When you think of energy efficiency, what do you think of? Lights, motors, variable frequency drives? Most likely you have never thought about efficient transformers, however, transformer efficiency can be very important to consider. Unlike lights, motors, and most other equipment which only consume energy when they are being used, a transformer will use energy every hour of the year, regardless if the equipment attached is on or off.
It is important to realize that the efficiency listed on the transformer nameplate is the operating efficiency only at the rated condition. There are two primary factors that determine what the actual operating efficiency of a transformer will be.
- The transformer itself — Transformer losses are characterized as core losses and coil losses. The core losses are the losses that occur all the time, even if nothing is connected to the transformer. The coil losses are resistance losses and will increase as the load on the transformer is increased. Therefore, the efficiency of the transformer will depend on the specific loss characteristics of the transformer at the load that it will see.
- The type of equipment connected to the transformer — Many transformers are designed and rated for a linear load condition. A motor would be a linear load, while computers, VFDs, and other electronic equipment would be non-linear loads. Non linear loads can as much as triple the losses and the cost to operate a transformer, unless it is specifically designed to meet this this type of load.
This means that not all transformers with the same efficiency will cost the same to operate. The chart below shows the operating cost of three transformers. All three transformers are 98% efficient; however, not all three transformers will cost the same to operate.
In the chart, the 75 kVA transformer with the 150°F temperature rise (red line) is more efficient at low load conditions; while the 75 kVA transformer with the 80°F temperature rise(blue line) is more efficient at higher load conditions. Both 75 kVA transformers are more efficient at all load conditions than the oversized 150 kVA transformer (green line). In the chart above, at a 40 kVA load, the 150 kVA transformer (green line) will cost twice as much to operate per year as a 75 kVA transformer (red line), even though both transformers are at the same nominal efficiency. Plus, it would cost more to purchase in the first place.
If you are planning on purchasing a transformer, it is important that you match the transformer to the loads that it will meet. Otherwise you may spend too much up front and too much every hour after that.