The classical electrical enclosure has normally been a closed, dust-tight box whose design was primarily dictated by the sensitivity of electromechanical controls to dust, dirt and corrosion. Heat and temperature buildup were rarely serious considerations with such controls. However, the advent of solid state controls for both logic and power handling duty mandates a different design approach to electrical enclosures:
- Dust, dirt and corrosion are not serious concerns for a properly designed solid state power control.
- Temperature extremes, however, can be catastrophic for any semiconductor device as well as its associated electronic components.
The 50 °C ambient temperature, a generally accepted international standard, is a very demanding requirement when one adds a 50 °C temperature rise to the semiconductor case and is left with only a 25 °C working range to the 125 °C “zero-current” rating of normal SCRs. For a silicon diode this is no problem, since most diode designs are flat-rated to at least 130 °C and their “zero-current” rating does not occur until 200 °C. Obviously, the more diodes one can substitute for SCRs, the more reliable the control will be. In addition, many electronic components have only a 1000 hour life at a typical specification temperature of 65 °C, and many optoelectronic isolators malfunction at only 60 °C.
Therefore the end user must carefully consider the electrical enclosure for proper internal cooling, usually by convection to the outside ambient. Low power solid state controls of less than 80 amps per phase, or three phase motor starters up to 50 HP, can be successfully installed in conventional sealed, dust-free enclosures with derating and/or with an isolated heatsink enclosure design. However, a typical sealed enclosure of substantial size (30 × 20 × 10 in, or 76 × 51 × 25 cm) was tested for its internal ambient temperature at various power control outputs: even a relatively benign 60 amp three phase external load caused the inside ambient to exceed 50 °C with only 22 °C outside the enclosure.
The same enclosure with a ventilation screen* on the bottom and on each top side would have only a small increase of its inside ambient temperature, simply because outside air is allowed to flow freely, using the buoyancy forces of natural convection, up through the bottom enclosure screen, past a vertically mounted solid state heatsink and out the top vents.
Finally, one should not be complacent about simply using a larger sealed enclosure and/or employing internal fans to cool the solid state control. Fans add even more air friction source energy to the enclosure ambient, and may only delay the inevitable overtemperature condition when the silicon forward drop wattage is not properly dissipated to the surrounding ambient.
* Typically, 10 in2 (65 cm2) of inlet area and an equal outlet area for every 50 amp phase is sufficient in most industrial plant applications.
References
- McAdams, W. H., Heat Transmission, McGraw-Hill, 1954.
- Schmidt, E., Zeitung des Kalte-Ind No. 35, p. 213, 1928.
- Kennard, R. B., Nat. Bur. Stds. J. Research No. 8, p. 787, 1932.
- Phillips, N. V., Semiconductor Data Book, 1972.
- EG&G/Wakefield, Cat. 100/30M, 1981.