I know the more farads a capacitor has generally the more energy it can store but is that energy current trying to figure out how to view it I understand in a A/C unit they are used for phase shift. It doesn't produce current I get that, but it can handle more charge for the inrush. Am I looking at this correctly?
Does a Larger Farad capacitor handle more current?
- Thread starter kjroller
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winnie
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- Springfield, MA, USA
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- Electric motor research
In an AC circuit (as opposed to DC), the greater the microfarad rating, the greater the current flow through the capacitor.
In this application, we aren't really depending upon the capacitor to _store_ energy but rather to provide a phase shift, and that requires storing energy for 1/4 cycle then releasing it for 1/4 cycle then storing it again.
For this application you can think of the capacitor as being sort of a 'resistor with phase shift', and the greater the farad rating of the capacitor the lower the ohms value of the resistor. Now for motor start or run application, capacitor is connected to a motor coil, so the greater the capacitance the greater the current flow through the coil, and that might be a problem for the motor.
In this application, we aren't really depending upon the capacitor to _store_ energy but rather to provide a phase shift, and that requires storing energy for 1/4 cycle then releasing it for 1/4 cycle then storing it again.
For this application you can think of the capacitor as being sort of a 'resistor with phase shift', and the greater the farad rating of the capacitor the lower the ohms value of the resistor. Now for motor start or run application, capacitor is connected to a motor coil, so the greater the capacitance the greater the current flow through the coil, and that might be a problem for the motor.
So would a capacitor increase the startup current of the motor ?
Would this be a accurate description?
Most start capacitors have a much higher MFD (microfarad) rating than the run capacitor, meaning they can store and release much more current. They are also generally electrolytic capacitors instead of oil-filled metal film-type, like a run capacitor.
All this adds up to a start capacitor being able to store and release a lot of current into the start winding, but it only stays in the circuit for a short period of time without damaging itself because it cannot dissipate heat easily like the metal film run capacitor.
The start capacitor is wired in parallel with the run capacitor, as shown in the image above. The potential relay contacts are closed on startup, which means the capacitor is in the circuit with all that electron storage capacity. When the compressor contactor closes, a large amount of current can move into that start winding because there is a larger “membrane” (see part 1 of this series) that can store and release energy. This extra current moving through the start winding helps get the compressor started more quickly,
But the start capacitor must be pulled out of the circuit very quickly to avoid overheating itself or damaging the compressor start winding.
Single-phase compressor start windings are not designed to carry high continuous current like the run winding. If the start capacitor were to stay in the circuit too long, the current on the start winding will stay high and risk damaging the start winding.
Various types of relays and controls can remove a start capacitor from the circuit, but the most common is a potential relay. The potential relay coil is either connected between start and common or start and run, and it is sized to open up when the motor reaches about 80% of its full speed.
The potential relay opens based on an effect called “Back EMF,” which leads us to our next thought experiment.
Most start capacitors have a much higher MFD (microfarad) rating than the run capacitor, meaning they can store and release much more current. They are also generally electrolytic capacitors instead of oil-filled metal film-type, like a run capacitor.
All this adds up to a start capacitor being able to store and release a lot of current into the start winding, but it only stays in the circuit for a short period of time without damaging itself because it cannot dissipate heat easily like the metal film run capacitor.
The start capacitor is wired in parallel with the run capacitor, as shown in the image above. The potential relay contacts are closed on startup, which means the capacitor is in the circuit with all that electron storage capacity. When the compressor contactor closes, a large amount of current can move into that start winding because there is a larger “membrane” (see part 1 of this series) that can store and release energy. This extra current moving through the start winding helps get the compressor started more quickly,
But the start capacitor must be pulled out of the circuit very quickly to avoid overheating itself or damaging the compressor start winding.
Single-phase compressor start windings are not designed to carry high continuous current like the run winding. If the start capacitor were to stay in the circuit too long, the current on the start winding will stay high and risk damaging the start winding.
Various types of relays and controls can remove a start capacitor from the circuit, but the most common is a potential relay. The potential relay coil is either connected between start and common or start and run, and it is sized to open up when the motor reaches about 80% of its full speed.
The potential relay opens based on an effect called “Back EMF,” which leads us to our next thought experiment.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I'd expect a larger farad capacitor to increase the starting current.
That looks like a reasonable description.
It also raises a very important point: the higher farad value means greater current flow, not greater current handling capacity. This is akin to the resistor analogy I made. If you apply 100V to a 1 ohm resistor, 100A will flow. The number of ohms doesn't tell you how many amps the resistor can actually handle, just how much will flow when the volts are applied.
Apply 100V to a 1 ohm 10kW resistive heating element and you will be operating right to spec. Apply 100V to a 1 ohm 5W resistor and you will quickly have a small ball of plasma.
-Jonathan
So instead of looking at it like "storing a charge" how most people say it, it stores current? As it stores current the voltage differential will continue to grow which is why it causes a phase shift which both a run or start cap can normally do however a start cap will help provide more current to the start winding of the motor causing more initial torque until the centrifugal switch kicks it out and then the run cap takes over continuing the phase shift and better power factor? If the start capacitor is left in then the current would be to high and windings would burn up?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Not 'stores current', but passes current.
A capacitor stores charge, so when current flows through it the charge builds up. If you apply a DC voltage (with some series resistance) then the charge will build up until you reach an equilibrium state, the current will die away and the charge will be constant. Usually when you put a capacitor into a DC circuit you care about the stored charge.
But if you apply an AC voltage to a capacitor, the charge continuously builds up and drains, and then builds up with the opposite polarity and then drains. When we put a capacitor in an AC circuit, what we care about is the current flow, not the stored charge (which is constantly changing and regularly passing through zero).
Yes to the start cap leading to higher starting current and higher torque.
Yes to the start cap leading to too much current for continuous operation.
A capacitor stores charge, so when current flows through it the charge builds up. If you apply a DC voltage (with some series resistance) then the charge will build up until you reach an equilibrium state, the current will die away and the charge will be constant. Usually when you put a capacitor into a DC circuit you care about the stored charge.
But if you apply an AC voltage to a capacitor, the charge continuously builds up and drains, and then builds up with the opposite polarity and then drains. When we put a capacitor in an AC circuit, what we care about is the current flow, not the stored charge (which is constantly changing and regularly passing through zero).
Yes to the start cap leading to higher starting current and higher torque.
Yes to the start cap leading to too much current for continuous operation.
Joethemechanic
Senior Member
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- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
They are in series with the inductance (winding) to improve power factor they have to be in parallel with the inductance.
Chances are a start capacitor would burn up before the winding. They are a type of electrolytic. Now if too large of a run cap was left in the circuit, yeah it could conceivably burn out the winding
Start caps are usually plastic cased "non-polarized aluminium electrolytic capacitors"
Run caps are always metallic cased
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Depending on the style of motor, Cap Start, Cap Start Cap Run, or Permanent Split you can blow a cap or burn a winding or both.
Sometimes you can lose the running winding and the motor will start, the starting circuit will open and the motor will coast till the starting switch closes and it will start again and so on until a cap blows or someone notices.