Instrumentation & Measurement Magazine 26-3 - 50

temperature change rate in K/min on the temperature controller,
the next measuring point can be reached more quickly
by short heating pulses controlled by the temperature controller.
By automating the measurement process, this long
measurement time becomes secondary, and the low nitrogen
consumption and thus the low operating costs have a beneficial
effect.
Modification Options of the Cooling
Chamber
Fig. 6. Measured cooling temperature Tca
for characterization of test
specimens with non-negligible power dissipation (adjustable soak time 20 min,
maximum cooling gradient 4 K/min).
The disadvantage of this cooling option is that, due to the design,
the nitrogen supply via the capillary tube connecting the
cryo-tank to the cooling chamber is constant during the entire
measuring period. In our case the nitrogen consumption
is about 4 liters/hour. However, by using a cryogenic needle
valve with actuator control instead of the capillary tube, the
nitrogen consumption can be reduced. This will be discussed
in the following section " Modification Options of the Cooling
Chamber. "
If components without significant power dissipation are
to be characterized, an operating mode can be used in which
nitrogen consumption is minimized. In this operating mode,
the cooling chamber is cooled down to the minimum cooling
temperature within one hour, as shown in Fig. 7. Without further
nitrogen supply, the cooling chamber will gradually heat
up due to the non-ideal thermal insulation. The warm-up process
takes place with a very long thermal time constant. This
means that measurements that require a time well below this
warm-up time constant can be performed directly during
the warm-up process. If necessary, the long resulting measurement
time of more than 8 hours can be reduced by short
heating pulses. Thus, by setting a target temperature and a
In addition to this initial setup of a cost-effective cryogenic
cooling chamber discussed in this paper, there are several
modification options. For future use in an application such
as a cryo-electric propulsion system of a hydrogen-powered
aircraft, the lifetime and reliability of the cryogenic power
electronic system is a critical issue. To carry out the required
long-term measurements, a constant tank pressure is necessary
to prevent the liquid nitrogen supply being interrupted.
For this operation, it is advisable to employ a cryo-tank with
a pressure build-up controller to maintain the internal tank
pressure and therefore the liquid nitrogen flow rate at a constant
level.
To optimize liquid nitrogen consumption, a cryogenic control
valve with actuator can be used instead of the capillary
tube, which controls a needle valve and thus enables precise
adjustment of the nitrogen flow rate. However, this involves
additional acquisition and maintenance costs and may be
more susceptible to errors. Another possibility for reducing the
liquid nitrogen flow rate is to use a thinner capillary. However,
care must be taken that the internal diameter is not too small to
ensure that the liquid nitrogen does not evaporate in the capillary
but instead takes place in the heat exchanger, to transfer
its entire cooling capacity to the circulating gaseous nitrogen.
Conclusion
This paper presents a cost-effective design of a cryogenic cooling
chamber based on the principle of a wind tunnel, suitable
for testing power electronic components, materials and power
converters at very low temperatures. Cooling is achieved by
forced convection of cryogenic gaseous nitrogen, and ice formation
on the test specimen is avoided by rewarming the
test specimen to room temperature before opening the cooling
chamber. Two operating modes of the cooling chamber
are presented, which allow the cooling temperature to be kept
constant over a longer period or to minimize nitrogen consumption.
The minimum achievable cooling temperature is
about -190 °C for power losses of the test specimen up to 10 W
and about -175 °C at 100 W.
References
[1] H. Gui, R. Chen, J. Niu, Z. Zhang, L. M. Tolbert, F. Wang, B. J.
Blalock, D. Costinett and B. B. Choi, " Review of power electronics
components at cryogenic temperatures, " IEEE Trans. Power
Electron., vol. 35, no. 5, pp. 5144-5156, 2020.
Fig. 7. Measured cooling temperature Tca
specimens with negligible power dissipation.
50
for characterization of test
[2] S. Büttner and M. März, " Profitability of low-temperature power
electronics and potential applications, " Cryogenics, vol. 121, 2022.
IEEE Instrumentation & Measurement Magazine
May 2023
Instrumentation & Measurement Magazine 26-3

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