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COST vs. EXCITING POWER OF 50/60HZ TRANSFORMERS

Product News Wednesday, September 16, 2009: Agile Magnetics

Regional Manufacturing Specialists Inc. (www.regionalmfg.com) understands that rising energy costs coupled with new technologies are driving demand for increased efficiencies from transformers. Achieving these efficiencies may increase your up-front investment, but they provide a better ROI in the long run. An often overlooked consequence of using cheaper materials is the loss of energy through exciting power. In this analysis we will discuss how to greatly reduce the exciting power of a transformer as well as show the justification for the increased material costs of doing so.

Exciting power is the power consumed by the transformer to create its magnetic field, whether it is delivering power or just consuming it in a no-load condition. Less expensive materials are attractive when striving to meet a build price, but they tend to absorb more energy in a no-load state. This, therefore, is wasted power which ultimately translates into unnecessary costs.

Using a 1KVA autotransformer as an example, we can design a 120V to 60V step-down transformer using 1 ¾” laminations with a 3” stack using M50 material. To operate at a flux density of 13KG, this will use 10.2lb of laminations and 3.3lb of copper magnet wire. The result is a transformer that meets the output voltage and current requirements at a lower price point. This design will support an excitation power of approximately 20W. In a highly competitive bid situation, where long term costs are not being considered, this would be the materials configuration of choice.

When total life-time operation costs are weighted more heavily than up-front costs, the excitation power factors become critical. No-load conditional losses (an expensive, non-value added, fixed overhead cost) can be greatly reduced by using a higher grade lamination material and by reducing the flux density. Flux density is reduced by increasing the number of turns of the windings, and/or increasing the cross- sectional area of the laminated core stack.

Now let’s look at building our 1KVA example with higher grade materials that carry higher up-front costs. To lower the no-load losses, we will use a 2 1/8 inch lamination with a 3 inch stack using M06 material. By increasing the lamination size from 1 ¾” to 2 1/8 inches we have increased the amount of core material used from 10.2lb to 11.40lb. M06 material is roughly twice the cost of M50 material. We have also increased the amount of copper magnet wire that is required because of the larger core and the increased number of turns. The result is using 6.1lb of copper compared to 3.3lb in the original design. The net up-front cost increase is doubled by implementing the more energy efficient design; however, operating at a flux density of 5.2KG, the excitation power has now been reduced to less than 1W from 20W!

Based on material costs at the time this article was written, there would be a $150 price tag for the M50 design, and $300 for the M06 design. At $0.11/KW-hour the break- even point is about two months. Considering the useful life of a transformer is often between ten and twenty years, the savings will be substantial. For instance, assuming operating usage is consistent over a ten year life cycle, using the higher grade materials would produce $9000.00 in savings or a 6000% ROI.