Author Topic: Voltage drop on only 1 120v leg, 7.5KW ST head  (Read 954 times)

cujet

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Voltage drop on only 1 120v leg, 7.5KW ST head
« on: September 21, 2023, 02:00:25 AM »
Today, I tried to start my small portable air compressor on my 8/1 powered ST genset. Attempt 1, resulted in lots of groaning and no air compressor action. On a whim I tried the other 120V leg and the compressor started right up every time.

When using leg 2, The 240V meter shows 238V with the compressor load. And a similar number with the 1500W heat gun.

However, the 240V meter shows 218V when the heat gun is used on leg 1.

Thoughts?
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ThomasEriksen

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Re: Voltage drop on only 1 120v leg, 7.5KW ST head
« Reply #1 on: September 21, 2023, 09:51:03 PM »
First thought: high resistance due to bad connection, check resistance

Powdermonkey

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Re: Voltage drop on only 1 120v leg, 7.5KW ST head
« Reply #2 on: September 22, 2023, 12:00:52 AM »
Hey CUJET- My suggestion would be to check the wiring going to, and condition of the #1 brush.  Check the condition of the copper contacts as well.  I don't know, but I'd also suspect that the AVR has split circuits, with the "controller" referencing #2.  Might switch those leads to the brushes, and see if #2 acts like the old #1, and #1 acts like the old #2.  That COULD tell you whether the problem is within the AVR and wiring to the brush, or the brush contact itself.  Right?

BruceM

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Re: Voltage drop on only 1 120v leg, 7.5KW ST head
« Reply #3 on: September 22, 2023, 03:19:44 AM »
Thomas's  "check connections" and PowderMonkey's comments are both good, the latter if you have an AVR with 120V, single leg sensing, which is a bad idea. 

You didn't mention an AVR.  Is is using the stock harmonic excitation?
Is this a new problem or have you never tested large 120V , asymmetric loads?  I ask because many (smaller than your ST-7) ST heads moan and vibrate badly with such a load, and regulation not good.

My first thought is to go over the basics via ohm meter:

I'd check the resistance of each set of the stator windings looking for a difference in values and documenting the resistance for future reference..

I don's see brushes or  the rotor as likely culprets due to the 120V leg voltage asymmetry.  But it never hurts to check connections, and document resistance of the rotor coils for future reference.

Best Wishes,
Bruce


 

cujet

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Re: Voltage drop on only 1 120v leg, 7.5KW ST head
« Reply #4 on: September 22, 2023, 04:03:08 AM »
I will have to double check, but I believe the ST 7.5 does not have any aftermarket voltage regulator. Just the standard excitation (Z?) winding.

It is not a new problem, in that I've never tried to start this air compressor on this genset before. I have run both legs of the generator with 1500W heaters, a balanced load, with fairly good results. But I think this is the first time I actually looked at the voltage meter and noticed that one leg is weaker than the other.

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38ac

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Re: Voltage drop on only 1 120v leg, 7.5KW ST head
« Reply #5 on: September 22, 2023, 11:59:45 AM »
I'm not electrically smart enough to help with your issue but my ST15 has a 4 to 5 volt differance between the 120 legs. It is running on the harmonic with no AVR. It has never caused problems in use that I am aware of.
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BruceM

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Re: Voltage drop on only 1 120v leg, 7.5KW ST head
« Reply #6 on: September 22, 2023, 01:00:40 PM »
38ac's good experience with the ST-15 for asymmetric 120V loads is normal-  it seems the smaller ST heads have more of an issue and most are best run at all 230V or all 120V.  I was hoping your 7.5KW ST head would be OK.  (And it almost is.) I use a step down transformer for 120V loads at my shop to avoid the problem. There is a great deal of variation in "ST" rotors and stators, at one time I had three ST-3's on my bench and all 3 of the stators were  different in length and wiring configuration.

Maybe for your application just knowing which leg handles the big asymmetric load better will be enough?