.
The neat thing is that capacitors and inductors store this energy in opposition to each other. As if each is one cylinder on a two lung motorcycle: one?s sucking when one?s blowing. The energy flows back and forth between the inductor's magnetic field and the capacitor's electric field over the power lines. (Note that none of this energy ever makes it to the load.)
Why does this matter? To make a motor work it has to have a magnetic field to carry the energy across the air space between the winding and the rotor. To make a transformer work it has to have a magnetic field to carry the energy from one winding to the other. Without the field they don?t work. The magnetic field is the transmission fluid needed to get the power from the line to the shaft. I like to think of the VARS from the capacitors or the utility generators as that transmission fluid. Without the fluid, the transmission is dead; the engine may run but no power is going to get to the wheels. To get power flow, you have to first top off the transmission fluid.
When you first energize a motor or a transformer there?s a rush of current to initially build up that magnetic field, then the current settles down to building and collapsing the magnetic field. That current has to come from the utility generators or it could come from a local capacitor. There are fewer wires between the local capacitor and the load than between the load and the generator, hence less voltage drop. That is how the capacitor improves the voltage, it provides the magnetic ?transmission fluid? locally, reducing the current needed from the utility. Less current means less votlage drop.
Why do single-phase motors have capacitors? Filling the ?capacitor tank? creates a lag in the voltage wave. The current fills up the capacitor's tank instead of going to the load. This makes the voltage on the winding downstream of the capacitor peak a little later than the normal voltage so its magnetic field also peaks a little later. Delaying a sine wave peak is the same as shifting the phase. That is how we get the phase shift needed for the motor to develop torque. Two magnets or magnetic fields that are aligned don?t produce torque to line up. They are already in line. When we move one magnetic field by shifting its phase, the rotor?s field tries to align itself, producing the torque. The capacitor helps get that phase shift at the start. On some motors the capacitor is used to keep the motor going, others are designed so the moving rotor helps generate the needed pahse shift.
Small motors can use other means to get that phase shift (shaded poles). Larger single-phase motors like well pumps need the capacitors to help get started and sometimes to help hold the voltage up.
Capacitors in ballasted light fixtures store and supply energy for the ballasts? magnetic field in the same way, reducing the fixture current by improving the power factor.
A resistive load or an incandescent light do not have much of a magnetic field so they don?t need capacitors.
Capacitors do many more and different things. I hope these wordy examples make it a little easier to understand.
Bob Wilson
Think of a capacitor as a tank for electrical charge, just like a surge tank in a water system. It stores up charge and releases it like a battery. (Just smaller and faster and it can?t hold a charge for long.) In some applications, like the furnace air cleaner, or your computer power supply, the capacitor is used to smooth out the HV DC power that is choppy from being rectified from the AC line. The capacitor smoothes the DC voltage (pressure) the same way a surge tank on a water system smoothes out the water pressure.
On AC, the capacitor is charging and discharging 120 times a second, filling and emptying its tank on every half cycle. It stores up energy in one quarter cycle and puts it back in the next quarter cycle and starts storing up energy with the opposite charge in the next quarter cycle, etc. (Not quite technically correct but close enough for an example.) Think of a tank getting filled and emptied.
An inductor or anything that creates a magnetic field (like a transformer, a motor, a lighting ballast or a conductor carrying a current) also stores up energy in its magnetic field. This also happens 120 times a second on AC. The inductor/transformer/cable/motor builds up its magnetic field by pulling energy from the line and then puts it back as the magnetic field reduces and then builds back up in the opposite direction.
The neat thing is that capacitors and inductors store this energy in opposition to each other. As if each is one cylinder on a two lung motorcycle: one?s sucking when one?s blowing. The energy flows back and forth between the inductor's magnetic field and the capacitor's electric field over the power lines. (Note that none of this energy ever makes it to the load.)
Why does this matter? To make a motor work it has to have a magnetic field to carry the energy across the air space between the winding and the rotor. To make a transformer work it has to have a magnetic field to carry the energy from one winding to the other. Without the field they don?t work. The magnetic field is the transmission fluid needed to get the power from the line to the shaft. I like to think of the VARS from the capacitors or the utility generators as that transmission fluid. Without the fluid, the transmission is dead; the engine may run but no power is going to get to the wheels. To get power flow, you have to first top off the transmission fluid.
When you first energize a motor or a transformer there?s a rush of current to initially build up that magnetic field, then the current settles down to building and collapsing the magnetic field. That current has to come from the utility generators or it could come from a local capacitor. There are fewer wires between the local capacitor and the load than between the load and the generator, hence less voltage drop. That is how the capacitor improves the voltage, it provides the magnetic ?transmission fluid? locally, reducing the current needed from the utility. Less current means less votlage drop.
