**Transformers**

Transformers are one of the most basic yet practical devices used today. No matter where you are there is always a transformer nearby. They are used throughout alternating-current (A.C.) systems from generating plants to the doorbell in your home. Power companies use transformers to increase the voltage for their long distance power lines, the voltage is than reduced by other transformers before the power enters your house.

The method of transferring electrical energy by a transformer is done indirectly. Electrical energy is first converted into magnetic energy, then reconverted back into electrical energy at a different voltage and ampacity. Because of this conversion process, the transformer can perform duties which have made it invaluable in the field of electricity.

The method of transferring electrical energy by a transformer is done indirectly. Electrical energy is first converted into magnetic energy, then reconverted back into electrical energy at a different voltage and ampacity. Because of this conversion process, the transformer can perform duties which have made it invaluable in the field of electricity.

Mutual-Induction

Transformers are based on the principle of “mutual-induction.” When current flows through a wire a magnetic field is produced. A good example of this is an “electro-magnet.” By wrapping an insulated wire around an iron bar and hooking this wire to a battery, a magnetic field is induced in the iron bar making it a temporary magnet !

This principle also works in reverse. When a conductor passes through a magnetic field, a current flow will be induced through the wire. Interesting ! So, it seems that magnetism and electricity are closely related. You can’t have one without the other. This relationship can be very useful.

A transformer uses both of these methods of “induction” at the same time. A basic transformer consists of two separate windings of insulated wires wound around a common iron core. The power source or supply is hooked to the primary winding, the load to be served is hooked to the secondary winding. When the primary winding is energized an electromagnetic field builds up and then collapses in the iron core, this field cuts through the secondary coil winding inducing power to the load hooked to the secondary. This power buildup and collapse is called magnetic flux and occurs at a frequency of sixty times a second (60 hz) in an A.C. circuit.

If the transformer is running perfectly, the power introduced on the primary will be equal to the power used on the secondary. You might be saying, “What good is a transformer if it uses as much power, or wattage, as it produces” ?

Now, here’s the magic ! By altering the number of windings on the primary and secondary, we can alter the amount of volts and amps between the source and the load. If we have a motor rated 240 volts, but a source voltage of 480 volts, we can use a transformer to reduce our source voltage by one-half. And, we can even increase our amps if needed.

The current in the secondary coil always changes by the inverse of the ratio by which the voltage changes. In other words, if the voltage is doubled, the current is cut in half. If the voltage is raised to ten times its original value, the current in the secondary coil will be reduced to one-tenth the value of the current in the primary coil.

If the transformer is running perfectly, the power introduced on the primary will be equal to the power used on the secondary. You might be saying, “What good is a transformer if it uses as much power, or wattage, as it produces” ?

Now, here’s the magic ! By altering the number of windings on the primary and secondary, we can alter the amount of volts and amps between the source and the load. If we have a motor rated 240 volts, but a source voltage of 480 volts, we can use a transformer to reduce our source voltage by one-half. And, we can even increase our amps if needed.

The current in the secondary coil always changes by the inverse of the ratio by which the voltage changes. In other words, if the voltage is doubled, the current is cut in half. If the voltage is raised to ten times its original value, the current in the secondary coil will be reduced to one-tenth the value of the current in the primary coil.

Transformer Ratio

Here’s how it works ! Every winding on the primary side will cause voltage to be induced into each winding on the secondary. By altering the number of windings (or turns) on either the primary or secondary side we will automatically alter the voltage ratio. Check this formula…

Here’s an example…

Notice that there are twice as many turns on the primary side (16/8 or 2:1) than on the secondary side. Also, there are twice as many volts on the primary side (8/4 or 2:1) than the secondary side. We call this a 2:1 step-down transformer because we are stepping the voltage down by a two to one ratio.

