Anybody else have yucky "sinusoidal" wave forms from their Chinese ST head?

[mobile_bob]

is it possible that the distorted waveform is caused by the harmonic winding?

almost looks like that is what is taking place as the waveform appears to have the step in about the same place on either side of the waveform.

the old delco 4 pole rotor alternator that i rewound having 12 volts dc applied to the rotor, put out a very clean sinus waveform. the stator was skewed i might also note.

i wonder now if the alternator would not have had a more distorted waveform if i had used some form of active voltage regulation in place.

i would expect the waveform of the ST head to change, perhaps for the better under load (resistive) and certainly alter its shape under an inductive or capacitive load.

anyone done any waveform tests under different types of loads? or loads folks are likely going to see such as a mix of all three?

bob g

[fattywagonman]

Hi Bruce,
I think there are several availble... Here's a couple manufactures

http://www.hammondmfg.com/CVR.htm

http://www.aelgroup.co.uk/pdf/ds_agt.pdf

http://www.clrwtr.com/Sola-Hevi-Duty-Transformers-Single-Three-Phase.htm

[oldnslow]

Hi Martin. Your webpage is great reading. Nice KISS design on the Oliver upwind burner and furnace. You can get it up and running in less than a day. Lots of good reading on your pages, thank you.

[BruceM]

Thanks for the links Wagon Man.  These also appear to be ferro resonant transformers.  In the power industry, CVT's are different, so I got confused. (My most common state of mind.)
Best Wishes,
Bruce M

[Doug]

We use something.... I think called a Solas brand in the mines.
I ignore them because they work and cause me no touble.
I just don't know if its worth the trouble to buy one of these passive filters to clean up the power of an ST when no one here has actualy said it causes them problems...

Doug

[GuyFawkes]

Just had a phone call from a mate who reminded me of something I had forgotten about triangular / sawtooth waveforms from AC generators, they are a sign of all the even harmonics adding up, which is a sign of a mechanical "source" for the "symptom"

So either the way it is constructed / wound or mebbe the way the AVR works in conjunction with the construction, still sawtooth is nicer than square wave for all sine wave based stuff.

[Doug]

I'm trying to think of how a winding could be botched to create an even harmonic. Triplins are easy, get the span wrong or try and wind on a stator with an ugly distribution factor (wrong stator for the pole-speed-phase combination ). But even harmonics thats a mystery to me I don't know what would creat that. Is there an electrical engineer in the house???

Doug

[chuckwagon337]

Newbie here..

I saw this same type of wave form two years ago when I first hooked up my 10kw ST generator to the house using a manual transfer switch.  During initial testing I was using a Fluke 41b Harmonics analyzer to monitor things.  I was concerned  with what this distortion was doing to my 3 ton central Air system system and other items in the home.  I also found out real quickly that my two UPS's for the computers hate this and will shut my system down to protect them.  This is frustrating since I live in South East Florida and am subject to up to a week or more of no power after a hurricane passes through. 

So I did some investigating into if the A/C system was causing the problem.  Replaced a contactor in the unit that had heavily pitted contact points yet still working.  This didn't change the wave form.  I did notice that sometimes when the A/C unit tried to start that it would bog down the generator.  So I talked to a good friend of mine who is an A/C contractor.  He suggested to install a time delay relay in the system to allow the head pressure to settle down before start-up.  And since the unit is about 8yrs old I also added a hard start capacitor.  All changed, the wave form is still rough, but I feel I may have added several more years to the A/C unit anyway. 

During this same period of time I used an Amprobe DMII chart recorder to monitor what was going on also with the ST output.  Obviously I'm happy with the setup (Changfa/ST) but not happy with what I was seeing on my meters.  My biggest concern is still anything that has electronics in them and the possible damage this will cause over a given period of time.  The chart readings show the same irratic indicators on both amperage and voltage.  Balancing your loads is very important.  I bought the new voltage regulator from George at Utterpower, but have not replaced the origional yet to see if there is any changes.  That's next on my list of To Do's....I'm not an engineer, just an Electrician DIYer.

 

kevin

[europachris]

I'm sure the 'weird' waveform is directly attributable to the design of the generator head.  It's a very basic, simple design with straight rotor slots and certainly no fancy engineering.  Brush type self-excited heads are about as dirt simple as you can get.  Whether electronics really care much about it is open for debate.  I'd be willing to guess that few things in your house are going to have an issue with the power quality.

The upside is that the ST head is rugged and cheap.  About the only thing that can go wrong besides needing a set of brushes now and again is the bearings, which are just standard ball bearings.  The stock units are likely low quality compared to some top notch parts (which are still made in China anyway).  I've never heard of anyone burning up an ST head.

