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Old 26th December 2006, 08:32 AM   #1
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Default Hammond Transformer Winding Resistance -- 369GX

Hi Folks,

I've searched here and on the web, and I can't find winding resistance specifications for Hammond power transformers, specifically the 369GX.

Can anyone give me an idea of the resistance value I should plug into PSUD2? Even a rough idea would help me.

Thanks, and Hippy Holidays!
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Old 26th December 2006, 02:05 PM   #2
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Per unit impedance of a small power transformer is maybe 8%, with X/R ratio of 12. Based on the VA rating of the xfmr, you can get practical values.

For instance, if you have a 120V, 250VA transformer, the Z = 0.08*(120^2/250) = 4.6 ohms. So XL reactance would be about 4.55, R about 0.38 (ballpark is fine)

XL of 4.55 would be L = 4.55/377 = 12mH.

Someone check my math. Not sure how PSUD works, but these numbers give you close approximations on paper.
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Old 26th December 2006, 06:40 PM   #3
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PSUD2 asks for the DC resistance. It is used for computing the peak current and the regulation. Unfortunately the DC resistance is not specified on Hammonds web site. I looked through my transformer collection and I don't have one of those to measure.

The DC resistance specified on some of their SE OPT's is wrong, so you need to measure one. Maybe someone out there has one, and an ohmeter.
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Old 26th December 2006, 08:48 PM   #4
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Quote:
Originally posted by zigzagflux
Per unit impedance of a small power transformer is maybe 8%, with X/R ratio of 12. Based on the VA rating of the xfmr, you can get practical values.

For instance, if you have a 120V, 250VA transformer, the Z = 0.08*(120^2/250) = 4.6 ohms. So XL reactance would be about 4.55, R about 0.38 (ballpark is fine)

XL of 4.55 would be L = 4.55/377 = 12mH.

Someone check my math. Not sure how PSUD works, but these numbers give you close approximations on paper.

Hi ZigZagFlux,

Many thanks for the help--it's just the sort of approximation I can use.

I'm having trouble understanding your equations--could you coach me just a bit more?

Specifically, when you say "per unit impedance", what is the unit?

From your equation "Z = 0.08*(120^2/250)", it looks like per unit of: (V^2)/P = (V^2)/IV = V/I = Z_inferred. Correct?

So is the unit "inferred impedance", calculated from the transformer's voltage and power ratings?

Pluggin my numbers in, I get Z = 0.08*(120^2/65) = 17.7, and R = 17.7/12 = 1.5

Many thanks for your help--I'm hoping this will get me in the ballpark.
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Old 26th December 2006, 08:49 PM   #5
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Quote:
Originally posted by tubelab.com
PSUD2 asks for the DC resistance. It is used for computing the peak current and the regulation. Unfortunately the DC resistance is not specified on Hammonds web site. I looked through my transformer collection and I don't have one of those to measure.

The DC resistance specified on some of their SE OPT's is wrong, so you need to measure one. Maybe someone out there has one, and an ohmeter.
Hi Tubelab,

Thanks for your help. I also searched for DC resistance measurements, at Hammond and the web in general, and came up blank.

Thanks again!
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Old 26th December 2006, 10:27 PM   #6
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Dr D:

Yes, you are correct, with the following clarification:

I looked up your transformer, and the secondary is 225V, with a VA rating of 50. However, it is understood that 16VA is used by the filament winding. So it looks like 34VA is the rating through the 225V winding: those are the numbers you want to use for the equations.

So Z = 0.08*(225^2/34) = 119 ohms. If you don't care about L versus R, then consider it all R for the purposes of PSUD. Do not divide by 12 unless you assign separate values for L and R.

Quick sanity check: if I short circuit the secondary, I would expect to get 225/119 = 1.9A output. Sounds reasonable, though the impedance of a transformer this small could be twice as large.

If you want to measure the impedance, it's a snap with two DMM's. Put a variac on the primary. Short the secondary with a low impedance ammeter (meter on 10A scale). Increase primary voltage until the rated secondary current is obtained on the meter. You are now applying "Impedance Voltage" to the xfmr. Measure this voltage with the second DMM. Let's assume you get 14V

Impedance of your transformer is therefore 14/120 = 0.116 per unit, or 11.6%. This would be the best way to get an accurate PSUD result.

Edit: measuring the secondary DC resistance will not give you the correct number for regulation purposes. Note our ballpark value was 119 ohms; if you would actually measure the secondary resistance, you'll probably get something less than 2 ohms. The measured winding resistance does not reflect the behavior through the transformer, and is not even related to the above. I would also guess that PSUD needs the 119 ohm value, instead of the 2 ohm value, as it correctly reflects the regulation characteristics of the transformer. (i.e. 8% IS the regulation of the transformer). By definition.
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Old 27th December 2006, 01:23 AM   #7
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From the help screen in PSUDII:

The transformer model is simplistic - it utilises an AC voltage source, in series with an effective impedance (strictly, a resistance). Attributes not modelled include winding capacitance, variation in resistance with temperature, core losses, and leakage effects.

