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Old 30th September 2012, 11:34 PM   #11
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You guys have me curious - I have a couple of Musical Power Supplies power transformers. I'll take one to work tomorrow and see how it stacks up with respect to exciting current. 100ma+ is definitely too much exciting current.
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Old 1st October 2012, 09:27 AM   #12
AJT is offline AJT  Philippines
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Quote:
Originally Posted by wknight View Post
Hi
I have bought the power transformer Musical Power Supplies PT325.2i
It heats without load.
It takes about 200mA without load.

If somebody use it, please tell me about your experience.

thanks
how big is your traffo? core area? 200mA is not such a big deal if that is a big traffo....high exciting current is an indication of fewer primary turns...not all of this current heats up your traffo...

how hot is hot? can you touch it for 5 seconds without your hand feeling pain?
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Old 1st October 2012, 10:49 AM   #13
wknight is offline wknight  Israel
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core area 8 cm^2
It heats up to ~50C (122F) without load.
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Old 1st October 2012, 12:08 PM   #14
AJT is offline AJT  Philippines
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Quote:
Originally Posted by wknight View Post
core area 8 cm^2
It heats up to ~50C (122F) without load.
that's one hot traffo, what is your ambient temp? if 25*C then that is just a 25*C temperature rise, not too far off...

a saturated transformer will burn out and emit smoke by itself...

if you are able to run your traffo without load without bringing out smoke, then it is probably the nature of that traffo....to me a traffo that heats up on idle is one traffo that has not enough primary turns so that idle current is higher otherwise....
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Old 2nd October 2012, 10:53 AM   #15
wknight is offline wknight  Israel
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Quote:
Originally Posted by wrenchone View Post
You guys have me curious - I have a couple of Musical Power Supplies power transformers. I'll take one to work tomorrow and see how it stacks up with respect to exciting current. 100ma+ is definitely too much exciting current.
Have you checked it ? What is the result ?
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Old 6th October 2012, 04:44 PM   #16
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Just signed up here at DIYAudio to participate in this discussion. He's caught us with the EBay store closed for vacation and offline until today. My apologies for the poor timing.

I had asked WKNIGHT to provide a reading off the heater windings to assure that he did not get a 120Vac version by mistake. His results were:
6.3vac -> 6.85vac without load
6.3vac -> 6.6vac with 1.5A load
So he has the right primary. If there was a shorted turn the PT would heat up and fail quickly. The core is a 1x1 interleave of 29M6 laminations, so air-gap would not be an issue.

The PT325.2, PT275.2 & PT190.2 families have consistently been built to an MPS design since 2009 using the same manufacturer and several hundred have been sold and used successfully in Amp builds from 0.5W to 25W. The "0.2" indicates a second generation design, where enhancements were made in 2008 to increase heater outputs and increase primary turns to reduce saturation losses.

Upon receipt in the USA, each PT is inspected, installed into USA made end bells with Stainless hardware and tested at no-load and loaded operating conditions before packing. Admittedly the 220/240v versions are a minority of the MPS business, but around 75pcs of this version have been sold and used successfully. I'm confident in the source, construction and processes used.

Sunday night I can get a comparison exitation current reading on a regular PT325.2. I'd agree that 122F is quite warm. In the normal set-up load testing the load resistors run that hot....but the transformer temp rise is negligable. If there's something wrong here, we'll get another one out to him.
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Old 9th October 2012, 08:37 PM   #17
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I just tried a PT275-2 Rev D, and it's running over 100ma excitation current at 60 Hz, rated voltage, using a Chroma sine wave AC source and a Yokogawa WT210 volt/amp/wattmeter. This sounds somewhat high, thout I'm going to cart in some other power transformers for comparison.
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Old 10th October 2012, 04:59 AM   #18
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Currently I don't have a great AC Ammeter on hand. I am getting about 100 to 120mA ac no load current on the PT190.2, PT275.2 and PT325.2 transformers reading on a 10A scale. They all have the same primary and core, so I'd expect they'd run close. The temp rise though is under 5C though. Putting my load test resistors on the HiV windings and pulling another 200mA Ac on the primary and there's still no significant temp rise.

If you get some more precise readings on the PT275.2 please share.

So there is likely something wrong with the unit WKnight has. It's possible the HiV windings are seeing some turn-to-turn arcing or shorting. A quick overload or voltage spike could potentially cause that type of issue without a dead short. It's hard to tell without measuring output voltage under load.

I've sold out the original 25pc lot of PT325.2i's from China, so I'll build one here in the US and send it off to WKnight. We'll see if this resolves the heat-up issue. We don't want him using that PT as-is.

Matt
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Old 10th October 2012, 10:55 AM   #19
DF96 is offline DF96  England
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Quote:
Originally Posted by MuzicalMatt
The "0.2" indicates a second generation design, where enhancements were made in 2008 to increase heater outputs and increase primary turns to reduce saturation losses.
Is it possible that your Chinese factory is working to the old design, either by accident or deliberately?

I'm no transformer expert, but 100mA primary current off-load seems rather high; maybe you need to increase primary turns some more?
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Old 10th October 2012, 07:39 PM   #20
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What I will do when I have the time is to plot the excitation current vs. input voltage. This should be linear. If you're near stauration, the excitation current will take a sharp jog upward as you increase input voltage. This is the behavior I noticed with the 275-2. I will quantify it a bit better. I also have a Hammond rated for somewhat higher power (270FX), and it will get the same treatment
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