Author Topic: Why do conventional portable generators produce such ugly power?  (Read 1453 times)

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Offline james_s

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I was marveling recently at the nice clean sine wave that comes out of my little Honda inverter generator, it's actually noticeably nicer looking than what comes out of the wall form the utility. Then I remembered years ago looking at the output of a cheap conventional generator, the type with a 2 pole self excited alternator that screams away at 3600 RPM and being surprised at how ugly it was. More sine-like than those crappy "modified sine" inverters that are just a square wave with dead time but nowhere near as clean as utility power. This got me wondering just why they are so nasty? Is it something to do with the way the field excitation works? The ones I've been inside all use a brushless arrangement that has a wound rotor with a rectifier mounted right on the spinning part with no electrical connection to the stator. It has occurred to me that I don't actually know how the voltage is regulated on these, is anyone familiar with the gory details? I recall that some (all?) of them have a capacitor connected to a winding on the stator, I fixed one once by replacing the capacitor and I remember during my initial testing I tried a motor run cap of similar but not identical value and the generator then worked but the output voltage was about 20% high.
 

Offline David Hess

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Re: Why do conventional portable generators produce such ugly power?
« Reply #1 on: August 17, 2020, 01:48:57 am »
Inverter-generators modulate the field to control the output and this feedback makes the output low impedance, which is also why the output frequency can be fixed while the alternator frequency is variable.  Standard generators only adjust the field to control the output power which is much slower responding and their output impedance is higher, so they have poorer load regulation.

They do make faster responding generators which better regulation but I am not sure how they are different.
 
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Offline james_s

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Re: Why do conventional portable generators produce such ugly power?
« Reply #2 on: August 17, 2020, 04:35:39 pm »
Inverter generators are permanent magnet, at least the small Honda ones are. They generate 3 phase AC which is rectified into a DC bus of around 275V to feed the inverter, which handles the regulation electronically and also serves as the governor controlling the throttle.

My question is about the old style conventional generators, these have a wound rotor with a rectifier on it which like an induction motor has no physical connection to the stator. They cannot directly control field excitation because the current in in the rotor must be induced, the resulting output waveform is pretty nasty though, I assume it would be much better if the field was powered by DC fed via slip rings as is typical of larger generators.
 

Offline David Hess

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Re: Why do conventional portable generators produce such ugly power?
« Reply #3 on: August 17, 2020, 11:58:58 pm »
Inverter generators are permanent magnet, at least the small Honda ones are. They generate 3 phase AC which is rectified into a DC bus of around 275V to feed the inverter, which handles the regulation electronically and also serves as the governor controlling the throttle.

The ones I have seen are *not* permanent magnet.  Instead the field of the alternator is modulated at 60 Hz and the rectified output is switched at the zero crossing points to produce AC.  Since the field controls the output frequency, the alternator has many more than the 2 or 4 poles of a conventional 3600 RPM or 1800 RPM and the engine RPM can be variable without affecting the output frequency.

The main advantage of this is that the control circuits only have to control the field current which is a tenth or lower than the output current.
 

Offline ahbushnell

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Re: Why do conventional portable generators produce such ugly power?
« Reply #4 on: August 18, 2020, 12:01:09 am »
Inverter generators are permanent magnet, at least the small Honda ones are. They generate 3 phase AC which is rectified into a DC bus of around 275V to feed the inverter, which handles the regulation electronically and also serves as the governor controlling the throttle.

The ones I have seen are *not* permanent magnet.  Instead the field of the alternator is modulated at 60 Hz and the rectified output is switched at the zero crossing points to produce AC.  Since the field controls the output frequency, the alternator has many more than the 2 or 4 poles of a conventional 3600 RPM or 1800 RPM and the engine RPM can be variable without affecting the output frequency.

The main advantage of this is that the control circuits only have to control the field current which is a tenth or lower than the output current.
Do you have a reference on that? 
 

