Keeping SCR controls cool

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:

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 functions without any special precautions (usually, but not always: if the fan in a personal computer fails, the user must fix it quickly to keep the computer working 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 125 °C. Rate an SCR control for a 50 °C case temperature rise and a 50 °C ambient temperature, and there is little room for error: if either goes over the 50 °C 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: its ability to dissipate waste heat to limit the case temperature rise;
• the user’s installation: installed so that the ambient doesn’t exceed 50 °C.

Choosing an SCR control

When selecting an SCR control, keep in mind certain key points about the design and construction of power electronic devices.

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. The design must minimize surface irregularities which create turbulent airflow across the heatsink, since such disturbances greatly reduce its cooling ability 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 airflow.
3. Fans are not necessary to achieve a 50 °C 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 semiconductor blocks and SSRs.

1. At current levels above 25 amps per phase, dissipating waste heat from encapsulated blocks is very difficult, because the silicon pellets are only a few millimeters 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 outlets should be openings at the top of each side. Louvers and fine-mesh screens over the openings reduce the volume of airflow 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 it is 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 heat and generate much more of it, are seldom given such special consideration.

1. Well-ventilated enclosures are meaningless if they are mounted next to or above a furnace that keeps the air around them at 50 °C 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 other heat generating equipment is placed inside without ensuring 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. By carefully selecting a control in light of the design considerations above, and taking care that it is properly installed, solid state control users can eliminate overtemperature failure as a concern and help ensure the maximum service and reliability from their investment.