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Power semiconductors such as thyristors (SCRs), solid state relays and diodes all dissipate waste heat when in conduction. Manufacturers specify a maximum allowable operating temperature for their apparatus. It is up to the user to ensure the electical enclosure is adequately cooled so that this temperature is not exceeded. In most cases, this achieved by fan cooling.

This information is for general purposes only, please contact us on 03 9720 4522 or sales@practicalcontrol.com.au to discuss your specific requirements.

Waste heat dissipation

Thyristors (SCRs) and diodes dissipate waste heat when conducting due to on-state slope resistance and threshold voltage. Waste heat dissipation is described by the equation

P_{T }= V_{T(TO)}.i_{T} + r_{T}.i^{2}_{T}

(where V_{T(TO}_{)}
is the threshold voltage and r_{T}
is the on state slope resistance).

The linear portion of this equation (V_{T(TO)}.i_{T)} tends to dominate. A conservative estimate of heat dissipation is 1W per amp per phase.

Typically, thyristor manufacturers publish curves or tables giving heat dissipation for an assembly. The engineering challenge is to limit the internal temperature rise within the enclosure so that it does not exceed the maximum temperature rating of the thyristor. This article concentrates on fan cooling or forced convection, by far the most common means of cooling power electronics. Cooling can also be achieved by natural convection, water cooling, air conditioning, or peltier modules.

Calculation of temperature rise within the enclosure

From the first law of thermodynamics, power dissipated by the electronics = power removed by the fan cooling. Hence:

P_{ }= V̇ρC_{p}ΔT

Where:

P = power dissipated by the semiconductor devices in kW

V̇ = volumetric flow rate of air through the enclosure in m^{3}/sec

ρ = density of air in kg/m^{3} (taken to be 1.13 kg/m^{3})

C_{p} = specific heat of air (taken to be 1.006 kJ/kgK)

ΔT = T_{enclosure} - T_{external ambient}

Rearranged, this gives us ΔT = P/V̇ρC_{p}_{ .}

Let's use this in a practical example.

Assume we have a CD Automation CD3000RE 350 amp-rated thyristor running at 300 A in a fan cooled enclosure with an air flow of 710m^{3/}h or 0.19722m^{3}/s.

From the manufacturer's data, we can see the power dissipation at 350 A is 1260 W.

Because we know the heat dissipation is very nearly linear we can safely say power dissipation at 300A = 1260W × 300/350 = 1080 W or 1.08 kW.

Plugging these figures back into our equation, we get ΔT = 1.080kW/1.13kg/m3 x 1.006kJ/kgK x 0.19722m3/s = 4.82°C.

This means that for an external ambient temperature of 35°C, our internal enclosure temperature will be 35°C + 4.82°C = 39.82°C. Under these conditions we are running well within the safe temperature range of the thyristor, which can run at up to 40°C at its full rated current.

Thyristor derating curve

Let's now rephrase the question and ask ourselves, what is the maximum external ambient temperature the thyristor can run at for a maximum current of 300 amps.

This is where the derating curve issued by the manufacturer comes in handy. In the case of our example, K = 300A/350A = 0.86 which gives us a maximum operating temperature or maximum internal enclosure temperature of about 47°C.

We know that our temperature rise ΔT is 4.82°C, so we can run our thyristor at 300A at an external ambient temperature right up to47°C - 4.82°C = 42.18°C .

Fan selection

Care needs to be taken with fan selection. Manufacturers' selection tables typically give fan flow in free air, i.e with no back pressure.

To accurately determine fan flow rate, a fan curve is required where fan air flow is plotted against back pressure and fan filter back pressure is plotted against flow. Fan airflow diminishes with back pressure and back pressure increases with flow.

The point where these two lines intersect will give the actual air flow expected in the real world. Below is the fan and filter curve for the Rittal SK3245 fan we used in our example calculation.

The free air flow rate is just under 900m^{3}/h, but this reduces to 710m^{3}/h due to the restriction of the filter.

Fan cooling at high altitudes

Air density diminishes with altitude. We can see from our equation that cooling capacity decreases (i.e ΔT increases) as air density ρ decreases. For just about any location within Australia altitude will not have a significant effect on air density, but this is not always the case elsewhere in the world. For example, there are mine sites in Chile located at 3000m above sea level where air density reduces to 0.825kg/m^{3}.

If we take our previous example ΔT = 1.080kW/0.825kg/m^{3} x 1.006kJ/kgK x 0.19722m^{3}/s = 6.6°C, almost a 2°C increase over our previous example. This reduction in cooling efficiency needs to be taken into account when designing a cooling system for higher altitudes.

Fan arrangement

Normally fans are placed low at the bottom of the enclosure with the exhaust filter higher up, with the fan blowing cool air into the enclosure rather than sucking out hot air. Having the air flowing from bottom to top adds to the airflow caused by natural convection.

Blowing cool air in allows the fan motor to run cooler and also subjects the inside of the enclosure to positive pressure, helping to minimise dust ingress.