High-Temperature Differential Pressure Switch

http://www.dwyer-inst.com/

Another possibility is to monitor the phase angle of the supply to the motor using a motor underload unit; if the phase angle goes below a certain figure then it ain't working!

(Other instrument suppliers are available)

Tried it but as they are fractional hp motors you can stall them or disconnect the impeller blades and not detect a change in current.

Simple ammeter, then?

Who are you calling a simple amateur?

I'd think the differential pressure from a fan would be fairly low unless there is a lot of ducting. I'd think maybe a hall effect sensor or some sort of movement sensor might be a better bet?

We use Dwyer pressure switches and gauges in our air filters to tell us if the filters need to be changed. We've also used current sensing for alarm purposes (under current).

The best solution we've used to sense if a fan is rotating is to put an inductive sensor to detect a couple of bolts welded onto the shaft. As the fan turns, the sensor is triggered and as long as the pulses keep coming, we know the fan is still turning.

In one installation, we went a little further. We put an inductive sensor on the fan's shaft and connected it to a frequency limit module. If the fan's belts failed, the fan's speed will drop. The module will sense that and trigger an alarm.

The problem is the 90 C ambient temperature

???!!!

90 deg C that's not v hot..ok its hot but not that hot.

So don't use plastic sensors! It doesn't have to be the inductive type. All we're interested in is in having it sense the bolt heads on the shaft. If needed, you can use an optical sensor wrapped in an insulated cylinder with a glass face. You'll need something on the shaft to sense but that's not much of a problem.

Alternative:

Put a magnet (or two or three) on the shaft and put a sensing coil close to them. As the shaft turns, an electrical current is produced that you can use to detect the spinning of the shaft.

Almost all computer fans have a built in hall effect switch with a signal output lead to warn the computer if the fan stops turning...

Just look in your BIOS PC health section, it will display the rpm of the CPU fans etc... and show the minimum limit.

As others have said why not use that method?

John.

http://content.honeywell.com/sensing/products/di/

loads of options

Here is a unique solution which has worked for us .

it is robust and realy cheap.

All you need is a low power heater mounted on an insulated holder with a sensor ( either a thermocouple , RTD or temperature switch).

as long as the fan is running, the temperature is low and can be read by the thermocouple or RTD or result in no activation of the thermostat.

If the fan fails , then there is no cooling of the heater and the sensor device wil detect it.

Hpe this helps.

Take a look at Ashcroft Gauges. The 316SS D/P switch with Viton seal is good for 300° F (150° C).