ST heads for battery bank charging

[veggie]

There seems to be a lot of discussion lately about using the ST head for charging at various voltages.
This fellow seems to have solved the problem.
http://www.navitron.org.uk/forum/index.php?topic=952.0

Is this a valid solution.? Seems fairly simple. (KISS in other words)

veggie

[mobile_bob]

yes in theory it should work, but

you are limited to the nameplate amperage whether you are at 12volts or 120volts
and you may be thermally limited due to decreased airflow from the fan, which might be very significant

cutting the rpm in half dramatically reduces the airflow, it is not a linear function

i would not be surprised that the ampacity of the st head would be cut by at least 50% at lower rpm.

if you can live with that, then it is a simple method that has merit and application.

bob g

[veggie]

Instead of 12 vdc, how about going with 48 volts an then adding a step down transformer for higher amperage and charge rates. From there to a solar charge controller to take gentle care of the batteries?

Having a system that can charge at low speed and then, with the flick of a switch and an engine speed change, give full 230 VAC for emergency power is very compelling. Or is this being a bit too optimistic ?

veggie

[mobile_bob]

don't let me rain on our parade, believe me there are countless ways to skin the cat.

until we explore all the various methods, do the testing, and document the results
its hard to say for sure what method works best and for what reasons.

so many variables, voltage, amperage needed, run times, to name a few

we need more out of the box thinking, which will lead to more options

options are always welcome in my opinion

bob g

[V5CVBB]

Thanks for posting the link.  I had been curious about the relationship between excitation voltage and output voltage. 

I had thought of this but never considered the reduced airflow.  Good catch.

[mobile_bob]

the reduced airflow at lower rpm could be remedied with a ducted electric fan
to increase airflow.

another thing i am unsure of is the st heads ability to produce power in a linear fashion
over a wide rpm range, it might be well less than half power at half rpm.

also care in making sure the field current is not exceeded is going to be a concern i would have
i am not sure how much over the field can take without burnout, so
the use of some sort of regulator or current limit scheme would seem prudent?

the last thing i want is any part of a suggestion that might damage an otherwise good st head,
damage is all to easy when one starts to experiment in operating parameters well outside of the oem
design.

so it would seem to me, in order to run at reduced speed and charge at a lower voltage, we need
to assure the following

1. adequate airflow, and probably some sort of temp sensor to monitor stator temps

2. some dynamic control over excitation current, by dynamic i mean some sort of regulator and overcurrent
protection

3. then do enough testing under real world conditions, with the head operating in a thermally stable temperature range
in an ambient temperature that is as high as it will ever be asked to run in, so that we are sure it is stable.

4. some method of output amperage control, that would keep the stator from an over current condition should the batteries be
lower than anticipated or batteries low and a load comes online.

just some things to consider fwiw, and there are likely other concerns as well?

bob g

[BruceM]

Bob's raised most of the important issues, but I'll add one more.

Most battery manufacturers warn against chargers with high ripple current.  This is the AC component of rectified DC.  AKA "Lumpy DC". 

In the article above the guy is running at half speed, with only 25Hz output.  The money saved by avoiding a transformer will be spent several times over trying to filter the output, since filter size and cost grows exponentially with lower frequency. (This is also why modern switching supplies with 100KHz and above speeds are so small and cheap, and some small DC-DC converters now switch at 1MHz.) 

So while controlling the excitation to regulate the ST output voltage is a good idea, and reducing the Lister(oid) speed at reduced power loads is a good idea, reducing the single phase ST generator head speed is not such a good idea. 

If you were starting with a 3 phase head, which is much better for generating DC and would require little or no filtering, then reducing speed could be considered, assuming you address the issues of cooling and loading that Bob has mentioned.

BruceM

[mbryner]

Pardon my ignorance, but could you do what the guy in the link did, but run at full speed?  If the excitation voltage is dialed down, does the output Vrms go down accordingly?   Would that eliminate the need for a step down transformer?   Still need filtering of course.

Marcus

[BruceM]

Sure, you can just reduce the excitation to get any output AC voltage you want.  But your output current will be limited to the rated max voltage current.  Only a transformer can convert watts in (volts x amps) to watts out while converting voltage.

[mbryner]

Wait, wait, wait!  I'm getting excited here!  Here's another way to skin a cat.   My ST head is a 7.5 kw.  A listeroid 6/1 can only sustain 3000 watts anyway.   So, let's say I put a dimmer switch into the excitation coil circuit, dial the dimmer down until the output from the ST head is the appropriate voltage (for rectification and filtration into your desired charge controller).  Then measure the resistence across the dimmer, and insert appropriate resister instead.   No?  Add a switch to short the resister when normal ST output desired.  True, the ST 7.5 is now approximately an ST 5, but what does that matter.

[BruceM]

Marcus-  If you are considering this for a 48V battery bank, then OK, though forget the dimmer. A variac would be better.  The harmonic winding puts out 4 sharp spikes per revolution, two positive and two negative going. The dimmer's triac control isn't set up to handle the very short duration, or the higher voltage peak.  A small variac will not complain, however, and could be used permanently.

I'm assuming you're still going to feed a PWM solar charge controller to do the battery charge regulation, right?  Otherwise, you'd need a special AVR design.

 

[mbryner]

Yes, for 48 V battery bank charging.

No, I have a Outback MX80 which is MPPT not PWM.

Remember, I was going to use the toroidal transformers to step down, then big transformer windings as a inductive filter.   Thanks for the tip on the variac.

BTW, about those toroidal transformers I bought on e-bay:  the seller never sent them.   That was 1 month ago.   He won't reply.   I'm going to have to file a claim and complaint.   Arggh.

Marcus

[BruceM]

The MPPT controller also does PWM of sorts (they use a buck/boost converter).

Bummer about the transformers.  I've had very few deadbeat sellers, but sometimes it happens.

[mbryner]

Hey, I have to retract what I said about the toroid transformers.   One came in the mail today and the seller wrote that he has had a delay in shipping.   

Using a dimmer, I would put it in the circuit in place of the resistor,  btwn F2R and R1 on the schematic in the ST manual ( http://utterpower.com/ST_manual.pdf ).   If I were to use a variac, would it be placed in the circuit in the same place?

[BruceM]

Yes, the dimmer (a dual SCR or Triac part-wave switching control device) would go in place of the dropping resistor, but a dimmer will likely fail on the first surge load, which boosts the harmonic spike over 400 volts, the dimmer is designed for 180V peak.   The dimmer is also not designed to switch a wave with only a millisecond or two time duration.

The Variac would be connected to the harmonic winding on the input side, and it's output side fed to the bridge diodes.  One problem- you may have to turn it all the way up to get the generator started.

A Variac can also use the AC output as the input.  Same situation- you have to turn it up very briefly to get it started.  Bill Rogers helped me with this years ago-  it's amazing how NOT using the harmonic cleans up the AC waveform of the ST heads.  For a steady load, the Variac connected to the AC ouput of the ST will let you dial in any output voltage you want.  Connected to the harmonic, it should give you fair regulation despite changes in load.