Brushless Self-Exciting Alternator - Power Factor Rating Explanation? - Page 1
Forum section: electronics / general

author: DMinMN
author_rank: Newbie
timestamp: December 14, 2022, 09:02:55 pm
content: Hello All!I salvaged a diesel powered APU (auxiliary power unit) out of the scrap yard. For those unfamiliar with these units, they are mounted on the side of semi trucks to provide auxiliary power without the need for the large diesel engine running. This APU curiously is only setup for electrical generation, however. All the other ones I've seen also had coolant lines fed to the cab for auxiliary heat, and some with air conditioning lines also. This unit has an AC alternator with a nameplate rating of 120V at 50A. My question has to do with the rating of alternator, and not necessarily the unit itself. Context is fun, however. Attached a picture (the alternator is the lower left pulley) for my fellow nerds.The datasheet covers two different models of this alternator. it covers the model I have - BL105, and another model, the SL105. Both units are self-exciting brushless alternators. The difference seems to be the wiring of the rotor and auxiliary windings. Here are the applicable sections out of the alternator's owner's manual (too big to upload,here is a link to the entire manualif anyone is interested):BL105 model information from datasheet:SL105 model information from datasheet:The BL105 is said to be "designed to operate with single phase loads at or near a power factor of 1.0", whereas the SL105 is "designed for loads at or near unity power factor, it has a capability of operation at 0.8 power factor." My question is WHY. More specifically,my questions are as such:What about the SL105's voltage regulator, rectifier, and choke allow it to handle more reactive power?What about the BL105's lack of voltage regulation, rectifier, and coke don't allow it to handle reactive power?What would happen to the BL105 if a reactive load was put on it? Why does it specify it needs to be at unity power factor, or thereabouts?My curiosity is purely just that; curiosity.] I would like to venture a guess at my last question. I am assuming that the reason the BL105 cannot handle reactive loads has something to do with the way it is excited? If the load takes too much reactive power, it robs the magnetic field, and the generator cannot continue to self-excite? So I would venture to guess that the answer to my question of what would happen if too large of a reactive load was put on the BL105, is that it would simply loose excitation, and the field would probably have to be re-flashed? I would venture to guess that the SL105's voltage regulation is somehow able to handle the reactive power better somehow? How exactly?I am new to the forum, and happy to be here. Thank you for the welcome, and I look forward to being part of the community.Edit: Tried to get pictures embedded inline. No dice - maybe next time

author: f4eru
author_rank: Super Contributor
timestamp: December 14, 2022, 09:28:21 pm
content: The passively excited one is probably highly dependent on the load impedance for the excitation to stay stable.The actively regulated one can accept variation it seems.

author: DMinMN
author_rank: Newbie
timestamp: December 15, 2022, 01:19:51 pm
content: That is much more elegant way of saying what I was thinking. Thank you.I recall hearing that certain power stations will intentionally over-excite their alternators to handle large reactive power demands. Clearly the actively excited alternator variation of what I have has a way to vary the excitation. So it must do the same thing as large power stations, on just a much smaller scale. I though excitation only regulated the output voltage. So does an alternator's ability to handle reactive loads simply boil down to a voltage drop issue?How would one determine how much reactive power a passively excited alternator can handle? Is experimentation the only way? I have to imagine it would supply some reactive power, but at a much reduced output rating.

author: f4eru
author_rank: Super Contributor
timestamp: December 17, 2022, 08:00:54 pm
content: I think it is absolutely not comparable.In a power station, the excitation is finely regulated, adjusted, monitored, optimized, and separately generated.In a simplistic generator like this, the excitation relies more or less on the working regime of the generator itself to keep going. it's a locally stable equilibrium.it's like a drink glass on a platter: you can tilt the platter only a certain angle before it spills(let's call it "phase" instead of "angle")

author: Jeroen3
author_rank: Super Contributor
timestamp: December 17, 2022, 08:43:39 pm
content: Power stations run against the grid (very low impendace), and more excitation is more current, when that is not matched with the prime mover power you get inductive power factor.In this case, island operation, more excitation is just more output voltage.The BL105 one is a clever coupling of magnetics that will generate the excitation based on the output current. The characteristics of these magnetics are different when the phase angle it too much off the nominal design values, and may cause a change in voltage. The advantage is that it responds fast and doesn't need any 'expensive' control parts.The SL105 has an AVR that will usually measure the output and adjust the excitation voltage via a PID loop. This one looks to be on the rotor, that's a strange place...?Usually an AVR can provide you with voltage adjust and a wide capability curve and possibly even monitoring and limiting features.

author: DMinMN
author_rank: Newbie
timestamp: December 19, 2022, 04:42:38 pm
content: Ok, it would seem that my understanding is still lacking. Clearly I need to learn more so I can ask smarter questions. If anyone can suggested reading sources, I would appreciate it.Quote from: f4eru on December 17, 2022, 08:00:54 pmHmm. So I'm comparing apples and oranges. Good to know.The simple generator I have is a "Locally stable equilibrium." That makes sense to me, but only at a 10000ft view. What exactly is in equilibrium? I have an idea, but fear I have too many gaps in my understanding. Here's my (probably incorrect) understanding:When the rotor is stopped, it still contains residual magnetism.When the rotor begins to spin, this residual magnetism induces a current, which creates a larger magnetic field.This magnetic field then collapses, where it is stored in the excitation capacitor.Then the energy stored in the capacitor drains and rebuilds the magnetic field.This buildup and collapse of the magnetic field into and out of the capacitor creates an oscillation that repeatsIf this crude understanding is correct, is the "equilibrium" the oscillation between the excitation capacitor and the inductance of the auxiliary winding?Quote from: Jeroen3 on December 17, 2022, 08:43:39 pmOk, so in my case ("island operation") with the BL105, the concern with poor power factor is the resultant output voltage? So if I run a relatively large load with poor power factor, there would be greater and greater voltage drop? With the SL105's AVR, it adjusts the excitation voltage to increase the output voltage to offset the voltage drop created by the poorer power factor? Is that correct?So it's not that the BL105can'thandle inductive loads, it's just that the more reactive the load, the greater the voltage drop will be?Again, if anyone has some suggested reading, I am all ears. I've been finding articles to read, and have purchased some power generation books. They don't seem to answer the questions I'm trying to ask, though (maybe i'm asking the wrong questions).