Transformers: Liquid Dielectric or Not?

There (apparently) should actually be three categories - but perhaps the tariff is targeted at disposal problems. Many liquid dielectrics have proven (or unproven) troublesome - see PCB. Whether your tariff applies to oil-filled I couldn't guess - so no help there.

Oil filled, Liquid Dielectric, or Dry.

The oil (or liquid dielectric) is used for cooling, so they are filled with liquid and therefore - in a can. So look for smooth sides or cooling fins.

Here are some dry types

http://www.spangpower.com/dry-general.htm

a transformer is a electrically reactive device. It can have amps flowing through the coil windings with a voltage applied but yet very little real power is flowing.

Direct current has only true power IE amps times volts equals watts.

Transformers have a 90 degree phase angle lag between the voltage and the current of the AC power.

Amps going through the actual coil resistance is what builds up heat. Not the actual wattage being transformed from one voltage to another.

VA or volt amperes are the apparent power limit of a transformer. Watts are the real power. A purely resistive load will give you true power and apparent power numbers that are equal.

Capacitive or reactive loads give you apparent power numbers.

its why a 1000 watt electric motor that is 80 % efficient will draw a true power of 1250 watts. But could have an apparent power (measured line volts x Line Amps) of 2000 VA or higher. That motor would need a transformer with a 2000 VA rating not at 1250 VA rating.

The VA Vs wattage misconseption is why so many people think electric moters and transformers are infficent. They are not!

Sorry to interrupt but

very little real power is flowing.

gives a bit of misconception.

How I will take it rather is that the transformer in fact converts the current of primary (or input side) to the secondary and that in turns converts the voltage to by the turns ratio

n₁I₁ = n₂I₂ (approximately - assuming no leakage flux and ...)

and

V₁I₁ = V₂I₂

are the two governing equation.

As we see here the power factor (that converts the VI= Volt Ampere or KVA whatever be the unit) is missing here. And that's why the transformers are rated in KVA and not KW

TCMTECH,

Your definitions on real and apparent power are not quite exact.

Real power is not only in DC.

In order to have real power in AC it is sufficient to have a 0 degree phase difference between current and voltage. This is the case if you have a heater(resistor) fed in AC.

The statement: "Amps going through the actual coil resistance is what builds up heat. Not the actual wattage being transformed from one voltage to another" - the load (Wattage) is what causes the current flowing through the coils (Amps).

Also "Transformers have a 90 degree phase angle lag between the voltage and the current of the AC power." is not right - this angle (between U and I) is the angle that intervenes in the calculation of the power factor. For an usual power factor of 0.8 this angle is 36.8 deg (cos 36.8 = 0.8). For 90 deg the power factor would be 0 (no real power, only reactive power).

Thanks EDIGNAN and TCMTECH, an obviously complex subject is becoming somewhat clearer. I'm going to read more on the sites provided.

Thanks again for the help.

I think that I have an easier definition and clarification for you Apothicus.

In this case dielectric and insulator can be considered synonyms. The dielectric insulates the electrical energy from the case of the transformer. To insulate the case from this energy, conduction through the dielectric should not be easy. Air, the most common non-liquid dielectric, will conduct electricity if the voltage potential is sufficiently high, think lightning. The voltage that air will conduct changes greatly depending on a host of variables like humidity, pressure, particulate suspension. Also air transfers heat relatively poorly. Liquid dielectrics, typically oil based, will withstand considerably higher voltages than air for the same volume. (Remember this is the volume between the transformer and the transformer case.) Liquids will also transfer heat to the case much more readily than air. But these advantages become a drawback because if the liquid leaks, one now has air for a dielectric. Arcing will likely occur to the transformer case after a leak happens.

Now that I've defined the differences, here's how to guess the difference from some indicators. Few small transformers use liquid dielectrics, so the power capability is a place to start. A simple rule of thumb, if your power one thing (lamp, computer, drill press, single dwelling) you probably do not have a liquid dielectric. If you are powering a factory, a group of homes you probably have a liquid dielectric. If at the connection points a rubber gasket can be seen between the ceramic insulator and the case, then this is likely a liquid dielectric. If air vents exist on the transformer case to permit air entry, probably not a liquid dielectric. If the transformer has thick cooling fins as part of the case design you probably have a liquid dielectric.

One last thing previously mentioned, some earlier transformer dielectric oils were extremely hazardous substances, PCB and PBB. Most today now use mineral oil but previous problems are what made your need to know for import/export concerns.