Portable generator excitation  - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: February 17, 2023, 08:31:49 am
content: I'm curious about the alternator portion of the typical portable generators (5-20kW) people use to power their homes during power outages. Do they use permanent magnets for the excitation field or do they use some sort of regulator that bootstraps initially?

author: richard.cs
author_rank: Super Contributor
timestamp: February 17, 2023, 03:21:11 pm
content: They use a wound excitation field, because that then allows automatic voltage regulation by controlling that field. Modern designs almost always take the main output from the stator, so the current for the field on the rotor has to get there somehow.There are lots of different techniques, one simple example is to use sliprings to bring the field current onto the rotor from an off-board regulator, which is powered either from its own stator winding or the main output (or a tap on that output). More sophisticated approaches let you design-out the brushes, for example you have a stationary excitation winding which is used to induce a small amount of current into an auxiliary winding on the rotor, which is then rectified by diodes on the rotating assembly to feed the rotating field winding. There's an extra magnetic step (which also has power gain) but you've gotten rid of some unreliable sliding contacts. Portable generators that have a 12V output for battery charging sometimes also use that winding to power the field.

author: jmelson
author_rank: Super Contributor
timestamp: February 17, 2023, 04:25:48 pm
content: I have an ~ 2700 KVA generator, and as far as I can tell, it uses permanent magnet excitation (I have never had it apart).Jon

author: themadhippy
author_rank: Super Contributor
timestamp: February 17, 2023, 05:02:21 pm
content: QuoteThats bigger than the units we had as back up for a large hospital,or do you mean 2700VA / 2.7KVA?

author: Jester
author_rank: Frequent Contributor
timestamp: February 17, 2023, 11:18:47 pm
content: Quote from: richard.cs on February 17, 2023, 03:21:11 pmThanks for the great explanation.Do you have a sense for the voltage/current/power required for the excitation field? I know on the really BIG utility type its fairly low ~1% I imagine a fair bit more for say a 10-20kW generator?

author: johansen
author_rank: Super Contributor
timestamp: February 17, 2023, 11:33:24 pm
content: Unfortunately the majority of cheap generators achieve regulation by saturation.The rotor actually wastes 600 watts of power in many 5 to 6kw generators.It is excited by a 150vac open circuit auxiliary winding in the stator, through a full wave rectifier, which sends approximately 120vdc to the rotor, which draws 4 amps.years ago i took a 5kw generator head apart and replaced the single phase, 3 pound stator winding with a 3 phase, 5 pound stator winding (and there was plenty of room left in the core). still have it. tried to use it as a rotary phase converter and couldn't get it to stay synchronized with the grid, as it has no damper windings. it doesn't "pull in" like they are supposed to.years later i found out that the salient pole, synchronous generator has a regulation of about 30%. the reason why is not super intuitive. but basically, it takes a given number of amp turns in the field to excite a 1T flux density in the stator core. lets say its 500.now put a load on the generator, 250 amp turns in the stator. your flux density in the motor just dropped to .5T, and the voltage under load dropped in half. this is assuming a small air gap. now add the air gap, so it takes 1000 amp turns in the rotor, and now your voltage drop under load is only 25%.basically for every amp turn in the stator you need an equal amount more added to the rotor, to keep the magnetic field strength the same.permanent magnet motors have a large amount of amp turns stored in the "air gap" of the magnet itself. as such the amp turns in the stator under load are a smaller proportion to the amp turns in the magnet.now the magic of saturation... you can size your rotor so the cross section of the core in the middle of the rotor is about the same as the stator core. now you can shove 20,000 amp turns through the rotor, but the field strength will never exceed 1.9T in the steel, and say, 1.7T in the stator core (due to leakage flux)now as the amp turns in the stator oppose the amp turns in the rotor, the rotor core starts to drop out of saturation, and instead of looking like an air gap at 1.9T it starts helping the field strength stay above 1.7T... now you only have a 10% drop under full load.open circuit voltage of such a generator is typically 260vac and drops to 220vac at the rated "5KW" capacity.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: February 18, 2023, 11:39:11 am
content: Quote from: johansen on February 17, 2023, 11:33:24 pmVery interesting. Thanks for sharing.

author: jmelson
author_rank: Super Contributor
timestamp: February 18, 2023, 04:26:53 pm
content: Quote from: themadhippy on February 17, 2023, 05:02:21 pmYeah, you're right! How about 2.7 KVA? I put 2 wheels on it, and it is rope-pull start. So, obviously NOT 2700 KVA. That would be the size of a semi, and have at least 16 cylinders.Jon