ST Service factor

[veggie]

Has anyone taken an ST head above it's rating ?

Part 1:
I am wondering if the ST types have a service factor.
Some heads state a "rated" wattage and an intermittent "peak" wattage (eg: 4000/4800)
Does an ST-5 burn up at 5.1 kw's ?

Part2:
Can an ST head operate continuous at it's rating ? (eg: ST-5 running continuous at 5kw)

veggie

[tinkerer]

good question Veggie. I've wondered the same thing.

[mobile_bob]

my opinion, based one what i have learned
or rather what i think i have learned

the st series heads were built to service lighting loads, so they are rated
such as 5kwatts, 7.5kwatts etc... this they are to provide for on a continuous basis
however...

this is powering a resistive load, or power factor of unity

with our mixed loads we are generally never running at unity, so unless you know
what your powerfactor is in its worst case, i would assume a power factor of 0.8

this means the ST heads should be derated to about 80% of full rated load, powering common
mixed loads, so a continuous rating of 5kw x .8pf = 4kwatts continuous.

in order to keep the unit from overheating one would need to keep his loads to not much over 4kwatts on an st5 powering a mixed load with a pf of 0.8

of course that also means if you are running mostly all modern power factor corrected appliances/loads, then perhaps your mixed load power factor is higher than 0.8 maybe 0.9
in which case you can run the  head at 4.5kwatts continuously... this about as high as i would suggest running one continuously, because it is unlikely a mixed load is going to be much better than 0.9 and very doubtful that they it will be corrected to over .95pf

having said that, i would expect that there might be some st heads that can run continuously at higher continuous ratings and probably more likely there are many more that won't.

in testing of an ST7.5 into a resistive load bank, power factor 0.99 (near unity) the head tolerated 7.5 kwatts continuously without getting much more than warm, (105 degree F)
so this particular head probably would do more than that if i needed it to.  however it is an
older heavy st head, rather than one of the newer and lighter built heads.

there is also something else to be addressed in the rating of the head

with the st head using harmonic excitation, it could be that the ratings are such that at full load
the regulation is acceptable, going over it might result in a lower output voltage... if this is the case then the head is not thermally limited by the rated load but regulation limited.  if the head uses some sort of voltage regulator then all bets are off.

in any event it is hard to imagine them engineering and building a head with more capability than what they would advertise, so i would lean toward derating rather than pushing the head over its limits, (taking into account the discussion regarding power factor concerns)

note:  mixed load reference means a mix of resistive and inductive loads, such as incandescent lighting and motor loads, or resistance heaters and computer power supply loads (newer comp power supplies are power factor corrected so this typically is not the problem it used to be.)

fwiw
bob g

[Jens]

Has anyone taken an ST head above it's rating ?

Part 1:
I am wondering if the ST types have a service factor.
Some heads state a "rated" wattage and an intermittent "peak" wattage (eg: 4000/4800)
Does an ST-5 burn up at 5.1 kw's ?

Part2:
Can an ST head operate continuous at it's rating ? (eg: ST-5 running continuous at 5kw)

veggie

Part1: 5.000000001 kw
Part2: as Bob said

IMHO, the 'rated/peak' thing is, using your example above, 4000 continuous and 4800 for a short span such as 10 minutes or whatever.
In any case, the ratings are only guidelines because they do not specify the conditions such as ambient temperature. If you run this test on a nice winters day with -30 ambient and 100 mph ambient air ducted over and through the head then I am sure things will be quite different.

Oh, don't forget that if you have a 220V head, this assumes even loading on the two legs (and the power factor correction)

Look at the ratings as a guidline only because you have no idea what the insulation on the wires are rated at and the ratings have no information under what conditions they are applicable and I am sure that there will be some "Sears factor" applied for the advertised rating. I would not want to go above 5 kw*0.8 on a 20C day (assuming that the generator doesn't sit in a room with limited air supply) and I would take the intermittent rating * 0.8 as the maximum motor starting load for a 220V motor (half that for a 110 motor)

My 2 cents anyway .....