Transformers
Transformers
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11 Secrets about Losses in Transformer you don’t know before


11 Secrets about Losses in Transformer in detail

In this article you will know more about losses in transformer and how to reduce them. Previously we are always used to review through the articles everything that is new and unique in the world of electrical engineering, we began to recognize the electric transformer, its principle of work and the equivalent circuit.

Then we touched on the important subject of the phasor diagram of transformer and then we know in detail the important tests to be performed to ensure the safety of the transformer.

All of these topics now lead us to a very important stage of the transformer.

In previous topic we talked about the efficiency of the transformer and how this efficiency effect on the lifetime of the transformer.

Today, we will discuss with all available scientific methods an important subject but it is one of the most powerful subjects through which we will reveal some hidden secrets >>> Let’s Go.

Losses in transformer

The losses of any machine in a simple scientific way are the amount of difference between the input power to the machine and the output power coming out of it.

But we will not let things go this way

and we should go deeper and deeper into the transformer

to see what exactly happens to consume this power within the transformer

and the factors affecting it

Gather all your mental strength now

  walk with me

to deepen more and know more all the secrets of the losses in the electrical transformer

The first secret: We previously knew the ideal transformer and its characteristics and it has become clear to us that it is the ideal transformer found only in the world of theories and has no practical presence on the ground.

So, the efficiency of the ideal transformer is 100%, so the losses in the ideal transformer is 0%.

That means the input power is equal to the output power and this is theoretically only.

The second secret: Most of the scientific sites that dealt with this subject has classified the losses in transformer into two parts, namely iron loss, copper loss, but the secret here is that the losses within the transformer is divided into 6 categories.

losses in transformer

• Hysteresis losses.
• Eddy current losses.
• Dielectric losses.
• Copper losses.
• Stray losses.
• Conductor eddy current losses.
Now I will reveal the set of secrets for each type of losses in transformer in detail.

No Load losses in transformer:

The third secret about No load losses in details:
When we disconnect the loads from the transformer while still connecting it to the source, the transformer continues to draw power from the source. We consider this power as a losses because there is no output.

But this power has a key benefit … is making the transformer energized and ready for service.

This power is withdrawn when the transformer is not loaded; it also remains a lost power while loading the transformer with the same value … “we have proved this through conducting experiments in a previous topic >>> no load test

We can divide These losses into three main types:

Lot of sites classified no load losses as hysteresis losses and eddy current losses only … but no load losses divided into 3 types of losses:
a. Hysteresis Losses.
b. Eddy current Losses.
c. Dielectric losses.

Hysteresis Losses

The fourth secret about hysteresis losses in details:
This loss is part of the core losses and is also a type of no load losses, that is, whether the transformer is connected to a load or not.

The idea of this type of loss is simply that “every time the magnetic field is reflected, a fraction of the power is lost as a result of hysteresis”

So, what the meaning of hysteresis?

This word means the tendency of magnetism to stay in its old state.

And the fact that a part of the magnetism remains in the iron core is “residual flux”, that we have lost a part of the magnetic power within the iron core this power is called “hysteresis losses” and their effect is represented in the circuit equivalent to the real transformer in the form of a winding with an impedance Xm.

When passing the sinusoidal current in the primary winding it produces as we know the magnetic flux.

This magnetic flux quite similar to the voltage that created … and it has the same sinusoidal curve.

sinusoidal curve

In the first half of the sinusoidal cycle … this magnetic flux causes magnetization of the magnetic material in the core, which we can consider to be internally composed of a group of domains that are aligned in one direction because of the magnetic field as in the following figure (1.1)

When the direction of the magnetic field is reversed in the second half of the second cycle of the sinusoidal curve, the domains inside the core must change direction and the poles must align parallel in the new direction of the field.

Thus we can consider hysteresis loss as the lost power during the friction of the particles of matter and the movement of its poles in each cycle of the magnetic field.

hysteresis losses

In fact, the energy consumed in the row of poles of the magnetic material comes from the input power and of course does not transfer this energy to the secondary side, which does not move to the load and then consider it a lost energy.

Calculation of Hysteresis losses:

We conclude from this that the stronger the magnetic field and the higher the frequency, the higher the energy consumed in the row and the change of the direction of the electrodes inside the material.

We can calculate the value of hysteresis losses from the following equation:

hysteresis loss equation

Such that:
Kh : constant depends on material and its quality
Bmax : peak value of the flux density.
F: frequency of the supply.
V : volume of the core
We can calculate the value of hysteresis loss in another way, through the area within the loop known as hysteresis loop as in the following figure:

hysteresis loop

So, the larger the area within the loop, the greater the hysteresis losses.

From the previous figure:

It is clear that we can reduce hysteresis losses by improving the quality of the material used in the core.

Eddy current losses

The fifth secret in details about eddy current losses:
We all know that magnetic materials such as iron also have the ability to connect the electrical current and not only to pass the magnetic flux … When the flux lines cut off the wires of copper winding, they generate a great e.m.f under the law of Faraday .

When they cut off these winding, they also cut off Iron core also generates an electric current called eddy current.

as in the following figure

eddy current

Of course, these eddy currents are not desirable. They do not reach to the load and cause the iron core to warm unnecessarily.

Usually, eddy currents cause about 50% of the iron core loss.

This type of loss depends mainly on the type of magnetic material, frequency and magnetic field density.

