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Author Topic: ST Head at 50 HZ  (Read 5791 times)

veggie

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ST Head at 50 HZ
« on: February 08, 2016, 06:27:11 PM »
A couple of questions relating to ST performance...

What is the typical VOLTAGE output of a single phase 60 hz ST2 head wired for low voltage? is it 110V, or 115v, or 120v ?
Is the relationship linear between speed and voltage on an ST head ?

The ST Head will be driving an IOTA 55 amp, 12 volt battery bank charger.
IOTA states that the charger can operate anywhere between 50hz and 60hz.
Allowable input voltage to the charger is 108 to 130 VAC.

The engine/generator are direct coupled at 1800 rpm (60hz) and the engine produces far more power (and noise) than needed.
I want to know if I can slow down my diesel & ST Head to 1500 rpm (50hz) and still charge the batteries.

If the speed/voltage relationship is linear then I could theoretically reduce the engine speed to 1500 and still charge batteries and make 50 hz. power.
But the resulting voltage would be too low for the IOTA charger.
eg:   115 volt X (50/60) = 95 volts.... too low for the IOTA

so.... in order to get at least 108 volts, I would have to slow the engine down to 1674 rpm or 55 hz.

That is ... IF the relationship of ST speed and voltage are linear and the initial 60 hz voltage is 115VAC (or is it 110v ? or is it 120V?)


Thanks,
Veggie


« Last Edit: February 08, 2016, 06:29:29 PM by veggie »
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Tom

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Re: ST Head at 50 HZ
« Reply #1 on: February 08, 2016, 07:59:55 PM »
Some ST heads can be wired for 50 or 60 hz operation. So if you want to run it at 50 hz and your head has that option do that. Otherwize I think that charger will just pull more amps at the lower voltage to maintain it's out put setting. Also keep in mind a ST head with no AVR will start out at a higher voltage and drop a bit as it warms up. Mine starts under load at like 125 and drops to 115. That's an ST5 running at a steady 3kw charging batteries.
Tom
2004 Ashwamegh 6/1 #217 - ST5 just over 3k hours.

BruceM

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Re: ST Head at 50 HZ
« Reply #2 on: February 08, 2016, 08:15:03 PM »
Hi Veggie,
You should be fine at 50Hz, just connect the 50Hz windings, and ADD CAPACITANCE on the DC side of the bridge diode as needed to boost the ouput voltage sufficiently.  You can use motor run caps most safely.  Even a small amount will boost voltage.  If you go to higher levels of capacitance, the voltage boost tapers off, but you can then use electrolytic caps of 100V rating or so, since the peak voltages will be reduced.  Peak voltages of my ST-3 harmonic with no capacitance were in the 400V p-p range on 1/2 hp motor starting surge.  As you add capacitance, the peak will diminish substantially.  

The IOTA has notoriously horrible power factor, chopping the upper part of the waveform off, but the stock ST generator harmonic regulation system copes with this well as long as you have enough head room on the capacity.  You certainly should be OK for your plan.

Since most switching supplies will run on 120VDC quite nicely (my home is 120VDC), it might be possible to reduce your speed even further, by converting to filtered (inductor and caps) DC before the IOTA.  If the harmonic was insufficient at say 40Hz, a modified version of my simple AVR design would keep the  DC level up to whatever is needed.  A bit of a project, but perhaps your 50Hz solution is sufficient.

Another way to allow allowed lower frequency would be to open up the IOTA and increase the high voltage capacitance after the bridge rectification.  

Best Wishes,
Bruce

« Last Edit: February 09, 2016, 04:31:21 PM by BruceM »

mike90045

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Re: ST Head at 50 HZ
« Reply #3 on: February 08, 2016, 08:17:32 PM »
The older Iota chargers were not frequency dependent, and would actually regulate high voltage DC, so maybe a trial with your hand on the throttle to see where it cuts off would be worthwhile.   If it's a new one with PF correction, I don't know how that would work with odd frequencies,

BruceM

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Re: ST Head at 50 HZ
« Reply #4 on: February 08, 2016, 08:22:00 PM »
I forgot to answer your specific question.  There is so much variation in output voltage and waveform quality between different ST heads, even by the same manufacturer, that any answer your question would not necessarily apply to your particular set.  You will likely get somewhere between 115 and 130V at your proposed load at 50Hz using the 50 Hz windings.  

You can boost voltage by adding capacitance, and reduce voltage by adding a power resistor in series with the harmonic output/field coil excitation.  

My experience is that most of the newer PF correction switching power supply designs work quite well on 120VDC input.

veggie

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Re: ST Head at 50 HZ
« Reply #5 on: February 09, 2016, 12:12:23 AM »

Ok, so if I understand you guys correctly....

The simplest way without adding caps or resistors would be to simply wire the ST for 50hz and let her fly ??
This would give me nominal 115 volts at 50 hz  at 1500 rpm?

correct ?

Veggie
- 6/1 GM90 Listeroid - Delco 33si Alternator
- Changfa R175 - Lease/Neville Alternator
- JiangDong R165 Air cooled - 2 kw
- Changfa S195 (Waiting for a project)

buickanddeere

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Re: ST Head at 50 HZ
« Reply #6 on: February 09, 2016, 12:28:51 PM »
I would isolate the factory excitation system and use a brush type regulator that is both supplied from and senses voltage from the alternator's main output terminals.  It will increase field excitation current as rpm drops to hold output terminal voltage, down  until the excitation field is saturated before output voltage drops.
   Keep in mind if near rated current is being drawn form the alternator at less than rated rpms . There is also less cooling air flowing through that alternator .

38ac

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Re: ST Head at 50 HZ
« Reply #7 on: February 09, 2016, 01:01:48 PM »
My experience with ST heads is not all that broad, 5 heads from 5 to 20 KW.  One of them had the manually operated rheostat for voltage adjustment, I like that feature. As I remember the voltages at 60 hz  all ran between 118 to 125 after warm up, 5-10 volts higher at start up.  Although I have never tried to make science out of it the voltage drops off quickly when RPM drops, I dont think you will find it to be linear if you simply drop RPMs to 1500. Be aware that not all ST heads have the extra leads that correct the voltage for 50Hz operation, out of the 5 I have experience with 3 did, 2 did not. Might be a U.S. market deal?
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BruceM

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Re: ST Head at 50 HZ
« Reply #8 on: February 09, 2016, 03:41:51 PM »
Yes, Veggie, assuming you have the 50Hz connections on your head, just doing that should be sufficient.  After that, if it's still low, you could try adding a motor run cap across the bridge DC side to boost voltage a bit more. 

If you don't have 50Hz winding outputs (higher voltage taps on stator) on your ST-2,  then the other possibility is as B&D suggests, find an AVR for 120V, using the mains as the excitement source.  The problem with CGG's bargain 120V AVR is that Tom has zero technical info for it.  It might work at 50Hz...buy you'd have to ask Tom about that.  I also don't know how well it works on very poor PF loads...no telling if it does RMS voltage regulation or not.