Load Pull from Transformer

Load withdrawal from the transformer is an absolute event to provide a voltage output from the secondary right.

In previous transformer trainings, we assumed that the transformer was ideal, that is, there were no core losses or copper losses in the transformer windings.However, in real-world transformers, since the transformer is put on a "load", there will always be losses related to the installation of transformers.

First, let's look at what happens to a transformer in this "load-free" state, that is, when no electrical charge is connected to the secondary winding and therefore the secondary current does not flow.

It is said that a transformer is "load-free" when the secondary side winding is on circuit, in other words, when nothing is connected and the transformer load is zero.When an AC sinusoidal supply is connected to the primary winding of a transformer, a small IOPEN current will flow through the primary coil winding due to the presence of the primary supply voltage.

When the secondary circuit is on, nothing is connected, while a rear EMF with primary winding resistance moves to limit the flow of this primary current.Obviously, this loadless primary current (Io) should be sufficient to maintain sufficient magnetic field to produce the required back emk.

Transformer's "Load-Free" Open Circuit Status

Load Pull from Transformer
Transformer's "Load Free" Status

The amperemeter above will show a small current flowing through the primary winding, although the secondary circuit is open circuit.This loadless primary current consists of the following two components:

  • An intra-phase current that feeds core losses (vortex current and hysteresis), I E.
  • A small current, the voltage that creates the magnetic flux, I M at 90 o.
Load Pull from Transformer
Phaser Diagram of Loadless Transformer

Note that this loadless primary current is very small compared to the normal full load current of Io transformers.Also due to the iron losses found in the core, as well as the small amount of copper losses in the primary winding, Io does not lag behind the supply voltage, the Vp will be exactly 90 o , ( cosφ = 0 ), some small quantities (phase angle difference).

Transformer "Load"

When an electric charge is connected to the secondary winding of a transformer, and therefore the transformer load is greater than zero, a current flows into the secondary winding and load.This secondary current is caused by the induced secondary voltage established by the magnetic flux generated in the nucleus from the primary current.

The secondary current determined by the characteristics of the payload, I S , Φ S in the transformer core that flows in the opposite direction of the main primary area, creates a self-induced secondary magnetic field, Φ P.These two magnetic fields are opposite each other, resulting in a combined magnetic field with less magnetic force than the single field produced by the primary winding alone when the secondary circuit is open circuited.

This combined magnetic field reduces the rear EMF of the primary current's primary winding, which causes a slight increase in IP.The primary current continues to increase until the core magnetic field returns to its original power, and for a transformer to function correctly, there must always be a balanced condition between the primary and secondary magnetic fields.This causes the power to stabilize and be the same on both the primary and secondary sides.

Load Pull from Transformer
Load-dependent transformer

We know that the rotation rate of a transformer indicates that the total induced voltage in each winding is proportional to the number of rotations in that winding, and also that the power output and power input of a transformer are equal to the volt multiplication amperage, (V xI).Then:

Load Pull from Transformer

But we also know that the voltage ratio of a transformer is equal to the rotational rate of a transformer: "voltage ratio = rotation rate".Then the relationship between the voltage, current and number of turns in a transformer can be connected and therefore given as follows:

Transformer Rate

Load Pull from Transformer
  • Here:
  • N P /N S = V P /V S – represents the voltage ratio
  • N P /N S = I S /I P – represents the current rate

Note that the current is inversely proportional to both the voltage and the number of turns.This means that with the loading of a transformer in the secondary winding, to maintain a balanced power level along the transformer windings, the current is lowered if the voltage is raised, and vice versa.In other words, "high voltage – lower current" or "lower voltage – higher current".

Since a transformer ratio, the number of primary and secondary windings, is the relationship between the voltage in each winding and the current passing through the windings, we can rearrange the transformer ratio equation above to find the value of any unknown voltage, (V) current, ( I ) or number of rotations, as shown.

Load Pull from Transformer

The total current drawn from the feed by the primary winding is the vector sum of the unburdened current, Io and additional supply current as a result of secondary transformer loading, I 1 and one angle behind the supply voltage.Φ .We can show this relationship as a phaser diagram.

