ST Wavfeform question

[veggie]

Hi all,

I have been digging around the older posts but I could not find an answer to this question...
I have an ST-2  2kw head that I was thinking of using on a small project.
I am wondering what is the typical quality of an ST waveform ?
Are they usually OK?  Perhaps rated as "average".?
I would assume that this is something that is "built-in" to the winding quality and cannot be adjusted if there is a distorted wave?
(I do not own an oscilloscope)

My other option on this project is to use a Voltmaster 3600 rpm brushless unit which is typically a very good generator, however given the choice I would prefer an 1800 rpm ST.

What's your experience?

Veggie

[BruceM]

Veggie, the waveforms of the ST heads are often usable, with THD of 15-20%. (Poor for generators.)  Some have terrible spikes on the peaks which cause trouble for electronics, some have very bad ratcheting or discontinuities/spikes along the waveform.  For running 1/4 hp or more  induction motors, none of these will be problematic though some small motors (as in timers) may moan and get hot.

The waveform distortion variations seem to be caused by the stator windings, of which there are many different versions. I had 3 different ST-3 stators on my bench at one time,  and no two were the same, even in physical dimensions of the laminated core.  This seems to be a situation where China is running head to head with Rajkot in variability and lack of QC.  Even recent units from CGG exhibit wild variation including improper (high) harmonic output to the point where an AVR isn't optional. (Although that unit has a decent waveform.)

There is no way to tell what you have short of looking at it with an o'scope and load testing it.  Even the cheapest handheld digital ones will work and are a useful tool.

[veggie]

Thanks BruceM,

I got a chuckle from your comment about China running head to head with Rajkot.  

Thoughts of a huge corporate merger come to mind where ST heads and Listeroids are all built in the same factory.
The managers all rejoice because they share the same business philosophy.
"just throw (or hammer) he darn thing together and ship it to the west....from there it's out of our hair !"  

I suppose it's a bit of a crap shoot as to whether one gets a good ST or a dud.

cheers,
Veggie

[mike90045]

And don't forget the newfangled aluminum windings.

[vdubnut62]

I wondered why the kitchen ceiling fan bitched about the the ST 'lectricity, it moans like a lost soul. Everything else seems happy except for the heat pump it just sulls up and pouts.
I gotta get one of those whizbang handheld sillyscopes, if I can only figure out how to use it, oh well I can alway post an image and let one of the more Learned Ones interpret.
Ron.

[BruceM]

Most of the digital o'scopes have an auto mode where the processor will figure out time scale (horizontal) and voltage range (vertical) for you.
Even the cheapest 1 Mhz o'scope is adequate for this type of thing. 

The ST waveform problems do not seem to plague the 3 phase units.  There aren't a lot of good choices for 3000KW single phase 1800 rpm generator heads. I know Gary recommends the Stamford clones for larger heads, and I highly value his expertise and experience.   

[buickanddeere]

  Running an aftermarket voltage regulator/excitation supply from the main AC output windings usually provides cleaner power and superior voltage regulation vs the "harmonic windings.  Idling back the single cylinder rims and using extra pulley to maintain the desired frequency will develop lumpy waveforms.
  750rpm of twin is a long way from 500rpm of single as a prime movers.

[BruceM]

The more common waveform distortions of the ST head are "harmonic hump", ratcheting, and spikes near the voltage peaks.  The harmonic hump is bad on my ST-3 and was eliminated by using the mains for excitation with an AVR of my own design to also address "Lister Flicker", which is the 10 Hz power stroke speed effect on frequency (and voltage). Lister flicker is not a wave form issue.  It's just a voltage and frequency regulation issue.  Most equipment doesn't seem to mind the frequency variation at the power stroke much.

Harmonic hump is highly variable between units and is again a stator winding issue. Current in the harmonic winding is affecting the adjacent mains windings and causes sometimes serious distortion that makes smaller motors growl and get hot.
It can sometimes be fixed by adding capacitance to the DC side of the bridge, as this shifts the current timing.  That fix was developed by Ronmar.

Adding lots of capacitance to AC is a serious problem for reactive power current and power loss, but some have improved their spikes/ratcheting by adding on fairly high levels of capacitance across the mains - in the order of 30-60 uF of motor run capacitors. This is not such a good idea on the ST-3 but may be tried on the ST-5 as there really is no other simple fix. The ST-5 can carry the extra reactive power loss better as it has so much "head room" when used with a 6/1 Listeroid.

A better or rewound stator for the ST-3 would be an item I'd buy in a minute.