Leading and Lagging AC Power

[OllieCramer]

Hi,

I'm very new to electronics and power systems and have started learning about the Complex, Apparent, Real and Reactive power and realise that the power factor effectively defines how efficient a system can be. To increase the efficiency (useful power outputted) the power factor needs to be as close to 1 as possible.

My question is that if an Inductive load causes Lagging (θv – θi > 0) and a Capacitive load causes Leading (θv – θi < 0) surely you could make a combination of the two meaning that the power factor would be closer to 1 (making the system more efficient)?

As I'm new to this I'm not really sure if this makes much sense as question or if I'm just looking at it in the wrong way!

Thanks,

Ollie

 

[Ratch]

Hi,

I'm very new to electronics and power systems and have started learning about the Complex, Apparent, Real and Reactive power and realise that the power factor effectively defines how efficient a system can be. To increase the efficiency (useful power outputted) the power factor needs to be as close to 1 as possible.

My question is that if an Inductive load causes Lagging (θv – θi > 0) and a Capacitive load causes Leading (θv – θi < 0) surely you could make a combination of the two meaning that the power factor would be closer to 1 (making the system more efficient)?

As I'm new to this I'm not really sure if this makes much sense as question or if I'm just looking at it in the wrong way!

Thanks,

Ollie

When you say something is lagging or leading, you should specify what (voltage or current) is lagging or leading. Anyway, yes, a circuit with too much inductance can be corrected by adding a capacitor to the circuit. That is called power factor correction. There are plenty of references on how to do that in textbooks and the web.

Ratch

 

[OllieCramer]

Thanks for the quick response!

Another quick question: When the current and voltage are not in phase the power factor is less than one meaning that the system is not 100% efficient. Where does the inputted energy go? Does it exit the system as heat?

Thanks
Ollie

 

[Ratch]

Thanks for the quick response!

Another quick question: When the current and voltage are not in phase the power factor is less than one meaning that the system is not 100% efficient. Where does the inputted energy go? Does it exit the system as heat?

Thanks
Ollie

No, it is stored and released in the electric field of the capacitor and the magnetic field of the inductor. The reactive power is taken in and given back to the circuit twice each cycle. A perfect capacitor or inductor will not consume any resistive power (heat) . That is true no matter what the phase of the voltage to current is.

Ratch

 

[Arouse1973]

Hi Ollie
Yes your correct. When you have a current in an inductor it stores energy in the magnetic field created around it. When the power is removed this energy has to go somewhere. If it was an ideal inductor it would just stay there. But all real inductors have resistance so this energy is converted to heat. This is what Ratch mentioned about PFC, it takes this energy from the magnetic field and stores it in a capacitor to use it again in the circuit.
Adam

 

[Ratch]

Ideal or not, in phase or not, regardless of the frequency, energy will be taken in and released twice during each each sinusoidal cycle.

Ratch

 

[hevans1944]

Power factor correction is a serious concern to utilities delivering power to customers. Reactive power (voltage that is not in phase with the current) cannot be billed for revenue purposes because it isn't real power. Nevertheless, reactive current causes resistive losses in the power lines... also not billed because these losses occur ahead of the power metering used for billing. The problem typically occurs in large industrial plants with lots of induction motors that present an inductive load to the power transmission system when the motors are not fully loaded. It is solved by placing large capacitors across the power lines to cancel out the inductive reactances of the motors. Large reactive currents flow in these power-factor correction capacitors, but up-stream where the power is generated the load looks resistive instead of inductive and reactive currents are therefore suppressed.

Similar problems occur on a smaller scale, but multiplied by millions of consumers, when appliance power supplies present a reactive load to the incoming power lines. Thus there is good reason (from the power company's point of view) to incorporate power-factor correction circuits in consumer electronics. You will need tons of math and theory to fully understand all this.

 

[Tha fios agaibh]

An opposite example is how telephone companies use inductors every 1/2 mile or so to counteract line losses.
This is due to capacitance, caused by running wires for long distances side by side.

 

[Arouse1973]

Ideal or not, in phase or not, regardless of the frequency, energy will be taken in and released twice during each each sinusoidal cycle.

Ratch

I thought that an ideal inductor and core required no magnetising current, so there would be no need for PFC. I thought phase was important because this is the part that allows the load to use this magnetising current when the field collapses. When it's out of phase, say 90 degrees this current can't be used by the load so part of it is converted to heat in the resistance of the coil and part of it goes back into the source. The addition of the capacitor acts as a storage device to give the energy back to the inductor and core to establish the magnetic field once more. I am not a power electronics expert so please feel free to correct me.
Thanks
Adam

 

[Tha fios agaibh]

Not sure what you mean by "requiring" magnetizing current.
Any inductor, or conductor for that mater with an electric current running thru it has a magnetic field around it.

Remember the left hand rule?

 

[Ratch]

I thought that an ideal inductor and core required no magnetising current, so there would be no need for PFC.

Any time a charge moves, a current exists, and a magnetic field if formed (see image below). Therefore, all currents are "magnetizing". Magnetic fields are concentrated by forming conductors into coils.

I thought phase was important because this is the part that allows the load to use this magnetising current when the field collapses.

A phase difference means reactance is present. If you are talking about a series resistive load, then the resistor will dissipate the energy according to the current in the series circuit. The reactance of the inductance will diminish the current and less energy will be dissipated by the load resistor. This will require more source voltage bring the current up to what it would be without the reactance of the inductance.

When it's out of phase, say 90 degrees this current can't be used by the load so part of it is converted to heat in the resistance of the coil and part of it goes back into the source.