Offline NiHaoMike

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Re: Why do conventional portable generators produce such ugly power?
« Reply #5 on: August 18, 2020, 01:30:28 am »
That might be the case with some cheap inverter generators, since it's a really cheap way to get a sine wave.
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Online Circlotron

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Re: Why do conventional portable generators produce such ugly power?
« Reply #6 on: August 18, 2020, 02:46:11 am »
For a conventional alternator with an ugly sine wave, I'd say the cause is partially the shape of the rotor pole pieces. The manufacturer hasn't done his homework. Rough enough is good enough. Maybe also the stator iron is approaching saturation at max voltage? If you dialed the voltage down and the waveform improved, that would verify that.
 

Offline bdunham7

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Re: Why do conventional portable generators produce such ugly power?
« Reply #7 on: August 18, 2020, 03:43:59 am »
I was marveling recently at the nice clean sine wave that comes out of my little Honda inverter generator, it's actually noticeably nicer looking than what comes out of the wall form the utility. Then I remembered years ago looking at the output of a cheap conventional generator, the type with a 2 pole self excited alternator that screams away at 3600 RPM and being surprised at how ugly it was. More sine-like than those crappy "modified sine" inverters that are just a square wave with dead time but nowhere near as clean as utility power. This got me wondering just why they are so nasty? Is it something to do with the way the field excitation works? The ones I've been inside all use a brushless arrangement that has a wound rotor with a rectifier mounted right on the spinning part with no electrical connection to the stator. It has occurred to me that I don't actually know how the voltage is regulated on these, is anyone familiar with the gory details? I recall that some (all?) of them have a capacitor connected to a winding on the stator, I fixed one once by replacing the capacitor and I remember during my initial testing I tried a motor run cap of similar but not identical value and the generator then worked but the output voltage was about 20% high.

I won't pretend to be a inductive motor/generator design expert, but the main factor I know of with the inexpensive portable generators is that a cruder design and construction (winding pitch) will yield a lot of 5th harmonics.  5th harmonic distortion looks ugly and is readily visible on a scope.

There is a separate, but not always visually obvious, winding set that inductively transfers power to the rotating field.  I suppose that could get ugly too, but I would guess that the rectifiers and the inductance of the armature would mostly even out any variations.

They do make faster responding generators which better regulation but I am not sure how they are different.

Fewer field windings, less inductance and more field current to shorten up the time constant.  The specific type of generator you are referring to would need quite a high ratio of field current to output current.  I'm not sure it fairly could be called an 'inverter generator'. 


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Offline james_s

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Re: Why do conventional portable generators produce such ugly power?
« Reply #8 on: August 18, 2020, 04:23:03 am »
The ones I have seen are *not* permanent magnet.  Instead the field of the alternator is modulated at 60 Hz and the rectified output is switched at the zero crossing points to produce AC.  Since the field controls the output frequency, the alternator has many more than the 2 or 4 poles of a conventional 3600 RPM or 1800 RPM and the engine RPM can be variable without affecting the output frequency.

The main advantage of this is that the control circuits only have to control the field current which is a tenth or lower than the output current.

The only inverter generators I've ever dug into are the Honda EU series and they are definitely permanent magnet. I have the service manual for the EU2000i in front of me now and the stator is bolted to the block under the flywheel which has a ring of magnets under it, it's essentially the same construction as a brushless outrunner motor. I assumed they all worked this way since it seems like the logical way to design one but I have not attempted to check.

Seems like modulating the field at 60Hz would result in a lumpy output and it would require having brushes with slip rings or a rotary transformer, that seems like a very unusual way of doing things. I'd be curious to see such a generator though and see how it performs. Maybe it becomes more practical at higher power levels? The little Honda is only 1600W continuous but it's a fantastic little machine, whisper quiet, I can carry it with one hand and it will run all day on a gallon of gas. You can also wire two of them in parallel and they will auto-sync and load share. Spendy though.
 

