Fan Failure Alarm

[Izzul]

Hi guys,

I would like to ask your help. 

I would like to make an electronic circuit where it can detect the fan failure and indicates it with the light. 

 

The specs are as follow:

-240VAC cooling fan (4 unit)

-1 tower light (green & red)

 

I want to make the light indicate green when all the fan are working but the red when 1 or more of the fan fault. 

 

Thank you guys for your cooperation. I really appreciate it. 

[audioguru]

Can the wind from a fan push a wing that pushes on a changeover switch? Then when the fan is slow or it stops (even if its electrical power gets cutoff) the changeover switch changes the light from green to red. The switches must use diodes and resistors and DC to make an "OR gate". The DC from the OR gate can activate triacs that turn on and off the lights. 

[Izzul]

Thank you for your good idea!

 

But in this case, I don't think that method is effective because there are multiple fan running simultaneously and the external air also may cause error signal. 

[pebe]

Behind each of the four fans you could put a thermistor and wire them all in series. Feed them through a current limiting resistor from a voltage source. Power will be dissipated in them and they would normally get warm, but the fans will keep them cool.

Monitor the voltage across the thermisters. Failure of one or more fans will result in a change in that voltage.

[Izzul]

Behind each of the four fans you could put a thermistor and wire them all in series. Feed them through a current limiting resistor from a voltage source. Power will be dissipated in them and they would normally get warm, but the fans will keep them cool.

Monitor the voltage across the thermisters. Failure of one or more fans will result in a change in that voltage.

That's pretty good idea. Doable I guess.
But actually I'm looking for internal detection solution.
Is it possible ?

[Hero999]

What do you mean by internal detection?

You could measure the current through the fan motors and if it's too high or too low, it's a reasonable indication that the fan isn't working properly. It's not as reliable as monitoring the air speed but it's better than nothing.

[Izzul]

What do you mean by internal detection?

You could measure the current through the fan motors and if it's too high or too low, it's a reasonable indication that the fan isn't working properly. It's not as reliable as monitoring the air speed but it's better than nothing.

How can I measure the current through  the fan motors ? Can you give me the concept or something ?
And how can I make them indicate the faulty fan where there are more than 1 fan ?

[Hero999]

How can I measure the current through  the fan motors ? Can you give me the concept or something ?
And how can I make them indicate the faulty fan where there are more than 1 fan ?

A low value resistor connected in series with the fan can be used to measure the current flowing through it. The voltage across the resistor, therefore the current, can be calculated using Ohm's law.

Each fan will need to have its own current sensing resistor.