Discuss Can someone once and forall tell me what a capacitor does in an AC circuit?Thanks. in the UK Electrical Forum area at ElectriciansForums.net
here endeth the first lesson.There are different ways to look at it, but one analogy is:
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.
- Capacitors try to prevent voltage changing
- Inductors try to prevent current changing
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?There are different ways to look at it, but one analogy is:
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.
- Capacitors try to prevent voltage changing
- Inductors try to prevent current changing
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?
If I had a capacitor across a winding in a three phase motor, then what difference would that make?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?
Yes, but not in any sane way.Yes but can I use capacitors to increase voltage across motor windings?
So a capacitor is used to generate a rotating magnetic field in an electric motor, how?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)
Does that means the second and third windings receive current after the first?So a capacitor is used to generate a rotating magnetic field in an electric motor, how?
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.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.
Thank you for sharing your knowledge with me.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.
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.
Reply to Can someone once and forall tell me what a capacitor does in an AC circuit?Thanks. in the UK Electrical Forum area at ElectriciansForums.net
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