OK, yes there is a considerable amount of iron within this car alternator being tested.
That means the Inductive Reactance of this alternator would be higher than a Dual Rotor Air Core Axial Flux generator. RIGHT?
But how much higher?"
Magnitudes in some cases. from uH to mH
"How does the Inductive Reactance compare to the DC Coil resistance for this type of car alternator?
Is it ?
a) significantly less
b) about equal
c) significantly more"
Ok, there is a bit here to look at.
The impedance caused by the inductance of the circuit is frequency dependant. So at low frequency, the difference between stator R and stator R+XL is insignificant.
As F increases, so too will the impedance caused buy the inductive reactance of the circuit.... thats why it is an impedance not a resistance, it is dynamic in nature.
So the answer is all of the above... depending on the frequency.
Which brings the next part of the question.
Agreed, the DC Coil Resistance would cause energy to be wasted as heat
and the Inductive Reactance should not waste the energy as heat.
If there was significant Inductive Reactance (as compared to the sum of the coil resistance + load resistance) then could this be observed and measured because the AC Voltage would be "out-of-phase" with the AC Current ?"
The answer is ...yes... need to measure synchronous impedance, and subtract the resistance losses, and then whats left will be mechanical losses,inductive reactance and the real elephant in the room with iron cores... armature reactance. (even though the inductive reactance acts like a choke... current limiter).
There are many how too's on how to measure the reactive components, open circuit to short circuit measurements will give you most of the answers.
Think of it like this.
If we use a magnet to induce a voltage in a wire, and load that wire, a current will flow in that wire. Simple enough too....
But, whenever we have a current flowing we create a magnetic field....so now the wire with the induced EMF is now an electromagnet whose field is in opposition to the magnetomotive force that induced it. (MMF).
We can see that now we have an inducing field and a repelling field produced by that field.... it's war. As long as the inducing field (rotor MMF) is stronger than the repelling field (Back MMF), we can continue to drive up the rpm, and get power out of the stator.
When the stator is carrying enough current to create back MMF sufficient to prevent further MMF from seeing the stator, we get a state where no matter how fast you spin the shaft, no more EMF can be induced in the stator.... we CURRENT LIMIT at this point.
If the air gaps are large (like in an axial flux with neos) and the stator coils are without iron (axial flux with neos again) and we have a strong magnetising field (neos etc) then we have a machine that will follow the ohms law pretty well, and we can ignore the armature reactance, and the inductive reactances....... but...
In a car alternator we have tight gaps for the field to operate over, and we have iron core in the stator with slots. These slotted iron cores allow much lower magnetising current to be used, and much tighter air gaps ( 1mm instead of 20mm). The stator wire is wound around these slots/poles, and that allows the flux induced by the rotor to cut the coils at one instant, as it flicks and drags from pole to pole........ in an axial, the coil pole is physically spread.....and without an iron core of any kind to focus the back MMF against the magnetising field.... and so the field penetrates the legs of the coils gradually, not all at once.
So a combination of gap (bigger is harder to focus the back MMF), and an iron core focussing the back MMF, we have a completely different animal to deal with.
As a car alternator, when these problems arise, we simply increase the flux in the magnetising field (rotor), and that papers over the problems for the most part.....
A car alternator will still current limit from armature reactance, depending on the max flux the rotor can create with full battery voltage across it ... for a car usually 5A is about it for the 14v driving emf. The rotor resistance limits it to that, and so the AMP TURNS that can create the field is limited to this value.
The inductive reaction from just the frequency in a car alt does not cause quite the results that back MMF does in most cases, as F is not usually so high as to bring it to the fore front, it will usually be just be a creeping output loss on the back of the armature reactance. In a car alt, the turns are very low, the core very small, so inductance is low.
In short, in an iron core machine, it will almost always be the back MMF of the armature reactance (and XL) that defines the upper limit, and the resistance that defines low power performance. in commercial generators, they tend to balance the copper loss with the armature reactance (and XL)losses.
It is really an excellent safety valve, as if it did not occur, then a short in a power alternator would probably destroy it, and bust shafts pronto.
Power factor losses are a vector thinggy, and so should not bother the alternator at first glance.... but it you need say 10 amps available to vectorially get 7 real amps in phase with the voltage, we still have to make the 10 amps.... Now there is no voltage associated with the last three amps so no power... so we should not require any more motor to develop it... but no... sadly.
Current exhibits as torque, so even though no power should be associated with the phantom current, the motor will feel it. Folks charging batteries with no PFC stages will notice this behaviour. The 10 amps needs to be carried to the work site, and the resistance in the wires, fuses, transmission whatever will exhibit a voltage drop, producing a voltage proportionally(albeit small), and our phantom current has a small in phase voltage now... not much in the way of watts, as V is small, but the 3 amps still exists, and torque will need to be found to support it...... motor labours accordingly. I would think that the 7 amps to 10 amps will give the ratio of change of torque in this instance...... because torque is proportional to current, as rpm is to stator voltage.
It is the stators back MMF fighting against the rotors MMF that makes the shaft hard to turn when load is applied..... it is only caused by current flow, as only current makes flux.... in ampere turns.... ideally, EMF has no effect ( does in the real world... we need to overcome internal resistance / impedance for the current to flow... but you get my point?)