Electric Transformer Efficiency: 4 Answers For Beginners

Electric Transformer Efficiency: 4 Answers For Beginners

What is transformer efficiency?

The proportion of a transformer’s output power to the input power is referred to as the efficiency of a transformer. The impact of losses on transformers can be measured using the term transformer efficiency, usually expressed in percentages. The following formula can be used to determine the efficiency of a transformer:

Efficiency of transformer= η = (Output Power/Input Power) X 100

The power transformer’s efficiency typically ranges between 97 and 99 percent. The power delivered to the load and resistive, eddy current, the loss of hysteresis, and flux losses must be equal to the power input. The input power must always be higher than the output power.

Are Transformers more efficient when cooled?

The cooler the transformer the higher ts efficiency. The efficiency of a transformer will change with the load if the transformer runs on 50% of its rated load value. However, it will remain cooled. Therefore, its efficiency will remain comparatively high when we overload the transformer from its rated load capacity due to losses in transformer heat produced and its efficiency decrease.

The cooling system plays a crucial function in prolonging the transformer’s lifespan. A properly functioning cooling system capable of dispersing the heat generated by the transformer is essential.

This cooling system can keep the heat produced within the transformer within acceptable limits and play a significant role in prolonging the life span of the transformer.

Thus, a properly-designed cooling system that can dissipate the heat generated by the transformer is exceptionally crucial.

A cooling system like this can keep the heat generated within the transformer to the permissible limits and plays a significant role in prolonging the life span of the transformer.

There are various cooling methods used to prolong the life span of the transformer. Let’s take an overview of them:

ONAN Method:

ONAN, “Oil Natural Air Natural”, is the most efficient method of the cooling transformer. This method is where the hot oil, by absorbing heat from the transformer’s windings, is pumped into the tank at the top of the transformer.

The hot liquid pumped to the upper tank of the transformer is then cool naturally through the exchange of heat via conduction, radiation, and convection techniques into the air.

The moment the oil in the tank cools, it flows to the radiators of the transformer. The flow of cold and hot oil continues when there is an electrical load on the transformer.

The heat’s rate at which the heat is dissipated depends on the area in the tank; these kinds of transformers are usually equipped with substantial circulation tanks.

Furthermore, an additional surface to allow for rapid heat dissipation is added by way of radiators and tubes to the tank to ensure that the transformer is cooled rapidly.

ONAF Method:

ONAF, “Oil Natural Air Forced”, is a technique used to help disperse the heat off the surface of radiators and is even more efficient. With the ONAN method, heat generated by heated oil escapes by the tank’s surface naturally and then to the air.

In this process, it is utilized to force air to speed up the cooling process through the aid of fans. The fans that blow air onto the cooling surface can disperse the heat quicker than air that absorbs the heat of the hot surface.

This lets you put more load to the transformer but without exceeding the heat levels allowed within the transformer—the fans used in this kind of cooling system to cool the windings of the transformer faster.

This means that the transformer’s performance and longevity can be increased using this method.

OFAF Method:

OFAF, “Oil Forced Air Forced”, is a technique used to cool transformers more quickly than ONAF method.

In the ONAF method, the air is forced onto the hot surface to cool it down; however, oil circulation within the radiators is still based on the natural convection process, which is extremely slow. However, in the OFAF method, oil is forced to move faster and allows for the device’s cooling to occur quicker.

In this kind of cooling system, pumping is used to increase the flow of oil, which causes the oil circulation to be more rapid when contrasted with natural convection. In the process, the speed of the flow of oil inside the radiators rises, increasing the rate of heat dissipation out of the transformers.

OFWF Method:

OFWF “Oil Forced Water Forced” is a highly sophisticated technique compared to the methods discussed previously. In this technique, water is the medium for heat transfer instead of air. We are all aware that in any weather condition, the temperature of the water is less than the temperature of the air. This is the main reason employed in this kind of coolant system.

The oil that circulates through the transformer’s radiators passes through a chamber of water, in which cooling water jets are poured onto the pipes that contain heated oil.

The heat exchange happens quicker through this process, and the oil is more extraordinary faster, making the cooling system more efficient.

How does transformer efficiency vary with load?

The efficiency of a transformer can depend on the load; it may vary according to the variations in load. The efficiency rises with a rise in output power up to a specific value and, after a particular value of output power, efficiency decreases.

