Exhaust fan alarm

M

MrozElec

New User
Location
PA
Occupation
Electrician
I was asked to install alarms for a large exhaust fan in a caustic area in a plant. What they want to do is have an alarm go off if the motor is not working as the can not hear when it is running or not. There is a switching circuit for the fan in the work area but they leave it on all of the time. I know I will have to add a relay to power up the lights/speakers from the load side if the switching circuit going through the NC contact so if there is loss of power on the coil of the relay and the switch is on the alarms will sound. The problem I am having is I'm not sure how I should power the coil. If I come off of the load side of the disconnect by the motor that only tells me that a fuse blew and while that would show that there is a problem the motor could go bad and not blow a fuse. I thought about using a current transformer off of one of the legs because if it is not drawing any amperage that would be a better judge of whether or not the motor is working. The problem is as I understand it CT transform the current to a more manageable amperage but the voltage will also drop to millivolts which I think would not provide appropriate voltage to energize the coil. Are there any other Ideas as to how I can achieve my goal? Any advice is welcome.
 
My suggestion would be to monitor the air differential pressure across the fan directly, with an differential pressure switch.

An adjustable differential switch with sensing tubes will tell you whenever the fan is not moving air for whatever reason, electrical, mechanical, missing belts, clogged filters...


Used these for many years on many fan types for proving operation. Mount the switch diaphragm in a vertical position, run 1/4" copper tubing to both the suction and discharge side of the fan housing for sensing fan air pressure.

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If you don't want to monitor the air and if the fan is belt driven, I wonder about using something like a PMP25 from Load Controls. They are designed for pump motors and have used them for cutting off the pump power when the pump runs dry. It monitors the power used by the pump motor and it has points you can set for alarm or shut down. It addresses all of the concerns when using just a current switch
I don't see why it would not work for a fan motor. However, it is probably cheaper to just sense the air flow as others have suggested. The only advantage of something like this is it can be installed in or at the motor starter without additional field wiring to a remote air flow sensor.
 
Depending on how critical this is, I also agree with monitoring air flow/pressure as tripped breakers, broken belts, etc. could give you false detection of what you are looking for if only relying on a aux contact on the motor starter.
 
MTW said:
My suggestion would be to monitor the air differential pressure across the fan directly, with an differential pressure switch.

An adjustable differential switch with sensing tubes will tell you whenever the fan is not moving air for whatever reason, electrical, mechanical, missing belts, clogged filters...


Used these for many years on many fan types for proving operation. Mount the switch diaphragm in a vertical position, run 1/4" copper tubing to both the suction and discharge side of the fan housing for sensing fan air pressure.

View attachment 2576578

View attachment 2576577
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I’ve used those before for the same reason on Home Depot tool rental maintenance rooms. They originally were using sail switches, but dust would collect on them and fail. WalMart uses them as dirty filter alarms.
 
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