Can i use a Reed switch to detect if my motor is spinning?

[Holewski]

Hey Everyone.
Im designing a product that has an analog electrical system in which i have multiple fans.
im thinking of using a Reed switch to let me know if my Fans are working.
My thought was that when a motor is spinning, it is creating a magnetic field that would close my reed switch. if one of my multiple fans mechanically failed the motor wouldn't be turning which would mean the magnetic field would be much weaker, thereby opening the switch which then would be wired to give me an indication that i have a faulty fan. Is this correct? can i do this?

 

[Darkwood]

You would be better using a proximity sensor looking at a passing metallic object on the shaft or impeller with a frequency converter watching it, other methods are encoders but we start to add more cost, is this safety critical?

 

[Holewski]

You would be better using a proximity sensor looking at a passing metallic object on the shaft or impeller with a frequency relay watching it, other methods are encoders but we start to add more cost, is this safety critical?

Thank you for your reply, I'll explain a little further. they are Kind of are safety critical. im an industrial designer at a a fireplace company. one of my colleagues is designing a project that has multiple fans. They create an air wash across glass. This cools the glass so it doesn't burn anyone who touches the glass. they are mandatory fans. so the fireplace will not operate if the fans are not working.
If there is a mechanical issue with one of the fans the circuit would still be complete meaning there wouldn't be an indication that i had a faulty fan. which is why i wanted a "sensor" that would inform me which one of my many fans stopped spinning.
Do you think its worth testing a Reed switch in this way? would it be sensitive enough to detect the changes in magnetic field?

im not the lead on this project and im trying to find a solution to work with the way the project lead has it designed. This system is stepping outside of what we normally are comfortable with. so just seeing what is available to us.

 

[Megawatt]

Thank you for your reply, I'll explain a little further. they are Kind of are safety critical. im an industrial designer at a a fireplace company. one of my colleagues is designing a project that has multiple fans. They create an air wash across glass. This cools the glass so it doesn't burn anyone who touches the glass. they are mandatory fans. so the fireplace will not operate if the fans are not working.
If there is a mechanical issue with one of the fans the circuit would still be complete meaning there wouldn't be an indication that i had a faulty fan. which is why i wanted a "sensor" that would inform me which one of my many fans stopped spinning.
Do you think its worth testing a Reed switch in this way? would it be sensitive enough to detect the changes in magnetic field?

im not the lead on this project and im trying to find a solution to work with the way the project lead has it designed. This system is stepping outside of what we normally are comfortable with. so just seeing what is available to us.

@Holewski how many fans are you talking about and how is the power distribution to the fans. I just can’t imagine all fans plugged into outlets

 

[PEG]

Hi,just from a belts and braces perspective,if the temperature of the glass,is the critical issue,i would start with a safety device,which dealt with that primary concern.

A fan shaft turning,or a magnetic field,may or may not,have a bearing on your primary risk.

A sensor placed at the glass,or checking the air temperature or flow,from the hot zone,may be more reliable.

Most heating devices,are expected to have fail-safes,built in to the design.

Can i ask,what is fuelling these fireplaces?

 

[Lucien Nunes]

Is this correct? can i do this?

No. A well designed electric motor does not 'leak' much magnetic flux to the outside world, it should ideally all be contained within the core of the motor, although some does in practice leak out. But the amount of flux leaking out does not vary much with whether the motor is turning or stalled, so a reed switch would not be able to detect the operational status of the fan. DC fans produce a similar amount of magnetic flux even when they are not connected to power, since it comes from permanent magnets.

Fans in computers etc have a tacho pulse output that is derived from the motor electronics. If you can use fans of this type, it is a simple matter to convert the pulse train from the tacho wire to a go / no go signal. If not, you will have to add a sensor, either proximity / optical as Darkwood mentions, or your own tacho coil and rotor. A slotted photointerruptor is not expensive and can sense a simple sector vane attached to the shaft.

 

[marconi]

My thoughts on a solution are to keep it very simple ( - and use readily available, cheap, standard products- ) and to detect what you want to avoid - a harmful to the touch high temperature of the glass. This could be achieved using either thermal fuses or thermal switches. I have provided links below. The idea is to have thermal fuses/switches closed when the temperature is safe and to open when the temperature rises above the safe threshold. You can buy these devices to operate at different temperatures. Some open and then have to be replaced. Some have to be reset. Others reset automatically so be careful what to you select for your application. The devices (Normally closed) are wired in series with the the hold-on logic of the heater/gas valve start/open relay/contactor in order that the gas is shut off immediately and then cannot be turned on again without maintainer intervention. Others can explain how to do this. You may want to have two thresholds - one for a warning and the other to cut off the power/gas. And you may decide to sense a number of spots on the glass in which case place a sensor at each one and then wire them in series. I hope this is clear enough.

A4-5AF | +130°C 5 A Thermal Fuse, Limitor, 250V ac | RS Components - https://uk.rs-online.com/web/p/products/1245061?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_UK_EN_Fuses_And_Sockets_And_Circuit_Breakers-_-Fuses%7CNon-Resettable_Thermal_Fuses-_-PRODUCT_GROUP&matchtype=&pla-544712137170&gclid=Cj0KCQiAk7TuBRDQARIsAMRrfUYuau7gDhpgNbNuxiC6qip_eFJX46nMzJFwpGyYEoBHBnwo-AuqnWoaAhUREALw_wcB&gclsrc=aw.ds

thermal cutouts | Rapid Online - https://www.rapidonline.com/Catalogue/Search?Query=thermal%20cutouts

Thermal Fuses | Rapid Online - https://www.rapidonline.com/Catalogue/Search?tier=Thermal%20Fuses

Thermostatic Switches | RS Components - https://uk.rs-online.com/web/c/automation-control-gear/industrial-switches/thermostatic-switches/

 

[James]

Similar to above, just as you use a flame detector to confirm a gas appliance has not gone out, you could use something similar to lock out the heat input if the temperature went above a set limit.

 

[Lucien Nunes]

Whilst the thermal fuse is simple and robust and perhaps should be fitted regardless, overheating actually has to occur before it responds, so the unit will start from cold even if the fans are kaput. Better to prove the airflow before enabling the burner, which might be easily done with a vane switch depending on how concentrated the airflow is. By sensing the airflow rather than fan rotation, stifled air inlets or filters will also be picked up.

 

[Spoon]

I'd go with @marconi suggestion above. It's a nice an cheap option.
If you have money to spare then vane switches are good but expensive. If you have 4 fans and you can find a vane switch for about £25 then this still adds £100 onto the price.
How many fans do you have per item?