Still more xfrmr loading:
- Thread starter rattus
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Still more xfrmr loading:
Three men checked into a hotel, and each paid $10 in cash (this is a very old story
Three men checked into a hotel, and each paid $10 in cash (this is a very old story
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Still more xfrmr loading:
I answered that in the fourth paragraph here.
In an "ideal" (lossless) transformer the voltages in the winding are shifted 90? from the current. They have to be or it wouldn?t be ?lossless.? Initially, the induced magnetic field stores the energy and then returns it as the field collapses.
If the two secondary windings were ?in phase? with each other the magnetic fields would store and collapse at exactly the same instantaneous time and ?system? energy transfer rate in RMS values would be our old friend 2080W (to the load) and (apparently) 2080VA to the transformer. However, since the two windings are phase shifted, storage and collapse occur at different instantaneous times, but the effective (apparent) transfer rate is constant in RMS values. Since it is a lossless phenomenon, for a 120? shift that rate is 1200VAR.
Now for a really interesting question, what would it be for a 60, 90, 150 or 180? shift?
[ March 15, 2005, 11:41 AM: Message edited by: rbalex ]
I answered that in the fourth paragraph here.
In an "ideal" (lossless) transformer the voltages in the winding are shifted 90? from the current. They have to be or it wouldn?t be ?lossless.? Initially, the induced magnetic field stores the energy and then returns it as the field collapses.
If the two secondary windings were ?in phase? with each other the magnetic fields would store and collapse at exactly the same instantaneous time and ?system? energy transfer rate in RMS values would be our old friend 2080W (to the load) and (apparently) 2080VA to the transformer. However, since the two windings are phase shifted, storage and collapse occur at different instantaneous times, but the effective (apparent) transfer rate is constant in RMS values. Since it is a lossless phenomenon, for a 120? shift that rate is 1200VAR.
Now for a really interesting question, what would it be for a 60, 90, 150 or 180? shift?
[ March 15, 2005, 11:41 AM: Message edited by: rbalex ]
Re: Still more xfrmr loading:
rbalex,
I have trouble wrapping my mind around your explanation. Seems to me that the primary and secondary load currents do not affect the magnetic field. The primary voltage and primary inductance provide the magnetic field, right?
Now, if we apply line to neutral impedances to obtain the same current and phase, we have some reactive components which I can understand. Perhaps we have some virtual reactances as a result of the phase shifts--something like we see in active filters. What say you?
rbalex,
I have trouble wrapping my mind around your explanation. Seems to me that the primary and secondary load currents do not affect the magnetic field. The primary voltage and primary inductance provide the magnetic field, right?
Now, if we apply line to neutral impedances to obtain the same current and phase, we have some reactive components which I can understand. Perhaps we have some virtual reactances as a result of the phase shifts--something like we see in active filters. What say you?
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Still more xfrmr loading:
Lets go back to basic physics: What generates the magnetic field around any conductor?
Lets go back to basic physics: What generates the magnetic field around any conductor?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Still more xfrmr loading:
Aw shucks, Paul, why?dya have ta go an? give away my secret?
As to the 1200 VAR, all reactive currents are manifestations of energy exchange between (or among) the magnetic fields of all inductive components (e.g., generators and transformers) and the electric fields of all capacitive components (e.g., capacitor banks and synchronous condensers). Since this specific example only had two inductors (i.e., the two secondary windings), and since the load was purely resistive, and since we are ignoring any capacitive or inductive reactance in the lines, then the 1200 VAR is being circulated back and forth between the two windings. It didn?t ?go? anywhere, any more than my $1.00 went anywhere.
Aw shucks, Paul, why?dya have ta go an? give away my secret?
As to the 1200 VAR, all reactive currents are manifestations of energy exchange between (or among) the magnetic fields of all inductive components (e.g., generators and transformers) and the electric fields of all capacitive components (e.g., capacitor banks and synchronous condensers). Since this specific example only had two inductors (i.e., the two secondary windings), and since the load was purely resistive, and since we are ignoring any capacitive or inductive reactance in the lines, then the 1200 VAR is being circulated back and forth between the two windings. It didn?t ?go? anywhere, any more than my $1.00 went anywhere.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Still more xfrmr loading:
The motion of charge within the conductor. It may be voltage that causes the charge to move, but it is "charge in motion" that creates a magnetic field.
Re: Still more xfrmr loading:
The ideal transformers can be replaced with zero impedance AC sources. There are no reactive components in such sources. What happens to the reactive power? I think it is pumped back and forth between these sources. Maybe I should try superposition?
The ideal transformers can be replaced with zero impedance AC sources. There are no reactive components in such sources. What happens to the reactive power? I think it is pumped back and forth between these sources. Maybe I should try superposition?
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Still more xfrmr loading:
You're falling into the model trap again. An ideal transformer has plenty of impedance; its just all inductive.
Re: Still more xfrmr loading:
rbalex,
Tell me Master about the inductances of which you speak. There is no leakage inductance, only an infinite primary inductance and an infinite core loss equivalent resistance. I have always believed that an ideal transformer was equivalent to a zero impedance source. But then I have made mistakes before--thought I was wrong!
Seriously now, if we do set up this problem with AC sources, what happens to the reactive power?
rbalex,
Tell me Master about the inductances of which you speak. There is no leakage inductance, only an infinite primary inductance and an infinite core loss equivalent resistance. I have always believed that an ideal transformer was equivalent to a zero impedance source. But then I have made mistakes before--thought I was wrong!
Seriously now, if we do set up this problem with AC sources, what happens to the reactive power?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Still more xfrmr loading:
What happens is just what you said earlier:
What happens is just what you said earlier:
That is all that ever happens with reactive power. A load and the conductors to the load might also participate in the pumping back and forth, and would any power factor correction devices.
- Status