With no loads, the Transformer draws a small amount of energy, primarily due to the current of magnetization. In a transformer designed to maximize efficiency, the energy input with no secondary load is usually 1 to 2 percent of rated power. This is because the power loss at no load occurs regardless of the secondary load’s power output. However, the efficiency is, in fact, zero. It is the result of the output power multiplied by the input power. Of is zero.

The transformer that loads are variable (like distribution transformer) is designed to maximize efficiency at around 75% of the load.

If it’s continuously operating at or near full load (likepower transformers), the design is to achieve maximum efficiency, or at least close to the maximum load.

When the workload on the transformer rises, the efficiency of the transformer increases. When the load is around 75%, it will be the most efficient.

When the output rises above 75 percent, the voltage drop within the windings of the transformer becomes more extensive, and the efficiency decreases. An efficient transformer efficiency can operate between 90 and 95 percent efficiency when fully loaded.


A transformer with a 500 KVA rating has 2500 watts of loss in iron and 7500 watts loss of copper when fully loaded. The power factor taken 0.8. Calculate:

  • Transformer efficiency at full load
  • The transformer’s efficiency is at its highest,
  • output KVA, which corresponds to the highest efficiency,


Transformer Efficiency at Full Load:

Transformer rating = 630 KVA.

The output power of the transformer is 630,000 × 0.8 = 504,000 Watts.

Iron losses (Pi) = 2500 W

Full load loss of copper (P cu) = 7500 W

= [(output power)/ (output power + Pi +Pcu)] x 100

= [(504,000)/504,000+2500+7500)] × 100

Transformer Efficiency at Full Load=98%

The transformer’s efficiency is at its highest:

For maximum efficiency, Copper loss = Iron losses = 2500 W

= [(output power)/ (output power + Pi +Pc)] * 100

Thus, maximum efficiency = [(504,000)/ (504,000 + 2500 +2500)]* 100

Maximum Efficiency of Transformer= 99%

Output KVA, which corresponds to the highest efficiency:

full load KVA x √(Pi/Pc)
= 630 x √ (2500/7500)
= 630 x √0.333 = 363.5 KVA

What is all day efficiency of transformer?

The efficiency discussed so far is the transformer’s ordinary, commercial, or power efficiency. But the distribution transformer does not give an accurate idea about the transformer’s performance because the distribution transformer’s load fluctuates throughout the day.

This transformer is energized for twenty-four hours, but it delivers a very light load for a significant portion of the day.

The efficiency of such transformers (like distributor transformers) cannot be evaluated through power efficiency. However, it can be assessed through a specific type of transformer efficiency referred to in energy efficiency and all-day efficacy. The efficiency of the whole day is calculated by calculating the amount of energy used during 24 hours.

The efficiency for the whole day of a transformer can be defined as the ratio of energy output (in units of kWh) to energy input (in units of) for 24 hours.

To determine the efficiency throughout the day of the transformer, it is necessary to know about load cycle of the power transformer is used.


A 20 KVA transformer operating on domestic power, which could be considered to be of a unity power factor. It has a full-load efficiency of 95.3 percent, with the loss of copper equal twice the iron loss. Calculate the efficiency of its all-day by following the day-to-day cycle:

  • No-load during 10 hours.
  • Half-load for 8 hours.
  • The total load for 6 hours.


Full output of load = 20x 1 = 20 KW

Full load input = output/efficiency = (20/95.3) 100 x 20.986 KW

Total losses = Pi + Pcu = Input – Output = 20.986 – 20 = 0.986 KW

Then You have to calculate P cu = 2P I (given)

So, the equation P 1 plus 2P I = 0.986 KW

Or Iron losses (Pi) = 0.3287 KW

Copper losses at full load (P cu) = 2 times 0.3287 = 0.6574 Kilowatts

kWh of output over 24 hours is = {(1/2) x 20 x 8} + (1 x 20 x 6) = 200 KWH

Iron losses over 24 hours = 0.3287 × 24 hours = 7.89 kW

Copper loss during 24 hours=Cu losses of 8 hours for half load + Cu losses for 6 hours when at full load

= {(1/2)2 x 0.6574 x 8} + (0.6574 x 6)

= 5.259 KWH

Input over the 24 hours (kWh) output over 24 hours + copper and iron losses for 24 hours

= 200 + 7.89 + 5.259 = 213.149 KWH

All-day performance of the transformer = (kWH output over 24 hours ÷kWH input during 24 hours) *100

All-day performance of the transformer = (200/213.149) = 93.83%

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