ST sinewave distortion figure

[emerald]

Does anyone have the sinewave distoration figure from an ST alternator they run, or measured it?

I am trying to get a feel for what is the norm and what is poor.

[okiezeke]

Emerald,
My first ST to go on line had lost all residual magnetism, so I was forced to try a lot of different ways to excite the field.  It became very clear that the quality of the power out was largely proportional to the quality of the DC being fed to the field windings.  The AVR that came with the ST was pretty dirty.  I tried a DC motor controller, a variac to a bridge rectifier to a 10,000uf electrolytic cap filter, and a commercial AVR from high tech hicks in toronto.  Same conclusion: dirty in, dirty out.  I hope to get some pictures of the output from the chinee canadian AVR, and will share them...  My best quality so far has been from the variac rig which had minimal distortion only on the peaks and valleys of the output.  around 10% of the output amplitude.  Everything in the house worked fine on it, even computers.

Zeke

[ronmar]

I have never measured the distortion, but Zeke is exactly right with the garbage in-garbage out analogy.  Another thing that effects the output waveform is drawing energy from the magnetic field via the harmonic winding to power the field windings. This adds some distortion to the final AC output.  Fed with pure DC, or the drive signal from an AVR, the AC waveform is pretty clean.  I posted some pics of my St head with harmonic excitation on coppermine, but I just checked and that site appears to be messed up right now and and I was unable to get any pics displayed.

I wrote up a review for Trigzy(Hightechhicks.com) on that chinese AVR which includes O-scope shots.  He was going to convert it to PDF with the pics in it so we could link to it.  One of the pics is of harmonic and AVR power overlayed and the AVR does have a much nicer sinewave.  It also drastically reduces the flicker from the slow speed single engine I am powering mine with.  I woud expect him to have a link to it up soon. 

I had the occasion last summer to be doing some generator work on my job and took the same O-scope to look at the Onan set we were installing and loadbanking.  Overlay comparisons of my ST-5 and the 12KW Onan showed very similar waveform paterns.  I have modified mine with filter caps to cleanup the DC being fed to the field so it is a little cleaner output than a bone stock harmonically excited ST.  The generator tech who was doing the Onan loadbanking saw nothing wrong with the ST power waveforms and was rather surprised when I told him it was from a chinese ST head. My St with filtered DC to the field powers everything in my house just fine, including computers and UPS's(2 triplights and a APC).

Ron   

[emerald]

Thanks for the coments guys. This sounds like a job to rewire the alternator rotor to take a 12v feed from the starting battery and power the AVR from that source too. Thats a clean source of DC we will get. If the harmonic winding is introducing distortion to the final output I would ask how that winding is laid out. If the spacing is off it will cause a problem and I used see that on PMG's that were home brewed. Its real easy to screw up the sinewave with coils and magnets not worked out right.

Two alternators came in here last week and one was obviously bought on price. And at first glance the owner must have thought he got the bargain of the year. For the output and continuous rating it was a good price. But when some appliance refused to work with it, the success of a cheap deal with pretty good quality parts soon fell down on build quality and I didnt want to judge it too harshly until I asked here for other owners input.

Its a pretty honest and old fashioned design and I see nothing wrong with that. Its just let down by low build quality. A little like some engines.

Emerald

[listerdiesel]

Can't comment directly on the ST, but mil-spec generators generally are separately excited, mainly so that they will start with a a load across the output which would normally prevent the voltage build-up from residual magnetism in the machine.

Feeding from a battery is OK, or a well-smoothed DC fed from the output, but do bear in mind that the field windings can only dissipate a certain amount of heat, and it can be very easy to overdrive the field, especially at low rpm and high loadings on the output.

It's not difficult to produce a regulator, there are plenty of other discussions on here about that, but making it controllable and resistant to spikes etc is another matter, and takes some thought.

I think GE published some details in one of their thyristor app notes in the 1970's, when Markon and Stamford went into solid-state stuff in a big way.

Peter
 

[emerald]

Most of the mil electrical stuff I have laid hands on was all 24VDC and that too is a relatively easy voltage to change an ST alternator for, but as the starter motor on these few home brew gensets are 12VDC its best to stick to whats available.

Heat buildup will happen if too much current is drawn and a current limiting setup would cure that. I dont want to introduce lots of electronics here and if possible make a few alternations to whats already there and clean up the sinewave to whatever final outcome happens as a result. Not to mention the legal can of worms that is opened when new controls are introduced. The owners themselves can decide on deep they want to get into these.

PMG is best for this althought the prime mover sets the output voltage and a guy would want a very well behaved governor. I costed up building a direct drive model for a CS and the thing would be big, and costly on magnets and copper wire if it is built to a high standard. A 1500RPM PMG would also be expensive, although less so over a <700RPM unit. Both would still cost more than an ST alternator, even a well manufactured and high quality item.

The old addage holds true, you get what you pay for.

Emerald