Hey All! First post. I am at a site and have a couple of exhaust fans. I want to be able to turn on a plc input if the exhaust fan has power turned on. Someone mentioned using dry contacts. I don't believe the fan itself has any auxilary contacts I could use. Any recommendations on what to do? Thanks.
1/3HP 115V exhaust fan running?
- Thread starter jump3mb
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gar
Senior Member
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- Ann Arbor, Michigan
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- EE
210810-1906 EST
jump3mb:
You need to accurately define what you want to know.
If you simply want to know if AC voltage is present at the fan, then I would put a relay at the fan that changes state when when voltage to the fan is present. Whether the output contact should be a NO or NC will depend upon various factors. I would suggest a reed relay or at least silver contacts, not silver-cadmium-oxide. If you use a solid-state relay, then you may need a shunt resistor at the PLC input.
If you want to know if current is flowing, then use a current sensing relay.
If you want to know if current is in an acceptable range, then you need a more complex current sensor.
.
jump3mb:
You need to accurately define what you want to know.
If you simply want to know if AC voltage is present at the fan, then I would put a relay at the fan that changes state when when voltage to the fan is present. Whether the output contact should be a NO or NC will depend upon various factors. I would suggest a reed relay or at least silver contacts, not silver-cadmium-oxide. If you use a solid-state relay, then you may need a shunt resistor at the PLC input.
If you want to know if current is flowing, then use a current sensing relay.
If you want to know if current is in an acceptable range, then you need a more complex current sensor.
.
hillbilly1
Senior Member
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If you want to know if it’s actually working, a pressure differential switch is the way to go. I had a customer whose engineers used a sail switch to interlock the power in a tool wash down room, that frequently had gas fumes. Problem was, dust and debris would build up on the sail, and shut down the power to the room even though the exhaust fan was running. Changed it to a PDS, and no more problems.
qcroanoke
Sometimes I don't know if I'm the boxer or the bag
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- Sorta retired........
Our marketing dept. often sells fan failure alarms.
When asked what do you mean by fan failure they say power failure.
I don't subscribe to that. I think a fan failure alarm is a no air movement alarm. You can make this a very complicated alarm......
hillbilly1
Senior Member
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- North Georgia mountains
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Exactly, that customer that I was talking about has belt driven fans, and if the belt breaks, motor keeps running….but the fan doesn’t.
Thanks all. There are three fans. One will be on all the time. The other two turn on at different times. We do have pressure differential switches on each one. We wanted to use a combination the two. Maybe activate a relay and use dry contact to get signal that fan has power going to it. Then also look at pressure differential switch status. So if dry contact is on telling us fan has power and is running, and we dont see change in pressure within a certain amount of time then we know there is an issue with one of the fans and have an 'HVAC' problem. So seems like a relay with some dry contacts would work in addition to PDS. Tie the dry contacts to an input in the plc to tell us if fan is actually getting power and use input for PDS to determine if its actually turning.
DrSparks
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Sounds like you want an airflow proving switch. One like this:
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