Having an accurate approximation of the transformers impedance may help improve the accuracy of PSUDII. I have just used the measured resistance value with good results, but my numbers are not far from your numbers. I measured a Hammond 274BX (198VA, incorrectly marked by Hammond) and I get 43 (86 / 2)ohms. I measured a 270FX (138 VA) and get 48 (98 / 2) ohms. Your 50 VA transformer would likely be close to the 119 ohm value if not higher. I measured a small transformer (about 50 VA) of unknown origin and got 141 (282 / 2) ohms.

It is not even clear how PSUDII is specifying this impedance. I have been using the entire secondary resistance divided by two for full wave center tapped circuits and the entire secondary resistance for bridge rectifier circuits. Many of my simulations have been very close (within a few volts) but I am more concerned with ripple than the actual output voltage. The resistance value affects this too.

In fact Hammond specifies their transformers with 115 or 117 AC volts input. My line voltage is at least 120 and usually runs 124 to 126 volts. I routinely get a voltage that is 10% higher that the specified value on Hammond transformers. Other users have complained of this also. I measure the actual voltage and use that value.
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Old 27th December 2006, 02:11 AM   #8
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Quote:
Originally posted by tubelab.com
The transformer model is simplistic - it utilises an AC voltage source, in series with an effective impedance (strictly, a resistance).

It is not even clear how PSUDII is specifying this impedance. I have been using the entire secondary resistance divided by two for full wave center tapped circuits and the entire secondary resistance for bridge rectifier circuits.

Tubelab:
I checked out your website: very nice. I may just try one of your single ended designs someday (when money is more plentiful). You must have about 25 years on me with tube circuit design.

The info you posted is good news: the way I read it, it says PSUD is calculating the proper way with effective impedance, enough at least to produce good output voltage results. I also would say you are correct in specifying half the secondary for center tapped designs, and all for full bridge.

One thing that would interest me (being as that I don't have PSUD) is comparing the results of measured DC secondary winding resistance versus PSUD's requested actual impedance (though only modelled as resistance). My reason for the curiosity is as follows:

The secondary resistance of the transformer is, for all intents and purposes, irrelevant. I can wind two transformers identically, one with #24 gauge and another with #20 guage, all other parameters being the same (volts per turn, core size, geometry, etc). These two transformers will have different winding resistances, but will measure almost identically with regards to regulation. (We're talking power xfmrs here, not audio).

The inductance of a transformer, based on X/R ratios, overwhelmingly determines the impedance (regulation) of a xfmr. The choice for winding gauge comes down to size, cost, physical constraints, and load losses (significant in big transformers). It has very little effect on the regulation, other than a general correlation.

Maybe it is this general correlation that works out well for you. Maybe as a percentage the xfmrs regulation is a small part of the PSUD results anyway. Don't know, but it would be nice to find out.

At the end of the day, the measurement of real impedance is a very easy one, only requiring a variac and extra multimeter. I would encourage people to use it if you're serious about modelling. I'm too cheap, and use equations.
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Old 27th December 2006, 03:10 AM   #9
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Quote:
One thing that would interest me (being as that I don't have PSUD)
That is easy (if you use a PC) go here and download it, it is free:

http://www.duncanamps.com/software.html

I am guessing that the DC resistance is the dominant factor in the budget minded transformers that we currently use. Ideally the DC resistance and the inductive reactance should be considered.

Quote:
The secondary resistance of the transformer is, for all intents and purposes, irrelevant. I can wind two transformers identically, one with #24 gauge and another with #20 guage, all other parameters being the same (volts per turn, core size, geometry, etc). These two transformers will have different winding resistances, but will measure almost identically with regards to regulation. (We're talking power xfmrs here, not audio).
The resistance of the secondary (plus the primary resistance divided by the turns ratio) are effectively a resistor in series with a perfect transformer. This resistance is in series with the inductive reactance presented by the transformer. This total impedance serves to limit the start up current that the rectifier sees as well as the peak current. The minimum source impedance is specified in the tube manual. PSUDII will warn you if the rectifier current is over spec. I believe that PSUDII is too conservative (probably because most people just use the resistance value), but it keeps people from blowing stuff up.

I use PSUDII to evaluate different filter topologies for ripple performance. I tend to choose transformers based on cost and availability, then make the circuit fit the transformer. I have about 2000 pounds of transformers that I have collected over the years. If I have to buy a transformer it will be one of the cheap Allied units (80% of my amps) or one of the Antek toroids from Ebay. 50 ohms for an 800 volt transformer.

The experiments that you propose are easy enough, I will try this when I have the time. Antek measures their transformers slightly differently. They short all secondaries, turn up the input voltage until rated current is drawn
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Old 27th December 2006, 11:37 AM   #10
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Quote:

(if you use a PC)
Mac user, though PC's at work.
Quote:

DC resistance is the dominant factor in the budget minded transformers that we currently use
Agreed.

Deville, have you all your questions answered, or did we threadjack you?
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