At 90° phase difference, there is no resistance in the series circuit, and therefore no energy will be dissipated, assuming a perfect inductor.

The addition of the capacitor acts as a storage device to give the energy back to the inductor and core to establish the magnetic field once more. I am not a power electronics expert so please feel free to correct me.
Thanks
Adam

A series circuit with an inductance and no capacitance or a circuit with a capacitance and no inductance will still receive and give back power from/to the circuit twice every cycle. A circuit with both capacitance and inductance will still do the same, except more of the power to build the fields will come from the power stored in the opposite reactance instead of the voltage source. If the circuit is in resonance, then the reactances cancel, and ALL the energy needed to build the fields comes from the opposite reactance. This allows the resistor to dissipate the maximum of power like it would if no capacitance or inductance were present. The idea is minimize the amount of power needed to build the fields from being supplied from the voltage source.

Ratch

 

[Arouse1973]

Magnetising current is the current needed to ensure the core produces the desired magnetic field by aligning the magnetic domains. It's the setup current for the magnetic field. It is defined as Ho / 0.4*pi*N for a wound core. If not used this is wasted current because it has to be given again each time the magnetic field collapses and re-establishes. That's my understanding, but please advise if this is wrong.
Thanks
Adam

 

[Arouse1973]

Any time a charge moves, a current exists, and a magnetic field if formed (see image below). Therefore, all currents are "magnetizing". Magnetic fields are concentrated by forming conductors into coils.


A phase difference means reactance is present. If you are talking about a series resistive load, then the resistor will dissipate the energy according to the current in the series circuit. The reactance of the inductance will diminish the current and less energy will be dissipated by the load resistor. This will require more source voltage bring the current up to what it would be without the reactance of the inductance.



At 90° phase difference, there is no resistance in the series circuit, and therefore no energy will be dissipated, assuming a perfect inductor.



A series circuit with an inductance and no capacitance or a circuit with a capacitance and no inductance will still receive and give back power from/to the circuit twice every cycle. A circuit with both capacitance and inductance will still do the same, except more of the power to build the fields will come from the power stored in the opposite reactance instead of the voltage source. If the circuit is in resonance, then the reactances cancel, and ALL the energy needed to build the fields comes from the opposite reactance. This allows the resistor to dissipate the maximum of power like it would if no capacitance or inductance were present. The idea is minimize the amount of power needed to build the fields from being supplied from the voltage source.

RatchView attachment 19311

Thanks Ratch I will look at this in detail later. Oh my last reply should have been to @Tha fios agaibh sorry I didn't reply directly.
Cheers
Adam

 

[Ratch]

Magnetising current is the current needed to ensure the core produces the desired magnetic field by aligning the magnetic domains. It's the setup current for the magnetic field. It is defined as Ho / 0.4*pi*N for a wound core. If not used this is wasted current because it has to be given again each time the magnetic field collapses and re-establishes. That's my understanding, but please advise if this is wrong.
Thanks
Adam

What is Ho? I think you have got a hold of some nonlinear phenomenon that most of us have not heard about.

Ratch

 

[Arouse1973]

Hi Ratch. Ho is the operating peak magnetizing force.
Thanks
Adam

 

[hevans1944]

An opposite example is how telephone companies use inductors every 1/2 mile or so to counteract line losses.
This is due to capacitance, caused by running wires for long distances side by side.

The inductors are added to level the frequency response of the twisted-pair copper wires. Line losses are a result of copper wire resistance and cannot be prevented. Line losses are compensated by adding bi-directional line-balanced amplifiers to the loop circuit every few miles. This limits distance from the central office to the subscriber to a few miles because line amplifiers are not normally used with subscriber lines, only with long-distance and trunk lines. Also see this Wikipedia article.

 

[Tha fios agaibh]

Thanks Adam, I understand what your saying, and agree, but don't understand the "ideal inductor" part. Wither your talking about a magnitizing current, or an amount less than that, your just talking about varying degrees of magnetic fields and inductance that can cause a low power factor. Right?

 

[Tha fios agaibh]

The inductors are added to level the frequency response of the twisted-pair copper wires. Line losses are a result of copper wire resistance and cannot be prevented. Line losses are compensated by adding bi-directional line-balanced amplifiers to the loop circuit every few miles. This limits distance from the central office to the subscriber to a few miles because line amplifiers are not normally used with subscriber lines, only with long-distance and trunk lines. Also see this Wikipedia article.

I admit that was not a good example to use here, and should have said signal loss instead of line loss.
But it's related in a way; Yes, frequency response is improved (restoring higher frequencies) by adding inductors (voice coils) to counteract capacitance which is caused by long spans of conductors.
These inductors compensate for high capacitance where voltage is lagging current.

 

[Arouse1973]

Thanks Adam, I understand what your saying, and agree, but don't understand the "ideal inductor" part. Wither your talking about a magnitizing current, or an amount less than that, your just talking about varying degrees of magnetic fields and inductance that can cause a low power factor. Right?

Hi John
What I am trying to explain, which might be wrong, lol, is with the ideal inductor and core it has such a large permeability that it requires virtually no current to establish the magnetic field. This has a much lower amount of wasted energy that it would almost be of no gain to use PFC for an ideal inductor and core. That's all I was on about, probably not relevant for the OP anyway

Thanks
Adam

 

[Tha fios agaibh]

Thanks Adam, I see what your saying.
Academically, It's common to ignore some factors in order to simplify things for the student trying to learn the basics.
Such as conductor resistance, impedance, or efficiency in a circuit.
I believe your ideal inductor could be designed perfectly, and and be 100% efficient, but It would still effect the power factor because of its inductance.
Naturally, wasted energy and power factor are not the same thing.

John