Hi,

Can someone please explain how if an inductor in an ac circuit is meant to result in the current lagging the voltage by 90deg then why is it that adding a resistance suddenly changes the angle. I guess it is shown mathetically, but intuitively I can't see it, it seems as though the resistance should just have a resistive loss associated and not affect the phase angle of resulting current waveform.

Thanks

 

Hi,

Can someone please explain how if an inductor in an ac circuit is meant to result in the current lagging the voltage by 90deg then why is it that adding a resistance suddenly changes the angle. I guess it is shown mathetically, but intuitively I can't see it, it seems as though the resistance should just have a resistive loss associated and not affect the phase angle of resulting current waveform.

Thanks

If you understood the concept of impedance as the vector sum of resistance and reactance it would possibly be a bit more transparent. Anything represented by a vector quantity can be resolved into components which make their individual contributions and can be combined in a linear fashion.

 

Thank you for your response.

If I have a circuit with a resistor and an impedance in it, if I viewed the current waveform at a point after the resistor, then I viewed it at a point after the inductor would I get different a different result for the angle between the current and the voltage?

 

Since they are in series, the current everywhere will have the same phase angle.

The voltage across the inductor will lead the current through the inductor by 90°. Doesn't matter what you put before or after it.

The voltage across the resistor will be in phase with the current through the resistor. Doesn't matter what you put before or after it.

But the voltage across the combination of the inductor and the resistor will be the vector sum of the voltages across each individually, so that angle, relative to the phase angle of the current, will depend on the ratio of the reactance to the resistance.

 

The reason the angle changes with resistance.......is because of time. In an inductor......without any resistance.....the inductance will take 1/4 of cycle time.....to establish and store an magnetic field. This is why the current will lag by 90 degrees/or 1/4 cycle. It takes 1/4 cycle time to charge inductor.

When you add resistance........the inductor can not charge as fast.......therefore the lag doesn't reach the full 90 degrees.

Does that make any sense?

 

Think of an inductor and resistor in series. Then connect to an A.C. supply. Any current flowing through the inductor creates a magnetic field, which causes a self-induced voltage which opposes the flow in current (actually, the change in current). This leads mathematically to a 90 degree phase shift. If the inductance were to be large, so that its impedance to the A.C. were much greater than the resistance, the phase angle is nearly 90 degrees. However if the inductance is reduced, so that the impedance reduces, then if it were quite low it becomes almost zero. SO changing the ratio of inductance to resistance causes the phase to move from 0 degrees (pure resistance) to 90 degrees (pure inductance) and for values in between the phase angle is somewhere between these limits too. More inductance-like, closer to 90 degrees, more resistor-like, closer to zero.

 

Thank you all for your responses.

I guess what I am trying to get my head around is what the power factor will look like in different sections of a circuit. If I have an inductor in a circuit followed by a resistor in series. The inductor puts the voltage and current out of phase by X degrees, the phase shift will also be the same phase shift in the resistor is that correct? Since has already had a lag due to the inductance?

Or another example, If I added a capacitor to the circuit would the overall phase shift now be reduced throughout the whole circuit or would there still be the same lag experienced in the inductor and then the capacitor would then reduce the shift bringing it back close to unity power factor.

Do I view the phase shift as lumped or as though the power factor undergoes a change along the circuit depending on what component?

Thanks

 

A phase angle is the mis-alignment of peak current with peak voltage in time.

 

I guess what I am trying to get my head around is what the power factor will look like in different sections of a circuit.

That really is not something you try to look at.
Power factor is only of interest for the complete circuit.

The voltage phase and amplitude does vary of course at various nodes, but the current is the same in every component in a series circuit.
The current through an inductor always lags the voltage across the inductor by 90°.
The current through a capacitor always leads the current through the capacitor by 90°.

If I added a capacitor to the circuit would the overall phase shift now be reduced throughout the whole circuit or would there still be the same lag experienced in the inductor and then the capacitor would then reduce the shift bringing it back close to unity power factor.

The overall applied voltage vs. current phase shift would be reduced.
At resonance the inductive reactance cancels the capacitive reactance leaving zero circuit phase shift and giving a unity power factor.

 

Ok. Why is it that it is advised to supply a circuit with a power source that supplies power at the same power factor as the circuit?

 

The "power factor" is not controlled or set by the power supply. One does not match a power supply to a power factor.

Are you sure that you are using the right term?

 

Ok. Why is it that it is advised to supply a circuit with a power source that supplies power at the same power factor as the circuit?

You cannot change the power factor of the source. You can only adapt the load. Doing so may be beneficial for large inductive loads, like large motors. Problem is that the load characteristics of such a large motor are not constant. So you are stuck with adding a fixed capacitance to a changing inductive load. Like a stopped watch that tells the correct time twice a day, the correction only applies to a single condition of the inductive load.

 

Hmm. Maybe I have misunderstood. But my understanding was that generators such as diesel synchronous sets and even inverter based supplies from solar pv are able to change the power factor that they export at when connecting to the utility, the data sheet gives a range over which they could be set to export at. Is this an example of matching the supply pf to the circuit pf?

 

Hi.

Can someone explain this, if every inductor creates a 90deg shift between the voltage and the current, does that mean that if I put enough inductors in series they can create a combined shift and put the voltage and current completely out of phase?

 

No. Think of it in this way. If you put two inductors in series you end up with a larger inductance. The phase difference between current and voltage is still 90°.