ST-5 typical wave form

[veggie]

On this forum I have seen many comments about the lousy waveform of the ST heads (single phase heads).
Unfortunately, I don't have an oscilloscope so I can't check this for myself.
Are the ST's really that bad ?
So many of use use them and we rarely get comments about appliances being unable to run.

So, how is the waveform of the single phase ST's.?
How far does it deviate from the utility mains "near perfect" wave?
Anyone got a oscilloscope picture of a single phase ST waveform ?

veggie

[Ronmar]

I have some I saved a few years ago, as far as waveform goes, it isn't horrible.  I had these saved on the O-scope and had the opportunity to compare them to the outputs of some of the sets i work with, and they compared pretty well.  Perhaps the biggest issue is they are straight rotors, so show a little slot noise in the waveform, and the harmonic signal is fed back into the field unfiltered(it is really ugly pulsed DC).  

Here is my stock ST-5, harmonically excited, under no load and 1.5KW of load.



Here is my ST-5 with harmonic winding disconnected under 1.5KW of load with pure DC being fed as excitation:


And here is one under 1.5KW being fed with a pulsed AVR(note the little spike in the waveform):



The pure DC and AVR make a much nicer waveform, as shown in this overlay of AVR and harmonic excitation under 1.5KW load.


As you can see, the harmonic excitation distorts the output waveform somewhat, I believe mostly because of the really poor waveshape being fed back into the field from the unfiltered rectified DC.  It looks much better with a little bit of filtration added to the field input in the form of electrolytic filter caps.  Here is one under 1.5KW with 1500UF of capacitance in the circuit.

[veggie]

Ronmar,

Thanks for posting those pics. Very useful.

1]
There is no voltage scale on the Y axis. In your first picture (unloaded) does the wave top out at 120 volts?

1a]
In the second picture (stock head at 1.5 kw) the peak of the wave seems to be at a lower voltage ?
How much lower is hard to discern, but there does seem to be a voltage drop when loaded.

2]
Is the wave form of the unmodified ST5 acceptable for general use (ie: appliances, lighting, electric motors) ?

veggie

[Ronmar]

There is a scale, it is just real hard to see along the edges.  I turned off the graticule when I saved the images as it made the waveform a little easier to see.  Unloaded it tops out a little above 120.

Yes, the output drops under load, and the waveshape changes significantly, as the unfiltered field input is very unstable and I believe it  cannot maintain the waveshape up to the peak.  The filtered input and that from the AVR provides a more consistent field IMO and allows the waveform to better maintain it's shape under load. 

Stock, mine ran all my appliances but would not keep a UPS online, which prompted my experimentation into why the waveshape shifted.  With filtered field input, it is acceptable by the UPS's I use, unless i overload it.

[bearhawk]

Hi Ronmar,
Are the inputs of the AVR from the output of the main windings (120v) so you can buypass the excitation coils? 

How did you "Here is my ST-5 with harmonic winding disconnected under 1.5KW of load with pure DC being fed as excitation:" as that looks great!

[Ronmar]

Hi Ronmar,
Are the inputs of the AVR from the output of the main windings (120v) so you can buypass the excitation coils? 

How did you "Here is my ST-5 with harmonic winding disconnected under 1.5KW of load with pure DC being fed as excitation:" as that looks great!

The AVR I was testing was connected across both legs(240V), and bypassed the harmonic winding alltogether.

The pure DC excitation was done with some series gel-cell batts, just to show what the waveform looked like with pure DC.  No output voltage regulation whatsoever, I just added power to the field to get near 120V output on each leg into a fixed 1.5KW resistive load.