OK ! We can transform the ratio of volts with a transformer, so what about trans-forming amps ? The ratio of current is also changed in a transformer, but in the opposite direction. Watch this…

OK ! We can transform the ratio of volts with a transformer, so what about trans-forming amps ? The ratio of current is also changed in a transformer, but in the opposite direction. Watch this…

In the above transformer, the voltage is stepping down by a ratio of 2:1 (or 480 to 240 volts) while the current increases by a ratio of 1:2 or (2 to 4 amps). So, what is actually changing in an ideal transformer is the ratio of volts to amps. What doesn’t change in a transformer is wattage. Look at this…

To find primary watts we’ll call them volt-amps to differentiate them from secondary watts, we multiply primary volts times amps (480 x 2 = 960 vA). To find secondary watts, we multiply secondary volts times amps (240 x 4 = 960 watts). Ideally, transformers do not alter power or wattage, again, they only alter the ratio of volts to amps.

Since it is so easy to increase or decrease voltage and current (merely by altering the turns ratio of a transformer), one might assume that power, (or wattage) might be increased or decreased. This assumption is not valid, since it violates the law of conservation of energy. It’s impossible to get as much power out of a transformer as is put into it, because no device can be made to operate at 100% efficiency; there is always some loss. If we can assume that a transformer runs at 100% efficiency, the amount of transformed power is neither increased or decreased, only the current to voltage ratio is changed.

Since it is so easy to increase or decrease voltage and current (merely by altering the turns ratio of a transformer), one might assume that power, (or wattage) might be increased or decreased. This assumption is not valid, since it violates the law of conservation of energy. It’s impossible to get as much power out of a transformer as is put into it, because no device can be made to operate at 100% efficiency; there is always some loss. If we can assume that a transformer runs at 100% efficiency, the amount of transformed power is neither increased or decreased, only the current to voltage ratio is changed.

Transformer Ladder

Yes, we can utilize the Ohms Law Ladder to do transformer calculations...

The ladder works on the primary side (replacing volt-amps for watts) by multiplying each step up the ladder and dividing each step down the ladder. This works the same on the secondary. The ratio of volts from primary to secondary can also be used. Can you determine primary volt-amps, primary amps and secondary watts for the transformer below ?

Remember, since the voltage ratio is 2:1, the amps ratio will be the opposite, 1:2. Also, the wattage will be the same on both the primary and secondary.

Although we can calculate resistance, it usually isn’t very important (except for calculating the resistance of loads connected to the secondary).

Efficiency

As we all know nothing works perfectly. Although transformers are pretty amazing there is some loss in power due to inefficiencies built into transformers. Here are the three main causes for power losses in the operation of a transformer:

Eddy Currents are local short-circuit currents induced in the iron core by alternating magnetic flux, causing the core to produce heat. This effect is minimized by cutting the core into thin layers and laminating each layer.

Hysteresis is the lagging of the magnetic molecules in the core in response to alternating magnetic flux. This lagging condition is due to the fact that it requires power to reverse magnetic molecules; they do not reverse until the flux has attained sufficient force to reverse them. Their reversal results in friction, and friction produces heat in the core which is a form of power loss. Hysteresis is minimized by the use of special steel alloys properly annealed.

The I2R Loss is sometimes referred to as "copper loss." It is the power lost from the resistance of the copper conductor windings. This represents the greatest loss in the operation of a transformer.

The intensity of power loss in a transformer determines its efficiency. The efficiency of a transformer is reflected in power (wattage) losses between the primary (input) and secondary (output) windings. Here are three formulas for determining power losses due to efficiency...

Eddy Currents are local short-circuit currents induced in the iron core by alternating magnetic flux, causing the core to produce heat. This effect is minimized by cutting the core into thin layers and laminating each layer.

Hysteresis is the lagging of the magnetic molecules in the core in response to alternating magnetic flux. This lagging condition is due to the fact that it requires power to reverse magnetic molecules; they do not reverse until the flux has attained sufficient force to reverse them. Their reversal results in friction, and friction produces heat in the core which is a form of power loss. Hysteresis is minimized by the use of special steel alloys properly annealed.

