Transformer efficiency: Minimizing transformer losses

Matching a transformer to its anticipated load is a critical aspect of reducing energy consumption.


Figure 1: How engineers approach electrical designs can significantly affect transformer losses. Courtesy: CFE MediaIn 2002, NEMA issued a Standard TP-1 in support of the U.S. Dept. of Energy’s guidelines for more energy efficient electrical devices. This standard was based on a previous U.S. Environmental Protection Agency study showing that the typical dry type transformer under normal operating conditions was loaded to approximately 35% of its nameplate rating. Therefore, TP-1 established a table of minimum efficiencies for various sized transformers when operating linear loads (see Table 1). These efficiencies are really quite incredible as they range from 97% to 98.8%. What TP-1 does not tell you is that it is very unlikely that you will ever see such efficiencies in actual installations. In addition, TP-1 does not tell you that using these very efficient transformers will impact your electrical designs significantly. 

Because of the differences among the efficiencies shown in TP-1 and what really happens with real transformers in real applications, the approach you take in your electrical design could be significantly different when attempting to design an electrical system with minimized losses. This article offers suggestions regarding how you approach your electrical designs to maintain minimum losses in the system transformers (see Figure 1). It will also show areas in which you will have greater losses than those shown in TP-1—no matter which design direction you might choose.


TP-1 was developed using linear loads. However, in today’s business environment, most of the loads are nonlinear (rich in harmonic content). Computers, fluorescent light fixtures, printers, elevators, or variable frequency drives for motors generate harmonics. Applying harmonically rich loads to transformers can double or triple their total losses. For example, a 75 kVA transformer that would normally have 2% losses at 35% loading would actually have 4% to 6% losses. Therefore, the 26 kVA load (35% of the 75 kVA) would have losses totaling more than 1.5 kW.

Table 1: Dry-type, low-voltage transformer efficiency chart. Courtesy: Lovorn Engineering Assocs.Core and coil losses

Transformer losses are a combination of core losses and coil losses. The core losses consist of those generated by energizing the laminated steel core. These losses are virtually constant from no-load to full-load, and for the typical 150 C rise transformer are about 0.5% of the transformer’s full-load rating. The coil losses are also called load losses because they are proportional to the load on the transformer. These coil losses make up the difference between the 0.5% losses for the core and range from 1.5% to 2% of the total load.

Typically, the total losses for a 75 kVA transformer are about 1,000 W at 35% loading or 1.3%. The actual losses when the transformer is fully loaded can be more than 3,000 W for linear loads and 7,000 W for nonlinear loads. This amounts to 4% and 9.3% respectively—considerably more than the NEMA TP-1 table for minimum efficiencies for a 75 kVA transformer. While the overall concept for requiring more energy-efficient transformers is quite good, engineers may want to be very careful about transformer selection when the anticipated operating conditions do not match the base criteria that were used in developing the TP-1 table. 

By selecting transformers with lower temperature ratings, that is, 115 and 80 C rise instead of the standard 150 C rise transformers, the core and load losses will change. To reduce the temperature rise, the core is increased in size. This increases the core losses but reduces the load losses, so, according to the anticipated operating point, the total losses may be higher or lower than the standard transformer. Due to the smaller core losses, the total losses for the 150 C transformer are less than the total losses of the 80 C transformer up to about 60% loading. With transformer loading above 60%, the total losses are less than those of a 150 C transformer of the same size (see Figure 2). 

A good compromise between core and load losses is the 115 C rise transformer. While the core losses are somewhat higher than those in the 150 C transformer, they are less than the 80 C transformer core losses. Correspondingly, the load losses are less than the 150 C transformer, allowing the total losses to be less than those of the 150 C transformer under normal operating conditions (see “Know the loss data, loading when specifying transformers”). 

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Anonymous , 06/27/13 11:00 AM:

My opinion on footnote at the bottom the June 2013, Article "Minimizing Transformer losses" gives me the opinion that the author did not try very hard to get transformer loss data. I was able to find data from several vendors on the internet without to much effort. For example,

One item that should have mentioned was the definition of efficiency.
Paraphrasing the efficiency definition in NEMA TP-1,

Efficiency = Power OUT/Power IN

%EFF = 100 x kVA x Output/(kVA x Output + kW losses at Output loading)

A reader could have been mislead in believing percent base is the transformer full load rating. In the article 1000W at 35% loading on a 75kVA transformer would be 1000*100/(0.35*75000)=3.8% losses and not 1.3% as stated.

As mentioned the transformer loss are the sum of the no-load losses and load losses. The load losses are proportional to the square of the load on the transformer and not protortional to the load. Since most of the load losses are I^2*R, transformer with low impedance and resistance tend to have lower load losses. Since the curve 'total losses(kW)' verses 'Transformer loading*%)' did not state the basis for the curve it could be missleading. For the curve to be useful the percent coil resistance would have to be the same. Generally the lower the %impedance to lower the full load losses. On small transformers the user does not have much control over the impedance since the small transformers tend to be off the shelf types. Sometimes the same size transformer from different vendors can significantly different impedance. Lower impedance also means higher fault currents which is another item that has to be considered.
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