Capacitors
- Thread starter patpappas
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I'll start:
I'll start:
All caps operate on the same principle, but there are many variations for different applications.
A "start" capacitor shifts phase in order to start a single phase motor. After starting, it is switched out of the circuit.
A "run" capacitor also shifts phase but is not switched out.
These caps may be used alone or in parallel.
The PF cap draws a leading current to compensate for the lagging PF from inductive loads.
I'll start:
All caps operate on the same principle, but there are many variations for different applications.
A "start" capacitor shifts phase in order to start a single phase motor. After starting, it is switched out of the circuit.
A "run" capacitor also shifts phase but is not switched out.
These caps may be used alone or in parallel.
The PF cap draws a leading current to compensate for the lagging PF from inductive loads.
patpappas said:
Not exactly. All caps operate on the basis of an electric field between two conductive plates. They may be made of metallic foil plates separated by paper, Mylar, etc, or they may be ceramic or mica discs with metallized plates, or they may be aluminum electrolytic or tantalum electrolytic, or they may use air as a dielectric as in radio turner caps. I have already told you more than I know about caps. There are many varieties on the market.
For example, some integrated circuits use builtin caps made from a sandwich of silicon, silicon dioxide, and silicon. Rectifier filter caps usually are usually electrolytic or tantalytic. Decoupling caps on a PC board are usually ceramics, and the list goes on.
The run and start caps are probably different because the run cap requires a moderate size and 100% duty cycle while the start cap requires a larger value and is active only during startup of the motor.
BTW, I do not claim to be an expert on this matter.
Let me chime in here with a limited degree of knowledge. I work with 450kw uninterruptible power modules, and the difference I see in capacitor construction is readily visible. AC capacitors that we use have no "coating" on the outside of the capacitor, just plain shiny tin cans. DC caps have a plastic-type coating that is supposedly there to assist in containing an explosion. Usually the DC capacitors have a rubber blow-off button on the top for pressure release.
Also as a note, we "condition" the DC capacitors by partially charging them up 20% and then discharge to zero, repeat in 20% increments to full voltage, prior to putting the DC capacitors into service. Supposedly the AC capacitors condition themselves, due to the nature of the AC sine wave. I hope I helped.
Dennis
Also as a note, we "condition" the DC capacitors by partially charging them up 20% and then discharge to zero, repeat in 20% increments to full voltage, prior to putting the DC capacitors into service. Supposedly the AC capacitors condition themselves, due to the nature of the AC sine wave. I hope I helped.
Dennis
DC Caps:
DC Caps:
Dennis, I believe you are referring to an electrolytic cap which utilizes an electrochemical reaction. These are polarized caps, that is you must guarantee that the positive terminal is always positive relative to the negative terminal. The can is often used as the negative terminal and must be insulated.
To my knowledge, no other cap utilizes this electrochemical reaction and therefore need no conditioning.
DC Caps:
Dennis, I believe you are referring to an electrolytic cap which utilizes an electrochemical reaction. These are polarized caps, that is you must guarantee that the positive terminal is always positive relative to the negative terminal. The can is often used as the negative terminal and must be insulated.
To my knowledge, no other cap utilizes this electrochemical reaction and therefore need no conditioning.
LarryFine
Master Electrician Electric Contractor Richmond VA
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The way a capacitor on AC shifts phase is because the current in a capacitor is the greatest while the alternating voltage is changing the most, which is when the voltage is at the lowest (nearest zero).
That means that, while current through a simple resistance peaks at the same time as the voltage does, the current through a capacitor peaks 90 degrees earlier, while the voltage is changing polarity.
That means that, while current through a simple resistance peaks at the same time as the voltage does, the current through a capacitor peaks 90 degrees earlier, while the voltage is changing polarity.
Neat trick!rattus said:
rattus, Steve66, LarrFine,
I am not sure what happened to a reply I tried to post a moment ago, but I'll try again. Thanks for your information and explanations. We do utilize electrolytical capacitors on the DC portion. They are marked for polarity, and I saw what can happen if polarity is not obeyed.
It's useful, for me, to utilize your knowledge and experience to confirm (or correct) what I think and have been doing. Thanks,
Dennis
I am not sure what happened to a reply I tried to post a moment ago, but I'll try again. Thanks for your information and explanations. We do utilize electrolytical capacitors on the DC portion. They are marked for polarity, and I saw what can happen if polarity is not obeyed.
It's useful, for me, to utilize your knowledge and experience to confirm (or correct) what I think and have been doing. Thanks,
Dennis
thunder15j
Member
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- Cali
Start capacitors
Start capacitors
Regarding start capacitors for single phase motors....
I notice that working with single phase 3 wire submersible pump motors that the larger motors, 3 horsepower and above, that the start capacitors are rated at 330 volts. The supply voltage is at 240 volt but with my multimeter (DVOM) I see voltages other than 240 volts. For that matter, when working with any motor that has a start cap., I see voltages other than one might expect to see. Usually higher than the supply voltage. Could someone shed some light on the subject for me? Thanks.
I also will never forget the blast sound of a run capacitor that served as both start and run capacitor that exploded on me. That capacitor metal case came blasting away from the motor. The original cap had failed and I put a lessor rated capacitor on the motor to see if the motor was alright. I also didn't replace the capacitor cover. The motor started and ran for about ten seconds and then pow. When I came back later and put the right capacitor on, the motor ran fine.
Start capacitors
Regarding start capacitors for single phase motors....
I notice that working with single phase 3 wire submersible pump motors that the larger motors, 3 horsepower and above, that the start capacitors are rated at 330 volts. The supply voltage is at 240 volt but with my multimeter (DVOM) I see voltages other than 240 volts. For that matter, when working with any motor that has a start cap., I see voltages other than one might expect to see. Usually higher than the supply voltage. Could someone shed some light on the subject for me? Thanks.
I also will never forget the blast sound of a run capacitor that served as both start and run capacitor that exploded on me. That capacitor metal case came blasting away from the motor. The original cap had failed and I put a lessor rated capacitor on the motor to see if the motor was alright. I also didn't replace the capacitor cover. The motor started and ran for about ten seconds and then pow. When I came back later and put the right capacitor on, the motor ran fine.
In a 240V system, the peak voltages are +/- 339V. Apparently these caps are rated in terms of actual peak voltage with a safety factor built into the ratings.
Overvoltage typically causes the dielectric to break down and short the plates, then it goes kaboom.
Other caps are typically rated in DC working volts and one should specify that voltage well above the maximum expected.
Overvoltage typically causes the dielectric to break down and short the plates, then it goes kaboom.
Other caps are typically rated in DC working volts and one should specify that voltage well above the maximum expected.
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