I have two regulated switching power supplies inside my control station which is a 1/4" plywood box 20” wide x 18” deep by 12” high and it has 1/4" holes drilled in all sides for ventilation.

One of the power supplies is 12VDC, 29A and the other 36VDC, 10A. Input for both is 110VAC. According to the manufacturer each power supplies has a thermostatically control fan inside to regulate its temperature.

I noticed today after I turn on the power that the 36VDC power supply gets its fan going after about 10 minutes and the fan cycles on/off every about 5 minutes. However after about 30 minutes the fan stayed on until I finished my work (about 6 hours) and shut off the power. During this time I was only jogging the axes on my CNC to check / install the limit switches and the ambient temperature was about 18C.

I found that strange because on my previous CNC I had a similar power supply 12-32VDC, 14A and the fan inside the unit never started even in the summer and when I was using the CNC a lot.

Is this normal or it’s a sign that the unit will overheat and shut off or burn?

What's the temperature in the box?
 I can't imagine a plywood box with any number of holes drilled in it would be allowing the supply to cool properly. Perhaps you need a fan installed on the box to ventilate it.
Heat is the enemy of electronics. Best keep it happy.

Sage

I don’t have a thermometer inside the box but I will assume it is the ambient 18C. Had a similar box but smaller size on my other CNC and never had a problem. The holes on the side (a total of 82) allow for plenty of air to circulate.

In addition, there is a small fan over the 4 motor drivers blowing air over the heat sink on the drivers.

Also if it was too hot inside the box why the 12VDC power supply unit fan does not go on?

Still I would have like to know if it’s normal for my 36VDC power supply to have its internal cooling fan going. Perhaps been 36V it must dissipate the heat somewhere?

I finish today all wiring inside the control station and I added a fan I had from a computer case I think it’s about 5”x5” with a 3 speed switch. With the switch set at MED the 36VDC cooling fan now was not always on but it was cycling on/off

Presumably the 36v supply is the one running your steppers?
If so then the 36 volt supply would be dissipating much more power.
For an example, say the 36volt supply supplies 3 amps. Then 36v x 3a would be 108 watts. (plus some more for losses).
If the 12 volt supply were also supplying 3amps then even though it is supplyig the same amount of current it would only dissipate 12v x 3a is 36 watts.
I assume the 12v supply is only for logic and control signals so it probably runs along with a very small load probably less than an amp.

More power dissipated means more heat to get rid of. Hence the fan running.

That's about all I can surmise from your description.

If the supply is in fact in ambient temperature air then it is designed to run the fan to keep itself cool. If you are overloading it then it will have other defences that will kick in like current limiting, voltage foldback or even a fuse to cut out.
I wouldn't worry about it too much. It'll let you know if it has problem.
As for your comparison to other supplies; all supplies are different designs and there may be components due to the particular design that are under more stress. Perhaps the designer decided they'd like to keep those at lower temperatures earlier than the other design.

Sage

Thank you Sage, sounds good what you are saying

Yes the 36V is for my 4 motor drivers and BOB and the 12V is for 8 LED lights I will have in the near future plus the 2 fans.

I didn’t know about the watts formula but now I can see the big difference

Much appreciated