PROTECTING YOUR SCR CONTROLS FROM FAILURE

Heat is the main enemy associated with SCR Control Failure. In fact, heat is the main enemy of all solid state devices. Almost all semiconductors, including transistors, IC/s and thyristors (SCRs and TRIACs) will be destroyed at an internal junction temperature of 125ºC. Diodes can take a higher internal junction temperature (130-200ºC). This is one reason Avatar Instruments uses SCR-Diode combinations whenever possible.

Heat is generated by voltage drops across a power device (SCR/TRIAC). This drop can be anywhere between 1 and 2 volts depending on the device. The more current (amps) put through the device, the higher the watts the device will dissipate in the form of heat. This heat must be removed or the device will fail. 

All Avatar SCR Controls are designed to run at full rated amperage at 50ºC / 122ºF ambient.

DETERMINING THE TYPE OF ENCLOSURE FOR YOU SCR POWER CONTROL

First, determine the heat generated by the SCR Controller. The basic formula is as follows: For each controlled leg (C), multiply the amperage load (I) by 1.2.

C x I x 1.2 = watts generated @ maximum load

• A single phase 60 Amp device would generate 72 watts of heat. 60 x 1.2 = 72.
• A 3 phase, 2 leg 60 Amp (A3Z) control would generate 144 watts. 2 x 60 x 1.2 = 144
• A 3 phase, 3 leg 60 Amp phase angle control would generate 216 watts. 3 x 60 x 1.2 = 216

The following data applies to a painted steel enclosure. To calculate what size enclosures you need, you must know what the temperature of your work environment will be. When the SCR is turned on, the inside of the enclosure will be warmer than the ambient air outside the enclosure. This temperature rise above ambient temperature is represented by (TR) in our equation. With a maximum working temperature of 122º F, for the SCR, care must be taken. For our example we will use the 216 watts (W) for the 3 leg 60 Amp SCR. And let’s use a 10ºF temperature rise (TR) over ambient as your target. The final sum is the exposed surface area in square feet (SF) you need to maintain the 10ºF rise.

Formula: W x 4 / TR = SF
Example: (216 x 4) /10 = 86.4

86.4 are the exposed square feet of your enclosure needed to run the SCR Control of an ambient temperature of 112º F. (TR) value in example of 10ºF is the allowable rise. 122ºF (50ºC) – 10ºF = 112 ºF. If you decide to use a vented enclosure, area 50 (recommended), the basic formula is 10 square inches of inlet and outlet for every area 50 amperes per controlled leg. Since heat rises, vent holes should be at the top and bottom of the enclosure.

FAN COOLING 

After looking at the size of the enclosure needed for a large SCR, you may want to consider a fan(s) to keep the enclosure at safe operating temperatures. To determine the size of the fans needed, use this simple formula:

Formula: (W x 3.16) / TR = CFM

Example: To figure out what size fan you need if using a device generating 216 watts (W) where you want to keep the temperature rise to only 10ºF above ambient (TR), use the following formula:

Example: (216 x 3.16) / 10 = 68.25 CFM Fan

Remember, you will need air inlet and outlet holes with fan cooling.

Finally, plastic enclosures act as an insulator. You will most likely destroy an SCR control if you install it inside a plastic box. The only reliable way to use a plastic enclosure is to have a “thru-hole mounting” style heat sink, with the heat sink’s cooling fans outside of the box.

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