CTT's Question and Answer of the Week 8/19/05

[Jumijo]

This week's question is another regarding a troublesome transformer. This is not the moisture issue from last week.

http://www.trains.com/Content/Dynamic/Articles/000/000/006/116gobyt.asp

Jim

[ChiefEagles]

Thanks Jim. Had forgot about it.

[cnw1995]

This answer is a good reminder about what I learned from the Chief and lionelosi - that power outputs vary, and are not necessarily the same as the max. possible.

[lionelsoni]

This might be a good place and time to mention a "short"coming of just about all traditional train transformers. (I don't know about the newer ones.)

Every transformer I have ever dealt with or seen the schematic of has a single circuit breaker, invariably located in series with the common. This breaker will trip when the sum of the load currents exceeds its setting. Unfortunately, except for a transformer with a single output in use, this does not necessarily mean that the load current is interrupted.

For example, suppose that a train is running between two blocks powered from separate outputs, which are set to significantly different voltages. The fault current will flow from one wiper of the transformer, through the pickups bridging the gap between blocks, and back to the other transformer wiper, not through the circuit breaker, allowing the transformer or wiring to burn up if the condition lasts longer than an instant, as if the train happened to stop in neutral at the right (wrong!) place.

Another example: Suppose a derailment shorts one output to ground and trips the breaker. A train or accessory running on another output of the same transformer will then see the difference between its former voltage and the voltage set for the derailed train. This could be either a decrease in voltage or an increase, depending on the settings. A second derailment, perhaps resulting from an unexpected increase in the voltage to the second train, could produce a heavy fault current which, with the circuit breaker already tripped, could do considerable damage.

Obviously, the manufacturers have been taking a "short"cut (sorry) here, for about as long as they have been putting circuit breakers in the transformers. The only real cure for this problem is to add an individual circuit breaker in series with each output and to jumper around the built-in one. Fortunately for anyone who wants to go to that trouble, automatically-resetting low-voltage DC circuit breakers are easy to find at any automotive-parts store.

[Jumijo]

QUOTE: Originally posted by cnw1995

This answer is a good reminder about what I learned from the Chief and lionelosi - that power outputs vary, and are not necessarily the same as the max. possible.

Are you saying that each individual transformer may vary from another in the total output of watts, and that the stated rating is not always what the transformer is capable of?

As far as circuit breakers in dual units go, I was looking into purchasing a dual powered transformer a while back. A hobby shop guy advised me to purchase two strong single units instead. I asked him his reasoning and he told me that if one side goes on a dual, you can't run any trains while it's being repaired. If one single goes down, you still have one to run trains with. That made a lot of sense to me.

Jim

[lionelsoni]

I wouldn't expect individual transformers of a particular model number to vary, unless the manufacturer made a design change without changing the number.

The ratings that Lionel published were the power into the transformer, not the output. Transformers of the size that we use are usually designed undersized so that the magnetic circuit saturates substantially in normal operation. This causes the primary winding to draw a heavy magnetization current, whether or not the transformer secondary winding is even loaded. That current heats the primary, wasting a fair amount of the power put in. As long as the transformer is not too large, no harm is done, except that the user gets rather less out than he thinks he paid for.

I have had a large isolation transformer sitting around for years which I finally put to use to reduce my line voltage to 110, as compared with the modern 120 or even 125. I hope this reduces the strain on my old transformers by keeping them to a saturation level that they were designed for 60 years ago or even maybe a little less.

Since the main effect of overloading the transformer is to heat it up at a rate that it cannot sustain, short periods of overloading with time to cool down in-between do no real harm to the transformer. So you can get 250 watts, for example, or more from a Z or ZW, just not continuously.

[ChiefEagles]

Why I have fast acting circuit breakers for each output between the transformer and the source. They work great too. Several derailments have "tested" them.

[cnw1995]

Sorry, Jim, I wasn't clear. I meant exactly what Jim Nelson said. It is also interesting to calculate the load on a transformer - engine with 2 motors for example, lighted cars, accessories, etc.

[Anonymous]

Although not precise, my panel meters(amp) indicate that my rebuilt 275 watt and 120 volt input ZWs deliver about 180 watts output (10 amps)continously . I used inline 10 amp Potter- Brumfield breakers or cartridge fuse blocks as overload protection on old ZWs.
Currently the ZWs are on the bench in favor of PowerHouses.