Offline james_s

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Re: Why do conventional portable generators produce such ugly power?
« Reply #9 on: August 18, 2020, 04:26:41 am »
For a conventional alternator with an ugly sine wave, I'd say the cause is partially the shape of the rotor pole pieces. The manufacturer hasn't done his homework. Rough enough is good enough. Maybe also the stator iron is approaching saturation at max voltage? If you dialed the voltage down and the waveform improved, that would verify that.

They've been making these things for decades, surely if it was as simple as that somebody would have come up with something that worked? Then everyone else would just copy that.

The regulation response is usually pretty reasonable since the engine is screaming along at full mechanically governed speed regardless of load. I don't know how the regulation actually works though.
 

Online Circlotron

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Re: Why do conventional portable generators produce such ugly power?
« Reply #10 on: August 18, 2020, 05:56:26 am »
For a conventional alternator with an ugly sine wave, I'd say the cause is partially the shape of the rotor pole pieces. The manufacturer hasn't done his homework. Rough enough is good enough. Maybe also the stator iron is approaching saturation at max voltage? If you dialed the voltage down and the waveform improved, that would verify that.

They've been making these things for decades, surely if it was as simple as that somebody would have come up with something that worked? Then everyone else would just copy that.
That's probably just it. It does work. Plug your appliance into the generator and it works, despite the ugly waveform. Sure, a transformer might be a bit noisy with a non-sinusoid, not that you could hear it above the noise of the generator, but traditional power tools with universal brush motors, light globes, and nowadays stuff with SMPS's just don't care. Hence the expression "rough enough is good enough."

How many generator users were actually quite satisfied until they attached a scope to the output?
 

Offline Jeroen3

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Re: Why do conventional portable generators produce such ugly power?
« Reply #11 on: August 18, 2020, 06:50:27 am »
For a conventional alternator with an ugly sine wave, I'd say the cause is partially the shape of the rotor pole pieces. The manufacturer hasn't done his homework. Rough enough is good enough. Maybe also the stator iron is approaching saturation at max voltage? If you dialed the voltage down and the waveform improved, that would verify that.
Yeah, its only lack of iron. 
Less copper and less iron = smaller size = less weight = less $$$.
If you buy a marine generators (typically 3 times underspecified) you get a much better waveform. What happens in cheap generators is that the waveform moves from sinusoidal to triangular/trapezoidal. (low harmonics are often not required by standards)

Regulation may have something to do with it when you have a bush excited system. Rare these days. Most are brushless.

Here some oscillograms of a cheap 20kva generator. In the unloaded waveform your can already see the trigger of the AVR thyristor, there barely is any energy in the iron. Imagine what happens when you load it with some PFC supplies!
The red one is the half winding supply for the AVR, which is wound only in half the slots, hence why it's even more awful.
Obviously this kills whatever is attached to in days.
« Last Edit: August 18, 2020, 06:53:56 am by Jeroen3 »
 

Online Circlotron

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Re: Why do conventional portable generators produce such ugly power?
« Reply #12 on: August 18, 2020, 07:42:03 am »
What if you put a reasonably large power factor capacitor across the output? Say enough that it pulled 20% of the ampere rating? That should iron out any HF fuzz, and the higher the harmonic the greater the current thought the cap. Would it do anything weird like upset the voltage regulation or cause some kind of instability because it might cause a bit of phase shift in the feedback mechanism?
 

Offline Jeroen3

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Re: Why do conventional portable generators produce such ugly power?
« Reply #13 on: August 18, 2020, 08:55:19 am »
Capacitors won't help. They filter high frequency, what you see on those images is all below 5 khz.
What helps is a reactor (series transformer) with basically the same weight as the generator to compensate for cheaping out on the iron inside.