Transformer Loading Current

Load Pull from Transformer

If I S and Io currents are given, we can calculate the primary current I P by the following methods .

Load Pull from Transformer

Transformer Loading Question Example 1

A single-phase transformer has 1000 turns in its primary winding and 200 turns in its secondary winding.The "load-free" current of transformers from the feed is 3 Amps in a power factor of 0.2 latency.The primary coil current is Ip, and calculate the corresponding power factor when the secondary current is 0.8 inductive 280 Amps.

Load Pull from Transformer
Load Pull from Transformer

You may have noticed that the phase angle of the primary current, the φ P,the phase angle of the secondary current, is almost the same as the φ S.This is because the 3 amps of unburdened current is very small compared to the larger 56 amps, which are pulled by the primary winding from the feed.

When drawing X-L and R Phaser diagrams of transformer windings with both impedances, these impedances should be taken into account, since these internal impedances cause voltage drops in the transformer windings.Internal impedances are caused by the resistance of the bandages and the fall of an induced induced called leaky reassurance caused by leaky flux.These internal impedances are given as follows:

Load Pull from Transformer

Therefore, the primary and secondary windings of a transformer have both resistance and reactance.Sometimes, combining all these impedance values on the same side of the transformer may be more convenient to make math calculations a little easier.

It is possible to move the primary impedances to the secondary side or the secondary impedances to the primary side.The combined values of the R and L impedances are called "Reference Impedances" or "Reflected Values".The goal here is to group the impedances within the transformer together and, as shown, have only one R and XL reference value to the primary or secondary side in our calculations.

Combining Transformer Impedances

Load Pull from Transformer

To indicate resistance or reactance from one side of the transformer to the other, we must multiply or divide by the square of the rotation rate (Rotation Rate 2).Therefore, by referring (or reflecting) impedances (resistance and reassurance) on the primary side of the transformer on the secondary side, we multiply the rotation rate square by N 2, and when referring to the secondary side of the primary impedances, we need to divide them. rotates the square of the ratio.The second reflection reduces R and X by an amount determined by the primary N, thus increasing the second where R and X.This reference or reflection of impedances applies in the same way to the connected load resistance and reassurance.

For example, specifying a 2Ω secondary resistance to the primary party with a rotation rate of 8:1 will result in a new primary resistance value: 2 x 8 2 = 128Ω, and the primary resistance will result in 2Ω. secondary resistance value: 0.03125Ω .

Transformer Voltage Regulation

Voltage regulation of a transformer is defined as a change in secondary terminal voltage when the transformer load is maximum, that is, when the full load is applied while the primary supply voltage is kept constant.Regulation determines the voltage drop (or increase) that occurs within the transformer as the load voltage decreases too much as a result of the high transformer load and therefore affects its performance and efficiency.

Voltage regulation is expressed as the percentage (or per unit) of load-free voltage.Then, if E represents load-free secondary voltage and V represents fully loaded secondary voltage, the percentage arrangement of a transformer is given as follows:

Load Pull from Transformer

For example, if a transformer gives 100 volts without load and the voltage drops to 95 volts at full load, the regulation is 5%.The E – V value will depend on the inner impedance of the winding, which includes its resistance, R and, more importantly, ac reagent X, current and phase angle.

In addition, voltage regulation usually increases as the power factor of the load becomes more delayed (inductive).Voltage regulation for transformer loading can be positive or negative in value, that is, with load-free voltage as a reference, there may be changes and changes in regulation as the load is applied, or changes and changes with full load as a reference. increases in regulation when the load is reduced or removed.

In general, the regulation of the core type transformer when the transformer load is high is not as good as the shell type transformer.This is due to the fact that the shell type transformer has a better flux distribution due to the interlocking of the coil windings.

In the next lesson about transformers, we will look at the Multi-Winded Transformer with multiple primary windings or multiple secondary windings and see how we can connect two or more secondary windings together to provide more voltage or more current.