4.3 Transformer Losses, Efficiency, Cooling & Connections

4.3 Transformer Losses, Efficiency, Cooling & Connections

Core Losses vs. Copper Losses

Every real transformer wastes some input energy as heat rather than delivering it all to the load. These losses fall into two distinct categories that behave very differently with load, and a master electrician must be able to tell them apart to correctly size, load, and troubleshoot a transformer.

Core losses (also called iron losses) occur in the magnetic core itself and consist of two components:

• Hysteresis loss — energy consumed repeatedly magnetizing and demagnetizing the core material as the AC flux reverses direction every half-cycle. It depends on the area of the core material's B-H hysteresis loop and on frequency; using high-grade silicon steel with a narrow hysteresis loop minimizes this loss.
• Eddy-current loss — circulating currents induced directly in the conductive core material by the changing flux (the same Faraday's-law mechanism that induces voltage in the windings). These circulating currents dissipate energy as I²R heat inside the core itself. Building the core from thin laminations, each insulated with a thin varnish or oxide coating, breaks up the conductive path and sharply reduces eddy-current loss compared to a solid iron core.

Core losses depend only on the applied voltage and frequency — both essentially fixed at rated conditions — so core losses are present continuously whenever the transformer is energized, whether or not it is supplying any load. For this reason, core losses are called constant losses (or no-load losses), and they can be measured directly with an open-circuit test.

Copper losses (also called I²R losses) are the resistive heating losses in the primary and secondary winding conductors. Because load current flows through both windings, copper losses rise with the square of the load current — double the load current and copper losses quadruple. Copper losses are therefore called variable losses, and they are measured with a short-circuit test.

This distinction matters directly for loading decisions: a lightly loaded transformer wastes proportionally more energy on constant core losses relative to its useful output, while a heavily overloaded transformer suffers rapidly escalating copper losses (and winding heating) that can shorten insulation life well before the nameplate current is exceeded.

Efficiency

Efficiency is the ratio of useful output power delivered to the load, divided by the total input power drawn from the source, expressed as a percentage:

Efficiency (%) = (Output Power / Input Power) × 100 = [Output / (Output + Core Loss + Copper Loss)] × 100

Worked Example

A distribution transformer delivers 90 kW to its connected load at full rated capacity. At that load, the core loss is 1 kW and the copper loss is 2 kW.

Input power = Output + losses = 90 + 1 + 2 = 93 kW

Efficiency = (90 / 93) × 100 = 96.77%

Because core loss stays essentially fixed while copper loss grows with the square of load, a transformer's efficiency actually peaks at a specific load point — maximum efficiency occurs at the load where copper loss equals core loss — not necessarily at full rated load. This is a useful design fact: utilities try to size distribution transformers so their typical daily loading sits near this maximum-efficiency point rather than at either extreme.

Cooling Methods

The rate at which a transformer can safely dissipate its core and copper losses as heat directly determines its continuous current-carrying capacity — the nameplate kVA rating. Two broad cooling families are in common use:

• Dry-type (air-cooled) transformers rely on natural or forced air circulation over the windings and core, with no liquid coolant. Self-cooled dry-type units are commonly designated AA (air natural, air natural); some add fans for forced-air cooling (AFA) to boost their rating. Dry-type units are favored indoors — in commercial and institutional buildings, electrical rooms, and near occupied spaces — because they eliminate the fire and spill risk of liquid coolant, at the cost of a somewhat lower current-carrying capacity for a given physical size compared to oil-immersed units.
• Oil-immersed (liquid-filled) transformers submerge the core and windings in mineral oil (or a less-flammable synthetic/natural ester fluid), which serves double duty as both an electrical insulator and a coolant. Because oil circulates heat away far more effectively than air, oil-immersed units can carry substantially more current for a given size than a comparable dry-type unit — common designations include ONAN (oil natural, air natural), ONAF (oil natural, air forced, using fans on the radiators), and OFAF (oil forced, air forced, using pumps and fans). Oil-immersed transformers dominate outdoor utility distribution and substation power transformers, but require fire walls, containment berms, or oil-containment pits, plus periodic oil testing, because of the fire and environmental risk of a large liquid-oil inventory.

Because cooling capability sets the current a transformer can carry without overheating its insulation, undersized or obstructed cooling (blocked vents, low oil level, failed cooling fans) forces a real-world derating of the nameplate kVA — a practical field consideration for a master electrician sizing or troubleshooting a transformer installation.

Three-Phase Connections

Three single-phase transformers (or the three windings of one three-phase unit) can be connected on each side in either a wye (star) or delta configuration, giving four common combinations:

For a master electrician, the delta-wye connection is the one most frequently encountered in the field: it is the standard configuration for pole-mounted and pad-mounted distribution transformers that step utility primary voltage down to a grounded, neutral-bearing secondary serving ordinary building loads.