Loss CalculationTransformer losses consist of two parts. No-load loss, also referred to as "core loss," is the power consumed to sustain the magnetic field in the transformer's steel core. This includes eddy current losses due to circulating currents induced in the core steel and hysteresis losses due to the magnetic properties of the steel. Core loss is unrelated to the level of load on the transformer and is present at all times that the transformer is energized.
Load loss is associated with full-load current flow in the transformer windings, due primarily to the resistance of the winding material. Because transformers traditionally used copper windings, load loss is also referred to as "copper loss." Following Ohm's law for power in a resistor, P=I2R, copper loss varies with the square of the load current.
For a given transformer, the manufacturer can supply values for no-load loss, PNO LOAD, and load loss, PLOAD. The total transformer loss, PTOTAL, at any load level can then be calculated from:
PTOTAL = PNO LOAD+ (% Load/100)2 x PLOAD
The Old Standard: Full Load Temperature RiseIn a standard dry-type transformer of the size typically used for step-down service in buildings, the load loss is between four and six times the value of the no-load loss. This has led previous efforts at improving transformer efficiency to concentrate on reducing load loss, and the result has been to reduce winding resistance by using copper instead of aluminum and increasing the winding cross section. The reduced loss results in a lower operating temperature at full load, and these transformers have been marketed on the basis of lower temperature ratings, which provide longer life as well as greater efficiency. The standard rating for a dry-type transformer is 150?C (rise above a 40?C ambient); transformers with reduced losses having temperature ratings of 115?C and 80?C have been available for many years.
The New Standard: Part-Load EfficiencyStudies undertaken by the EPA and electrical equipment manufacturers identified that the average dry-type transformer in a commercial building is between 30% and 40% loaded. At this level, the no-load loss contribution to total loss is equal to or greater than that of the load loss, and transformer optimized to reduce total losses at full load may not be very efficient at all. In fact, this is often the case with 115?C and 80?C designs, which may have higher no-load loss than a standard 150?C design.
This led to a new industry standard for transformer efficiency, published by the National Electrical Manufacturers Association (NEMA) in 1996 as TP-1, "Guide for Determining Energy Efficiency for Distribution Transformers." In this standard, minimum efficiencies are established for low-voltage transformers between 15 kVA and 1,000 kVA for operation at 35% load. Exceptions are allowed for types of transformers for which the efficiency requirement is not economically attainable or would conflict with other critical design criteria. Exceptions include autotransformers, drive isolation transformers, rectifier transformers, and general-purpose transformers designed for non-linear loads (K-rated transformers).
Transformers meeting or exceeding the NEMA TP-1 Class 1 efficiency levels may be marketed as energy efficient and are eligible for the "Energy Star" label. This label identifies products determined by the Department of Energy (DOE) and the Environmental Protection Agency (EPA) to meet state-of-the-art energy conservation targets.