Generators also can't deal with a negative pf, the regulation stops working. (when on mains it becomes a motor)
« Last Edit: August 18, 2020, 08:56:59 am by Jeroen3 »
 

Offline David Hess

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Re: Why do conventional portable generators produce such ugly power?
« Reply #14 on: August 19, 2020, 12:55:10 pm »
The only inverter generators I've ever dug into are the Honda EU series and they are definitely permanent magnet. I have the service manual for the EU2000i in front of me now and the stator is bolted to the block under the flywheel which has a ring of magnets under it, it's essentially the same construction as a brushless outrunner motor. I assumed they all worked this way since it seems like the logical way to design one but I have not attempted to check.

They might include some permanent magnets in the rotor assembly to provide startup current.  But the schematics of the Honda and other inverter based generators that I can find all show a field winding.

Quote
Seems like modulating the field at 60Hz would result in a lumpy output and it would require having brushes with slip rings or a rotary transformer, that seems like a very unusual way of doing things.

Slip rings to drive the field winding have been routine for more than 100 years and are very reliable.  Non-inverter based generators use them without problems.
 

Offline MarkL

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Re: Why do conventional portable generators produce such ugly power?
« Reply #15 on: August 19, 2020, 03:10:38 pm »
...
They might include some permanent magnets in the rotor assembly to provide startup current.  But the schematics of the Honda and other inverter based generators that I can find all show a field winding.
The magnets are for generation.  Take a look at the rotor/stator assembly, attached.

Screen shot taken from this video, if you want to see more:

 

The magnets are quite strong.  He has trouble separating the two.
« Last Edit: August 19, 2020, 03:13:14 pm by MarkL »
 

Offline G7PSK

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Re: Why do conventional portable generators produce such ugly power?
« Reply #16 on: August 30, 2020, 09:06:19 pm »
I used to make gen sets,started as a hobby when a teenager and made commercially later on. None of the units I ever shipped out had bad waveforms or voltage regulation,it is down to the type of engine and governor and the quality of the alternator. I always used Markon and Newage units never had any problems with them, it is the cheap mostly Chinese built sets ( some Italian) that have bad waveforms. Very often these are just capacitor excited units with virtually no regulation and those which claim electronic regulation usually work by clamping the field short with a thyristor or mosfet so every time the voltage rises beyond limit the field is shorted out and you get a spike and another when the voltage drops and the short is switched off. You need to load such sets to somewhere around 80% load for best results. But at the end of the day what you are talking about is a generator set that cost £200-00 and a similar capacity unit at £1000-00 to £1500-00. you get what you pay for and most people don't want to pay which is why I dropped any thing below 15KVA and concentrated on industrial and agricultural until I retired ten years ago.
 

Offline james_s

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Re: Why do conventional portable generators produce such ugly power?
« Reply #17 on: September 10, 2020, 06:22:36 am »
The magnets are for generation.  Take a look at the rotor/stator assembly, attached.

Screen shot taken from this video, if you want to see more:


The magnets are quite strong.  He has trouble separating the two.

And that is all there is too, on the back of the back of the engine where the output shaft would normally be it is capped off, the crankshaft does not protrude out of the engine casing, the entire generator portion is that stator mounted under the flywheel and the magnets that rotate around it. It's quite a remarkable machine, I have not done a teardown on mine but I have removed the muffler to clean out a mud dauber nest at which point the back of the engine is exposed. I have seen a video of someone doing a major repair on a similarly designed Generac 2000W suitcase and it is a very similar layout. I have no idea how other inverter generators are built but now I'm curious to get a closer look at some.

I was actually given another conventional generator a couple weeks ago and was surprised to see this one does have slip rings and a little voltage regulator board, I have not scoped the output but it is noticeably better made than the other units I've messed with, maybe I just had the misfortune to deal with really cheap generators. I need to re-home this recent acquisition, I got it running easily enough and it works fine but at 5kW it's much bigger than I need and compared to the little Honda it's obnoxiously loud. I've been spoiled by inverter generators, the 3600 RPM screamers are obsolete.
 


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