ST3 Noise and vibration

[veggie]

I spoke too soon when I installed my new ST3 head.
When I test ran the unit (unloaded) it was quiet and smooth.
Then I tested it with a balanced 240 volt ac load. Again smooth and quiet.
So I thought all was good.

Well ... today I tested it with a 1200 watt resistive space heater on one leg and only a 75 watt light on the other leg.
The growling and vibration was so bad that I thought something had broken in my system !
Terrible vibration every time I unbalanced the two legs, but just fine when the power is balanced.
A] Is this normal for an ST3 (do they need a balanced load all time) ?
B] Can it be fixed ??

Veggie

Here's some pictures.
The unit is wired for 230 volt and I am loading one leg more than the other when the vibrations and noise occurs.

[BruceM]

The ST-3's that I have experience with all can not tolerate ANY load imbalance.  Nothing to be done except to run either 230 or 120V. If you need both, step up or down via transformer for the smaller load, or step down near point of use if going some distance.  I recommend Antekinc.com for high efficiency toroidal transformers. 

https://www.antekinc.com/an-154115-1500va-115v-transformer/

I have also used smaller transformers in parallel to increase capacity with a suitable fuse for each transformer. The Antek transformers will handle huge surge loads.  A warning-  because of the high efficiency wound silicon steel cores, they do have high inrush current so soft start may be required on something like an inverter.  The ST-3 has no trouble with them.

I will note that I have seen significant variation in ST-3 stators, in particular, along with great differences in harmonic winding and clearance between rotor and stator. These affect waveform distortions, regulation, etc.  A pity no one has yet identified a single source in China that is reliable and has consistent higher quality. 

[veggie]

Thanks BruceM,

Based on your comments I will reconfigure to 120 volt as most of my loads are that voltage.
That growling/vibration is really terrible !
Wiring for 120vac should allow me to pull the full load without issue.

cheers

[veggie]

@BruceM,

You nailed it ! Thanks.
I rewired to 120 volts and all noise and vibration disappeared.
If you look at the pictures above, the 4-prong 240 volt receptacle in the orange box now has two 20A, 120 volt receptacles.
Full load and still quiet. No vibration.

My house transfer switch requires split phase 240, so this generator will be mostly useless as a backup house generator.
Fortunately it powers the greenhouse 120vac electrical + heat loads, so it still has a job.
Hmmmm.... time for another generator build for the house ? 

Now, here's a question for you (and anyone else).
The listeroid/ST-3 will be powering a greenhouse that is not grid connected.
It's electric outlets are not connected to the house, so they are not wired to a source that has ground and neutral bonded.
The ST-3 head has a floating neutral (not bonded to ground)
When powering the greenhouse, the whole system is a floating neutral.

When using the ST heads for home service the floating neutral is desired, but for powering anything other than a house, do we bond the ground with neutral?  or not?
As a test, I made a plug which connects ground to neutral, and plugged it into the generator outlet while running. No observable change.
However, the safety aspect changed. For the better? or for the worse? I don't know.

Here's another (but similar) example:
A listeroid with ST head powering a 120 volt 75 amp (24 volt)  battery charger for a large battery bank (1900 watts). No grid connection at all.
The charger is plugged directly in to a 120 outlet on the 120VAC ST head using standard 20A 120 volt plug with L1, Neutral, and Ground prongs.
Should the ground be connected to the neutral somewhere at the origin of the power ?

cheers

[BruceM]

Glad that it was just the usual ST-3 split phase snafu.  They can really moan and groan with sounds that say imminent failure.

You can carry a split phase situation with the ST-3 by adding a transformer with dual 115V windings, or adding two transformers  with 115V secondaries if you need more than 1500W of power.  The load on each 115 split phase must be below the rating for that secondary winding.  Spendy and a bit wasteful (40 watts iclle current for 2- 1500W tranformers) but its the best you can do with an ST-3, typically.

You can drive the Antec primaries with either 115 (primaries in parallel)  or 230V (primaries in series).

Regarding grounding of the neutral.  Sure, its nice to have the neutral grounded in one and only one location- if you ever intend to power the greenhouse from the house service - or someone might do that via extension cord, then making the greenhouse a normal code compliant sub panel with separate neutral and grounds being supplied by a grounded neutral/safety ground generator is probably best- assuming generator unplugged if house power is connected.  If the greenhouse was remote and so the generator is the only potential power source, then it could be set up just like main panel with the grounding rod connecting to the ungrounded generator ground and neutral tied to that at the panel.

Multipoint grounding of the neutral is a no-no as it causes current to flow through the safety ground which can then act as an "alternate neutral".  Bad news as the safety ground is NOT intended to be a current carrying wire within the home, it should be a relatively safe earth ground except only for clearing faults in an emergency.

[veggie]

Hi BruceM

Yes, you confirmed my understanding of neutral bonding.
ONLY one point in the system can have the neutral/ground connection.
In my case the greenhouse is off grid.

veggie

[38ac]

Veggie,  I'm surely not smart enough to help you with electrics but wanted to say I'm sure glad to see Bruce posting.
My ST15 doesnt care how the 110 loads are split but an ST-5 I have paired with a 6/1 will protest loudly if the 110  loads get nore than a little bit imbalanced

[veggie]

Veggie,  I'm surely not smart enough to help you with electrics but wanted to say I'm sure glad to see Bruce posting.
My ST15 doesnt care how the 110 loads are split but an ST-5 I have paired with a 6/1 will protest loudly if the 110  loads get nore than a little bit imbalanced

Nothing like a good electrical question to pull Bruce out of hiding 
Greetings to you both, 38AC and BruceM