We can calculate according to the following formula:

eddy current formula

Such that :
Ke is constant depends on the type and thickness of the material

Reduce Eddy Current Losses:

The value of the thickness of the magnetic material strongly effects on the Eddy current loss … So, the less thickness the greater the electrical resistance and the less the current passing through the iron core … Therefore all the iron cores of all electrical transformers should be in the form of lamination separated from each other and compressed together.

transformer lamination core1transformer lamination core2

In terms of magnetism, the lamination set gives the area of the cube of the iron core to withstand the passage of the flux and in the electric sense, it is high resistance to the small area of the section in each slice (usually does not exceed the thickness of the single lamination is 0.35 mm) there is only a small current and then can be The way to reduce the eddy current.

Dielectric losses:

The seventh secret in detail about Dielectric losses:
The dielectric losses are classified under no load losses … they are present during loading and at no load condition of transformer.

The insulation materials used to isolate the conductors from each other inside the transformer cause a kind of capacitors known as the stray capacitors.

They are capacitors that are not visible to the eye and are not held by hand, but do the same work as the real capacitor and cause some kind of loss of energy.

In fact we can represent the ideal capacitor by capacitance only without resistance … where there is no loss in power, it is charged by the first half of the sine wave and then discharge in the second half and then charged and discharged and so on … And therefore does not lose anything of the active power so the angle between the voltage and the current 90 degrees.

But this is only theoretically possible … There is no capacitor in fact, only capacitance, but always with a small resistance.

So, the angle between voltage and current is less than 90 degrees by a small angle called delta (δ).

Tan δ is a standard measure to give an indication of the magnitude of the capacitor spacing as an ideal capacitor.

The smaller the angle, the closer the capacitor is to the ideal capacitor.

Thus, dielectric losses are proportional to the other with tan δ and this amount called “dissipation factor” … and we can calculate the dielectric losses from the following equation:

dielectric losses equation

Such that:
Pd: Dielectric losses in Watt.
F: applied frequency in HZ.
C: Capacitance in Farad.
V: Operating Voltage in r.m.s
Tan δ: dissipation factor.

Severity of dielectric loss

Although dielectric loss is a small fraction of the losses in transformer, it is one of the most dangerous types of losses in transformer because temperature strongly effects on tan δ.

“The higher the temperature the more tan δ”

The more tan δ increased dielectric losses and increased More and more temperature until thermal breakdown occurs.

Load losses in transformer:

The eighth secret in details about Load losses:
These set of load losses appears only during the loading of the transformer as a result of the passage of the load current through the winding.

Therefore it consists mainly of copper losses in the resistance of winding, either in primary or secondary winding.

The importance of calculating the load losses in it is a fundamental element when estimating the size of the transformer, the heat resulting from the passage of the current in the winding raise the temperature to dangerous degrees … So it is necessary to work to reduce these losses, which is often by reducing the value of resistance winding.

We can calculate load losses through short circuit test of transformer … you can check this here >>> short circuit test of transformer.

Load losses divided into three main types:
a. Copper losses.
b. Stray losses.
c. Leakage flux losses.

Copper losses:

The ninth secret about Copper losses in details:
These losses are the first type of load loss in the sense that it does not appear as an effective value until after loading the transformer.

The more the transformer loads, the more power losses are high.
It is known that the copper winding (primary and secondary) have a certain resistance and therefore the passage of a current which causes loss of power is calculated from the following equation:

P= I² * R

For accuracy the loss in the copper winding at no load condition because of the passage of no load current.

The no load current is the current that passes through the primary winding only at the rated voltage with the fact that the other winding Open and often within 1% to 2% of full load.

Effect of temperature on copper losses

As the temperature rises, the value of winding resistance increases in contrast to the eddy current loss, which decreases with increasing temperature.

Statistically, each 1 ° C increase can cause a rise in losses of 0.4%.

Effect of types of currents on copper losses

It is known that the value of the resistance is calculated from the law (R = ρ *L /A)   … but this law is absolutely true in the case of DC Current only if the current AC is considered a kind of approximation acceptable and not accurate … because the AC tends to pass at the ends of the connector Away from its center (Skin effect) as in the following figure

skin effect

 

The concentration of the current in the sides makes the area of the actual section of the connector less than the engineering area of the conductor and then Rac increases the value of Rdc and this is the skin effect.

Therefore, the copper losses increase more if the current of AC and of course shows this effect clearly whenever the area of the section of the conductor is larger … We can neglect this effect In small conductors.

Stray losses

The tenth secret about stray losses in details:
These stray losses produces as a result of the leakage flux … We know that the flux that generates when the current passes in a winding is not completely coupled to the other winding but there is a missing part.

We can express this missing part of the flux as leakage flux.

This leakage flux may cut off the external iron parts of the transformer and create the eddy current.

This eddy current causes the hotness of these non-current metal parts. This is a type of energy loss that shows an effect in large transformers only.

In addition to the previous losses types, there are other types, although less effective than the previous types, but the total sometimes represents 5% of the value of losses in transformer.

Conductor eddy current losses

The eleventh secret about conductor eddy current losses in details:
This type of losses is generated in the copper conductors of the winding due to leakage flux but has small values and its effect is weak … its value is calculated by experimentation and measurement and not by equations.

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Electrical Engineer