The I2R Loss is sometimes referred to as "copper loss." It is the power lost from the resistance of the copper conductor windings. This represents the greatest loss in the operation of a transformer.

The intensity of power loss in a transformer determines its efficiency. The efficiency of a transformer is reflected in power (wattage) losses between the primary (input) and secondary (output) windings. Here are three formulas for determining power losses due to efficiency...

Here’s a simplified way of determining the Efficiency (Eff.) formulas above...

Just put your finger on the “W” to find Watts (output) = VA x Eff., or “vA” to find Volt-Amps (input) = W/Eff., or “Eff.” to find Efficiency = W/VA.

Here’s a sample problem: Find the efficiency of a transformer with a primary of 3,000 vA and a secondary of 2,400 watts.

Transformer Types

A byproduct of the efficiency and power factor losses is excessive heat. Several methods are used to dissipate this heat from transformer cores and windings to the outside. Some transformers are designed for air-cooling. This type is known as dry-type transformers. They are designed with sufficient air spaces (in and around the coils and core) to allow sufficient air circulation for cooling. Some dry-type transformers depend on a fan for air circulation.

Most transformers use a coolant for heat transfer. Oil is the most commonly used coolant, but in applications where oil would present a fire hazard, askarel coolants must be used. Askarel is a term for coolants that includes all the synthetic non-combustible insulating coolants manufactured under different trade names. A coolant conducts heat to the sides of the tank, and the tank conducts it to the outside. To aid in dissipation of heat to the outside, some tanks are corrugated or equipped with fins to increase the radiating surfaces.

Most transformers use a coolant for heat transfer. Oil is the most commonly used coolant, but in applications where oil would present a fire hazard, askarel coolants must be used. Askarel is a term for coolants that includes all the synthetic non-combustible insulating coolants manufactured under different trade names. A coolant conducts heat to the sides of the tank, and the tank conducts it to the outside. To aid in dissipation of heat to the outside, some tanks are corrugated or equipped with fins to increase the radiating surfaces.

__Large transformers__use additional methods for cooling. Some are equipped with vertically spaced tubes (around the tank). The warm coolant has a natural circulation in the tank and tubes, which vents heat to the outside.__Transformers are used to efficiently distribute electricity from generating plants to industrial, commercial and residential areas. Step-up transformers boost voltages up to 765,000 volts for easy transmission, using small sized conductors. Step-down transformer are use to meet local higher current demands at 480, 277, 240, 208 and 120 volt requirements.__

Power DistributionPower Distribution

__Autotransformers__use one continuous winding through both the primary and secondary on the same iron-core. The primary and secondary serve in the same magnetic circuit causing current to flow in parts of the same winding. The main advantage of autotransformers are economical construction, and operating efficiency in low ratio situations like reduced-voltage motor starters.__Current Transformers (CT’s)__are used when the a.c. currents are too large for measuring instruments such as power company kilowatt-hour meters. They work on the same principle as the clamp-on ammeter by sensing current flow through a conductor without having to break the circuit.__Constant-Current Transformers__produce a constant secondary amperage to a load even though the primary input amperage changes. By using a movable primary coil, air space between coils can be varied. This causes magnetic leakage between the coils, and varies current flow in the secondary. A typical example of the use of these transformers are series street-lighting systems.Transformer Size Chart

Transformer Calculations

Remember this gadget for transformer calculations. Using this guy will save us from having to look-up or memorize all of the previous formulas we have used so far...

Notice the ladder on the primary is only useful with primary side values (vA, volts, amps and resistance). The secondary side ladder values (watts, volts, amps and resistance) are only useful on the secondary side. With the diagram at the bottom of the chart, we can cross-over between the primary and secondary taking efficiency into account. Remember that with 100% efficiency, primary volt-amps and secondary watts would be exactly the same.

Transformer Sample Problem

Problem: A single-phase transformer has 120 volts on the primary with 24 volts on the secondary. The transformer is feeding a 500 watt load with an efficiency of 95%.