Can someone once and forall tell me what a capacitor does in an AC circuit?Thanks.

[Pcg127]

Can someone tell me once and for all, what a capacitor does in an AC circuit?

 

[plugsandsparks]

It accumulates charge and then discharges to the best of its ability.
Hope this helps

 

[telectrix]

this will help.

https://www.electronics-tutorials.ws/capacitor/cap_8.html

 

[loz2754]

Lots of things. It can smooth the bump of a rectified waveform.
It can block DC.
It can be used in conjunction with an inductor to reduce EMI.
And ashit ton of other stuff.

Why the "once and for all"? Has this been on your mind for a while?

 

[telectrix]

can also be used to correct power factor of an inductive load.

 

[telectrix]

a useful function is to charge up to a dc voltage, put in toolbox. plumbers and customers' carpet crawlers and cats will then learn to keep their grubby mitts out.

 

[pc1966]

There are different ways to look at it, but one analogy is:

• Capacitors try to prevent voltage changing
• Inductors try to prevent current changing

So when you try to increase the voltage across a capacitor you need to put current in to charge it, this makes it hard to change voltage suddenly as the required current is given by I = C * dV/dt so a capacitor provides "smoothing" and stores electric charge in doing so.

An inductor need a voltage applied in order to change the current flow. So if you try to interrupt current in an inductive circuit (e.g. motor, relay coil, etc) you get a high voltage generate opposing that. The voltage is given by V = L * dI/dt and that can cause damage to switches or solid state controls, but is sometimes of use in cases such as car ignition coils.

The rate of change terms (dV/dt and dI/dt) for the useful case of a sine wave is given by 2 * PI * f which leads to the concept of capacitive and inductive reactance, analogous to resistance in opposing current flow, but not in-phase with the voltage as it is the changing aspect that matters. That is at its maximum when the voltage is crossing zero.

 

[telectrix]

There are different ways to look at it, but one analogy is:

• Capacitors try to prevent voltage changing
• Inductors try to prevent current changing

So when you try to increase the voltage across a capacitor you need to put current in to charge it, this makes it hard to change voltage suddenly as the required current is given by I = C * dV/dt so a capacitor provides "smoothing" and stores electric charge in doing so.

An inductor need a voltage applied in order to change the current flow. So if you try to interrupt current in an inductive circuit (e.g. motor, relay coil, etc) you get a high voltage generate opposing that. The voltage is given by V = L * dI/dt and that can cause damage to switches or solid state controls, but is sometimes of use in cases such as car ignition coils.

The rate of change terms (dV/dt and dI/dt) for the useful case of a sine wave is given by 2 * PI * f which leads to the concept of capacitive and inductive reactance, analogous to resistance in opposing current flow, but not in-phase with the voltage as it is the changing aspect that matters. That is at its maximum when the voltage is crossing zero.

here endeth the first lesson.

 

[Pcg127]

There are different ways to look at it, but one analogy is:

• Capacitors try to prevent voltage changing
• Inductors try to prevent current changing

So when you try to increase the voltage across a capacitor you need to put current in to charge it, this makes it hard to change voltage suddenly as the required current is given by I = C * dV/dt so a capacitor provides "smoothing" and stores electric charge in doing so.

An inductor need a voltage applied in order to change the current flow. So if you try to interrupt current in an inductive circuit (e.g. motor, relay coil, etc) you get a high voltage generate opposing that. The voltage is given by V = L * dI/dt and that can cause damage to switches or solid state controls, but is sometimes of use in cases such as car ignition coils.

The rate of change terms (dV/dt and dI/dt) for the useful case of a sine wave is given by 2 * PI * f which leads to the concept of capacitive and inductive reactance, analogous to resistance in opposing current flow, but not in-phase with the voltage as it is the changing aspect that matters. That is at its maximum when the voltage is crossing zero.

Yes but can I use capacitors to increase voltage across motor windings?

 

[Pcg127]

Yes but can I use capacitors to increase voltage across motor windings?

Lots of things. It can smooth the bump of a rectified waveform.
It can block DC.
It can be used in conjunction with an inductor to reduce EMI.
And ashit ton of other stuff.

Why the "once and for all"? Has this been on your mind for a while?

If I had a capacitor across a winding in a three phase motor, then what difference would that make?

 

[pc1966]

Yes but can I use capacitors to increase voltage across motor windings?

Yes, but not in any sane way.

Usually a capacitor is used with a motor either to phase-shift the supply so a single phase can generate a "rotating" magnetic field, or as power factor correction so the motor plus capacitor is nearly resistive (i.e. PF close to 1)

 

[Pcg127]

Yes, but not in any sane way.

Usually a capacitor is used with a motor either to phase-shift the supply so a single phase can generate a "rotating" magnetic field, or as power factor correction so the motor plus capacitor is nearly resistive (i.e. PF close to 1)

So a capacitor is used to generate a rotating magnetic field in an electric motor, how?

