Converting Volts to Watts?

[Anonymous]

I have a transformer that came with my sond Thmas teh tank engine set. It is listed at 18VAC and 1.8 A. Can someone help me translate this into watts? I am having a hard time with watts vs amps vs cycles and want to know if the output here is closer to my postwar 110 or 45 Watt transformer. Does that make sense? [?]

[Anonymous]

Red,

Watts = Volts x Amps so:

Watts = 18 x 1.8 (Thomas Tk. Set data)
Watts = 32.4 say 32 watts total.

Hope this helps.
Steve

[waltrapp]

This thread gives me a perfect opportunity to ask a question that's been bugging me: My brother-in-law asked me to go to a train show with him so he could buy a new transformer to run 2 very small 4'x8' independent loops. He had 2 transformers and claimed that the 2 40Watt loaners that I lent him were better than what he had. However, he NEVER talked about Watts, he ALWAYS talked about VOLTS. I didn't challenge him other than to say that I"ve always thought about the 'power' of a transformer in terms of watts. He said 'nonsense'.

Given that he's a bit 'smarter' than me (he works in the nano-technology field) I didn't dare question his statement even though I kept wondering.

It seemed to me that many transformers that he looked at gave fairly equivalent ranges in terms of Volts. Some said 8 to 18 V. But yet I can't believe that these smaller transformers can come anywhere near the capabilities of my 'V' or 'ZW'.

Should I have pushed him harder to think Watts?

as always - thanks, walt

[Chris F]

The VAC rating is the maximum voltage avaiable from the transformer, while the Amps rating is the amperage at which the internal circuit breaker should open (after a certain time delay). Steve gave you the straight dope on calculating Watts (sometimes called VA, or Volt-Amps), but I wanted to mention Wattage rating for postwar transformers.

Because of Federal regulations from the '70's (created because of "Watt-wars" in stereo receiver ads), Wattage ratings for modern transformers are for continuous operation. Postwar transformer ratings were not so regulated, so they refer to Watts output for short-term operation. At this maximum output, the transformer will heat up and the circuit breaker will open. For continuous operation, the output is approximately 65-70% of maximum. For example, a Type RW transformer is rated for 110 Watts, but only 70 Watts for continuous operation (per Greenberg's Repair and Operating Manual for Lionel Trains, 1945-1969). Your 45-Watter thus would have a continuous rating of about 32 Watts, the same as your "Thomas transformer".

[Chris F]

Walt,

Think of Voltage as the electrical "pressure" that wants to push electricity through the wires. If there is no current flow (Amps), there is no power produced, no matter what the voltage. The correct term for power is Watts, which, as Steve noted, is a function of both Voltage and Amperage.

Obviously, expertise in one area does not imply expertise in other areas (I've found that out about myself, too!). I like my dentist, but I wouldn't let him wire my layout.

Push back!

[Big_Boy_4005]

Nice explaination Chris.

Red caboose mentioned cycles in the original post, and for the purpose of most discussions about toy train operation, cycles have absolutely no meaning. Cycles also known as Hertz are used to describe Alternating Current, and the North American Standard is 60. The current that comes from a standard wall outlet is ~120V, 60Hz.
Your transformer reduces the voltage to a safe 20V max, but has no effect on the 60Hz.

If I remember correctly, the early postwar model V and Z transformers were able to deliver up to 25V to the track, which is why some people like them better than the classic ZW, even though they aren't as pretty.

[Anonymous]

Thanks guys, I knew I could get some help here, although I am confused about one last point. In this months issue of OGRR, it talks about the draw of wattage from a typical engine, cars and other accessories. It mentioned that most single motor engines draw 35-40 watts of power. Add in light bulbs, accessories, etc. and you have a good idea of what kind of power you really need from you transformer measured in Watts.

So today I am running a K-Line GG1 with Railsounds and using my Thomas transformer and it is running just fine, yet according to the article, this engine and caboose alone should have exceeded the wattage from the transformer. Does this just mean that eventually the transformer would have shut down if I ran it long enough? Or am I missing something here...

[Chris F]

The 35-40 Watt specification was a maximum you might expect when running full throttle. You could exceed the wattage if you run the GG-1 too fast.

When CTT reviews locomotives, a couple of graphs are included. One shows Amps vs. Volts, while the other shows scale MPH vs. Volts. The higher the Volts, the higher the speed and the higher the Amps.

For example, a K-Line NYC Hudson ran 20 MPH at 1.1 Amps and 8 Volts, so the Watts consumed was about 9. The same locomotive ran 85 MPH at 2.2 Amps and 18 Volts, so the Watts consumed was about 40.

To get an idea how maximum Wattage requirements can vary, consider two other K-Line products. An MP-15 ran at 141 MPH (!) using 18 V and 0.5 A, for a 9W power consumption. A C&O 2-6-6-6 ran at 87 MPH using 18 V and 3.2 A, for a 58W power consumption.

Because the CTT reviews usually are not available until some time after the product is shipped, many of us already have the locomotive before its Wattage requirements are known. That's why many of us go to the larger transformers - just to be sure.