Why do single-phase motors have capacitors? Filling the ?capacitor tank? creates a lag in the voltage wave. The current fills up the capacitor's tank instead of going to the load. This makes the voltage on the winding downstream of the capacitor peak a little later than the normal voltage so its magnetic field also peaks a little later. Delaying a sine wave peak is the same as shifting the phase. That is how we get the phase shift needed for the motor to develop torque. Two magnets or magnetic fields that are aligned don?t produce torque to line up. They are already in line. When we move one magnetic field by shifting its phase, the rotor?s field tries to align itself, producing the torque. The capacitor helps get that phase shift at the start. On some motors the capacitor is used to keep the motor going, others are designed so the moving rotor helps generate the needed pahse shift.
Small motors can use other means to get that phase shift (shaded poles). Larger single-phase motors like well pumps need the capacitors to help get started and sometimes to help hold the voltage up.
Capacitors in ballasted light fixtures store and supply energy for the ballasts? magnetic field in the same way, reducing the fixture current by improving the power factor.
A resistive load or an incandescent light do not have much of a magnetic field so they don?t need capacitors.
Capacitors do many more and different things. I hope these wordy examples make it a little easier to understand.
Bob Wilson
bphgravity
Senior Member
- Location
- Florida
Re: capacitors
What I find interesting is that a "charged" capacitor has more energy than an "uncharged" one but exactly the same net-charge, and the same quantity of + and - particles inside it.
What I find interesting is that a "charged" capacitor has more energy than an "uncharged" one but exactly the same net-charge, and the same quantity of + and - particles inside it.
Ed MacLaren
Senior Member
Re: capacitors
Excellent explanations.
Sometimes a mechanical analogy can be useful. The lagging current, and the storage of energy, in an inductive circuit is similar to the effect that inertia has on the motion of an object.
In a capacitive circuit, the current is leading the voltage, and the effect can be compared to that of a spring in a mechanical assembly.
Ed
Excellent explanations.
Sometimes a mechanical analogy can be useful. The lagging current, and the storage of energy, in an inductive circuit is similar to the effect that inertia has on the motion of an object.
In a capacitive circuit, the current is leading the voltage, and the effect can be compared to that of a spring in a mechanical assembly.
Ed
Re: capacitors
More about capacitors:
At the risk of repeating what has been said so far, here is a partial listing of capacitor uses:
Capacitors?
The capacitor is used in various ways, to wit:
In power supply filters, they store charge which smooths out the voltage ripples from rectifiers.
In capacitor banks, they draw a leading current to compensate for lagging current from inductive loads.
In motors, they provide phase shift for starting and running.
In coupling circuits, they provide a path for AC signals while blocking the DC component.
They provide a low impedance bypass for AC signals around resistors.
They are key components in oscillators and tuned circuits. Radios are usually tuned with variable capacitors.
In RC circuits, they provide time delays.
In certain communication circuits, they convert analog signals to digital and vice versa.
They provide noise suppression in many applications.
They isolate low level analog and digital circuits from power supply noise.
There are, as you might expect, many variations of capacitor designs for the myriad of applications.
[ May 27, 2005, 12:59 AM: Message edited by: rattus ]
More about capacitors:
At the risk of repeating what has been said so far, here is a partial listing of capacitor uses:
Capacitors?
The capacitor is used in various ways, to wit:
In power supply filters, they store charge which smooths out the voltage ripples from rectifiers.
In capacitor banks, they draw a leading current to compensate for lagging current from inductive loads.
In motors, they provide phase shift for starting and running.
In coupling circuits, they provide a path for AC signals while blocking the DC component.
They provide a low impedance bypass for AC signals around resistors.
They are key components in oscillators and tuned circuits. Radios are usually tuned with variable capacitors.
In RC circuits, they provide time delays.
In certain communication circuits, they convert analog signals to digital and vice versa.
They provide noise suppression in many applications.
They isolate low level analog and digital circuits from power supply noise.
There are, as you might expect, many variations of capacitor designs for the myriad of applications.
[ May 27, 2005, 12:59 AM: Message edited by: rattus ]
Re: capacitors
Ed's analogies were used in analog computers where a mechanical problem was converted into an analogous electrical circuit. This circuit was then stimulated and the response plotted.
Advances in digital computers have pretty much made the analog computer obsolete.
Ed's analogies were used in analog computers where a mechanical problem was converted into an analogous electrical circuit. This circuit was then stimulated and the response plotted.
Advances in digital computers have pretty much made the analog computer obsolete.
Re: capacitors
Bryan, it is the position of those charges that count. In the uncharged state, each plate carries no net charge. In the charged state, one plate carries an excess of electrons and the other plate carries a deficit. The electric field between these unbalanced charges holds the energy. In either case, there is zero net charge on the capacitor.
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