 

[Lucien Nunes]

The current through a capacitor is proportional to the rate of change of voltage across it (I=C.dv/dt). A capacitor in a series circuit can therefore be used to change the phase of the current in that circuit relative to the supply voltage. If you have two identical windings fed from a common AC supply, one with a capacitor in series and one without, the phase of the currents will be different (assuming the capacitor is of a suitable capacitance) and so will be the phase of the magnetomotive forces and hence fluxes created by the windings. Place the two windings mechanically at an angle to one another in the motor and they will create a rotating flux. The radial axis along which the peak flux passes will progress around the stator, first aligning with the capacitor winding then with the non-capacitor winding, repeating for each half-cycle of AC.

 

[Pcg127]

So a capacitor is used to generate a rotating magnetic field in an electric motor, how?

Does that means the second and third windings receive current after the first?

 

[Lucien Nunes]

Yes. However many windings are in use, either two or three, they should all receive a sinusoidal current where the relative phase angles correspond to the physical angles of displacement of the coils around the stator in electrical degrees (which may be a multiple of the number of mechanical degrees, depending on how many pole-pairs the motor has.) Single-phase capacitor motors typically have two windings not three, in which case the currents might be arranged to differ by 90 electrical degrees i.e. a quarter of a cycle. A 3-phase motor re-purposed to run on single-phase by adding capacitors, should be arranged to receive three peaks 120 electrical degrees apart, as per a normal 3-phase supply.

 

[Pcg127]

How does a capacitor across a one of the windings in 3 phase motor running off a single phase supply cause it to rotate and generate a 3 phase supply?

 

[Lucien Nunes]

Although purpose-made motors and generators differ in detail, fundamentally they are identical machines that can serve each other's roles up to a point. What determines whether a machine works as one or the other is where the power is put in (mechanically or electrically) and taken out. Connect a machine to the grid and attach a brake, and it will run as a motor converting electrical power to mechanical. Replace the brake with a Diesel engine and the same machine will run as a generator.

A phase converter has multiple windings and does both at once. The winding energised by the single-phase supply receives power and functions as a motor, turning the rotor. The rotation causes the non-energised windings to function as a generator, from which current (and hence power) is delivered to the load. Part of the load power is converted from electrical to mechanical and back to electrical. The phase relationships of the three output phases are created by the mechanical positions of the windings relative to the rotating flux vector in the converter, just as in a mechanically-driven generator.

That is true as far as real power flow is concerned. There are many subtleties and complexities about phase converters due to the reactive power flows, but you will need some AC circuit theory knowledge to analyse those and the functions of the capacitors in a practical converter setup.

 

[Pcg127]

Although purpose-made motors and generators differ in detail, fundamentally they are identical machines that can serve each other's roles up to a point. What determines whether a machine works as one or the other is where the power is put in (mechanically or electrically) and taken out. Connect a machine to the grid and attach a brake, and it will run as a motor converting electrical power to mechanical. Replace the brake with a Diesel engine and the same machine will run as a generator.

A phase converter has multiple windings and does both at once. The winding energised by the single-phase supply receives power and functions as a motor, turning the rotor. The rotation causes the non-energised windings to function as a generator, from which current (and hence power) is delivered to the load. Part of the load power is converted from electrical to mechanical and back to electrical. The phase relationships of the three output phases are created by the mechanical positions of the windings relative to the rotating flux vector in the converter, just as in a mechanically-driven generator.

That is true as far as real power flow is concerned. There are many subtleties and complexities about phase converters due to the reactive power flows, but you will need some AC circuit theory knowledge to analyse those and the functions of the capacitors in a practical converter setup.

I understand much better now,but do the capacitors in parallel with the none energised windings increase voltage the to same value of the single phase supply (230v) so you get 415v phase to phase at the motor output.

 

[Lucien Nunes]

They provide excitation current for the converter. An induction machine driven as a generator won't generate in isolation, because there is no source of excitation power to set up the magnetic flux and hence rotor current. When connected to an AC supply network the machine draws magnetising vars from the network, while delivering in-phase real power watts back to the network.

For an induction generator working as a stand-alone, the magnetising vars can instead come from capacitors connected across the output, which produce a current that leads the stator voltage. For the rotary phase converter the excitation provided by the supply is not equal on all phases, hence an asymmetric arrangement of capacitors is used to compensate for this and provide excitation that gives as near a balanced and correctly-phased output as possible.

 

[Pcg127]

They provide excitation current for the converter. An induction machine driven as a generator won't generate in isolation, because there is no source of excitation power to set up the magnetic flux and hence rotor current. When connected to an AC supply network the machine draws magnetising vars from the network, while delivering in-phase real power watts back to the network.

For an induction generator working as a stand-alone, the magnetising vars can instead come from capacitors connected across the output, which produce a current that leads the stator voltage. For the rotary phase converter the excitation provided by the supply is not equal on all phases, hence an asymmetric arrangement of capacitors is used to compensate for this and provide excitation that gives as near a balanced and correctly-phased output as possible.

Thank you for sharing your knowledge with me.