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***** KEEPING SCR CONTROLS COOL *****
Technical Information Publication 203
Solid state devices, from pocket calculators to personal computers to SCR power controls to solid state motor starters,
generate heat during operation because there is a voltage drop across conducting silicon semiconductors. That voltage
drop shows up as waste heat, which can be calculated by using Ohm's Law:
Volts x Amps = Watts
Assuming a 2-volt drop (typical for power SCRs), two watts of heat are generated for every amp of current, waste
heat that must, without exception, be dissipated to prevent the semiconductors generating it from overheating,
degrading, and ultimately failing.
With low-power products, the manufacturer is usually able to design in a heat-dissipation mechanism that will function
without any special precautions (usually, but not always: if the fan an a personal computer fails the user must
fix it quickly if he wants his computer to function much longer). But when large amounts of power are involved
- like a single-phase 150 amp power control or a 100-hp motor starter - keeping semiconductors cool is more difficult.
Power SCRs have a zero-current temperature rating of 125C. Rate an SCR control for a 50C case temperature rise
and a 50C ambient temperature, and there is little room for error: if either goes over the 50C limit, the lifetime
of the SCRs in question becomes very short indeed.
The life expectancy of an SCR control, then, is affected by:
- the manufacturer's design (ability to dissipate waste heat to limit the case
temperature rise)
- the user's installation (installed so that the ambient doesn't exceed 50C)
Choosing an SCR Control
When selecting an SCR control certain key points about the design and construction of power electronic devices
should be kept in mind:
A. HEATSINKS: heat dissipation via airflow over aluminum heat exchangers called
"heatsinks" is the most common, most economical and - properly designed - the most reliable way to cool
SCR controls.
1) Convection heat transfer - air moving across the fin surfaces - accounts
for 85% of total heat transfer from a heatsink. Design must minimize surface irregularities which create turbulent
air-flow across the heatsinks, since such disturbances greatly reduce the cooling ability of a heatsink and cannot
be offset by additional fins and /or mass.
2) Fins must be spaced so that dust and dirt cannot accumulate and cut down air-flow.
3) Fans are not necessary to achieve a 50C ambient temperature rating below 600 amps/phase when the heatsink is
well designed. As mechanical devices with a nominal 2000 hour MTBF, fans are a weak link in any cooling system
and should be avoided.
B. SEMICONDUCTORS: Individual semiconductors mounted separately on heatsinks are easier to cool than encapsulated
semiconductors blocks and SSRs.
1) At current levels above 25 amps per phase, dissipation of waste heat is
very difficult from encapsulated blocks because the silicon pellets are only a few mm apart.
2) Power SCRs should be replaced by diodes whenever possible in three-phase controls. Diodes generate less waste
heat AND have a higher temperature withstand rating than equivalently rated SCRs.
Using SCR Controls
Heat is the number one threat to industrial solid state equipment. Even the most careful design can be nullified
by failure to observe basic installation guidelines.
ENCLOSURES: SCR controls, unlike their electromechanical
counterparts, are not susceptible to (non-conductive) dust or dirt, so the traditional dust-tight electrical enclosure
is not necessary; in fact, it is to be avoided at all cost since it will trap waste heat and lead to overheating
failure.
1) Enclosures for SCR controls must be ventilated with 10 in2 of inlet and
outlet area for each 50 amp power phase. The inlet vent should be in the bottom of the enclosure; the outlet should
be openings at the top of each side. Louvers and fine-mesh screens over the openings reduce the volume of air flow
and should be avoided.
2) a common misconception is that an oversized enclosure is an acceptable substitute for ventilation. Field studies
clearly show that regardless of how large an enclosure may be, if not ventilated the temperature inside will eventually
reach levels dangerous to most solid state components.
LOCATION: often overlooked is where an SCR control is installed.
Computers are infamous for their inability to tolerate even the slightest overtemperature conditions, so they are
placed in special air-conditioned rooms; but SCR controls, which are just as vulnerable to, and generate much more,
heat, are seldom given such special consideration.
1) Well-ventilated enclosures are meaningless if the enclosures are mounted
next to or above a furnace that keeps the air around it at 50C or more. Similarly, putting a solid state control
in an enclosure on a roof in direct sunlight, or in a metal building in the middle of a field, will counteract
cooling.
2) A well-ventilated enclosure won't help if one solid state control is mounted directly over another. Heat rising
from the lower one will make it very difficult to keep the upper one cool.
3) A ventilated enclosure in a mild ambient area may not be enough if the other heat generating equipment is placed
inside without ensuring that there is enough ventilation to get rid of the extra waste heat.
Solid state power electronic technology has been around since the late 1950s, and in that time
has proven many times over its potential for reliable, economical operation with minimum maintenance requirements.
By carefully selecting a control in light of the design considerations discussed above, and taking care to ensure
that it is properly installed, solid state control users can eliminate overtemperature failure as a concern, and
help to ensure the maximum service and reliability from their investment.
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