An improved thermometric method of measuring local power dissipation
David A. Ball, H.O. Lorch · Journal of Scientific Instruments · 1965
The power dissipated at any part of an electrical machine or component may, under some conditions, be determined by switching it on and measuring the rate of rise of temperature before appreciable heat diffuses to or from other parts at different temperatures. This note deals with the problem of measuring the loss in a small region of a sheet of magnetic steel. A thermocouple, 0.001 in. thick, is used in conjuction with a d.c. amplifier to give a useful sensitivity of full scale deflection on the output meter for 0.0025 degC; this enables the loss in a region of oriented silicon-steel magnetized at 50 c/s to 15 kG to be measured after switching on for as little as 0.1 sec. Due to time delays in the thermocouple and amplifier, some 0.5 sec elapses before sensibly full output is reached, and this means that the area of the strip significantly affecting the reading is not less than about 0.2 in. radius. Loss-induction curves of transformer steel test strips are conveniently measured by switching on the magnetization for 1 or 2 sec and noting the change in deflection of a meter. The apparatus could measure eddy current losses in copper and aluminium magnetic screens and in tank walls, where the loss density is usually small.