60Hz Transformer Core Loss

[Evan.Cornell]

I've got a 40VA 120VAC->24VAC transformer that pulls about 4W according to my KillAWatt when completely unloaded. Is this amount of transformer loss a reasonable value?

[Benta]

Looks a bit high to me. A better estimation would be to measure temperature rise. The Killawatt thingys are not really accurate at very load loads.

[WattsThat]

While I expect you've got some measurement error in the 4 watt number, 10% magnetizing current is not unreasonable for a low cost HVAC style transformer. That’s an assumption on my part since 40VA is not an industrial rating.

Why do you care? It’s not like you can save much energy with a better transformer, those things are designed down to a cost point with the minimum amount of copper and iron required and efficiency is low on the list of concerns.

[Evan.Cornell]

I don't really care... power consumption isn't really relevant to the project, but I was more just curious. Thanks!

[TimFox]

Does your KillAWatt measure power or just primary current?  All transformers have a finite magnetizing inductance, which pulls current (90 deg out of phase from voltage) from the source when the secondary is unloaded.  The actual core loss due to going round and round the hysteresis curve, eddy-current loss, and copper losses due to current through the windings are actual power.

[bdunham7]

I've got a 40VA 120VAC->24VAC transformer that pulls about 4W according to my KillAWatt when completely unloaded. Is this amount of transformer loss a reasonable value?

What are the PF (power factor) and AMP readings?  Can you post a photo of the transformer?  Whether it is a 'reasonable' loss sort of depends on what it is and how it is used.

[Benta]

AFAIK the Killawatt thingys measure kW, kvar, PF, etc.

[Evan.Cornell]

Reads PF=0.31, I=0.1A, V=124.2V, that works out to 3.85W, pretty close. Transformer gets decently warm, so I think that number is reasonably accurate.

[bdunham7]

Reads PF=0.31, I=0.1A, V=124.2V, that works out to 3.85W, pretty close. Transformer gets decently warm, so I think that number is reasonably accurate.

The voltage is a bit high, not worrisome, just a factor.  I forgot--can you measure the DC resistance of the primary?

[Evan.Cornell]

The voltage is a bit high, not worrisome, just a factor.  I forgot--can you measure the DC resistance of the primary?

I measure 20.8ohms.

[bdunham7]

So your current phase angle (theta) is arccos (0.31) or 72 degrees.  Look here:

https://www.electronics-tutorials.ws/transformer/transformer-loading.html

You can figure the resistive loss as I^2R (0.10A) x (0.10A) x (20.8R) = 0.208 watts.  The core losses are the balance.

The idle current is a significant portion of the full-load primary current.  This means the transformer is not terribly efficient at low loads and could be close to or in core saturation.  This isn't necessarily a flaw as some transformers are designed this way.  And being a bit over on the input voltage would make that worse.

[floobydust]

Cheaper 60Hz transformers operate at high flux density- less iron and copper required, but core losses are higher and worse yet at high line. Between 115VAC and 125VAC, I have measured around 30-40% increase in primary current for 20-75VA transformers.
Several watts is dissipated with no load, worse nowadays with utilities having high mains voltage and transformer manufacturers competing on price.

[langwadt]

Cheaper 60Hz transformers operate at high flux density- less iron and copper required, but core losses are higher and worse yet at high line. Between 115VAC and 125VAC, I have measured around 30-40% increase in primary current for 20-75VA transformers.
Several watts is dissipated with no load, worse nowadays with utilities having high mains voltage and transformer manufacturers competing on price.

are there that many transformer supplies left, with the stricter no load and minium efficiency requirements?

[floobydust]

Low cost is king and the "race to the bottom" leaves us with transformers that run hot and have terrible  regulation. I find it a PITA to design them in now because no-load current and % regulation specs are missing, to hide the cheapness. What I do is sample a few brands and chose the heaviest lol.

I used to stay with a particular good brand such as Hammond, but they changed to off-shore low cost manufacturing in Asia and quality went awry. I'm not sure why a coil-winding machine running in North America verses Asia is so different in labour costs.

