DC Motor Air Switch question

[BearB]

I have a 40hp DC Motor on a Digital Drive and I would like alarm maybe with a flashing light, not shutdown if the Motor Blower is not operating. This Drive is stand alone not connected to PLC or DCS. The Motor Thermal Switch is going back to the Drive but I would like to catch the problem before the motor heats. I was considering an air Switch with contacts to operate a flashing light to get maintenance to address the Motor Blower before the Process is effected. But I am not sure about the reliability of air switches for this small of an application. Has any one have similar experience?

 

[waross]

Use either a sail switch or a zero speed switch or slip detector on the fan shaft. I saw a lot of problems with differential pressure switches to prove air flow. That's probably why I haven't seen a differential pressure switch on a fan for decades.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BearB]

Thanks for the feedback Bill. This site is always helpful.

 

[Compositepro]

It would be better to use a temperature switch on the motor. This will detect a problem due to more causes and a temperature switch is more robust cheaper, and more reliable.

 

[waross]

Compositepro;
I understand that the motor already has a temperature switch. BearB wants an alarm that will act before the motor heats.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

If the blower is separately powered, maybe a Current Sensing Relay on the fan power leads? No (or too low) current means the fan is not moving air. Covers a failure AND a blockage, but might miss a bad bearing. Still, too high of current for too long could detect that. So maybe an Over/Under current relay. Is the blower AC or DC?




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[edison123]

How about a speed encoder for the fan with an alarm when the fan is not running at its rated speed along with jraef's current sensor ? That should cover any seized fan motor bearings and the fan motor running without the fan (some one forgot to put the fan there !)

Of course, you must be already having a mechanism of starting the dc motor along (or better, before) with the fan motor.

 

[itsmoked]

You should reconsider the air switches and vanes. 40hp is far more than you need for useful air/vane switching. They're visually testable and easy to adjust. They are simple and easily repairable. They prove far more than most other methods. They can show thrown blades, missing fans, shafts spinning in the fans, and even down-range changes like closed dampers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

When I worked in mining we always used Dwyer Photohelic switches across the motor filter to prove air flow, I never saw one of these fail.
You could also use a cheaper Dwyer switch to monitor the fan pressure.
Often it's not the switch that fails but a bad installation detail.
I recommend that whatever switch you use, verify the switches operation by using it's non activated state as a permissive in the fan start circuit, if it doesn't change state when the fan stops you don't have permission to start it again and if it doesn't activate when you start the fan the DC motor wont start. If they both start together you may need a timer to bypass the switch for a few seconds.
Regards
Roy

 

[BearB]

Thanks all for the feedback.