[Anonymous]

Dear RedCaboose5525,
Warning: you are drawing out the physics-teaching-monster within me.

Okay: to answer your questions I am going to have to review some stuff.

1.) The basic unit of force is the Newton (a force is any push or pull, and can also take the forms of weight, tension, and many other things). The Joule is the measure of work, which is force times distance (W=fd). And so, we find that 1 Joule=1 Newton times 1 meter (J=nm), and as such a Joule is sometimes referred to as a Newton-meter.

A Coulumb just a measure of electrons (or sometimes protons), and is equal to 6,250,000,000,000,000,000 electrons (I can't figure out how to use exponents on this forum, but that's 6.25-times-10-to-the-eighteenth electrons).

And so, a Volt is one Joule/coulumb (one Joule per one Coulumb)(V=J/c). And so, we see that a Volt is a measure of the force applied onto a given amount of electrons.

2.) Amperes (commonly called Amps) are a measure of the number of electrons passing a given point during a given time. I have told you what a Coulumb is, and I certainly hope you know what a second is. And so, an Ampere is a coulumb per second, or A=c/s.

3.) A Watt is a Volt times an Ampere, or Watt=va. Through some cross-canceling, we see that this is also equivalent to Watt=J/s, since the coulumbs cancel (I use Watt to avoid confusion with "W," which is the symbol for work).

4.) Finally, cycles apply only to AC electricity. The cycle is the number of changes in current direction per second, which is measured in Hertz. A Hertz (a measure of frequency) is the number of something per second, and so it is used in a number of applications.

I hope I have helped,
Daniel

[lionelsoni]

Here are some considerations that have not been discussed:

The speed is more closely related to the voltage applied for modern can-motor locomotives. These use permanent-magnet DC motors fed from rectifiers in the locomotives, which try to run at a speed porportional to the voltage. Traditional series-wound universal moters, on the other hand, tend to slow down with increased load, as the field winding takes more of the voltage and the armature less. (This is actually the same way that prototype traction motors behave.) Thus more voltage may be required to keep a universal-motor locomotive up to speed with a heavy load than an can-motor locomotive would need.

Two transformers may put out the same voltage; but, if one is more powerful than the other (more watts), as you draw current from them, the voltage of the less powerful one will begin to drop faster than the other's. Thus it may be necessary to have a more powerful transformer in order to keep the voltage up to what you need.

[waltrapp]

RedCaboose522: I'm glad that 'your' thread turned back to what you wanted to discuss. I was bit worried that my question would land up hi-jacking your thread and turning its direction. Luckily I got my answer and the thread is still 'yours'!

- walt

[jkerklo]

There is another consideration for train transformers.

The transformer wattage rating is for the INPUT: how much power the transformer is consuming from the wall plug. This is not the same as the
delivered OUTOUT power. Train transformers, particularly, are inefficient because of the variable voltage mechanism. (And also because less iron was used than should have been.)

Try this. Set the transformer power lever about 3/4 of the way to the max. Connect an AC volt meter to the output. Then add a small load, in the form of a train, and watch the voltage go down. Then add another load, and the voltage will go down some more. Finding "loads" can be tricky, but engines on the bench can be used).

I have been meaning to actually measure the efficiency, (Output Volts * Amps)
/ (Input Volts * Amps) some day, but I would guess it is less than 70%.

This means that a train that consumes 30 watts, actually needs (at least) 43 watts of rated input power. (The heating limit previously mentioned also applies to set an upper limit).

This effect is less if a fixed-voltage terminal is used for the test.

I have played with KWs, VWs, and Zws. Similar results.

JK

www.Three-Rail.com

[Chris F]

JK,

You're definitely right about Input vs. Output Watts in reference to Postwar transformers. Greenberg's Postwar service manual indicates output wattage is about 65% on input wattage.

However, it is my understanding that modern transformer power ratings actually are continuous output Watts (or Volt-Amps). This can be verified by checking the data plate on the transformer.

[Anonymous]

Well, I am ready to build my own nuclear power plant after this thread. Many thanks to everyone. I have my answers and more. I only wi***his thread was around about 25 years ago for my high school physics class. I sue could have used you then!

[lionelsoni]

Another wrinkle: Power transformer design is typically a compromise between size and efficiency. The core always saturates to some extent; and you can make the transformer smaller if you allow it to saturate more, drawing more magnetizing current and turning more power into heat in the primary winding. This is the main reason for the typical inefficiency of train transformers. It would be intolerable in the power-distribution network, because of the great cost in lost energy and the difficulty of keeping the transformers cool; but it is no big deal on a small scale.

Why this matters is that the heating due to saturation depends not on the current drawn but on the voltage applied, as you can observe by noting that even an unloaded transformer gets warm after it is on for a while. That voltage has gone up historically, from 110 volts long ago, through 115, to the present standard of 120, which itself is now often exceeded. This means that old transformers, which may have been just tolerably inefficient in their day, are now closer to the edge. They may be the only electrical appliances anywhere close to their age in your house!

