A transformer generates heat from circulating currents induced in the magnetic materials (despite the use of laminations to reduce them). This effect is pretty much independent of the load. It also generates heat from the currents flowing through the wire windings, which have some resistance. This effect depends on the loading, but is significant even with no load because of the magnetizing current in the primary winding. The heat from these effects raises the temperature of the transformer, but slowly, because it has considerable thermal mass. The materials from which it is made put a limit on how hot the transformer can be allowed to get ultimately.
It is perfectly reasonable that a transformer could heat up to that limit after a long period of operation at 180 watts. But it is also reasonable that the same transformer could put out much more power, even the 95 extra watts that Lionel claims for the later ZW, if it has not yet reached its temperature limit. The copper windings would generate about twice as much heat (in proportion to the square of the current) so the temperature would rise faster than with the 180-watt load. But, until the whole thing reached its temperature limit, no harm would be done.
Notice that the Z and the two ZWs are all rated at 180 watts continuously, but 250, 250, and 275 for short times. That probably reflects someone’s judgment that the later transformer’s wiring, which does not have much thermal mass, could stand the higher short-term current, rather than any change in the capability of the actual transformer component.
Several countries in Europe use the volt-amp rating instead of watts.
Far as using volt-amp ratings on train transformers it sounds to me like the company doing this is trying to deceive people about the true rating of the transformer.
It’s not malicious. It better describes what the transformer can do. If I were to put a 180-watt load with a unity power factor on the 18-volt secondary of a transformer rated for a resistive load of 180 watts, I would draw 10 amperes. But, if my power factor were .5 (60-degree lead or lag), I would draw 20 amperes and destroy the transformer. Specifying the transformer as a 180-volt-ampere transformer instead would tell me that it is designed to put out only 10 amperes into a load of any power factor and that I really need a 360-volt-ampere transformer to get the 180 watts of power that I want for my (very reactive) load.
Since there is no limit in principle to how low the power factor can be, a transformer would literally have to be designed to handle infinite current for any finite power rating. Toy trains probably have fairly reasonable power factors, not too far below 1; so a transformer rated in watts probably won’t be called on for infinite current. But I don’t see any harm in specifying it in volt-amperes for those who understand the difference.
You could specify a toy-train transformer as capable of putting out, for example, 180 watts at a power factor of .9. But I think that would be even more of a puzzle than volt-amperes for most customers.
Bob, in the thread titled "4851 Transformer–AC or DC?" I mention hoping you can chime in on Lionel’s cryptic ratings from the MPC/Fundimensions/Kughn period… 50 watts equals 7.5 VA? I don’t think this can be close to right…
Any idea how they made their conversions/interpretations of the power ratings? K-Line did something similar w/ identical transformers - a 110 watt became like 40 or 50 VA w/o changing anything else.
Also, the MW & RS-1 transformers outperform by a longshot any 4150/4850/4851 50 watt transformers. I’ve used these w/ 2333’s, eg, and they pull very well(better than any 1033/1044). The manuals, especially for the RS-1, seem to interchange the use of “VA” & “Watts”, but for our purposes, it seems a 50VA transformer has A LOT more punch than a 50 watt, or even a 90 watt 1033/1044/4090.
Here is UL’s answer to the question why toy transformers can be listed in either watts or volt-amps. I’m not sure it clears all up but it agrees with the reasoning expressed so far. The “ratings as shown below” were in a gif format I’m still fussing with but they were pretty mundane.
Jim
Thanks for your question. Toy Transformers are investigated per UL 697 Standard. For a Listed Toy Transformer, the electrical ratings should be included on the transformer. The requirements for electrical ratings are shown below. The output can be rated in either Watts or VA. I understand your point with respect to power factor and differences between True Power and Apparent Power. In a circuit having both reactive and resistive components, the Watts will always be less than or equal to the VA rating. However, UL 697 contains a Power Input and Output Test, which is specifically conducted in WATTS. Part of the test checks for verification of manufacturer’s rating (in Watts)…and it needs to be within 90%. Further, if a manufacturer rates it in VA, then we assume a best-case power factor of one (1), where Watts would equal VA. Since testing is based on Watts, I don’t see the potential for inflation of the nameplate ratings.