An ideal transformer is one with NO energy loss. But in practice, Transformers do undergo power losses due to various reasons. As the electrical transformer is a static device, kinetic loss in the transformer normally does not come into the picture. We generally consider only electrical losses and magnetic losses in transformer. Loss in any machine is broadly defined as the difference between input power and output power.
Table of Contents
TYPES OF LOSSES IN TRANSFORMER
All types of transformer losses are discussed below:
No-load losses in transformer
Losses incurred in the transformer, even when no energy is being transformed are referred to as No-load losses. These are also referred to as core losses. This loss is calculated based on the amount of power required to magnetize the core of the transformer.
Since most Transformers are energized 24*7, no-load losses are present at all times, whether a load is supplied by the transformer or not. When it is lightly loaded, no-load losses represent the greatest portion of the total losses. No-load losses are caused by the magnetizing current needed to energize the core of the transformer and do not vary according to the loading on the transformer. They are constant and occur 24 hours a day, 365 days a year, regardless of the load, hence the term no-load losses.
They can be categorized into five components: hysteresis losses in the core laminations, eddy current losses in the core laminations, I2 R losses due to no-load current, stray eddy current losses in core clamps, bolts, and other core components, and dielectric losses.
Hysteresis Loss:
This is the type of loss that occurs due to loss of energy in the process of continuous magnetization and demagnetization of a material. Because of the AC supply input at the terminal, the iron core gets magnetized and demagnetized in each cycle and some energy is lost in the process. Imperfection in the material increases the energy expenditure because of internal friction of the domain wall motion. The use of thin lamination, and proper heat treatment of the core material can reduce this loss.
Eddy Current Loss:
Whenever any electrical conductor is placed in a constantly changing magnetic field, it experiences an EMF induced. Due to this circulating currents start to flow in the conductor, in the case of the transformer, it is the iron core. This current takes up energy to produce a lot of heat in the system and results in less efficiency of the transformer.
Hysteresis losses and eddy current losses contribute over 99% of the no-load losses, while stray eddy current, dielectric losses, and I2 R losses due to no-load current are small and consequently often neglected. Thinner lamination of the core steel reduces eddy current losses.
The biggest contributor to no-load losses is hysteresis losses. Hysteresis losses come from the molecules in the core laminations resisting being magnetized and demagnetized by the alternating magnetic field. This resistance by the molecules causes friction that results in heat. The Greek word, hysteresis, means “to lag” and refers to the fact that the magnetic flux lags the magnetic force. The choice and type of core material often decided at the designing stage reduces hysteresis.
Dielectric Loss:
Dielectric loss occurs in the insulating medium of the transformer. Oil is used as a dielectric/insulating medium. Due to the continuous operation of the transformer, the dielectric strength of the oil is reduced. When the quality of the oil deteriorates, it causes some losses known as Dielectric losses which decrease the overall efficiency of the transformer.
Frequent checks on the quality and condition of the dielectric medium can help avoid such losses.
Load losses in transformer
Load losses, on the other hand, are the losses incurred while carrying a load. These include winding losses, stray losses due to stray flux in the windings and core clamps, and circulating currents in parallel windings. Because load losses are a function of the square of the load current, they increase quickly as the transformer is loaded. Load losses represent the greatest portion of the total losses when a transformer is heavily loaded.
Load losses vary according to the loading on the transformer. They include heat losses and eddy currents in the primary and secondary conductors of the transformer.
Heat losses, or I2R losses in transformer
Heat losses in the winding materials contribute the largest part of the load losses. They are created by the resistance of the conductor to the flow of current or electrons. The electron motion causes the conductor molecules to move and produce friction and heat.
The energy generated by this motion can be calculated using the formula:
Watts = (volts)*(amperes) or VI.
According to Ohm’s law, V = IR,
or the voltage drop across a resistor equals the amount of resistance in the resistor, R, multiplied by the current, I, flowing in the resistor.
Hence, Heat losses = (I) * (RI) or I2R.
Transformer designers cannot change I, or the current portion of the I2 R losses, which are determined by the load requirements. They can only change the resistance or R part of the I2 R by using a material that has a low resistance per cross-sectional area without adding significantly to the cost of the transformer.
Most transformer designers have found copper the best conductor considering the weight, size, cost, and resistance of the conductor. Designers can also reduce the resistance of the conductor by increasing the cross-sectional area of the conductor. Here the use of copper conductor having low resistance & low I2R losses can be considered at design stage having better properties over the Aluminum conductor as given below:
Stray Losses in transformer
The stray losses in the transformer comprise winding stray losses, viz. eddy loss and circulating current loss; the loss in the edge stack (smallest packet of the core limb); and the loss in structural parts, viz. frame, flitch plate, and tank. Core loss at the impedance voltage being insignificantly low, is not considered in the present analysis.
In the case of large generator transformers, stray losses due to high current carrying leads also become significant. As the total stray losses with shielding measures in large rating transformers are of the order of 20-25% of the total load losses, it is imperative to estimate stray losses accurately as control over these gives a competitive advantage. Measures like using judiciously designed magnetic shunts help reduce stray losses effectively.
How can losses in transformer be reduced?
Using magnetically softer core material can help reduce the hysteresis loss by promoting efficient magnetization and demagnetization of the core.
Eddy-current losses are reduced by using a core composed of thin laminated sheets of magnetic
material that are coated with an insulating material and pressed or bolted together instead of a
solid core. The laminations offer high resistance to current flow; hence, the eddy currents and the
resulting losses are reduced.
The use of better insulating materials can help reduce the stray losses which are more prominent in the upper side of the transformer tank, near bushings. Also, use of magnetic shunts for these sections can redirect the leakage flux away thereby reducing the stray losses.
Adequate natural cooling can help lower the auxiliary losses in transformer.
FAQs
Q: Which losses are influenced by voltage and current?
A: Core loss is basically a function of voltage and load loss are function of current.
Q: what is hysteresis loss?
A: It is a function of frequency. It results in heating of the core and occurs due to continuous magnetization and demagnetization of the core.
Q: What factor contributes to eddy current loss in a transformer?
A: The circulating current that is induced in the core by alternating magnetic fields results in eddy current loss. This current results in heat and contributes to power loss.
Q: What are copper losses?
A: Copper losses are I2 R losses that are caused because of the resistance of the copper winding.
Q: What causes stray loss and dielectric loss?
A: Stray losses are caused by leakage flux which induces current in the structural parts of the transformer.
Dielectric losses occur in the insulating medium due to changes in an electric field.
