200 C Demonstration Transformer Operates at 50 kHz

Abstract

A compact, high temperature demonstration transformer was constructed, using a moly permalloy powder core and Teflon-insulated copper wire. At 50 kHz and 200 C, this 1:2 ratio transformer is capable of 98% efficiency when operating at a specific power of 6.1 kW/kg at 4 kW. This roughly 7 cm diameter transformer has a mass of 0.65 kg. Although Teflon is unstable above 200 C, about the same electrical performance was seen at 250 C. A plot of winding loss vs. frequency illustrates the need to control these losses at high frequency. COMPACT HIGH TEMPERATURE TRANSFORMER OPERATES EFFICIENTLY AT 200 C Readily available magnetic and wiring materials suitable for use at elevated temperatures were used to construct a transformer that demonstrates the consequences of pushing these materials beyond their conservative temperature ratings. The critical elements above 200 C are the magnetic material and wire insulation. The present transformer was based on a moly permalloy powder (MPP) core wound with Teflon insulated copper wire. Commercial MPP cores are normally not rated for operation above about 140 C, although they are regularly equipped with 200 C coatings. Their intrinsic magnetic material, being roughly 80-20 % Ni-Fe, is known to have good soft magnetic characteristics up to 300 C. But the properties of the binder material may be expected to have a deleterious influence at such high temperatures. Occasional measurements at NASA Glenn indicate that over short durations (hours, not thousands of hours) certain MPP cores have nearly the same core loss and inductance at 250 C as at 23 C. Long-term stability under high temperature is unknown. Above 200 C, no wire insulation is known that is compact, has a high dielectric strength, and is stable and mechanically durable. Ceramic beads or sleeves may be acceptable in high frequency designs that can use relatively few turns of winding. Such windings have been used to characterize Supermendur up to 900 C in a NASA magnetic materials testing program. Teflon insulation was chosen here, for its good dielectric strength and ease of winding. Indeed, Teflon film is used as the dielectric in some commercial, hermetically sealed capacitors rated up to 200 C. Above 200 C, Teflon materials start to outgas rapidly and soften and hence would not be suitable for long duration aerospace applications at such temperatures. However, in the present, short-duration demonstration tests, Teflon insulation served quite well to 250 C. Using Teflon insulated windings on an MPP core, 98% efficiency was shown to 250 C, for a 1:2 ratio transformer capable of power densities over 6 kW/kg, when operated at 50 kHz. This is all fairly predictable and nothing new was found. Nevertheless, the demonstration reviews an important point regarding the distribution of core and winding losses at high frequencies.