I use a type Z and a B30, which get quite warm. I am seriously considering reducing the voltage that I run them on.

[jkerklo]

Am am curious about lionelsoni/Bob Nelson's comment regarding line voltage.

Having designed entertainment products, I am aware that line voltage can be different in different places, but have not been aware that line voltage has been increasing over the years.

Can you expand on this a bit?

John Kerklo
TCA 94-38455
www.Three-Rail.com

[lionelsoni]

Apparently Edison started with 110 and 220 volts. See
http://www.ieee.org/organizations/history_center/faqs.html
Many still use these numbers informally to refer to the modern voltages. You can see these numbers in the transformer specifications in old Lionel catalogs.

Around the middle of the century, the voltages had risen to 115 and 230. One still sees these numbers on some modern appliances.

The National Electric Code now considers 120 and 240 to be the proper nominal voltages. But the NEMA plug specifications use 125 and 250:
http://www.leviton.com/sections/techsupp/nema.htm
This may be a harbinger of higher voltage to come.

I have dragged out of storage a 70-pound Signal-Transformer-brand DU-3 isolation transformer that I rescued over 30 years ago (and that is amazingly still in production, with specifications on the internet:
http://www.belfuse.com/Data/DBObject/pgs28_29.pdf). It will handle 28 amperes and give me taps at 120, 110, and 104 volts. I am now wiring it up.

[jkerklo]

Thanks Bob. That IEEE/History URL is a gem. I have bookmarked it.

I measured my house voltage: 123.8 Volts. Not with a lab meter, but should be close.

Seems a bit high. I might have to look at one of those isolation transformers. Wish I could find a use for some of the junk I have been carting around for 30 years.

John Kerklo
TCA 94-38455
www.Three-Rail.com

[Anonymous]

"Should I have pushed him harder to think Watts?"

Try this water supply analogy with him. Voltage is the size of the pipe supplying water to a sink while watts is the pressure of the water coming out.

[lionelsoni]

I disagree, KJ.

[jkerklo]

I always thought the water supply analogy worked only for those that already understood volts and amps.


John Kerklo
TCA 94-38455
www.Three-Rail.com

[Anonymous]

Actually, the voltage is equivalent to the pressure and the current (amps) is equivalent to the flow rate (gallons/minute). The wire guage is equivalent to the diameter of the pipe and the voltage rating of the wire (insulation thickness) is equivalent to the pressure rating (thickness) of the pipe. Watts is the amount of power and is the product of voltage times current.

Daniel Lang

[Anonymous]

Daniel Lang's analogy is much more accurate.

My post was to try and draw a very rough, simplistic analogy for someone who sounded as though they didn't know anything about electricity at all.

Ok. So how many rivets are in a PRR K4 boiler? ;)

[lionelsoni]

For John Kerklo,

I checked my 1929 Lionel catalog this morning: 110 and 220. And my 1953 NEC: 115 and 230.

My step-down isolation transformer is now all wired up and working. I grounded the output circuit to the input circuit's equipment ground, rather than to the grounded conductor, after some deliberation.

[jkerklo]

Not sure of the grounding.

I looked at the spec of that isolation transformer and it can run as an autotransformer. Common on the neutral side may be a good idea.

I think I would want green-wire ground to be common through to the train transformers as well.

John Kerklo
TCA 94-38455
www.Three-Rail.com

[Anonymous]

With 30 years in power distribution, I must state that volts times amps DOES NOT equal watts. It equals only volt-amps. Watts must include the power factor (the cosine of the angle that the voltage lags or leads the amperage). Volt-amps is referred to as "apparent power", watts is "real power" The inductance of the electric motor will cause the voltage to lag the current. We use an assumed power factor of 85% in relation to motors. It can be much lower under some circumstances.

[lionelsoni]

The current in an inductor lags the voltage.

[BentnoseWillie]

QUOTE: Originally posted by lionelsoni

The current in an inductor lags the voltage.

ELI the ICeman lives!

[:o)]

[lionelsoni]

Although I used a rather large isolation transformer that I had on hand, it should be possible to reduce the line voltage (without isolation) using a much smaller transformer, if anyone is interested in doing that. A 6.3- or 12.6-volt filament transformer with a 15-ampere secondary would do the job. The secondary would be wired in series with the hot side of the branch circuit, bucking the line voltage. I would connect the primary to the load side, to give the filament transformer the same voltage-reduction benefit as the train transformers.

[jkerklo]

Another way to solve the problem of high input voltage.

I run a layout where six to eight trains are all running on the layout at the same time, without operator attention. I have discovered that the trains run increasingly faster as they warm up. The difference is quite noticeable.

To more easily adjust for this, I installed a variac in the primary side, feeding all the transformers (4 KW, 4 ZW, 1Mailine industries).

The overall speed of the layout can be slowed as the trains speed up.

The variac could also serve to adjust for high, or changing input line voltage. The variac should probably only be used on conventional transformers.


John Kerklo
TCA 94-38455
www.Three-Rail.com