The downside is that a 'real' head, such as the Marathon Lima MAC series, which has skewed rotor poles, separate brushless exciter built onto the rotor, etc. is going to run you about $1500 for a 5kW size.  Yikes! :-o  I never got back to the salesman to see if there were 'quantity' discounts, etc. or other way to get it cheaper.  But, that would be the cat's meow for a gen head.

Chris

[europachris]

Something else I thought of on the way home tonight....the ST head only has a single phase exciter.  Therefore, you rectify 60Hz AC and get 120Hz 'DC' ripple, since there are no capacitors to filter out the ripple.

The Marathon Lima MAC series heads have a 3 phase brushless exciter.  This will make a 360Hz 'DC' ripple, and a much lower peak to average ripple current than the single phase rectified excitation.

My thought is that if you are trying to make a constant field strength with DC current with a lot of ripple, your field will not be constant and that has to affect the purity of the sine wave AC output of the head.  I believe the inductance of the field windings will help smooth out the ripple somewhat, but there must be some improvement that can be made.

I wonder how it would work if someone put 100uF of capacitance across the field winding to filter the DC?  That would increase the DC output as the cap would charge to the peak voltage value coming out of the rectifier.  Therefore you would have to fiddle with the series resistance to maintain your desired voltage regulation.

Might make a noticeable difference in the quality of the AC output from the ST head.  Then again, it might not to a dag-gone thing.  I don't run an ST head, so I can't try it out.  Anyone want to be a guniea pig?

On that same note, I need to drag my scope out to the garage this weekend and see what my Markon 2-pole brushless head does for output quality.....

Chris

[trigzy]

Just a guess here, but if I remember correctly the field current on a ST head is "just a few amps", so a 100uF probably isn't large enough to smooth the voltage out with that big of a load.  I'd be interested to see the effects, but I dont have an ST head, and high capacitance capacitors at that high of a voltage aren't exactly free....


Steve

[europachris]

Does anyone know what the field voltage is during operation on the ST head?

If it's under 100V, you can get a LOT of capacitance very cheaply with some basic electrolytic caps from www.mouser.com or www.digikey.com. 

Also, you can try a local photo processing store that handles the disposable cameras and get a pile of photoflash caps out of them.  They have more than enough voltage capability, and parallel enough to get the capacitance needed.

Or, if anyone has a variable voltage DC power supply you could use that to energize the field for test purposes.

Chris

[Andre Blanchard]

Probably have a lot better luck with and LC filter.

 Bridge        Added        Added      Slip
  Rect.      Inductors       Cap.      Rings         Field  
   _________( ) ( ) ( )________________=[]_____________
   |                           |                       }
   +                         -----                     }
   -                         -----                     }
   |________( ) ( ) ( )________|_______=[]_____________}

[mjn]

If my memory is correct the field voltage is 50volts at 2 amps. Using the formula found here http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=9809 A quick calculation shows that assuming a 2 amp draw at 120hz, I will need at least 1000uf to keep the ripple around 10v.

I'll check my junk box to see if I have enough 100V electrolytics to get 1000uf.  Most of my big caps are low voltage.  I'll stick in what I have to see if it helps.

Andre.. I assume that the field coil itself will act as the inductor in the circuit. 

I'll take pictures of the field and main A/C waveforms before and after adding the capacitors.   This weekend I'll be busy hauling firewood.  Once I have a few loads in, I'll feel free enough to go play with my generator.

On a somewhat related note, my wife reports that the microwave oven does not work as well when running on generator power.  I did some crude tests (frozen snack) and she is right.  The microwave oven takes at least twice as long to heat something when on generator power.  I don't have any explanation because the 1000 watt load of the oven is well within the capabilities of my generator.  Everything else in the house is completely happy with my home made power.  The ceiling fans buzz a bit, but that is all. Anybody have any ideas?

Martin

[europachris]

The microwave oven takes at least twice as long to heat something when on generator power.  I don't have any explanation because the 1000 watt load of the oven is well within the capabilities of my generator.  Everything else in the house is completely happy with my home made power.  The ceiling fans buzz a bit, but that is all. Anybody have any ideas?

Martin

It could be something with the waveform distortion affecting the output circuit of the magnetron.  I'm not real familiar with how microwave ovens are set up, but it's basically a radio transmitter, and transmitters have tuned tank circuits to generate the RF.  The distortion could be screwing up the tuning and causing reduced output.

Sounds plausible, but then again, I could be talking out my a$$, too.  At least I'm a licensed ham radio operator, so I can pretend that I know what I'm talking about