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In Reply to: RE: OHMS LAW..... posted by jea48 on July 13, 2012 at 21:04:00
Yes I agree it makes sense just wonder how Jon arrived at what he did for the SU-2 in balanced.
There has to be a hint in what he wrote in first paragraph for the SU-2 in the link below.
"Unit #2
Balanced AC Power only
For this, we use the Signal SU-2 model, which will provide balanced AC power at up to 16 amps of current draw (it is also rated at 2 kVA, but the wiring places more stress on the core). This model can not be readily converted to simple isolation use, unless you were taking 240 VAC down to 120 VAC."
Follow Ups:
I was in contact with Signal's application engineers, and specifically stated the exact scenario that I was concerned with, including wiring diagrams, etc. The figures I came up with were derived from my discussions with the Signal engineers. They also make sense to me, with what I know about magnetics and transformer physics, etc., or I would have questioned the Signal engineers more thoroughly.
I stand by my figures as posted at my original web site.
Jon Risch
Hi Jon,
Thank you for your response to this thread.
If I understand correctly the two secondary windings, of the SU-2, are apparently wound with #14 awg copper wire minimum. Would you agree?
E / I = R
120V / 16 amps = 7.5 ohms
With the SU-2 wired as you described in your 1999 paper ecir38 should be able to connect a resistive load of 7.5 ohms across the 120V secondary and the output voltage should measure within a 5% VD.
(120V unloaded, 114V loaded output.)
Primary mains voltage would need to be checked unloaded as well as loaded for VD and the % loaded output VD drop adjusted accordingly.
Would not this test tell anyone reading this thread that the SU-2 xfmr is indeed capable of handling a connected load of 16 FLA continuous for 3 or more hours as was represented to you by the Signal Transformer Company engineer in 1999 without damaging the xfmr?
Thank you for your input,
Jim
RE the first part, I agree that, yes such a measurement should tell where the transformer stands with heavy load current.
Re the last long sentence, the 3 hour continuous draw current capability is moot.
Music is not a sine wave, nor is it continuous. Unless the amp is Class A, the transformer will never see a full rated load continuous load current.
If the amplifier was Class A, then the transformer might see 1/2 the load current represented by the power amp load rating into a given nominal impedance, with some increase above that for the brief moments when the current draw of the Class A power amp slightly exceeded the draw of 1/2 of full rated power.
With almost any other power amp, the current draw will average out to less than 1/3 full rated power current draw, and more typically, 1/8 of full rated load current draw.
UL, which are known to be severely conservative in their testing and ratings, uses the 1/8 power point of full rated load, and the current draw at that point for the "average" power draw from the line for most modern power amps, with the exception of a Class A type power amp.
Switching amps are even less current draw that that criteria.
If you use the SU-2 with a power amp that at full rated power output into a given load, draws at or less than the continuous current rating of the transformer (per Signal), there is not going to be a problem. No way.
Even with 10% THD clipping point (which sounds absolutely terrible to any one with audiophile/music lover pretensions), the power draw is still only a moderate fraction of the full rated power current draw for the power amp.
Music has a crest factor, so does a sine wave. The crest factor is the ratio of the peaks to the RMS value of the signal.
For a sine wave, it is 3 dB. For even the most highly compressed Rock/DubStep/Reggae whatever, the crest factor is going to be no worse than 10 dB, and is usually higher. That means for a 1000W power amp, the average RMS signal power is going to be 100W when the amp is clipping.
10% THD clipping only raises that average to maybe 120W.
Old school rock and roll has a higher crest factor, as does jazz, classical, etc. A ballpark figure for most other music that is not heavily compressed and clipped during mastering is around 15-25 dB crest factor, which puts our hypothetical 1000W power amp, clipping when it reaches an average power of from 31 to 4 watts.
Not exactly burning up the wires, eh?
Jon Risch
RE the first part, I agree that, yes such a measurement should tell where the transformer stands with heavy load current.Re the last long sentence, the 3 hour continuous draw current capability is moot.
No it is not moot.... If the Signal xfmr Engineer represented to you the SU-2 was capable of handling a connected FLA load of 16 amps he has to stand behind his claim.
Saying now that he meant short pulses, bursts, of current up to 16 amps is a horse of a different colour.As for the definition of continuous load.....
NEC 2011
Article 100 Definitions
A load where the maximum current is expected to continue for 3 hours or more.
When Signal Transformer gives the FLA data for their xfmrs I assume it meets the definition of continuous load.Jon, the only reason I have pushed the issue here is for a reader of this thread that might come away thinking the SU-2 is capable of handling a total connected load of up to 16 amps when the 2 secondary windings are wired in series... As you have pointed out in your response it is not.
I personally would not use the SU-2 to power a power amp.
A 20 amp dedicated circuit will easily handle short pulses, bursts, of current of 75 amps or more. The branch circuit breaker won't even see it....
Will there be corresponding dips in the voltage at the outlet recept?
How much depends on the size of the current pulse and the length and size of the wire used for the branch circuit. One reason many use #10 awg wire.You won't do that with the SU-2......
Jim
Edits: 07/16/12 07/16/12
Thanks for chiming in Jon.
Specific instructions for use as a balanced AC power unit:
Connect the AC input power to the primary windings. There are a set of two rows of four terminals, marked "0", "208", "220", and "240". Place the "0" terminals in parallel (connected together) and the "240" terminals in parallel. These are the input terminals for the AC power from the line cord, which should run through a safety device such as a fuse or circuit breaker. See the common section at the end for details. For the secondary, connect the AC outlet neutrals to one of the "0" terminals, then connect the "120" terminal of that row to the "0" terminal of the other secondary side row of terminals, and connect the AC outlet hot to the remaining "120" terminal.
*The two primary windings are connected in parallel. Fed by 120V....
*The two secondary windings are connected in series. The outer 2 leads will output 120V nominal. Each outer lead to center tap will output 60V.
Here the manufacture design and FLA data info must be used.
If the winding wire size is the same as the DU-2, (not knowing for sure), then each winding has a max rating of 9 amps each.
With the secondary windings in series, yes the voltage is doubled from 60V to 120V (nominal), but putting the two windings is series will not double the available output current rating. Current is the same in all parts of a series circuit.
I admit I could be missing something here with the Signal SU-2 isolation xfmr.....
> > > > > > > > > > > > > > > > > > > > > > > > > > >
I did catch something Jon missed in his instruction wiring the DU-2 for use as a separately derived 120V isolation transformer. He forgot to mention one lead of the 120V secondary shall be intentionally connected to earth, thus becoming the Neutral, The Grounded Conductor....
The connection must be connected to the main electrical service grounding electrode system. In the case of a cord and plug fed iso xfrm the wall recept safety equipment ground can be used.
(There is a code change in the latest edition of NEC 2011 regarding using the safety equipment ground for the neutral earth connection of a separately derived power system.... Call me lazy I haven't read the change yet.)
> > > > > > > > > > > > > > > > > >
A comment on Technical Grounds.
(No separate isolated ground rods allowed.)
A technical ground does not mean the secondary neutral conductor of a transformer does not have to be connected to the main service grounding electrode system of the main electrical service. Per code the grounded center connection of a balanced AC grounded 60/120 system is called the neutral conductor. The two 120V leads of the secondary, that feed the two hots of the recept, are called the ungrounded conductors.
Yes you can drive a separate ground rod/s, (Technical ground / Aux Ground), but per NEC code the ground rod/s shall connect back to the main system ground with a minimum size #6 copper wire. (See link below.)
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