I have used the big HPS Imperator machine tool industrial transformers when I need a conservative design in a control panel.

[NiHaoMike]

Why do you care? It’s not like you can save much energy with a better transformer, those things are designed down to a cost point with the minimum amount of copper and iron required and efficiency is low on the list of concerns.

That's most certainly true for an aftermarket replacement part, but for a part designed to be integrated into a product, that can cause it to fail standby power requirements.

[james_s]

IIRC at no load the core losses will be higher. As you put some load on the transformer that draws some of the flux out of the core. It would not surprise me if the core is saturating with no load at all, especially if your line voltage happens to be a tad on the high side. Try connecting a resistive load of at least 10% or so of the transformer's VA rating and see if the losses drop.

[duak]

I believe james_s is right on the money.  I understand that microwave oven transformers are generally designed for full load and can saturate at no load.

I got a bunch of vacuum tube telco exchange equipment in the early 70s and found the quality of the components to be top notch.  Wasn't this stuff designed for 20 years of 24/7?   The power transformers ran remarkably cool compared to, say, consumer audio or TV stuff of similar vintage.

I haven't bought any new Hammond stuff since the 90s.  Their potted toroidal transformers easily met their specs.  I hope the trademarked product names like Imperator doesn't mean the end is nigh.  (Anyone remember jumping the shark?).

lifted from wiki: Avē Imperātor, moritūrī tē salūtant ("Hail, Emperor, those who are about to die salute you") is a well-known Latin phrase quoted in Suetonius, De vita Caesarum.

[H713]

Lots of cheap transformers are running right on the edge of core saturation, and at 124 VAC you might be pushing it slightly farther into saturation. Not ideal, but it's probably fine if it doesn't get too hot. I will agree that the current you're reading is pretty high for being just magnetizing current on a transformer that size- I read about 350 mA on a "750 VA" (it weighs 50 pounds... by my calculations it's more like a 2400 VA transformer).

I've had good luck with Hammond tube output transformers. Price is a bit high, but the fit and finish has been fine.

I have not heard good things about their power transformers. A lot of them seem to run ridiculously hot even when running at less than 50% load.

For Toroids, I have had good experiences with Antek for the price.

[trobbins]

If the magnetising current waveform is peaking due to onset of saturation, then even more doubtful that the current measurement is representative (especially given a reported sensitivity of 0.1A, although my plugin monitor has a mA resolution), and doubtful about the PF value as well.

[H713]

If the magnetising current waveform is peaking due to onset of saturation, then even more doubtful that the current measurement is representative (especially given a reported sensitivity of 0.1A, although my plugin monitor has a mA resolution), and doubtful about the PF value as well.

Agreed, it would be nice to get a scope waveform of the primary current. Of course, this would require a differential probe or current transformer.

[floobydust]

In the early 1990's Hammond switched over their 166 family from Canadian to off-shore manufacturing and caused a shitstorm because those parts were really cheap and ran noisy and hot. It caused problems for any manufacturer using them. I flamed Ross Hammond and probably his dad, said if you want a cheap line, instead change the series part number so we aren't duped. He said they made a mistake and had to roll back the assembly and specs. 167's are made in china but to their specs and I find them OK.

Saturation due to primary (line) voltage isn't some sudden happening, it does ramp up.
Here I took an old made in Canada 167M25 115VAC 25VCT 3A 75VA up to 190VAC until it literally leaped off the bench. PF 0.47 at 90VAC input, PF 0.33 at 110VAC (3.3W) (5.8W at 120VAC), PF 0.13 at 145VAC (13.3W).
made in china 166JA12 115VAC 12VCT 1A 12VA, core does do worse. Similar PF 0.333 at 110VAC input.
Neither show any compression of output voltage which I expected. The primary current (magnetization) waveform is always ugly, sharp distorted peak not a sine wave at all and need true RMS to measure it.

The problem is the transformers have old 115VAC design/spec yet are getting 125VAC, so they waste several watts at lo load and there is no "eco" spec or product line that I know of.